Sumatra Andaman Research Articles

Trade-off between seismic source detail and crustal heterogeneities in spherical 3D finite element modeling: a 2004 Sumatra earthquake case study

Finite-element methods are a powerful numerical simulation tool for modeling seismic events, as they allow three-dimensional complex models to be solved. We used a three-dimensional finite-element approach to evaluate the co-seismic displacement field produced by the devastating 2004 Sumatra–Andaman earthquake, which caused permanent deformations that were recorded by continuously operating GPS networks in a region of unprecedented area. Previous analysis of the static displacement fields have focused on the heterogeneous distribution of moment release on the fault plane; our intention here is to investigate how much the presence of crustal heterogeneities trades-off seismic source details. To achieve this aim, we adopted a quite simple source model in modeling the event. The key feature of our analysis is the generation of a complex three-dimensional spherical domain. Moreover, we also carried out an accurate analysis concerning the boundary conditions, which are crucial for finite-element simulations.

Transtension to transpression: A study of strain evolution in Andaman Islands based on GPS measurements following Great Sumatra–Andaman Earthquake, 2004

We use campaign mode and continuous GPS data from 2005 to 2008 to examine the evolving strain pattern in Andaman Islands after the Great Sumatra–Andaman earthquake. Two and half years after the earthquake, the post-seismic period changes to inter-seismic period. At Port Blair, post-seismic displacement rate reduced to about 95% with clockwise rotation of azimuths and towards North Andaman, velocity vector gradually rotates towards north with drop in magnitude. The inter-seismic displacement was dominated by trench parallel movement. Strain calculations on inclined transpession model shows deformations at Andaman were highly heterogeneous both in post and inter-seismic periods with dominant strike-slip and dip-slip shearing along with layer parallel shortening and extension. Post-seismic and inter-seismic periods had suffered simultaneous subduction perpendicular extension and contraction leading to transtension and transpression deformational environments respectively. Occurrence of earthquakes during post and inter-seismic periods were results of interplay between simple shear and pure shear components of the strain matrix.

Ocean contribution to co-seismic crustal deformation and geoid anomalies: Application to the 2004 December 26 Sumatra–Andaman earthquake

Large earthquakes do not only heavily deform the crust in the vicinity of the fault, they also change the gravity field of the area affected by the earthquake due to mass redistribution in the upper layers of the Earth. Besides that, for sub-oceanic earthquakes deformation of the ocean floor causes relative sea level changes and mass redistribution of water that have again a significant effect on the gravity field. To model these deformations, sea level changes and gravity field perturbations self-consistently we use an adapted version of the sea level equation (SLE) that has been used for glacial isostatic adjustment studies. The sea level equation, next to our normal mode model for seismic solid earth modeling, allows us to compute a gravitationally self-consistent solution for the co-seismic relative sea level, surface deformation and geoid height changes. We apply our geographically detailed models to the case of the 2004 December 26 Sumatra–Andaman earthquake. Recent studies that have modeled the ocean mass effect on co-seismic gravity change for this specific earthquake show model results that indicate a broad negative change in geoid height around the fault due to ocean water redistribution (de Linage et al., 2009; Melini et al., 2010). Our model results for the ocean contribution to geoid height differ from these studies in the sense that we find a pattern similar to the elongated dipole pattern of the solid earth model outputs for gravity and vertical deformation, together with a relatively small broad negative geoid height change. We explain the relation between outcomes for geoid height, relative sea level and vertical deformation of the ocean floor and we confront our model results with a least squares estimation of the co-seismic discontinuity in GRACE-derived gravity field time series. We show that taking into account the contribution of ocean water redistribution to the co-seismic geoid height change next to a compressible solid earth model is essential to explain the predominant negative co-seismic geoid anomalies from the GRACE gravity field solutions. Besides, we introduce a detailed approach to modeling an earthquake in a normal mode model that better approximates realistic continuous slip on the fault plane than models that do not distribute slip with depth. To demonstrate the importance of the slip distribution we show the differences in outcomes for modeled geoid height and vertical deformation.

The Mw 7.5 2009 Coco earthquake, north Andaman region

The recent 10 August 2009 Coco earthquake (Mw 7.5), the largest aftershock of the giant 2004 Sumatra Andaman earthquake, occurred within the subducting India plate under the Burma plate. The Coco earthquake nucleated near the northwestern edge of the 2004 Sumatra-Andaman earthquake rupture under the unruptured updip segment of the plate boundary interface. The earthquake with predominant normal motion on approximately north-south to northeast-southwest oriented plane is very similar to the 27 June 2008 Little Andaman earthquake which occurred in the South Andaman region near the trench. We provide the only available estimate of coseismic offset due to the 2009 Coco earthquake at a survey-mode GPS site in the north Andaman, located about 60 km south of the Coco earthquake epicentre. The not so large coseismic displacement of about 2 cm in the ESE direction is consistent with the earthquake focal mechanism and its magnitude. We suggest that, like the 2008 Little Andaman earthquake, this earthquake too occurred on one of the approximately north-south to northeast-southwest oriented steep planes of the obliquely subducting 90°E ridge which was reactivated in normal motion after subduction, under the favourable influence of coseismic and ongoing postseismic deformation due to the 2004 Sumatra-Andaman earthquake. Another notable feature of this earthquake is its relatively low aftershock productivity. We suggest that the earthquake occurred very close to the aseismic region of the Irrawaddy frontal arc of very low seismicity where pre-existing faults are not so critically stressed and because of which the earthquake could trigger only a few aftershocks in its immediate vicinity.

Searching for Records of Past Earthquakes Under Water

European Science Foundation Research Conference: Submarine Paleoseismology—The Offshore Search of Large Holocene Earthquakes; Obergurgl, Austria, 11–16 September 2010; The 2004 Sumatra–Andaman Islands earthquake and tsunami highlighted the urgent need to better understand the long‐term history of underwater earthquakes and tsunamis and to better define hazards for the significant populations in coastal areas. As a result, a European Science Foundation Research Conference was convened to review research in the rapidly expanding field of marine and lake paleoseismology. The 4‐day‐long meeting brought together scientists engaged in interdisciplinary research from paleoseismology and marine science. Sixty‐six participants from 21 countries attended the conference, which took place at the Innsbruck University Centre, in Austria. The conference consisted of three main parts: (1) new techniques and results for on‐fault and off‐fault investigations, which summarized the most advanced marine methods for mapping, imaging, and dating underwater paleoseismic events; (2) case studies from different active tectonic continental margins, such as subduction zones, and from lakes or coastal environments that monitor tectonic systems; and (3) the contributions of marine and lake paleoseismology to seismic hazard assessment.

Stress and Seismicity Changes on the Sunda Megathrust Preceding the 2007 Mw 8.4 Earthquake

Abstract The Bengkulu M w 8.4 earthquake on 12 September 2007, close in time and space to the 2004 M w 9.2 Sumatra–Andaman and 2005 M w 8.7 Nias megathrust events, suggests that it could be a triggered earthquake. It was located in the southern portion of the historic 1833 M w 8.9 rupture. However, it appears perplexing that the portion of the Sunda subduction zone between the Nias and Bengkulu rupture patches, which last ruptured in a 1797 M w 8.7 event, did not recur first. Coulomb failure stress (CFF) modeling of the 2004 and 2005 megathrust earthquakes and subsequent postseismic relaxation processes fails to explain why the 2007 patch ruptured before the northern 1797 segment. Surprisingly, the much smaller 2000 M w 8.0 Enggano earthquake produced a much larger positive CFF change at the 2007 hypocenter and may help to explain the southern location of the 2007 earthquake. Investigation of changes in seismicity rates in the region following the 2004–2005 events shows that the megathrust earthquakes may have dynamically triggered slip near the northern end of the 2007 rupture zone. A large increase in seismicity levels following the 2000 earthquake may also have influenced the eventual initiation point of the 2007 earthquake. Regrettably, the section of the Sunda megathrust near Siberut that last ruptured in 1797 still poses a great seismic hazard to the region as the only segment not to have ruptured in the sequence of twenty-first century megathrust earthquakes.

Role of crustal heterogeneity beneath Andaman–Nicobar Islands and its implications for coastal hazard

The Andaman–Nicobar (A–N) Islands region has attracted many geo-scientists because of its unique location and complex geotectonic settings. The recent occurrence of tsunamis due to the megathrust tsunamigenic north Sumatra earthquake (Mw 9.3) with a series of aftershocks in the A–N region caused severe damage to the coastal regions of India and Indonesia. Several pieces of evidence suggest that the occurrence of earthquakes in the A–N region is related to its complex geodynamical processes. In this study, it has been inferred that deep-seated structural heterogeneities related to dehydration of the subducting Indian plate beneath the Island could have induced the process of brittle failure through crustal weakening to contribute immensely to the coastal hazard in the region. The present study based on 3-D P-wave tomography of the entire rupture zone of the A–N region using the aftershocks of the 2004 Sumatra–Andaman earthquake (Mw 9.3) clearly demonstrates the role of crustal heterogeneity in seismogenesis and in causing the strong shakings and tsunamis. The nature and extent of the imaged crustal heterogeneity beneath the A–N region may have facilitated the degree of damage and extent of coastal hazards in the region. The 3-D velocity heterogeneities reflect asperities that manifest what type of seismogenic layers exist beneath the region to dictate the size of earthquakes and thereby they help to assess the extent of earthquake vulnerability in the coastal regions. The inference of this study may be used as one of the potential inputs for assessment of seismic vulnerability to the region, which may be considered for evolving earthquake hazard mitigation model for the coastal areas of the Andaman–Nicobar Islands region.

Estimation of the rupture velocity and fault length of the 2004 Sumatra–Andaman earthquake using a dense broadband seismic array in Taiwan

The rupture process of the disastrous Sumatra–Andaman earthquake of 26 December 2004 was analyzed by array processes for teleseismic P-waves recorded by a dense broadband seismic array in Taiwan with epicentral distances of close to 31°. The azimuthal variation from the BATS array center to both ends of the rupture fault is approximately 21°, which is larger than that reported previously for seismic arrays used to image the rupture process of this earthquake, thereby providing a high spatial resolution in studying the source rupture behavior. Two array-processing methods were used to analyze teleseismic P-wave trains. Both analyses were based on data recorded by a broadband network, covering a region of 200 × 400 km, with the aim of evaluating the rupture behavior of the earthquake. Consistent results from both analyses indicate that the earthquake had a rupture duration exceeding 500 s, with major asperities encountered at 80, 260, and 330 s after the initiation of rupturing. We traced the ruptured fault for more than 1200 km from the point of initial rupture. The average rupture velocity was approximately 3.0 km/s and the major northward rupture propagation began at 80 s after the initiation of rupturing.

Aftershock sequences of two great Sumatran earthquakes of 2004 and 2005 and simulation of the minor tsunami generated on September 12, 2007 in the Indian Ocean and its effect

We present the seismic energy, strain energy, frequency–magnitude relation (b-value) and decay rate of aftershocks (p-value) for the aftershock sequences of the Andaman–Sumatra earthquakes of December 26, 2004 (M w 9.3) and March 28, 2005 (M w 8.7). The energy released in aftershocks of 2004 and 2005 earthquake was 0.135 and 0.365% of the energy of the respective mainshocks, while the strain release in aftershocks was 39 and 71% for the two earthquakes, respectively. The b-value and p-value indicate normal value of about 1. All these parameters are in normal range and indicate normal stress patterns and mechanical properties of the medium. Only the strain release in aftershocks was considerable. The fourth largest earthquake in this region since 2004 occurred in September 2007 off the southern coast of Island of Sumatra, generating a relatively minor tsunami as indicated by sea level gauges. The maximum wave amplitude as registered by the Padang, tide gauge, north of the earthquake epicenter was about 60 cm. TUNAMI-N2 model was used to investigate ability of the model to capture the minor tsunami and its effect on the eastern Indian Coast. A close comparison of the observed and simulated tsunami generation, propagation and wave height at tide gauge locations showed that the model was able to capture the minor tsunami phases. The directivity map shows that the maximum tsunami energy was in the southwest direction from the strike of the fault. Since the path of the tsunami for Indian coastlines is oblique, there were no impacts along the Indian coastlines except near the coast of epicentral region.

U–Pb zircon age of the Andaman ophiolite: implications for the beginning of subduction beneath the Andaman–Sumatra arc

Abstract: The Andaman ophiolites form the basement of the Andaman Islands, which is a part of the outer forearc that links the Indo-Burma accretionary complex to the north with the Java–Sumatra trench–arc system to the SE. Upper mantle harzburgite and dunite are overlain by a cumulate peridotite–gabbro complex, high-level intrusive rocks and a tholeiitic volcanic series. The upper crust in the South Andaman ophiolite shows also a prominent andesite–dacite volcanic suite, suggesting arc volcanism built onto ocean crust. U–Pb zircon dating of a trondhjemitic rock from Chiriya Tapu in South Andaman Island using laser ablation inductively coupled mass spectrometry reveals an age of crustal formation of 95 ± 2 Ma. The trondhjemites have geochemistry comparable with that of plagiogranites associated with ophiolite complexes, and ε Nd values around +7 further confirm that they are derived from depleted mantle melts. Basaltic pillow lava and basaltic dykes that cut the trondhjemites have mid-ocean ridge basalt-like trace-element geochemistry. The new data show that the Andaman volcanic arc was built on Cenomanian ophiolite–oceanic crust and that subduction was initiated at this time along Tethys, at least from Cyprus through Oman to the Andaman Islands.

Wavelet analysis of the seismograms for tsunami warning

Abstract. The complexity in the tsunami phenomenon makes the available warning systems not much effective in the practical situations. The problem arises due to the time lapsed in the data transfer, processing and modeling. The modeling and simulation needs the input fault geometry and mechanism of the earthquake. The estimation of these parameters and other aprior information increases the utilized time for making any warning. Here, the wavelet analysis is used to identify the tsunamigenesis of an earthquake. The frequency content of the seismogram in time scale domain is examined using wavelet transform. The energy content in high frequencies is calculated and gives a threshold for tsunami warnings. Only first few minutes of the seismograms of the earthquake events are used for quick estimation. The results for the earthquake events of Andaman Sumatra region and other historic events are promising.

Sedimentological and geomorphological effects of the Sumatra–Andaman Tsunami in the area of Khao Lak, southern Thailand

The Sumatra–Andaman Tsunami left distinctive sedimentological and geomorphological signatures in the area of Khao Lak. Fine-grained sediments, predominantly layers of cohesive, carbonate-rich, fine-sandy silt with thicknesses of 1–10 cm, erosionally overlying pre-tsunami sandy soils and sediments, represent the most common tsunami deposits in the study area. Petrographically, they differ significantly from other coastal sediments and affiliated soils. Due to their grain size and corresponding clay mineral content, muddy shelf sediments (sub-wave base) are indicated as a main source. The present results suggest that indications of shelf influence, although varying regionally, might contribute to the identification of fine-grained tsunami sediments and their differentiation from storm sediments. However, the observed differences of tsunami sediments to soils and other coastal sediments, especially with respect to carbonate mineralogy, might disappear in short geological time under conditions of intensive weathering and bioturbation. At Cape Pakarang, hundreds of boulders with up to 24 tons were deposited on the foreshore and upper shoreface. Applying Nott’s (Earth Planet Sci Lett 210:269–276, 2003) formulas, minimum flow velocities of 3.9 m/s are required to transport the largest boulders. The devastating tsunami effect of both, onshore flow and backflow, is documented by damaged human constructions. Geomorphological effects include intensive widening of estuary mouths and the development of erosional channels. Now, estuary mouths are reduced, and erosional channels cut off from the sea due to the formation of a post-tsunami beach ridge.

Temporal and spatial precursors in the ionospheric global positioning system (GPS) total electron content observed before the 26 December 2004 M9.3 Sumatra–Andaman Earthquake

We report seismo‐ionospheric precursors of anomalous decreases in the total electron content (TEC) appearing day 5 prior to an M9.3 earthquake, the largest one in the last five decades, which occurred in Sumatra‐Andaman, Indonesia on 26 December 2004. Sequences of global ionosphere maps of the TEC derived from worldwide ground‐based receivers of the global positioning system (GPS) are used to statistically study the temporal and spatial precursors of the earthquake. It was found that the temporal precursor of the GPS TEC around the epicenter was significantly reduced during the afternoon period on d 5 before the earthquake. The spatial precursors prominently, persistently, and simultaneously appear around the epicenter and its conjugate areas of the Sumatra‐Andaman earthquake.

Sharpening the tomographic image of the subducting slab below Sumatra, the Andaman Islands and Burma

SUMMARY Taking advantage of the increased ray coverage due to seismicity following the 2004 December and 2005 March great earthquakes, an improved iterative regional–global tomographic method was applied to the Sumatra–Andaman and adjacent regions to better constrain the 3-D mantle velocity heterogeneity in the region. Velocity and hypocentral parameters were iteratively perturbed to sharpen the image of the subducted slab. Several iterations were performed, and the effects of source mislocation were considered in the iterative process, an issue usually neglected in global tomography. We find that source relocation between iterations increases the amplitudes of slab anomalies and sharpens the definition of slab geometry beyond what can be achieved by a fixed-source iterative inversion alone. In addition, extensive restoration tests of synthetic data were conducted that emphasize enhancements obtained by our iterative process. These tests show significant increases in amplitude and decreased smearing of synthetic slab features. Thus, when applied to the real data, similar enhancements are inferred in the resulting model, which better illustrates the complex slab geometry in the upper-mantle and transition zone regions along the Sumatra, Andaman and Burma subduction zones.

Numerical modelling of tsunami propagation with implications for sedimentation in ancient epicontinental seas: The Lower Jurassic Laurasian Seaway

Tsunamis are frequent events in modern marine environments but evidence of their passing in subtidal ancient epicontinental sea deposits is elusive. It has been suggested that this is due to mis-identification or poor preservation potential. Herein a numerical modelling approach is used to show that tsunami propagation in one large ancient epicontinental sea was hindered by the damping effect of shallow bathymetries and reflection, refraction and diffraction from emergent landmasses. The Imperial College Ocean Model (ICOM) is used for this study and is first validated against data from the Sumatra–Andaman Tsunami of December 2004. A palaeobathymetric dataset is then presented for the Hettangian (Lower Jurassic) Laurasian Seaway with idealised tsunami sources situated on the continental shelf and within the adjacent oceanic basin. Results show that tsunamis forced from within ocean basins adjacent to the epicontinental sea are rapidly attenuated over the continental slope and fail to propagate great distances onto the shelf. Similarly, the sedimentological effect of tsunamis forced from within the epicontinental sea is also restricted. It is concluded that tsunami deposits in ancient epicontinental seas are most likely to occur in relative proximity to the source region and this must contribute to their scarcity in the geological record.

Poroelastic stress-triggering of the 2005 M8.7 Nias earthquake by the 2004 M9.2 Sumatra–Andaman earthquake

The M9.2 Sumatra–Andaman earthquake (SAE) occurred three months prior to the M8.7 Nias earthquake (NE). We propose that the NE was mechanically triggered by the SAE, and that poroelastic effects were a major component of this triggering. This study uses 3D finite element models (FEMs) of the Sumatra–Andaman subduction zone (SASZ) to predict the deformation, stress, and pore pressure fields of the SAE. The coseismic slip distribution for the SAE is calibrated to near-field GPS data using FEM-generated Green's Functions and linear inverse methods. The calibrated FEM is then used to predict the postseismic poroelastic contribution to stress-triggering along the rupture surface of the NE, which is adjacent to the southern margin of the SAE. The coseismic deformation of the SAE, combined with the rheologic configuration of the SASZ produces two transient fluid flow regimes having separate time constants. SAE coseismic pore pressures in the relatively shallow forearc and volcanic arc regions (within a few km depth) dissipate within one month after the SAE. However, pore pressures in the oceanic crust of the down-going slab persist several months after the SAE. Predictions suggest that the SAE initially induced MPa-scale negative pore pressure near the hypocenter of the NE. This pore pressure slowly recovered (increased) during the three-month interval separating the SAE and NE due to lateral migration of pore fluids, driven by coseismic pressure gradients, within the subducting oceanic crust. Because pore pressure is a fundamental component of Coulomb stress, the MPa-scale increase in pore pressure significantly decreased stability of the NE fault during the three-month interval after the SAE and prior to rupture of the NE. A complete analysis of stress-triggering due to the SAE must include a poroelastic component. Failure to include poroelastic mechanics will lead to an incomplete model that cannot account for the time interval between the SAE and NE. Our transient poroelastic model explains both the spatial and temporal characteristics of triggering of the NE by the SAE.

Normal mode detection and splitting after Sumatra–Andaman earthquake

Abstract Apart from life loss and damages which earthquakes cause, their signature in the collected data either seismic and/or gravity enables us to define their location and dislocation (i.e., geometric parameters), time of occurrence, and magnitude. In addition, the Earth’s interior physical parameters, such as density profile and anelasticity can be defined. Gravity data contribute additional knowledge about the Earth’s interior through careful analyses of superconducting gravimeter (SG) records particularly after strong earthquakes. In this paper, SG data recorded after the Sumatra–Andaman earthquake in December 26, 2004 at eight SG stations, are used to investigate the properties of the long-period seismic modes: their frequencies, amplitudes, and quality factors. These parameters are estimated very precisely in this study for the spheroidal modes S 0 0 , S 0 2 and S 0 3 . In addition, for the first time we observe the toroidal modes T 1 4 and T 1 5 .

Slow rupture in Andaman during 2004 Sumatra-Andaman earthquake: a probable consequence of subduction of 90°E ridge

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.

A sea level equation for seismic perturbations

SUMMARY Large earthquakes are a potentially important source of relative sea level variations, since they can drive global deformation and simultaneously perturb the gravity field of the Earth. For the first time, we formalize a gravitationally self-consistent, integral sea level equation suitable for earthquakes, in which we account both for direct effects by the seismic dislocation and for the feedback from water loading associated with sea level changes. Our approach builds upon the well-established theory first proposed in the realm of glacio-isostatic adjustment modelling. The seismic sea level equation is numerically implemented to model sea level signals following the 2004 Sumatra–Andaman earthquake, showing that surface loading from ocean water redistribution (so far ignored in post-seismic deformation modelling) may account for a significant fraction of the total computed post-seismic sea level variation.

Is the Northern Bay of Bengal Tsunamigenic?

The giant tsunami generated by the great 26 December 2004 Sumatra–Andaman earthquake and the resultant destruction have necessitated appropriate assessment of tsunami potential in the northern Bay of Bengal, where high population density in the coastal area (∼100 million) makes this region very vulnerable if a large tsunami were to occur. Here we examine whether the India–Burma plate boundary in the Arakan and Irrawady region can produce a tsunamigenic earthquake. We find: (a) the region is characterized by oblique plate motion leading to strike-slip dominated earthquakes with low tsunami generating potential; (b) the deformation front associated with the plate boundary between India and Sunda plates in the northern Bay of Bengal is either landward or in shallow water in the Arakan region and, hence, a great earthquake is unlikely to displace large amounts of water to create a significant tsunami; (c) convincing evidence that the 1762 Arakan earthquake generated a large tsunami is lacking; and (d) no large tsunami has affected the region in the past 2000 years. We conclude that while a great earthquake could occur in the Arakan region, the physiographic situation may not lead to generation of a large tsunami.