Geophysical Research Letters

Coseismic offsets due to the 11 April 2012 Indian Ocean earthquakes (Mw 8.6 and 8.2) derived from GPS measurements

Authors


Abstract

[1] The 11 April 2012 earthquake (Mw 8.6) in the Indian Ocean, about 100 km west off the Sumatra subduction zone, is the largest intraplate strike-slip earthquake in the known history. Two hours later, it triggered another great earthquake of Mw 8.2 in its vicinity. The earthquakes reflect the internal deformation of the diffused plate boundary between India and Australia caused by the differential plate motion between them. The slip occurred on conjugate planes, and the presence of some of them has been reported from the swath bathymetry and satellite magnetic anomalies. We estimate coseismic offsets due to these earthquakes at continuous GPS sites in the Andaman-Nicobar region and at other International GNSS Service (IGS) sites around the earthquakes source region. The sites on the Andaman Islands, which are about 900–1200 km to the north of the earthquake epicenters, experienced predominantly southward coseismic offset of up to 3 cm. The nearest site, Campbell Bay, on Great Nicobar Island, about 500 km to the north of the earthquake, documented an ESE offset of about 4 cm. The coseismic offsets are consistent with the finite-fault slip models derived from back projection of the seismic waves recorded by the global networks.

1 Introduction

[2] In less than 1 decade, 11 earthquakes of Mw ≥ 7.5 have occurred in the Andaman-Sumatra and adjoining regions. At least five of them were great (Mw ≥ 8), and at least four of them caused damaging tsunami (Figure 1). Their occurrence in quick succession and several aspects related to their source processes have highlighted our lack of understanding about them. The 26 December 2004 Sumatra-Andaman earthquake (Mw 9.2) had the longest rupture of about 1400 km in the recorded history of earthquakes and exhibited slow slip along its northern part [Lay et al., 2005; Gahalaut et al., 2006; Chlieh et al., 2007; Shearer and Bürgmann, 2010]. The rupture was almost unilateral and had a sharp rupture boundary at its southern edge. However, 3 months later, it triggered another great earthquake, the 28 March 2005 Nias earthquake (Mw 8.6), with abutting rupture along the southern edge [Franke et al., 2008]. Unlike its predecessor, the Nias earthquake rupture did not extend till the trench [Briggs et al., 2006; Konca et al., 2007], and we assumed that the shallow portion resisted rupture but afterward it crept aseismically. Farther southwest along the subduction zone, another great earthquake occurred on 12 September 2007, the Mw 8.5 Bengkulu earthquake (also known as Kepulauan earthquake) [Konca et al., 2007, 2008; Ambikapathy et al., 2010]. This left about 200 km of unruptured segment of the subduction zone which last ruptured in 1797 [Konca et al., 2008]. Even during the 2007 Bengkulu earthquake, the shallow part of the subduction zone did not participate in strain release and the tsunami caused by the earthquake was not damaging. However, 3 years later, the 25 October 2010 Mentawai earthquake (Mw 7.8) occurred on the shallow part of the subduction zone which is just updip of the 2007 Bengkulu earthquake rupture [Lay et al., 2011; Hill et al., 2012]. It caused an unexpected tsunami which killed more than 400 people. So the shallow portion of the plate boundary fault, updip of the 2005 Nias earthquake, crept aseismically, but 300 km southeast along the same subduction zone, a similar region produced a tsunami earthquake. The latest in the list from the region is the 11 April 2012 earthquake (Mw 8.6) which occurred in the Indian Ocean, about 100 km west of the Sumatra subduction zone (Figure 1) [Meng et al., 2012; Yue et al., 2012; Pollitz et al., 2012]. It was followed by another great earthquake (Mw 8.2). Unlike the previous four plate boundary events, these earthquakes occurred in the Wharton Basin which is the diffused plate boundary zone between the Indian and Australian plates [Gordon et al., 1998]. In this article, we report GPS measurements of coseismic offsets due to this earthquake sequence at sites on the Andaman and Nicobar Islands and at other IGS sites around the earthquake epicenter. The estimated coseismic offsets are consistent with the rupture models derived from seismological methods.

Figure 1.

The 11 April 2012 earthquakes and coseismic offsets derived from GPS measurements at various IGS sites and at permanent GPS sites in the Andaman-Nicobar region (shown with black arrows with error bars). The bold gray arrows represent the compressional regime of the diffused plate boundary region (shaded with light gray color) between the Indian and Australian plates [Gordon et al., 1998]. The yellow dashed lines denote the rupture planes of the 11 April 2012 earthquakes [Yue et al., 2012]. Arrows with different colors show the simulated coseismic offsets due to the slip models by Yue et al. [2012] using the layered spherical earth [Pollitz, 1997]. The purple stars are other earthquakes discussed in the text. The north-south gray lines indicate the fracture planes in the Wharton and Central Indian basin.

2 The 11 April 2012 Earthquakes

[3] The 11 April 2012 earthquake (Mw 8.6) occurred in the Indian Ocean, about 100 km west of the Sumatra subduction zone (Figure 1). Two hours later, another earthquake with Mw 8.2 occurred about 180 km SWS of the first one. The main shock and all the aftershocks exhibited strike-slip motion with high stress drop. The earthquake is the largest ever strike-slip earthquake which occurred in an intraplate region [Gahalaut, 2012; McGuire and Beroza, 2012; Meng et al., 2012; Yue et al., 2012; Delescluse et al., 2012; Ishii et al., 2013]. The combined rupture models of the two earthquakes have been estimated using back projection source imaging techniques [Meng et al., 2012; Yue et al., 2012; Satriano et al., 2012]. The source zone appears to be very complex with slip on several conjugate and en echelon faults trending NNE-SSW and WNW-ESE. The slip was as large as 30 m, and the rupture extended up to a depth of at least 40 km into the upper mantle. It radiated large-amplitude Love waves which triggered global aftershocks [Pollitz et al., 2012]. The earthquake occurred in the diffused plate boundary region between the Indian and Australian plates rather than the discrete plate boundary. The region experiences northwest-southeast compression due to India-Eurasia collision in the north and approximately east-west extension due to the enhanced slab pull force on the Australian plate in the Sumatra subduction zone [Gordon et al., 1998; Yue et al., 2012; Delescluse et al., 2012]. These forces are considered to be leading to the detachment of India from the Australian plate in this region [Delescluse et al., 2012]. Focal mechanisms of earthquakes from the region are consistent with it, in which left-lateral and right-lateral motion occurs on the NNE-SSW- and WNW-ESE-oriented conjugate planes, respectively, besides occasional reverse faulting on the NE-SW-oriented planes [Gordon et al., 1998]. The region is marked by several north-south-trending fracture zones. It hosts the approximately north-south trending 90°E ridge which is a plume-fed aseismic ridge formed due to the movement of the Indian plate over the Kerguelen hot spot [Gordon et al., 1998; Vogt, 1973]. There are several fracture zones and transform faults that were formed due to the opening of the Wharton Basin which continued until about 45 Ma [Gordon et al., 1998]. These NNE-SSW-oriented planes as well as the conjugate planes have been mapped using swath bathymetry and magnetic anomalies (Figure 2) [Graindorge et al., 2008; Deplus et al., 1998; Meng et al., 2012; Yue et al., 2012].

Figure 2.

Satellite magnetic anomalies and swath bathymetry data. (a) North-south-oriented planes can be seen on the magnetic anomalies. These planes extend right up to the trench and appear to influence the tectonics of the subduction zone. Near the trench, they are characterized by strike-slip faulting (marked by yellow ellipses). If extended farther north, they coincide with the low-slip regions of the 2004 Sumatra-Andaman earthquake rupture [Chlieh et al., 2007]. The pink dashed line with arrow marks the northern limit of the fast slip during the 2004 Sumatra-Andaman earthquake rupture [Lay et al., 2005], and it coincides with the north-south plane which accommodated part of the slip during the 11 April 2012 earthquakes. The slab pull force along the arc is also shown [Lallemand et al., 2005]. (b) Swath bathymetry data [Graindorge et al., 2008] showing a north-south-oriented fault. A, B, C, and D in the figure are the seismic lines across the fault. These near-vertical normal faults are now reactivated as left-lateral strike-slip faults.

3 GPS Measurements of Coseismic Offsets Due to the 2012 Indian Ocean Earthquake

[4] The Andaman-Nicobar permanent GPS network in India was established after the 2004 Sumatra-Andaman earthquake. The sites on the Andaman and Nicobar Islands are located about 500–1200 km north of the 2012 Indian Ocean earthquake. The nearest site is at Campbell Bay on Great Nicobar Island (CBAY), which is about 500 km north from the earthquake epicenter. We processed daily files, 5 days before and 5 days after the 2012 earthquake, from these sites using GAMIT/GLOBK software [Herring et al., 2010a, 2010b]. We also included other IGS sites located within 2500 km from the earthquake epicenter. We estimated coordinates in International Terrestrial Reference Frame 2008 [Altamimi et al., 2011] by including several IGS sites. The offsets reported here (Table 1) are the cumulative effect of the two earthquakes (Mw 8.6 and 8.2). The maximum offset is at CBAY, 41 ± 5 mm toward N115°. Other sites of our network on the Andaman Islands reported offsets of 17–30 mm predominantly toward the south.

Table 1. Coseismic Offsets Estimated From GPS Measurements at Sites in the Andaman-Nicobar Region and at IGS Sites
GPS SiteLatitudeLongitudeNorth (mm)East (mm)
CBAY (Nicobar Island)7.05°N93.93°E−17.8 ± 5.637.5 ± 6.2
HBAY (Andaman Islands)10.69°N92.57°E−29.4 ± 4.02.9 ± 4.3
PORT (Andaman Islands)11.63°N92.73°E−25.7 ± 4.02.4 ± 4.2
HAVE (Andaman Islands)12.03°N92.99°E−16.6 ± 4.01.8 ± 4.4
MBDR (Andaman Islands)12.90°N92.90°E−18.3 ± 3.7−0.5 ± 4.1
PALK (Sri Lanka)7.27°N80.70°E−4.9 ± 3.6−0.5 ± 4.0
COCO (Indian Ocean)12.18°S96.83°E16.1 ± 3.8−4.9 ± 4.2
BAKO (Java)6.49°S106.84°E1.8 ± 4.0−4.2 ± 4.6
XMIS (Indian Ocean)10.45°S105.68°E5.1 ± 3.8−4.36 ± 4.1
NTUS (Singapore)1.346°N103.67°E4.6 ± 3.919.4 ± 4.3
CUSV (Thailand)13.74°N100.53°E5.0 ± 4.03.4 ± 4.4
DGAR (Indian Ocean)7.26°S72.37°E−5.3 ± 4.1−10.3 ± 4.7

[5] In the Sumatra region, the GPS sites of the Sumatra GPS Array also documented the coseismic offsets due to this earthquake (Figure S2 in the supporting information). The sites in the southeast Sumatra region showed coseismic zoffsets in the northwest direction, whereas those in the northwest Sumatra region showed coseismic offsets in the northeast direction. The largest offset is about 301 mm at site LEWK located about 310 km from the Mw 8.6 earthquake epicenter.

[6] Because of the limited GPS sites around the earthquake epicenter and due to the complex nature of rupture and slip, we did not attempt to estimate the earthquake slip model from these offsets. We used finite-fault slip models derived from the back projection of seismic waves [Yue et al., 2012]. We used a layered spherical earth model [Pollitz, 1997] to simulate the horizontal displacement due to the earthquake source model. Yue et al. [2012] reported a combined source model for the two earthquakes which consists of variable slip on six planes. The total seismic moment of their source model is 9.94 × 1028 dyn cm, which is equivalent to Mw of 8.63. We calculated the seismic moment for each slip plane and then estimated the equivalent uniform slip on each plane. Our equivalent source model is given in Table 2. The simulated offsets are quite consistent with that estimated from the GPS measurements (Figures 1 and S2). At sites in the Andaman Islands, the azimuth of the simulated coseismic displacement is in N170°, whereas the azimuth of the offsets derived from the GPS measurements is predominantly in the south direction (N179°). However, the offset at CBAY on Great Nicobar Island, the nearest site to the main shock, is remarkably consistent with that estimated from GPS observations. Sites in the Andaman-Nicobar region and the IGS site at NTUS are located in the cusp region of the simulated coseismic displacement where their direction changes quite rapidly (Figure 1). A slight change in the source location, orientation of rupture planes, direction of slip (rake) on the rupture planes, and distribution of slip on the source may change the pattern of the simulated coseismic displacements in the cusp region quite significantly, leading to better fit in the data. However, we do not attempt it here due to the complex source model for this earthquake and the limited number of observations of coseismic offsets from GPS measurements.

Table 2. Source Model for the Earthquake Sequence [Yue et al., 2012]
Slip PlaneStrike (deg)Dip (deg)Rake (deg)Depth of Top Edge (km)Depth of Bottom Edge (km)Equivalent Uniform Slip (m)Moment (dyn cm) × 1027
a10685−1755.0954.9022.641.8
b286751705.8554.158.215.4
c01680−105.3854.622.55.3
d01680−105.3854.621.84.11
e109801805.3854.628.025.0
f111741805.9754.033.37.83

4 Concluding Discussion

[7] The slip model for the 2012 earthquakes has been revised (T. Lay, personal communication, 2013). The slip is larger in this model, and it yields a total seismic moment of 13.6 × 1028 dyn cm, equivalent to Mw of 8.72. We tested this model and found that the simulated displacements are larger than that estimated from GPS measurements in the Andaman-Nicobar region (Figure S1) as well as in the Sumatra region (Figure S2). Thus, the original model suits better as far as the coseismic offsets derived from GPS measurements are concerned.

[8] Slip models of the 2012 earthquakes suggest that majority of the slip occurred on the approximately east-west oriented fault [Yue et al., 2012]. Although such fracture planes have been reported from the swath bathymetry and magnetic anomalies data, the north-south-oriented planes are the most abundant and unambiguous (Figure 2). These north-south-oriented planes extend right up to the trench and appear to locally influence the tectonic processes of the Sumatra subduction zone. Near the trench, they are characterized by earthquakes with strike-slip faulting (marked by yellow ellipses in Figure 2), a feature similar to the 2012 earthquake sequence. Prior to the current earthquake sequence, a large-magnitude earthquake (Mw 7) with a similar focal mechanism occurred off Nicobar Island [Rajendran et al., 2011] which appears to be located on the extension of one such north-south-oriented plane. Therefore, earthquake focal mechanisms with strike-slip-type motion in the frontal arc are an indirect evidence for the northward extension of these fracture zones (Figure 2). These north-south-oriented features also appear to have influenced the 2004 Sumatra-Andaman earthquake rupture. The most prominent such feature, the subducting 90°E ridge, probably slowed down the 2004 earthquake rupture in the Andaman Islands [Gahalaut et al., 2010]. The other north-south faults, east of the 90°E ridge, if extended into the frontal arc, appear to coincide with the low-slip regions of the 2004 Sumatra-Andaman earthquake rupture. One of them coincides with the change in rupture velocity during the 2004 earthquake rupture in a manner similar to that observed during the 23 June 2001 Peru earthquake [Robinson et al., 2006] and 1 April 2007 Solomon Islands earthquake [Furlong et al., 2009].

[9] Prior to the 2012 Indian Ocean earthquake, two similar but with lesser magnitude earthquakes occurred in the Wharton Basin. The 18 June 2000 earthquake (Mw 7.8) occurred more than 1800 km southeast-south of the 2012 earthquake. This earthquake too involved slip on conjugate faults [Robinson et al., 2001]. Majority of the slip appears to have occurred on the approximately east-west trending fault which displaced the nearby COCO IGS site by about 24 ± 4 mm toward the east. The 10 January 2012 earthquake (Mw 7.2) occurred within the source region of the recent Indian Ocean earthquake. This earthquake too involved slip on conjugate planes, as evident from the aftershocks of this earthquake. However, the majority of the slip again appears to have occurred on the approximately east-west oriented plane (G. Hayes, Finite fault model, preliminary result of the Jan 10, 2012 Mw 7.2 off the west coast of northern Sumatra, Indonesia Earthquake, 2012, http://earthquake.usgs.gov/earthquakes/eqinthenews/2012/usc0007ir5/finite_fault.php).

[10] The 11 April 2012 Indian Ocean earthquakes occurred in the region which demarcates the diffused plate boundary between the Indian and Australian plates [Gordon et al., 1998]. The NNW-SSE compression resulting from the relative motion between the two plates is consistent with the earthquake focal mechanisms. The slab pull force along the Andaman-Sumatra subduction zone (Figure 2) changes significantly from the Andaman-Nicobar segment (~21 × 1012 N m−1) to the Sumatra segment (~25 × 1012 N m−1) [Lallemand et al., 2005]. The differential slab pull force in the region, along with the ambient NNW-SSE compression, facilitated the occurrence of this earthquake sequence [Yue et al., 2012; Delescluse et al., 2012]. It has been proposed that the great earthquakes in the Andaman-Sumatra subduction zone increased the stress in the region of this intraplate earthquake sequence [Delescluse et al., 2012; Wiseman and Bürgmann, 2012]. However, this is a unique event which gave us a glimpse of the process that operates over geological timescales in which the Indian and Australian plates will be detached.

Acknowledgments

[11] We dedicate this work to P. Mahesh, a bright student within our group, who passed away on 5 February 2013. He participated in the Andaman field work and was actually installing the Campbell Bay permanent site when the 11 April 2012 earthquake occurred. Our work on the Andaman GPS is financially supported by the Ministry of Earth Sciences, India. Amit Bansal, Sapna Ghavari, Rajeshwar Rao, L. Prem Kishore, and M. Narsaiah helped in the field work in the Andaman-Nicobar region and data management. We thank J. Curray and Stephen Miller for providing information regarding swath bathymetry images and Thorne Lay and Han Yue for the discussion and providing the revised source model for the earthquake. Fred Pollitz helped in running the STATIC-1D program. We thank an anonymous reviewer and Roland Burgmann for very constructive reviews. GPS data and processed GPS time series from SuGAR sites were provided by Sylvain Barbot of EOS, Singapore, and we appreciate the efforts of the scientists and technical staff of EOS and LIPI, involved in running SuGAR. B.K. and R.K.Y. were supported by the CSIR fellowship. This is a CSIR-NGRI-GENIAS contribution.

[12] The Editor thanks Roland Burgmann and an anonymous reviewer for their assistance in evaluating this paper.

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