Geophysical Journal International
Geophys. J. Int. (2010) 180, 1181–1186
doi: 10.1111/j.1365-246X.2009.04449.x
Slow rupture in Andaman during 2004 Sumatra–Andaman
earthquake: a probable consequence of subduction of 90◦ E ridge
V. K. Gahalaut, C. Subrahmanyam, B. Kundu, J. K. Catherine and A. Ambikapathy
National Geophysical Research Institute, Council of Scientific and Industrial Research, Uppal Road, Hyderabad 500007, India.
E-mail: [email protected]
Accepted 2009 November 8. Received 2009 October 30; in original form 2009 June 2
SUMMARY
One of the most enigmatic features of the 2004 Sumatra–Andaman earthquake was the slow
rupture speed and low slip on the northern part of the rupture under the Andaman region.
We propose that the aseismic 90◦ E Ridge (NER) on the Indian Plate obliquely subducts
under the Andaman frontal arc region. Though other possibilities also exist, we hypothesized
that this ridge probably acted as a structural barrier influencing rupture characteristics of
the earthquake. Here we present several features of the Andaman region that favour NER
subduction under the region, which include (i) comparatively shallow bathymetry and trench
depth, (ii) low seismicity, (iii) significant variation in the azimuths of coseismic horizontal
offsets due to the 2004 Sumatra–Andaman earthquake, (iv) lack of post-seismic afterslip on
the coseismic rupture in the Andaman frontal arc region, (v) low P wave with only small
decrease in S wave speed from tomographic studies, (vi) gravity anomalies on the Indian Plate
indicating continuation of the ridge under the Andaman frontal arc and (vii) lack of back arc
volcanoes in the Andaman region.
I N T RO D U C T I O N
The 2004 Sumatra–Andaman earthquake (M w 9.2) ruptured about
1400-km-long frontal arc of the Sumatra–Andaman subduction
zone between northwest of Sumatra and the Andaman region (Lay
et al. 2005). The earthquake caused large coseismic offsets, reaching 6m in the source region (Gahalaut et al. 2006; Subarya et al.
2006), and also caused the worst tsunami damage in history, in
which more than 200 000 people lost their lives in the countries
around the earthquake source region. Analyses of the seismological waveform and the tide gauge data recording tsunami, suggest
that the rupture speed was about 2.0 km s−1 in the northern one
third part of the rupture under the Andaman region whereas it was
about 2.5–2.7 km s−1 in the southern part under the Nicobar and
North Sumatra region (Guilbert et al. 2005; Lay et al. 2005; Fujii &
Satake 2007; Tolstoy & Bohnenstiehl 2006). Thus the rupture was
slow in the Andaman region and even the slip on the rupture was low
here (Gahalaut et al. 2006; Chlieh et al. 2007). Slow slip under the
Andaman region probably continued for about half an hour with low
slip (Gahalaut et al. 2006; Singh et al. 2006; Chlieh et al. 2007),
though a few studies suggest that it was not the case (e.g. Vigny
et al. 2005). The reason for slow rupture and low slip under the
Andaman region is not known. Further, whether slow rupture speed
during great and major earthquakes is an intrinsic property of the
material in the region or it is unique to the 2004 Sumatra–Andaman
earthquake, is not known. The last great or major earthquake in
the region occurred in 1941, M 7.7, (Pacheco & Sykes 1992) for
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Journal compilation which no seismological data exist to suggest whether slow rupture occurred during that earthquake as well. Though the reports
are scanty, it appears that even that earthquake did not generate a
tsunami (Rajendran et al. 2007). There may be various reasons for
that earthquake not to be tsunamigenic, and one of them could be
slow rupture during the earthquake.
In this paper, we examine bathymetry, trench morphology, seismicity, coseismic and post-seismic deformation data, volcanic arc,
gravity, and available results of tomographic studies in the Andaman
region to suggest that the almost N–S trending 90◦ E Ridge (NER)
impinges the south Andaman region very obliquely and at least
some part of it obliquely subducts under the Burma Plate, which
influenced rupture characteristics of 2004 Sumatra–Andaman earthquake, including its slow speed and low slip in the Andaman
region.
B AT H Y M E T RY A N D G R AV I T Y
A S S O C I AT E D W I T H N E R S U B D U C T I O N
The aseismic plume-fed NER, formed during the passage of the
Indo-Australian Plate over hotspot(s) (Curray et al. 1982), is an
approximately N10◦ trending feature with a surface width of about
300 km and is the most prominent feature of the Indian ocean. The
ridge rises by about 2–3 km from the ocean floor. Bathymetry data
reveals that the ridge extends at least up to 10◦ N latitude (Fig. 1)
in the north direction and it appears to impinge the frontal arc at
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GJI Seismology
Key words: Earthquake dynamics; Seismicity and tectonics; Subduction zone processes;
Continental margins: convergent; Dynamics and mechanics of faulting; Asia.
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V. K. Gahalaut et al.
Figure 1. Bathymetry (Smith & Sandwell 1997) and general tectonics of the Andaman–Sumatra region. Surface projection of the 2004 Sumatra–Andaman
earthquake rupture in the Andaman Nicobar and Sumatra frontal arc (Chlieh et al. 2007) is shown by fine hatching. Star shows the 2004 earthquake epicentre.
Region of slow rupture in the Andaman region (Lay et al. 2005) is shown by double verging arrow. Black arrows show horizontal coseismic offsets from GPS
measurements (Gahalaut et al. 2006; Subarya et al. 2006) due to the 2004 Sumatra–Andaman earthquake. The azimuths of coseismic horizontal offsets are
quite consistent in the Nicobar and Sumatra region whereas in the Andaman region they show some variation. Black triangles show active volcanoes in the
backarc (Natawidjaja et al. 2004). The gap in volcanic arc in the Sumatra region (black double verging arrow in Sumatra) is due to possible subduction of the
Wharton ridge. Note general absence of volcanoes in the Andaman region. Bathymetry in the Andaman frontal arc region is shallower than that in the Nicobar
and Sumatra region. This is also shown in the inset where bathymetry across two west to east vertical cross sections in the Andaman and Nicobar regions are
shown on the top panel. East of the trench, bathymetry in the Andaman region is shallower by almost 2 km. The other two panels in the inset show shallow
seismic sections along two lines Seis1 and Seis2 (Curray et al. 1982; Gopala Rao et al. 1997). Two reflectors in these sections correspond to ocean bottom
surface and the NER. BV: Barren volcano.
about 8–10◦ N latitude. Further north of 10◦ N latitude, continuity of
the ridge is inferred from the seismic data as it is buried under the
Bay of Bengal sediments (Subrahmanyam et al. 2008). Bathymetry
of the island arc in the Andaman region is shallower than that of
Nicobar and Sumatra island region in the south (Fig. 1). Even the
trench is very prominent in the Sumatra–Nicobar region till about
8◦ N whereas, in the Andaman region it is very shallow and further
north it is totally lost as the Bengal fan sediments fill the trench.
Shallow bathymetry of the frontal arc in the Andaman region may
be ascribed to the subduction of buoyant NER.
The NER is associated with a positive free air gravity anomaly
of about 20–80 mGal and can be traced in the north direction up to
16◦ N latitude and possibly beyond on the subducting Indian ocean
(Subrahmanyam et al. 2008). The NER, as seen in the free air
gravity anomaly map, appears to impinge the subduction zone at
about 7–9◦ N latitude (Fig. 2), where the trench slightly deviates
from its convex trend and forms a small cusp (Vogt et al. 1976;
Guzman-Speziale & Ni 1996). Seismic data from this region also
attest interaction of NER (Gopala Rao et al. 1997; Curray 2005) on
the Indian Plate with the trench, confirming the results of gravity and
bathymetry. Unfortunately, the available shallow seismic sections
along east-west lines crossing the Andaman–Sumatra trench, do not
provide deeper information, particularly east of the trench under the
accretionary prism. Nevertheless, these sections show evidence of
presence of the ridge under the sediments on the Indian Plate, and
the ridge appears to impinge the trench in the Andaman region. In
Fig. 1, we show a couple of sections in the Andaman region (Curray
et al. 1982; Gopala Rao et al. 1997). However, as these seismic
sections are restricted to very shallow depth and gravity anomalies
in the frontal arc zone are dominated by the subduction process,
these data do not allow us to comment on the presence of subducted
ridge under the frontal arc island belt.
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Journal compilation Slow rupture during 2004 earthquake
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Figure 2. (a) Free air anomaly from satellite gravity data (Sandwell & Smith 1997). The 90◦ E ridge is marked with high anomaly of about 20–80 mGal and
appears to impinge the subduction zone at about 6–10◦ latitude. Contours of only 0 and positive anomalies are shown with an interval of 20 mGal. (b) Relative
location of 2004 Sumatra–Andaman earthquake rupture (Gahalaut et al. 2006; Chlieh et al. 2007) and zone of afterslip during 2005–2007 on the plate interface
(Gahalaut et al. 2008). In the Andaman region the two zones do not overlap at all while in southern part, they do. (c) Seismicity of the region from 1973 to 2008
with M > 4 from USGS catalogue. The 2008 June 27 Little Andaman earthquake with M w 6.6, and 2009 August 11 Coco earthquake with M w 7.5, and their
focal mechanisms are also shown. Earthquakes falling in the pink shaded region were used to quantify level of seismicity in the next panel. (d) Cumulative
earthquakes (Scholz & Small 1997) in the frontal part. We considered only those earthquakes in the frontal arc which occurred within 150 km from the trench
(shaded region in (c) and thus avoided earthquakes from the backarc and Andaman opening regions. m1 and m2 (number of earthquakes/degree latitude)
are the slope of best fit lines in the Andaman and Nicobar–Sumatra regions, respectively. The error in these estimates corresponds to residual mean square.
Note very low seismicity at about 8–10◦ N latitude and lower level of seismicity in the Andaman region north of it, as evident from the lower value of m1 .
The blue curve, representing the level of seismicity, is the derivative of black curve, which also show lead to similar interpretation. The arrow marks the region
of the lowest seismicity which can also be noted in (c), at around 10◦ N latitude.
EFFECT OF NER SUBDUCTION ON
F R O N TA L A R C S E I S M I C I T Y A N D B A C K
A RC V O L C A N I S M
We analyse seismicity of the frontal arc to assess the effect of
NER subduction under the Andaman frontal arc. Vogt et al. (1976)
and Scholz & Small (1997) reported decrease in seismicity within
subduction zones where a seamount or ridge subducts under the
trench. Guzman-Speziale & Ni (1996) reported a significant decrease in the number of earthquakes in the frontal arc region at
around 8–10◦ N latitude where the NER appears to impinge on the
subduction zone. We analyse seismicity in the frontal arc region
of the Andaman–Sumatra subduction zone using USGS earthquake
catalogue from 1973 to 2008 with M ≥ 4. Following Scholz &
Small (1997), we considered latitudinal variation in the number
of earthquakes in the frontal arc which occurred within 150 km
from the trench (Fig. 2c). By doing this we avoided earthquakes
from the backarc and Andaman opening regions. We find that in
the Andaman frontal arc region (between latitude 10–15◦ N) about
109 ± 15 earthquakes occurred per degree latitude whereas in the
Nicobar and Sumatra frontal arc (between latitude 2–9◦ N) more
than 152 ± 30 earthquakes per degree latitude occurred during
the same period. Thus our simple analysis using method of Scholz
& Small (1997) shows that earthquake production in the frontal
arc in the Andaman region is about 28 per cent lower than that
in the Nicobar and Sumatra region. It probably implies relatively
higher coupling in the Andaman region (Scholz & Small 1997)
due to possible subduction of the buoyant NER. Focal mechanisms
of some of the aftershocks of 2004 Sumatra–Andaman earthquake
in the Andaman region (see Fig. S1), particularly the recent aftershocks of 2008 June 27 (M w 6.6) and its own aftershocks and 2009
August 11 occurring in the frontal arc region near the trench, show
predominant normal motion on the approximately north–south ori
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2010 The Authors, GJI, 180, 1181–1186
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Journal compilation ented steep planes (Fig. 2c). The NER on the Indian Plate is characterized by the left lateral strike slip motion on the north–south
oriented steep planes (Delescluse & Chamot-Rooke 2007). These
pre-existing planes on the NER were probably reactivated as normal faults under the frontal arc due to the flexural bending of Indian
Plate as it subducts (Franke et al. 2008). Further, the coseismic slip
during the 2004 Sumatra–Andaman earthquake and the ongoing
afterslip in the post-seismic period (Gahalaut et al. 2008; Catherine
et al. 2009) favoured normal slip on these planes. Another important
feature of the seismicity in the Andaman region, which may or may
not be linked to the presence of the ridge under the Andaman region,
is the absence of earthquakes beyond 200 km depth (Engdahl et al.
2007). Such earthquakes are common in the Nicobar and Sumatra
region. Richards et al. (2007) attributed the absence of these earthquakes in the Andaman region to a possible subhorizontal tear in
the subducting Indian Plate.
Subduction of a buoyant ridge may also lead to decrease in the
number of active volcanoes in the backarc (Vogt et al. 1976; Nur
& Ben-Avraham 1982). In the entire Andaman region there is only
one such volcano, namely the Barren volcano, while in the Sumatra
region such volcanoes are abundant, particularly the regions which
are devoid of such ridges (Fig. 1). Even the lone Barren volcano is
found to be depleted in Ba concentration despite its high (491 ppm)
concentration in the Andaman sediments (Luhr & Haldar 2006).
Thus even this volcano appears to be genetically different as compared to the other volcanoes in the Sumatra arc region. The longer
spatial gap in the absence of active volcanic arc in the region implies oblique subduction of the ridge. An arc normal subduction of
a ridge generally results in a narrow gap in active volcanic arc, as in
the eastern south Pacific, western Pacific, and Tonga regions (Nur
& Ben-Avraham 1982).
Another aspect which is also linked to the seamount or ridge
subduction along the convergent plate margin is the backarc rifting.
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Wallace et al. (2005) studied several such subduction zones (Papua
New Guinea, New Zealand, Tonga, Vanuatu and the Marianas) and
noted spatial correlation between rotation of the convergent margin
block and the transition from subduction to collision. They suggested that such transition exerts a torque on the upper microplate,
causing it to rotate rapidly. They suggested that such rotation may
also lead to backarc rifting. Although, data to constrain block rotation of the Andaman frontal arc are not available and other possibilities also exist for the Andaman Sea opening in the backarc
region, in view of the studies by Wallace et al. (2005), the possibility that backarc rifting in the Andaman sea is due to rotation of the
Andaman frontal arc and NER subduction, may not be ruled out.
In a recent tomographic study, Miller & Lee (2008) analysed
perturbations in P-wave, shear wave speed, and bulk-sound speed
in the upper mantle of the subducting Indo-Australian Plate. In the
Andaman region at about 60–160 km, they found that the slab is
characterized by a low P-wave signal, with only a small decrease in
the S wave speed, and a modest decrease in bulk-sound speed. They
suggested that the negative P wave and bulk-sound speed anomalies
in the slab, along with only a small decrease in S wave speed cannot
be explained by thermal variations alone and hence it implies for a
possible change in composition. They suggested that these anomalies correspond to orthopyroxene-rich zones within the peridotitic
lithospheric mantle, which was possibly formed by the interaction
of upwelling magmas with preexisting oceanic lithospheric mantle beneath the NER before subduction. Thus tomographic studies
(Kennett & Cummins 2005; Miller & Lee 2008) provide an indirect evidence of the subduction of NER and its presence under the
Andaman region.
H E T E RO G E N E O U S N E R U N D E R
A N DA M A N A N D S T RO N G C O U P L I N G
Gahalaut et al. (2006) and Subarya et al. (2006) reported GPS measurements of coseismic horizontal offsets in the Andaman-Nicobar
and Sumatra region due to the 2004 Sumatra–Andaman earthquake,
which are of the order of 3–6 m (Fig. 1). The error, not more than
a few cm, involved in these measurements is insignificant in comparison to the magnitude of the coseismic offset at each site. The
azimuths of these coseismic horizontal offsets are very consistent in
the Nicobar and Sumatra region. However, in the Andaman region,
there appears a significant variation in the azimuths of coseismic
offsets. We suggest that such variation in azimuth in the Andaman
region may occur due to the presence of some heterogeneity under the region. Analysis of post-seismic deformation that occurred
during 2005–2006 (Gahalaut et al. 2008) also indicates that the
Andaman region is distinctly different from the region lying further
south. In the Andaman region afterslip, which is used to simulate
post-seismic deformation, occurred downdip of the coseismic rupture on the plate interface, whereas in the Nicobar region, regions
of afterslip and coseismic rupture overlapped (Fig. 2). Lack of afterslip on the coseismic rupture in the Andaman frontal arc region
probably implies strong coupling. Subduction of a buoyant and heterogeneous feature, like seamount or ridge, will lead to additional
increase in the normal stress (σ ) across the subduction interface
which enhances seismic coupling (Scholz & Small 1997). In the
Andaman region we estimate it to be about 50 MPa, assuming
height (w) and width (B) of the NER and elastic thickness (T e ) of
the Indian Plate to be 3, 100 and 10 km, respectively using following
relation (Turcotte & Schubert 1982)
σ = 192w D/B 4 ,
where flexural rigidity, D = ET e 3 /12(1 − ν 2 ), E is the Young’s modulus (3.2 × 104 MPa) and ν is the Poisson’s ratio (0.25). We suggest
that enhanced coupling due to the subduction of heterogeneous material may lead to low slip and significant variation in the azimuth
of coseismic offsets and may also resist afterslip in the Andaman
frontal arc region. Low slip on the part of the rupture over the subducted ridge has earlier been reported by Spence et al. (1999). In
case of the great 1996 November 12 Peruvian earthquake (M w 8.0),
they found that relatively larger slip on rupture occurred south of the
Nazca ridge as compared to the region of enhanced coupling over
the ridge. Enhance seismic coupling may also lead to an increase in
the recurrence interval of great earthquakes in the region (Scholz
& Small 1997). Although historical records of occurrence of great
earthquakes in the Andaman region are poor, it appears that great
earthquakes are less frequent in the Andaman region than in the
Sumatra region (Lay et al. 2005; Gahalaut et al. 2006). We suggest
that in addition to the reduced plate motion rate, increased obliquity in the convergence and increasing age of the subducting Indian
lithosphere plate in the Andaman region, subduction of NER may
also be one of the factors affecting earthquake recurrence interval.
C O N C LU D I N G D I S C U S S I O N
Subduction of topographic features, for example, ridges and
seamounts, has been studied extensively and is considered to locally increase coupling (e.g. Mogi 1969; Kelleher & McCann 1976;
Cloos 1992; Cloos & Shreve 1996; Scholz & Small 1997; Bilek
et al. 2003; Collot et al. 2004). These features have been found to
act as asperities rupturing during earthquakes (Abercrombie et al.
2001; Husen et al. 2002; Bilek et al. 2003) and in some cases
as barrier to incoming rupture front (Kodaira et al. 2000, 2002;
Cummins et al. 2002). Such topographic features have also been
found to slow down the rupture velocity (Bilek et al. 2003) as in
the case of 1983 Osa earthquake (M w 7.4) in Cocos ridge segment.
In an another case, Robinson et al. (2006) suggested that the 2001
Peru earthquake (M w 8.4) rupture was stalled by a fracture zone
which was associated with slow rupture speed and low slip.
Based on the above evidences of the presence of NER under the
Andaman frontal arc and the available case histories, we suggest
that the low slip and slow rupture speed in the Andaman region
during the 2004 Sumatra–Andaman earthquake may be explained
by the presence of a structural barrier in the form of NER. We
suggest that such a barrier in the Andaman region not only retarded
the rupture speed at its northern end but also helped in arresting its
further propagation in north in a way similar to that near the southern
rupture end in Sumatra region, where rupture terminated at a sharp
boundary. Considering the fact that the NER is remarkably linear
only in the region lying south of the equator, it is possible that in the
Andaman region the wider ridge deviates from this trend (Sclater &
Fisher 1974) and only some part of it or its eastern flank subducts
under the Andaman region.
We acknowledge that all the evidences cited here in support of the
presence of NER under the Andaman region and its effects on seismicity and earthquake rupture processes, though appear appealing,
yet they are all indirect and there may be some additional processes
responsible for such effects, for example, tear in the subducting slab
and subducted Bengal fan sediment thickness, etc. Richards et al.
(2007) proposed that there is a subhorizontal tear in the subducted
Indian slab under the Andaman Nicobar region which becomes
progressively shallower in the north. Thus the subducted slab width
decreases from Nicobar to Andaman and is the least (about 200 km)
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Journal compilation Slow rupture during 2004 earthquake
in the frontal arc region north of Andaman. This might have caused
termination of rupture in the north (Kundu & Gahalaut 2010) and
also might have slowed down the rupture as it propagated northward
in the Andaman region. Another important factor, which might have
affected the rupture velocity in the Andaman is the possible subduction of Bengal fan sediments. In the Andaman region, the sediment
thickness, deposited by Ganga and Brahmaputra rivers, is of the order of about 6 km, which decreases in south and increases in north
(Curray et al. 1982; Gopala Rao et al. 1997). It is possible that
these sediments on the Indian Plate might have subducted under the
Andaman frontal arc region. There are two contrasting views on how
the sediment subduction affects the earthquake characteristics. Ruff
(1989) proposed that subduction of sediments at elevated temperature and pressure makes a homogeneous and strong contact zone
between the two plates, thereby increasing coupling and causing
great earthquakes. On the other hand, Kanamori (1986) and Polet
& Kanamori (2000) suggested that such unconsolidated sediments
may slow down the rupture process and may eventually terminate the
rupture. At this point of time, it is difficult to choose between the various causes of slow rupture of 2004 Sumatra–Andaman earthquake
in the Andaman region. However, mounting evidence of presence
of NER and anomalous rupture characteristics of the earthquake
in the Andaman region makes it an attractive hypothesis that the
NER under the Andaman frontal arc region possibly influenced the
rupture characteristics of the 2004 Sumatra–Andaman earthquake.
Deep seismic reflection surveys across the Andaman region using
ocean bottom seismographs may provide additional and direct evidence for the presence of NER under the Andaman region and its
possible influence on the rupture characteristics.
AC K N OW L E D G M E N T S
Constructive comments by Roland Bürgmann, an anonymous reviewer and the Editor, Yehuda Ben-Zion, led to significant improvement in the paper. Financial support for the work was provided by
the Ministry of Earth Sciences. CS was supported by an Emeritus Scientist Grant from the Council of Scientific and Industrial
Research, India.
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S U P P O RT I N G I N F O R M AT I O N
Additional Supporting Information may be found in the online version of this article:
Figure S1. Fault plane solutions of aftershocks of 2004
Sumatra–Andaman and 2005 Nias earthquakes. CMT solutions
from Harvard catalogue till July 2008 are plotted here. Two fault
plane solutions, marked as red, with predominant normal slip correspond to the 2008 June 27 Little Andaman aftershock (M w 6.6) and
the 2009 August 11 Coco aftershock (M 7.5). Note several normal
slip dominated focal mechanism solutions with slip on predominantly N–S oriented planes in the Andaman Nicobar frontal arc.
Background tectonic image is from Curray (2005).
Please note: Wiley-Blackwell are not responsible for the content or
functionality of any supporting materials supplied by the authors.
Any queries (other than missing material) should be directed to the
corresponding author for the article.
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2010 The Authors, GJI, 180, 1181–1186
C 2010 RAS
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