Sumatra-Andaman Subduction Zone Research Articles

Co-seismic ionospheric disturbances due to 2004 Sumatra-Andaman earthquake

The Coseismic Ionospheric Disturbances (CID) due to the 26th December 2004 earthquake of Mw 9.2, which occurred in the Sumatra-Andaman subduction zone, are analyzed using cGPS-aided Total Electron Content (TEC) measurements. For the CID analysis, data from nearby seven Sumatran GPS Array (SuGAr) and two International GNSS Stations (IGS) located to the south of the epicenter, at a distance of 500–1000km (near-field) and two IGS stations located to the north-west of the epicenter at a distance of 2000km (far-field) are considered. The CIDs with a propagation velocity of 595–694m/s arrived within 2–10min after the earthquake, depending upon the distance of a station from the epicentre. Variations in the CIDs can be prominently seen at the nearest cGPS Station SAMP immediately after the earthquake. NTUS, being the farthest station shows some small variations. The delay in the occurrence of variations at GPS sites can also be associated with rupture propagation. Because all the stations used in our analysis are located south of the epicenter and rupture of the earthquake propagated in the north, the trend of rupture propagation could not be analyzed clearly.

Coda wave attenuation in Northern Aceh, Indonesia from a single station measurement: preliminary results

Sumatra-Andaman subduction zone is one of the most active tectonic areas in the world. Understanding seismic risk in this region is important for hazard mitigation. One of the crustal physical properties is attenuation. Attenuation structure may affect the seismic propagation through the medium. It expresses the seismic wave amplitude decay when wave propagates through a medium. In this preliminary result, we report the seismic attenuation characteristics in Northern Aceh, Indonesia. We estimate coda waves attenuation from a single station from GE network. using a single back-scattering model. We analyze about 123 waveforms from local earthquakes (less than 3°) with magnitudes between 3 and 6 using a single back-scattering model. The depth of the events is selected to be shallower than 40 km depth. We estimate coda quality factor (Qc ) for each event at frequencies 1 – 8 Hz. The coda window is estimated from two times of S-wave arrival time with different window length ranging from 20 to 80 s. Reference coda quality factor (Q0 ) is selected at frequency of 1 Hz. We obtain that Q0 varies from 74.34 ± 10.04 to 160.2 ± 5.96 at 20 s and 80 s window lengths, respectively. The frequency dependence factor (η) decreases from 1.12 ± 0.06 to 1.05 ± 0.03 at 20 s and 80 s window length, respectively. An increase of Q0 with window length may suggest the depth dependent of seismic wave attenuation at reference frequency of 1 Hz. Low Q0 and high η are also consistent with this region’s high tectonic activity.

Strike-slip seismicity at the Andaman-Sumatra Subduction Zone: Role of the fracture zones and age of the subducting lithosphere

A better comprehension of the shallow strike-slip seismicity along the Sunda Trench is achieved by superimposing the Wharton basin fracture zones, magnetic isochrones, and focal mechanisms to the geometry of the subducting plate. The shallow strike-slip seismicity is generally associated with the north-south trending fracture zones of the basins, which segment the subducting lithosphere into eight main compartments named A-H. Strike-slip earthquakes with magnitudes higher than 4.5 Mw in the time period of 2004–2019 are examined with respect to the thickness and density of the subducting lithosphere, estimated from the lithospheric age. Bulk density ranges from ~3258 to ~3272 and lithospheric thickness from ~74 to ~112 km between the younger fossil spreading axis and the older Compartment H, respectively. ‘Subductability’, a parameter calculated from lithospheric thickness and bulk density, is coined to represent the easiness of subduction and varies for each spreading compartment depending on the lithospheric age. We propose the correlation between the shallow strike-slip seismicity and the contrasts of lithospheric thickness, density and subductability. Considering the available seismic records from the past four decades, a prominent cluster of strike-slip earthquakes is observed along Fracture Zone B-C, which displays a strong subductability contrast, whereas other fracture zones displaying only minor seismicity correspond to low subductability contrasts. Despite a high subductability contrast, slab tearing on the subducting Investigator Fracture Zone between Compartments G and H prevents the generation of strike-slip earthquakes. This result emphasizes the importance of physical and mechanical properties of the subducting lithosphere in localizing the seismicity at subduction zones.

Megathrust Slip Behavior for Great Earthquakes Along the Sumatra-Andaman Subduction Zone Mapped From Satellite GOCE Gravity Field Derivatives

During the last two decades, space geodesy allowed mapping accurately rupture areas, slip distribution, and seismic coupling by obtaining refined inversion models and greatly improving the study of great megathrust earthquakes. A better understanding of these phenomena involving large areas of hundreds of square kilometers came from the last gravity satellite mission that allowed detecting mass transfer through the Earth interior. In this work, we performed direct modeling of satellite GOCE (Gravity Field and Steady-State Ocean Circulation Explorer) derived gravity gradients up to degree/order N = 200 of the harmonic expansion and then corrected this by the effect of topography. Cutting off the model up to this degree/order allows inferring mass heterogeneities located at an approximate depth of 31 km, just along the plate interface where most (but not all) significant slip occurs. Then, we compared the vertical gravity gradient to well-constrained coseismic slip models for three of the last major earthquakes along the Sunda interface. We analyzed seismic rupture behavior for recent and for historical earthquakes along this subduction margin and the relationship of the degree of interseismic coupling using the gravity signal. From this, we found that strong slip patches occurred along minima gravity gradient lobes and that the maximum vertical displacements were related quantitatively to the gravity-derived signal. The degree of interseismic coupling also presents a good correspondence to the vertical gravity gradient, showing an inverse relationship, with low degrees of coupling over regions of relatively higher density. This along-strike segmentation of the gravity signal agrees with the along-strike seismic segmentation observed from recent and historical earthquakes. The thermally controlled down-dip ending of the locked fault zone along central Sumatra also presented an inverse relationship with the density structure along the forearc inferred using our modeling. From this work, we inferred different mass heterogeneities related to persistent tectonic features along the megathrust and along the marine forearc, which may control strain accumulation and release along the megathrust. Combining these data with geodetical and seismological data could possibly delimit and monitor areas with a higher potential seismic hazard around the world.

Barrier Islands Resilience to Extreme Events: Do Earthquake and Tsunami Play a Role?

Barrier islands are indicators of coastal resilience. Previous studies have proven that barrier islands are surprisingly resilient to extreme storm events. At present, little is known about barrier systems’ resilience to seismic events triggering tsunamis, co-seismic subsidence, and liquefaction. The objective of this study is, therefore, to investigate the morphological resilience of the barrier islands in responding to those secondary effects of seismic activity of the Sumatra–Andaman subduction zone and the Great Sumatran Fault system. Spatial analysis in Geographical Information Systems (GIS) was utilized to detect shoreline changes from the multi-source datasets of centennial time scale, including old topographic maps and satellite images from 1898 until 2017. Additionally, the earthquake and tsunami records and established conceptual models of storm effects to barrier systems, are corroborated to support possible forcing factors analysis. Two selected coastal sections possess different geomorphic settings are investigated: (1) Lambadeuk, the coast overlying the Sumatran Fault system, (2) Kuala Gigieng, located in between two segments of the Sumatran Fault System. Seven consecutive pairs of comparable old topographic maps and satellite images reveal remarkable morphological changes in the form of breaching, landward migrating, sinking, and complete disappearing in different periods of observation. While semi-protected embayed Lambadeuk is not resilient to repeated co-seismic land subsidence, the wave-dominated Kuala Gigieng coast is not resilient to the combination of tsunami and liquefaction events. The mega-tsunami triggered by the 2004 earthquake led to irreversible changes in the barrier islands on both coasts.

Preliminary tsunami hazard map for Africa

Major earthquakes display a significant tsunamigenic seismic activity (with Mw > 7) that may affect Africa continent. These events are from the far-field tectonics plates or even nearby tectonics boundaries. The main target of this study was to choose the most dangerous tsunamigenic zones according to the historical tsunami events and to update the tsunami hazard map of Africa published in UNISDR (2009). This was done by using nested bathymetry grids to reevaluate the maximum wave height using high-resolution (15 arcs per second) data near the African coastlines. Mirone version 2.10 software is used in these calculations. Four tsunamigenic zones sources affecting the African coastal zones have been tested at Andaman-Sumatra subduction zone, Makran trench zone, Western and Eastern Hellenic arcs. These tsunamigenic source zones were responsible for huge tsunamis generated from large historical earthquakes on 26 December 2004; 27 November 1945; 8 August 1303; and 21 July 365. Two to 4 m was the calculated maximum wave height resulted from the scenario 1 which arrived to the coasts of Tanzania, South Africa, and South Madagascar, while scenario 2 resulted in maximum wave height of 1–2 m toward the Somalian coast. The scenarios 3 and 4 were responsible for the maximum wave height of 2–4 m at the Egyptian and Libyan coasts. Preparing an early warning system will be required necessarily for the whole of Africa to overcome possible future high tsunami risk to the African coastal cities.

A note on stress rotations due to the 2004 Mw 9.2 Sumatra–Andaman megathrust earthquake

Here we study the evolution of stress and its rotations in the Sumatra–Andaman Subduction Zone (SASZ) after the 2004 Mw 9.2 megathrust earthquake, using moment tensor stress inversions. Models indicate returning of North Andaman stress regime to the inter-seismic oblique-compression, after a short stint of extension. Likewise, Little Andamans experience extension indicating a trench-ward guidance of extensional stresses. Similarly, Nicobar regime is presently normal-oblique, from early post-seismic reverse-oblique, and North Sumatra experiences pure-reverse regime. The influence of the Wharton Basin stress field and 2012 earthquakes on the SASZ fore-arc deformation is also probed. A near-complete co-seismic stress-relief is observed at Nicobar, followed by North Sumatra and Little Andaman. Co-seismic and post-seismic model comparisons indicate strong correlation with zones of co-seismic stress relief and regions of rapid post-seismic reloading. The North Andaman shows a northward compressive shear than the margin-normal component of subduction, as evident from prominent oblique stress regime with lower co-seismic stress relief and lack of post-seismic back rotations. At Andaman Spreading Ridge (ASR), transition from oblique to extensional stress indicates lateral shear to back-arc spreading enhancement.

Field surveys and numerical modelling of the 2004 December 26 Indian Ocean tsunami in the area of Mumbai, west coast of India

ABSTRACTIn the aftermath of the 2004 Indian Ocean (Sumatra-Andaman) tsunami, numerous survey teams investigated its effects on various locations across the Indian Ocean. However, these efforts were focused only on sites that experienced major destruction and a high death toll. As a consequence, some Indian Ocean coastal megacities were not examined. Among the cities not surveyed was Mumbai, the principal west coast port and economical capital of India with a population of more than 12 million. Mumbai is at risk of tsunamis from two major subduction zones in the Indian Ocean: the Sumatra–Andaman subduction zone (SASZ) and the Makran subduction zone (MSZ). As a part of the present study, we conducted a field survey of the 2004 Indian Ocean tsunami effects in Mumbai, analysed the available tide gauge records and performed tsunami simulations. Our field survey in 2018 January found run-up heights of 1.6−3.3 m in the Mumbai area. According to our analysis of tide gauge data, tsunami trough-to-crest heights in Okha (550 km to the north of Mumbai) and in Mormugao (410 km to the south of Mumbai) were 46 cm and 108 cm, respectively. Simulations of a hypothetical MSZ Mw 9.0 earthquake and tsunami, together with the Mw 9.1 Sumatra–Andaman earthquake and tsunami, show that the tsunami heights generated in Mumbai by an MSZ tsunami would be significantly larger than those generated by the 2004 Sumatra–Andaman tsunami. This result indicates that future tsunami hazard mitigation for Mumbai needs to be based on a potential large MSZ earthquake rather than an SASZ earthquake.

A 1000-yr-old tsunami in the Indian Ocean points to greater risk for East Africa

Abstract The December 2004 Sumatra-Andaman tsunami prompted an unprecedented research effort to find ancient precursors and quantify the recurrence time of such a deadly natural disaster. This effort, however, has focused primarily along the northern and eastern Indian Ocean coastlines, in proximal areas hardest hit by the tsunami. No studies have been made to quantify the recurrence of tsunamis along the coastlines of the western Indian Ocean, leading to an underestimation of the tsunami risk in East Africa. Here, we document a 1000-yr-old sand layer hosting archaeological remains of an ancient coastal Swahili settlement in Tanzania. The sedimentary facies, grain-size distribution, and faunal assemblages indicate a tsunami wave as the most likely cause for the deposition of this sand layer. The tsunami in Tanzania is coeval with analogous deposits discovered at eastern Indian Ocean coastal sites. Numerical simulations of tsunami wave propagation indicate a megathrust earthquake generated by a large rupture of the Sumatra-Andaman subduction zone as the likely tsunami source. Our findings provide evidence that teletsunamis represent a serious threat to coastal societies along the western Indian Ocean, with implications for future tsunami hazard and risk assessments in East Africa.

Seismic Cycles and Trend Predictions of Earthquakes in Sumatra–Andaman and Burmese Subduction Zones using Temporal b-value and Hurst Analysis

The annual b-value fluctuation patterns in Burmese subduction zone and Andaman–Sumatra subduction zone are evaluated from earthquake data (January 1990 to June 2016; Mw ³ 4.3) to identify seismic cycles with sequential dynamic phases as described in the ‘elastic failure model’ of Main et al. (1989). Two seismic cycles have been identified in Andaman–Sumatra subduction zone, one started in 1990 and ended on 2004 with occurrence of great Sumatra earthquake (Mw 9.0) and the other started in 2005 and continuing till date with the phase of crack coalescence and fluid diffusion (3A&B). Similarly, the subduction zone of Burma shows evidence of one incomplete seismic cycle within 1990–2016 and presently undergoing the crack coalescence and fluid diffusion (3A&B) phase. The analysis has prompted to subdivide the area into thirteen smaller seismic blocks (A to M) to analyse area specific seismic trend and vulnerability analysis employing Hurst Statistics. Hurst plots with the dynamic phases of ‘elastic failure model’ of earthquake generation is compared to assess the blocks with high seismic vulnerability. The analysis suggest that north Andaman zone (block G) and north Burma fold belt (block M) are seismically most vulnerable. Moreover, the seismic vulnerability of Tripura fold belt and Bangladesh plain (block K) is equally high.

Evaluating the state of stress and seismic hazard in Thailand and vicinity through finite element modeling

Thailand is surrounded by seismically active plate boundary zones and large-offset crustal faults, but there is comparatively little seismic activity within its borders. On 5 May 2014, a moment magnitude (MW) 6.2 earthquake occurred in the Mae Lao district of northern Thailand. To better anticipate future seismic hazards from events like this, we model the state of stress in Thailand and its vicinity using a finite element approach. We identify northern Thailand as a region with significant stress accumulation, consistent with being caused by convergence at the Sumatra-Andaman subduction zone and southward escape of the Himalayan Orogen. The stress field in this region is compatible with the kinematics of the Mae Lao event. Our modeling also shows that the magnitude of the stress field throughout southeast Asia is particularly sensitive to the stage of the earthquake cycle at the Sumatra-Andaman subduction zone, although the Sumatra-Andaman subduction zone produces a smaller effect on the orientations of stresses in the source region of the Mae Lao earthquake. Finally, we find that the stress change effect from the Mae Lao earthquake is geographically limited and the broader region remains in nearly the same state of stress as prior to the event.

Earthquakes as Expressions of Tectonic Activity

Earthquakes occur as the natural outcome of the processes that govern a dynamic planet, one that supports plate motions. The internal structure of the Earth that can sustain the convective -ow of the mantle and the presence of a strong, fractured outer shell, the lithosphere that can glide over the underlying asthenosphere are important components of the plate tectonic framework, and therefore, the global distribution of earthquakes. With an introduction to the ideas of plate tectonics and earthquake terminology, this article introduces the significance of the India-Eurasia plate boundary and its seismic productivity. The most active continent-continent collision boundary on the Earth defined by the Himalaya and the Andaman-Sumatra subduction zone are part of this plate boundary. Parts of the Indian continental interior regions are also noted for some destructive earthquakes. This article provides a brief overview of the background processes, terminology, and some highlights on Indian earthquakes studies, and points to academic opportunities in the area of earthquake studies.

Seismic Risk Analysis for Critical Infrastructure: The Case Study of a Medical Center and its Supporting Systems in Yangon, Myanmar

Myanmar has a great strike-slip active fault called the “Sagaing Fault Zone” besides the Sumatra-Andaman Subduction Zone. Major cities (Yangon, Naypyitaw, Bago and Mandalay) are at risk along this fault. Recently, in 2012, Thabeikkyin earthquake with Magnitude of 6.8 caused collapse of many residential housings and ground failures near Mandalay. Therefore more attention should be paid for Yangon which has no large earthquakes since 1930 and is the largest not only in population but also in socio-economic activity. One of the most important concerns after an earthquake is to survive under any disastrous conditions. The medical care is requested not only for emergent injured people after an earthquake, but also for various types of patient and aged people from several weeks to longer periods. So medical center must be always functional before and after earthquake. For this purpose, medical buildings should be structurally resilient and also be functional for medical services by sustainable supply of electric power, water and any other delivery service which can be carried out by urban lifeline systems. This research is to investigate the structural vulnerability of hospital buildings and facilities, to assess the performance of urban lifeline systems and to check the operational capability of medical services in which surgical capability and life safety management method should be discussed. The water supply system is adopted as a typical lifeline system in Yangon in this study. One sample medical center in Yangon is adopted to carry out this analysis. Finally, the performance of medical services after the earthquakes can be assessed in a probabilistic manner.

Source parameters of the Bay of Bengal earthquake of 21 May 2014 and related seismotectonics of 85°E and 90°E ridges

Source parameters of the Bay of Bengal earthquake of 21 May 2014 have been studied using full waveform inversion. Its source mechanism thus determined the orientation of the strike slip faulting as NW-SE/NE-SW. The occurrence of past earthquakes along the NE-SW nodal plane suggested its preference as the main fault which could result from the transmission of stresses from the Indian plate boundary. High stress drop of this earthquake (216 bar) is attributed to its location in the intraplate region, strike slip faulting and focus in the colder upper mantle. Comparison of the stress drop of deeper focus Hindukush earthquakes with that of the Bay of Bengal earthquake showed a smaller felt radius due to fractured lithosphere in the Himalayas vis-a-vis more efficient propagation of seismic waves in the peninsular region from the source region of this recent earthquake. The seismological evidence presented for the 85°E and 90°E ridges shows the predominance of strike slip faulting with thrusting on both the ridges. Integrating their source mechanism with that of the May 2014 earthquake, it could be inferred that the Bay of Bengal region (excluding Andaman Sumatra subduction zone) is characterised predominantly by strike slip faulting in the region north of latitude 20°N and strike slip with thrusting in the remaining portion.

The 2005 and 2010 Earthquakes on the Sumatra–Andaman Trench: Evidence for Post‐2004 Megathrust Intraplate Rejuvenation

Great earthquakes on oblique subduction zones accommodate slip on strike-slip faults on the overriding and subducting plates, leading to increased intraplate seismicity. After the 2004 Sumatra earthquake, there was an increase in activity in the Wharton basin (WB), which is considered to be part of a diffuse plate boundary. Here, the intraplate deformation occurs in response to northwest-southeast directed compression as opposed to the north-northeast-south-southwest directed plate motion along the Sumatra-Andaman subduction zone. Earthquakes here mostly occur by left-lateral strike slip on north-south (N-S) trending fossil fractures, although ruptures on east-west planes are not uncommon. Here, we discuss the 24 July 2005 M-w 7.2 and the 12 June 2010 M-w 7.5 earthquakes, both of which were left-lateral strike-slip events sourced in the northern WB. Our source models for these events, based on teleseismic waveform inversion, favor slip on the N-S fossil fracture systems, consistent with previous studies in the WB. Although the former sourced at similar to 16 km depth was not tsunamigenic, the latter (similar to 34 km) triggered a small tsunami, possibly due to its oblique slip at similar to 5 km depth. We use conceptual models of plate deformation to suggest that both these events have resulted from post-2004 redistribution of stresses and rejuvenation of intraslab fracture systems within the WB, an actively deforming oceanic intraplate region.

Probabilities of Earthquake Occurrences along the Sumatra-Andaman Subduction Zone

AbstractEarthquake activities along the Sumatra-Andaman Subduction Zone (SASZ) were clarified using the derived frequency-magnitude distribution in terms of the (i) most probable maximum magnitudes, (ii) return periods and (iii) probabilities of earthquake occurrences. The northern segment of SASZ, along the western coast of Myanmar to southern Nicobar, was found to be capable of generating an earthquake of magnitude 6.1–6.4 M

Probabilistic tsunami hazard assessment along Oman coast from submarine earthquakes in the Makran subduction zone

The Sultanate of Oman is among the Indian Ocean countries that were subjected to at least two confirmed tsunamis during the twentieth and twenty-first centuries: the 1945 tsunami due to an earthquake in the Makran subduction zone in the Sea of Oman (near-regional field tsunami) and the Indian Ocean tsunami in 2004, caused by an earthquake from the Andaman Sumatra subduction zone (far - field tsunami). In this paper, we present a probabilistic tsunami hazard assessment for the entire coast of Oman from tectonic sources generated along the Makran subduction zone. The tsunami hazard is assessed taking into account the contribution of small- and large-event magnitudes. Results of the earthquake recurrence rate studies and the tsunami numerical modeling for different magnitudes were used through a logic-tree to estimate the tsunami hazard probabilities. We derive probability hazard exceedance maps for the Omani coast considering the exposure times of 100, 250, 500, and 1000 years. The hazard maps consist of computing the likelihood that tsunami waves exceed a specific amplitude. We find that the probability that a maximum wave amplitude exceeds 1 m somewhere along the coast of Oman reaches, respectively, 0.7 and 0.85 for 100 and 250 exposure times, and it is up to 1 for 500 and 1000 years of exposure times. These probability values decrease significantly toward the southern coast of Oman where the tsunami impact, from the earthquakes generated at Makran subduction zone, is low.

Precursory seismic quiescence along the Sumatra-Andaman subduction zone: past and present

In this study, the seismic quiescence prior to hazardous earthquakes was analyzed along the Sumatra-Andaman subduction zone (SASZ). The seismicity data were screened statistically with mainshock earthquakes of M w ≥ 4.4 reported during 1980–2015 being defined as the completeness database. In order to examine the possibility of using the seismic quiescence stage as a marker of subsequent earthquakes, the seismicity data reported prior to the eight major earthquakes along the SASZ were analyzed for changes in their seismicity rate using the statistical Z test. Iterative tests revealed that Z factors of N = 50 events and T = 2 years were optimal for detecting sudden rate changes such as quiescence and to map these spatially. The observed quiescence periods conformed to the subsequent major earthquake occurrences both spatially and temporally. Using suitable conditions obtained from successive retrospective tests, the seismicity rate changes were then mapped from the most up-to-date seismicity data available. This revealed three areas along the SASZ that might generate a major earthquake in the future: (i) Nicobar Islands (Z = 6.7), (ii) the western offshore side of Sumatra Island (Z = 7.1), and (iii) western Myanmar (Z = 6.7). The performance of a stochastic test using a number of synthetic randomized catalogues indicated these levels of anomalous Z value showed the above anomaly is unlikely due to chance or random fluctuations of the earthquake. Thus, these three areas have a high possibility of generating a strong-to-major earthquake in the future.

A 6600 year earthquake history in the region of the 2004 Sumatra-Andaman subduction zone earthquake

In order to investigate the possibility of a long-term paleoseismic history from offshore sedimentary records in Sumatra, we collected 144 deep-sea sediment cores in the trench and in lower slope piggyback basins of the Sumatra accretionary prism. We used multibeam bathymetry and seismic reflection data to develop an understanding of catchment basins, turbidity current pathways, and depositional styles, as well as to precisely locate our gravity cores, piston cores, Kasten cores, and multicores. We use detailed physical property data, including computed tomographic X-ray, gamma density, magnetic susceptibility, grain-size analysis, faunal analysis, and smear slides, to evaluate the turbidite stratigraphy and sedimentology at each site. We use radiocarbon age control for piggyback basin sites above the carbonate compensation depth, and use 210 Pb and 137 Cs to evaluate the timing of the most recent sedimentary deposits. Using well-log correlation methods and radiometric age control, we test for potential correlations between isolated sites in piggyback basins and the trench. We find evidence for very young surface turbidites along the northern Sumatra margin, most likely emplaced within the past few decades at the seafloor in both the 2004 and 2005 earthquake rupture zones, with no overlying hemipelagic sediment. Based on the young soupy deposits, lack of oxidation, and 210 Pb and 14 C age determinations, we interpret the uppermost turbidite in 21 cores within the 2004 rupture area to have been deposited within a few years of collection in 2007, and most likely as a result of the 2004 moment magnitude (M w ) ∼9.2 earthquake. The likely 2004 turbidite has a distinctive stacked structure of three major fining-upward sequences observed at several basin and trench sites, similar to the pattern of moment release in the 2004 earthquake. We observe rapid die out of the 2004 and 2005 deposits with distance from the slip zones, from local sources of sediment supply, and in the segment boundary between the slip zones. Many individual turbidites show strong similarities between isolated sites, as well as having similar emplacement times. Based upon radiocarbon age control and lithostratigraphic correlations between isolated basin and trench core sites, we interpret that 43 turbidites can be linked spatially over a distance of ∼230 km within the southern portion of the 2004 rupture zone. Sampling at deep-water sites isolated from terrestrial and shallow-water sediment sources, as well as potential storm or tsunami wave triggers, limits potential mechanisms for initiating turbidity currents to plate boundary, crustal, or slab earthquakes. Other potential triggers, such as tectonic oversteepening, random self-failures, gas hydrate destabilization, are unlikely to be correlative between any two isolated sites. The most probable explanation for the similarity of timing, turbidite sequences, and individual turbidite structure in isolated basin and trench stratigraphic sequences is a seismogenic origin. The mean emplacement time for turbidites (likely triggered by Great earthquakes, magnitude > ∼8) in the 2004 rupture region for the past 6.6 ± 0.14 k.y. is 160 yr for 43 turbidites. The ages of 8 of the 10 uppermost turbidite deposits, spanning the past ∼1500 yr, are largely consistent with the terrestrial paleoseismic and/or tsunami records in Thailand, Sumatra, India, and the Andaman Islands, suggesting either coincidence or a common origin. The mean interseismic time from the turbidite record for this same period is 170 yr, comparable to the ∼210 yr recurrence for regional tsunami. The turbidite record, at 180 yr (6 events), compares reasonably well to the average for all events on northern Simeulue of 220 yr, and is identical to the tsunami interval of 180 yr for the same time period (6 events). Of the 43 correlated turbidites in the 2004 earthquake region, 13 are well correlated in our cores along strike lengths of 150 km or greater, and satisfy criteria for robustness; 24 turbidites correlated along a shorter strike distance may represent other plate boundary earthquakes of shorter spatial extent and may include turbidite beds sourced from crustal and slab earthquakes.

Stratigraphic evidence for earthquakes and tsunamis on the west coast of South Andaman Island, India during the past 1000 years

Stratigraphic records from west coast of South Andaman Island revealed evidence of three historical earthquakes and associated transoceanic tsunamis during past 1000yrs, in addition to the Mw 9.3 tsunamigenic earthquake of 26 December, 2004. Our finding suggests that along with Sumatran arc segment the Andaman–Arakan segment is also capable of generating mega-subduction zone earthquakes and transoceanic tsunamis. To study the near sub-surface stratigraphic succession we excavated shallow trenches and obtained geoslices from two sites around Collinpur (sites 1 and 2). The exposed succession comprised 11 lithounits (Unit a — youngest and k — oldest) of alternating sequence of coarser units overlain by peaty soils and some of these are indicative of deposition during paleo-tsunami events.Event I that predated AD 800, and is marked by a 35–40cm thick deposit of fine gravel to coarse sands along with broken shell fragments (Unit k). Event II dated around AD 660–800, is represented by 20–25cm thick coarse sand and broken shell fragments (Unit i). Based on stratigraphic evidences of land-level changes, this event is attributed to a near source rupture along Andaman–Arakan segment, accompanied by a transoceanic tsunami. Event III, occurred around AD 1120–1300, is marked by a 50cm thick sand deposit (Unit g). The 2004 tsunami resulted in deposition of 15cm thick medium to coarse sand at the same location. We infer that the 2004 tsunami and Event III resulted in different styles of sedimentation at the same site.Four events at Collinpur along with the record of a subsidence event of AD 1679 from the east coast of Andaman, close-to, Port Blair (Malik et al., 2011), suggest that mega-subduction zone earthquakes and associated tsunamis recur at an interval of 300–500 years at variable locations along the Sumatra–Andaman subduction zone.