West Andaman Fault Research Articles

A Comparison of Geologic Structure Detection of Sumatera Island Using Goce Satellite Gravity Data and Sgg-Ugm-2 Data

GOCE gravity satellite data can be used for regional fault detection because the observation area is wide and not limited by area. In this study, GOCE satellite data is used to detect geological structures on the island of Sumatra, the results of which are used as the basis for disaster mitigation. GOCE data and SGG-UGM-2 were processed using the GOCE User Toolbox (GUT) software to produce a gravity disturbance map and a complete bouguer anomaly map. The GOCE obtained results were validated using the SGG-UGM-2 high-resolution gravity model data. The calculation results obtained that the gravity disturbance value from the GOCE data was around -140 to 200 mGal, while the value of the gravity disturbance from the SGG-UGM-2 data was around -180-300 mGal. The GOCE gravity disturbance map and the SGG-UGM-2 can detect the Subduction Trench, Mentawai Fault, and West Andaman Fault on Sumatra Island with negative values, while the Sumatran Fault Zone (SFZ) along Sumatra Island with positive values ​​in line with the presence of mountain ranges. The results of the SGG-UGM-2 data processing for the gravity disturbance are more detailed than GOCE because the SGG-UGM-2 data degree is higher than that of GOCE. GOCE complete bouguer anomaly value is around 40-560 mGal, while the value of complete bouguer anomaly SGG-UGM-2 is around 60-560 mGal. The complete bouguer anomaly maps from GOCE and SGG-UGM-2 can detect patterns from the Subduction Trench, Mentawai Fault, and West Andaman Fault but cannot clearly detect SFZ. The complete bouguer anomaly can also detect differences between oceanic and continental crust. The GOCE and the SGG-UGM-2 complete bouguer anomaly map show almost similar patterns and the ability to detect geological structures for sub and regional Sumatra Island. In addition, GOCE data detect geological structures more clearly than GRACE data.

A Complex Structure in the northwestern of Sumatra Fore-arc

The Sumatran offshore region is known as an active seismogenic zone. Consequently, there were many records of great events that caused a big catastrophe, such as the 2004 Sumatra-Andaman earthquake. In order to investigate the geometry and nature of the earthquake ruptures and structures, we utilize the bathymetry and Multi-Channel Seismic (MCS) profiles from the SO-198 research project in 2008 (R/V Sonne) and KH-10-5 research project in 2010 (R/V Hakuho-Maru). We find that the area along the Sunda Trench to the fore-arc high in the northwestern Sumatra are characterized by parallel anticlinal ridges with distinctive vergence regimes. These vergence regimes classified as seaward, mix and landward vergence system. Our data interpretation recognized structures such as Main thrust, Lower thrust, Middle thrust, Upper thrust, piggy-back basin and West Andaman Fault within this region. Our conclusion suggest that the anticlinal seaward and landward vergence are considered as the product of recent deformation rupture. The co-seismic slip from the past earthquake ruptures tends that it was occurred from the deeper part in the entire northwestern of Sumatra fore-arc.

Fore-arc high and basin evolution offshore northern Sumatra using high-resolution marine geophysical datasets

We present a morphotectonic study of the offshore northern Sumatra fore-arc high and fore-arc basin and an evolutionary model of the area based on high-resolution marine geophysical datasets. We show that the landward slope of the fore-arc high and the western edge of the Aceh fore-arc basin host a set of deeply-rooted, seaward-dipping backthrusts: main backthrust (MBT) and frontal backthrust (FBT), which we call north Sumatra backthrust system (NSBS). The FBT is imaged reaching the seafloor throughout the study area, whereas the MBT is imaged as a blind fault. Many landward-dipping imbricated thrusts (fore-arc high thrusts-FHT) are also observed below the fore-arc high, which along with the system of backthrusts have been responsible for uplifting the fore-arc high, shortening the Aceh Basin. On the landward slope of the fore-arc high, a strike-slip fault zone is imaged (termed West Andaman Fault - WAF - in accordance with previous studies), is readily interpreted as the product of the slip-partitioning at this oblique convergent margin, with a fraction of the arc-parallel slip localizing at the boundary between fore-arc high and fore-arc basin. Our results allow us to establish the evolutionary history of the thrust faults, strike-slip fault zone and the fore-arc basin. We suggest that the major structural features including backthrusts (MBT, FBT), fore-thrusts (FHTs) and the fore-arc high were developed during the Miocene, and that the growth of the fore-arc high accelerated in the Pliocene onwards owing to a sudden increase in sediment volume provided by the Nicobar fan.

Interpretation of ship-track magnetic data near Narcondam and Barren Island volcanoes revisited

We interpret the legacy ship-track magnetic and bathymetric profile data to investigate the role of the lithospheric scale West Andaman Fault (WAF) in the dormant Narcondam and active Barren Island volcanoes. We use data-based automated interpretation of total magnetic intensity (TMI) profiles using a contact model in order to delineate geometrical and physical parameters of several faults including the WAF and then investigate its implications in recent volcanism at the Barren Island. Our data-based interpretation of TMI data requires transformation of it into the first order analytical signal and tilt angle, which in turn, requires estimation of the first order horizontal and vertical derivatives of TMI data. We ensure robust estimation of the first order horizontal and vertical derivatives using Savitzky–Golay derivative filter and Hilbert–Noda transformation, respectively. We have identified several anomaly segments of the TMI anomaly profiles and used data-based quantitative interpretation in each segments with a contact model. We then make a comprehensive interpretation using bathymetry, geological setup and the results from quantitative data-based interpretation of magnetic data, and ascertain possible role of the WAF in Barren Island volcanism.

CONTINUITY OF GREAT SUMATRAN FAULT IN THE MARINE AREA REVEALED BY 3D INVERSION OF GRAVITY DATA

The Great Sumatran Fault (GSF) is a 1900-km-long fault extending from Lampung, Indonesia, to India's Andaman Islands. The fault location is not only on the land but also in the marine area. Previous studies were only focused on the land area of Sumatra and Andaman Islands even though the marine fault has also impacted earthquakes and tsunamis such as in 2004. As an effort to disaster risk mitigation, this study used the gravity method to map and study the continuity of the GSF in the marine area from the Aceh Province, Indonesia, to the Andaman Islands, India. The gravity data were obtained from Topex with a resolution of 1.85 km/px. Based on the Bouguer data, the subduction zone in the western part of the Indian Ocean is observed with the anomaly of 500–700 mGal, while the residual structure of GSF, relative to the subduction zone, only comes to clarity through a horizontal derivative transformation with anomaly 130-250 mGal. To delineate the fault's geometry, the data were inverted by GRABLOX 1.6 using Singular Value Decomposition and Occam methods. The 3D modeling results also clearly show the contrast density between regional faults such as subduction zones on the Westside of the West Andaman Fault (WAF). The GSF faults can also be well demonstrated at 50 km depth. Based on these results, the gravity Topex is potentially used as a preliminary study of the GSF activity in the marine area.

Fore-arc basin deformation in the Andaman-Nicobar segment of the Sumatra-Andaman subduction zone: Insight from high-resolution seismic reflection data

The Andaman-Nicobar region is the northernmost segment of the Sumatra-Andaman subduction zone and marks the western boundary of the Andaman Sea, which is a complex active back-arc extensional basin. We present the interpretation of a new set of deep seismic reflection data acquired across the Andaman-Nicobar fore-arc basin, from 8°N to 11°N, in order to better understand its structure and evolution, focusing on (1) how obliquity of convergence affects deformation in the fore arc, (2) the nature and role of the Diligent Fault (DF), and (3) the Eastern Margin Fault (EMF). Despite the obliquity of convergence, back thrusting and compression seem to dominate the Andaman-Nicobar fore-arc basin deformation. The DF is primarily a back thrust and corresponds to the Mentawai and West Andaman Fault systems farther in the south, along Sumatra. The DF is expressed in the fore-arc basin as a series of mostly landward verging folds and faults, deforming the early to late Miocene sediments. The DF seems to root from the boundary between the accretionary complex and the continental backstop, where it meets the EMF. The EMF marks the western boundary of the fore-arc basin; it is associated with subsidence and is expressed as a deep piggyback basin, containing recent Pliocene to Pleistocene sediments. The eastern edge of the fore-arc basin is the Invisible Bank (IB), which is thought to be tilted and uplifted continental crust. Subsidence along the EMF and uplift and tilting of the IB seem to be related to different opening phases in the Andaman Sea.

West Andaman Fault determined to be strike slip, not back thrust

Before the devastating tsunami hit Sumatra on 26 December 2004, not much was known about the fault zones located offshore Sumatra in the Indian Ocean. Since then, geologists have set out to study the subduction margin's structure and dynamics for future mitigation of earthquake hazards.

The West Andaman Fault: A complex strain‐partitioning boundary at the seaward edge of the Aceh Basin, offshore Sumatra

AbstractAlong the Sumatran subduction zone, oblique strain is partitioned between the Sunda megathrust and the strike‐slip Sumatran Fault located near the volcanic arc. Additional strike‐slip motion may localize adjacent to the forearc basin system, forming the Mentawai and West Andaman Fault Zones. An alternative interpretation of the West Andaman Fault, as primarily a backthrust, has also been proposed. We combine a 2‐D multichannel seismic survey (SUMUT) acquired aboard the R/V Sonne in 2008 with previous bathymetry and seismic surveys to characterize the West Andaman Fault adjacent to the Aceh forearc Basin. Two‐dimensional Kirchoff prestack time migration of SUMUT profiles provides the clearest images to date of this portion of the West Andaman Fault. In these images we interpret inward dipping fault segments along the West Andaman Fault indicative of a transpressional flower structure. A reflector at the base of the Aceh Basin sediments (believed to be base Neogene in age) continues trenchward of the fault zone for >15 km and has geometry inconsistent with the geometry expected if a major backthrust were to daylight at the edge of the basin. Therefore, we conclude that the West Andaman Fault is predominantly strike‐slip in nature. As such, the West Andaman is likely part of a system of faults including the Sumatran Fault that accommodate the significant shear component of strain in this oblique subduction zone. Additionally, the location of the West Andaman Fault within the forearc may be controlled by the rheologic contrast between the marginal plateau and the forearc basin.

Back‐arc extension in the Andaman Sea: Tectonic and magmatic processes imaged by high‐precision teleseismic double‐difference earthquake relocation

AbstractThe geometry, kinematics, and mode of back‐arc extension along the Andaman Sea plate boundary are refined using a new set of significantly improved hypocenters, global centroid moment tensor (CMT) solutions, and high‐resolution bathymetry. By applying cross‐correlation and double‐difference (DD) algorithms to regional and teleseismic waveforms and arrival times from International Seismological Centre and National Earthquake Information Center bulletins (1964–2009), we resolve the fine‐scale structure and spatiotemporal behavior of active faults in the Andaman Sea. The new data reveal that back‐arc extension is primarily accommodated at the Andaman Back‐Arc Spreading Center (ABSC) at ~10°, which hosted three major earthquake swarms in 1984, 2006, and 2009. Short‐term spreading rates estimated from extensional moment tensors account for less than 10% of the long‐term 3.0–3.8 cm/yr spreading rate, indicating that spreading by intrusion and the formation of new crust make up for the difference. A spatiotemporal analysis of the swarms and Coulomb‐stress modeling show that dike intrusions are the primary driver for brittle failure in the ABSC. While spreading direction is close to ridge normal, it is oblique to the adjacent transforms. The resulting component of E‐W extension across the transforms is expressed by deep basins on either side of the rift and a change to extensional faulting along the West Andaman fault system after the Mw = 9.2 Sumatra‐Andaman earthquake of 2004. A possible skew in slip vectors of earthquakes in the eastern part of the ABSC indicates an en‐echelon arrangement of extensional structures, suggesting that the present segment geometry is not in equilibrium with current plate‐motion demands, and thus the ridge experiences ongoing re‐adjustment.

Elastic thickness structure of the Andaman subduction zone: Implications for convergence of the Ninetyeast Ridge

We use the Bouguer coherence (Morlet isostatic response function) technique to compute the spatial variation of effective elastic thickness (Te) of the Andaman subduction zone. The recovered Te map resolves regional-scale features that correlate well with known surface structures of the subducting Indian plate and the overriding Burma plate. The major structure on the India plate, the Ninetyeast Ridge (NER), exhibits a weak mechanical strength, which is consistent with the expected signature of an oceanic ridge of hotspot origin. However, a markedly low strength (0<Te<3km) in that region, where the NER is close to the Andaman trench (north of 10°N), receives our main attention in this study. The subduction geometry derived from the Bouguer gravity forward modeling suggests that the NER has indented beneath the Andaman arc. We infer that the bending stresses of the viscous plate, which were reinforced within the subducting oceanic plate as a result of the partial subduction of the NER buoyant load, have reduced the lithospheric strength. The correlation, Te<Ts (seismogenic thickness) reveals that the upper crust is actively deforming beneath the frontal arc Andaman region. The occurrence of normal-fault earthquakes in the frontal arc, low Te zone, is indicative of structural heterogeneities within the subducting plate. The fact that the NER along with its buoyant root is subducting under the Andaman region is inhibiting the subduction processes, as suggested by the changes in trench line, interrupted back-arc volcanism, variation in seismicity mechanism, slow subduction, etc. The low Te and thinned crustal structure of the Andaman back-arc basin are attributed to a thermomechanically weakened lithosphere. The present study reveals that the ongoing back-arc spreading and strike-slip motion along the West Andaman Fault coupled with the ridge subduction exerts an important control on the frequency and magnitude of seismicity in the Andaman region.

New insights on the offshore extension of the Great Sumatran fault, NW Sumatra, from marine geophysical studies

Over the last 20 years, the Great Sumatran Fault (GSF) has been studied on land, but we have very little information about its offshore extension NW of Sumatra and its link with the West Andaman Fault to the north. The problem is further complicated by its vicinity to the volcanic arc. Here we present detailed analyses of the offshore extension of the GSF based on recently acquired high‐resolution bathymetry, multichannel seismic reflection data and some old single channel seismic reflection data. Our findings demonstrate that the branches of the GSF near Banda Aceh proceed further northwestward producing two 15–20 km wide adjacent basins. The southwestern transpressional Breueh basin is 1–2 km deep and has a flower structure with a push‐up ridge in the center, suggesting the presence of an active strike‐slip fault. The presence of strike‐slip earthquakes beneath this basin further suggests that one active branch of the GSF passes through this basin. The northeastern transtensional Weh basin is up to 3.4 km deep and the absence of recent sediments on the basin floor suggests that the basin is very young. The presence of a chain of volcanoes in the center of the basin suggests that the Sumatran volcanic arc passes through this basin. The anomalous depth of the Weh basin might be a site of early back‐arc spreading or may have resulted from pull‐apart extension. We examine all these new observations in the light of plate motion, local deformation and possible seismic risk.

The 2005 volcano‐tectonic earthquake swarm in the Andaman Sea: Triggered by the 2004 great Sumatra‐Andaman earthquake

A 6 day duration earthquakes swarm occurred in the Andaman Sea, 31 days after the giant 2004 Sumatra‐Andaman earthquake (Mw 9.2). The swarm occurred less than 100 km from the eastern edge of the 2004 earthquake rupture and is the most energetic ever recorded in the world. The earthquakes swarm appear to have occurred on en echelon fault system bounded by the two main right‐lateral strike‐slip faults, namely, the Seulimeum Strand of Sumatra Fault system (SEU) and the West Andaman Fault (WAF). At the beginning of the swarm, earthquakes with predominantly strike‐slip focal mechanisms occurred which were followed by earthquakes with predominantly normal faulting focal mechanisms having significant deviatoric component. Highbvalue, presence of double slope in the Gutenberg‐Richter relation, presence of monogenetic submarine volcanoes in the region of the swarm and the earthquake focal mechanisms suggest that the swarm was of volcano‐tectonic origin and fluid flow played an important role in its occurrence. Indeed, our modeling suggests that it was triggered by the 2004 Sumatra‐Andaman earthquake through poroelastic relaxation of the coseismic stresses.

The ISLANDS Network in the Andaman-Nicobar Subduction Zone

### Tectonics The Andaman–Nicobar Islands form the central part of the 5,000-km-long Burma–Sunda–Java subduction complex. The islands, situated to the southwest of the Indian subcontinent, currently overlie one of the most seismically active regions of the world. The complexity of this active tectonic region can be attributed to several factors, including oblique subduction, arc volcanism, and the ill-defined Andaman spreading ridge in the Andaman Sea. The oceanic lithosphere of the Indian plate subducts eastward below the oceanic part of the Burmese plate in southeast Asia (Curray et al. , 1979) identified by the Andaman–Sumatra trench and accretionary subduction complex ridges to the west of the Andaman Islands. The Andaman Sea, lying east of these islands, is a pull-apart basin rather than a typical backarc basin (Curray, 1989). The region comprises the West Andaman fault (WAF), the Sumatra fault system, and a spreading center spanning 10°–12°N in the Andaman Sea (Fig. 1). Also, the Sagaing fault, situated further east and characterized by transform faulting, demarcates the eastern boundary of the Burmese plate (Curray et al. , 1979). The central lowlands are separated from the eastern highlands of Burma by this Sagaing fault, which continues to the Andaman Sea rift zone. The young Barren and Narcondam volcanoes within the Andaman Sea, lying 150 km east of the main islands, are interpreted as a part of the inner arc. The Narcondam volcano is calm, with no historically known activity (Pal et al. , 2007). However, the Barren is designated as an active volcano that erupted in March 1991, after lying dormant for nearly two centuries and subsequently erupting in 1994–1995, 2005–2006, and 2008–2009 (Rajendran et al. , 2003; Sheth et al. , 2010). The region between the Andaman Islands and the volcanic arc is described as the foredeep sedimentary trough (Dickinson and Seely, 1979 …

Tectonic and volcanic implications of a cratered seamount off Nicobar Island, Andaman Sea

The region of the Nicobar earthquake swarm of January 2005 has been explored during a recent cruise using multibeam swath bathymetry, seafloor imaging and TV-guided sampling to decipher the seafloor morphology, nature and tectonic frame work. A seamount with well-developed crater at the summit was discovered near to the center of the Nicobar swarm. Rock samples collected by TV-guided grab from the seamount crater are dacite, rhyolite and andesite type with a veneer of ferromanganese oxide coating. The aggregates recovered from the slope consisted of manganese globules. The geochemistry of the globules suggests 66–97% Mn-oxide indicative of hydrothermal origin. The morphology of the seamount, seafloor video footage and geochemistry of the seabed samples suggest that the cratered seamount has erupted in the recent geological past and is dormant at present. This is the first documented report of submarine arc-volcanism in the Andaman Sea. This finding substantiates the prediction of a submarine volcano east of Nicobar Island. Study of the seismicity and the stress pattern in the region suggest that the earthquake swarm in the region occurred due to 2004 Sumatra–Andaman megathrust earthquake. The derived stress pattern suggests that the 2004 earthquake greatly reduced the normal stress in the region of West Andaman fault from the planes or unclamped them. Thus unclamping of the region by the 2004 Sumatra Andaman earthquake led to the initiation of the swarm through predominantly strike slip faulting. The newly discovered dormant submarine volcano indicates the volcanic nature of the region. We suggest that this submarine volcano is a link between sub-aerial volcanoes of Barren–Narcondam Islands of the Andaman Sea and the volcanoes of Sumatra.

Fractal dimension and b-value mapping in the Andaman-Sumatra subduction zone

The Andaman-Sumatra subduction zone is seismically one of the most active and complex subduction zones that produced the 26 December 2004 mega thrust earthquake (Mw 9.3) and large number of aftershocks. About 8,000 earthquakes, including more than 3,000 aftershocks (M ≥ 4.5) of the 2004 earthquake, recorded during the period 1964–2007, are relocated by the EHB method. We have analysed this large data set to map fractal correlation dimension (Dc) and frequency-magnitude relation (b-value) characteristics of the seismogenic structures of this ~3,000-km-long mega thrust subduction zone in south-east Asia. The maps revealed the seismic characteristics of the Andaman-Sumatra-Java trenches, West Andaman fault (WAF), Andaman Sea Ridge (ASR), Sumatra and Java fault systems. Prominent N–S to NW–SE to E–W trending fractal dimension contours all along the subduction zone with Dc between 0.6 and 1.4 indicate that the epicentres mostly follow linear features of the major seismogenic structures. Within these major contours, several pockets of close contours with Dc ~ 0.2 to 0.6 are identified as zones of epicentre clusters and are inferred to the fault intersections as well as asperity zones along the fault systems in the fore arc. A spatial variation in the b-value (1.2–1.5) is also observed along the subduction zone with several pockets of lower b-values (1.2–1.3). The smaller b-value zones are corroborated with lower Dc (0.5–0.9), implying a positive correlation. These zones are identified to be the zones of more stress or asperity where rupture nucleation of intermediate to strong magnitude earthquakes occurred.

Morphology and tectonics of the Andaman Forearc, northeastern Indian Ocean

SUMMARY The Andaman Sea has developed as the result of highly oblique subduction at the western Sunda Trench, leading to partitioning of convergence into trench-perpendicular and trench-parallel components and the formation of a northward-moving sliver plate to accommodate the trench parallel motion. The Andaman forearc contains structures resulting from both components of motion. The main elements of the forearc are the accretionary prism and outerarc ridge, a series of forearc basins and major N–S faults. The accretionary prism is an imbricate stack of fault slices and folds consisting of ophiolites and sediments scrapped off the subducting Indian Plate. The western, outer slope of the accretionary prism is very steep, rising to depths of 1500–2000 m within a distance of 30 km. There is a difference in the short wavelength morphology between the western and eastern portions of the accretionary prism. The outer portion consists of a series of faulted anticlines and synclines with amplitudes of a few 100 to ∼1000 m and widths of 5–15 km resulting from ongoing deformation of the sediments. The inner portion is smoother with lower slopes and forms a strong backstop. The width of the deforming portion of the accretionary prism narrows from 80 to 100 km in the south to about 40 km between 10 ◦ N and 11 ◦ 30 � N. It remains at about 40 km to ∼14 ◦ 40 � N. North of there, the inner trench wall becomes a single steep slope up to the Myanmar shelf. The eastern edge of the outerarc ridge is fault bounded and, north of the Nicobar Islands, a forearc basin is located immediately to the east. A deep gravity low with very steep gradients lies directly over the forearc basin. The West Andaman Fault (WAF) and/or the Seulimeum strand of the Sumatra Fault System form the boundary between the Burma and Sunda plates south of the Andaman spreading centre. The WAF is the most prominent morphologic feature of the Andaman Sea and divides the sea into a shallow forearc and a deeper backarc region. The Diligent Fault runs through the forearc basin east of Little Andaman Island. Although it has the general appearance of a normal fault, multichannel seismic data show that it is a compressional feature that probably resulted from deformation of the hanging wall of the Eastern Margin Fault. This could occur if the forearc basins were formed by subduction erosion of the underlying crust rather than by east–west extension.

Seismic imaging of forearc backthrusts at northern Sumatra subduction zone

Forearc tectonics at accretionary convergent margins has variously been studied using analogue and numerical modelling techniques. Numerous geophysical investigations have targeted the subsurface structure of active forearc settings at convergent margins. However, several critical details of the structure, mode of tectonic evolution and the role forearcs play in the subduction seismic cycle remain to be further understood, especially for large accretionary margins. In this study, we present a high-resolution deep seismic reflection image of the northern Sumatran subduction forearc, near the 2004 December 26 Sumatra earthquake epicentral region. The profile clearly demarcates the backthrust branches at the seaward edge of the Aceh forearc basin, along which the inner forearc continues to evolve. Sharp bathymetric features at the seafloor suggest that the imaged backthrusts are active. Coincident wide-angle seismic tomographic image of the Sumatra forearc allows us to image the geometry of the seaward dipping backstop buttress, with which the imaged backthrust branches are associated. The presence of forearc backthrusting confirms model predictions for the development of backthrusting over seaward dipping backstops. The West Andaman fault at the seaward edge of Aceh basin appears to be a shallow tributary of the backthrust and sheds light on the complex deformation of the forearc. Uplifting along the backthrust branches may explain the presence of forearc islands observed all along Sumatran margin and help further constrain the tectonic models for their evolution. Moreover, if these backthrusts slip coseismically, they would contribute to tsunamigenesis and seismic risk in the region.

Structural evolution and strike-slip tectonics off north-western Sumatra

Based on new multi-channel seismic data, swath bathymetry, and sediment echosounder data we present a model for the interaction between strike-slip faulting and forearc basin evolution off north-western Sumatra between 2°N and 7°N. We examined seismic sequences and sea floor morphology of the Simeulue- and Aceh forearc basins and the adjacent outer arc high. We found that strike-slip faulting has controlled the forearc basin evolution since the Late Miocene. The Mentawai Fault Zone extends up to the north of Simeulue Island and was most probably connected farther northwards to the Sumatran Fault Zone until the end of the Miocene. Since then, this northern branch jumped westwards, initiating the West Andaman Fault in the Aceh area. The connection to the Mentawai Fault Zone is a left-hand step-over. In this transpressional setting the Tuba Ridge developed. We found a right-lateral strike-slip fault running from the conjunction of the West Andaman Fault and the Tuba Ridge in SSW-direction crossing the outer arc high. As a result, extrusion formed a marginal basin north of Simeulue Island which is tilted eastwards by uplift along a thrust fault in the west. The shift of strike-slip movement in the Aceh segment is accompanied by a relocation of the depocenter of the Aceh Basin to the northwest, forming one major Neogene unconformity. The Simeulue Basin bears two major Neogene unconformities, documenting that differences in subsidence evolution along the northern Sumatran margin are linked to both forearc-evolution related to subduction processes and to deformation along major strike-slip faults.

Stress change and effective friction coefficient along the Sumatra‐Andaman‐Sagaing fault system after the 26 December 2004 (Mw= 9.2) and the 28 March 2005 (Mw= 8.7) earthquakes

The 2004 Aceh and 2005 Nias events are the two greatest earthquakes of the past 40 years with a total rupture of 1700 km long and a coseismic slip reaching up to 25 m. These two earthquakes have caused large stress perturbations which significantly altered seismic activity in the Sumatra‐Andaman region. Using both detailed mapping of failure planes and various slip distributions, we calculate this stress change along the Sumatra‐Andaman‐Sagaing fault system from central Sumatra to southern Myanmar. The static Coulomb stress change ΔCFFand the observed seismic activity are in very good agreement with a Coulomb index ∼ 20% greater than the one obtained for random events. Compared to previous studies, this high Coulomb Index confirms two important issues on the use of static stress change criterion: unsuited to study near‐field aftershocks and only relevant for aftershocks analysis on large and mature faults at a time scale of several months. The calculated ΔCFFdistribution suggests that the 2004 and 2005 earthquakes inhibit failure on the North Andaman rift and on the Sagaing fault, while failure is encouraged along the transform Andaman zone, the central Andaman rift, the West Andaman fault, the Sumatra fault system, and the offshore thrust faults west of Sumatra Island. The maximum value of ∼15–20 bar (1.5–2 MPa) for ΔCFFis reached in the northern part of the Sumatra fault system. This high value together with the lack of major earthquake in the last 170 years result in a high seismic hazard for this region. Our results are also consistent with temporal evolution of both earthquakes’ location and focal mechanism prior to and after the events. In particular, we explain the occurrence and the mechanism of seismic swarms observed in the central Andaman rift and along the west Andaman fault. Finally, our calculations reveal that the seismicity in the Andaman rift zone can only be explained ifμ′ &gt; 0.5. This result leads to two end‐member models: one with a constant and high fault friction and one with spatial variations, for which friction may depend on either the nature of the lithosphere (oceanic versus continental) or the fault type.

Seismicity, gravity anomalies and lithospheric structure of the Andaman arc, NE Indian Ocean

The Andaman arc in the northeastern Indian Ocean defines nearly 1100 km long active plate margin between the India and Burma plates where an oblique Benioff zone develops down to 200 km depth. Several east-trending seismologic sections taken across the Andaman Benioff Zone (ABZ) are presented here to detail the subduction zone geometry in a 3-D perspective. The slab gravity anomaly, computed from the 3-D ABZ configuration, is a smooth, long-wavelength and symmetric gravity high of 85 mGal amplitude centering to the immediate east of the Nicobar Island, where, a prominent gravity “high” follows the Nicobar Deep. The Slab-Residual Gravity Anomaly (SRGA) and Mantle Bouguer Anomaly (MBA) maps prepared for the Andaman plate margin bring out a double-peaked SRGA “low” in the range of − 150 to − 240 mGal and a wider-cum-larger MBA “low” having the amplitude of − 280 to − 315 mGal demarcating the Andaman arc–trench system. The gravity models provide evidences for structural control in propagating the rupture within the lithosphere. The plate margin configuration below the Andaman arc is sliced by the West Andaman Fault (WAF) as well as by a set of sympathetic faults of various proportions, often cutting across the fore-arc sediment package. Some of these fore-arc thrust faults clearly give rise to considerably high post-seismic activity, but the seismic incidence along the WAF further east is comparatively much less particularly in the north, although, the lack of depth resolution for many of the events prohibits tracing the downward continuity of these faults. Tectonic correlation of the gravity-derived models presented here tends to favour the presence of oceanic crust below the Andaman–Nicobar Outer Arc Ridge.