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PUBLICATIONS
Geophysical Research Letters
RESEARCH LETTER
10.1002/2016GL068590
Key Points:
• Subducting Indian ocean plate has been
imaged beneath the Andaman-Nicobar
trench
• Evidence of tear in subducting Indian
ocean plate using Ps and Sp
conversions
• The lithospheric thickness varies from
~40 to 70 km all along the Island from
north to south
Supporting Information:
• Supporting Information S1
Correspondence to:
P. Kumar,
[email protected]
Citation:
Kumar, P., G. Srijayanthi, and M. Ravi
Kumar (2016), Seismic evidence for
tearing in the subducting Indian slab
beneath the Andaman arc, Geophys. Res.
Lett., 43, 4899–4904, doi:10.1002/
2016GL068590.
Received 8 MAR 2016
Accepted 28 APR 2016
Accepted article online 2 MAY 2016
Published online 18 MAY 2016
Seismic evidence for tearing in the subducting
Indian slab beneath the Andaman arc
Prakash Kumar1, G. Srijayanthi1, and M. Ravi Kumar1
1
CSIR-National Geophysical Research Institute, Hyderabad, India
Abstract Segmentation of a subduction zone through tearing is envisaged as an inevitable consequence
of the differential rate of slab rollback along the strike of convergent plate boundaries. It is a key feature that
controls plate tectonics and seismogenesis in a subduction setting. Globally, lithospheric tears are mostly
recognized by seismic tomography and seismicity trends. However, such an intriguing feature has never been
imaged with high resolution. Here we present seismological evidence for tearing of the Indian oceanic plate at
shallow depths along the Andaman arc. Our image of the subducted plate using the shear-wave receiver
function technique reveals three distinct plate segments. The middle lithospheric chunk has an abrupt offset of
~20 km relative to the northern and southern segments along the entire stretch of Andaman-Nicobar Islands.
We interpret that this abrupt offset in the base of the lithosphere is caused by the tearing of the subducted
oceanic plate. For the plate age of ~80 to 60 Myr, the lithospheric thickness varies from ~40 to 70 km.
1. Introduction
Oceanic plates are created at mid-oceanic ridges and consumed along the trenches by subduction. Along the
trenches, the oceanic plates are in continuous motion. In such convergent tectonic settings, the subducting
slabs are migrated backward [Jarrard, 1986; Schellart et al., 2007] with a variable rate along the strike. As a consequence of such rollback, the subducted slabs experience deformation in terms of bending and/or tearing.
This creates a discontinuity in the geometry of the subducting slab along the trench [Spence, 1977] and
usually occurs in the case of a retreating lithospheric segment [Rosenbaum and Piana Agostinetti, 2015].
The important factors, which control slab rollback, are the buoyancy of the lithosphere and the contrasting
physical properties with respect to the ambient asthenosphere [Garfunkel et al., 1986]. Other factors that control the segmentation of the slab are the topographic anomalies within the subducting plate, such as ridges,
fracture zones, and seamount chains [Kodaira et al., 2000], the major structures abutting the over-riding plate
[Ryan and Scholl, 1993] and large-scale variations in the buoyancy of the subducting plate related to its
thermal age [Yáñez and Cembrano, 2004]. Wilson [1965] opined that tearing is inevitable in a tectonic setting
where subduction terminates, thus creating an offset between the adjacent segments of the oceanic
lithospheric plate. Slab-tear is an important entity that controls plate tectonics, in particular the tectonic
activities in convergent plate boundaries. It possibly provides pathways for ascending magmas [Gvirtzman
and Nur, 1999; Rosenbaum et al., 2008; Gasparon et al., 2009].
Tearing of oceanic plates has been documented in many subduction zones of the world [Isacks et al., 1969;
Forsyth, 1975; Chatelain et al., 1992], based on the observations or interpretations of the abrupt break in seismicity [Reyners et al., 1991] or the sudden change in the velocity anomalies evidenced in tomographic models
[Lucente et al., 1999; Benoit et al., 2011; Giacomuzzi et al., 2012]. Examples include the South Sandwich trench
[Barker, 2001], Tonga trench [Millen and Hamburger, 1998], Lesser Antilles [Clark et al., 2008], Gibraltar arc
[Meighan et al., 2013], and north Apennines [Rosenbaum and Piana Agostinetti, 2015].
©2016. American Geophysical Union.
All Rights Reserved.
KUMAR ET AL.
Other than the observation of seismicity distribution and seismic images from low-resolution tomographic
studies, such an intriguing structural feature of the Earth has never been imaged with high resolution, using
body waves. We designed a seismic experiment to image the nature and geometry of the subducting Indian
plate beneath the Andaman-Nicobar Island arc (as shown in Figures 1 and S1 in the supporting information)
by installing broadband seismic instruments. In the aftermath of the mega 2004-Sumatra earthquake (Mw 9.3)
followed by the deadly Tsunami, this region has attracted the attention of seismologists world over. Further,
in the years 2005, 2009, and 2010, other mega earthquakes having Mw greater than 7.5 rocked this region. In
order to understand the genesis of the mega thrust earthquake of 26 December 2004, Shapiro et al. [2008]
produced a shear velocity model and imaged the highly oblique subducting slab. However, this study
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did not focus on tearing of the slab.
North of the Andaman Islands, slab
tear between the Burma and
Andaman arc regions has been
documented based on the vertical
distribution of seismicity [Richards
et al., 2007]. Pesicek et al. [2010]
reported the lack of a clear subduction in the same region and further
suggested that the vertical offset of
lithosphere might be responsible
for the northward termination of
the rupture zone of the 2004 mega
earthquake.
The Andaman region is characterized by the existence of an active
spreading center in the back arc
Figure 1. Topographic map with the station disposition and seismic profiles.
beneath the Andaman Sea [e.g.,
The inverted triangles (white) are the locations of broadband seismic stations.
Khan and Chakraborty, 2005] and
AB, CD, and EF are the three profiles along which the migrated receiver
the interaction of the linear 90E
function images are constructed in Figures 2 and 3. The seismicity (gray dots)
and major earthquakes (thick black circles) are also shown. The pink dots are the ridge with the trench (Figure 1). The
2005 swarm earthquakes observed after the 26 December 2004 earthquake
complexity of the region imposes a
(Mw, 9.0–9.3). The plate boundary and the contours (blue lines) of oceanic
major challenge in mapping this zone.
crustal age (gray contours) (http://www.ngdc.noaa.gov/mgg/ocean_age/ocean_age_2008.html) of the Indian ocean region are also shown. The red triangles Seismologically, this region is one of
the least studied in the world. In order
are the locations of the volcano [taken from Curray, 2005].
to image the plate geometry beneath
the Andaman-Nicobar Islands, we applied the shear-to-compressional (Sp) converted wave technique to a large
number of teleseismic earthquake waveforms. We also present the compressional-to-shear (Ps) images as an
independent support to our findings (supporting information).
2. Data and Method
The seismological data from the Andaman-Nicobar Islands are from an 11-station seismic network being
operated by the Council for Scientific and Industrial Research-National Geophysical Research Institute and
the India Meteorological Department. The operation periods of our experiment are different for different
stations (Table-S1 in the supporting information), and the stations are located only on the islands covering
the entire Andaman-Nicobar region. The mean elevation of the region is close to or below the mean sea level
(Figure 1). In order to generate images of the plate architecture using S-receiver functions (SRF) [Farra and
Vinnik, 2000], we use teleseismic waveforms registered at these broadband seismic stations. The SRFs are
generated from the vertical component of ground motion recorded at the seismic station. The advantage
of SRFs is that they are devoid of S wave multiple reflections within the time window of arrival of converted
phases from the shallower discontinuities. This provides a clean image of the subsurface. In recent times, the
SRF technique has become an effective tool to map the uppermost mantle discontinuities such as the
lithosphere-asthenosphere boundary (LAB) and has been applied in diverse tectonic settings. The distance
ranges of the teleseismic earthquakes whose waveforms are selected for the analysis are 60°–85° for S waves
and 85°–120° for SKS waves:). To ensure good signal-to-noise ratios, the earthquakes having magnitude
greater than 5.7 Mb from all the available back-azimuths are chosen (Figures S2 and S3 in supporting information). Further, the S phases on the radial component of the waveforms are visually inspected and only
those which show good SV arrivals are retained. The waveforms are then filtered with a low-pass filter with
a corner frequency of 0.25 Hz for S wave data and rotated into the radial and transverse components using
the back-azimuth information. To isolate the weak Sp phase from the parent S phase, we further rotate the
Z-R-T components into L-Q-T components using an incidence angle derived from the minimum SV energy
on the P component at zero time, in order to have an optimal isolation of P-SV-SH [Kumar et al., 2006;
Kumar and Kawakatsu, 2011]. In the presence of large noise and strong lateral heterogeneity, the incidence
KUMAR ET AL.
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Figure 2. S-receiver function data along two horizontal profiles (a) AB and
(b) CD (Figure 1). The bathymetry with station locations (black inverted triangles)
is also shown at the top of each profile (see wiggle plots in Figure S3). The
earthquakes (Mb > 5 ) are well within the lithosphere. The observation shows
two separate slab segments from the Indian ocean side and from the
Andaman Sea side dipping in opposite directions.
10.1002/2016GL068590
angles determined in this way may
be erroneous, and therefore, we discard the events with incidence angles
>52°. This coordinate rotation
ensures a clean decomposition of Sto-p conversions. Once the traces
are rotated into P and SV components, we employ source normalization using deconvolution adopting a
time domain deconvolution technique [Berkhout, 1977] to generate the
S-receiver functions. In order to
achieve a time to depth migration,
all the receiver function amplitudes
are placed at their proper conversion
points by tracing the ray from the
source to the receiver using the global IASP91 Earth model [Kennett and
Engdahl, 1991].
3. Seismic Images
Figures 2 and 3 show the migrated images below the Andaman-Nicobar Islands down to the uppermost mantle. While generating these images, we have taken care of the artifacts arising due to the lateral heterogeneity
in structure, since the S-to-p conversion points sample a wide spatial extent (see Figure S4 for piercing points for
S-to-p conversions). Figure 2, containing near east-west sections along the profiles AB and CD (see Figure 1),
reveals that there are at least two prominent phases corresponding to conversions from the Moho (red-positive
polarity, crust-mantle boundary) and the LAB (blue-negative polarity, marked in the figure) representing a
velocity increase across the Moho and decrease across the LAB. The wiggle plots of these figures are
presented in Figure S5 to demonstrate the robustness of the observation. A close examination of the section
down to 20–30 km depth reveals that there are two oppositely dipping features, i.e., one from the Indian
ocean side and the other from the Andaman back arc side. The thickness of the 80–90 Ma old Indian ocean
plate subducting below the Andaman-Nicobar Island region is found to be ~50 km (Figure 2), which is somewhat lower than that predicted by the thermal model. The primary reason for this discrepancy in the thickness might be due the deviation from a normal oceanic mantle due to the presence of sea-mounts and
ridges reported here [e.g., Kamesh Raju et al., 2004]. The presence of Nintyeast Ridge might affect the plate
structurally. The nature and size of the velocity jump across the oceanic LAB have been discussed by
Kawakatsu et al. [2009] in the context of Pacific and Philippine sea plates. In the context of converting conversion times to the depths to the interfaces, issues such as (i) thickness of the warmer younger plate tending
to be overestimated by using a 1-D reference velocity model, (ii) presence of azimuthal anisotropy, and
(iii) regional variational in Vp/Vs ratios [Kumar and Kawakatsu, 2011] cannot be ruled out. Further, the Indian
oceanic plate in this region may not be a representative of the normal oceanic plate.
The migrated images (Figure 2) show a complex lithospheric architecture beneath the Andaman islands. In
both the images (Figures 2a and 2b), two separate lithospheric fragments are seen and the seismicity
(Mb > 5) along the respective profiles is well within the lithosphere.
In order to image the nature of the Indian plate subduction beneath the Andaman trench, we constructed
north-south images along a curved profile (Figure 1). The imaging (Figures 3) brings out the complex tectonic
scenario, especially at the crustal level. At a deeper level, the base of the lithospheric plate is evident at a
depth of about 50 km, except between 7 and 8° latitude, where it deepens down to ~70 km (see Figure S5
for wiggle plots and Figure S6 for comparison with PRF images). In Figure S5 binning and stacking of the
traces are performed based on the piercing points of Sp conversions at depths of 20 and 60 km with a bin
step of 0.05° and 0.1° with overlap of 0.1°, respectively, and bins which contain more than 5 traces are only
stacked. The north-south section reveals that there are three distinct segments of the subducting Indian plate
along the trench that is (i) to the north of the intersection of the 90°E ridge with the Andaman trench, (ii)
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Figure 3. Receiver function image along the north-south profile EF parallel to the trench, with the station locations
(red inverted triangles). The average thickness of the lithosphere (blue color) is ~50 km from north to south along the arc,
except in the vicinity of ~8 deg latitude, where it deepens to ~80 km. Interestingly, the positive phase (red) which is at
~20 km all along the profile suddenly deepens to ~40 km, where the base of the lithosphere is deepest. The seismicity
(Mb > 5, green dots) with mega earthquakes of 2005 and 2010 is well within the lithosphere. This abrupt offset of the
middle part of the lithosphere has been interpreted as caused by the tearing in oceanic slab (interpretive model in bottomright inset). The numerals 1, 2, and 3 denote the three distinct lithospheres observed in our seismic sections corresponding
to those in the inset cartoon.
between the Andaman and Nicobar islands, and (iii) on the Nicobar side. An abrupt offset of ~20 km is
observed in the middle part of the lithosphere with respect to the adjacent segments. In order to demonstrate the robustness of the observation, we further show the stacked receiver functions from three representative stations, which are in the north, middle, and south segments of the arc. The station stacks displayed in
Figure 4 (also see Figure S7 for Ps stacks) show that the phase, which we identified as a Sp conversion from
the LAB, has a substantial amplitude above the ±2 SE.
4. Discussion
The seismic images reveal clear evidence for an offset in the subducted lithosphere relative to the adjacent
downgoing slabs along the arc. We interpret this offset as being caused by tearing in the oceanic lithosphere.
Seismicity and tomography studies have already indicated tearing in other places along the Burma-AndamanSumatra trench [e.g., Dasgupta et al.,
2003], which plays a significant role in
shaping the tectonics in this region. The
seismicity trends in our study region also
vindicate the possibility of a slab tear
(Figure S8). The hypocentral distribution
in the tear region shows somewhat deeper seismicity, i.e., tending toward the
trench. Such tears could have served as
Figure 4. S-receiver function stacks from three stations CMBY, KAMO, and vents for the outpouring of the asthenoHUTB plotted at their geographical locations. The stacks are generated
spheric material. This is substantiated by
after applying moveout correction with a reference slowness of 6.4 s/°
the presence of numerous volcanoes in
using the IASP91 global Earth model. The short vertical bars are the ±2 SE
these regions [Curray, 2005]. Also, lava
in time. The thin lines on either side of the wiggles indicate the error in
amplitudes. A similar stack section has also been shown for P-to-s conversions samples from the Barren islands reveal
their affinity toward the mantle or deep
in Figure S7.
KUMAR ET AL.
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mantle origin [e.g., Alam et al., 2004]. The cartoon shown as an inset in Figure 3 summarizes our observations.
The thickness of the oceanic plate as mapped by the Sp conversions (also by Ps conversions) varies between
~40 and 70 km along the arc. The two slabs, one on the northern side and the other on the southern side of
8° latitude, have gentler dips, whereas the middle segment has a steeper subduction angle with an offset of
~20 km relative to the other two adjacent segments. This is typically governed by the surface features of the
oceanic slab like the age-variability of the Indian plate along the trench (Figure 1), the existence of active
spreading in the back arc beneath the Andaman Sea, and an anomalously strong strain partitioning [e.g.,
Socquet et al., 2006], in which the oblique Sumatra-Andaman subduction is accommodated. Since the
downgoing slabs are in constant motion, a Subduction Transform Edge Propagator [Govers and Wortel, 2005]
develops in the downgoing slab as a result of slab roll back. It is possible to observe such features with structural
variability in the gradients of the gravity anomaly map (see Figure S9), where, along the arc, the anomaly
suddenly diminishes around the place where we found tearing in the slab. Such gaps in slabs have been
observed at deeper depths [Hafkenscheid et al., 2001] and mostly attributed to slab detachment. The downgoing rate of the slab is controlled by the presence of 90°E ridge, which meets the trench in the latitude
range of about 8–10°N. The differential dips in the Mariana slabs have been attributed to the tearing in the slab
[van der Hilst and Seno, 1993]. However, this is the first ever demonstration of tear in the subducting slab along
the Andaman-Nicobar Island trench. Such a feature not only affects the local seismicity pattern in the region but
also modulates the stress regimes. Results from GPS studies affirm this notion by demonstrating that the largest
horizontal displacement (Figure S10) [Chlieh et al., 2007; Gahalaut et al., 2006; Subarya et al., 2006] due to the
rupture generated by the 26 December 2004 Sumatra-Andaman earthquake (Mw 9.0–9.3) occurred exactly in
the area where we have imaged the tearing of the lithosphere. It seems that such close correlation between
the slip and the plate configuration may indicate the irregularity of the surface that defines the top of the
subducting Indian plate. The interesting point is the correlation between the two independent observations;
however, at present their relationship cannot be ascertained. It can only be authenticated by more complex
modeling incorporating realistic structure of the region.
Further, such tearing of the plate may provide clues about the anatomy of the Indian oceanic plate. The plate
reconstruction models suggest eastward motion of the plate with varying degree of obliquity and rate of
subduction [Sieh and Natawidjaja, 2000]. The subduction initiated due to the convergence between the
Australia/Indian plate and the Eurasian/Sunda plates orthogonally. However, the plate progressively moved
obliquely westward in order to accommodate the arc [Curray et al., 1979; McCaffrey, 1992]. Such oblique
subduction pattern implies the southward migration of the bottom tip of the northern slab diminishing
the slab gap over time in the deeper part of the mantle. We further propose that such slab discontinuities
could be the locales for the intense seismicity. For example, the 2005-swarm activity, due to the rupturing
generated by the 26 December 2004 earthquake (Mw 9.0–9.3), occurred exactly in the region of tearing.
Acknowledgments
We thank INCOIS (Ministry of Earth
Sciences) for funding the project. Seismic
data analysis was performed in Seismic
Handler (K. Stammler, http://www.seismic-handler.org/). Plots are generated
using Generic Mapping Tool [Wessel and
Smith, 1965]. We thank. V.K. Gahalaut for
fruitful discussions. The manuscript has
been benefited immensely from the constructive comments by two anonymous
reviewers. Michael Wysession has
handled the manuscript excellently.
KUMAR ET AL.
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