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

eruptive history of earth s largest quaternary caldera toba indonesia clarified single grain laser fusion sup 40 ar sup 39 ar analyses of individual sanidine phenocrysts from the two youngest toba indonesia tuffs yield mean ages of 73 plus minus 4 and 501 plus minus 5 ka in addition glass shards from toba ash deposited in malaysia were dated at 68 plus minus 7 ka by the isothermal plateau fission track technique these new determinations in conjunction with previous ages for the two oldest tuffs at toba establish the chronology of four eruptive events from the toba caldera complex over the past 1 2 m y ash flow tuffs were erupted from the complex every 0 34 to 0 43 m y culminating with the enormous 2500 3000 km sup 3 youngest toba tuff eruption caldera formation and subsequent resurgence of samosir island timing of this last eruption at toba is coincident with the early wisconsin glacial advance the high precision sup 40 ar sup 39 ar age eruption of such magnitude may provide an important marker horizon useful as a baseline for research and modeling of the worldwide climatic impact of exceptionally large explosive eruptions

seismicity and attenuation in the central vanuatu new hebrides islands a new interpretation of the effect of subduction of the d entrecasteaux fracture zone the spatial distribution of the intermediate depth earthquakes in the central vanuatu new hebrides islands includes a remark able gap in seismic activity located between depths of 100 and 200 km and having a lateral extent of about 150 km no well located earth quakes determined from teleseismic d ata during a 16 year period nor any microearthquakes located by a local seismograph network during a 5 year period have occurred in the gap a zone of attenuation of high frequency shear waves over laps the seismicity gap no contortion of the benioff zone in the region of the gap indicative of major disruption of the subducted plate is apparent however the gap is close to the extrapolated location of the subducted part of the d entrecasteaux fracture zone dfz the dfz is a ridgelike bathymetric feature on the oceanic plate being subducted beneath the island a rc a pogitive thermal anomaly associated with the dfz may cause the gap of intermediate depth seismic ity and produce anomalous absorption of high frequency shear waves alternatively a zone of strong scattering associated with the subducted part of the dfz may also account for the anoma lous attenuation

p s velocity and vp vs ratio beneath the toba caldera complex northern sumatra from local earthquake tomography summary in this paper we investigate the crustal and uppermost mantle structure beneath toba caldera which is known as the location of one of the largest cenozoic eruptions on earth the most recent event occurred 74 000 yr bp and had a significant global impact on climate and the biosphere in this study we revise data on local seismicity in the toba area recorded by a temporary passcal network in 1995 we applied the newest version of the lotos 07 algorithm which includes absolute source location optimization of the starting 1 d velocity model and iterative tomographic inversion for 3 d seismicp s or thevp vs ratio and source parameters special attention is paid to verification of the obtained results beneath the toba caldera and other volcanoes of the arc we observe relatively moderate for volcanic areas negative p and s velocity anomalies that reach 18 per cent in the uppermost layer 10 12 per cent in the lower crust and about 7 per cent in the uppermost mantle much stronger contrasts are observed for the vp vs ratio that is a possible indicator of dominant effect of melting in origin of seismic anomalies at a depth of 5 km beneath active volcanoes we observe small patterns 7 15 km size with a high vp vs ratio that might be an image of actual magmatic chambers filled with partially molten material feeding the volcanoes in the mantle wedge we observe a vertical anomaly with low p and s velocities and a high vp vs ratio that link the cluster of events at 120 140 km depth with toba caldera this may be an image of ascending fluids and melts released from the subducted slab due to phase transitions however taking into account poor vertical resolution these results should be interpreted with prudence although the results show clear signatures that are quite typical for volcanic areas low velocity and high vp vs ratio beneath volcanoes we do not observe any specific features in seismic structure that could characterize toba as a super volcano

submergence and uplift associated with the giant 1833 sumatran subduction earthquake evidence from coral microatolls the giant sumatran subduction earthquake of 1833 appears as a large emergence event in fossil coral microatolls on the reefs of sumatra s outer arc ridge stratigraphic analysis of these and living microatolls nearby allow us to estimate that 1833 emergence increased trenchward from about 1 to 2 m this pattern and magnitude of uplift are consistent with about 13 m of slip on the subduction interface and suggest a magnitude mw of 8 8 9 2 for the earthquake the fossil microatolls also record rapid submergence in the decades prior to the earthquake with rates increasing trenchward from 5 to 11 mm yr living microatolls show similar rates and a similar pattern the fossil microatolls also record at least two less extensive emergence events in the decades prior to 1833 these observations show that coral microatolls can be useful paleoseismic and paleogeodetic instruments in convergent tectonic environments

effects of the heterogeneous subducting plate on seismicity constraints from b values in the andaman sumatra java subduction zone abstract the andaman sumatra java subduction zone is characterized by significant spatial variations in subducting plate age subduction velocity and slab bending with i the uplifting controlled by the ninety east ridge in the north ii trench eroding affected by the roo rise in the south and iii the highly deformed warton basin in front of the trench cut through by a series of strike slip faults the entire subducting plate exhibits strong spatial heterogeneity in this paper we investigated the effect of the heterogeneous subducting plate on seismicity based on the comparison between the spatial variations in the b value of the gutenberg richer law and in the tectonic features of the subducting oceanic plate we used the international seismological centre ehb bulletin isc ehb from 1970 to 2013 to conduct a detailed study of b value spatial variations in the andaman sumatra java subduction zone via the inter comparison of tectonic factors we found that the subduction zone was tectonically segmented there are good positive correlations between b value and subducting plate age in two tectonic domains after removing the influence of local tectonic irregularities which implies that the subducting plate density generally controls the stress and coupling between the plates the subduction of local tectonic irregularities such as the fracture zones and seamounts has significantly affected seismicity which prevents the occurrence of large earthquakes and causes the b value to rise in addition the anomalous tectonic environments such as the spreading center and the thrust fault zones could also impact spatial variations of the b value therefore seismicity in subduction zones is a result of the combined impacts of complex tectonic features understanding and clarifying the mechanisms of these tectonic features in relation to b values can help us to better assess seismic risk in subduction zones

spatial aftershock distribution of the 26 december 2004 great sumatra andaman earthquake in the northern sumatra area 1 in july 2005 about 6 months after the 2004 great sumatra andaman earthquake 20 ocean bottom seismometers were deployed in the northern sumatra area more than 1000 events were identified during the 12 day recording period after relocation the seismicity shows different patterns on each side of the lower splay fault east of this feature and beneath the aceh basin and fore arc the deep earthquakes outline the subduction megathrust fault west of it the aftershocks distribution is strongly influenced by the active n s oceanic fracture zones of the subducted plate two n s trending clusters of 10 to 50 km deep earthquakes observed below the lower wedge are interpreted as reactivated oceanic fracture zones the postseismic activity suggests a significant influence of the n s active fracture zones of the oceanic plate on the toe of the wedge explaining the dextral wrenching of wedge thrusts in the morphology the megathrust fault does not extend to the trench but outcrops east of these active oceanic fractures and seems to merge upward along the lower splay fault where shallow earthquakes are observed

evolution of the eastern indian ocean since the late cretaceous constraints from geosat altimetry we propose a new model for the tectonic evolution of the eastern indian ocean from the late cretaceous to the present two types of data are used to improve previously published reconstructions first recent reinterpretations of seafloor magnetic anomalies between australia and antarctica and in the wharton basin provide new constraints on spreading rates and the timing of major reorganizations second vertical deflection profiles i e horizontal gravity anomaly derived from 22 repeat cycles of geosat altimeter data reveal the tectonic fabric associated with fracture zones these new geosat data provide tight constraints on paleospreading directions for example three prominent fracture zones can be traced from south of tasmania to the george v basin antarctica providing an important constraint on the relative motions of australia and antarctica through the late eocene in addition the geosat profiles are used to locate the conjugate continental margins and continent ocean boundaries of australia and antarctica as well as the conjugate rifted margins of kerguelen plateau and broken ridge based on a compilation of magnetic anomaly data from the crozet basin the central indian basin the wharton basin and the australian antarctic basin ten plate tectonic reconstructions are proposed reconstructions at chrons 5 11 ma 6 21 ma 13 36 ma and 18 43 ma confirm that the southeast indian ridge behaved as a single plate boundary since chron 18 the constraints from the geosat data provide an improvement in the fit of the kerguelen plateau and broken ridge at chron 20 46 ma to avoid overlaps between broken ridge and the kerguelen plateau prior to their breakup our model includes relative motions between the northern and southern provinces of the kerguelen plateau finally we examine the implications of our model for the relative motions of india australia and antarctica on the tectonic evolution of the kerguelen plateau and broken ridge and the adjacent labuan basin and diamantina zone as well as the emplacement of the ninetyeast ridge and the kerguelen plateau over a fixed hot spot

the fine structure of the subducted investigator fracture zone in western sumatra as seen by local seismicity the sumatran margin suffered three great earthquakes in recent years aceh andaman 26 december 2004 mw 9 1 nias 28 march 2005 mw 8 7 bengkulu 12 september 2007 mw 8 5 here we present local earthquake data from a dense amphibious local seismic network covering a segment of the sumatran margin that last ruptured in 1797 the occurrence of forearc islands along this part of the sumatran margin allows the deployment of seismic land stations above the shallow part of the thrust fault in combination with ocean bottom seismometers this station geometry provides high quality hypocentre location for the updip end of the seismogenic zone in an area where geodetic data are also available in this region the investigator fracture zone ifz which consists of 4 sub ridges is subducted below the sunda plate this topography appears to influence seismicity at all depth intervals a well defined linear streak of seismicity extending from 80 to 200 km depth lies along the prolongation of closely spaced ifz sub ridges more intermediate depth seismicity is located to the southeast of this string of seismicity and is related to subducted rough oceanic seafloor the plate interface beneath siberut island which ruptured last in 1797 is characterised by almost complete absence of seismicity

basal ice microbiology at the margin of the greenland ice sheet basal ice at the margin of the greenland ice sheet was studied with respect to its physical characteristics and microbiological community the basal ice contained high concentrations of dissolved ferrous fe and must therefore be anoxic oxygen consumption experiments indicate that 50 of the oxidation was due to biological activity while the rest could be attributed to chemical processes most likely weathering reactions with ferrous fe at least six different fe containing mineral sources were detected in basal ice together with potential bioavailable fe nanoparticles an active denitrifier population was identified due to formation of 30 n dinitrogen gas after amendment of anoxic sediment slurries with 15 n no3 sulfate reduction could not be detected the solid ice facies contained an abundant 10 8 cells cm 3 and complex microbial community that harbored representatives of at least eight major phyla within the domain bacteria the clone library was dominated by members of the subdivision of proteobacteria of which the largest proportion was affiliated to the genus rhodoferax that comprises facultative aerobic iron reducers the second most abundant phylum was bacteroidetes the solid ice facies had many physical similarities with the overlying debris rich banded ice facies indicating that they formed by similar subglacial processes and harbor similar microbial communities this study extends our knowledge of life in subglacial environments such as beneath ice sheets genbank accession numbers hm439882 hm439950 hq144215 hq144221

strain partitioning during oblique plate convergence in northern sumatra geodetic and seismologic constraints and numerical modeling global positioning system gps measurements along the subduction zone of northern sumatra 2 s to 3 n reveal that the strain associated with the oblique convergence of the australian plate with eurasia is almost fully partitioned between trench normal contraction within the forearc and trench parallel shear strain within a few tens of kilometers of the sumatran fault kinematic analyses of interplate earthquake slip vectors provide slip rates on the sumatran fault within a few millimeters per year of gps and geologic rates giving us more confidence in the use of slip vectors for inferring slip partitioning elsewhere the inferred slip rate on the sumatran fault is 1 3 less than the full margin parallel component of plate motion an across forearc rotation in the slip vectors suggests that the missing arc parallel shear occurs seaward of the geodetic network between the forearc islands and the trench simple finite element models are used to explore the conditions under which the change in the principal strain rate directions between the forearc and the arc region can occur modeling suggests that neither a preexisting strike slip fault nor a zone of thermally induced lithospheric weakness in the overriding plate is needed for strain partitioning to occur in general forearc slivers form over the region of interplate coupling and are driven along strike by the basal shear a volcanic arc can help the partitioning process by localizing the margin parallel shear strain in the upper plate if its crust and mantle are weaker than its surroundings interplate slip vectors and geodetic results from sumatra together suggest that the highest coupling on the plate boundary occurs beneath and seaward of the forearc islands consistent with inferences about the rupture zones of great nineteenth century earthquakes there the sumatra example suggests that geodetic measurements of interseismic margin parallel shear strain at oblique convergent margins can be used to map the landward extent of the relatively high basal stress beneath the overriding plate if one can correct for strain localization caused by weak upper plate strike slip faults

geodetic observations of interseismic strain segmentation at the sumatra subduction zone deformation above the sumatra subduction zone revealed by global positioning system gps geodetic surveys shows nearly complete coupling of the forearc to the subducting plate south of 0 5 s and half as much to the north the abrupt change in plate coupling coincides with the boundary between the rupture zones of the 1833 and 1861 mw 8 thrust earthquakes the rupture boundary appears as an abrupt change in strain accumulation well into the interseismic cycle suggesting that seismic segmentation is controlled by properties of the plate interface that persist through more than one earthquake cycle structural evidence indicates that differences in basal shear stress may be related to elevated pore pressure in the north

p and s wave velocities of consolidated sediments from a seafloor seismic survey in the north celtic sea basin offshore ireland a geophysical survey was conducted over a hydrocarbon prospect in the north celtic sea basin using a small array of ocean bottom seismographs obss the purpose of this study was to determine the ratio of p compressional to s shear wave velocity of consolidated sedimentary rocks in order to constrain possible subsurface variations in pore fluid content the ratio of vp and vs is known to be particularly sensitive to lithology porosity and pore fluid content making it a useful parameter for evaluating hydrocarbon prospects obss offer a relatively cheap and time effective means of acquiring multi component data compared with ocean bottom cables in this contribution we demonstrate the ability of an obs survey comprising three pairs of two obss spaced at 1 6 km to recover lateral variations in the vp vs ratio a key requirement of this type of study is that s waves will be generated by mode conversions in the subsurface since they cannot be generated in nor travel through fluids in this survey the contrast in physical properties of the hard seabed of the north celtic sea basin provided a means of generating converted s waves two dimensional ray tracing and forward modeling was used to create both vp and vs models along a profile crossing the blackrock prospect in the north celtic sea basin these models comprise four layers and extend to a maximum depth of 1 1 km the observed northward decrease in the vp vs ratio at depths of 500 1000 m below the seafloor in the study area is interpreted to represent lateral variation in the amount of gas present in the pore space of upper cretaceous chalks and shales overlying the prospective reservoir

studying deep sources of volcanism using multiscale seismic tomography abstract this paper presents an overview of several examples of multiscale studies of different volcanic systems including intracontinental collisional and subduction related volcanic areas regional tomographic models of the upper mantle are presented for three regions with the cenozoic intracontinental volcanism in eurasia namely europe southern siberia and eastern arctic in all cases recent volcanic fields correlate with low velocity anomalies in the mantle the modeling results show that these volcanic manifestations are related to large hot areas in the mantle and not to thin column shaped contrasted plumes as presumed by many collisional type volcanism is discussed with an example of the caucasus and the surrounding areas the regional tomographic model for this region reveals a lack of mantle lithosphere beneath the collision zone volcanism in this area is most likely due to the direct heating of the thick crust by the hot asthenosphere a dominantly felsic composition of the crust facilitates upward transport of the fluids and melts that leads to volcanic eruption subduction related volcanic systems are discussed for three different scale levels an example of a regional model of the mantle beneath the sunda arc reveals the general configuration of the slab middle scale tomographic models in areas of the toba caldera central java and central andes display feeding paths of the arc volcanoes from the slab the feeding paths are apparent in all of the considered cases but have different shapes depending on the unique features of the subduction regimes an example of a detailed local scale study of magmatic sources in the crust is discussed for the kluchevskoy volcano group it is found that the main volcano kluchevskoy is fed through a complex three layered system of magma sources another volcano of the same group bezymianny seems to be connected with the mantle through a direct short lived channel which appears only during the eruption periods a time dependent tomography study performed for this group over a period of ten years clearly reveals the correlation between the changes in the seismic structures and the stages of the volcanic process namely preparation activation and relaxation

teleseismic double difference relocation of earthquakes along the sumatra andaman subduction zone using a 3 d model 1 we have extended the double difference seismic tomography method to teleseismic distances with 3 d ray tracing conducted through nested regional global velocity models and applied the method to relocate seismicity from the sumatra andaman region before and after the great earthquakes of 2004 and 2005 we tested the algorithm s accuracy using both independent local data and an alternate relocation method and found good agreement between the results the use of depth phases differential times and a realistic 3 d velocity model improves the accuracy and precision of epicenters and focal depths systematically shifting them perpendicular to the trench and shallower respectively the relocations refine the location of the megathrust and other faults the patterns of aftershocks and their relation to slip during the two great earthquakes in addition the relocations reveal several discrete features not readily discernible in the scatter of teleseismic catalogs including an arcuate narrow band of earthquakes presumed to define the updip rupture limit in the 2005 event and a lineation at depth tracing the subduction of the investigator fracture zone when viewed in conjunction with tomography results the geometry and structural features of the subduction zone are revealed in unprecedented detail

heterogeneous coupling of the sumatran megathrust constrained by geodetic and paleogeodetic measurements geodetic and paleogeodetic measurements of interseismic strain above the sumatran portion of the sunda subduction zone reveal a heterogeneous pattern of coupling annual banding in corals provides vertical rates of deformation spanning the last half of the 20th century and repeated gps surveys between 1991 and 2001 and continuous measurements at gps stations operated since 2002 provide horizontal velocities near the equator the megathrust is locked over a narrow width of only a few tens of kilometers in contrast the locked fault zone is up to about 175 km wide in areas where great interplate earthquakes have occurred in the past formal inversion of the data reveals that these strongly coupled patches are roughly coincident with asperities that ruptured during these events the correlation is most spectacular for rupture of the m w 8 7 nias simeulue earthquake of 2005 which released half of the moment deficit that had accumulated since its previous rupture in 1861 suggesting that this earthquake was overdue beneath the mentawai islands strong coupling is observed within the overlapping rupture areas of the great earthquakes of 1797 and 1833 the accumulated slip deficit since these events is slowly reaching the amount of slip that occurred during the 1833 earthquake but already exceeds the slip that occurred during the 1797 earthquake thus rerupture of part of the mentawai patch in september 2007 was not a surprise in contrast coupling is low below the batu islands near the equator and around enggano island at about 5 s where only moderate earthquakes m w 8 0 have occurred in the past two centuries the correlation of large seismic asperities with patches that are locked during the interseismic period suggests that they are persistent features this interpretation is reinforced by the fact that the large locked patches and great ruptures occur beneath persistent geomorphologic features the largest outer arc islands depth and convergence rate dependent temperature might influence the pattern of coupling through its effect on the rheology of the plate interface but other influences are required to account for the observed along strike heterogeneity of coupling in particular subduction of the investigator fracture zone could be the cause for the low coupling near the equator

hydrothermal minerals and microstructures in the silangkitang geothermal field along the great sumatran fault zone sumatra indonesia detailed study of core samples of silicic tuff recovered from three geothermal wells along the strike slip great sumatran fault zone near silangkitang north sumatra supports a model for enhanced hydrothermal circulation adjacent to this major plate boundary fault two wells a and c were drilled nearly vertically 1 km southwest of the eastern i e the principal fault trace and the third directional well b was drilled eastward from the site of well a to within 100 m of the principal fault trace the examined core samples come from depths of 1650 2120 m at measured well temperatures of 180 320 c the samples collected near the principal fault trace have the highest temperatures the largest amount of secondary pore space that correlates with high secondary permeability and the most extensive hydrothermal mineral development secondary permeability and the degree of hydrothermal alteration decrease toward the southwestern margin of the fault zone these features indicate episodic localized flow of hot possibly co 2 rich fluids within the fault zone the microstructure populations identified in the core samples correlate to the subsidiary fault patterns typical of strike slip faults the geothermal reservoir appears to be centered on the fault zone with the principal fault strands and adjoining highly fractured and hydrothermally altered rock serving as the main conduits for vertical fluid flow and advective heat transport from deeper magmatic sources

lateral variation in slab orientation beneath toba caldera northern sumatra the investigator fracture zone ifz subducts beneath toba caldera the earth s largest quaternary caldera in northern sumatra suggesting a possible relationship between them locations of sub crustal earthquakes based on arrival times of p and s waves at a seismograph network surrounding toba reveal the geometry of the subducted slab and the ifz beneath toba a vertical tear of less than 20 km in the slab across the ifz as previously suggested cannot be ruled out but the large scale geometry of the slab is dominated by a broad bend of slab contours parallel to the concaveseaward indentation of the trench the slab shape is probably a response to the trench curvature can explain the change in trend of the volcanic arc near toba and may cause shallowing of the forearc basin near nias island the decrease in radius of curvature of the slab contours is not accompanied by an observable decrease in dip angle possibly resulting in lateral compressive stress in the slab the high rate of seismicity along the subducted investigator fracture zone that intersects the slab obliquely to its plunge direction is uncommon at subducted fracture zones and is likely caused by such lateral stress in the slab

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

a study of the hydrocarbon distribution and lower cretaceous greensand prospectivity in blocks 48 15 48 17 48 18 and 48 19 north celtic sea basin

implications of the diffuse deformation of the indian ocean lithosphere for slip partitioning of oblique plate convergence in sumatra oblique plate convergence between indian ocean lithosphere and continental crust of the sunda plate is distributed between subduction on the sunda megathrust and upper plate strike slip faulting on the sumatran fault zone in a classic example of slip partitioning over the last decade a destructive series of great earthquakes has brought renewed attention to the mechanical properties of these faults and the intervening forearc crustal block while observations of forearc deformation over the earthquake cycle indicate that the forearc crust is fundamentally elastic the spatial pattern of slip vector azimuths for earthquakes sourced by rupture of the sunda megathrust is strongly inconsistent with relative motion of two rigid plates permanent and distributed deformation therefore occurs in either the downgoing lithospheric slab or the overriding forearc crust previous studies have inferred from geodetic velocities and geological slip rates of the sumatran fault that the forearc crust is undergoing rapid trench parallel stretching using new geological slip rates for the sumatran fault and an updated decadal gps velocity field of sumatra and the forearc islands we instead show that permanent deformation within the forearc sliver is minor and that the sumatran fault is a plate boundary strike slip fault the kinematic data are best explained by diffuse deformation within the oceanic lithosphere of the wharton basin which accommodates convergence between the indian and australian plates and has recently produced several large earthquakes well offshore of sumatra the slip partitioning system in sumatra is fundamentally linked with the mechanical properties of the subducting oceanic lithosphere

the great sumatra andaman earthquakes imaging the boundary between the ruptures of the great 2004 and 2005 earthquakes segmentation along convergent margins controls earthquake magnitude and location but the physical causes of segment boundaries and their impact on earthquake rupture dynamics are still poorly understood one aspect of the 2004 and 2005 great sumatra andaman earthquakes is their abrupt termination along a common boundary this has led to speculation on the nature of the boundary its origin and why it was not breached for the first time the boundary has been imaged and with newly acquired marine geophysical data we demonstrate that a ridge on the subducting indo australian oceanic crust may exert a control on margin segmentation this suggests a lower plate influence on margin structure particularly its segmentation the ridge is masked by the sedimentary cover in the trench its most likely trend is nne ssw it is interpreted as a fracture zone on the subducting oceanic plate a ramp or tear along the eastern flank of the subducting fracture zone beneath simeulue island may be considered as an intensification factor in terms of rupture propagation barrier

a fluid inclusion study of cement and vein minerals from the celtic sea basins offshore ireland abstract fluid inclusion petrography and microthermometry have been carried out on jurassic sandstone cements quartz calcite and dolomite and later fracture filling calcite quartz veins from the north celtic sea and the fastnet basins fluid inclusions in the sandstone cements are small 1 8 m monophase liquid only or liquid rich two phase liquid vapour aqueous inclusions all display irregular to elliptical morphologies the two phase aqueous inclusions homogenise to the liquid phase between 60 122 c and have salinities ranging from 7 0 16 0 wt nacl equivalent indicating precipitation of the cements by moderate salinity waters in contrast the veins host both aqueous and petroleum inclusions the aqueous inclusions are liquid rich two phase liquid vapour inclusions 2 30 m long that are typically primary and show irregular and negative crystal shape morphologies their homogenisation temperatures to liquid range from 50 145 c and salinities from 6 20 wt nacl equivalent indicating broadly similar salinities coupled with a wider range of minimum trapping temperatures to those recorded from the cements yellow brown two phase liquid vapour petroleum inclusions are hosted by vein calcite from several wells in the central north celtic sea basin some of these inclusions appear to be primary and all are spatially associated with the aqueous inclusions the two phase liquid vapour petroleum inclusions homogenise mostly to the liquid between 50 130 c petroleum inclusions in the vein calcites indicate that petroleum was transported via fractures at a late stage through the jurassic and cretaceous strata of the celtic sea basins and apparently not through the sandstone matrix

slab geometry of the cascadia subduction zone beneath washington from earthquake hypocenters and teleseismic converted waves using earthquake hypocenters obtained from the washington regional seismograph network and analysis of ps converted phases from teleseisms at special temporary stations located south of the olympic peninsula we have developed a preliminary model of the structure of the subducted slab beneath western washington and southern british columbia the slab is smoothly arched beneath puget sound subducting at a dip of about 10 12 at the top of the arch north and south of the arch crest the slab dips 15 20 subcrustal earthquakes are localized near the top of the arch this deformation from planar slab geometry is caused by the change in azimuth of the trench or hinge line of the slab from almost ns south of 47 north latitude to approximately n30 w north of 48 north the slab appears to be easily flexed probably due to small elastic thickness but its geometry reflects conservation of surface area and minimization of in plane strain crustal seismicity in the puget sound basin may result indirectly from the presence of the arch in the underlying juan de fuca plate

the sipoholon geothermal field and adjacent geothermal systems along the north central sumatra fault belt indonesia reviews on geochemistry tectonics and natural heat loss abstract the sipoholon system in the tarutung basin is one of five geothermal systems associated with a c 100 km stretch of the sumatra fault system sfs in north central sumatra indonesia that exhibits parallel sets of dextral strike slip faults associated with seismic activity characteristics of discharged thermal fluids differ significantly between the systems the gas rich sipoholon field transfers only c 19 mw heat by ca so4 hco3 springs with sio2 fluid equilibria pointing to temperatures of 50 80 c at intermediate depths most of its thermal springs t in contrast all four thermal systems in the sarulla block concession to the southeast of the sipoholon field are convective high temperature systems that transfer significant heat to the surface fumaroles steaming ground thermal springs hot pools and thermal lakes discharge at least 50 mw heat at three of the four prospects where the temperatures of springs and fumaroles often reach boiling point many hot springs of the sarulla block systems discharge dominantly na hco3 cl type waters with partially equilibrated cation compositions a result of fluid rock equilibration and mixing processes sio2 fluid equilibria indicate 130 to c 200 c with cation geothermometers pointing to a high reservoir temperature 200 260 c for all four systems this was confirmed by the first deep exploratory wells drilled c 20 years ago in three prospects that encountered t 250 c at bottom hole 1 5 km depth the structural setting of the five geothermal fields is complex at tarutung it is associated with the narrow tarutung basin filled with volcanic debris and ash the basin exhibits a partly exposed dextral strike slip fault along its ne margin and can be interpreted in terms of a trans tensional pull apart basin two of the adjacent sarulla block systems are associated with large volcanic debris basins namora i langit and silangkitang that exhibit half graben structures in profiles normal to two large active faults of the sumatra fault system sfs the other two systems donotasik and sibualbuali are hosted by an elongated graben structure parallel and within the sfs and by a stratovolcano respectively

a comparative study of the sumatran subduction zone earthquakes of 1935 and 1984 a m s 7 7 earthquake struck the western equatorial coast of sumatra in december 1935 it was the largest event in the region since the two devastating giant earthquakes of 1833 and 1861 historical seismograms of this event from several observatories around the world provide precious information that constrains the source parameters of the earthquake to more precisely quantify the location geometry and mechanism of the 1935 event and to estimate the coseismic deformation we analyze the best of the available teleseismic historical seismograms by comparing systematically the records of the 1935 earthquake with those of a smaller event that occurred in the same region in 1984 first we constrain the source parameters of the 1984 event using teleseismic records then we compare the records of the 1935 event with those of 1984 from the same sites and instruments to do this we choose several time windows in the corresponding seismograms that contain clearly identifiable phases and deconvolve the modern event from the older one the deconvolutions result in very narrow pulses with similar sizes thus confirming similar locations and mechanisms for the events the initiation of the 1984 event was on the subduction interface at a depth of 27 2 km its m 0 is 6 5 x 10 19 n m m w is 7 2 the sense of slip was nearly pure thrust on a plane dipping 12 the 1935 event also involved rupture of the shallow subduction interface but was about five times larger m 0 3 3 x 10 20 n m m w 7 7 and initiated a few kilometers to the southeast along strike the 1935 rupture propagated unilaterally toward the southeast the along strike rupture length was about 65 km from these source parameters we calculate the surface deformations assuming an elastic multilayered medium these deformations compare favorably with those actually recovered from paleoseismic data in the form of coral microatolls

sedimentology of the cretaceous greensand quadrants 48 and 49 north celtic sea basin a progradational shoreface deposit

local magnitude coda magnitude and radiated energy of volcanic tectonic earthquakes from october 2010 to december 2011 at sinabung volcano indonesia in august 2010 sinabung volcano began erupting after more than a thousand years of dormancy following several weeks of phreatic eruptions the eruptions ceased and sinabung entered what became an inter eruptive period of dominantly seismic unrest while standard equations for understanding the size of an earthquake local magnitude ml coda magnitude mc and seismic energy release er have long been developed it is best practice to fine tune these relations for a given region and period of study to more accurately describe seismicity and to directly compare it with other volcanic systems more accurate descriptions of magnitudes and energy release are vital to accurate volcanic eruption forecasting and evaluation of seismic and volcanic risk in this study we use high frequency volcano tectonic vt earthquakes recorded on a temporary three component network installed between october 2010 and december 2011 in the region around sinabung volcano to better constrain the seismic parameters of and better understand this previously unstudied volcano we determine region specific formulas for ml mc and er as follows mathrm m mathrm l log 10 a 1 1252 log 10 r 0 0280 r 2 5427 kern0 5em mathrm m mathrm c 0 7764 log 10 t coda 0 0676 r 0 7185 kern0 5em mathrm and kern0 5em log 10 left mathrm e mathrm r right 1 5720 mathrm m mathrm l 11 5258 where a r and tcoda are maximum amplitude on a wood anderson seismogram hypocentral distance km and the coda duration s respectively constants in the ml equation have physically interpretable meanings the constant for the geometrical spreading term log10r term equals one for perfect spherical spreading of the waveform our value is greater than one and thus suggests that wavefronts spread at a slightly different rate than for simple spherical spreading the constant for the attenuation term r term is consistent with locally mapped attenuative deposits limestones and tuffs and previous 3d tomographic results our mc equation differs from a previous study likely because different data in a different time period were used earthquake hypocenters are consistent with those located in previous tomographic studies and we interpret the earthquakes in this study as distal vt earthquakes induced by continued magmatic intrusion at sinabung over the period of october 2010 december 2011

earthquakes plate subduction and stress reversals in the eastern aleutian arc plate subduction beneath the 1500 km long segment of the eastern aleutian arc between kodiak and atka islands 154 w and 176 w longitude is studied with observations from teleseismic data the primary data base consists of hypocenters of earthquakes for the period 1965 1975 carefully selected from the bulletins of the international seismological centre and of 44 new focal mechanism solutions the principal results of this study are that hypocenters of intermediate depth earthquakes in the eastern aleutians appear to define a weakly developed double seismic zone at depths between 70 and 170 km additional evidence for a double seismic zone comes from focal mechanisms which generally show downdip directed p axes for earthquakes in the upper zone and downdip directed t axes in the lower zone major features of the double zone can be explained by thermoelastic stresses in the downgoing plate the observed predominant downdip stress polarity at intermediate depths in the descending plate reverses along strike of the arc this stress reversal coincides in map view with a change from a continental to an oceanic arc the coincidence may result from spatial differences either in the coupling between the plates at shallow depths or in the rheology of the surrounding oceanic versus continental mantle alternatively the stress reversal may be related to the time since the last great earthquake if the observed stress polarity relates to the elapsed time since the last great earthquake it may be a significant observation for long to intermediate term prediction of great thrust earthquakes in subduction zones portions of the eastern aleutian arc where downdip tension predominates contain one or more seismic gaps that appear to have a high probability for great earthquakes in the next few decades a moderate sized bathymetric feature in the pacific plate the amlia fracture zone modifies subduction and produces a bend or offset in the downgoing plate as well as an offset in the line of active volcanoes in the upper plate subduction of the fracture zone appears also to cause formation of summit basins on the arc platform and some aseismic behavior of a 100 km long segment of the main thrust zone

lake toba volcano magma chamber imaged by ambient seismic noise tomography 1 for the first time ambient noise tomography is used to clearly image the magma chamber beneath lake toba caldera one of the largest quaternary calderas on earth using data from 40 seismic stations deployed between may and october 2008 around lake toba empirical green s functions are extracted from long term cross correlations of continuous records these functions are dominated by rayleigh waves whose group velocities can be measured in the period range from 2 5 to 12 seconds arrival times of these waves are picked for a given period and inverted using 2 d tomography to calculate lateral variations in velocity for the given period this was done for six different periods which all correspond to different sampling depths thus the six 2 d models presented together provide information on velocity variations with depth the results show a low velocity body coincident with the lake toba caldera representing the magma chamber under the volcano the chamber is observed to have a complex 3 d geometry with at least two separate sub chambers underlying the caldera other results include a deep low velocity body possibly another magma chamber south west of the lake with an upper limit of 7 km depth the maximum depth to which this body reaches could not be resolved the sumatra fault marks a velocity contrast but only down to depths not greater than 5 km the reliability of the results was further confirmed by checkerboard recovery tests

complex slab subduction beneath northern sumatra 1 new data provided by the 2004 2005 sumatra andaman great earthquake sequences allow us to image with improved detail the p wave velocity structure beneath sumatra and adjacent regions below northern sumatra we find that the slab is folded at depth exhibiting geometry similar to that of the volcanic arc and the trench at the surface we speculate that this fold plays a major role in the segmentation of the sumatra megathrust and may impede rupture propagation in the region north of sumatra significant slab material in the mantle transition zone is imaged for the first time and we infer the presence of a major tear between the upper mantle and transition zone there

high pressure metamorphism and uplift of the olympic subduction complex the discovery of the critical assemblage lawsonite quartz calcite indicates that a significant part of the cenozoic olympic subduction complex of northwestern washington state formed by underplating at a depth of about 11 km the deep structural level exposed in this area is attributed to the presence of a 10 km high arch in the underlying juan de fuca plate we postulate that this arch was formed when the southern cordilleran coastline swung westward as a result of middle miocene to recent extension in the basin and range province

seismic history and seismotectonics of the sunda arc historic records of the last 300 years reveal two great interplate earthquakes 1833 mw 8 1861 mw 8 8 which ruptured major segments of the sumatra fore arc in western indonesia a significant percentage of interplate slip along this portion of the plate boundary appears to occur seismically the ends of these rupture zones are coincident with clusters of large ms 7 and moderate 6 ms 7 shocks and with heterogeneities in the plate interface as inferred from geologic and geophysical data the northern extent of the 1833 rupture zone is coincident with a group of moderate and large earthquakes a structural arch in the fore arc and a fracture zone on the subducted plate the southern portion is coincident with a very prominent cluster and the along strike extension of a sediment filled trough on the subducted plate prior to this study sumatra was characterized as relatively aseismic as inferred from the lack of great earthquakes in the instrumental record of this century java and the lesser sunda islands had major ms 6 earthquakes in the historic record but none are of the same magnitude as the great events near sumatra there is no clear evidence that shocks in the java fore arc were interplate events yet geophysical data indicate recent uplift of the trench slope break and a change in strike of the trench axis where the northernmost flank of the roo rise a prominent bathymetrie high appears to be interacting with the fore arc this feature was formed at a ridge crest and is more buoyant and thus more difficult to subduct than the surrounding seafloor there is no increase in seismicity associated with this tectonism however that may indicate a lack of contact between the crystalline portions of the plates at this time we infer that a majority of slip on the plate interface near java occurs aseismically the variation in occurrence of great interplate earthquakes along the sunda arc may be interpreted in terms of a model that attributes variation in interplate coupling to the age of the subducted lithosphere great interplate earthquakes occur near sumatra where the age of the youngest crust is 46 ma the characteristics of the sunda arc and analogies with pacific arcs imply that the entire length of sumatra has the potential to produce great thrust earthquakes its seismic potential should be considered high as there has been no recurrence of the great events of 1833 and 1861 in contrast no such events have been reported off java and the lesser sunda islands where the oldest crust is 152 ma the plate interface near java and the lesser sunda islands should be considered to have low seismic potential

distribution of magma beneath the toba caldera complex north sumatra indonesia constrained by three dimensional p wave velocities seismicity and gravity data 1 we estimate the one and three dimensional p wave velocity structure beneath the toba caldera complex a 30 100 km topographic depression in north sumatra using arrival time data of local earthquakes recorded by a 40 station seismic network that operated for 4 months inversions reveal the presence of p velocities up to 37 below normal that likely map the distribution of magma within this subduction related volcanic system considered the world s largest in the upper 10 km of crust the largest low velocity region underlies the southern two thirds of the depression and coincides with a gravity low centered over the resurgent dome a smaller volume of low velocities is observed in the upper crust under the north end of the depression separating the two regions is a zone of locally high velocities indicating that the shallow subcaldera magma system is composed of two separate reservoirs not a single one that extends the entire length of the caldera complex above each low velocity region is a postcollapse volcano that erupted mostly mafic lavas after the last major caldera collapse 74 kyr ago a low velocity column below one of these volcanoes can be traced into the uppermost mantle and corresponds with a planar distribution of low frequency earthquakes in the 20 to 40 km depth range the low frequency earthquakes apparently record the migration of melt in the mafic roots of this large volume crustal magma system

teleseismic relocation and assessment of seismicity 1918 2005 in the region of the 2004 mw 9 0 sumatra andaman and 2005 mw 8 6 nias island great earthquakes the m w 9 0 2004 sumatra andaman islands and m w 8 6 nias island great earthquake sequences have generated over 5000 catalog reported earthquakes along 1700 km of the sumatra andaman and western sunda regions studies of prior regional seismicity have been limited to global catalog locations that often have poorly constrained epicenters and depths approximately 3650 teleseismically well recorded earthquakes occurring in this region during the period 1918 2005 are relocated with special attention to focal depth reduced uncertainties of epicenters and depths in the region on the order of 15 and 10 km respectively foster interpretation of focal mechanism data and provide additional details about the subducting indian and australian plates the revised earthquake dataset reveals a sharp delineation between aftershocks of the 2004 and 2005 earthquakes near simeulue island and a steepening in slab dip from south to north the downdip width of the aftershock zone of the 2004 m w 9 0 earthquake varies from 200 km at its northern end to 275 km at its southern end and events located between 35 and 70 km focal depth occur more frequently in the southernmost section of this aftershock zone outer rise and near trench normal and strike slip faulting earthquakes also increase in frequency following the 2004 and 2005 earthquakes earthquake swarms triggered along the andaman backarc spreading center both north of sumatra and near siberut island 100 km south of the nias island aftershock sequence illustrate the complex and variable nature of seismicity following these great earthquakes

systematic variation in anisotropy beneath the mantle wedge in the java sumatra subduction system from shear wave splitting the tectonic context of south east asia is dominated by subduction one such major convergent boundary is the java sunda trench where the australian indian plates are being subducted beneath the eurasian plate we measure shear wave splitting in local and teleseismic data from 12 broadband stations across sumatra and java to study the anisotropic characteristics of this subduction system which can provide important constraints on dynamical processes involved splitting in s waves from local earthquakes between 75 and 300 km deep show roughly trench parallel fast directions and with time lags 0 1 1 3 s 92 0 6 s splitting from deeper local events and sks however shows larger time lags 0 8 2 0 s and significant variation in fast direction in order to infer patterns of deformation in the slab we apply a hybrid modelling scheme we raytrace through an isotropic subduction zone velocity model obtaining event to station raypaths in the upper mantle we then apply appropriately rotated olivine elastic constants to various parts of the subduction zone and predict the shear wave splitting accrued along the raypath finally we perform grid searches for orientation of deformation and attempt to minimise the misfit between predicted and observed shear wave splitting splitting from the shallow local events is best explained by anisotropy confined to a 40 km over riding plate with horizontal trench parallel deformation however in order to explain the larger lag times from sks and deeper events we must consider an additional region of seismic anisotropy in or around the slab the slab geometry in the model is constrained by seismicity and regional tomography models and many sks raypaths travel large distances within the slab models placing anisotropy in the slab produce smaller misfits than those with anisotropy outside for most stations there is a strong indication that inferred flow directions are different for sub sumatran stations than for sub javanese with 60 change over 375 km the former appear aligned with the subduction plate motion whereas the latter are closer to perpendicular parallel to the trench direction there are significant differences between the slab being subducted beneath sumatra and that beneath java age of seafloor maximum depth of seismicity relative strength of the bulk sound and shear wave velocity anomaly and location of volcanic front all vary along the trench we speculate therefore that the anisotropy may be a fossilised signature rather than due to contemporary dynamics

converted wave imaging of the toba caldera indonesia 1 receiver functions calculated from data recorded by the 1995 passcal toba seismic experiment in northern sumatra indonesia and the permanent station psi reveal the presence of a low seismic velocity zone at between 8 and 14 km depth which is interpreted to be the top of a magma reservoir the moho under the toba caldera lies near 30 km with a localized thickened region that reaches 39 km beneath the eastern edge of the caldera with an abrupt shallowing to the northeast this thickened crust may be where hot gabbroic material underplates the base of the crust to serve as the heat source for shallower volcanism alternatively the thickened crust and moho offset may be due to localized crustal thickening when this region was situated in a sharp restraining bend of a now extinct segment of the sumatran fault

persistent termini of 2004 and 2005 like ruptures of the sunda megathrust to gain insight into the longevity of subduction zone segmentation we use coral microatolls to examine an 1100 year record of large earthquakes across the boundary of the great 2004 and 2005 sunda megathrust ruptures simeulue a 100 km long island off the west coast of northern sumatra indonesia straddles this boundary northern simeulue was uplifted in the 2004 earthquake whereas southern simeulue rose in 2005 northern simeulue corals reveal that predecessors of the 2004 earthquake occurred in the 10th century ad in ad 1394 2 and in ad 1450 3 corals from southern simeulue indicate that none of the major uplifts inferred on northern simeulue in the past 1100 years extended to southern simeulue the two largest uplifts recognized at a south central simeulue site around ad 1422 and in 2005 involved little or no uplift of northern simeulue the distribution of uplift and strong shaking during a historical earthquake in 1861 suggests the 1861 rupture area was also restricted to south of central simeulue as in 2005 the strikingly different histories of the two adjacent patches demonstrate that this boundary has persisted as an impediment to rupture through at least seven earthquakes in the past 1100 years this implies that the rupture lengths and hence sizes of at least some future great earthquakes and tsunamis can be forecast these microatolls also provide insight into megathrust behavior between earthquakes revealing sudden and substantial changes in interseismic strain accumulation rates

development of small gas fields in the kinsale head area a number of potential development prospects have been identified in the area of the kinsale head gas field in the celtic sea using the recent development of the ballycotton gas field as a model this paper discusses how similar fields might be developed as satellites of the existing production platforms the ballycotton gas field was discovered in march 1989 by an exploration well drilled in block 48 20 by the western pacesetter iv a further two wells confirmed the extent of the gas accumulation see taber et al 1995 the field is located approximately 25 miles off the coast of cork and 8 miles north of the kinsale head gas field fig 1 which up to that time was ireland s only producing gas field colley et al 1981 the existence of gas processing facilities relatively close to the discovery well was a major factor in determining the technical and commercial viability of the ballycotton gas field owing to the relatively small size of the field and the predicted initial flow rate of over 50 mmscfd which could be obtained from the primary lower cretaceous a sandstone reservoir it was decided to tie the discovery well 48 20 2 back to one of the kinsale head platforms this was made possible by means of sub sea completion techniques whereby the necessary production equipment is installed underwater at the well site and the well is remotely controlled from a mother platform which also processes the gas this obviates the need for a permanent platform or a floating production system both