from partial to full strain partitioning along the indo burmese hyper oblique subduction abstract the andaman nicobar trench and its onshore prolongation the indo burmese wedge is the least studied segment of the india australia subduction new offshore geological and geophysical data have recently been collected along the burma scarp during a marine survey conducted with the r v marion dufresne andaman cruise swath bathymetric mapping combined with shallow and deep seismic show that the dominant active tectonics is dextral strike slip faulting accompanied by various amounts of shortening the southern portion of the burma scarp is transpressive north andaman islands the central portion is pure strike slip on nne oriented segments southern myanmar the northern portion shows sedimentary wedge growth along nw segments both onshore and offshore arakan yoma prism further north the wedge bends to the west while dextral shear faults more or less parallel to the sagaing fault develop within the internal part of the wedge and elongated ns folds form in its external part abutting onto the shillong plateau we propose a simple kinematic model involving evolution from partial to full partitioning from south to north along the west burma scarp wbs and we test it quantitatively using the most recent geodetic results in the south half of the 3 5 cm year of india motion is taken at the trench itself the other half being accommodated onto a single shear fault the sagaing fault in myanmar in the north where the bangladesh fold system developed dextral strike slip faults are activated within the arakan yoma belt and at the accretionary prism backstop contact resulting in full partitioning there faults accommodating the oblique component of motion of india are progressively migrating in space from far field faults andaman sagaing to near trench faults arakan yoma belt and trench itself the burma sliver being buttressed by the eastern himalayas this kinematic extends back to 4 ma which is the time of initiation of the last pulse of oceanic accretion in the andaman basin

evidences for cessation of indian plate subduction in the burmese arc region the issue of whether subduction is still active in the india burma plate boundary zone has been rather controversial while the presence of an eastward dipping indian lithospheric slab is undisputed different opinions have been voiced regarding the continuance of subduction at present analysis of the harvard cmt data in comparison with major subduction zones of the world demonstrates that the burmese arc is a unique region where there is a subducted slab but the direction of plate motion is nearly perpendicular to the down dip direction we propose a major right lateral shearing of the indian plate along with its subducted slab past the burmese plate in the nne direction

mentawai fault zone off sumatra a new key to the geodynamics of western indonesia the geodynamic evolution of the western part of the sunda arc is controlled by the change from frontal subduction of the indo australian plate along java to oblique subduction along sumatra this obliquity gives rise to the sumatra fault zone that links the accretionary zone of the andaman sea to the sunda strait previous studies have shown a decrease of displacement rate of the movement along the fault zone from north to south consequently it has been proposed that the area between the subduction zone and the fault zone i e the sumatra sliver platelet was deformed an oceanographic cruise on the indonesian ship r v baruna jaya iii was designed to study this area seismic reflection data reveal the existence of a major strike slip fault parallel to the sumatra fault zone that we called the mentawai fault zone located in the fore arc area just east of the mentawai islands it is at least 600 km long thus the sumatra sliver plate appears to be composed of several strips that move toward the northwest to accommodate the oblique subduction

new insights into the tectonic evolution of the andaman basin northeast indian ocean multibeam swath bathymetry data acquired over an area of about 30 000 km2 together with magnetic and single channel seismic data have been analyzed to understand the tectonic evolution of the andaman basin northeast indian ocean swath bathymetry data revealed several morphotectonic features that divide the basin into a complex western part comprising arc parallel seamount chains and n s trending fault systems and a relatively smooth eastern part a sw ne trending spreading ridge bisects the basin an offset of about 11 8 km at 94 21 e longitude divides the spreading ridge into sw and ne segments high resolution bathymetry and magnetic data enabled us to identify two finer scale segments over the sw part of the ridge that display distinct topographic fabric compared to the ne segment thin sediment cover and presence of axial seamounts characterize sw segments of the ridge whereas the ne segment has thick sediment cover and well maintained rift valley kinematics of basin evolution derived from the identification of the seafloor spreading magnetic anomalies suggest very slow initial opening rates of about 1 6 cm yr and an increase in the spreading rate to about 3 8 cm yr from anomaly 2 to present although this interpretation relies only on magnetic anomalies in the two westernmost segments it is suggested that the true seafloor spreading started about 4 ma in the andaman backarc basin rather than the 11 ma postulated previously we infer based on the integrated analysis of swath bathymetry magnetic and seismological data that the basin has evolved as a consequence of extrusive tectonics that prompted extension and rifting along the plane joining the sagaing and semengko fault systems extension by seafloor spreading during the past 4 myr resulted in the formation of the deep andaman backarc basin this phase has also experienced westward propagation of the spreading center

crustal structure of the central sunda margin at the onset of oblique subduction summary the convergent margin of the central sunda arc in indonesia was the target of a reflection and refraction seismic survey conducted in 1998 and 1999 along two seismic lines across the subduction complex off southern sumatra and off sunda strait coincident multichannel and wide angle data were collected complemented by two refraction strike lines in the forearc basin off sumatra the combined analysis of the acquired data allows us to present a detailed model of the subduction zone where initiation of strain partitioning occurs due to the onset of oblique subduction the dip of the subducted plate is well defined along both dip lines and a lateral increase from 5 to 7 from beneath the outer high off sumatra to sunda strait is supported by complementary gravity modelling the downgoing slab is traced to a depth of more than 30 km on both reflection dip lines a clearly developed backstop structure underlying a trench slope break defines the landward termination of the active accretionary prism and separates it from the outer high active subduction accretion is supported by laterally increasing velocities between the deformation front and the active backstop structure seismic velocities of the outer high are moderate along both lines 5 8 km s 1 at 20 km depth suggesting a sedimentary composition reduced reflectivity beneath a rugged top basement traced along the outer high of both dip lines supports a high degree of deformation and material compaction several kilometres of sediment has accumulated in the forearc domain although a distinct morphological basin is only recognized off southern sumatra and is not developed off sunda strait the bathymetric elevation of the java shelf that is encountered in the southern sunda strait corresponds to increased velocities of a basement high there and is connected to extensional structures of the sunda strait transtensional basin differences observed in the morphology of the forearc domain are also reflected in the lower crustal structure off southern sumatra the velocity depth model clearly indicates a continental type crust underlying the forearc basin whereas lower velocities are found beneath the sunda strait forearc domain off sumatra some 3 d constraint on the upper plate structure is gained from the refraction strike lines which in addition is supported by synthetic data modelling

the west andaman fault and its influence on the aftershock pattern of the recent megathrust earthquakes in the andaman sumatra region 1 distinctly different rupture patterns of december 2004 and march 2005 megathrust earthquakes occurred in the andaman sumatra region suggest strong influence of tectonic and structural elements we have analysed the shipboard gravity bathymetry and seismic data across the west andaman fault waf a major tectonic feature in the andaman sea to infer the crustal structure and to examine its influence in controlling the aftershock pattern our results support the idea of singh et al 2005 that waf forms a lithospheric scale boundary and together with other tectonic elements modulate the occurrence of large earthquakes and their rupture pattern the active strike slip motion along the waf presence of backarc spreading coupled with increased obliquity of subduction in the andaman sector reduce the probability of occurrence of major or great earthquakes north of 10 n

comment on the great sumatra andaman earthquake of 26 december 2004 lay et al research articles 20 may 2005 p 1127 estimated a 600 km length for the tsunami source region adding tide gauge data from paradip the northernmost of the indian east coast stations and therefore the most critical constraint on the northern extent of the source we estimate that its length was greater by 30

structural geology and the seismotectonics of the 2004 great sumatran earthquake y the authors acknowledge funding support from the australian research council discovery project dp120103554 a unified model for the closure dynamics of ancient tethys constrained by geodesy structural geology argon geochronology and tectonic reconstruction and linkage project lp130100134 where to find giant porphyry and epithermal gold and copper deposits the research was also supported by the satellites seismometers and mass spectrometers initiative within the research school of earth sciences at anu bob engdahl is thanked for his support during earlier versions and for providing data

geophysical characteristics of the ninetyeast ridge andaman island arc trench convergent zone abstract the convergence tectonics of the ninetyeast ridge ner upon the andaman island arc trench system is examined through an analysis of etopo2 bathymetry satellite derived free air gravity and seismic data oblique subduction and the buoyancy forces arising from subduction of the ner render the subduction processes near the andaman arc highly complex the bathymetric expression of the ner is visible up to lat 10 n but seismic reflection data indicate that it extends up to about lat 17 n the gravity anomalies are strongly positive over the exposed segment of the ridge but are subdued over the buried portion there is a prominent break in the continuity of the trench gravity low where the ner seems to impinge upon the island arc further a strong curvilinear belt of negative anomalies just behind and running parallel to the island arc associated with the forearc basin is a dominant feature of the gravity map an offset in the continuity of this strong negative anomaly occurs at about the same latitude where the ner seems to be converging upon the island arc seismic reflection data indicate that the ner is very close to the trench flexural modeling of the gravity anomalies for the subducting indian ocean lithosphere loaded by sediments and the ner indicates that the ner is at the starting phase of its collision with the island arc and may not have started affecting the subduction process itself we infer that the en echelon block structure of the ner in the proximity of the convergent zone is a consequence of complex strike slip and subduction related tectonic forces

seismicity gravity anomalies and lithospheric structure of the andaman arc ne indian ocean abstract 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

seismology speed and size of the sumatra earthquake we now have a clearer picture of the seismic features of last year s gigantic event our seismological results reveal that indonesia s devastating sumatra andaman earthquake on 26 december 2004 was 2 5 times larger than initial reports suggested second only to the 1960 chilean earthquake in recorded magnitude they indicate that it slowly released its energy by slip along a 1 200 km fault generating a long rupture that contributed to the subsequent tsunami now that the entire rupture zone has slipped the strain accumulated from the subduction of the indian plate beneath the burma microplate has been released and there is no immediate danger of a similar tsunami being generated on this part of the plate boundary although large earthquakes on segments to the south still present a threat

rupture process of the 2004 sumatra andaman earthquake the 26 december 2004 sumatra andaman earthquake initiated slowly with small slip and a slow rupture speed for the first 40 to 60 seconds then the rupture expanded at a speed of about 2 5 kilometers per second toward the north northwest extending 1200 to 1300 kilometers along the andaman trough peak displacements reached 15 meters along a 600 kilometer segment of the plate boundary offshore of northwestern sumatra and the southern nicobar islands slip was less in the northern 400 to 500 kilometers of the aftershock zone and at least some slip in that region may have occurred on a time scale beyond the seismic band

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

the great sumatra andaman earthquake of 26 december 2004 the two largest earthquakes of the past 40 years ruptured a 1600 kilometer long portion of the fault boundary between the indo australian and southeastern eurasian plates on 26 december 2004 seismic moment magnitude mw 9 1 to 9 3 and 28 march 2005 mw 8 6 the first event generated a tsunami that caused more than 283 000 deaths fault slip of up to 15 meters occurred near banda aceh sumatra but to the north along the nicobar and andaman islands rapid slip was much smaller tsunami and geodetic observations indicate that additional slow slip occurred in the north over a time scale of 50 minutes or longer

elastic thickness structure of the andaman subduction zone implications for convergence of the ninetyeast ridge abstract 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

a late miocene early pliocene biogenic silica crash in the andaman sea and bay of bengal abstract variations in the mass accumulation rate of biogenic silica bsi in continental margin sediments can be used to reconstruct relative changes in productivity through time in these settings in the northern bay of bengal a lack of long sedimentary records has historically precluded this type of reconstruction the acquisition of 21 new long sedimentary records during the 2006 indian national gas hydrate program nghp expedition 01 has made it possible for the first time to reconstruct paleoproductivity in this important region of the world that is dominated by intense changes in the geological and biological fluxes largely driven by tectonic and climate related mechanisms in the research presented here fluctuations in the mass accumulation rate of biogenic opal during the past 9 4 myrs are reconstructed using continental margin sediment cores from the andaman sea site nghp 01 17a and the northern bay of bengal site nghp 01 19 within these records a biogenic silica crash is recorded at 6 ma and is consistent with previous geotectonic geochemical and paleontological studies of the southern indian ocean and pacific ocean that suggest connectivity and thus exchange of nutrient rich water masses between the eastern tropical indian ocean and western tropical pacific ocean was diminished as a result of the tectonic restriction of the northerly sector of the indonesian throughflow itf the biogenic silica crash at sites 17 and 19 is consistent with a decrease in surface water productivity that may have been driven by the reduction of nutrient rich pacific waters delivered to the andaman sea and bay of bengal via the northerly route of the itf following the bsi crash at 6 ma subsequent recovery of the bsi mass accumulation rates at sites 17 and 19 occurred and was perhaps renewed by an enhanced supply of nutrient rich freshwater from the nearby irrawaddy and mahanadi rivers which could have occurred during a documented increase in the intensity of the indian monsoon at 5 ma although recovery is noted at both core locations biogenic silica mass accumulation rates did not fully recover in the andaman sea this could be explained by the restricted nature of the andaman basin and its more distal location from a major source of nutrient rich freshwater

neotectonics of the sumatran fault indonesia the 1900 km long trench parallel sumatran fault accommodates a significant amount of the right lateral component of oblique convergence between the eurasian and indian australian plates from 10 n to 7 s our detailed map of the fault compiled from topographic maps and stereographic aerial photographs shows that unlike many other great strike slip faults the sumatran fault is highly segmented cross strike width of step overs between the 19 major subaerial segments is commonly many kilometers the influence of these step overs on historical seismic source dimensions suggests that the dimensions of future events will also be influenced by fault geometry geomorphic offsets along the fault range as high as 20 km and may represent the total offset across the fault if this is so other structures must have accommodated much of the dextral component of oblique convergence during the past few million years our analysis of stretching of the forearc region near the southern tip of sumatra constrains the combined dextral slip on the sumatran and mentawai faults to be no more than 100 km in the past few million years the shape and location of the sumatran fault and the active volcanic arc are highly correlated with the shape and character of the underlying subducting oceanic lithosphere nonetheless active volcanic centers of the sumatran volcanic arc have not influenced noticeably the geometry of the active sumatran fault on the basis of its geologic history and pattern of deformation we divide the sumatran plate margin into northern central and southern domains we support previous proposals that the geometry and character of the subducting investigator fracture zone are affecting the shape and evolution of the sumatran fault system within the central domain the southern domain is the most regular the sumatran fault there comprises six right stepping segments this pattern indicates that the overall trend of the fault deviates 4 clockwise from the slip vector between the two blocks it separates the regularity of this section and its association with the portion of the subduction zone that generated the giant m w 9 earthquake of 1833 suggest that a geometrically simple subducting slab results in both simple strike slip faulting and unusually large subduction earthquakes

sedimentation rates from calcareous nannofossil and planktonic foraminifera biostratigraphy in the andaman sea northern bay of bengal and eastern arabian sea abstract in this study we determined calcareous nannofossil and planktonic foraminifera events in sediments recovered during the 2006 indian national gas hydrate program nghp expedition 01 selected bioevents permitted the assignment of orbitally calibrated ages derivative sedimentation rate estimates and identification of sedimentary discontinuities in the andaman sea at hole nghp 01 17a a late miocene to recent record was recovered but with a significant hiatus during the latest miocene and pliocene sedimentation rates here vary from 50 m myr during the pleistocene to 130 m myr for the late miocene in the northern bay of bengal at hole nghp 01 19a the base of the cored record also reaches the late miocene and extends to recent but the same hiatus missing the pliocene is present at this hole a the early pleistocene at hole nghp 1 19a shows sensible variation in sedimentation rates ranging between 130 m myr and 10 m myr during the last 0 5 ma for the late miocene the sedimentation rate was ca 50 m myr in the western bay of bengal at hole nghp 01 10b d calcareous nannofossils were sparse and only foraminifera datums are available the base of the section is latest early pliocene to late pliocene and the majority of the record is considered younger than late pliocene due to the scarcity of calcareous nannofossils and the few foraminifera datums sedimentation rates were not determined here however hole nghp 01 16a in the same region recorded the highest occurrence of pseudoemiliania lacunosa an event that confirms a pleistocene age allowing us to estimate a sedimentation rate between 110 m myr and 450 m myr along the western peninsular indian margin at hole nghp 01 01a a continuous record from the holocene to the early oligocene indicates sedimentation rates of ca 25 m myr until the middle miocene and a sensible reduction downward reaching 4 m myr in the early miocene to the early oligocene no hiatuses were observed the large changes in sedimentation rates observed in these cores reflect long timescale changes in the environment likely induced by changes in the strength of the indian monsoon system

tectonics and history of the andaman sea region abstract the andaman sea is an active backarc basin lying above and behind the sunda subduction zone where convergence between the overriding southeast asian plate and the subducting australian plate is highly oblique the effect of the oblique convergence has been formation of a sliver plate between the subduction zone and a complex right lateral fault system the late paleocene collision of greater india and asia with approximately normal convergence started clockwise rotation and bending of the northern and western sunda arc the initial sliver fault which probably started in the eocene extended through the outer arc ridge offshore from sumatra through the present region of the andaman sea into the sagaing fault with more oblique convergence due to the rotation the rate of strike slip motion increased and a series of extensional basins opened obliquely by the combination of backarc extension and the strike slip motion these basins in sequence are the mergui basin starting at 32 ma the conjoined alcock and sewell rises starting at 23 ma east basin separating the rises from the foot of the continental slope starting at 15 ma and finally at 4 ma the present plate edge was formed alcock and sewell rises were separated by formation of the central andaman basin and the faulting moved onshore from the mentawai fault to the sumatra fault system bisecting sumatra

on the mechanism of seismic decoupling and back arc spreading at subduction zones to address the problem of the great variability of the mechanical state of subduction zones we investigate the mechanics of back arc spreading and seismic decoupling back arc spreading is assumed to be due to rifting of the upper plate and hence occurs when trench normal tension reaches a critical value seismic decoupling is assumed to occur when the normal stress at the frictional interface is decreased by an amount sufficient to cross the friction stability transition two forces are important in this problem the first is a small component of the slab pull force which remains unbalanced by the subduction resistance and exerts a vertical suction force at the trench the second is a sea anchor force exerted on the slab that resists its lateral motion assumed to occur at the upper plate velocity both forces contribute to the coupling problem only the sea anchor force is responsible for back arc spreading the unbalanced slab pull force is determined from a force balance for subduction the sea anchor force is computed as the hydrodynamic resistance to the facewise translation of an elliptical disc through a viscous fluid the model predicts three regimes seismically coupled compressional arcs with advancing upper plates seismically decoupled extensional arcs with retreating upper plates and strongly extensional arcs which also have back arc spreading this model is applied in detail to the izu bonin marianas system it shows that back arc spreading occurs when the integrated tension in the upper plate exceeds a value of about 1 1013 n m 1 and requires a residual tension of about a third that to drive the back arc spreading once rifting is completed it shows why the plate interface near guam is seismically coupled while the plate boundary everywhere farther north is decoupled when applied globally it successfully predicts the state of seismic coupling and back arc spreading in more than 80 of the world s subduction zones of the remaining half can be seen to contain additional complications not included in the model about 10 of cases remain unexplained but some of these may have incorrectly determined seismic coupling coefficients

constraints on 2004 sumatra andaman earthquake rupture from gps measurements in andaman nicobar islands the 26 december 2004 sumatra andaman earthquake mw 9 0 9 3 is the greatest earthquake of the modern seismological era the rupture characteristics of the earthquake particularly in the andaman nicobar region are not well resolved from seismological or far field geodetic data here in this article we present campaign mode global positioning system gps measurements of coseismic displacements at 13 sites in the andaman nicobar islands before and after the 2004 sumatra andaman earthquake these measurements provide improved estimates of rupture characteristics in the region coseismic horizontal ground displacement of 1 5 6 5 m towards the southwest and coseismic vertical displacement mostly subsidence of 0 5 2 8 m occurred along the andaman nicobar islands with maximum displacements in the nicobar islands we estimate coseismic slip under the andaman and nicobar islands as 3 8 7 9 m and 11 15 m respectively the length of the rupture is estimated to be about 1500 km with a width varying from 120 km under middle andaman island to 160 km under great nicobar island gps measurements during january 11 22 2005 from port blair suggest rapid afterslip in the postseismic period limited gps data available from 1995 measurements at two sites in andaman provide evidence of strain accumulation that varied significantly in the 10 yr preceding the earthquake d 2005 elsevier b v all rights reserved

structural evolution and strike slip tectonics off north western sumatra abstract 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 m w 9 2 and the 28 march 2005 m w 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 delta cff and the observed seismic activity are in very good agreement with a coulomb index similar to 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 delta cff distribution 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 similar to 15 20 bar 1 5 2 mpa for delta cff is 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 if mu 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

discussion basin infill architecture and evolution from magnetostratigraphic cross basin correlations in the southeastern pyrenean foreland basin the chronostratigraphy of the middle eocene upper eocene successions of the southeastern pyrenean foreland basin has been revised by taberner et al 1999 1 this contribution was based on the correlation of the magnetic polarity stratigraphy mps for several partial stratigraphic sections

seismic coupling and uncoupling at subduction zones seismic coupling has been used as a qualitative measure of the interaction between the two plates at subduction zones kanamori 1971 introduced seismic coupling after noting that the characteristic size of earthquakes varies systematically for the northern pacific subduction zones a quantitative global comparison of many subduction zones reveals a strong correlation of earthquake size with two other variables age of the subducting lithosphere and convergence rate the largest earthquakes occur in zones with young lithosphere and fast convergence rates while zones with old lithosphere and slow rates are relatively aseismic for large earthquakes results from a study of the rupture process of three great earthquakes indicate that maximum earthquake size is directly related to the asperity distribution on the fault plane asperities are strong regions that resist the motion between the two plates the zones with the largest earthquakes have very large asperities while the zones with smaller earthquakes have small scattered asperities this observation can be translated into a simple model of seismic coupling where the horizontal compressive stress between the two plates is proportional to the ratio of the summed asperity area to the total area of the contact surface while the variation in asperity size is used to establish a connection between earthquake size and tectonic stress it also implies that plate age and rate affect the asperity distribution plate age and rate can control asperity distribution directly by use of the horizontal compressive stress associated with the preferred trajectory i e the vertical and horizontal velocities of subducting slabs are determined by the plate age and convergence velocity indirect influences are many including oceanic plate topography and the amount of subducted sediments all subduction zones are apparently uncoupled below a depth of about 40 km and we propose that the basalt to eclogite phase change in the down going oceanic crust may be largely responsible this phase change should start at a depth of 30 35 km and could at least partially uncouple the plates by superplastic deformation throughout the oceanic crust during the phase change

state of the art of research in kinetic modelling of oil formation and expulsion abstract the last ten years have seen a rapid development in the understanding of petroleum generation and expulsion by a combined use of natural series and experimental simulation this knowledge is now used to develop mathematical models which reproduce the kinetics of petroleum generation and migration in the purpose of basin evaluation by a combination of observation of homogeneous geochemical series experimental simulation and mathematical modelling it has been now established that parameters governing the quantity and composition of petroleum are temperature type of organic matter time or heating rate catalysis by clay minerals which would produce important amounts of isoalkanes is not necessary to produce petroleum the role of minerals although still controversial is rather to retain the heavy fraction of petroleum pressure seems to have a limited influence within the range for effective source rocks kerogen is a tremendously complex macromolecule which is now insufficiently characterized to develop fundamental predictive models of thermal cracking i e involving intermediates such as radicals and olefins such models presently apply only to the cracking of simple molecules hexane hexadecane at limited degrees of cracking for this reason kinetic models are still empirically calibrated on pyrolysis experiments and or on natural maturation trends the most popular models involve a series or a continuous distribution of parallel reactions to describe primary cracking i e direct formation of mobile compounds from kerogen these are generally calibrated against open system pyrolysis with various temperature programs consideration of natural maturation trends i e decrease in petroleum potential may be used also for calibration but discrepancies of organic matter properties availability of deep samples and uncertainties on thermal history limit the effectiveness of such methods whatever the method main activation energies are found in the range 45 60 kcal mol secondary cracking which refers generally to cracking of mobile compounds cannot be modelled from open system pyrolysis nor from natural series unknown residence times but only from closed system pyrolysis although models of primary and secondary cracking still need experimental improvements and more realistic kinetic schemes their ability to predict quantities and composition of petroleum depends primarily on migration which is poorly known indeed migration controls the residence time of petroleum in hot zones of sedimentary basins and thus determines the extent of oil conversion into gas as reproduced by recent numerical models preliminary studies documenting the interactions between thermal cracking and expulsion are shown from haltenbanken area and paris basin in the last few years a kind of agreement has been reached that primary migration was most likely occuring in a separate phase from water and that a sufficient concentration of petroleum must be generated for expulsion to start indeed high expulsion efficiencies over 50 are systematically found for rich and mature source rocks a further understanding of primary migration requires observations of natural series petrophysical studies of source rocks porosities under 10 permeabilities in the nanodardy range thermodynamic investigation of unconventional multiphase organic systems kerogen liquid s gas and experimental simulation under stress this on going research should enable models to track correctly migration paths and thermal history of petroleum fluids as necessary to model their composition

the potential for giant tsunamigenic earthquakes in the northern bay of bengal the sumatra andaman earthquake and indian ocean tsunami of 2004 came as a surprise to most of the earth science community while it is now widely recognized that the risk of another giant earthquake is high off central sumatra just east of the 2004 earthquake there seems to be relatively little concern about the subduction zone to the north in the northern bay of bengal along the coast of myanmar formerly burma phil cummins suggests however that giant earthquake activity is possible in this region and that a large and vulnerable population may be facing a significant earthquake and tsunami hazard there is a high potential for giant earthquake activity along the coast of myanmar indications that this may be the case include the tectonic environment historical earthquake activity and stress and crustal strain observations these findings suggest that giant earthquakes do occur off the coast of myanmar and that a very large and vulnerable population is thereby exposed to a significant earthquake and tsunami hazard the great sumatra andaman earthquake and indian ocean tsunami of 2004 came as a surprise to most of the earth science community although it is now widely recognized that the risk of another giant earthquake is high off central sumatra1 just east of the 2004 earthquake there seems to be relatively little concern about the subduction zone to the north in the northern bay of bengal along the coast of myanmar here i show that similar indicators suggest a high potential for giant earthquakes along the coast of myanmar these indicators include the tectonic environment which is similar to other subduction zones that experience giant megathrust earthquakes stress and crustal strain observations which indicate that the seismogenic zone is locked and historical earthquake activity which indicates that giant tsunamigenic earthquakes have occurred there in the past these are all consistent with active subduction in the myanmar subduction zone and i suggest that the seismogenic zone extends beneath the bengal fan i conclude therefore that giant earthquakes probably occur off the coast of myanmar and that a large and vulnerable population is thereby exposed to a significant earthquake and tsunami hazard

distribution of rees and yttrium among major geochemical phases of marine fe mn oxides comparative study between hydrogenous and hydrothermal deposits abstract mineralogy and geochemistry of the ferromanganese oxides collected from two seamounts in the andaman sea have been investigated with special reference to the solid phase partitioning behavior of rare earth elements rees bulk elemental compositions indicate that the oxides from a volcanic cratered seamount csm are hydrothermal while those from another seamount sm2 are hydrogenous in origin the distributions of rees and yttrium in four geochemical phases viz easily exchangeable mn oxide fe oxyhydroxide and residual of csm oxides were distinctly different from that of sm2 oxides in csm oxides the rees and yttrium are mainly associated with fe oxyhydroxide phase while in sm2 oxides both mn oxide and fe oxyhydroxide phases contain the major fraction of these elements the exchangeable and residual phases of both hydrothermal and hydrogenous oxides have extremely low ree content the shale normalized negative ce anomalies and relative enrichment of heavy ree characteristic of hydrothermal oxides are retained in fe oxyhydroxide phase in hydrogenous oxides the positive ce anomalies occurred in both fe and mn oxide phases while negative y anomalies were found only in fe oxyhydroxide phase the phase distribution of rees and yttrium in hydrothermal oxides is described in terms of co precipitation and adsorption of their halide complexes and available free ions in hydrothermal solutions instead adsorption of major carbonate species from seawater primarily regulates the partitioning of rees among the co existing phases of hydrogenous oxides

back arc extension in the andaman sea tectonic and magmatic processes imaged by high precision teleseismic double difference earthquake relocation 1 the 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

burma plate motion 1 plate motion in the indo burmese arc andaman sumatra region of burma plate is poorly resolved this is mainly due to lack of relevant data and complex tectonics of the region we analyze 1 azimuths of coseismic displacements due to the 2004 sumatra andaman and 2005 nias earthquakes 2 estimates of interseismic deformation in the indo burmese arc andaman sumatra and sagaing fault regions all based on gps measurements 3 long term plate motion rates across sumatra fault system sagaing fault and andaman sea from geomorphological and other geophysical studies and 4 the earthquake focal mechanisms in the region we suggest that the ssw motion of sunda plate with respect to indian plate may be partitioned into the dextral strike slip motion across the sagaing fault in the north and sumatra fault system in south in the back arc region and the arc normal motion across the sumatra subduction zone which becomes oblique in andaman and southern indo burmese arc region and dextral in the northern indo burmese arc region of the fore arc under the rigid plate approximation we estimate a pole for india burma plate pair at 27 1 n 82 1 1 e with an angular velocity of 0 845 0 12 ma and for burma sunda at 22 3 1 1 n 109 3 2 5 e with an angular velocity of 0 67 0 12 ma thus the plate motion in the northern and southern regions of burma plate namely the indo burmese arc and andaman sumatra arc may be explained by a single pole and does not require a boundary between the two