Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
eve.tsanghinsun@univ-brest.fr
Context
Ocean surface
Data
Methods
Hydrophone
Results
Next ?
Seafloor
Oceanic crust
2495 AUH events mb >2.5
292 ISC events mb>3.4
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
Context
Data
Methods
Results
Next ?
X
The Indian Ocean is characterized by three mid-oceanic
ridges with contrasted spreading rate:
• The Southwest Indian Ridge (SWIR) is the slowest of the
three with a full rate of ~15 mm/yr and a complex
morphology, mainly governed by tectonic processes with
little magma supply.
• Seismicity is evenly distributed along ridge segments and
transform faults. In 2012, some ridge segments seemed
almost inactive, especially south to the triple junction
• The Central Indian Ridge (CIR) ridge has a slow to
intermediate spreading rate of 35 mm/yr .
• Seismicity is also evenly distributed along ridge segments
and transform faults.
• The Southeast Indian Ridge (SEIR) is the fastest of the
three with a spreading rate of 60 mm/yr.
• Seismicity is mainly located on transform faults except for
one very active ridge segment.
1/3
Due to their remote location relative to the seismological
networks, the low-magnitude activity of these spreading
ridges remains poorly known.
Seismicity reported in 2012
by land-based catalogs
CIR
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
X
2012 seismic catalogs
CIR
• ISC bulletin (red points) reports
183 events
• Global CMT catalog reports only
19 large earthquakes
Seismicity is mainly located along
the three spreading ridges.
Events are mainly located on
transform faults along the SEIR
and the CIR.
A segment of the SEIR near 29°S
shows an unusual activity.
Seismic gaps appear along some
SWIR segments.
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
Context
Data
Methods
X
The hydroacoustic approach
Submarine earthquakes produce seismic wave that
propagate through the oceanic crust and convert to
acoustic T-waves at the seafloor-ocean interface. These
acoustic phases propagate laterally and eventually
penetrate the SOFAR channel, a low sound velocity
layer in the water column.
The SOFAR channel acts as wave guide and carries
the T-waves energy over very long distances (> 1000
km) with little attenuation.
Hence hydrophones can detect smaller (mb > 2.5)
earthquakes than land-based stations.
Results
Up to several thousands of km
Sound speed Ocean surface
Next ?
500-1300 m
SOFAR channel
T-wave
Depth
Ocean
Focus
P and S waves
Oceanic crust
2/3
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
Sound speed
X
Up to several 1000 of km
Ocean surface
500-1300 m
SOFAR channel
T-wave
Depth
Ocean bottom
Focus
P and S waves
Oceanic crust
Deployment of a hydrophone in the SOFAR (Sound Fixing And Ranging) channel.
The hydrophone is moored in the SOFAR channel axis. The line is composed of an expandable weight, an acoustic
release, an adjustable line and a buoy hosting the instrument. The purpose of the buoy is to keep the line stretched
and to recover it when released from the weight.
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
Context
Data
Methods
Results
Next ?
X
The OHASISBIO array
• The array is composed of 7
autonomous hydrophones deployed at
5 sites encompassing large sections of
the three Indian spreading ridges.
• The long-term deployment of the array
takes advantage of the yearly voyage of
RV Marion Dufresne to the French
southern islands.
• This study is based on data collected
between February 2012 and February
2013.
• Our analysis presents the low-level
seismicity recorded along the three
MOR and discussed the nature of some
clusters of events.
3/3
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
X
The OHASISBIO array :
5 sites
7 instruments
The monitored area is
about 2000 x 2000 km2
1000 km
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
X
Sketch of the hydrophone instrument. The housing is a solid titanium tube containing batteries,
electronic board and data storage devices. The sensor itself is attached to the buoy frame.
Detailed view of the electronic board and sensor.
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
Context
Data
Methods
Results
Next ?
Hydroacoustic data
Results
Data analysis
• A total of 7776 acoustic events were located in the whole
Indian Ocean Basin.
• Events are more accurately located inside the array (40°S20°S and 52°E - 80°E ) than outside.
• Fig. 2 displays the uncertainties in the location and time
origin for the whole catalog and study area, based on
events detected by 4 or more hydrophones.
• Covered period: 4 Feb. 2012 – 16 Feb. 2013
• Continuous
• Sampling: 240 Hz
• 3 bytes dynamic (224 range)
• Clock drift in the order of 10-8 s/s
• Data from the 7 hydrophones are jointly visualized
using the Seasick package from NOAA/PMEL
• Arrival times are hand-picked on spectrograms (fig. 1)
• Source location and origin times are estimated with a
nonlinear least squares methods using regional sound
speed models.
• Location errors for latitude, longitude and origin time
are estimated through a bootstrap approach based on
Monte Carlo simulation techniques.
2)
1)
X
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
X
NCRO3
WKER1
WKER2
WKER3
NEAMS
SWAMS
MAD
Example of time arrival picking for an
earthquake that occurred along the
SWIR on 8 October 2012 at 10h26.
As the signal is scrolling on the screen,
the analyst identifies signals on each
hydrophone
(here all the 7
instruments) and manually picks the
maximum of energy on each
spectrogram.
Time
The main difficulty is to associate
events originating from the same
source.
Frequency
0-50 Hz
The advantage of using a spectral
representation is that it provides
information on the nature of the
source, making it easier to distinguish
different types of signal.
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
X
Distribution of the uncertainties in latitude, longitude and origin time
• for the whole catalog (blue) and
• for the study area (red dots in green square) inside the hydrophone array
Most events are relatively well located, with uncertainties less than 2 s on the
origin time and 2.7 km (0.025°) in latitude or longitude.
The map shows all the events located with 4 hydrophones and more.
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
Source Level and completeness
Context
Data
As for the magnitude of earthquakes, a source level of
completeness (SLc), similar to the Mc for earthquakes, can
be derived from fitting a Gutenberg-Richter law-like
adapted to acoustic events:
AUH
log( N)  a  bSL
Methods
Results
For our analysis of the seismicity along the three ridges,
only events with SL >= SLc were considered.
• N is the number of events with a source level greater or
equal to SL;
• a is a constant dependent on the total number of events

and is related to the seismic activity;
• b is analog to the b-value of the Gutenberg-Richter law
and is the slope of the distribution.
Next ?
The resulting SLc = 230 db (with 1100 events).
Data with SL>256 were excluded from the regression due
to the clipping of the sensors for large events.
The Gutenberg-Richter distribution from the ISC catalog for
the year 2012 yields Mc=3.8 (100 events)
Extrapolating the latter with slopes b = 1.0 and b = 1.8, we
find that the SLc corresponds to a magnitude of
completeness of Mc = 3.1 +/- 0.25.
1/3
ISC
X
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
X
a) Cumulative number of events from the
OHASISBIO AUH catalog as a function of
source level (SL).
The SLc is defined as the minimum SL
where the distribution follows a linear
relationship. Here the SLc is ~230 db,
which corresponds to ~1100 events.
b) Cumulative number of events for the ISC
bulletin of 2012.
The b-value is 1.8.
Extrapolating the result from the AUH catalog
and assuming a slope b = 1 and b = 1.8 yields
to an estimated magnitude of completeness
of 3.1 +/- 0.25.
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
Context
Data
Methods
Results
Identification of clusters
Clusters of events were identified using 2 consecutive
single-link cluster (SLC) analyses.
The method is based on a nearest neighbor grid search
and links pairwise elements regarding to the shortest
distance. The result of an SLC is best displayed in a
dendrogram.
First we extracted all events that were linked with an
inter-event distance of 20 km. A cluster is defined if it
contains more than 10 events. Space-clustering analysis
yields to 26 clusters.
Second, among each subset of events, we search for
events linked by an inter-event arrival time up to 3 days.
With the same criterion on the minimum number of
events we found 6 clusters (table).
For analyzing the regional seismicity, the catalog was
declustered, in order to look at the sporadic seismic
activity, which reflects the long term patterns of the
seafloor spreading.
Next ?
C6
C7
Clusters at 20 km
C2
C1
C3
2/3
C4
X
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
Context
Data
Methods
X
Seismicity distribution and cluster analysis
Earthquakes sequences can be classified into 2 main types of clusters, related to the tectonic settings:
• Earthquake swarms, where magnitudes are roughly equivalent for all events and none of them can be described
as a possible mainshock.
• A large earthquake followed by smaller aftershocks and/or preceded by foreshocks may reflect a predominantly
tectonic origin. Such cluster is often associated with slip on transform fault. For these clusters, the cumulative
number of aftershocks generally follows a modified Omori law (MOL):
N(t)  K(c  t) p
Results
Next ?
Where K, c and p are empirically estimated constant. The parameter p describes the decay rate of events following
the mainshock and indicates the stress state of the region. The p-value generally ranges between 0.6 and 2.5 in
continental domain but seems to be higher for oceanic ridges.

The seismicity distribution along the three spreading ridges were computed for both AUH and ISC catalogs. We
computed the number of events along the axis in windows of 20 km width. Distances are computed from the
triple junction. The resulting distribution was then smoothed with a Gaussian filter to account for location
uncertainties. To examine the similarity between the ISC and the AUH profiles, correlation has been examined
through the Spearman rank correlation coefficient :
  1
6 Di
2
n(n 2  1)
Where Di is the difference of ranks i from both data sets and n the number of samples. This correlation coefficient
is a good tool tom compare similar trend in a non linear distribution.
3/3
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
Cluster
Time of mainshock (or first event)
Time Interval
N
p
c
K
KS stat
AIC
C1
SWIR, 8 Nov 2012 , mb 5.1
[0.001, 2.96]
32
2.7 (0.82)
0.343 (0.217)
10 (7.97)
0.21
-117
C2
SEIR, 7 Feb 2013, mb 5.1
[0.02, 3.1]
7
2.7 (0.53)
0.777 (1.423)
10 (19.26)
0.43
12
C3
S/A hotspot, 12 Nov 2012, Mw 5.3
[0.003, 1.6]
27
1.08 (0.38)
1.211 (1.776)
51.3 (16.31)
0.18
-172
C4
S/A hotspot, 18 Sept 2012, mb 4.1
[0.01, 1.35]
11
2.2 (0.6)
0.548 (1.393)
10 (18.53)
0.33
-26
C6
SEIR, 5 Jul 2012, mb 4.6
[0.12, 9.2]
144
0.23 (0.11)
0.01 (2.293)
21 (9.23)
0.12
-508
C7
SEIR, 12 Dec 2012, Ms 4.6
[0.003, 3.1]
45
2.7 (0.36)
0.264 (0.065)
10 (5.67)
0.23
-306
Decay parameters for each cluster (using Matlab® toolbox Z-MAP developed by the ETH Zurich)
X
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
Context
X
Spatio-temporal distribution of the seismicity
Data
Methods
Results
Next ?
SEIR
SWIR
CIR
Different time-space behaviors of the seismicity :
• High time and spatial clustering on the SEIR, all along the ridge. It appears that zones of time and space
clustering are active all year long;
• No clustering on the SWIR on the first 1000 km, then spatial clustering but few time clustering;
• No specific time clustering along the CIR and a weak trend for spatial clustering
1/5
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
X
Spatio-temporal distribution of the seismicity along the SEIR.
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
X
Spatio-temporal distribution of the seismicity along the SWIR.
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
X
Spatio-temporal distribution of the seismicity along the CIR.
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
X
Context
Data
Methods
Results
Next ?
Background seismicity
The background seismicity is different for
each spreading ridge.
Whereas the intermediate SEIR and slow CIR
exhibit similar behavior on the short and
long term seismicity, the ultra-slow SWIR
shows more variability in time.
In the space domain, events cluster in the
vicinity of transform faults along the two
fastest MOR and for the SEIR in the vicinity
of the St-Paul and Amsterdam hotspot.
Along the SWIR, events are more scattered
and rather associated with abyssal hills
faulting activity.
1342 events detected by more than 4 AUH
328 events detected by 3 AUH
342 clustered events
19 Global CMT solutions
C6
C7
Clusters
Among the 6 clusters, only two can be
classified as shock/aftershocks sequences.
Despite a high uncertainty level, results
suggest that most of the rupture processes
are complex and certainly mix tectonic and
magmatic events.
2/5
C2
C1
C3
C4
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
Context
Data
Methods
Results
Next ?
X
Seismicity along the SEIR
The seismicity is relatively homogeneous at a large wavelength but
does exhibit clear variability at a segment scale.
Peaks of activity appear at transform faults and in the vicinity of
the S/A hotspot reflecting little thermal influence of the hotspot.
The “acoustic” seismicity in one year is similar to the 40 year
“teleseismic” seismicity with Spearman rank correlation
coefficient 0.6 for unclustered data and 0.7 for declustered data.
C6
C7
C2
The seismicity recorded on land over 40 years is equally
represented by the seismicity recorded in one year by
autonomous hydrophones.
The SEIR seismicity is dominated by the transform fault activity
and most of the segments appears as seismic gaps, except for one
at 29˚S where 2 large clusters occurred.
3/5
C3
C4
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
X
Close-up on the SEIR seismicity
In one year, AUH detected:
•463 events recorded by 4 or more hydrophones (red)
•112 events recorded by 3 hydrophones (black).
•And 5 clusters (green).
Available focal mechanism solutions from the GCMT catalog are also shown.
C6
C7
• Seismicity appears scattered along the ridge, with only few events on ridge
segments and most events clustered in the vicinity of transform faults.
• One ridge segment at 29˚S, however, is particularly active during the year. The
two largest clusters, C6 et C7, are located in the middle of this segment. Further
analysis shows that there is a clear magmatic influence in these sequences and
reveals the strong influence of magmatic accretion mode for this fast spreading
rate ridge.
• The other clusters, C2, C3 and C4 are mainly associated with transform faults
activity.
C2
Click on a cluster for details.
C3
C4
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
X
a)
b)
c)
a) Seismicity rate for the year 2012 as a function of distance from the Rodrigues triple junction.
Dotted lines indicate transform faults.
b) Number of ISC events since 1964 as a function of distance from the triple junction.
Because of the small number of detected events the seismic rate is not computed.
c) Comparison between the AUH (one year) and the ISC (40 years) seismo-acoustic.
The two profiles are highly correlated
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
X
Context
Seismicity along the SWIR
Data
Methods
Results
Next ?
Earthquakes along the SWIR are less scattered
than along the SEIR. However fewer events are
recorded and only one cluster, C1, is found and
seems to be associated with the Gazelle transform
fault.
A weak correlation is found between the long
term seismicity recorded since 1964 and the AUH
seismicity profile. The spearman rank correlation
coefficient is 0.3 for unclustered data and 0.4 for
declustered data. Those values show that that
there is a clear difference of space variability
between the short and long term seismicity along
this ridge.
The seismicity rate is in average similar to those
observed along other ultra-slow spreading centers
(e.g. MAR) and exhibits time and space variability.
4/5
C1
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
X
Click on a cluster for details.
Gauss FZ
Gazelle FZ
• The eastern part of the SWIR is morphologically characterized
by the absence of transform offsets but presents abyssal hills.
Seismicity in this area seems to be strongly correlated with
abyssal hill faulting. Events are mainly located off-axis.
• The low-level seismicity rate is less than along the SEIR but
more constant. This is in good agreement with a tectonic
dominant mode of accretion.
• On the western part, seismicity is less scattered and more
clustered along transform faults. The Melville fault zone
clearly marks a limit for the seismic signature of the ridge.
Atlantis II FZ
In one year, AUH detected:
•245 events recorded by 4 or more hydrophones (red)
•83 events recorded by 3 hydrophones (black).
•And 1 clusters (green).
Available focal mechanism solutions from the GCMT catalog
are also shown.
C1
Melville
FZ
Close-up on the SWIR seismicity
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
X
a)
b)
c)
a) Seismicity rate for the year 2012 as a function of distance from the Rodrigues triple junction.
Dotted lines indicate transform faults.
b) Number of ISC events since 1964 as a function of distance from the triple junction.
Because of the small number of detected events the seismicity rate is not computed.
c) Comparison between the AUH (one year) and the ISC (40 years) seismo-acoustic.
The two profiles are poorly correlated.
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
X
Context
Seismicity along the CIR
Data
Methods
Results
Next ?
The seismicity is limited here to the first 600 km of the CIR.
Ridge segments appear mainly aseismic and events are
clustered around the transform faults.
The seismicity rate is low, even for fault zones. Clusters of
events were found but with no more than 10 events, which
makes their interpretation difficult.
The correlation between the 40 year ISC and one year AUH
seismicity is poor, as for the SWIR.
5/5
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
X
Close-up on the CIR seismicity
In one year, AUH detected:
•266 events recorded by 4 or more hydrophones (red)
•75 events recorded by 3 hydrophones (black).
•And few small clusters.
Available focal mechanism solutions from the GCMT
catalog are also shown.
• 110 events with SL>SLc have been recorded.
Earthquakes mostly occurred along transform fault.
• Small clusters where found and seemed to be
associated with aftershocks sequence. However, their
interpretation is difficult due to the very few number
of events in each cluster. It is not clear whether this
observation is related to the recording limits of our
array (due to the geometry of the array) or to weaker
stress release after the mainshocks. A further
analysis is required to address these questions.
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
X
a)
b)
c)
a) Seismicity rate for the year 2012 as a function of distance from the Rodrigues triple junction.
Dotted lines indicate transform faults.
b) Number of ISC events since 1964 as a function of distance from the triple junction.
Because of the small number of detected events the seismicity rate is not computed.
c) Comparison between the AUH (one year) and the ISC (40 years) seismo-acoustic.
The two profiles are poorly correlated.
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
C1 : SWIR, Gazelle transform fault
Supposed mainshock : 08 November 2012 03:24:24 (AUH) - 03:24:28 (ISC), mb 5.1
The cumulative number of aftershocks seems to follow a MOL at the beginning of the sequence but clearly differs
after 0.5 days. Aseismic slip effects related to strength fault are suggested.
X
aseismic slip ?
See table for details
mb 5.1
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
C2: SEIR, transform fault
Supposed mainshock : 07 February 2013 07:38:12 (AUH) - 07:38:18 (ISC), mb 5.0
X
mb 5.0
See table for details
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
C3 : S/A hotspot
Supposed mainshock : 12 November 2012 23:02:14 (AUH) - 23:02:23 (ISC), Mw 5.3
This sequence occurred on a transform fault related to the St-Paul and Amsterdam hotspot. However, even if a transform mechanism is
provided by the GCMT, it was not possible to fit a MOL. The distribution of aftershocks in time suggest the influence of the hotspot on the
tectonic activity of the ridge. Magmatic intrusion or thermal effects may induced changes in the strength of transform fault.
Mw 5.3
See table for details
X
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
C4: S/A hotspot
Supposed mainshock : 18 September 2012 21:41:30 (AUH) - 21:41:38 (ISC), mb 4.1
The few number of events makes interpretation difficult.
X
mb 4.1
See table for details
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
C6: SEIR segment, ~29˚S
Supposed mainshock : 05 July 2012 01:56:35 (AUH)
The sequence is clearly not related to a classical shock/aftershock process. No fitting was found for the MOL. This cluster is defined as a
tecto –magmatic swarm. The distribution of events in time strongly suggest a magmatic episode, such as dyke intrusion, that induced
activation or reactivation of normal faults in the axial valley.
X
mb 4.6
Mw 4.9
Mw 5.0
See table for details
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
C7: SEIR segment, ~29˚S
Supposed mainshock :12 December 2012 02:10:07 (AUH) – 02:10:13 (ISC), Ms 4.6
The cumulative number of aftershocks seems to follow a MOL at the beginning of the sequence but clearly differs after 0.5 days. Aseismic slip
effects related to strength fault are suggested.
This sequence occurred 6 months after the large tecto-magmatic swam C6 and suggests the reactivation of faults, induced by fluids circulation.
X
Ms 4.6
aseismic slip ?
See table for details
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
Context
Further analysis and interpretation based on :
Data
- An additional AUH seismicity catalog for 2007
Methods
Results
Next ?
- Improved relocalizations of our catalog using the 2 permanent
hydroacoustic stations in the Indian Ocean (CTBTO)
- A detail analysis along ridge segments of
- Seismicity vs geomorphology
- Seismicity vs mantle Bouguer gravity anomaly (MBA) as a
proxy to the thermal regime of the spreading ridges
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques
Analysis of the low-level seismicity along the Southern
Indian Ocean spreading ridges by the OHASISBIO array
of hydrophones in 2012
Eve Tsang-Hin-Sun, Jean-Yves Royer, Alexey Sukhovich and Julie Perrot
University of Brest and CNRS, Laboratoire Domaines Océaniques