enhanced reflectivity of backthrusts in the recent great sumatran earthquake rupture zones in the last five years there have been three great megathrust earthquakes in the sumatra subduction zone and a part of the zone is still locked we have carried out deep seismic reflection surveys along the sw sumatran margin and have imaged backthrusts along four profiles down to 15 20 km depth we find that the seismic images of the backthrusts in the 2004 and 2007 great earthquake ruptured zones are brighter than those in the regions where the subduction zone is still locked we suggest that this enhanced reflectivity is due to the increase of fluid contents along the reactivated backthrusts during or soon after the great earthquakes if this interpretation is valid then the backthrusts observed in the forearc basins could be used to monitor fluid movement from mantle wedge and earthquake precursors before and after an imminent great earthquake in the sumatra locked zone

a self consistent model of melting magma migration and buoyancy driven circulation beneath mid ocean ridges numerical modeling and analysis are used to investigate the processes leading to the eruption of mantle derived magma at mid ocean ridges our model includes the following effects melting due to decompression magma migration by percolation and circulation of the mantle driven by both the oceanic plates and the distribution of buoyancy beneath the ridge the distribution of buoyancy is due to both the low density of the liquid and the difference in the density of the residual solids relative to unmelted mantle material the calculation of densities is based on a simple petrological model in which garnet spinel lherzolite melts to form a basaltic liquid and a harzburgite residue we find that the system spontaneously evolves to a state in which a rapid upwelling beneath the ridge axis faster than the plate velocity is confined laterally by stably stratified residual material beneath the newly formed plates this effect is exaggerated if a modest decrease in the shear viscosity of the solid upon melting is included our results provide a simple explanation for the narrowness of the zone of crustal formation at mid ocean ridges the model also predicts a transition from steady state to episodic crustal formation as the spreading velocity is reduced perhaps giving rise to along axis variations in the character of seafloor spreading the narrow rapid upwelling gives rise to substantial porosities at depths that are a large fraction of the depth to the solidus this may allow the liquid at depth to segregate into macroscopic channels which would account for the consensus from experimental petrology that the liquids parental to morb are derived from well below the base of the crust

applying benford s law to volcanology benford9s law predicts that the distribution of first digits of real world observations is not uniform but instead the lower digits 1 2 and 3 occur more frequently than the higher ones 8 9 it has been shown that the use of benford9s law may help as a validity check on databases and that the first digit rule may provide new ways to detect anomalous signals in data sets in fact nonconformity to benford9s law could be an indicator of 1 incompleteness 2 excessive data round off or 3 data errors inconsistencies or anomalies this law has long been known but has received little attention in earth sciences in this work we first test the conformity of three volcanology related data sets and then we consider the relevance and potential utility of using benford9s law to assess the integrity and authenticity of the presented volcanological data the first two data sets are the area in square kilometers and age in years of collapse calderas extracted from the collapse caldera database ccdb covering areas from 0 03 to 4700 km 2 and ages from a few years to 2000 ma the third data set is the duration in days of the volcanic eruptions that occurred between a d 1900 and 2009 according to the smithsonian9s global volcanism program catalogue http www volcano si edu results obtained indicate that the volcanological data sets of this study follow benford9s law the present analysis shows that excessive data round off data errors or anomalies may be detected when comparing the data with benford9s law expected frequencies

evidence of active backthrusting at the ne margin of mentawai islands sw sumatra summary the indo australian plate subducts obliquely beneath the sunda plate leading to a slip partitioning into pure thrust and strike slip motion just in the last 5 yr three pure thrust earthquakes of mw 8 4 have occurred along this subduction interface the great sumatra fault traversing the sumatra continental block takes up a significant part of the strike slip motion but the mentawai fault bounding the ne margin of mentawai islands has been suggested to accommodate a part of the strike slip motion although the great sumatra fault is active no seismicity has been observed along the mentawai fault using a combination of high resolution seismic reflection and bathymetry data here we show that the mentawai fault seems to be characterized by active sw dipping backthrusts the presence of recent steeply dipping thrust earthquakes suggests that these faults should be active combined with results from north in 2004 earthquake region and south of this study area our results suggest that backthrusting should play an important role in forearc evolution sw of sumatra we also observed several mass wasting sites at ne margin of the mentawai islands which could be erosional features or landslides triggered by earthquake activities localized uplift along the steeply dipping backthrusts at the ne margin of mentawai islands in the fully locked region could pose serious seismic and tsunami risks to the sw coast of sumatra in the future

tsunamigenic potential due to frontal rupturing in the sumatra locked zone abstract the sumatra subduction zone is one of the most seismically active subduction zones although there have been three m w 8 4 earthquakes in the region including the disastrous 2004 m w 9 2 sumatra andaman earthquake a 500 km long patch around mentawai islands is still locked and could produce a large megathrust earthquake if the rupture propagates to the subduction front as it most likely occurred during the 2004 earthquake it may lead to a devastating tsunami here we present high resolution reflection seismic data from the sumatra locked zone that shows the subduction interface down to 20 km depth the seismic data also show that the wedge is composed of two layers a shallow layer formed by mixed to landward vergent thrusts termed as pop ups and a deeper layer showing sub horizontal reflectors the lower layer is most probably formed by duplexes whose roof serves as a pseudo decollement for the mixed to landward thrust systems based on the seismic results we perform mechanical modeling in order to understand the formation of these structures and to retrieve the associated frictional properties we first show that the activation of the pseudo decollement requires 1 either a sudden increase of effective friction along the plate interface or an irregular geometry of the plate interface 2 a lower effective friction along the pseudo decollement than along the plate interface we then show that low effective frictional values 0 1 are required to reproduce the observed frontal structures the low effective friction along the pseudo decollement could either be due to the presence of a long term high pore pressure layer or to dynamic weakening associated with earthquakes since similar structures are present in the 2010 tsunami earthquake area we favor the dynamic weakening hypothesis according to the mechanical modeling if the next rupture propagates up to the toe rupturing the three most frontal pop up structures we could expect at least 5 5 to 9 2 m of frontal horizontal displacement and a frontal uplift of 2 to 6 6 m along the frontal thrusts this would amplify the uplift of the water column and as a consequence generate a large tsunami

deepening of the axial magma chamber on the southern east pacific rise toward the garrett fracture zone a wide aperture profile along the ridge axis from 14 29 s to 13 39 s 120 km to 30 km south of the garrett fracture zone is analyzed to constrain the thickness of layer 2a and the depth to the axial magma chamber reflector five areas along the 90 km line are examined in detail with several consecutive gathers being analyzed for each area to establish the degree of consistency within each area a genetic algorithm code is used to find a best fit model from a comparison of the data and wkbj synthetic seismograms one hundred starting models are generated using a predefined set of velocity nodes with a fixed window of allowable depth variations between nodes an evolutionary process favors the better fitting models in each generation and a satisfactory misfit is usually obtained within 40 generations within individual areas the models were in good agreement with the depth of a given velocity node generally varying by not more than 20 m the depth discretization interval for the models a consistent deepening trend of the axial magma chamber amc is observed across the five areas as the garrett fracture zone is approached the depth varies from 0 99 km at area 1 which is approximately 100 km south of the garrett to 1 23 km at area 5 which is approximately 40 km south of the garrett the depth to the axial magma chamber is highly sensitive to any ship wander off axis since layer 2a thickens rapidly off axis with age for the areas examined here layer 2a is observed to be relatively constant in thickness along the axis although it is about 40 m thicker over area 5 where the axial magma chamber is deepest this variation is within the scatter of previously detailed layer 2a measurements at 13 n on the east pacific rise where an effectively constant thickness is observed this implies that layer 2a thickening is not a significant factor along this profile and that the amc deepening is real rather than apparent theoretical modeling suggests that the depth to the lid of the axial magma chamber is related to the rate of heat supply at a given location thus the gradual consistent deepening of the axial magma chamber can be taken as an indication of a slightly reduced magma supply toward the garrett fracture zone which marks a major interruption of hundreds of kilometers of continuous ridge axis the deepening may also be interpreted as a downward limb from a central injection point however there is no indication of a similar downward trend in the other southern direction furthermore there is no accompanying systematic variation in axial depth or axial volume both of which are proposed to be indicators of central injection and along axis flow

penultimate predecessors of the 2004 indian ocean tsunami in aceh sumatra stratigraphic archeological and historical evidence we present stratigraphic archeological and historical evidence for two closely timed predecessors of the giant 2004 tsunami on the northern coast of aceh northern sumatra this is the first direct evidence that a tsunami played a role in a fifteenth century cultural hiatus along the northern sumatran portion of the maritime silk route one seacliff exposure on the eastern side of the lambaro headlands reveals two beds of tsunamigenic coral rubble within a small alluvial fan radiocarbon and uranium thorium disequilibrium dates indicate emplacement of the coral rubble after 1344 3 c e another seacliff exposure on the western side of the peninsula contains evidence of nearly continuous settlement from 1240 c e to soon after 1366 3 c e terminated by tsunami destruction at both sites the tsunamis are likely coincident with sudden uplift of coral reefs above the sunda megathrust 1394 2 c e evidence for which has been published previously the tsunami or tsunami pair appears to have destroyed a vibrant port community and led to the temporary recentering of marine trade dominance to more protected locations farther east the reestablishment of vibrant communities along the devastated coast by about 1500 ce set the stage for the 2004 disaster

velocity structure of a gas hydrate reflector seismic reflection profiles across many continental margins have imaged bottom simulating reflectors bsrs parallel to the seabed these are often interpreted as the base of a zone in which methane hydrate ice is stable waveform inversion of seismic reflection data can be used to estimate from seismic data worldwide the velocity structure of a bsr and its thickness a test of this method at a drill site of the ocean drilling program predicts that sediment pores beneath the bsr contain free methane for approximately 30 meters the hydrate and underlying gas represent a large global reservoir of methane which may have economic importance and may influence global climate

reply to discussion by p j barnett on generation of heavy mineral glacial indicator dispersal trains from a diabase sill nipigon region northwestern ontario larson and mooers 1753 barnett 2006 poses a number of questions regarding some of the field observations and assumptions we used in our interpretation of a heavy mineral dispersal train derived from an isolated eastern outlier of diabase of the nipigon sills in northwestern ontario larson and mooers 2005b he also brings attention to some details of the regional glacial geologic setting we did not address in our analysis and interpretation of the dispersal train and highlights new field data barnett and dyer 2005 relevant to interpretation of past glacial processes acting in the broader nipigon region we thank barnett for his comments and feel they serve in part to highlight the variety of problems regarding glacial erosion and entrainment and transport to which we believe our conceptual model of glacial indicator dispersal larson and mooers 2004 is able to contribute barnett 2006 brings attention to the fact that tills exposed at surface in the beardmore geraldton region contain a wide range 1 up to 23 in carbonate content thorleifson and kristjansson 1993 high carbonate contents in these tills are a clear signal of long distance transport of carbonate bearing debris from the hudson bay lowlands the occurrence of high carbonate tills in patches of thick streamlined drift has been interpreted by hicock 1988 and hicock and others 1989 to mean that all surface tills in this region were deposited at the base of a fastflowing ice stream however the ice stream model does not explain the widely disparate values in till carbonate occurring over distances of only a few kilometres we believe a different interpretation explains the same data larson and mooers 2005a tills sampled in our limited study area contained virtually none of the paleozoic carbonate and proterozoic greywacke clasts characteristic of high carbonate tills found in the region we believe this observation is inconsistent with an origin of the till forming material by longdistance transport of debris from the hudson bay lowlands by an ice stream and rather that it reflects local erosion and entrainment and short distance transport however the origin of the tills we sampled in our study area is not relevant to the interpretations we presented determining whether the till in our study area was generated by erosion and entrainment into the englacial basal debris layer of the ice sheet by erosion and entrainment in a subglacial deforming debris layer or by another process was beyond the scope of our study we deliberately made no explicit assertion regarding the processes by which diabase was eroded and entrained and transported we did however attempt to document the rates at which the erosion and entrainment process worked the only explicit assumptions we incorporated into our interpretations were of the debris mass per unit bed area m and of the velocity at which the basal debris layer was transported u these values were used to estimate the erosivity e recorded in the till sheet and indirectly the absolute erosion and entrainment rate e of diabase into the basal debris layer examination of the equations presented in larson and mooers 2004 and the modified equations presented in larson and mooers 2005b shows that increasing the estimated m results in increased estimates of the values of e and e increasing u likewise results in increased estimates of e however estimates of the erosion length scale and the relative rates of erosion of the diabase and greenstone are unaffected by varying these quantities we feel that the m and u estimates that we incorporated into our interpretation of the heavy mineral dispersal data represent reasonable values for an ice sheet and that the resulting e estimates are likewise reasonable it is important to note that the absolute values of erosion and entrainment we have estimated for diabase and greenstone lithologies lie within the range documented for modern glacial systems cf hallet et al 1996 barnett 2006 questions our conclusion that the diabase sill in our study area was significantly more resistant to glacial erosion and entrainment than the archean greenstones into which it was intruded notwithstanding the higher susceptibility of gabbro diabase to physical and chemical

seismic images of the sliver strike slip fault and back thrust in the andaman nicobar region 1 the sliver strike slip great sumatra fault gsf traverses mainland sumatra from the sunda strait in the southeast to banda aceh in the northwest and defines the present day plate boundary between the sunda plate in the north and the burmese sliver plate in the south it has been well studied on mainland sumatra but poorly north of banda aceh in the andaman sea here we present deep seismic reflection images along the northward extension of the gsf over 700 km until it joins the andaman sea spreading centre and we interpret these images in the light of earthquake gravity and bathymetry data we find that the gsf has two strands between banda aceh and nicobar island a transpression in the south and a deep narrow active rift system in the north dotted with volcanoes in the center suggesting that the volcanic arc is coincident with rifting farther north of nicobar island an active strike slip fault the andaman nicobar fault cuts through a rifted deep basin until its intersection with the andaman sea spreading centre the volcanic arc lies just east of the rift basin the western margin of this basin seems to be a rifted continental margin tilted westward and flooring the andaman nicobar fore arc basin the andaman nicobar fore arc basin is bounded in the west by back thrusts similar to the west andaman and mentawai faults the cluster of seismicity after the 2004 great andaman sumatra earthquake just north of nicobar island coincides with the intersection of two strike slip fault systems

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

convection and crystallization in magma cooled from above calculations are presented for the cooling from above of melts in the di an system in which the kinetics of crystallization are incorporated and play a dominant role the results indicate that even with no initial superheat whatsoever convection plays an important role in the cooling of magma chambers and allows substantial internal cooling crystallization and differentiation the calculations show in agreement with observations that in magma bodies hundreds of meters thick crystallization occurs predominantly in the interior or at the floor even though heat is lost only from the roof the ratio of the final thickness of the layer formed at the floor to that formed at the roof increases as the overall size of the chamber increases owing to the effects of convection

modelling the anisotropic seismic properties of partially molten rocks found at mid ocean ridges abstract the problem of modelling the seismic properties of mid ocean ridge rocks of the axial magma chamber amc and the low velocity triangle lvt immediately below it has been addressed using two samples from oman ophiolite as examples the specimens are a layered gabbro from the lower oceanic crustal sequence and a harzburgite from the upper most mantle section at the palaeo ridge axis the seismic properties have been simulated at a temperature of 1200 c and pressure of 200 mpa so that the basalt melt is above its solidus various effective medium methods are discussed in the perspective of modelling rocks that have a strong background elastic anisotropy due to crystal preferred orientation cpo and an introduced anisotropy due to oriented melt filled inclusions calculations using various methods show a wide range of predicted melt fractions for a given seismic velocity the self consistent scheme scs and differential effective medium dem are compared in some detail a tensorial model was developed using a poro elastic method of gassman at low seismic frequency and a standard dem with isolated basalt inclusions at high frequency at low frequency the basalt pore fluid is considered to be everywhere connected and the pressure uniform whereas at high frequency basalt inclusions are isolated assuming the medium to be a standard linear solid the low and high frequency velocities were used to calculate the anisotropy of attenuation calculations with spherical basalt inclusions show that the seismic velocities decrease and attenuation increase with increasing melt fraction the symmetry of the background anisotropy due to cpo is preserved but gradually reduced with increasing melt fraction for the seismic velocities observed for the axial magma chamber amc with v p of 3 5 to 4 7 km s the model predicts 50 70 melt for the low velocity triangle lvt below the amc with v p of 4 8 5 6 km s the model predicts 35 50 melt predictions from the observed attenuations q 1 at the amc of 0 05 0 02 and in the lvt of 0 02 0 01 are around 60 and 45 melt respectively seismic anisotropy has been modelled with ellipsoidal basalt pancake shaped inclusions with their circular sections in the xy foliation plane only a 2 3 of ellipsoidal basalt inclusions are required to over print the anisotropy of the background medium there is a rapid decrease in v p and increase of q 1 for propagation normal to the foliation z with increasing axial ratio of the inclusions with increasing axial ratio the model predicts decreasing amounts of melt for given velocity v p in the z direction for the lvt only 15 25 melt is predicted for a 10 1 inclusion instead of the 35 50 of the spherical inclusion

generation of a heavy mineral glacial indicator dispersal train from a diabase sill nipigon region northwestern ontario the heavy clinopyroxene mineral pigeonite forms a glacial indicator dispersal train originating from diabase intrusions in the nipigon region of northwestern ontario analysis and interpretation of the pigeonite dispersal pattern adjacent to the up ice portion of the diabase provides a number of insights into the nature of glacial erosion of bedrock and the generation of heavy mineral dispersal trains bedrock erosion and entrainment rates at the time of pigeonite dispersal train formation were high 3 14 mm a 1 suggesting that bedrock erosion was rapid yet spatially and temporally restricted contrasting erosion rates between the diabase and surrounding greenstone lithologies suggests that modern shield topography is not an assemblage of equilibrium bedforms with respect to the ice sheet this agrees with hypothesized low total erosion of shield bedrock during the pleistocene pigeonite grain size coarsens over the diabase source indicating that most of the pigeonite was quarried from outcrops as coar

structure and seismogenic properties of the mentawai segment of the sumatra subduction zone revealed by local earthquake traveltime tomography 1 on 12 september 2007 an mw8 4 earthquake occurred within the southern section of the mentawai segment of the sumatra subduction zone where the subduction thrust had previously ruptured in 1833 and 1797 traveltime data obtained from a temporary local seismic network deployed between december 2007 and october 2008 to record the aftershocks of the 2007 event was used to determine two dimensional 2 d and three dimensional 3 d velocity models of the mentawai segment the seismicity distribution reveals significant activity along the subduction interface and within two clusters in the overriding plate either side of the forearc basin the downgoing slab is clearly distinguished by a dipping region of highvp 8 0 km s which can be a traced to 50 km depth with an increased vp vs ratio 1 75 to 1 90 beneath the islands and the western side of the forearc basin suggesting hydrated oceanic crust above the slab a shallow continental moho of less than 30 km depth can be inferred suggesting that the intersection of the continental mantle with the subducting slab is much shallower than the downdip limit of the seismogenic zone despite localized serpentinization being present at the toe of the mantle wedge the outer arc islands are characterized by low vp 4 5 5 8 km s and high vp vs greater than 2 0 suggesting that they consist of fluid saturated sediments the very low rigidity of the outer forearc contributed to the slow rupture of the mw 7 7 mentawai tsunami earthquake on 25 october 2010

quantifying offshore fore arc deformation and splay fault slip using drowned pleistocene shorelines arauco bay chile most of the deformation associated with the seismic cycle in subduction zones occurs offshore and has been therefore difficult to quantify with direct observations at millennial time scales here we study millennial deformation associated with an active splay fault system in the arauco bay area off south central chile we describe hitherto unrecognized drowned shorelines using high resolution multibeam bathymetry geomorphic sedimentologic and paleontologic observations and quantify uplift rates using a landscape evolution model along a margin normal profile uplift rates are 1 3 m ka near the edge of the continental shelf 1 5 m ka at the emerged santa maria island 0 1 m ka at the center of the arauco bay and 0 3 m ka in the mainland the bathymetry images a complex pattern of folds and faults representing the surface expression of the crustal scale santa maria splay fault system we modeled surface deformation using two different structural scenarios deep reaching normal faults and deep reaching reverse faults with shallow extensional structures our preferred model comprises a blind reverse fault extending from 3 km depth down to the plate interface at 16 km that slips at a rate between 3 0 and 3 7 m ka if all the splay fault slip occurs during every great megathrust earthquake with a recurrence of 150 200 years the fault would slip 0 5 m per event equivalent to a magnitude 6 4 earthquake however if the splay fault slips only with a megathrust earthquake every 1000 years the fault would slip 3 7 m per event equivalent to a magnitude 7 5 earthquake

comment on subglacial erosion and englacial sediment transport modeled for north american ice sheets by d h d hildes g k c clarke g e flowers s j marshall

earthquake supercycles inferred from sea level changes recorded in the corals of west sumatra records of relative sea level change extracted from corals of the mentawai islands sumatra imply that this 700 kilometer long section of the sunda megathrust has generated broadly similar sequences of great earthquakes about every two centuries for at least the past 700 years the moment magnitude 8 4 earthquake of september 2007 represents the first in a series of large partial failures of the mentawai section that will probably be completed within the next several decades

blueschist metamorphism in an active subduction zone the high pressure low temperature metamorphic rocks known as blueschists have long been considered to form in subduction zones where the descent of a relatively cold slab leads to the occurrence of unusually low temperatures at mantle pressures until now however the link between blueschist facies rocks and subduction zones has been indirect relying on a spatial association of blueschists with old subduction complexes and estimates of the geothermal gradients likely to exist in subduction zones here we strengthen this link by reporting the discovery of blueschist facies minerals lawsonite aragonite sodic pyroxene and blue amphibole in clasts from a serpentinite seamount in the forearc of the active mariana subduction zone the metamorphic conditions estimated from the mineral compositions are 150 250 c and 5 6 kbar 16 20 km depth the rocks must have been entrained in rising serpentine mud diapirs and extruded from mud volcanoes onto the sea floor further study of these rocks may provide new insight into the tectonics of trench forearc systems and in particular the processes by which blueschist facies clasts come to be associated with forearc sediments in ancient subduction complexes

magmatic processes at superfast spreading mid ocean ridges glass compositional variations along the east pacific rise 13 23 s major and minor element analyses of 496 natural volcanic glass samples from 141 locations along the superfast spreading 150 mm yr east pacific rise epr 13 23 s and near ridge seamounts comprise 212 chemical groups we interpret these groups to represent the average composition of individual lava flows or groups of closely related flows groups slightly enriched in k2o t morb are distributed variably along the axis in contrast to the galapagos spreading center where t morb are extremely rare this result is consistent with the interpretation that t morb magmas arise from low melting temperature k rich heterogeneities in the subaxial epr mantle the galapagos spreading center which is migrating to the west in an absolute reference frame is underlain by mantle previously processed and depleted in the t morb component during melting events giving rise to earlier epr magmas excluding t morb there are nearly monotonic twofold increases in k ti and k p of axial lavas from 23 s to 13 s from 22 s to 17 s these gradients correlate with isotopic ratios but north of 17 s there is a reversal of isotopic gradients indicating recent decoupling of the isotopic and minor element ratios in the subaxial mantle a strong southward increase in degree of differentiation for approximately 200 km north of the large offset at 20 7 s correlates with a gradient in bathymetry consistent with previous interpretations that this offset is propagating to the south samples from recently abandoned ridges associated with this dueling propagator mainly carry the distinctive evolved fractionation signatures of rift propagation suggesting that propagating rift tips have been abandoned preferentially to failing rift tips glass compositional variations south of this offset are consistent with rift failure on the southern limb within 40 km of the offset and possibly also south of 22 s the latter region may be affected by deformation accompanying northward growth of the easter microplate near ridge seamounts on the pacific plate between 18 19 s comprise two distinct populations those aligned approximately parallel to the spreading direction are extremely variable in major element composition but consistently enriched in sr relative to nearby axial lavas smaller seamounts aligned approximately parallel to the direction of absolute plate motion are uniformly depleted in minor elements and sr relative to axial lavas the degree of differentiation of axial lavas between 18 19 s can be related to the structural development of the rift axis and or vigor of hydrothermal activity of individual segments glass compositional variations indicate that magmatic segmentation occurs on several different scales at the superfast spreading rate of this area primary magmatic segmentation mainly reflects mantle source variations the boundaries of which correlate with the largest physical offsets in the rise axis between the easter microplate and garrett transform zone a secondary magmatic segmentation defined by the along axis continuity of similar parental magma compositions or liquid lines of descent occurs with a length scale varying from 11 to 185 km with an average of 69 57 1 km the boundaries of these segments mainly occur at overlapping spreading centers all first second and third order physical offsets correspond to secondary magmatic segment boundaries but some secondary magmatic segment boundaries also occur at small fourth order ridge axis discontinuities the secondary magmatic segments define the length scale of mantle melting variations mainly variations in extent of melting but not the scale of melt extraction processes that feed the axis this scale must be smaller than that of the secondary magmatic segments and probably corresponds to the length scale of fourth order physical discontinuities along axis there is a good positive correlation of average secondary magmatic segment length with spreading rate for four well sampled areas varying from 20 to 150 mm yr secondary magmatic segments also become more variable in axial length with increasing spreading rate the average lengths of secondary magmatic segments are smaller than those predicted by gravitational instability considerations at all spreading rates superposed on the axial magmatic segmentation are variations reflecting subaxial magmatic temperature defined by extent of magmatic differentiation which bears little systematic relation to physical or other kinds of magmatic segmentation at 13 23 s the length scale of this variation is 217 60 1 km approximately corresponding to the wavelength of rolls in the gravity field observed off axis taken together the various kinds and scales of magmatic variations observed for this superfast spreading ridge suggest that regional temperature of the upwelling asthenosphere magma supply to the axis and crustal magmatic temperature reflect independent regionally decoupled processes

the september 2009 padang earthquake on 30 september 2009 the city of padang in indonesia was rocked by an earthquake with a moment magnitude of mw 7 6 despite its size the earthquake did not rupture the sunda megathrust and did not significantly relax the 200 years of accumulated stress on the mentawai segment the megathrust strain energy budget remains substantially unchanged and the threat of a great mw 8 5 tsunamigenic earthquake on the mentawai patch is unabated

release of mineral bound water prior to subduction tied to shallow seismogenic slip off sumatra plate boundary fault rupture during the 2004 sumatra andaman subduction earthquake extended closer to the trench than expected increasing earthquake and tsunami size international ocean discovery program expedition 362 sampled incoming sediments offshore northern sumatra revealing recent release of fresh water within the deep sediments thermal modeling links this freshening to amorphous silica dehydration driven by rapid burial induced temperature increases in the past 9 million years complete dehydration of silicates is expected before plate subduction contrasting with prevailing models for subduction seismogenesis calling for fluid production during subduction shallow slip offshore sumatra appears driven by diagenetic strengthening of deeply buried fault forming sediments contrasting with weakening proposed for the shallow tohoku oki 2011 rupture but our results are applicable to other thickly sedimented subduction zones including those with limited earthquake records

splay fault slip during the mw 8 8 2010 maule chile earthquake splay faults are thrusts that emerge from the plate boundaries of subduction zones such structures have been mapped at several convergent margins and their activity commonly ascribed to large megathrust earthquakes however the behavior of splay faults during the earthquake cycle is poorly constrained because typically these structures are located offshore and are difficult to access here we use geologic mapping combined with space and land geodesy as well as offshore sonar data to document surface fault ruptures and coastal uplift at isla santa maria in south central chile 37 s caused by the 27 february 2010 maule earthquake m w 8 8 during the earthquake the island was tilted parallel to the margin and normal faults ruptured the surface and adjacent ocean bottom we associate tilt and crestal normal faulting with growth of an anticline above a blind reverse fault rooted in the nazca south america plate boundary which slipped during the maule earthquake the splay fault system has formed in an area of reduced coseismic plate boundary slip suggesting that anelastic deformation in the upper plate may have restrained the 2010 megathrust rupture surface fault breaks were accompanied by prominent discharge of fluids our field observations support the notion that splay faulting may frequently complement and influence the rupture of subduction zone earthquakes

ultrasonic velocity and attenuation measurements on basalt melts to 1500 c role of composition and structure in the viscoelastic properties viscoelastic properties of basalt melts of a wide range of composition are determined from ultrasonic compressional wave velocity vp and quality factor qp measurements in the frequency range of 3 4 22 mhz and the temperature range of 1150 1500 c the basalt samples are grouped into two types low magnesia group a containing 5 9 wt mgo and high magnesia group b containing 18 23 wt mgo the vp values of the melts determined at 1400 c are in the range 2 53 2 64 km s for group a basalts and 2 79 2 86 km s for group b basalts the relaxed low frequency velocity vr is a principally linear function of the melt density least squares fits of the data give velocity density relations vr km s 0 691 0 712 g cm3 for group a basalts and vr 0 328 0 907 for group b basalts at 1400 c the mean molecular weights m are 65 0 0 3 and 58 2 0 5 g mol for groups a and b basalts respectively suggesting that m is also a principal factor determining the melt velocity the relaxed modulus mr of the basalt melts range from 16 5 to 22 8 gpa and is also a linear function of the melt density the values of longitudinal l and volume v viscosities are calculated from the measured vp and qp and the available shear viscosity s data the viscosities and related activation energies decrease almost linearly as nbo t nonbridging oxygens per tetrahedrally coordinated cation increases

seismic structure of the southern east pacific rise seismic data from the ultrafast spreading 150 to 162 millimeters per year southern east pacific rise show that the rise axis is underlain by a thin less than 200 meters thick extrusive volcanic layer seismic layer 2a that thickens rapidly off axis also beneath the rise axis is a narrow less than 1 kilometer wide melt sill that is in some places less than 1000 meters below the sea floor the small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge crest thermal models however the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate these observations indicate that the paradigm of ridge crest magma chambers as small sill like midcrustal bodies is applicable to a wide range of intermediate and fast spreading ridges

magma distribution across ridge axis discontinuities on the east pacific rise from multichannel seismic images detailed studies of the morphology of the east pacific rise axis have shown that its linearity is disrupted by many small but distinct non transform offsets including overlapping spreading centres oscs and deviations from axial linearity devals which display variable geochemical signals1 9 using multichannel seismic reflection profiling we have mapped the distribution of a bright shallow reflector that detrick et al 10 have associated with an axial magma chamber we have found that it is neither continuous across the 9 03 osc6 nor separated into two parallel bodies2 and that its lateral offset does not conform to that of the topographic offset associated with the 9 17 deval these observations provide important insight into a causative relationship between morphological and petrological segmentation in this region of the east pacific rise and we speculate that the discontinuities may be the morphological response to fluctuations in the spatial pattern of magma delivery

coseismic slip and afterslip of the great mw 9 15 sumatra andaman earthquake of 2004 we determine coseismic and the first month postseismic deformation associated with the sumatra andaman earthquake of 26 december 2004 from near field global positioning system gps surveys in northwestern sumatra and along the nicobar andaman islands continuous and campaign gps measurements from thailand and malaysia and in situ and remotely sensed observations of the vertical motion of coral reefs the coseismic model shows that the sunda subduction mega thrust ruptured over a distance of about 1500 km and a width of less than 150 km releasing a total moment of 6 7 7 0 10 22 n m equivalent to a magnitude m w 9 15 the latitudinal distribution of released moment in our model has three distinct peaks at about 4 n 7 n and 9 n which compares well to the latitudinal variations seen in the seismic inversion and of the analysis of radiated t waves our coseismic model is also consistent with interpretation of normal modes and with the amplitude of very long period surface waves the tsunami predicted from this model fits rela tively well the altimetric measurements made by the jason and topex satellites neither slow nor delayed slip is needed to explain the normal modes and the tsunami wave the near field geodetic data that encompass both coseismic deformation and up to 40 days of postseismic deformation require that slip must have continued on the plate interface after the 500 sec long seismic rupture the postseismic geodetic moment of about 2 4 10 22 nm mw 8 8 is equal to about 30 5 of the coseismic moment release evolution of postseismic deformation is consistent with rate strengthening frictional afterslip online material summary of geodetic data used in this study

benford s law in the natural sciences 1 more than 100 years ago it was predicted that the distribution of first digits of real world observations would not be uniform but instead follow a trend where measurements with lower first digit 1 2 occur more frequently than those with higher first digits 8 9 this result has long been known but regarded largely as a mathematical curiosity and received little attention in the natural sciences here we show that the first digit rule is likely to be a widespread phenomenon and may provide new ways to detect anomalous signals in data we test 15 sets of modern observations drawn from the fields of physics astronomy geophysics chemistry engineering and mathematics and show that benford s law holds for them all these include geophysical observables such as the length of time between geomagnetic reversals depths of earthquakes models of earth s gravity geomagnetic and seismic structure in addition we find it also holds for other natural science observables such as the rotation frequencies of pulsars green house gas emissions the masses of exoplanets as well as numbers of infectious diseases reported to the world health organization the wide range of areas where it is manifested opens up new possibilities for exploitation an illustration is given of how seismic energy from an earthquake can be detected from just the first digit distribution of displacement counts on a seismometer i e without actually looking at the details of a seismogram at all this led to the first ever detection of an earthquake using first digit information alone

significance of mud volcanism 1 mud volcanism and diapirism have puzzled geoscientists for 2 centuries they have been described onshore and offshore in many places on earth and although they occur in various tectonic settings the majority of the features known to date are located in compressional tectonic scenarios this paper summarizes the main thrusts in mud volcano research as well as the various regions in which mud volcanism has been described mud volcanoes show variable geometry up to tens of kilometers in diameter and several hundred meters in height and a great diversity regarding the origin of the fluid and solid phases gas predominantly methane water and mud may be mobilized at subbottom depth of only a few meters but in places can originate from several kilometers depth with minor crustal or mantle input the possible contribution of mud extrusion to global budgets both from quiescent fluid emission and from the extrusive processes themselves is important in regions where mud volcanoes are abundant such as the collision zones between africa and eurasia fluid flux through mud extrusion exceeds the compaction driven pore fluid expulsion of the accretionary wedge also quiescent degassing of mud volcanoes may contribute significantly to volatile budgets and hence to greenhouse climate

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

geological structure of the offshore sumatra forearc region estimated from high resolution mcs reflection survey abstract to investigate detailed fault distributions and shallow geological structure offshore northwestern sumatra we obtained high resolution multi channel seismic mcs reflection data around the sunda trench trench slope and forearc high regions offshore northwestern sumatra in general trench parallel anticlinal ridges are distributed from trench slope region to forearc high region two kinds of different vergence systems are characterized in the sumatra forearc region landward vergence is dominant in the lower trench slope region and seaward vergence is dominant in the forearc high region moreover piggyback or slope basins are recognized between anticlinal ridges deformation in the uppermost part of these basins that is referred to recent deformation in this paper can be identified not only along major thrusts but also between major thrusts and the lower trench slope suggesting these are related to recently active faulting several but the largest number of such deformation are distributed along a major thrust located in the middle of the forearc high region whereas few are done along other major thrusts

another potential source of destructive earthquakes and tsunami offshore of sumatra 1 we link geodetic data from the sumatran gps array sugar and earthquake focal mechanisms to show that a 900 km long backthrust arising from the sunda megathrust offshore of sumatra has recently become active following the series of great megathrust earthquakes of this past decade shallow failure of the mentawai backthrust explains coseismic displacements during moderate earthquake clusters in 2005 and 2009 these two clusters represent the first activity on the backthrust in more than 30 years existing paleogeodetic evidence of vertical deformation in past centuries is too sparse to characterize earlier major activity but leaves open the possibility of historic great backthrust earthquakes our geodetic evidence for rupture of the mentawai backthrust during the two recent earthquake clusters suggests that this large fault may well pose an additional seismic and tsunami hazard to the coastal communities of central sumatra

joint bulk sound and shear tomography for western pacific subduction zones abstract detailed regional body wave tomographic inversion of the western pacific region has been performed using p and s travel times from common sources and receivers with a joint inversion in terms of bulk sound and shear wave speed variations in the mantle this technique allows the separation of the influence of bulk and shear moduli and hence a more direct comparison with mineral physics information the study region is parameterized with cells of side 0 5 to 2 and 19 layers to a depth of 1500 km while the rest of the mantle was parameterized with 5 5 cells with 16 layers between the surface and the core mantle boundary a simultaneous inversion is made for regional and global structures to minimize the influence of surrounding structures on the regional image a nested iterative inversion scheme is employed with local linearization and three dimensional ray tracing through the successive model updates the results of the regional tomographic inversion reveal the penetration of a subducted slab below the 660 km discontinuity at the kurile kamchatka trench while flattening of slabs above this depth is observed in the japan and izu bonin subduction zones on both the bulk sound and shear wave speed images the penetration of a subducted slab down to a depth of at least 1200 km is seen below the southern part of the bonin trench mariana philippine and java subduction zones fast shear wave speed perturbations associated with the subducted slabs down to the 410 km transition zone are larger than the comparable bulk sound perturbations for all these subduction zones except the philippines the bulk sound signature for the subducted slab is more pronounced than for shear in the philippines talaud new guinea solomon and tonga subduction zones where penetration of the slab into the middle mantle is observed variation in the amplitude ratio between bulk sound and shear wave speed anomalies correlates well with the subduction parameters of the descending slab slabs younger than 90 ma at the trench show bulk sound dominance in the upper mantle while older slabs have a stronger shear wave speed signature spreading of the fast shear wave speed zone between 800 and 1000 km is observed in the areas of deep subducted slab penetration but has no comparable expression in the bulk sound images this high velocity feature may reflect physical or chemical disequilibria introduced to the lower mantle by subducted slabs

melt to mush variations in crustal magma properties along the ridge crest at the southern east pacific rise the determination of along axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle eruption history along the ridge crest and the process of crustal accretion seismic reflection images1 2 3 4 have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths 250 4 500 m but variations in its seismic properties and thickness have remained elusive despite several attempts to constrain these properties5 6 7 here we report that the melt sill along the southern east pacific rise which is about 50 m thick undergoes abrupt changes in its internal properties ranging from pure melt to mush the 60 km long ridge crest segment near 14 00 s is characterized by three 2 4 km sections containing pure melt embedded within a magma chamber rich in mush these small pure melt pockets may represent a fresh supply of magma from the mantle capable of erupting and forming the upper crust conversely the 80 90 of the magma chamber which is mushy is unlikely to erupt and may influence the lower crustal accretion

distribution of magma beneath the east pacific rise between the clipperton transform and the 9 17 n deval from forward modeling of common depth point data we have reprocessed seven cross axis common depth point cdp profiles between the clipperton transform and the 9 17 n deval deviation in axial linearity on the east pacific rise epr to understand the relationship between axial magma chamber amc width and seafloor morphology forward modeling of cross axis cdp profiles suggests a segmented amc in which significant variations in width occur across minor rise axis discontinuities e g devals the modeled rise segment widths bounded by the 9 53 n 9 35 n devals the 9 35 n 9 17 n devals and south of the 9 17 n deval were 0 7 km 1 0 1 2 km and 4 15 km respectively transition in amc width across these discontinuities is unclear due to the sparseness of cross axis line spacing however a simple association of devals with decreased magma supply is doubtful the minimum 250 m and maximum 4150 m amc widths are found near the 9 35 n and 9 17 n devals respectively the reprocessing of cdp profiles included repicking stacking velocities to ensure a proper stack of the amc reflector and its associated diffractions imaging postcritical reflections from the base of layer 2a finite difference time migration ray theoretical depth migration and travel time modeling of amc diffraction patterns constraints on amc width were derived from forward modeling techniques based on analytic raytracing velocity models were constructed from seabeam bathymetry and modified expanding spread profile esp velocity depth functions esp velocity models were altered to compensate for off axis thickening of layer 2a as imaged in the cdp reflection data continuous two dimensional velocity models constructed from modified esp velocity depth functions and seabeam bathymetry should account for ray bending at the seafloor basalt interface and any lateral velocity gradients induced by a thickening layer 2a stacked data were time migrated using a finite difference algorithm and extrapolated to depth using ray theoretical depth migration reflector positions were input into our forward modeling scheme to produce a zero offset travel time response of our migrated solution the travel time response was then overlain on the stacked section to ensure an adequate match especially to diffractions generated at the amc edge forward modeling of amc diffraction patterns reveals that original amc width estimates were too large the under migration of the amc reflector resulted from the conversion of stacking to interval velocities without accounting for topographic effects on individual cmp gathers thus resulting in improperly collapsed diffraction hyperbolae ship wandering relative to the amc edge can account for variations in amc reflector amplitude and dropout on the along axis cdp line the continuity of the amc appears unbroken across several ridge axis discontinuities between the clipperton transform and the 9 17 n deval which suggests an amc whose along axis dimension exceeds 75 km reflectivity modeling of cmp gathers suggests that the available data are consistent with a magma chamber comprising only a thin layer of melt overlying a zone of partially solidified crystal mush such a thin layer of melt might inhibit along axis mixing of magmas and thereby account for variations in magma composition along the rise crest this melt lens model for the amc would also produce strong diffraction patterns as imaged in the cdp data

mud diapirism and subcretion in the barbados ridge accretionary complex the role of fluids in accretionary processes mud diapirism has recently been recognized in several modern accretionary wedges it provides an important means of dewatering accretionary wedges and should be regarded as an important process for producing the melanges found in both modern and ancient accretionary terranes mud diapirism affects a large area of the barbados ridge accretionary complex the distribution of the mud diapirs appears to be primarily controlled by the presence of underconsolidated terrigenous submarine fan deposits that are being accreted to the complex the frequency of diapir occurrence decreases northward as the fan becomes thinner mud diapirs are absent from the very eastern most part of the complex formed from sediments accreted at its toe with the exception of a few mud volcanoes on the ocean floor in front of the complex the initiation of diapirism appears to be spatially coincident with the onset of subcretion or underplating of sediment to the base of the complex at a ramp between two levels of decollement it is proposed that the release of mud and pore water from the subcreted sediments is a direct or indirect cause of most of the mud diapirism in the accretionary complex there is a range of diapiric form dependent on the viscosity of the mud from mud volcanoes fed by low viscosity mud to higher viscosity mud ridges the diapirs in the eastern areas of the complex are generally mud volcanoes with narrow conduits feeding a surface mound mud ridges are prominent in the western parts of the complex this is interpreted as reflecting a general westward decrease in the fluid content of the accretionary complex bottom simulating seismic reflectors formed by gas hydrate are commonly developed in the areas of mud volcano occurrences the presence of the hydrate indicates that large volumes of methane are being generated at depth in these regions the generation of methane may be contributing to zones of overpressuring in the wedge methane may also be partly responsible for driving the diapiric material to the surface to form mud volcanoes ridges in the subducting oceanic crust beneath the accretionary complex locally enhance diapirism above their crests and southern flanks faults formed later in the development of the complex are more commonly associated with diapirism than those resulting from accretion at the toe of the wedge these later faults play an important role in controlling the sites of individual mud volcanoes chains of mud volcanoes and mud ridges

subduction system variability across the segment boundary of the 2004 2005 sumatra megathrust earthquakes subduction zone earthquakes are known to create segmented patches of co seismic rupture along strike of a margin offshore sumatra repeated rupture occurred within segments bounded by permanent barriers whose origin however is still not fully understood in this study we image the structural variations across the rupture segment boundary between the mw 9 1 december 26 2004 and the mw 8 6 march 28 2005 sumatra earthquakes a set of collocated reflection and wide angle seismic profiles are available on both sides of the segment boundary located offshore simeulue island we present the results of the seismic tomography modeling of wide angle ocean bottom data enhanced with mcs data and gravity modeling for the southern 2005 segment of the margin and compare it to the published model for the 2004 northern segment our study reveals principal differences in the structure of the subduction system north and south of the segment boundary attributed to the subduction of 96 e fracture zone the key differences include a change in the crustal thickness of the oceanic plate a decrease in the amount of sediment in the trench as well as variations in the morphology and volume of the accretionary prism these differences suggest that the 96 e fracture zone acts as an efficient barrier in the trench parallel sediment transport as well as a divider between oceanic crustal blocks of different structure the variability of seismic behavior is caused by the distinct changes in the morphology of the subduction complex across the boundary related to the difference in the sediment supply

dynamic rupture activation of backthrust fault branching we perform dynamic rupture simulations to investigate the possible reactivation of backthrust branches triggered by ruptures along a main thrust fault simulations with slip weakening fault friction and uniform initial stress show that fast propagation speed or long propagation distance of the main rupture promotes reactivation of backthrust over a range of branch angles the latter condition may occur separately from the former if rupture speed is limited by an increasing slip weakening distance towards the junction direction the results suggest a trade off between the amplitude and duration of the dynamic stress near the main rupture front for backthrust reactivation termination of the main rupture by a barrier can provide enhanced loading amplitude and duration along a backthrust rooted near the barrier facilitating its reactivation especially with a high frictional resistance the free surface and depth dependent initial stress can have several additional effects the sign of the triggered motion along the backthrust can be reversed from thrust to normal if a deeply nucleated main rupture breaks the free surface while it is preserved as thrust if the main rupture is terminated by a barrier at depth the numerical results are discussed in relation to several recent megathrust earthquakes in sumatra chile and japan and related topics such as branch feedbacks to the main fault the dynamic view on backthrust fault branching provided by the study fills a gap not covered by quasi static models or observations a specific examined case of antithetic fault branching may be useful for indicating a barrier like behavior along the main fault

seismic evidence of bending and unbending of subducting oceanic crust and the presence of mantle megathrust in the 2004 great sumatra earthquake rupture zone abstract in subduction zones the plate interface megathrust is typically poorly imaged at depths 12 km however its precise geometry and nature as well as the positions of updip and downdip limits of the seismogenic zone are important elements to understand the generation of megathrust earthquakes using deep marine seismic reflection and refraction data we observed discontinuous reflections off the top of the subducting oceanic crust down to 60 km depth in the 2004 great sumatra andaman earthquake rupture zone we find that the top of the downgoing plate does not dip gently into the subduction zone but instead displays a staircase geometry with three successive 5 15 km vertical steps spaced 50 km apart micro earthquake data indicate that most of the seismicity lies below this interface suggesting that the oceanic plate is deforming actively along part of the profile we also image a second reflector located 8 10 km below the top of the oceanic crust the forward modelling of the gravity data along the profile supports the presence of a high density material above this reflector the presence of a staircase shape for the top of the oceanic crust together with constraints from gravity data and earthquake data require that the megathrust goes through this second reflector this leads us to conclude that the megathrust is at least partly located in the oceanic mantle and that underplating of oceanic crust beneath the wedge and underplating of upper mantle beneath the forearc basin are taking place in this region

structural evolution of backthrusting in the mentawai fault zone offshore sumatran forearc 1 we present the interpretation of newly acquired high quality industry standard deep seismic reflection and swath bathymetry data to provide insight into the structural style and evolution of the mentawai fault zone mfz the mfz lies along the boundary between the accretionary wedge and the proposed continental backstop this zone exhibits arcuate ridges on the seafloor convex toward the east beneath these ridges the structures developed as landward vergent imbricated backthrusts in the inner part of the accretionary wedge and higher angle backthrusts that deformed the forearc basin sediments in the forearc high anticlines were developed due to the seaward vergent forearc high thrusts originating in the accretionary wedge the imbricated backthrusts may have initiated during the early middle miocene contemporaneously with the slide and back rotation of forearc high thrusts in the late miocene the higher angle backthrusts were initiated continuous contraction induced the frontal higher angle backthrusts and formed a fold thrust belt toward the east during the pliocene the folds and thrusts were disturbed by diapirs and mud volcanoes backthrusting and fold thrust belts developed in the mfz may explain the compressional features observed at the boundary between the accretionary wedge and continental backstop along the southern sumatra margin the backthrusts along the mfz are waning in activity and hence the risk of a large earthquake and associated tsunami at the present time should be small

partial rupture of a locked patch of the sumatra megathrust during the 2007 earthquake sequence the great sumatra andaman earthquake and tsunami of 2004 was a dramatic reminder of the importance of understanding the seismic and tsunami hazards of subduction zones 1 2 3 4 in march 2005 the sunda megathrust ruptured again producing an event 5 of moment magnitude mw 8 6 south of the 2004 rupture area which was the site of a similar event in 1861 ref 6 concern was then focused on the mentawai area where large earthquakes had occurred in 1797 mw 8 8 and 1833 mw 9 0 6 7 two earthquakes one of mw 8 4 and twelve hours later one of mw 7 9 indeed occurred there on 12 september 2007 here we show that these earthquakes ruptured only a fraction of the area ruptured in 1833 and consist of distinct asperities within a patch of the megathrust that had remained locked in the interseismic period this indicates that the same portion of a megathrust can rupture in different patterns depending on whether asperities break as isolated seismic events or cooperate to produce a larger rupture this variability probably arises from the influence of non permanent barriers zones with locally lower pre stress due to the past earthquakes the stress state of the portion of the sunda megathrust that had ruptured in 1833 and 1797 was probably not adequate for the development of a single large rupture in 2007 the moment released in 2007 amounts to only a fraction both of that released in 1833 and of the deficit of moment that had accumulated as a result of interseismic strain since 1833 the potential for a large megathrust event in the mentawai area thus remains large

plate boundary deformation associated with the great sumatra andaman earthquake the sumatra andaman earthquake of 26 december 2004 is the first giant earthquake moment magnitude m w 9 0 to have occurred since the advent of modern space based geodesy and broadband seismology it therefore provides an unprecedented opportunity to investigate the characteristics of one of these enormous and rare events here we report estimates of the ground displacement associated with this event using near field global positioning system gps surveys in northwestern sumatra combined with in situ and remote observations of the vertical motion of coral reefs these data show that the earthquake was generated by rupture of the sunda subduction megathrust over a distance of 1 500 kilometres and a width of 150 kilometres megathrust slip exceeded 20 metres offshore northern sumatra mostly at depths shallower than 30 kilometres comparison of the geodetically and seismically inferred slip distribution indicates that 30 per cent additional fault slip accrued in the 1 5 months following the 500 second long seismic rupture both seismic and aseismic slip before our re occupation of gps sites occurred on the shallow portion of the megathrust where the large aceh tsunami originated slip tapers off abruptly along strike beneath simeulue island at the southeastern edge of the rupture where the earthquake nucleated and where an m w 7 2 earthquake occurred in late 2002 this edge also abuts the northern limit of slip in the 28 march 2005 m w 8 7 nias simeulue earthquake

seismic analysis of the axial magma chamber reflector along the southern east pacific rise from conventional reflection profiling the thickness and internal properties of the magma sill located at the top of the axial magma chamber amc along the southern east pacific rise epr have been investigated through a combination of waveform modeling the near vertical incidence reflections from this body and analysis of reflection amplitude variation as a function of source receiver offset or slowness our results show that the amc reflector observed along the southern epr is best modeled by a thin 100 m thick sill of partial melt vs 0 km s sandwiched between higher velocity material and that the thickest sills are generally associated with the lowest p and s wave velocities the comparatively high p wave velocities and nonzero shear wave velocities inferred for this sill indicate that it is filled with partially molten magma which in some locations has a high crystal content this may have important implications for eruption mechanisms and along axis mixing of magma at the epr there is no simple relationship between morphologic indicators of magma supply e g axial depth or volume and sill thickness depth or velocity magma sill properties may be closely tied to the eruption and replenishment cycle of the amc and thus may vary on a much shorter spatial and temporal scale than axial morphology which reflects longer term variations in magma supply to the ridge

deformation and slip along the sunda megathrust in the great 2005 nias simeulue earthquake seismic rupture produced spectacular tectonic deformation above a 400 kilometer strip of the sunda megathrust offshore northern sumatra in march 2005 measurements from coral microatolls and global positioning system stations reveal trench parallel belts of uplift up to 3 meters high on the outer arc islands above the rupture and a 1 meter deep subsidence trough farther from the trench surface deformation reflects more than 11 meters of fault slip under the islands and a pronounced lessening of slip trenchward a saddle in megathrust slip separates the northwestern edge of the 2005 rupture from the great 2004 sumatra andaman rupture the southeastern edge abuts a predominantly aseismic section of the megathrust near the equator

seismic structure and indicators of magma budget along the southern east pacific rise in this paper we re examine the relationship between seismically constrained variations in crustal structure along the southern east pacific rise sepr and the segment scale variations in axial depth morphology basalt geochemistry and hydrothermal activity that have often been attributed to along axis differences in the supply of magma to the mid ocean ridge along 800 km of the fast spreading sepr good correlations exist between axial depth ridge cross sectional area mantle bouguer anomaly and the mgo weight percent of basalts recovered from the rise axis these correlations indicate along axis changes in crustal thickness and temperature consistent with variations in magma supply on time scales of 100 000 years in contrast we show that the depth and width of the midcrustal magma sill the thickness of seismic layer 2a and the intensity of hydrothermal venting are poorly correlated with regional variations in ridge depth and cross sectional area we suggest that the emplacement geometry width of the intrusion zone and flow lengths not magma supply controls extrusive layer seismic layer 2a thickness we hypothesize that magma lens properties and hydrothermal activity are closely linked to spreading events dike intrusion eruptions faulting which occur on much shorter timescales 10 100 years than the longer term variations in magma supply reflected in along axis changes in the shape and depth of the ridge axis

uniform accretion of oceanic crust south of the garrett transform at 14 15 s on the east pacific rise using migrated common depth point reflection profiles we find the structural differences along the ultrafast spreading 150 mm yr east pacific rise south of the garrett fracture zone are second order suggesting a remarkably uniform process of crustal accretion the rise axis south of the garrett transform is underlain by a narrow 1 0 km melt lens which shows great along strike continuity the depth of the axial melt sill is approximately 1200 m beneath the seafloor which is about 400 m shallower than along the slower spreading east pacific rise at 9 30 n this observation strengthens the argument that the depth to the top of the crustal velocity inversion is spreading rate dependent melt sill width however shows little variation along the east pacific rise suggesting no dependence of magma chamber size on spreading rate the melt reservoir decreases in width toward across the 14 27 s ridge axis discontinuity by a modest 250 300 m and appears to be continuous across this feature given the small aspect ratio 1 0 km by 50 m by tens of kilometers of the axial melt lens the previously recorded jump in mgo content across the 14 27 s offset is likely the result of a mixing boundary which is sustained through an along strike impedance in convection wide angle reflections originating at the base of seismic layer 2a assumed to coincide with the extrusive layer reveal a twofold to threefold increase 200 250 to 500 600 m in thickness within 1 2 km of the rise axis the pattern of extrusive thickening imaged south of the garrett transform is similar to that observed along the slower spreading 110 120 mm yr east pacific rise at 9 n outside of the neovolcanic zone mean extrusive thickness is relatively invariant along a profile and from profile to profile this implies a degree of temporal stability of the along strike magma supply when integrated over the 10 kyr that corresponds to the width of the neovolcanic zone the inferred uniformity of off axis mean extrusive thickness is inconsistent with the conjecture that decreases in axial volume toward the 14 27 s discontinuity are caused by long term reductions in magma supply second order differences in the style of extrusive thickening may be related to structural differences within the low velocity zone underlying the rise axis and or changes within the stress field in the overlying carapace which results in the diffuse emplacement of lavas near the rise axis images of mono on cross axis profiles may be traced to within 1 0 km of the melt sill edge this observation is in agreement with rise crest models which generate the lower crustal section through the advection of material down and outward from the axial melt lens rather than through cumulate deposition at the base of a large magma chamber

carbonate till as a soft bed for pleistocene ice streams on the canadian shield north of lake superior silty carbonate till derived from erosion of paleozoic carbonate and proterozoic rocks within and adjacent to hudson bay covers extensive areas of the canadian shield north of lake superior it is hypothesized that this carbonate till could have acted as low resistance substrata for overriding ice streams by deforming and or supporting high subglacial water pressures contrary to assumptions presented in some current models for ice flow within the laurentide ice sheet it need not be assumed that shield terrain in these areas acted as a rigid bed generating large basal shear stresses and inhibiting ice flow indeed erratic dispersal patterns long distance glacial transport and splayed patterns of ice flow indicators in areas of thick till cover may be better explained by rapid ice flow events or ice streams enhanced by the thickness distribution impermeability and susceptibility to deformation of fine carbonate till

uplift rate transients at subduction margins due to earthquake clustering coastal uplift is common in continental fore arc systems with elevated paleoshorelines indicating that uplift rates can vary dramatically over time on individual margins the origins of these changes in uplift rates are examined using a global data set of paleoshorelines together with 2 d numerical models of subduction systems empirical paleoshoreline data n 282 from eight subduction margins indicate that uplift rates are generally not steady state and varied by up to a factor of 20 during the late quaternary 125 ka on many subduction margins uplift rates increase to the present day a finding which we attribute to sampling bias toward those locations where holocene uplift rates have been highest with respect to other global margins which have undergone fast subsidence or no vertical motion e g a property akin to the so called sadler effect paleoshorelines and 2 d models suggest that transient uplift rates at subduction margins are mainly a short term 20 ka phenomenon that cannot be accounted for by plate boundary scale processes such as changes in the rates of plate convergence sediment underplating or isostatic unloading instead time variable uplift rates are ascribed to temporal clustering of large magnitude earthquakes on upper plate faults and to a lesser extent the subduction thrust the potential for future damaging earthquakes and tsunamis may have been underestimated at active subduction margins with no measureable holocene uplift and in such cases late quaternary paleoshorelines could provide an important constraint for hazard analysis

fore arc basin deformation in the andaman nicobar segment of the sumatra andaman subduction zone insight from high resolution seismic reflection data the andaman nicobar region is the northernmost segment of the sumatra andaman subduction zone and marks the western boundary of the andaman sea which is a complex active back arc extensional basin we present the interpretation of a new set of deep seismic reflection data acquired across the andaman nicobar fore arc basin from 8 n to 11 n in order to better understand its structure and evolution focusing on 1 how obliquity of convergence affects deformation in the fore arc 2 the nature and role of the diligent fault df and 3 the eastern margin fault emf despite the obliquity of convergence back thrusting and compression seem to dominate the andaman nicobar fore arc basin deformation the df is primarily a back thrust and corresponds to the mentawai and west andaman fault systems farther in the south along sumatra the df is expressed in the fore arc basin as a series of mostly landward verging folds and faults deforming the early to late miocene sediments the df seems to root from the boundary between the accretionary complex and the continental backstop where it meets the emf the emf marks the western boundary of the fore arc basin it is associated with subsidence and is expressed as a deep piggyback basin containing recent pliocene to pleistocene sediments the eastern edge of the fore arc basin is the invisible bank ib which is thought to be tilted and uplifted continental crust subsidence along the emf and uplift and tilting of the ib seem to be related to different opening phases in the andaman sea

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

the west andaman fault a complex strain partitioning boundary at the seaward edge of the aceh basin offshore sumatra along the sumatran subduction zone oblique strain is partitioned between the sunda megathrust and the strike slip sumatran fault located near the volcanic arc additional strike slip motion may localize adjacent to the forearc basin system forming the mentawai and west andaman fault zones an alternative interpretation of the west andaman fault as primarily a backthrust has also been proposed we combine a 2 d multichannel seismic survey sumut acquired aboard the r v sonne in 2008 with previous bathymetry and seismic surveys to characterize the west andaman fault adjacent to the aceh forearc basin two dimensional kirchoff prestack time migration of sumut profiles provides the clearest images to date of this portion of the west andaman fault in these images we interpret inward dipping fault segments along the west andaman fault indicative of a transpressional flower structure a reflector at the base of the aceh basin sediments believed to be base neogene in age continues trenchward of the fault zone for 15 km and has geometry inconsistent with the geometry expected if a major backthrust were to daylight at the edge of the basin therefore we conclude that the west andaman fault is predominantly strike slip in nature as such the west andaman is likely part of a system of faults including the sumatran fault that accommodate the significant shear component of strain in this oblique subduction zone additionally the location of the west andaman fault within the forearc may be controlled by the rheologic contrast between the marginal plateau and the forearc basin

seismic mapping of a magma chamber beneath the valu fa ridge lau basin we analyze a dense grid of seismic normal incidence and wide angle data collected over an active back arc spreading center in the lau basin of the southwest pacific our survey area covers the whole of a 35 km long morphological segment and a small overlapping spreading center a bright reflector which is coincident with a velocity inversion at a depth of 3 2 0 2 km below the seafloor is observed on every one of the 40 across axis profiles we interpret this reflector as being the roof of a crustal magma chamber the widest magma chamber reflector occurs beneath the overlapping spreading center where it extends up to 4 km being imaged beneath both ridges and in places beneath the overlap basin elsewhere the width of the reflector varies between 0 6 and 2 3 0 4 km the narrowest reflectors are observed beneath deviations devais of the ridge axis in addition to being observed on each of the kilometer spaced across axis profiles the reflector is also seen as a continuous event for at least 10 km on a profile along the ridge axis the reflection coefficient of the event is between 0 34 and 0 65 this requires the melt at the top of the chamber to have a p wave velocity less than 2 7 km s if the interface is planar or less than 3 8 km s if the interface is layered

active backstop faults in the mentawai region of sumatra indonesia revealed by teleseismic broadband waveform modeling abstract two earthquake sequences that affected the mentawai islands offshore of central sumatra in 2005 mw 6 9 and 2009 mw 6 7 have been highlighted as evidence for active backthrusting of the sumatran accretionary wedge however the geometry of the activated fault planes is not well resolved due to large uncertainties in the locations of the mainshocks and aftershocks we refine the locations and focal mechanisms of medium size events mw 4 5 of these two earthquake sequences through broadband waveform modeling in addition to modeling the depth phases for accurate centroid depths we use teleseismic surface wave cross correlation to precisely relocate the relative horizontal locations of the earthquakes the refined catalog shows that the 2005 and 2009 backthrust sequences in mentawai region actually occurred on steeply 60 degrees landward dipping faults masilo fault zone that intersect the sunda megathrust beneath the deepest part of the forearc basin contradicting previous studies that inferred slip on a shallowly seaward dipping backthrust static slip inversion on the newly proposed fault fits the coseismic gps offsets for the 2009 mainshock equally well as previous studies but with a slip distribution more consistent with the mainshock centroid depth 20 km constrained from teleseismic waveform inversion rupture of such steeply dipping reverse faults within the forearc crust is rare along the sumatra java margin we interpret these earthquakes as unsticking of the sumatran accretionary wedge along a backstop fault separating imbricated material from the stronger sunda lithosphere alternatively the reverse faults may have originated as pre miocene normal faults of the extended continental crust of the western sunda margin our waveform modeling approach can be used to further refine global earthquake catalogs in order to clarify the geometries of active faults

splay fault triggering by great subduction earthquakes inferred from finite element models we have investigated the influence that megathrust earthquake slip has on the activation of splay faults using a 2 d finite element method fem taking into account the effects of gravity and variations in the frictional strength properties of splay faults we simulated both landward dipping and seaward dipping splay fault geometries and imposed depth variable slip distributions of subduction events our results indicate that the two types of splay fault exhibit a similar behavior with variations in frictional properties along the faults affecting only the seismic magnitude the triggering process is controlled by a critical depth megathrust slip concentrated at depths shallower than the critical depth will favor normal displacement while megathrust slip concentrated at depths deeper than the critical depth is likely to result in reverse motion our results thus provide a useful tool for predicting the activation of secondary faults and may have direct implications for tsunami hazard research

structure of the lesser antilles subduction zone backstop and its role in a large accretionary system 1 the role of a backstop in subduction zones has been the subject of numerous laboratory and numerical modeling studies however few field observations exist revealing how backstops control deformation in subduction zones and accretionary wedge construction a seismic reflection and refraction survey acquired in 1998 with the r v maurice ewing reveals the geometry of the forearc igneous crust accretionary wedge and forearc basin structure of the northern guadeloupe area of the lesser antilles forearc an accreted block of buoyant crust accreted in the late miocene forms the toe of the overriding arc crust and forms the backstop we imaged the top of this surface beneath the forearc basin to its seaward edge where it meets the subducting oceanic crust the toe of the backstop was thrust upward and forms a steep buttress in contact with the lower half of the accretionary wedge the steep buttress produces a narrow inner deformation zone with minimal backthrusting of the accretionary complex landward over the backstop and a narrow 10 km transition between accreted and forearc basin sediment seismic reflections from the subducting crust and the decollement appear beneath the entire accretionary wedge and below the backstop toe separating the decollement and the subducting crust is an interval usually between 500 and 750 m of underthrust sediment carried underneath the accretionary wedge and subducted 15 km landward and beneath the toe of the backstop we speculate that the upturned geometry of the toe of the backstop and a weak fluid rich decollement may facilitate sediment subduction beneath the backstop and potentially into the mantle

seismic evidence for broken oceanic crust in the 2004 sumatra earthquake epicentral region the 2004 sumatra earthquake was one of the largest events to occur in a subduction zone in the past 50 years seismic reflection data for this subduction zone reveal thrust faults cutting across the entire oceanic crust this observation coupled with the hypocentres of aftershocks suggests that the megathrust the interface between the indo australian plate and the sunda plate currently lies in the oceanic mantle

megathrust earthquakes can nucleate in the forearc mantle evidence from the 2004 sumatra event current models predict that the seismogenic zone along subduction thrusts where the largest earthquakes nucleate and propagate does not extend to the forearc mantle below the crust of the upper plate stable sliding conditions have been shown to prevail there particularly along several circum pacific margins that underwent great megathrust earthquakes mw 8 5 during the twentieth century based on geophysical investigation we show that the great 2004 sumatra andaman earthquake mw 9 9 3 contradicts these models not only did it propagate downdip along the interface between the forearc mantle and the subducting plate but it actually nucleated along this reportedly aseismic part of the interplate contact petrological models can therefore underestimate the downdip extent of rupture zones to be expected in megathrust earthquakes and need to be revised to account for this observation albeit unusual

instantaneous global plate motion model from 12 years of continuous gps observations 1 we estimate a global plate motion model for 17 major and minor tectonic plates solely on the basis of analysis of data from 106 globally distributed continuous gps stations spanning the period from january 1991 to july 2003 site positions estimated from 24 hour segments of data are aligned day by day to the international gps service igs realization of the itrf2000 reference frame using a similarity transformation thereby ensuring that the itrf2000 no net rotation condition is preserved linear velocities of a carefully selected set of stations are estimated from the position time series along with annual and semiannual fluctuations and position offsets due to gps instrument changes a white noise plus flicker noise model is applied to estimate realistic uncertainties for the site velocities which are then propagated into the derived plate motion model parameters we also examine the vertical velocities in the site selection process at the scripps orbit and permanent array center we have implemented a procedure whereby the plate motion model is updated on a regular monthly basis to improve its precision and reliability as new data become available and as a baseline against which anomalous motions can be detected