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\begin{document}

\begin{frontmatter}

% Title, authors and addresses

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\title{Seismicity in the outer rise offshore southern Chile:
  indication of fluid effects in crust and mantle}

% use optional labels to link authors explicitly to addresses:
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\author[cam]{Frederik J. Tilmann\corauthref{cor1}}
\corauth[cor1]{Corresponding author}
\ead{tilmann@esc.cam.ac.uk} 
\DIFdelbegin %DIFDELCMD < \author[ifm]{%%%
\DIFdel{Ingo Grevemeyer}\DIFdelend \DIFaddbegin \author[ifm,ham]{\DIFadd{et al}\DIFaddend }
\DIFdelbegin %DIFDELCMD < \author[ifm]{Ernst R. Flueh}
%DIFDELCMD < \author[ifm]{J\"urgen Go{\ss}ler}
%DIFDELCMD < \author[ifm]{Martin Scherwath}
%DIFDELCMD < \author[ham]{Torsten Dahm}
%DIFDELCMD < %%%
\DIFdelend %DIF > \author[ifm]{Ingo Grevemeyer}
%DIF > \author[ifm]{Ernst R. Flueh}
%DIF > \author[ifm]{J\"urgen Go{\ss}ler}
%DIF > \author[ifm]{Martin Scherwath}
%DIF > \author[ham]{Torsten Dahm}

\address[cam]{Bullard Laboratories, University of Cambridge,
  Cambridge CB3 0EZ, UK}
\address[ifm]{IfM-GEOMAR, Kiel, Germany}
\address[ham]{Institute for Geophysics, University of Hamburg, Hamburg, Germany}

\begin{abstract}
% Text of abstract
\DIFdelbegin %DIFDELCMD < \remark{EPSL limits: 6500 words in main text, 10 figures, 80 refrences}
%DIFDELCMD < %%%
\DIFdel{(Type abstract here)}\DIFdelend %DIF > \remark{EPSL limits: 6500 words in main text, 10 figures, 80 refrences}
\DIFaddbegin 

%DIF > Hydration of the crust and mantle at the outer rise has been proposed as a significant
%DIF > pathway for water into the subduction system. Anomalously low heat flow values in
%DIF > some outer rise regions indicate the presence of an active hydrothermal system, multichannel
%DIF > seismic profiles have shown that large bending faults cross the Moho and
%DIF > penetrate the mantle, and Pn velocities measured on refraction profiles are reduced just
%DIF > seaward of the trench, consistent with partial hydration of the
%DIF > underlying mantle. Here,
%DIF > we present observations that suggest the presence of free fluids both within the crust
%DIF > and mantle of the Outer Rise of even very young plates, which lack the characteristic
%DIF > bulge due to their low elastic thickness, and where changes in seismic velocity are
%DIF > hard to see because of superposition with the plate-cooling
%DIF > signal. Specifically, 

\DIFadd{We examine the micro-earthquake seismicity recorded by two temporary arrays of ocean
bottom seismometers on the outer rise offshore southern Chile on young oceanic plate
of ages 14 Ma and 6 Ma, respectively. The arrays were in operation from
December 2004-January 2005 and consisted of 17 instruments and 12 instruments, respectively.
Approximately 10 locatable events per day were recorded by each of the arrays.
The catalogue, which is complete for magnitudes above 1.2-1.5, is characterized by
a high $b$ value, i.e., a high ratio of small to large events, and the data set is remarkable
in that a large proportion of the events form clusters whose members show a
high degree of waveform similarity. The largest cluster thus identified consisted of
27 similar events (average inter-event correlation coefficient$>$0.8
for a 9.5~s window),
and waveform similarity persists far into the coda. Inter-event spacing is irregular, but
very short waiting times of a few minutes are far more common than expected from
a Poisson distribution. Seismicity with these features (high $b$ value, large number of
similar events with short waiting times) is typical of swarm activity, which, based on
empirical evidence and theoretical considerations, is generally thought to be driven
by fluid pressure variations. Because no
pronounced bulge exists on the very young plate in the study region, it is unlikely
that melt is accessible from decompression melting or opening of cracks. Likewise no
pressure or temperature changes are imaginable which could have initiated dehydration
reactions. We thus infer the fluid to be derived from seawater, which enters through fractures in the
crust. 
Most of the similar-earthquake clusters are within the crust, but some of them locate
significantly below the Moho. If our interpretation is correct, this implies that water
is present within the mantle. Hydration of the mantle is also indicated by a decrease of $P_n$ velocities below the outer rise seen on a refraction profile through one of the arrays \mbox{%DIFAUXCMD
\citep{contreras-reyes07}
}%DIFAUXCMD
.
The deepest events within the array on the 6 Ma old
plate occur where the temperature reaches 500-600}\dg \DIFadd{C, consistent with the value observed
for large intraplate earthquakes within the mantle (650}\dg \DIFadd{C),
suggesting that the maximum temperature
at which these fluid-mediated micro-earthquakes can occur is similar or identical to that of
large earthquakes.
}\DIFaddend \end{abstract}

\begin{keyword}
subduction \sep outer rise \sep seismicity \sep fluids and earthquakes \sep serpentinization
% keywords here, in the form: keyword \sep keyword
\PACS 91.30.Dk \sep 91.30.Ga \sep 91.30.Ye \sep 91.50.Wy \sep 91.55.Tt
% PACS codes here, in the form: \PACS code \sep code
% 91.30.Dk Seismicity
% 91.30.Ga Subduction zones
% 91.30.Ye Oceanic crust seismology
% 91.50.Wy      Subduction zone processes
% 91.55.Tt      Role of fluids
\end{keyword}

\end{frontmatter}

\DIFaddbegin \DIFadd{\pagewiselinenumbers
}\DIFaddend % main text
\section{Introduction}

The outer rise forms an integral part of the subduction system with
regard to both material fluxes and stress distribution. \DIFaddbegin \DIFadd{The oceanic lithosphere is stressed by 
bending and remote loading from the subduction zone or possibly other nearby plate boundaries, so that crustal and even mantle earthquakes occur.
}\DIFaddend Faulting at the outer rise is thought to provide pathways for water into the
lithosphere, both its crustal part \citep{kirby96} and probably also
into the mantle  \DIFdelbegin \DIFdel{\mbox{%DIFAUXCMD
\citep{ranero03}
}%DIFAUXCMD
}\DIFdelend \DIFaddbegin \DIFadd{\mbox{%DIFAUXCMD
\citep{ranero03,ranero04,grevemeyer07}
}%DIFAUXCMD
}\DIFaddend ; it therefore exerts a strong
influence on melt generation and rheology further down the subduction
system.  \DIFdelbegin \DIFdel{The causative stress field }\DIFdelend \DIFaddbegin \DIFadd{Hydration of the mantle lithosphere at the outer rise can
potentially more than double the amount of water carried by the
downgoing slab \mbox{%DIFAUXCMD
\citep{rupke04}
}%DIFAUXCMD
. Dehydration of serpentinised mantle
has also been proposed to account for the lower plane of double
seismic zones \mbox{%DIFAUXCMD
\citep{peacock01}
}%DIFAUXCMD
.
 The  stress field at the outer rise }\DIFaddend is a superposition of bending
stresses and regional horizontal stresses \citep{christensen88}.
Because the latter varies through the seismic cycle of the interplate
megathrust \citep[e.g.][]{taylor96}, with trench-perpendicular
compression late in the cycle and tension early in the cycle,
\citet{christensen88} proposed that an 
increased numbers of outer rise thrust events indicates an
interplate-thrust close to the rupture limit, whereas many normal faulting events
are expected to follow great interplate earthquakes. However, actual
mechanisms also depend on the loading history \citep{mueller96b}, such that the
relationship is likely to be more complicated than suggested by this
simple idea.

In addition to
its significance to the subduction system, the outer rise region also
plays host to most of the world's oceanic intraplate  seismicity
(excluding intermediate and deep-focus earthquakes which might be
controlled by different frictional laws).  
%Because the temperature and
%composition of oceanic plates is comparatively well understood,
Intra-plate earthquakes in oceanic mantle have been used to show that
temperature is the dominant factor controlling the
depth of brittle faulting \citep{wiens83}, with 650\dg C being the
maximum temperature at which earthquakes are observed
\citep{mckenzie05}.
However, all previous studies of outer rise seismicity and oceanic
intraplate seismicity in general exclusively utilised teleseismic and
regional data \citep[e.g.][]{wiens83,christensen88}. This approach allows global
  coverage but limits the analysis to strong earthquakes ($m_b>\sim
  5.5$). This limitation poses particular problems for very young
  oceanic plates where few events of sufficient magnitude
  occur.   \DIFdelbegin %DIFDELCMD < \remark{better check this statement for other TJs}
%DIFDELCMD < %%%
\DIFdelend %DIF > \remark{better check this statement for other TJs}
Moreover, the finite source properties of the larger of these events
make it hard to ascertain the maximum depth of faulting (as opposed to
the hypocentre, see \citet{tichelaar92}).
Here, we take the complementary approach and use \DIFdelbegin \DIFdel{a dense array }\DIFdelend \DIFaddbegin \DIFadd{dense arrays }\DIFaddend of
ocean bottom stations to characterize micro-earthquake activity in the
outer rise of the Chilean subduction zone.

A number of fracture zones north of the Chile triple junction segment
the Nazca plate which subducts at the Chilean margin. No significant
variation \DIFdelbegin \DIFdel{exists }\DIFdelend \DIFaddbegin \DIFadd{has been resolved }\DIFaddend in 
%either the fossil spreading rate (5-12
%cm/yr  between the TJ and 40\dg S) 
% full spreading rates seaward of trenchmeasured from Ingo magnetic anomalies map: 
% 4.7 cm/yr at 47 S 
% 8.0 cm/y  at 43 S
% 12.3 cm/y at 41 S
%
%gmtdefaults -D >.gmtdefaults4
%gmtset PAGE_ORIENTATION portrait
%makecpt -Crainbow  -T0/200/20  > rate.cpt
%makecpt -Crainbow  -T10/40/2  > rate.cpt
%grdimage -Y4 /usr/local/gmt/data/agegrid/rate_1.6.grd -Crate.cpt -R-87/-70/-49/-35 -JM14.6 -Ba5g5f1 -K > rate_chile.ps
%grdcontour /usr/local/gmt/data/agegrid/rate_1.6.grd -R -JM -A10 -C5 -L0/40 -O -K >>rate_chile.ps
%psscale -Crate.cpt -B5 -D5/-2/10/1h -O >>rate_chile.ps
%
%grdimage -Y4 /usr/local/gmt/data/agegrid/age_1.6.grd -C/usr/local/gmt/data/agegrid/age.cpt -R-87/-70/-49/-35 -JM14.6 -Ba5g5f1 -K > age_chile.ps
%psscale -C/usr/local/gmt/data/agegrid/age.cpt -D5/-2/10/1h -O >>age_chile.ps
the convergence rate between the Nazca and South American
plates (7.9  - 8.0 cm/yr, near trench-perpendicular convergence of
N79E) \citep{demets94}. 
% octave:4> [vel,az]=rpm(-46,-76,'sa','nz','nuvel1a')
% octave:5> [vel,az]=rpm(-35,-74,'sa','nz','nuvel1a')
% octave:5> [vel,az]=rpm(-40,-75,'sa','nz','nuvel1a')
On the other hand, large variations in thermal structure are expected
because the square root of time dependence of plate cooling results in
strong age dependence for young plates.
%This leaves the age of the oceanic plate as
%the only large-scale tectonic parameter likely to influence processes
%at the outer rise. 
%  Because on the segment just north of the triple
%junction the spreading ridge is so close to the outer rise that no
%distinct outer rise region can be identified, 
\DIFdelbegin \DIFdel{We }\DIFdelend \DIFaddbegin \DIFadd{During cruise SO181 of R/V Sonne (December 2004-February 2005) we }\DIFaddend have placed seismic arrays on the second and
third youngest subducting 
segments (age at the trench 6 and 14 Ma at \DIFdelbegin \DIFdel{center }\DIFdelend \DIFaddbegin \DIFadd{centre }\DIFaddend of segment, respectively), which are
bounded, from south to north by the Guamblin, Guafo and Chiloe
fracture zones.  This
experiment forms part of the TIPTEQ project \DIFaddbegin \DIFadd{\mbox{%DIFAUXCMD
\citep{scherwath06}
}%DIFAUXCMD
}\DIFaddend , such that the passive
seismic data are complemented by refraction \DIFaddbegin \DIFadd{and heat flow }\DIFaddend profiles as
well as swath mapping and magnetic profiling. 

\section{Data and processing}

The northern array consisted of 7 ocean bottom hydrophones (OBH) and 10 four-component stations (ocean
bottom seismometers - OBS).
% STA1 no significant data obh13 
   The southern array consisted of 2 OBH and 10 OBS.  All stations
   were sampling continuously at 100 Hz, except OBH28 within the
   southern array \DIFdelbegin \DIFdel{(}\DIFdelend \DIFaddbegin \DIFadd{which sampled at }\DIFaddend 50 Hz\DIFdelbegin \DIFdel{). Two
   stations , }\DIFdelend \DIFaddbegin \DIFadd{. Most stations were operational for 5--6 weeks, but two
   stations, }\DIFaddend one in each array\DIFaddbegin \DIFadd{, }\DIFaddend did not return any useful data (see
   \DIFaddbegin \DIFadd{Table~\ref{tbl:stations} in }\DIFaddend the supplementary material for \DIFdelbegin \DIFdel{a table of }\DIFdelend station locations and
   instrument types). 
   The hydrophone records generally showed clear
   P onsets and, for most stations, could easily be picked on
   the unfiltered traces (\DIFdelbegin \DIFdel{see  supplementary Figure~\ref{figa:waveform}). 
  .  }\DIFdelend \DIFaddbegin \DIFadd{Figure~\ref{fig:waveform}). 
  }\DIFaddend Strong S arrivals were visible on all
   seismometer components (including the vertical) but were
   characterized by ringing, nearly monochromatic waveforms, presumably due to resonances within the shallow
   sediment. The P arrivals on the hydrophone channels were usually
   succeeded by a strong arrival on the \DIFdelbegin \DIFdel{horizontal }\DIFdelend \DIFaddbegin \DIFadd{seismometer }\DIFaddend channels 0.5--1~s
   later, with the time delay varying from station to station but
   being similar between events.  Similarly, S arrivals were preceded
   by precursory arrivals, which were particularly clear on the hydrophone
   channels, but \DIFdelbegin \DIFdel{also appeared }\DIFdelend \DIFaddbegin \DIFadd{sometimes also appeared weakly }\DIFaddend on the horizontal components, again
   separated by approximately 0.5--1~s from the main arrival.  The
   delayed and precursory phases are presumably related to
   mode-conversions at the basement. Active source data show that sediment
   thickness does not exceed 500~m in the area of the array
   \citep{ifmgeomar02} but the observed large delays are easily
   explained by extremely low S velocities ($<$100 m/s) in the
   uppermost layers of unconsolidated marine sediment \DIFdelbegin \DIFdel{. }%DIFDELCMD < \remark{needs a
%DIFDELCMD <      reference}%%%
\DIFdel{.  
}\DIFdelend \DIFaddbegin \DIFadd{\mbox{%DIFAUXCMD
\citep{hamilton80}
}%DIFAUXCMD
. 
}\DIFaddend % Care was taken during picking to pick the main
%   S arrival and not the precursory mode-conversion.

\subsection{Location and magnitude determination}

Before any further analysis timing and location of the ocean bottom
stations were corrected as described in the supplementary material. \DIFaddbegin \DIFadd{We
then carried out the following procedure.
}\DIFaddend \begin{enumerate}
\item Generation of  a preliminary list of events
with a STA/LTA trigger algorithm that  detects nearly
coincident changes in the amplitude at several stations.  
\item Manually inspect all trigger events and pick arrivals, assigning
a subjective weight to each pick. Remove
events, which are unclear, presumably not earthquakes, or cannot be
picked on at least three stations. 
\item Obtain a preliminary location of each event by linearised
inversion and using a 1-D velocity model derived from the refraction
data \DIFdelbegin \DIFdel{\mbox{%DIFAUXCMD
\citep[see supplementary material for velocity
model]{scherwath06}
}%DIFAUXCMD
.  
%DIF <  reference to EOS article
}\DIFdelend \DIFaddbegin \DIFadd{\mbox{%DIFAUXCMD
\citep{scherwath06}
}%DIFAUXCMD
.  
}\DIFaddend % STA2: grep '1$' /work/tilmann/SEISAN8/WOR/STA2_/STA2_alls20.nor | wc -l
% STA1: grep '1$' /work/tilmann/SEISAN8/WOR/STA1_/STA1_all.nor | wc -l
A total of 656 events were located by the northern array, and 1114
events by the southern array.
\item Based on preliminary locations, temporarily remove events which
  were recorded by less than 5 stations, have only picks for one phase
  type (P or S), or are far from the array (azimuthal gap
  $>300\dg$).
% STA2: grep '1$' /work/tilmann/SEISAN8/WOR/STA2_/STA2_goods20.nor | wc -l
% STA1: grep '1$' /work/tilmann/SEISAN8/WOR/STA1_/STA1_good.nor | wc -l
 This reduced event set comprised 240  events for the
  northern array, and 484 events for the southern array.

\item Apply the joint
  \DIFdelbegin \DIFdel{hypocenter }\DIFdelend \DIFaddbegin \DIFadd{hypocentre }\DIFaddend determination (JHD) method to the restricted event sets (using
  \textsc{Velest} \citep{kissling94}).  Station delays for S were large ($\sim 1$s) and did
  not correlate well with P delays but instead correlated with
  the observed delays between the P arrival and the delayed phase, and
between the S-precursor and main S arrival.  This observation is consistent
  with our interpretation of a zone of extremely low S velocities
  causing much of the delay between direct and mode-converted phases. 
\item For the southern array only, we further restrict the data set to
  those events with a gap of less than 200\dg\DIFaddbegin \ \DIFaddend and eliminating outlier
  picks 
  with residuals larger than 0.75~s (after JHD).
% STA2: grep '1$' /work/tilmann/SEISAN8/WOR/STA2_/inarray.sgl.s20.jhdfresh.nosed-vpvs18.nor | wc -l 
% 
  This procedure leaves 287 events. A minimum 1D-model is determined
  for this event set \citep{kissling94} \DIFaddbegin \DIFadd{(see Figure~\ref{figa:velocity} in the supplementary material for velocity
model)}\DIFaddend .  We tested the robustness of the \DIFdelbegin \DIFdel{minimum-1d
  }\DIFdelend \DIFaddbegin \DIFadd{minimum-1D
  }\DIFaddend model by randomly perturbing the starting model and comparing the resulting models and depth distributions.
  Only slight differences in average event depths are found
  ($<$1~km). Similarly, even with extreme variations in starting event
  depths,
%(all starting depths set to 15 km)
 the resulting final depth distributions and models are
  similar. \\
  Most of the events were located at the margin or outside the
  northern array, so that a minimum-1D model could not be determined.
  However, the velocity structure of the northern array is
  constrained by both trench-parallel and -perpendicular refraction
  lines \DIFdelbegin \DIFdel{\mbox{%DIFAUXCMD
\citep{scherwath06}
}%DIFAUXCMD
}\DIFdelend \DIFaddbegin \DIFadd{\mbox{%DIFAUXCMD
\citep{scherwath06,contreras-reyes07}
}%DIFAUXCMD
}\DIFaddend such
  that the lack of a minimum-1D model in the north should not
  compromise our ability to determine accurate
  locations. 
\DIFdelbegin %DIFDELCMD < \remark{probably should reference Contreras at this
%DIFDELCMD <     point. Is there anything suitable}
%DIFDELCMD < %%%
\DIFdelend \item Using the station terms and, in the south, minimum 1D-model
  \DIFaddbegin \DIFadd{(Table~\ref{tbl:stations}, Figure~\ref{figa:velocity} in supplementary material)
  }\DIFaddend derived in the previous two steps we relocate all earthquakes using
  the non-linear oct-tree search algorithm \citep{lomax00}.  This method efficiently
  explores the probability density function for location associated
  with a particular earthquake, and as such provides much better
  information on location errors than linearized inversions.  The
  maximum-likelhood location is retained as the catalogue location. 
\item 
Finally, moment magnitudes are estimated using the automated procedure
of \citet{ottemoeller03}, which was modified to account for the
requirements of the ocean bottom data (see supplementary material for
further information).  
\end{enumerate}

\subsection{Correlation and cluster identification}

We cross-correlate the waveforms from all events.  In many cases, an
extremely high degree of waveform similarity is observed between the traces of
different events, with correlation coefficients 
as high as 0.95 for a correlation window of 9.5 s, equivalent to more
than 60 oscillations at the dominant period of the data (Figure~\ref{fig:clust-waveform-histo}a).
We identify
all events with similar waveforms and group them into clusters using
hierarchical clustering with the dendrogram method \DIFdelbegin %DIFDELCMD < \remark{Ref needed}%%%
\DIFdelend \DIFaddbegin \DIFadd{\mbox{%DIFAUXCMD
\citep{hartigan75}
}%DIFAUXCMD
}\DIFaddend .  During the deployment
period, 41\% of events within the southern array exhibited a high
degree of waveform similarity \DIFaddbegin \DIFadd{(average correlation coefficient larger
than 0.8) }\DIFaddend with at least one other event, with the
largest cluster encompassing 27 events (Figure~\ref{fig:clust-waveform-histo}c). For the northern array,  32\%
of events are members of a cluster, and the largest cluster consisted
of 11 events \DIFaddbegin \DIFadd{(Figure~}\DIFaddend \ref{fig:clust-waveform-histo}b).   Within 
both arrays doublets were the most common type of cluster.
% Percentages from last line of merge_P_BP5-20P2_9-5.clusters

For all identified clusters, cross-correlation time
differences are evaluated using a short window of 0.4 s for the P
arrival and 0.6~s for the S wave (with lead-in times before the P or S
pick of 0.1 and 0.2~s, respectively). The short time window minimises
the impact of scattered arrivals, while the restriction of correlation
time measurements to known clusters eliminates the possibility of
obtaining spurious measurements with high correlation coefficients
which otherwise are expected to occur by chance for short time windows.
All events are relocated using the double-difference procedure with
both catalogue and correlation times but only approximately half the
events are sufficiently linked to neighbouring events to be included
in the double-difference inversion (see Figure~\ref{fig:epimaps}).

% Command to get these numbers
% run_all.csh analyse_clust_times
A cluster location for all clusters of three or more events is derived
by first aligning the waveforms, and then using the median of the
manual picks of individual events as the cluster arrival time.  This
procedure also allows an independent verification of the manual
picking errors.  If we define the picking error as the difference
between the earliest and latest pick within a nearly identical
waveform, the average picking error for P is 0.06-0.10~s (maximum: 0.58-0.77~s)
and the average error for S is 0.09-0.10~s (maximum 0.38-0.88).; the
lower bound pertains to the southern array and the upper bound to the
northern array.  
% except maximum S where it is the other way round.
 It has to be remembered that these picking errors are only valid for
 identified clustered events and might not be representative of the
 overall dataset. 

Lastly, for all those cluster locations 
with small azimuthal
gaps and good depth constraints we pick the polarity of arrivals on the hydrophone
component. The ability to view nearly-identical signals on several
traces strongly reduces the likelihood of misidentifications of
polarities due to the presence of noise. The station density is not
high enough to determine reliable focal mechanism solutions for
individual clusters but an estimate of the regional stress field was
made using the method of \citet{robinson00}, which attempts to fit the
first motions of all earthquakes simulataneously by varying the
direction of the regional P and T axes and using the assumptions that earthquakes
occur on optimally oriented fault planes.  Because bending might cause
a variation of the `regional' stress with depth, we separate the event set
into shallow crustal ($<$4 km below the seafloor) and deep
crustal/mantle earthquakes ($>$4 km \DIFdelbegin \DIFdel{) }\DIFdelend below the seafloor\DIFaddbegin \DIFadd{)}\DIFaddend .

Because most of the activity in the north is just outside the array,
the first motions do not provide a meaningful constraint on the
stress field there and were not picked.
\DIFdelbegin %DIFDELCMD < \remark{Vertical component not suitable because of low gain}
%DIFDELCMD < %%%
\DIFdelend %DIF > Vertical component not suitable because of low gain

\section{Results}

\subsection{\DIFdelbegin \DIFdel{Epicenters }\DIFdelend \DIFaddbegin \DIFadd{Epicentres }\DIFaddend and magnitude distribution}

The seismic activity recorded by the northern array is strongly
concentrated in a small area of about 10$\times$20~km$^2$
(Figure~\ref{fig:epimaps}a).  Event
rates within this area \DIFdelbegin \DIFdel{drop }\DIFdelend \DIFaddbegin \DIFadd{dropped }\DIFaddend sharply from 10 events per day to 1.7 per
day on December
\DIFdelbegin \DIFdel{23. (Event }\DIFdelend \DIFaddbegin \DIFadd{23 (Figure~\ref{fig:clust-statistics}b; event }\DIFaddend rates only take into account events in the reduced set with
magnitude
determinations).  As outside this region events \DIFdelbegin \DIFdel{occur }\DIFdelend \DIFaddbegin \DIFadd{occurred }\DIFaddend at  a nearly constant rate of
2.3 events per day throughout the experiment, and no  changes to the network configuration took
place near December 23 we \DIFdelbegin \DIFdel{are probably witnessing }\DIFdelend \DIFaddbegin \DIFadd{probably witnessed }\DIFaddend the tail end of an
earthquake swarm with event magnitudes up to $M_W$=2.7, but no clear
mainshock-aftershock sequences.   Even after December 23, the swarm
region \DIFdelbegin \DIFdel{remains }\DIFdelend \DIFaddbegin \DIFadd{remained }\DIFaddend the most active region in terms of number of events per
area.

In contrast, no single region dominates in the region of the southern
array, and the event rate is 10 
events/day throughout.  Some events form elongated groups that are
aligned sub-parallel to the pre-existing fabric; however, the clearest
of these groups,
between obh19 and obs24~(Figure~\ref{fig:epimaps}b), actually subtends an angle
of nearly 20\dg\ with pre-existing seafloor fabric.  

The frequency-magnitude distribution (Figure~\ref{fig:freq-mag}) of
events in both sub-arrays is
characterized by \DIFdelbegin \DIFdel{b values between }\DIFdelend \DIFaddbegin \DIFadd{$\beta$ values (slope of frequency-$\log(M_0)$ plot) between 1.0 and 1.1 for the
northern and southern array, respectively,
equivalent to $b$ values (for $M_W$) of
}\DIFaddend 1.5 and 1.7.
%b values this large are
%typically only  found in volcanic or geothermal
%areas, or in creeping sections of faults \citep{wiemer02}.  
There is
some suggestion in Figure~\ref{fig:freq-mag} that the \DIFdelbegin \DIFdel{b }\DIFdelend \DIFaddbegin \DIFadd{$b$ }\DIFaddend value of the northern array is biased upwards by
the swarm activity, but the number of earthquakes outside the swarm
region is too small for a reliable estimate. 
The magnitude of completeness $M_c$ differs by 0.4 magnitude units between
the southern and northern arrays.
The greater sensitivity of the southern array 
 is a sufficient  explanation for the larger total number of events
recorded by it.
  In fact, event rates for events with $1.5<M_W<2.5$
are very similar for both arrays if the swarm region \DIFdelbegin \DIFdel{of the northern array }\DIFdelend \DIFaddbegin \DIFadd{in the northern area }\DIFaddend is
excluded.   Given comparable station densitities within both arrays,
the cause of the different sensitivities is not clear; possibly, it
is related to the spatial distribution of seismicity: for the
southern array, most earthquakes are shallow and within the array;
for the northern array, the average earthquake depth is larger, and
most earthquakes occur outside the array. In the southern array, small
earthquakes are thus more likely to be recorded by a sufficient number
of stations to be included in the reduced set. 
An alternative explanation for the apparent different $M_c$ values 
could be a systematic difference in the average station
amplification factors (see \DIFdelbegin \DIFdel{appendix~\ref{sec:magnitude}}\DIFdelend \DIFaddbegin \DIFadd{supplementary material}\DIFaddend ) of both
arrays. This explanation would imply that seismicity rates are indeed
larger for the southern array.  A direct comparison of the magnitude
estimates for the \DIFaddbegin \DIFadd{only }\DIFaddend two events recorded by both arrays does indeed show
a systematic shift consistent with the latter explanation but
uncertainties are too large to be conclusive. 

\subsection{Event depths}

Within both arrays, most earthquakes occur within the crust but both
also show significant activity in the mantle
(Figures~\ref{fig:dephistos} and~\ref{fig:profiles}). Within the southern array, the deepest
well-located vents occur at a depth of
$\sim$10~km (Figures~\ref{fig:profiles}) below the seafloor, only slightly deeper than the
maximum depth of faulting at slow-spreading ridges
\citep[e.g.][]{toomey88,tilmann04}.  Asssuming purely conductive
cooling, this depth corresponds to temperatures of 500-600\dg
C, although hydrothermal activity  near the ridge or at the outer rise
could result in somewhat lower
 temperatures.  We find a
larger fraction of deep events in the region of the northern array
(Figures~\ref{fig:dephistos}) but
a more quantitative comparison is precluded by the small number of
events with well-constrained depths in the north. A later array
deployment landward of the trench located intra-oceanic-plate
earthquakes in the trench region of this segment (75.5\dg W) with a maximum depth of
$\sim$30~km \citep{lange07}. 

\subsection{Cluster analysis}

The location of cluster events appears to reflect the overall
distribution of seismicity.  \DIFdelbegin \DIFdel{There }\DIFdelend \DIFaddbegin \DIFadd{They }\DIFaddend are distributed through the
experimental region in the south, and are strongly concentrated within
the swarm region in the North (Figure~\ref{fig:clust-location}).  In
particular, cluster type events occur both within the crust and mantle
within the southern array.  Within the northern array, the fact that
most clusters locate outside or at the limit of the array and therefore
have poor depth determination precludes us from making a similar
assertion about the northern array.   Short intervals between events
belonging to the same cluster are common, with occasional reactivation
after a longer waiting times.  In fact, the recurrence interval
distribution follows a power-law (Figure~\ref{fig:clust-statistics}\DIFaddbegin \DIFadd{a}\DIFaddend ),
which indicates that the individual events of a cluster do neither occur
independently of each other, nor do they seem to recur at
quasi-periodic intervals.  The deficit of events at very short
recurrence intervals (below $\sim$100s) could possibly have arisen from our
processing method which often  misses events which occur in the
coda of a previous event.

The events within a cluster can differ in
magnitude by up to one magnitude unit\DIFdelbegin %DIFDELCMD < \remark{check and modify
%DIFDELCMD <   appropriately, I have a feeling it is somewhat more}%%%
\DIFdel{,
but the
largest event can occur at any time within the }\DIFdelend \DIFaddbegin \DIFadd{,
%DIF >  seems a reasonable number but two outliers in the south:
%DIF >  clust0545 : -1.32 - 0
%DIF >  clust0628m : -1.24 - 0
 but inspection shows that the
largest events of any cluster do not occur preferentially at or near
the beginning of the }\DIFaddend sequence, i.e. they
cannot be understood in terms of a mainshock with its aftershocks.
%DIF > (Figure~\ref{fig:clust-statistics}c and d).
This is also likely to be true when considering the swarm in the
northern array where the largest event occurs close to the end of the
sequence \DIFaddbegin \DIFadd{(Figure~\ref{fig:clust-statistics}b)}\DIFaddend .  We cannot categorically exclude a large earthquake before
the beginning of our survey (but small enough not to be \DIFdelbegin \DIFdel{registerd
}\DIFdelend \DIFaddbegin \DIFadd{registered
}\DIFaddend teleseismically), but the abrupt end of the swarm argues against its
interpretation as an aftershock sequence.

\subsection{Stress directions}

The stress fields estimated from the shallow and deep event set within
the southern array are both characterized by a sub-horizontal EW to
ENE-WSW oriented pressure axis but differ in the orientation of the
tension axis (Figure~\ref{fig:focmec}). However, the uncertainty
regions of both sets overlap, and fitting the whole dataset results in
stress direction estimates very similar to those of the shallow set
($\sigma_1$ az=260\dg, dip=10\dg; $\sigma_3$ azimuth=350\dg, dip=0\dg;) with only marginally reduced fit
(\%ok=92.9).  Although  mechanism type for individual clusters
could not be determined with any confidence the results were
also generally compatible with EW compressional stresses, and were in
particular generally {\em not} compatible with EW tensional stresses.

\section{Discussion}

Large $b$ values are typically found in volcanic or geothermal \DIFaddbegin \DIFadd{areas}\DIFaddend , or in
the creeping sections of faults \citep{wiemer02}.  
%\remark{not sure if the following is true, might be an artifact of
%  magnitude measurements (see Okal and Romanowics paper):
%$b$ values as large
%as in the present study have also been observed near segment-centres
%of slow-spreading mid-oceanic ridges where there is evidence of recent
%magmatic injection events (Table~2 of \citep{tilmann04}; note in this
%table $\beta$ values were quoted, where $b=\frac{2}{3}b$).}
Similarly, the presence of a large number of earthquakes with strong
waveform similarity over a short time span is typical of
swarm-like activity.  Repeating earthquakes are also found in or near creeping
sections of major faults \citep[e.g.][]{nadeau98} but are then
generally characterised by more regular, and larger, inter-event
times, and smaller variation in event magnitudes within a
cluster. Power law distributions of waiting times can also
arise in settings where elastic stress transfer dominates, but in this
case mainshock-aftershock sequences result in the largest earthquakes
preferentially occuring near the beginning of a sequence, contrary to
our observations \DIFdelbegin \DIFdel{, 
}\DIFdelend \DIFaddbegin \DIFadd{(e.g., Figure~\ref{fig:clust-statistics}b). 
}\DIFaddend 

Swarm-type seismicity has been
previously observed in the shallow crust following rainfall
\citep{kraft06}, in the mid-crust in central Europe \DIFdelbegin \DIFdel{\mbox{%DIFAUXCMD
\citep{kurz04}
}%DIFAUXCMD
}\DIFdelend \DIFaddbegin \DIFadd{\mbox{%DIFAUXCMD
\citep{spicak01}
}%DIFAUXCMD
}\DIFaddend , in
the mid and upper crust in California and Japan \citep{vidale06}, and
in the crust of the overriding plate of the Aegean subduction zone
\citep{becker06} and other settings.  In all \DIFdelbegin \DIFdel{these setting}\DIFdelend \DIFaddbegin \DIFadd{those examples}\DIFaddend ,  fluids
have been implicated in causing the swarm seismicity.
  \DIFdelbegin %DIFDELCMD < \remark{really need a better reference for
%DIFDELCMD < Vogtland swarm.}  %%%
\DIFdelend \citet{yamashita99} has developed a model which
explains how the presence of fluids can lead to swarm type activity.
In his model, overpressures within a fault allow earthquakes to
occur.  As the earthquake progresses, permeability is created \DIFdelbegin \DIFdel{in }\DIFdelend \DIFaddbegin \DIFadd{within }\DIFaddend the
fault zone, which allows the overpressures to diffuse away, thereby
arresting the earthquake before it becomes too large.  In this way
mainshock-aftershock sequences are suppressed, and smaller events are
more common. 

The dramatic rate change within the northern array hints that swarm
activity does indeed occur within our study region\DIFdelbegin \DIFdel{but }\DIFdelend \DIFaddbegin \DIFadd{. However, }\DIFaddend the presence of
 earthquakes with similar waveforms outside the area of this particular swarm as well
as the fact that a high rate of similar earthquakes is observed in
 both arrays within two different segments of the incoming plate
strongly suggests that this type of seismic activity is typical for
the outer rise region in Southern Chile, rather than just a fortuitous
recording of a rare swarm event.

Based on the occurence of cluster events up to 8-9~km below the
seafloor (for a $\sim$5~km crust), we thus infer
the presence of fluids within both the crust and the  mantle below our southern
array on the 6 Ma old plate. 
\DIFdelbegin \DIFdel{We do not have sufficient
depth control for a suitably large number of clusters within the northern array, but }\DIFdelend %DIF > We do not have sufficient
%DIF > depth control for a suitably large number of clusters within the
%DIF > northern array, but 
\DIFaddbegin \DIFadd{Although the database in the northern array is
weaker with regard to hypocentral depth, }\DIFaddend it seems reasonable to assume that cluster events
occur at least as deep as below the southern array. We will now
consider potential fluid sources. 
\begin{enumerate}
\item Melts. Minor \DIFdelbegin \DIFdel{magnetism }\DIFdelend \DIFaddbegin \DIFadd{magmatism }\DIFaddend has been observed on the outer rise of
  old plates \citep{hirano04}, and has been attributed to
  melts prevalent in the asthenosphere, which can migrate to the
  surface following flexure-induced fractuirng. 
  \citep{hirano06}.   However, observations of velocities in
  the low velocity zone below the oceans  in conjunction with
  measurements of the temperature dependence of seismic velocities at
  seismic frequencies indicate temperatures below the solidus, and in
  particular do not indicate the presence of melt \citep{priestley05,faul05}. 
   Furthermore, the elastic thickness of \DIFdelbegin \DIFdel{a }\DIFdelend \DIFaddbegin \DIFadd{very }\DIFaddend young plates
   is expected to be \DIFdelbegin \DIFdel{very }\DIFdelend small, particularly at 6 Ma, and thus little flexure should
  occur.   This   is borne out by the absence of an identifiable flexural bulge in either topography or
  gravity \DIFdelbegin \DIFdel{\mbox{%DIFAUXCMD
\citep[e.g.]{bry03}
}%DIFAUXCMD
}\DIFdelend \DIFaddbegin \DIFadd{\mbox{%DIFAUXCMD
\citep[e.g.][]{bry03}
}%DIFAUXCMD
}\DIFaddend .   We thus do not consider it likely that
  the fluids implied by the seismicity are pervasive asthenospheric
  melts.  The fluids \DIFdelbegin \DIFdel{also cannot be derived from }\DIFdelend \DIFaddbegin \DIFadd{are also unlikely to be }\DIFaddend remnant melt advected
  with the plate from the ridge axis because
  even at 6~Ma, a magma-filled sill or dike formed
  at the ridge would need to be several kilometres thick in order to
  escape solidification \citep{turcotte02}. 
% see page 166-168 of Turcotte&Schubert second edition for equation
% b=sqrt(t/(4*ka*lambda^2))  
% b : half-width of intrusion
%
% With the values of Turcotte and Schubert this gives b=14 km for 6 Ma
% - I am not really sure what rock these values pertain to but kappa
% seems roughly in line with lithospheric mantle, T constrast is
% a little too large:
% Thermal diffusivity kappa=  0.5 mm^2/s
% Latent heat of melting  L=  320 kJ/kg
% Temp excess of melt     Tmelt-T0= 1000 K
% Specific heat           c=  12.kJ
% b=sqrt(t/(4*ka*lambda^2))
 \DIFdelbegin %DIFDELCMD < 

%DIFDELCMD < %%%
\DIFdelend \DIFaddbegin \DIFadd{Volatiles related to processes at the ridge might feasibly
  remain in the crust, and maybe even the mantle, and could influence
  the  seismic activity at the outer rise.  However, the apparent
  similarity of clustering behaviour in the 14~Ma and 6~Ma old plates
  argues against a strong influence of the ridge on our observations. 
}\DIFaddend \item Water. As the minerals within the oceanic lithosphere experience
  no significant pressure change on approach to the outer rise area,
  it is not clear how water could be sourced from dehydration
  reactions.  It is thus necessary to transport water \DIFdelbegin \DIFdel{throughout }\DIFdelend \DIFaddbegin \DIFadd{into }\DIFaddend the
  seismically active region.   One possibility is that seawater is
  penetrating along major faults as proposed by \DIFdelbegin \DIFdel{\mbox{%DIFAUXCMD
\citet{ranero03}
}%DIFAUXCMD
and
  \mbox{%DIFAUXCMD
\citet{peacock01}
}%DIFAUXCMD
}\DIFdelend \DIFaddbegin \DIFadd{\mbox{%DIFAUXCMD
\citet{peacock01}
}%DIFAUXCMD
and
  \mbox{%DIFAUXCMD
\citet{ranero03}
}%DIFAUXCMD
}\DIFaddend .   In order for this mechanism to be feasible,
  some basement outcrops are required to serve as inflow sites,
 as the sedimentary blanket tends to be relatively
 impermeable. 
Basement is \DIFdelbegin \DIFdel{clearly }\DIFdelend exposed widely in the southern area
(Figure~\ref{fig:epimaps}\DIFdelbegin \DIFdel{a}\DIFdelend \DIFaddbegin \DIFadd{b}\DIFaddend ), but only isolated outcrops can be seen in
the northern area (Figure~\ref{fig:epimaps}a).  However,
\DIFdelbegin \DIFdel{\mbox{%DIFAUXCMD
\citet{fisher03}
}%DIFAUXCMD
had }\DIFdelend \DIFaddbegin \DIFadd{\mbox{%DIFAUXCMD
\citet{fisher03_juandefuca}
}%DIFAUXCMD
}\DIFaddend demonstrated that circulation cells can span
surprisingly large lateral distances with up to 50~km between in- and
outflow sites.  Heat flow measurements have been collected in the
northern area along a \DIFaddbegin \DIFadd{$\sim$10~km long }\DIFaddend trench-perpendicular profile \DIFdelbegin \DIFdel{\mbox{%DIFAUXCMD
\citep{heesemann07abs}
}%DIFAUXCMD
}\DIFdelend \DIFaddbegin \DIFadd{\mbox{%DIFAUXCMD
\citep[see
Figure~\protect\ref{fig:epimaps}a for approximate location]{contreras-reyes07}
}%DIFAUXCMD
}\DIFaddend . The
heat flow matches the expectation from a purely conductive model at
the westernmost measuring point but is much lower than expected (by 
$\sim$60 mW/m$^2$) at the easternmost point; the heat flow
measurements are well explained if the basement is cooled to near
seabottom temperatures, proving that water enters the crust, similarly
to what is observed in other outer rise areas with exposed
basement in Nicaragua and Central Chile \citep{grevemeyer05}.

  However, \DIFaddbegin \DIFadd{the heat flow measurement points are located $\sim$25~km seaward of the
  seismically active area and 
}\DIFaddend they cannot tell us how deep the water penetrates.  
A 250~km long trench-perpendicular  refraction
profile, approximately passing through obs03, obh06 and obh11, has
been shot through the array \citep{contreras-reyes07}. $P_n$
velocities in the western part of this profile are 8.1 km/s and above
as expected for oceanic upper mantle, but decrease to 7.8~km/s below
the outer rise region\DIFaddbegin \DIFadd{, at a broadly similar distance to the trench where we
observed the swarm activity (Figure~\ref{fig:epimaps}a)}\DIFaddend .
The decrease of mantle seismic velocities below the outer rise can be
explained by \DIFdelbegin \DIFdel{a }\DIFdelend partial hydration (serpentinisation) of the mantle 
\DIFdelbegin \DIFdel{lithosphere 
\mbox{%DIFAUXCMD
\citep{contreras-reyes07}
}%DIFAUXCMD
, }\DIFdelend \DIFaddbegin \DIFadd{\mbox{%DIFAUXCMD
\citep{carlson03}
}%DIFAUXCMD
, and thus serves as }\DIFaddend an independent indicator that fluids can
circulate down to mantle depths.   \DIFdelbegin \DIFdel{We note that similar }\DIFdelend \DIFaddbegin \DIFadd{Similar }\DIFaddend reductions of $P_n$
velocities in the outer rise region have been observed in Northern
Chile \citep{ranero04} and in Middle
America \citep{grevemeyer07}.  In the latter example, a small outer
rise passive network (4 hydrophones) recorded a number of earthquakes, whose best fit
solution was in the mantle but because of the small number of stations
and the lack of S wave observations, depth error bars were generally
too large to be certain, and it is not known whether \DIFdelbegin \DIFdel{the earthquakes
should }\DIFdelend \DIFaddbegin \DIFadd{those earthquakes
showed }\DIFaddend similar clustering behaviour and \DIFdelbegin \DIFdel{prevalence of small
earthquakes.
 Whereas we in }\DIFdelend \DIFaddbegin \DIFadd{a similarly large $b$ value.
 Whereas we---in }\DIFaddend accordance with most of these \DIFdelbegin \DIFdel{papers
prefer }\DIFdelend \DIFaddbegin \DIFadd{papers---prefer }\DIFaddend an interpretation where seawater percolates from the seafloor,
an alternative scenario envisages a deep source of water. The water
released from the downgoing slab crust by dehydration reaction could
partly remain in the slab and hydrate it \citep{abers03,robin07abs},
rather than percolating into the overriding plate, as is usually
assumed.  However, for this mechanism to serve as an effective source
of mantle hydration the deep hydration front must move all the up to
the surface, which would require the subduction zone to have been
active for a very long time.  It can also not explain the apparent
coincidence of outer rise faulting and the onset of reduced Pn
velocities\DIFdelbegin \DIFdel{, which is more clearly seen in Middle
America \mbox{%DIFAUXCMD
\citep{grevemeyer07}
}%DIFAUXCMD
than in Southern Chile. }\DIFdelend \DIFaddbegin \DIFadd{.
%DIF > , which is more clearly seen in Middle
%DIF > America \citep{grevemeyer07} than in Southern Chile.
A trench-perpendicular refraction profile has also been collected
within our southern array \mbox{%DIFAUXCMD
\citep{scherwath06}
}%DIFAUXCMD
but the interpretation
of $P_n$ velocities there is not straightforward because a potential
reduction of velocities due to serpentinisation is superimposed
on a gradual velocity increase due to plate cooling. 
}\DIFaddend \end{enumerate}
\DIFaddbegin \DIFadd{The EW compressive stress direction inferred from the $P$ wave first
motions of some cluster events is opposite to the stress field
expected for shallow events from bending, i.e. faults are activated as thrusts
rather than as normal faults.   As the methodology \mbox{%DIFAUXCMD
\citep{robinson00}
}%DIFAUXCMD
assumes a uniform stress field and imposes a
constant value for the coefficient of friction, which are both
essentially untestable assumptions, the reliability of the stress
field inversion is uncertain but in any case the observed
first motions cannot easily be reconciled with classic bending-related trench-perpendicular normal
faulting.  Given the small elastic thickness of the only 6~My old
plate and the absence of a noticable outer rise bulge, we should
probably not be surprised that the stress field is not primarily
controlled by bending, as expected bending stresses would be small.
Instead, we can expect regional tectonic stresses to be dominant, for
example due to ridge push, or the progression of the seismic cycle on
the megathrust \mbox{%DIFAUXCMD
\citep{christensen88}
}%DIFAUXCMD
.  In particular, outer rise thrust
events are thought to occur late in the seismic cycle.  Although the
megathrust landward of our southern array participated in the 1960
earthquake rupture, the inferred slip was less than half that of the slip
maxima near the northern end of the rupture \mbox{%DIFAUXCMD
\citep[also see Figure~\ref{fig:overview-map}]{barrientos90}
}%DIFAUXCMD
.  It is thus entirely
plausible that the southernmost part of the megathrust could have a slip
deficit, which would put the adjacent oceanic plate into compression. 
%DIF >  Engdahl hypocentres within southern array box
%DIF >  cd /work/tilmann/proposals/tipteq/abb2
%DIF >  awk '$8>-45.25 && $8<-44.15 && $9>-77.25 && $9<-5.0625' tmp.files/engdahl.EHB
%DIF >  DEQ  64  8  5   6 47 27.58  -44.377 -73.057  20.1  33.0 4.7 0.0 0.0  19  17   2 144    0.83   29.25    7.62  20.7 148.8 177.6  90 350 180 119 15.5
%DIF >  FEQ  65  6  4   8  5 36.13  -44.305 -75.768  25.0  33.0 5.2 0.0 0.0  41  37   0 143    1.01   15.50    0.00  11.8 102.9 135.2  91 181   1  93  9.9
%DIF >  LEQ  65 11 10   9 45 52.15  -44.889 -75.458  15.0  33.0 4.6 0.0 0.0  19  16   0 143    0.96   32.89    0.00  12.0 165.3 176.5 100 407  10 148 18.6
%DIF >  DEQ  73  7  5   4 24  8.46  -44.515 -76.282   6.3   0.0 5.0 0.0 0.0  29  23   3 143    1.05   16.55    5.46   5.2 136.5 150.4  87 167 177 118 10.7
%DIF >  DEQM 80  1 28  16 59 21.82  -44.991 -76.134  15.5  33.0 5.2 5.2 5.5  52  42   4 143    1.08   11.58    5.45  11.6  99.4 103.5 105 110  15  77  7.0
%DIF > *DEQM 83 11 28  19 10  8.95  -44.942 -76.106  22.6  26.0 5.4 4.6 5.6 155 143  20 143    0.90    6.50    2.46  11.6  54.2  54.4 102  62  12  48  4.2
%DIF >  DEQM 85  8  4   4 54  2.24  -45.071 -75.860  17.4  28.0 5.5 5.4 5.9 169 142  11 143    1.00    7.14    3.21  11.6  47.3  52.9  93  67   3  50  4.4
%DIF > ZDEQM 86 11 28  20 34 51.86  -45.235 -77.246  21.9  15.0 4.7 4.6 5.3  27  14   3 143    0.80   15.34    5.96  12.1 210.8 280.3 107 145  17 115  9.8
%DIF >  * marks earthquake with large difference between CMT and Engdahl locations
However, the only three available CMT solutions in the southern area whose
EHB location puts them unambiguously on the outer rise
 all show normal faulting with an
approximately trench-normal tension axis
(Figure~\ref{fig:overview-map}). These events are located landward
of the micro-earthquakes examined by us, so it is possible that there
is a change in stress regime as the plate approaches the trench and
descends but without a more detailed study, beyond the scope of
this work, we cannot exclude the possibility that the
micro-earthquakes are simply not representative of the long-term, large
scale stress field. 
}

\DIFaddend If our interpretation \DIFaddbegin \DIFadd{of the source of fluids }\DIFaddend is correct, it implies that hydration \DIFdelbegin \DIFdel{.
}\DIFdelend \DIFaddbegin \DIFadd{of slab
mantle lithosphere can occur at the outer rise of even very young
plates. Bending per se does not seem to be required, but the crust
needs to be pervasively faulted to be able to act as a pathway for the
water.     The temperature at the hypocentre of the deepest earthquakes
 (500-600}\dg \DIFadd{C) is entirely consistent with \mbox{%DIFAUXCMD
\citeauthor{mckenzie05}
}%DIFAUXCMD
's
 \mbox{%DIFAUXCMD
\citeyearpar{mckenzie05}
}%DIFAUXCMD
recent estimates of the maximum temperature
 for brittle faulting in oceanic mantle ($\sim 650\dg C$), which was  inferred from the global
 dataset of intra-plate and outer rise events \mbox{%DIFAUXCMD
\citep{wiens83}
}%DIFAUXCMD
,
 particularly given the small number of deep events in our dataset.
 It would thus appear that microearthquakes, even those triggered by
 fluids, cannot occur at temperatures substantially higher than those
 at which large earthquakes occur.  This finding implies that the
 maximum depth of micro-earthquakes can be used to infer temperature
 structure; conversely, the presence of fluids (as invoked by, for example,
 \mbox{%DIFAUXCMD
\citealp{reyners07}
}%DIFAUXCMD
) might not be 
 sufficient to explain seismicity at locations thought to
 be too hot.
%DIF > , such as the flanks of continental rifts.
 It is also consistent with the observation that the
 temperature limit of intermediate depth seismicity within subducting slab, which is probably
 also related to fluid-effects \mbox{%DIFAUXCMD
\citep[e.g.]{peacock01}
}%DIFAUXCMD
, is essentially
 identical to the limiting temperature for oceanic intraplate events \mbox{%DIFAUXCMD
\citep{emmerson07}
}%DIFAUXCMD
.
}\DIFaddend 

\DIFdelbegin %DIFDELCMD < \remark{
%DIFDELCMD < Next points:
%DIFDELCMD < 

%DIFDELCMD < source of fluids 
%DIFDELCMD <  a. melt (observed small amounts of magmatism on outer rise offshore
%DIFDELCMD <  Japan due to decompression melting but unlikely here because young
%DIFDELCMD <  plate age implies small elastic thickness and the absence of an
%DIFDELCMD <  identifiable bulge, hence no decompression takes place).
%DIFDELCMD <  b. water (evidence for rehydration in Contreras data (reduction of
%DIFDELCMD <  crust and mantle velocities). Likewise, the slab has not started
%DIFDELCMD <  descending so dehydration unlikely;  water supplied from the top
%DIFDELCMD <  through hydrothermal circulation.
%DIFDELCMD < 

%DIFDELCMD <  In STA2, a large number of outcropping ridges available for water inflow.  In STA1, blanketing by sediments
%DIFDELCMD <  at first sight makes this explanation problematic but Fisher(Nature,
%DIFDELCMD <  2003) paper shows discharge cna happen across 50~km guided by
%DIFDELCMD <  seamount - need to read this paper)
%DIFDELCMD < 

%DIFDELCMD <  Cluster earthquakes occur both in the crust and mantle: indicates
%DIFDELCMD <  that fluids penetrate mantle.
%DIFDELCMD < 

%DIFDELCMD <  Interpretation of stress direction: compressional stress from
%DIFDELCMD <  convergence: small elastic thickness implies bending stresses are
%DIFDELCMD <  relatively small
%DIFDELCMD < 

%DIFDELCMD < Max Earthquake depth similar to max depth of large EQ (within +- 100 C)
%DIFDELCMD < }
%DIFDELCMD < %%%
\DIFdelend \DIFaddbegin \section{\DIFadd{Conclusion}}
\DIFaddend 

\DIFaddbegin \DIFadd{Based on a temporary deployment of ocean bottom seismometers and
hydrophones on the outer rise of two segments of the Nazca plate  (14 Ma and 6 Ma old plate at the trench) as
it is subducting at the Southern Chilean margin, we observe that
micro-earthquakes occur throughout the crust and in the uppermost
mantle.  A significant part of
the seismicity has the hallmarks of fluid-triggered seismicity,
i.e. a large $\beta$ value ($>$1) and a high degree of clustering with a
large number of earthquakes with similar waveforms but irregular
recurrence intervals and the absence of mainshock-aftershock
patterns. The deepest earthquakes appear to occur at a temperature of
500-600}\dg \DIFadd{C, which, given the sampling, is consistent with the
temperature limit of 650}\dg \DIFadd{C for large intra-plate earthquakes below
the oceans. 
We interpret the fluids to be derived from seawater, which enters the
crust and mantle through pervasive faults, not necessarily
bending-related.  Corroborating evidence for the hydration of mantle lithosphere
comes from a coincident refraction line through the array on the 14 Ma
old plate \mbox{%DIFAUXCMD
\citep{contreras-reyes07}
}%DIFAUXCMD
,
which shows a reduction of $P_n$ velocity from more than 8~km/s to 7.8~km/s
near the seaward limit of seismicity, projected along strike.
}




\DIFaddend %\begin{acknowledgments}
\ack
We thank  
Capts. Kull and Mallon and their crews for excellent suppport of the deployment.
The SO181 shipboard scientific crew carried out initial processing of
the data.  
\DIFdelbegin \DIFdel{We thank ... for processing the bathymetric data
from the study region, and .. for picking part of the data. 
}%DIFDELCMD < \remark{Please enter names or let me know if you think it unnecessary}
%DIFDELCMD < %%%
\DIFdelend This is publication \DIFdelbegin \DIFdel{no }\DIFdelend GEOTECH-xxx of the \DIFaddbegin \DIFadd{R}\&\DIFadd{D programme }\DIFaddend GEOTECHNOLOGIEN program funded by
the German Ministry of Education\&Research \DIFdelbegin \DIFdel{and ..., }%DIFDELCMD < \remark{get number} %%%
\DIFdel{and }\DIFdelend \DIFaddbegin \DIFadd{(BMBF) and German Research
Foundation (DFG), grants 03G0181A and 03G0594E,  and }\DIFaddend Cambridge Department of Earth Sciences Publication xxxx. 
%\end{acknowledgments}


\bibliography{lit-base}

% Tables
%\end{article}

% Figures
\begin{figure}
\centering
\includegraphics[width=0.74\textwidth]{tipteq-abb2-paper}
\caption{Seismicity around and north of Chile triple junction. Focal
  mechanisms from Harvard catalogue are plotted at the centroid location \citep{dziewonski81a}. Circles show Engdahl
  solutions \citep{engdahl98}, with thin lines connecting CMT and Engdahl locations.  \DIFdelbeginFL \DIFdelFL{Red contour }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{Contour }\DIFaddendFL lines show co- and
  postseismic slip of the 1960 Chile earthquake based on geodetic data
  \citep{barrientos90}, contour spacing 5~m except for outermost
  contour, which corresponds to slip of 1~m.  White squares correspond
to station locations of the two outer rise arrays.}
\label{fig:overview-map}

\end{figure}

\begin{figure}
\DIFaddbeginFL \centering
\includegraphics[angle=-90,width=0.8\textwidth,totalheight=0.6\textwidth]{obseis-seminar04/tipteq/2005-01-08-1728-57-overview}
\caption{\DIFaddFL{Waveform example for $M_0=1.4\times 10^{11}$Nm ($M_W=1.4$)
  event recorded at stations obs27, obs29 and obs23 of the southern
  array, all at water depth of approximately 3250~m. BHH: pressure;
  SH1, SH2: horizontal components; SHZ: vertical component. A time of
  0~s corresponds to the inferred origin time of the event.  Picked P
  and S arrival times are marked by vertical lines. The phase arriving
  between 6 and 6.5 s is the $P$ wave water multiple; the phase
  arriving between 7.5 and 8.5~s is the $S$-converted-to-$P$ water
  multiple. %DIF > unfiltered data!
}}
\label{fig:waveform}
\end{figure}


\begin{figure}
{\sf \bf \DIFaddFL{a}}
\includegraphics[angle=-90,width=0.8\textwidth]{clust0547-obs24-overview}\\[1cm]

\parbox{0.5\textwidth}{{\sf \bf \DIFaddFL{b}} \DIFaddFL{North: age at trench = 14 Ma }\\
\includegraphics[width=0.3\textwidth]{clustnr-histo-STA1}}
\parbox{0.5\textwidth}{{\sf \bf \DIFaddFL{c}} \DIFaddFL{South:  age at trench = 6 Ma}\\
\includegraphics[width=0.3\textwidth]{clustnr-histo-STA2}}

\caption{\DIFaddFL{(a) Waveform example for 8-event cluster in the southern
  array.  Each trace shows the pressure trace from obs24, bandpass
  filtered 5-20~Hz, with the time axis origin corresponding to the
  origin time of the earthquake.  Event magnitude is estimated from
  relative amplitudes and the absolute magnitude estimate for
  the largest event of the
cluster. (b), (c) Histograms of number of events per cluster for the northern
(b) and southern (c) array. Additionally, the southern array had one
cluster consisting of 27 events.}}
\label{fig:clust-waveform-histo}
\end{figure}



\begin{figure}
\DIFaddendFL \begin{minipage}[t]{0.49\textwidth}
{\sffamily {\bfseries a} Age at trench = 14 Ma} \\
\DIFdelbeginFL %DIFDELCMD < \includegraphics[width=\textwidth]{STA1-map/STA1-map}
%DIFDELCMD < %%%
\DIFdelendFL \DIFaddbeginFL \includegraphics[scale=0.4]{STA1-map/STA1-map}
\DIFaddendFL \end{minipage}\hfill%
\begin{minipage}[t]{0.49\textwidth}
{\sffamily {\bfseries  b}  Age at trench = 6 Ma} \\
\DIFdelbeginFL %DIFDELCMD < \includegraphics[width=\textwidth]{STA2-map/STA2-map}
%DIFDELCMD < %%%
\DIFdelendFL \DIFaddbeginFL \includegraphics[scale=0.4]{STA2-map/STA2-map}
\DIFaddendFL \end{minipage}

\caption{Epicentral map of events located within the northern array
  (a) (recording period: 13 Dec 2004 - 22 Jan 2005) and southern array
  (b)  (recording period: 14 Dec 2004 - 27 Jan 2005). Where available, locations resulting
  from double difference relocation are shown (circles with thick
  borders), otherwise maximum likelihood locations from the non-linear
   algorithm are used (circles with thin borders).  
  The earthquake depth color scale is defined relative to sealevel but
  the color scales for a and b were chosen in such a way that the same depths
  below the seafloor will approximately correspond to the same color
  for both arrays.    Events
  with poor depth determination are shown in white. Events recorded by
  only three or four stations, or where one phase type was missing (P
  or S) have large epicentral errors and are shown as black dots; no
  magnitude was determined for these poorly located events.
  The shaded bathymetry is derived from shipboard
soundings during Sonne cruise SO181. 
\DIFdelbeginFL %DIFDELCMD < \remark{Is this correct for this
%DIFDELCMD <   area, or does it contain merged bathymetry from previous
%DIFDELCMD <   cruises. Also I don't think I have the final version, there seem to
%DIFDELCMD <   be a lot of trackline artifacts} %%%
\DIFdelFL{Dashed lines show }\DIFdelendFL %DIF > \remark{Is this correct for this
%DIF >   area, or does it contain merged bathymetry from previous
%DIF >   cruises. Also I don't think I have the final version, there seem to
%DIF >   be a lot of trackline artifacts} 
\DIFaddbeginFL \DIFaddFL{In (a), }\DIFaddendFL the \DIFaddbeginFL \DIFaddFL{dashed outline shows the }\DIFaddendFL ``swarm
region'' \DIFdelbeginFL \DIFdelFL{in a }\DIFdelendFL (see text), and the \DIFaddbeginFL \DIFaddFL{star shows the location where anomalously
low heatflow was measured \mbox{%DIFAUXCMD
\citep{contreras-reyes07}
}%DIFAUXCMD
; a refraction }\DIFaddendFL profile \DIFaddbeginFL \DIFaddFL{was shot along the }\DIFaddendFL line \DIFdelbeginFL \DIFdelFL{for
}\DIFdelendFL \DIFaddbeginFL \DIFaddFL{through stations obs03, obh06 and
obh11: along the green continuous section of the profile, $P_n$ velocity is larger than
8.1 km/s, along the orange dashed section, $P_n$ velocity is 7.8 km/s
\mbox{%DIFAUXCMD
\citep{contreras-reyes07}
}%DIFAUXCMD
. In (b), the dashed line indicates the location of the
cross-section in }\DIFaddendFL Figure~\protect\ref{fig:profiles}\DIFdelbeginFL \DIFdelFL{in b}\DIFdelendFL .}
\label{fig:epimaps}
\end{figure}

\begin{figure}
\begin{center}
{\sffamily {\bfseries a} event depth $<$4~km below sea floor}\\
\includegraphics[width=0.45\textwidth]{nl-restr-shallow-ctr-minimum-focmec}
\\
{\sffamily {\bfseries b} event depth $>$4~km below sea floor}\\
\includegraphics[width=0.45\textwidth]{nl-restr-deep-ctr-minimum-focmec}
\\
{\sffamily {\bfseries c} }\\
\includegraphics[width=0.35\textwidth]{nl-restr-shallow-deep-ctr-minimum-focmec-map}
\end{center}

\caption{Upper hemisphere regional pressure and tension axis
  directions derived from first motions using the \citet{robinson00}
  method. The black circle shows the the pressure direction
  ($\sigma_1$) and the \DIFdelbeginFL \DIFdelFL{gray }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{grey }\DIFaddendFL circle the tension direction ($\sigma_3$), with noughts and
  crosses indicating the 95\% confidence region. Small beachballs show
 the expected focal mechanisms for the two optimal fault planes
 (assuming a coefficient of friction $\mu=0.75$ in the Coulomb
 fracture criterion), with the numbers above signifying its strike, dip and rake, and the proportion of first motions consistent with
it. (c) Map of cluster locations used in  the shallow (light \DIFdelbeginFL \DIFdelFL{gray
}\DIFdelendFL \DIFaddbeginFL \DIFaddFL{grey
}\DIFaddendFL circles) and deep (dark \DIFdelbeginFL \DIFdelFL{gray }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{grey }\DIFaddendFL circles) stress inversions.}
\label{fig:focmec}
\end{figure}



\begin{figure}
\centering
\includegraphics[width=0.5\textwidth]{STA121nsw-GR-mc90}
\DIFdelbeginFL %DIFDELCMD < \remark{Change this over to moment on x axis and $\beta$ value instead
%DIFDELCMD <   of b}
%DIFDELCMD < %%%
\DIFdelendFL \caption{Frequency-magnitude plots (\DIFaddbeginFL \DIFaddFL{bottom $x$ axis: $\log_{10}M_0$, top $x$ axis: }\DIFaddendFL $M_W$). The \DIFdelbeginFL \DIFdelFL{b }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{$\beta$ }\DIFaddendFL values were estimated using the maximum
likelihood method \citep{aki65} where the magnitude of completeness
was determined using the goodness-of-fit criterion at the 90\% level,
 \citep{wiemer02}.  The southern array is characterized by a larger
 $b$ value and lower magnitude of completeness $M_c$ but the
 difference between the \DIFdelbeginFL \DIFdelFL{$b$ }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{$\beta$ }\DIFaddendFL values of the two arrays is not significant according to the
 \citet{utsu92} test. The figure also shows the frequency-magnitude
 plot for the northern array, when all events within the area of the
 suspected swarm are excluded.  The difference between the \DIFdelbeginFL \DIFdelFL{b }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{$\beta$ }\DIFaddendFL value for
 this set and the southern array is marginally significant (94\%
 confidence).}
% utsutest(45,1.25,220,1.69)  n1,b1,n2,b2

% even though the seismicity rate for events $M_W>1.5$ was
%actually larger at the northern array.
% NB note that there was a rate change within STA1 with the cluster
% having some swarm characteristics

\label{fig:freq-mag}
\end{figure}

\begin{figure}

\DIFdelbeginFL %DIFDELCMD < \begin{minipage}[t]{0.49\textwidth}
%DIFDELCMD < %%%
\DIFdelendFL \DIFaddbeginFL \begin{minipage}[t]{0.4\textwidth}
\DIFaddendFL {\sffamily {\bfseries a} North: age at trench = 14 Ma} \\
\includegraphics[width=\textwidth]{dephisto-inarray-sgl-ingo-STA1}
\end{minipage}\hfill%
\DIFdelbeginFL %DIFDELCMD < \begin{minipage}[t]{0.49\textwidth}
%DIFDELCMD < %%%
\DIFdelendFL \DIFaddbeginFL \begin{minipage}[t]{0.4\textwidth}
\DIFaddendFL {\sffamily {\bfseries  b} South: age at trench = 6 Ma} \\
\includegraphics[width=\textwidth]{dephisto-inarray-sgl-s20-minimum-Rset}
\end{minipage}


\caption{Depth distribution of events with
  Gap$<$200\dg\DIFaddbeginFL \ \DIFaddendFL after  joint
  \DIFdelbeginFL \DIFdelFL{hypocenter }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{hypocentre }\DIFaddendFL determination in the north (a), and minimum 1-D model
inversion in the south (b). The
  dashed line shows the approximate Moho depth based on refraction
profiles. 
%Note the different scale on the
%  x-axis (counts).  
Not all individual earthquakes represented in the
  histograms are necessarily well constrained, as is apparent from the
  bundling of events at \DIFdelbeginFL \DIFdelFL{3km }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{3~km }\DIFaddendFL in (a) but nearly identical depth distributions were
  obtained for a large variety of initial models and trial depths in
  the south
  (b).  As the number of earthquakes in the north is insufficient for
  a minimum 1D model inversion, the resulting depth distribution is
  somewhat dependent on the input model.  However, the actual input model
  is well constrained from active source data, and the increased
  incidence of deeper earthquakes in the north is found for all
  reasonable models. 
} 
\label{fig:dephistos}
\end{figure}


\begin{figure}
\centering
% [width=\textwidth]
% no re-scaling should be necessary for this figure -> preserves
% sensible scale
%\includegraphics{profile-true-nl-minimum}
\includegraphics{STA2-map/STA2-profile}
\caption{True-scale cross-section of events with robust depth determinations along
  profile line through the southern array. (The profile is
  approximately perpendicular to the spreading direction, see Fig.~\ref{fig:epimaps}\DIFaddbeginFL \DIFaddFL{b}\DIFaddendFL ). Contours show 100\dg C increments; the temperature field is derived
  from \DIFdelbeginFL \DIFdelFL{the cooling }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{\mbox{%DIFAUXCMD
\citeauthor{mckenzie05}
}%DIFAUXCMD
's \mbox{%DIFAUXCMD
\citeyearpar{mckenzie05}
}%DIFAUXCMD
}\DIFaddendFL plate \DIFaddbeginFL \DIFaddFL{cooling }\DIFaddendFL model \DIFdelbeginFL \DIFdelFL{of \mbox{%DIFAUXCMD
\citet{mckenzie05}
}%DIFAUXCMD
}\DIFdelendFL with the
  asthenospheric potential temperature adjusted to 1280\dg C, which is
  required to produce the observed 5~km thick crust. \DIFdelbeginFL \DIFdelFL{Gray }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{Grey }\DIFaddendFL circles
  indicate earthquakes within 10 km of the profile line, open circles
  correspond to earthquakes to the south of this swath, and black circles to
  earthquakes to the north. Circles with thicker borders indicate
  events which are parts of clusters relocated by the
  double-difference method.
\DIFdelbeginFL %DIFDELCMD < \remark{Need to add Moho line to this plot}
%DIFDELCMD < %%%
\DIFdelendFL \DIFaddbeginFL \DIFaddFL{The dashed line indicates the Moho as inferred from refraction data \mbox{%DIFAUXCMD
\citep{scherwath06}
}%DIFAUXCMD
.}\DIFaddendFL }
\label{fig:profiles}
\end{figure}


\begin{figure}
\DIFdelbeginFL %DIFDELCMD < {\sf \bf %%%
\DIFdelFL{a}%DIFDELCMD < }
%DIFDELCMD < \includegraphics[angle=-90,width=0.5\textwidth]{total-talk/clust0547-waveform}\\[1cm]
%DIFDELCMD < %%%
\DIFdelendFL \DIFaddbeginFL \parbox{0.5\textwidth}{{\sf \bf \DIFaddFL{a}} \DIFaddFL{North array }\\
\includegraphics[scale=0.4]{STA1-clust/STA1-clust}}
\parbox{0.5\textwidth}{{\sf \bf \DIFaddFL{b}} \DIFaddFL{South array }\\
\includegraphics[scale=0.4]{STA2-clust/STA2-clust}}
\DIFaddendFL 

\DIFdelbeginFL \parbox{0.5\textwidth}{%DIFDELCMD < {\sf \bf %%%
\DIFdelFL{b}%DIFDELCMD < } %%%
\DIFdelFL{North array }%DIFDELCMD < \\
%DIFDELCMD < \includegraphics[width=0.4\textwidth]{clustnr-histo-STA1}%%%
}
%DIFAUXCMD
\parbox{0.5\textwidth}{%DIFDELCMD < {\sf \bf %%%
\DIFdelFL{c}%DIFDELCMD < } %%%
\DIFdelFL{South array }%DIFDELCMD < \\
%DIFDELCMD < \includegraphics[width=0.4\textwidth]{clustnr-histo-STA2}%%%
}
%DIFAUXCMD
%DIFDELCMD < 

%DIFDELCMD < \remark{redo waveform example with date, time and rel magnitude label} 
%DIFDELCMD < %%%
%DIFDELCMD < \caption{%
{%DIFAUXCMD
\DIFdelFL{(a) Waveform example for 8-event cluster in the southern
  array.  Each trace shows the pressure trace from obs23 and is labelled with the origin time of the event and
  the event magnitude, determined relative to the $M_W$ estimate of
  the largest event of the
cluster. (b), (c) Histograms of number of events per cluster for the northern
(b) and southern (c) array. Additionally, the southern array had one
cluster consisting of 27 events.}}
%DIFAUXCMD
%DIFDELCMD < \label{fig:clust-waveform-histo}
%DIFDELCMD < \end{figure}
%DIFDELCMD < 

%DIFDELCMD < \begin{figure}
%DIFDELCMD < %%%
\parbox{0.5\textwidth}{%DIFDELCMD < {\sf \bf %%%
\DIFdelFL{a}%DIFDELCMD < } %%%
\DIFdelFL{North array }%DIFDELCMD < \\
%DIFDELCMD < \includegraphics[width=0.45\textwidth]{STA1-clust/STA1-clust}%%%
}
%DIFAUXCMD
\parbox{0.5\textwidth}{%DIFDELCMD < {\sf \bf %%%
\DIFdelFL{b}%DIFDELCMD < } %%%
\DIFdelFL{South array }%DIFDELCMD < \\
%DIFDELCMD < \includegraphics[width=0.45\textwidth]{STA2-clust/STA2-clust}%%%
}
%DIFAUXCMD
\DIFdelendFL \DIFaddbeginFL \vspace{\baselineskip}
\DIFaddendFL {\sf \bf c}
\DIFdelbeginFL %DIFDELCMD < \includegraphics[width=0.45\textwidth]{STA2-clust/STA2-clust-prof}
%DIFDELCMD < %%%
\DIFdelendFL 

\DIFaddbeginFL \includegraphics[width=0.6\textwidth]{STA2-clust/STA2-clust-prof}

\DIFaddendFL \caption{(a),(b) Epicentral distribution of \DIFdelbeginFL \DIFdelFL{clusters centres
  }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{doublet members }\DIFaddendFL (\DIFdelbeginFL \DIFdelFL{circles}\DIFdelendFL \DIFaddbeginFL \DIFaddFL{grey
  dots}\DIFaddendFL ) \DIFaddbeginFL \DIFaddFL{and cluster centres
  }\DIFaddendFL for clusters with three or more \DIFdelbeginFL \DIFdelFL{2 }\DIFdelendFL events \DIFaddbeginFL \DIFaddFL{(circles)}\DIFaddendFL . The cluster centres
  are determined from the \DIFdelbeginFL \DIFdelFL{median }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{medians }\DIFaddendFL of the manual picks in stacked
  waveforms; for a few clusters, these procedure failed because there
  were not a sufficient number of common picks between the cluster
  member events.   For these \DIFdelbeginFL \DIFdelFL{events }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{clusters }\DIFaddendFL and all doublets, \DIFaddbeginFL \DIFaddFL{instead }\DIFaddendFL the
  \DIFdelbeginFL \DIFdelFL{individual }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{member }\DIFaddendFL events are shown as grey dots.  Where cluster centre
  \DIFdelbeginFL \DIFdelFL{depth is }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{depths are }\DIFaddendFL well determined, the circles are filled appropriately.
 (c) \DIFdelbeginFL \DIFdelFL{Profile }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{Cross-section }\DIFaddendFL of clusters and \DIFaddbeginFL \DIFaddFL{member }\DIFaddendFL events for the southern array. Only those events and
 clusters with well-constrained depths are shown. \DIFaddbeginFL \DIFaddFL{The profile location
is shown as a dashed line in (b).}\DIFaddendFL }
\label{fig:clust-location}
\end{figure}


\begin{figure}
\DIFdelbeginFL %DIFDELCMD < {\sf \bf %%%
\DIFdelFL{a}%DIFDELCMD < }
%DIFDELCMD < \includegraphics[width=0.5\textwidth]{interevent/interevent-mod}
%DIFDELCMD < %%%
\DIFdelendFL \DIFaddbeginFL \centering 
\DIFaddendFL 

\DIFdelbeginFL %DIFDELCMD < \remark{Need to fix figure such that it can work in black and white -
%DIFDELCMD <   also might have to reduce line weight again slightly. Need to do
%DIFDELCMD <   time-magnitude plots for selected clusters}
%DIFDELCMD < %%%
\DIFdelendFL \DIFaddbeginFL \parbox{0.65\textwidth}{{\sf \bf \DIFaddFL{a}}  \\
\includegraphics[width=0.6\textwidth]{interevent/interevent-mod}}

\parbox{0.65\textwidth}{{\sf \bf \DIFaddFL{b}}  \\
\includegraphics[width=0.63\textwidth]{STA1-time-magnitude-swarm}}


%DIF > {\sf \bf a}
%DIF > \includegraphics[width=0.5\textwidth]{interevent/interevent-mod}

%DIF > \remark{Need to fix figure such that it can work in black and white -
%DIF >   also might have to reduce line weight again slightly. Need to do
%DIF >   time-magnitude plots for selected clusters}
\DIFaddendFL \caption{(a) Probability density distribution of waiting  times for all clustered events.  Events are binned by
inter-event times within the same cluster with logarithmically scaled
bin edges; each doublet
contributes one waiting time. One standard deviation error bounds are
estimated to be proportional \DIFdelbeginFL \DIFdelFL{$\sqrt(n)$ }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{$\sqrt{n}$ }\DIFaddendFL where $n$ is the number of
waiting times in the bin; this variance estimate \DIFdelbeginFL \DIFdelFL{is }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{would }\DIFaddendFL only \DIFaddbeginFL \DIFaddFL{be }\DIFaddendFL strictly
correct if the underlying distribution \DIFdelbeginFL \DIFdelFL{is }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{were }\DIFaddendFL Poissonian.  The continuous
line shows the Poisson distribution for a hypothetical cumulative
event rate equal to the combination of all clustered events; i.e. the
total number of events per day, equivalent to the total area underneath the
curve (as it would appear with linear axes), is the same for the
actual distribution and hypothetical Poisson distribution. (b)
Magnitude-time plot of the swarm area marked in
Figure~\ref{fig:epimaps}.\DIFdelbeginFL \DIFdelFL{(c),(d) Normalised magnitude vs event number
plots for all clusters with 6 or more events. The magnitude of the
largest event is set to 1, with the other magnitudes being shifted appropriately.}\DIFdelendFL }
%DIF >  (c),(d) Normalised magnitude vs event number
%DIF > plots for all clusters with 6 or more events. The magnitude of the
%DIF > largest event is set to 1, with the other magnitudes being shifted
%DIF > appropriately. If most clusters were main shock - aftershock
%DIF > sequences, the largest event would typically corresponds to a low
%DIF > event number.
\label{fig:clust-statistics}
\end{figure}


\clearpage

\DIFaddbegin {\bf \DIFadd{Supplementary material}}

\DIFaddend \appendix
\setcounter{figure}{0}
\DIFdelbegin %DIFDELCMD < \renewcommand{\thefigure}{A\arabic{figure}}
%DIFDELCMD < %%%
\DIFdelend \DIFaddbegin \setcounter{table}{0}
\renewcommand{\thefigure}{S\arabic{figure}}
\renewcommand{\thetable}{S\arabic{table}}
\DIFaddend 


%The following material will not form part of the published article but
%instead be deposited as supplementary online material

\section{Timing corrections and relocations}

\begin{enumerate}
\item 
The internal clocks of the dataloggers were synchronized with GPS time
just before deployment and just after recovery. The internal time was
then corrected assuming linear drift.  Although there is no way to
directly check the linearity assumption it is believed to be a good
one, as for any particular clock the deviation from the nominal rate
is primarily dependent on temperature which is nearly constant at the
seafloor, and the time spent on deck and sinking or rising\DIFaddbegin \DIFadd{, during
which large temperature variations can be experienced, }\DIFaddend is short
compared to the time on the seafloor.

 Typical total drift was less than
100 ms although a few stations had larger drift values.  One station
(OBS20) had experienced a time jump of over 7~s; 
the drift of this
station was determined from the slope of a plot of double-difference
times of highly correlated events against the inter-event time
(Figure~\ref{figa:obs20timing}).  After correction, this slope is
nearly zero; and inspection shows that the jump occurred before any
events had been recorded. 

\item The stations were relocated by fitting the water wave arrival
times from airgun shots near the station. \DIFdelbegin %DIFDELCMD < \remark{Is this true for all
%DIFDELCMD < the stations (e.g. obs20).  Were stations relocated in 3-D or just
%DIFDELCMD < along a profile.  What were typical relocations?}
%DIFDELCMD < %%%
\DIFdelend \DIFaddbegin \remark{Is this true for all
the stations (e.g. obs20)?.  Were stations relocated in 3-D or just
along a profile.  What were typical relocations?}
\DIFaddend \end{enumerate}


\DIFdelbegin \section{\DIFdel{Stress directions from first motions}}
%DIFAUXCMD
\addtocounter{section}{-1}%DIFAUXCMD
%DIFDELCMD < 

%DIFDELCMD < %%%
\DIFdelend \section{Earthquake magnitude determination}
\label{sec:magnitude}

Because of the low gain of the seismometer components, and the fact
that both vertical and horizontal seismometer components are dominated
by shear waves (or p-converted S-waves), whose amplitudes are stronly
dependent on site resonance effects (as can be seen from the `ringy'
waveform), we based all our magnitude estimates on hydrophone data.
We removed the nominal hydrophone response
(Table~\ref{tbl:hyd-response}), and \DIFdelbegin \DIFdel{multiplied the resulting trace by
$v_P \rho$ }\DIFdelend \DIFaddbegin \DIFadd{then multiplied by
$\rho v_P$ }\DIFaddend where $v_P$ and $\rho$ are the sound velocity \DIFdelbegin \DIFdel{and density
}\DIFdelend \DIFaddbegin \DIFadd{(1.5 km/s)
and density (1000 kg/m$^3$)
}\DIFaddend of water, respectively. The resulting trace would be identical to the
displacement for a vertically incident wave and zero impedance contrast
between water and seafloor.  These assumptions are not \DIFdelbegin \DIFdel{exactly
fulfilled
}\DIFdelend \DIFaddbegin \DIFadd{fulfilled
exactly }\DIFaddend but are not violated too badly because velocities and
densities of the uppermost sediments are only slightly larger than in
the water, and the low velocities cause most earthquake arrivals to
have steep raypaths below the receiver.  

'Pseudo-displacement' spectra were then calculated from the converted traces for 2~s windows
centred on the P wave arrival time.  Following standard seismological
procedure we assume the \DIFdelbegin \DIFdel{\mbox{%DIFAUXCMD
\citep{brune70}
}%DIFAUXCMD
}\DIFdelend \DIFaddbegin \DIFadd{\mbox{%DIFAUXCMD
\citet{brune70}
}%DIFAUXCMD
}\DIFaddend source model, which predicts
the frequency-dependent theoretical displacement spectrum $d(f)$ for a circular source at
station $i$:
\begin{equation}
d\DIFaddbegin \DIFadd{_i}\DIFaddend (f)=\frac{K G(r_i,h) A_i(f)}{4 \pi \rho v^3} \DIFaddbegin \DIFadd{\frac{M_0}{(1+f^2/f_0^2)}  }\DIFaddend S_i
\DIFdelbegin \DIFdel{\frac{M_0}{1+f^2/f_0^2} 
}\DIFdelend \end{equation}
where $\rho$ and $v$ are the density and velocity of the medium
($\rho=2.8$g/cm$^3$, $v_p=6.2$km/s), $G(r_i,h)$ the geometrical 
spreading factor for a source at epicentral distance $r_i$ and depth
$h$ (\DIFdelbegin \DIFdel{$G(r,h)=\sqrt{r^2+h2^2}$}\DIFdelend \DIFaddbegin \DIFadd{$G(r_i,h)=\sqrt{r_i^2+h^2}$}\DIFaddend , i.e. body wave spreading is assumed),
\DIFdelbegin \DIFdel{$A(f)=\exp[-\pi f Q (\kappa + T / Q)]$ }\DIFdelend \DIFaddbegin \DIFadd{$A(f)=\exp[-\pi f Q (\kappa + T_i / Q)]$ }\DIFaddend is the attenuation factor with 
$\kappa$ parameterising the near-receiver attenuation ($\kappa=0.015$),
\DIFdelbegin \DIFdel{$T$ }\DIFdelend \DIFaddbegin \DIFadd{$T_i$ }\DIFaddend being the travel time and $Q$ the quality factor ($Q_P=400$ with a
slight assumed frequency dependence). $K$ is a factor correcting for the
radiation pattern and free surface effect; usually this is set to
$2*0.6$ where 0.6 is the radiation pattern effect averaged over the
focal sphere, and 2 corrects for the free surface effect.  Instead, we
set $K$ to 0.6 as the seismic stations are effectively imbedded in the
media.  $S_i$ is a station term to account for site specific
amplification or differences in media properties; initially all
station terms are set to 0. 

Moment magnitude estimates for each station are
 then automatically \DIFaddbegin \DIFadd{generated }\DIFaddend by a genetic
 algorithm which searches for the combination of seismic moment $M_0$
 and corner frequency $f_0$ that best
 fits the observed spectrum corrected for attenuation \DIFdelbegin \DIFdel{, }\DIFdelend \DIFaddbegin \DIFadd{and }\DIFaddend geometric
 spreading, \DIFdelbegin \DIFdel{and radiation pattern }\DIFdelend using the method and code of
 \citet{ottemoeller03}.
We only make use of frequencies \DIFdelbegin \DIFdel{band }\DIFdelend above 4~Hz; as lower frequencies were dominated by noise for all
but the largest events, and the
lower limit is still likely to be above the corner frequency of all events.  
\DIFdelbegin %DIFDELCMD < \remark{ought to check this statement}  
%DIFDELCMD < %%%
\DIFdelend %DIF > \remark{ought to check this statement}  
Corner frequency measurements
are discarded as corner frequencies of most events are close to or
larger than the Nyquist frequency, and moment magnitudes derived from
the moments according to \citet{kanamori77}.   Finally, the individual station
moment magnitudes estimates are combined in order to estimate the
station terms and final earthquake moment magnitudes under the
constraint that the geometric average of the station terms \DIFdelbegin \DIFdel{$s_i$ }\DIFdelend \DIFaddbegin \DIFadd{$S_i$ }\DIFaddend be
1 (equivalently (\DIFdelbegin \DIFdel{$\sum_i \log_{10}s_i=0$}\DIFdelend \DIFaddbegin \DIFadd{$\sum_i \log_{10}S_i=0$}\DIFaddend ).  
The standard deviation of
log\DIFdelbegin \DIFdel{$_{10}s_i$ }\DIFdelend \DIFaddbegin \DIFadd{$_{10}S_i$ }\DIFaddend is 0.43 for the southern array and 0.27 for the northern
array (except obh10, see table~\ref{tbl:hyd-response}\DIFaddbegin \DIFadd{)}\DIFaddend , where this variation is likely 
\DIFdelbegin \DIFdel{due to be
}\DIFdelend dominated by site effects, e.g. \DIFdelbegin \DIFdel{site resonances variations in actual impedance
contrast }\DIFdelend \DIFaddbegin \DIFadd{variations in seafloor impedance
contrast and sedimentary layer thickness}\DIFaddend . Formal uncertainties of individual log$_{10}s_i$ values are
around 0.05 and formal uncertainties of average event moment
magnitudes are around 0.1 magnitude units. 
% Mw station term std.dev.0.29*1.5 (STA2) and 0.18*1.5
A total of four events located by the northern array were also located
by a contemporary land deployment \citep{lange07}.
 The land magnitudes, which are based on the Richter magnitude
formula and are generally close to magnitudes in the NEIC catalogue, are on average 0.35 magnitude units larger
than the ocean bottom magnitudes but because this comparison is based
on two different magnitude scales and involves  events located far
from the array, \DIFdelbegin \DIFdel{for which }\DIFdelend \DIFaddbegin \DIFadd{where }\DIFaddend the geometrical 
spreading and attenuation functions might no longer be appropriate, we
did not correct the ocean bottom magnitudes for this potential bias.
No events in the catalogue of the southern array  have been recorded
on land, and only two events have magnitude determinations from
stations of both arrays; these two events appear to be 0.4-0.5
magnitudes larger in the northern array.  Invariably\DIFaddbegin \DIFadd{, }\DIFaddend these events are
far away from one of the arrays such that it cannot be determined whether the
difference can be attributed to differences in the average station
terms of both arrays, or to inappropriate spreading/attenuation
assumptions for more distant events, and any conclusion based on a
comparison of \DIFaddbegin \DIFadd{just }\DIFaddend two events must remain highly uncertain in any case. 

\begin{table}
\DIFaddbeginFL \renewcommand{\baselinestretch}{1.0}

\DIFaddendFL \caption{List of stations}
\DIFaddbeginFL \label{tbl:stations}
%DIF >  STA2 delays
%DIF >  awk '{ print substr($0,1,5) " & " substr($0,6,7) " & "  substr($0,15,8) " & " 0-substr($0,25,5) " &      &             & " substr($0,37,5) "  & " substr($0,44,5) " &      &    &          \\\\"}' ~/work/SEISAN8/WOR/STA2_/sta_cor.out.minimum
%DIF >  STA2 number of picks
%DIF >  wo; cd STA2_
%DIF >  grep ' $' STA2_alls20.nor | awk 'substr($0,14,1) ~ /[ 123]/ && substr($0,11,1)=="P" { print substr($1,1,5)}' | sort | uniq -c
%DIF >  grep ' $' STA2_alls20.nor | awk 'substr($0,14,1) ~ /[ 123]/ && substr($0,11,1)=="S" { print substr($1,1,5)}' | sort | uniq -c
%DIF >  log_Si from run_all.csh

%DIF >  STA1 delays
%DIF >  awk '{ print substr($0,1,5) " & " substr($0,6,7) "  & "  substr($0,15,8) "  & " 0-substr($0,25,5) " &      &             & " substr($0,37,5) "  & " substr($0,44,5) " &      &    &          \\\\"}' ~/work/SEISAN8/WOR/STA1_/sta_cor.out.jhdfresh.ingo-STA1
%DIF >  STA2 number of picks
%DIF >  wo; cd STA1_
%DIF >  grep ' $' STA1_all.nor | awk 'substr($0,14,1) ~ /[ 123]/ && substr($0,11,1)=="P" { print substr($1,1,5)}' | sort | uniq -c | sort -k2.5
%DIF >  grep ' $' STA1_all.nor | awk 'substr($0,14,1) ~ /[ 123]/ && substr($0,11,1)=="S" { print substr($1,1,5)}' | sort | uniq -c |sort -k2.5
%DIF >  log_Si from run_all.csh

\small
\begin{tabular}{lrrrccrrrrr}
%DIF >  awk '{ print substr($0,1,5) 
\hline
\DIFaddFL{Station }& \DIFaddFL{Lat }& \DIFaddFL{Lon         }& \DIFaddFL{Depth }& \DIFaddFL{\makebox[0cm][l]{Instrument$^{(1)}$} }& &\DIFaddFL{. }& \DIFaddFL{\makebox[0cm][r]{Statics (s)} }&  \DIFaddFL{Pobs }& \DIFaddFL{Sobs }& \DIFaddFL{$\log_{10}S_i$ }\\
      &   \dg \DIFaddFL{S  }& \dg \DIFaddFL{W    }& \DIFaddFL{(m)   }& \DIFaddFL{Hydr     }& \DIFaddFL{Seism            }& \DIFaddFL{P     }& \DIFaddFL{S     }& \#   & \#   &           \DIFaddFL{$^{(1)}$   }\\
\hline
\DIFaddFL{\makebox[0cm][l]{\bf North array: age at trench = 14 Ma} }\\
\DIFaddFL{obs01 }& \DIFaddFL{42.6923  }&  \DIFaddFL{76.6613  }& \DIFaddFL{3543 }& \DIFaddFL{OAS     }& \DIFaddFL{Webb        }&  \DIFaddFL{0.56  }&  \DIFaddFL{0.74 }& \DIFaddFL{76   }&  \DIFaddFL{7 }& \DIFaddFL{-0.02    }\\
\DIFaddFL{obs02 }& \DIFaddFL{42.8258  }&  \DIFaddFL{76.6390  }& \DIFaddFL{3526 }& \DIFaddFL{OAS     }& \DIFaddFL{Owen 4.5Hz  }&  \DIFaddFL{0.31  }&  \DIFaddFL{0.33 }& \DIFaddFL{112  }&  \DIFaddFL{9 }&  \DIFaddFL{0.18    }\\
\DIFaddFL{obs03 }& \DIFaddFL{42.9590  }&  \DIFaddFL{76.6118  }& \DIFaddFL{3537 }& \DIFaddFL{OAS     }& \DIFaddFL{Owen 4.5Hz  }&  \DIFaddFL{0.21  }&  \DIFaddFL{0.76 }& \DIFaddFL{271  }& \DIFaddFL{283}&  \DIFaddFL{0.26    }\\
\DIFaddFL{obs04 }& \DIFaddFL{43.0908  }&  \DIFaddFL{76.5847  }& \DIFaddFL{3524 }& \DIFaddFL{DPG     }& \DIFaddFL{Webb        }&  \DIFaddFL{0.02  }&  \DIFaddFL{0.32 }& \DIFaddFL{263  }& \DIFaddFL{267}&  \DIFaddFL{---   }\\
\DIFaddFL{obs05 }& \DIFaddFL{43.0732  }&  \DIFaddFL{76.4025  }& \DIFaddFL{3566 }& \DIFaddFL{OAS     }& \DIFaddFL{Owen 4.5Hz  }&  \DIFaddFL{0.03  }&  \DIFaddFL{0.36 }& \DIFaddFL{551  }& \DIFaddFL{558}&  \DIFaddFL{0.05    }\\
\DIFaddFL{obh06 }& \DIFaddFL{42.9388  }&  \DIFaddFL{76.4288  }& \DIFaddFL{3559 }& \DIFaddFL{OAS     }& \DIFaddFL{---         }&  \DIFaddFL{0.21  }&  \DIFaddFL{---  }& \DIFaddFL{139  }& \DIFaddFL{---   }& \DIFaddFL{---      }\\
\DIFaddFL{obs07 }& \DIFaddFL{42.8052  }&  \DIFaddFL{76.4557  }& \DIFaddFL{3555 }& \DIFaddFL{OAS     }& \DIFaddFL{Owen 4.5Hz  }&  \DIFaddFL{0.48  }&  \DIFaddFL{0.68 }& \DIFaddFL{200  }& \DIFaddFL{264}& \DIFaddFL{-0.01    }\\
\DIFaddFL{obs08 }& \DIFaddFL{42.6730  }&  \DIFaddFL{76.4837  }& \DIFaddFL{3553 }& \DIFaddFL{OAS     }& \DIFaddFL{Owen 4.5Hz  }&  \DIFaddFL{0.30  }&  \DIFaddFL{0.72 }& \DIFaddFL{102  }&  \DIFaddFL{48}&  \DIFaddFL{---     }\\
\DIFaddFL{obs09 }& \DIFaddFL{42.6553  }&  \DIFaddFL{76.3002  }& \DIFaddFL{3590 }& \DIFaddFL{DPG     }& \DIFaddFL{Webb        }&  \DIFaddFL{0.42  }&  \DIFaddFL{0.93 }& \DIFaddFL{80   }&  \DIFaddFL{47}&  \DIFaddFL{---     }\\
\DIFaddFL{obh10 }& \DIFaddFL{42.7863  }&  \DIFaddFL{76.2743  }& \DIFaddFL{3627 }& \DIFaddFL{OAS     }& \DIFaddFL{---         }&  \DIFaddFL{0.47  }&  \DIFaddFL{---  }& \DIFaddFL{137  }& \DIFaddFL{---   }& \DIFaddFL{2.03     }\\
\DIFaddFL{obh11 }& \DIFaddFL{42.9203  }&  \DIFaddFL{76.2467  }& \DIFaddFL{3616 }& \DIFaddFL{OAS     }& \DIFaddFL{---         }&  \DIFaddFL{0.28  }&  \DIFaddFL{---  }& \DIFaddFL{166  }& \DIFaddFL{---   }& \DIFaddFL{0.03     }\\
\DIFaddFL{obs12 }& \DIFaddFL{43.0553  }&  \DIFaddFL{76.2193  }& \DIFaddFL{3590 }& \DIFaddFL{OAS     }& \DIFaddFL{Webb        }& \DIFaddFL{-0.04  }&  \DIFaddFL{0.70 }& \DIFaddFL{491  }& \DIFaddFL{311 }&   \DIFaddFL{---    }\\
\DIFaddFL{obh13 }& \DIFaddFL{43.1668  }&  \DIFaddFL{76.0993  }& \DIFaddFL{3613 }& \DIFaddFL{OAS     }& \DIFaddFL{---         }& \DIFaddFL{-0.63  }&  \DIFaddFL{---  }&   \DIFaddFL{2  }& \DIFaddFL{---  }&   \DIFaddFL{---     }\\
\DIFaddFL{obh14 }& \DIFaddFL{43.1828  }&  \DIFaddFL{76.2672  }& \DIFaddFL{3559 }& \DIFaddFL{OAS     }& \DIFaddFL{---         }& \DIFaddFL{-0.28  }&  \DIFaddFL{---  }& \DIFaddFL{568  }& \DIFaddFL{---   }& \DIFaddFL{-0.27    }\\
\DIFaddFL{obs15 }& \DIFaddFL{43.2000  }&  \DIFaddFL{76.4330  }& \DIFaddFL{3530 }& \DIFaddFL{OAS     }& \DIFaddFL{Webb        }& \DIFaddFL{-0.18  }&  \DIFaddFL{0.22 }& \DIFaddFL{327  }& \DIFaddFL{26 }&  \DIFaddFL{---     }\\
\DIFaddFL{obh16 }& \DIFaddFL{43.2165  }&  \DIFaddFL{76.5987  }& \DIFaddFL{3511 }& \DIFaddFL{OAS     }& \DIFaddFL{---         }&  \DIFaddFL{0.02  }&  \DIFaddFL{---  }& \DIFaddFL{268  }& \DIFaddFL{---   }&  \DIFaddFL{---     }\\
\DIFaddFL{obh17  \makebox[0cm][l]{$^{(2)}$}  }& \DIFaddFL{43.2333  }&  \DIFaddFL{76.7658  }& \DIFaddFL{3504 }& \DIFaddFL{HTI }& \DIFaddFL{--- }&  \DIFaddFL{0.00 }& \DIFaddFL{---  }& \DIFaddFL{190 }& \DIFaddFL{---}& \DIFaddFL{-0.21    }\\
\hline
\DIFaddFL{\makebox[0cm][l]{\bf South array: age at trench = 6 Ma} }\\
\DIFaddFL{obs18 }& \DIFaddFL{44.6080 }&  \DIFaddFL{76.9492 }& \DIFaddFL{3074 }& \DIFaddFL{HTI  }&  \DIFaddFL{Owen 4.5Hz  }& \DIFaddFL{-0.44  }&  \DIFaddFL{0.46  }& \DIFaddFL{322  }& \DIFaddFL{242   }& \DIFaddFL{---      }\\
\DIFaddFL{obh19 }& \DIFaddFL{44.7392 }&  \DIFaddFL{76.8982 }& \DIFaddFL{2787 }& \DIFaddFL{HTI  }&  \DIFaddFL{---         }& \DIFaddFL{-0.57  }&  \DIFaddFL{---  }& \DIFaddFL{310  }& \DIFaddFL{---   }& \DIFaddFL{---      }\\
\DIFaddFL{obs20 }& \DIFaddFL{44.8675 }&  \DIFaddFL{76.8455 }& \DIFaddFL{3025 }& \DIFaddFL{OAS  }&  \DIFaddFL{Owen 4.5Hz  }& \DIFaddFL{-0.56  }&  \DIFaddFL{0.24 }& \DIFaddFL{428  }& \DIFaddFL{682   }&  \DIFaddFL{0.44    }\\
\DIFaddFL{obs21 }& \DIFaddFL{44.9972 }&  \DIFaddFL{76.7932 }& \DIFaddFL{3106 }& \DIFaddFL{OAS  }&  \DIFaddFL{Owen 4.5Hz  }& \DIFaddFL{-0.53  }&  \DIFaddFL{0.10 }& \DIFaddFL{42   }& \DIFaddFL{288   }& \DIFaddFL{---      }\\
\DIFaddFL{obs22 }& \DIFaddFL{44.9597 }&  \DIFaddFL{76.6113 }& \DIFaddFL{3262 }& \DIFaddFL{OAS  }&  \DIFaddFL{Owen 4.5Hz  }& \DIFaddFL{-0.33  }&  \DIFaddFL{0.33 }& \DIFaddFL{363  }& \DIFaddFL{364   }& \DIFaddFL{-0.12   }\\
\DIFaddFL{obs23 }& \DIFaddFL{44.8332 }&  \DIFaddFL{76.6608 }& \DIFaddFL{3237 }& \DIFaddFL{OAS  }&  \DIFaddFL{Owen 4.5Hz  }& \DIFaddFL{-0.33  }&  \DIFaddFL{0.41 }& \DIFaddFL{808  }& \DIFaddFL{987   }& \DIFaddFL{-0.04    }\\
\DIFaddFL{obs24 }& \DIFaddFL{44.7005 }&  \DIFaddFL{76.7130 }& \DIFaddFL{3221 }& \DIFaddFL{OAS  }&  \DIFaddFL{Owen 4.5Hz  }& \DIFaddFL{-0.46  }&  \DIFaddFL{0.16 }& \DIFaddFL{604  }& \DIFaddFL{762   }& \DIFaddFL{-0.44    }\\
\DIFaddFL{obs25 }& \DIFaddFL{44.5722 }&  \DIFaddFL{76.7655 }& \DIFaddFL{3029 }& \DIFaddFL{HTI  }&  \DIFaddFL{Owen 4.5Hz  }& \DIFaddFL{-0.50  }&  \DIFaddFL{0.40 }&  \DIFaddFL{17  }&  \DIFaddFL{36   }& \DIFaddFL{---      }\\
\DIFaddFL{obs26 }& \DIFaddFL{44.5348 }&  \DIFaddFL{76.5845 }& \DIFaddFL{3017 }& \DIFaddFL{HTI  }&  \DIFaddFL{Owen 4.5Hz  }& \DIFaddFL{---    }& \DIFaddFL{---   }& \DIFaddFL{---  }& \DIFaddFL{---   }& \DIFaddFL{---      }\\
\DIFaddFL{obs27 }& \DIFaddFL{44.6653 }&  \DIFaddFL{76.5298 }& \DIFaddFL{3240 }& \DIFaddFL{HTI  }&  \DIFaddFL{Owen 4.5Hz  }& \DIFaddFL{-0.52  }& \DIFaddFL{-0.25 }& \DIFaddFL{536  }& \DIFaddFL{734   }& \DIFaddFL{---      }\\
\DIFaddFL{obh28 }& \DIFaddFL{44.7942 }&  \DIFaddFL{76.4772 }& \DIFaddFL{3255 }& \DIFaddFL{OAS  }&  \DIFaddFL{---   }& \DIFaddFL{-0.28  }&  \DIFaddFL{---  }& \DIFaddFL{565  }& \DIFaddFL{---   }&  \DIFaddFL{0.37    }\\
\DIFaddFL{obs29 \makebox[0cm][l]{$^{(2)}$} }& \DIFaddFL{44.8895  }&  \DIFaddFL{76.4260  }& \DIFaddFL{3263 }& \DIFaddFL{OAS  }& \DIFaddFL{Owen  }&  \DIFaddFL{0.00  }&  \DIFaddFL{1.04 }& \DIFaddFL{470  }& \DIFaddFL{535  }& \DIFaddFL{-0.21      }\\
\hline

\end{tabular}

\footnotesize
\begin{description}
\item[$^1$] \DIFaddFL{The mean of the log amplitude station terms (excluding
  obh10) is 0 by definition, see text.  
%DIF > Stations without a station term were not used in the seismic moment calculation.
}\item[$^2$] \DIFaddFL{The $P$ delay of stations obh17 and obs29 is 0~s by definition.  All other station delays are defined relative to the $P$ delay at these two stations.
}\end{description}

\DIFaddendFL \end{table}

\begin{table}
\caption{Hydrophone reponse in angular frequency convention for obs
  20,22, 23, 29 and 29, and obh 28 (southern array), and obs01, 02,
  03, 06 and 07, and obh 10, 11, 14, 16 (northern array)$^1$. }
\label{tbl:hyd-response}
\DIFaddbeginFL \renewcommand{\baselinestretch}{1.0}
\small
\DIFaddendFL \begin{center}
\begin{tabular}{lrr}
\hline
 & Real  & Im \\
\hline
Poles & 0.0   & 0.0 \\
Zeros & -20.0 & 0.0 \\
Gain (cnt/Pa) & 1050 & \DIFaddbeginFL \\
\hline
\DIFaddendFL \end{tabular}
\end{center}
\footnotesize
$^1$ Subsequent analysis of magnitudes and amplitudes showed that at obh10 amplitudes
were larger than expected by a factor of approximately 1000.  The
amplitude estimates for this station were accordingly corrected by
this factor to be more compatible with the remainding stations. 
\end{table}


\begin{figure}
%DIF <  velocity models
\centering
%DIF < \includegraphics[width=\textwidth]{}
\DIFdelbeginFL %DIFDELCMD < \caption{%
{%DIFAUXCMD
\DIFdelFL{Waveform example.}}
%DIFAUXCMD
%DIFDELCMD < \label{figa:waveform}
%DIFDELCMD < \end{figure}
%DIFDELCMD < 

%DIFDELCMD < \begin{figure}
%DIFDELCMD < %%%
%DIF <  velocity models
%DIFDELCMD < \centering
%DIFDELCMD < %%%
\DIFdelendFL \includegraphics[width=0.49\textwidth]{obs20-drift-before-correction}
\caption{Double-difference times for similar earthquakes of uncorrected obs20 picks
with corrected picks from all other stations \DIFaddbeginFL \DIFaddFL{plotted versus inter-event time}\DIFaddendFL . The double difference
times are determined by waveform cross-correlation with a window
length of 9.5 s.  Because 
%DIF > plot of double-difference time versus
%DIF > inter-even times appear to have 
the \DIFdelbeginFL \DIFdelFL{plot of double-difference time versus
inter-even times appear to have the }\DIFdelendFL same slope \DIFaddbeginFL \DIFaddFL{is obtained }\DIFaddendFL for double differences
of obs20 with all other stations  \DIFdelbeginFL \DIFdelFL{we think }\DIFdelendFL it is unlikely that the slope
represents a spatial trend \DIFaddbeginFL \DIFaddFL{(e.g. migration of hypocentres); instead}\DIFaddendFL ,  \DIFdelbeginFL \DIFdelFL{and but rather }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{it }\DIFaddendFL is \DIFdelbeginFL \DIFdelFL{indicative of the }\DIFdelendFL \DIFaddbeginFL \DIFaddFL{due to }\DIFaddendFL clock
drift at obs20.  This conclusion is \DIFaddbeginFL \DIFaddFL{further
}\DIFaddendFL supported by the fact that similar plots with other reference stations
all appear flat. \DIFaddbeginFL \DIFaddFL{In contrast, the scatter is probably at least partly due to small changes in event location.}\DIFaddendFL }
\label{figa:obs20timing}
\end{figure}


\begin{figure}
% velocity models
\DIFdelbeginFL %DIFDELCMD < \centering
%DIFDELCMD < %%%
%DIF < \includegraphics[width=\textwidth]{}
\DIFdelendFL \DIFaddbeginFL \begin{minipage}[t]{0.49\textwidth}
{\sffamily {\bfseries \DIFaddFL{a}} \DIFaddFL{Age at trench = 14 Ma}} \\
\includegraphics[width=0.8\textwidth]{vel/vel-STA1}
\end{minipage}\hfill%DIF > 
\begin{minipage}[t]{0.49\textwidth}
{\sffamily {\bfseries  \DIFaddFL{b}}  \DIFaddFL{Age at trench = 6 Ma}} \\
\includegraphics[width=0.8\textwidth]{vel/vel-STA2}
\end{minipage}
\vspace{1.5\baselineskip}

\begin{minipage}[t]{0.49\textwidth}
{\sffamily {\bfseries \DIFaddFL{c}} \DIFaddFL{Age 6 Ma, starting models}} \\
\includegraphics[width=0.8\textwidth]{vel/vel-STA2-ini}
\end{minipage}\hfill%DIF > 
\begin{minipage}[t]{0.49\textwidth}
{\sffamily {\bfseries  \DIFaddFL{d}} \DIFaddFL{Age 6 Ma, final models}} \\
\includegraphics[width=0.8\textwidth]{vel/vel-STA2-fin}
\end{minipage}

\DIFaddendFL \caption{\DIFaddbeginFL \DIFaddFL{1D velocity models used for the final inversion.  The model
  for the northern array (a) is an average of the two-dimensional
  trench-parallel and -perpendicular models from refraction profiling,
and the model for the southern array (b) has been derived by a linearised
inversion of the travel time picks from well-located events within the
array.  A set of 20 inversions with different starting models were run
to test the sensitivity of the final model on the starting model: (c)
shows the 20 starting models. The shade of the lines indicates the
initial rms
misfit of the travel time data for the model.  The thick line shows the starting
model for the preferred model. 
 (d) the final inversion results for the 20 runs are
shown.  The thick line shows the preferred model (same as (b)). The shade of the lines indicates the
final rms misfit. 
}\DIFaddendFL }
\DIFdelbeginFL %DIFDELCMD < \label{}
%DIFDELCMD < %%%
\DIFdelendFL \DIFaddbeginFL \label{figa:velocity}
\DIFaddendFL \end{figure}

% The Appendices part is started with the command \appendix;
% appendix sections are then done as normal sections
% \appendix

% \section{}
% \label{}

\end{document}

