the 2006 2007 kuril islands great earthquake sequence the southwestern half of a 500 km long seismic gap in the central kuril island arc subduction zone experienced two great earthquakes with extensive preshock and aftershock sequences in late 2006 to early 2007 the nature of seismic coupling in the gap had been uncertain due to the limited historical record of prior large events and the presence of distinctive upper plate trench and outer rise structures relative to adjacent regions along the arc that have experienced repeated great interplate earthquakes in the last few centuries the intraplate region seaward of the seismic gap had several shallow compressional events during the preceding decades notably an m s 7 2 event on 16 march 1963 leading to speculation that the interplate fault was seismically coupled this issue was partly resolved by failure of the shallow portion of the interplate megathrust in an m w 8 3 thrust event on 15 november 2006 this event ruptured 250 km along the seismic gap just northeast of the great 1963 kuril island m w 8 5 earthquake rupture zone within minutes of the thrust event intense earthquake activity commenced beneath the outer wall of the trench seaward of the interplate rupture with the larger events having normal faulting mechanisms an unusual double band of interplate and intraplate aftershocks developed on 13 january 2007 an m w 8 1 extensional earthquake ruptured within the pacific plate beneath the seaward edge of the kuril trench this event is the third largest normal faulting earthquake seaward of a subduction zone on record and its rupture zone extended to at least 33 km depth and paralleled most of the length of the 2006 rupture the 13 january 2007 event produced stronger shaking in japan than the larger thrust event as a consequence of higher short period energy radiation from the source the great event aftershock sequences were dominated by the expected faulting geometries thrust faulting for the 2006 rupture zone and normal faulting for the 2007 rupture zone a large intraplate compressional event occurred on 15 january 2009 m w 7 4 near 45 km depth below the rupture zone of the 2007 event and in the vicinity of the 16 march 1963 compressional event the fault geometry rupture process and slip distributions of the two great events are estimated using very broadband teleseismic body and surface wave observations the occurrence of the thrust event in the shallowest portion of the interplate fault in a region with a paucity of large thrust events at greater depths suggests that the event removed most of the slip deficit on this portion of the interplate fault this great earthquake doublet demonstrates the heightened seismic hazard posed by induced intraplate faulting following large interplate thrust events future seismic failure of the remainder of the seismic gap appears viable with the northeastern region that has also experienced compressional activity seaward of the megathrust warranting particular attention

normal faulting earthquakes beneath the outer slope of the japan trench after the 2011 tohoku earthquake implications for the stress regime in the incoming pacific plate 1 after the 2011 mw 9 1 tohoku earthquake numerous intraplate earthquakes occurred beneath the outer slope of the japan trench based on ocean bottom seismograph observations these earthquakes occurred in the oceanic crust and uppermost mantle of the pacific plate at depths shallower than about 40 km and had normal faulting focal mechanisms at all depths before the 2011 earthquake normal faulting earthquakes beneath the outer trench slope occurred only at depths shallower than 20 km whereas those at depths of around 40 km had reverse faulting mechanisms these observations suggest that the stress regime in the pacific plate was changed by the 2011 earthquake the tensional stresses that now extend to depths of about 40 km may play an important role not only in the occurrence of large normal faulting earthquakes but also in hydration of the uppermost mantle of the incoming pacific plate prior to the subduction

body wave inversion moment tensors and depths of oceanic intraplate bending earthquakes this paper develops a moment tensor inversion technique suited for analysis of long period body waves from distant shallow earthquakes in addition to the moment tensor the procedure resolves precise source depths 5 km and estimates the duration of faulting the technique is applied to digital sro dwwssn recordings of outer rise intraplate earthquakes these earthquakes result from bending of oceanic plates prior to subduction bending induces horizontal tension near the top of the lithosphere and horizontal compression near its base a neutral surface separates compressional from tensional regimes an examination of 11 bending earthquakes in the period january 1981 to july 1982 indicates that the depth to the neutral surface is not geographically uniform although the seismicity sample is very small the depth of the neutral surface appears to correlate with the position of the trench arc in the evolutionary sequence of uyeda and kanamori 1979 in particular neutral surfaces at strongly coupled chilean type plate boundaries are elevated as much as 15 km above those at weakly coupled mariana type boundaries this correlation implies that nonuniformities in neutral surface height are created by variations in regional compressive stress resulting from contrasts in plate plate coupling to account for the observed heights maximum interplate compressive stresses must be comparable with stresses derived from bending

crustal thickness variations and isostatic disequilibrium across the north anatolian fault western turkey we use teleseismic recordings from a dense array of seismometers straddling both strands of the north anatolian fault zone to determine crustal thickness p s velocity ratio and sedimentary layer thickness to do this we implement a new grid search inversion scheme based on the use of transfer functions removing the need for deconvolution for source normalization and therefore eliminating common problems associated with crustal scale receiver function analysis we achieve a good fit to the data except at several stations located in quaternary sedimentary basins where our two layer crustal model is likely to be inaccurate we find two zones of thick sedimentary material one north of the northern fault branch and one straddling the southern branch the crustal thickness increases sharply north of the northern strand of the north anatolian fault zone nafz where the fault nearly coincides with the trace of the intra pontide suture the velocity ratio changes across the southern fault strand indicating a change in basement composition we interpret these changes to indicate that both strands of the nafz follow preexisting geological boundaries rather than being ideally aligned with the stress field the thick crust north of the northern nafz strand is associated with low topography and so is inconsistent with simple models of isostatic equilibrium requiring a contribution from mantle density variations such as possible loading from underthrust black sea oceanic lithosphere

deep sulfate methane transition and sediment diagenesis in the gulf of alaska iodp site u1417 abstract sediment samples from the gulf of alaska goa iodp expedition 341 site u1417 have been analyzed to understand present and past diagenetic processes that overprint the primary sediment composition no sulfate methane transition zone smtz was observed at the zone of sulfate depletion but a 200 m thick sulfate and methane free sediment interval occurred between the depth of sulfate depletion 200 m and the onset of methanogenesis 440 m we suggest that this apparent gap in biogeochemical processing of organic matter is caused by anaerobic oxidation of methane fueled by sulfate which is released during dissolution of barites at the upper boundary of the methane rich layer beneath the methanogenic zone at 650 m depth pore water sulfate concentrations increase again indicating sulfate supply from greater depth feeding into a deep inverse smtz a likely explanation for the availability of sulfate in the deep sub seafloor at u1417 is the existence of a deep aquifer related to plate bending fractures which actively transports sulfate rich water to and potentially along the interface between sediments and oceanic crust such inverse diagenetic zonations have been previously observed in marine sediments but have not yet been linked to subduction related plate bending with the discovery of a deep inverse smtz in an intra oceanic plate setting and the blocking of upward methane diffusion by sulfate released from authigenic barite dissolution site u1417 provides new insights into sub seafloor pore fluid and gas dynamics and their implications for global element cycling and the deep biosphere

determinations of focal depths of earthquakes associated with the bending of oceanic plates at trenches abstract the earthquakes examined in this paper are all within the oceanic lithosphere and are associated with the bending of plates before subduction accurate determinations of the depth of these earthquakes are needed to study the stress pattern within a bending plate routinely determined depths of shallow sub oceanic earthquakes published in bulletins are unreliable the depths can be accurately determined to within a few kilometers if the original seismograms from these events are studied in some cases the reflected phases pp and pwp can be clearly identified there exists the possibility that the wave reflected at the water air interface pwp may be misidentified as pp leading to erroneous estimates of depth additional methods of analysis such as surface wave radiation patterns or the apparent frequency dependence of reflection at the crust water interface can remove this possible source of confusion one of the most powerful techniques for depth analysis is the modelling of long period waveforms the pattern of stresses within the bending oceanic lithosphere revealed by the depths and focal mechanisms of these intraplate earthquakes is one of horizontal deviatoric tension down to a depth of about 25 km with horizontal compression at greater depths

spreading rate dependence of gravity anomalies along oceanic transform faults gravity and bathymetry data from transform fault systems associated with mid ocean ridges that are spreading at different rates indicates that crustal thickening may occur under transform faults associated with intermediate and fast spreading ridges if confirmed this observation would require a change in our understanding of how magma accretion occurs along rapidly spreading mid ocean ridges mid ocean ridge morphology and crustal accretion are known to depend on the spreading rate of the ridge slow spreading mid ocean ridge segments exhibit significant crustal thinning towards transform and non transform offsets1 2 3 4 5 6 7 8 9 10 11 12 which is thought to arise from a three dimensional process of buoyant mantle upwelling and melt migration focused beneath the centres of ridge segments1 2 4 5 6 7 9 10 12 in contrast fast spreading mid ocean ridges are characterized by smaller segment scale variations in crustal thickness which reflect more uniform mantle upwelling beneath the ridge axis13 14 15 here we present a systematic study of the residual mantle bouguer gravity anomaly of 19 oceanic transform faults that reveals a strong correlation between gravity signature and spreading rate previous studies have shown that slow slipping transform faults are marked by more positive gravity anomalies than their adjacent ridge segments1 2 4 6 but our analysis reveals that intermediate and fast slipping transform faults exhibit more negative gravity anomalies than their adjacent ridge segments this finding indicates that there is a mass deficit at intermediate and fast slipping transform faults which could reflect increased rock porosity serpentinization of mantle peridotite and or crustal thickening the most negative anomalies correspond to topographic highs flanking the transform faults rather than to transform troughs where deformation is probably focused and porosity and alteration are expected to be greatest indicating that crustal thickening could be an important contributor to the negative gravity anomalies observed this finding in turn suggests that three dimensional magma accretion may occur near intermediate and fast slipping transform faults

seismic evidence of tectonic control on the depth of water influx into incoming oceanic plates at subduction trenches water transported by slabs into the mantle at subduction zones plays key roles in tectonics magmatism fluid and volatiles fluxes and most likely in the chemical evolution of the earth s oceans and mantle yet incorporation of water into oceanic plates before subduction is a poorly understood process several studies suggest that plates may acquire most water at subduction trenches because the ocean crust and uppermost mantle there are intensely faulted caused by bending and or slab pull and display anomalously low seismic velocities the low velocities are interpreted to arise from a combination of fluid filled fractures associated to normal faulting and mineral transformation by hydration mantle hydration by transformation of nominally dry peridotite to water rich serpentinite could potentially create the largest fluid reservoir in slabs and is therefore the most relevant for the transport of water in the deep mantle the depth of fracturing by normal fault earthquakes is usually not well constrained but could potentially create deep percolation paths for water that might hydrate up to tens of kilometers into the mantle restrained only by serpentine stability yet interpretation of deep intraplate mineral alteration remains speculative because active source seismic experiments have sampled only the uppermost few kilometers of mantle leaving the depth extent of anomalous velocities and their relation to faulting unconstrained here we use a joint inversion of active source seismic data and both local and regional earthquakes to map the three dimensional distribution of anomalous velocities under a seismic network deployed at the trench seafloor we found that anomalous velocities are restrained to the depth of normal fault micro earthquake activity recorded in the network and are considerably shallower than either the rupture depth of teleseismic normal fault earthquakes or the limit of serpentine stability extensional micro earthquakes indicate that each fault in the region slips every 2 3 months which may facilitate regular water percolation deeper teleseismic earthquakes are comparatively infrequent and possibly do not cause significant fracturing that remains open long enough to promote alteration detectable with our seismic study our results show that the stability field of serpentine does not constrain the depth of potential mantle hydration

active tectonics of the north and central aegean sea summary in this paper we examine the connection between the westward motion of turkey relative to europe and the extension in and around the aegean sea the principal new data available since the last attempt to synthesize the tectonics of this region by mckenzie 1978 are much improved focal mechanisms of earthquakes constrained by p and sh body wave modelling as well as by first motions these mechanisms show that the faulting in the western part of the aegean region is mostly extensional in nature on normal faults with a nw to wnw strike and with slip vectors directed nnw to nne there is evidence from palaeomagnetism that this western region rotates clockwise relative to stable europe in the central and eastern aegean and in nw turkey distributed right lateral strike slip is more prevalent on faults trending ne to ene and with slip vectors directed ne palaeomagnetic data in this eastern region is more ambiguous but consistent with very small or no rotations in the northern part and possibly anticlockwise rotations relative to europe in the south the strike slip faulting that enters the central aegean from the east appears to end abruptly in the sw against the nw trending normal faults of greece the kinematics of the deformation is controlled by three factors the westward motion of turkey relative to europe the continental collision between nw greece albania and the apulia adriatic platform in the west and the presence of the hellenic subduction zone to the south as the right lateral slip on the north anatolian fault enters the aegean region it splays out becoming distributed on several parallel faults the continental shortening in nw greece and albania does not allow the rotation of the western margin of the region to be rapid enough to accommodate this distributed e w right lateral shear and thus leads to e w shortening in the northern aegean which is compensated by n s extension as the southern aegean margin can move easily over the hellenic subduction zone the dynamics of the system once initiated is self sustaining being driven by the high topography in eastern turkey and by the roll back of the subducted slab beneath the southern aegean the geometry of the deformation resembles the behaviour of a system of broken slats attached to margins that rotate in spite of its extreme simplicity a simple model of such broken slats is able to reproduce quantitatively most of the features of the instantaneous velocity field in the aegean region including the slip vectors and nature of the faulting in the eastern and western parts the senses and approximate rates of rotation the overall extensional velocity across the aegean and the distribution of strain rates as seen in the seismicity and topography or bathymetry and using geodetic measurements as part of this study we re examined the relation between the surface faulting and the focal parameters determined seismologically for the three large 1981 gulf of corinth earthquakes and reassessed the evidence for associating particular earthquakes in the sequence with observed surface faulting an important result is that in one of the events there is a resolvable discrepancy between the slip vector measured at the surface and that determined from the seismic body waves

incoming plate faulting in the northern and western pacific and implications for subduction zone water budgets abstract the greatest uncertainty in the amount of water input into the earth at subduction zones results from poor constraints on the degree of mantle serpentinization in the incoming plate recent studies suggest that the depth of serpentinization within the incoming plate mantle is likely controlled by the depth of extensional faulting caused by lithospheric bending at the outer rise and trench we explore the maximum depth of extension within the incoming plate at northwestern pacific subduction zones in order to estimate the depth limit of serpentinization and to identify any significant variation between subduction zone segments we relocate trench earthquakes to identify which events occurred within the incoming plate and determine accurate depths for 63 incoming plate earthquakes occurring during 1988 2011 by inverting teleseismic broadband p and sh waveforms we observe that the top 10 15 km of the incoming plate mantle experiences extensional faulting at all of the subduction zones with a reasonable sample of earthquakes 60 of the total number of extensional earthquakes occur at crustal depths or within the top 5 km of the incoming plate mantle 80 occur above 10 km within the mantle and 95 occur above 15 km there is evidence for variation throughout the different regions of study for example extensional earthquakes occur down to 20 km below the crust in the western aleutians and izu bonin we propose that the incoming plate mantle is most strongly hydrated in the upper 5 km and that partial serpentinization exists regionally within the incoming plate mantle to 15 km making reasonable assumptions about the degree of serpentinization and incorporating previous estimates of crustal water we calculate that the total water carried into the northern and western pacific subduction zones is generally higher than previous estimates and is approximately 4 6 10 8 tg myr or 45 70 10 3 tg myr per kilometer of subduction zone

outer rise earthquakes and seismic coupling outer rise earthquakes can be separated into two basic categories based on focal mechanism compressional and tensional events reflecting compressional and tensional outer rise stress regimes we propose a model that connects the stress regime in the outer rise to the behavior of large subduction earthquakes and explains the focal mechanisms and both the temporal and spatial occurrence of outer rise earthquakes we find that compressional outer rise events tend to occur oceanward of seismic gaps in seismically coupled subduction zones or equivalently just prior to large subduction zone earthquakes the compressional stress regime is probably caused by slip in adjacent subduction zone segments there are two modes of occurrence for tensional outer rise earthquakes depending on whether they occur in coupled or uncoupled subduction zones in coupled subduction zones tensional events tend to occur after large subduction events as tensional stress from slab pull is temporarily transmitted to the outer rise in subduction zones that are inherently uncoupled tensional events can occur at any time therefore in uncoupled subduction zones the outer rise is always in tension and only tensional outer rise events occur there while in coupled subduction zones the outer rise is in a compressional state prior to large subduction earthquakes and a tensional state afterwards due to the large displacements

impact of bending related faulting on the seismic properties of the incoming oceanic plate offshore of nicaragua impact of bending related faulting on the seismic properties of the incoming oceanic plate offshore of nicaragua

source process of the great 1977 sumba earthquake in order to investigate the applicability of the asperity model to intraplate earthquakes we have studied the rupture process of the great 1977 sumba normal faulting earthquake mw 8 2 8 3 one of the largest earthquakes since 1963 this event has been variously interpreted as a plate detachment event i e rupture through the entire lithosphere or a shallow plate bending event we have analyzed long period body waves in order to determine the spatiotemporal rupture characteristics including the depth extent of the rupture area visible depth phases in the initial stage of weak radiation beginning about 15 s before the main pulse suggest that the earthquake nucleated near 29 km depth which corresponds to the maximum depth of the aftershock sequence the excitation of a strong tsunami implies that rupture extends to the surface while deconvolutions of long period p waves suggest a relatively shallow 50 km maximum depth extent however the existence of slip below this depth cannot be ruled out particularly if it preferentially radiates frequencies below the passband of the body waves used 0 02 hz for undiffracted phases the source time function of the principal rupture which initiated 30 km west of the epicenter of the precursor comprises a dominant initial short duration 24 s triangular pulse followed by several smaller pulses however slightly diffracted phases indicate a simple basic underlying character for the rupture consisting of the initial 24 s triangle superimposed on a longer duration 42 44 s trapezoid the absence of observable directivity in the deconvolved source time functions suggests a bilateral rupture mode although the body waves do not require a fault length greater than 80 km the aftershock area and surface wave results of zhang and kanamori 1988 imply a total length of at least 200 km the body wave source functions are well modeled by a 200 km long fault extending to 30 50 km depth on which slip nucleates at the center of the fault and has variable displacement along strike the peak moment release of the deconvolved source functions indicates higher displacement in the hypocentral area than in other areas of the fault the spatial relationship of the precursor and the main pulse is suggestive of rupture initiating at and propagating along the edge of an asperity prior to the rupture of the asperity itself the large size of the event may be due to an unusual stress environment arising from strong lateral gradients in seismic interplate coupling along the java timor arc

on the deformation cycle of a strongly coupled plate interface the triple earthquakes of 16 march 1963 15 november 2006 and 13 january 2007 along the kurile subduction zone 1 the great plate interface kurile earthquake of 15 november 2006 mw 8 3 was immediately followed by normal aftershocks in the outer rise of the central kurile subduction zone two months later at a distance 50 km from this event the 13 january 2007 mw 8 1 earthquake with a pure normal mechanism occurred in the outer rise of the central kurile the 15 november 2006 earthquake ruptured an area of 280 150 km2 with maximum slip of almost 7 m while the 13 january 2007 earthquake ruptured an area of 240 40 km2 with a maximum slip of almost 14 m these two significant events were preceded by a major reverse earthquake on 16 march 1963 mw 7 7 that took place close to the location of the 13 january 2007 earthquake these very different earthquakes demonstrate the different stages of the deformation cycle along strongly coupled plates at the central kurile arc and present a rare case of stress regime reversal over a short time span in order to understand the details of these three earthquakes we inverted for the source rupture processes of the earthquakes and delineated the location and dimension of asperities the role of gravity and topography anomalies and their correlation to the asperity distribution as well as the long term seismicity is studied in detail using trench parallel bouguer anomaly we have identified the differences in the asperities and the location of larger normal outer rise aftershocks as well as the location of 13 january 2007 earthquake we also show that the long term seismicity of the central kurile arc is confined to the intraslab and not the plate interface the static stress transfer for these earthquakes showed that the stress on optimally oriented thrust faults is increased in the northeastern part of the rupture area of the 15 november 2006 earthquake absence of major and great earthquakes to the northeastern immediate vicinity of the rupture area of the 15 november 2006 earthquake supports the hypothesis that this area is mature for the next strong earthquake along the kurile arc

a procedure for modelling shallow dislocation sources summary expressions for displacements on the surface of a layered half space due to an arbitrary oriented shear dislocation are given in terms of generalized ray expansions useful approximations of these expressions for shallow events as recorded at teleseismic distances for realistic earth models are presented the results of this procedure are used to generate synthetic p sv and sh waveforms for various assumptions of stress drop

hydration reduced lattice thermal conductivity of olivine in earth s upper mantle earth s water cycle enables the incorporation of water hydration in mantle minerals that can influence the physical properties of the mantle lattice thermal conductivity of mantle minerals is critical for controlling the temperature profile and dynamics of the mantle and subducting slabs however the effect of hydration on lattice thermal conductivity remains poorly understood and has often been assumed to be negligible here we have precisely measured the lattice thermal conductivity of hydrous san carlos olivine mg0 9fe0 1 2sio4 fo90 up to 15 gigapascals using an ultrafast optical pump probe technique the thermal conductivity of hydrous fo90 with 7 000 wt ppm water is significantly suppressed at pressures above 5 gigapascals and is approximately 2 times smaller than the nominally anhydrous fo90 at mantle transition zone pressures demonstrating the critical influence of hydration on the lattice thermal conductivity of olivine in this region modeling the thermal structure of a subducting slab with our results shows that the hydration reduced thermal conductivity in hydrated oceanic crust further decreases the temperature at the cold dry center of the subducting slab therefore the olivine wadsleyite transformation rate in the slab with hydrated oceanic crust is much slower than that with dry oceanic crust after the slab sinks into the transition zone extending the metastable olivine to a greater depth the hydration reduced thermal conductivity could enable hydrous minerals to survive in deeper mantle and enhance water transportation to the transition zone

teleseismic location techniques and their application to earthquake clusters in the south central pacific abstract improved methods for single and multiple event hypocenter determinations are developed and applied to the problem of locating earthquake clusters in the south central pacific ocean bayesian statistical methods are used to incorporate a priori information about arrival time variance into the derivation of hypocenter confidence ellipsoids permitting a more realistic calculation of critical parameters in the case where the number of stations is small the diagonal elements of certain projection operators called data importances by minster et al 1974 are used to evaluate network balance the hypocentroid of an event cluster is defined to be the average location of events within the cluster and the deviations of individual hypocenters from the hypocentroid are called cluster vectors the problem of estimating the cluster vectors can be decoupled from the problem of estimating the hypocentroid by a simple but fundamental mathematical result here termed the hypocentroidal decomposition theorem the algorithm based on this analysis appears to have many advantages over other published methods for multiple event location both in its efficient use of available information and its computational speed the application of this method to three clusters of shallow intraplate seismicity in the south central pacific designated regions a b and c demonstrates that the seismicity within each cluster is very localized the rms lengths of the cluster vectors for each group of epicenters are estimated to be only 9 6 and 12 km respectively estimates of the epicentroids are

intraplate seismicity and related mantle hydration at the nicaraguan trench outer rise summary we examine micro earthquake records from a dense temporary array of ocean bottom seismometers obs and hydrophones that has been installed from september to november 2005 at the trench outer rise offshore nicaragua approximately 1 5 locatable earthquakes per day within the array of 110 120 km show the high seismic activity in this region seismicity is restricted to the upper 15 km of the mantle and hence where temperatures reach 350 400 c which is smaller than values observed for large mantle intraplate events 650 c determination of moment tensor solutions suggest a change of the stress region from tensional in the upper layers of the oceanic plate to compressional beneath the neutral plane between both regimes is located at 6 9 km beneath moho and thus very shallow fluids which are thought to travel through the tensional fault system into the upper mantle may not be able to penetrate any deeper the earthquake catalogue which seems to be complete for magnitudes above mw 1 6 1 8 suggests a strong change of the lithospheric rheology when approaching the trench and b factors that is the ratio between small and large earthquakes increase significantly in the closest 20 km to the trench axis implying that the crust and upper mantle is massively weakened and hence ruptures more frequently but under less release of stress we explain this with a partly serpentinized upper mantle

alteration of the subducting oceanic lithosphere at the southern central chile trench outer rise hydrothermal circulation and brittle faulting processes affecting the oceanic lithosphere are usually confined to the upper crust for oceanic lithosphere created at intermediate to fast spreading rates lower crust and mantle rocks are therefore relatively dry and undeformed however recent studies at subduction zones suggest that hydration of the oceanic plate is most vigorous at the trench outer rise where extensional bending related faulting affects the hydrogeology of the oceanic crust and mantle to understand the degree of hydration we studied the seismic velocity structure of the incoming nazca plate offshore of southern central chile 43 s here the deep sea trench is heavily filled with up to 2 km of sediments seismic refraction and wide angle data complemented by seismic reflection imaging of sediments are used to derive a two dimensional velocity model using joint refraction and reflection traveltime tomography the seismic profile runs perpendicular to the spreading ridge and trench axes the velocity model derived from the tomography inversion consists of a 5 3 km thick oceanic crust and shows p wave velocities typical for mature fast spreading crust in the seaward section of the profile with uppermost mantle velocities as fast as 8 3 km s approaching the chile trench seismic velocities are significantly reduced however suggesting that the structures of both the oceanic crust and uppermost mantle have been altered possibly due to a certain degree of fracturing and hydration the decrease of the velocities roughly starts at the outer rise 120 km from the deformation front and continues into the trench even though the trench is filled with sediment basement outcrops in the outer rise frequently pierce the sedimentary blanket anomalously low heat flow values near outcropping basement highs indicate an efficient inflow of cold seawater into the oceanic crust hydration and crustal cracks activated by extensional bending related faulting are suggested to govern the reduced velocities in the vicinity of the trench considering typical flow distances of 50 km water might be redistributed over most of the trench outer rise area where trapped in faults seawater may migrate down to mantle depth causing up to 9 of serpentinization in at least the uppermost 2 km of the mantle between the outer rise and the trench axis

effect of trench outer rise bending related faulting on seismic poisson s ratio and mantle anisotropy a case study offshore of southern central chile summary several trench outer rise settings in subduction zones worldwide are characterized by a high degree of alteration fracturing and hydration these processes are induced by bending related faulting in the upper part of the oceanic plate prior to its subduction mapping of p and s wave velocity structures in this complex tectonic setting provides crucial information for understanding the evolution of the incoming oceanic lithosphere and serves as a baseline for comparison with seismic measurements elsewhere active source seismic investigations at the outer rise off southern central chile 43 s were carried out in order to study the seismic structure of the oceanic nazca plate seismic wide angle data were used to derive 2 d velocity models of two seismic profiles located seaward of the trench axis on 14 5 ma old crust p01a approximately parallel to the direction of spreading and p03 approximately parallel to the spreading ridge and trench axes we determined p and s velocity models using 2 d traveltime tomography we found that the poisson s ratio in the upper crust layer 2 ranges between 0 33 at the top of the crust to 0 28 at the layer 2 3 interface while in the lowermost crust and uppermost mantle it reaches values of 0 26 and 0 29 respectively these features can be explained by an oceanic crust significantly weathered altered and fractured relative high poisson s ratios in the uppermost mantle may be likely related to partially hydrated mantle and hence serpentinization thus the seismic structure of the oceanic lithosphere at the southern central chile outer rise exhibits notable differences from the classic ophiolite seismic model normal oceanic crust these differences are primarily attributed to fracturing and hydration of the entire ocean crust which are direct consequences of strong bending related faulting at the outer rise on the other hand the comparison of the uppermost mantle p wave velocities at the crossing point between the perpendicular profiles 90 km oceanward from the trench axis reveals a low degree of pn anisotropy 2 per cent

multiple plates subducting beneath colombia as illuminated by seismicity and velocity from the joint inversion of seismic and gravity data abstract subduction beneath the northernmost andes in colombia is complex based on seismicity distributions multiple segments of slab appear to be subducting and arc volcanism ceases north of 5 n here we illuminate the subduction system through hypocentral relocations and vp and vs models resulting from the joint inversion of local body wave arrivals surface wave dispersion measurements and gravity data the simultaneous use of multiple data types takes advantage of the differing sensitivities of each data type resulting in velocity models that have improved resolution at both shallower and deeper depths than would result from traditional travel time tomography alone the relocated earthquake dataset and velocity model clearly indicate a tear in the nazca slab at 5 n corresponding to a 250 km shift in slab seismicity and the termination of arc volcanism north of this tear the slab is flat and it comprises slabs of two sources the nazca and caribbean plates the bucaramanga nest a small region of among the most intense intermediate depth seismicity globally is associated with the boundary between these two plates and possibly with a zone of melting or elevated water content based on reduced vp and increased vp vs we also use relocated seismicity to identify two new faults in the south american plate one related to plate convergence and one highlighted by induced seismicity

faulting within the pacific plate at the mariana trench implications for plate interface coupling and subduction of hydrous minerals we investigate faulting within the incoming pacific plate at the mariana subduction trench to understand stresses within the bending plate regional stresses acting upon the plate interface and the extent of possible faulting induced mantle serpentinization we determine accurate depths by inverting teleseismic p and sh waveforms for earthquakes occurring during 1990 2011 with global centroid moment tensor gcmt solutions for earthquakes with mw 5 0 we determine centroid depths and source time functions and refine the fault parameters results from central mariana indicate that all earthquakes are extensional and occur at centroid depths down to 11 km below the moho at the southern mariana trench extensional earthquakes continue to 5 km below the moho one compressional earthquake at 34 km below the seafloor suggests stronger plate interface coupling here in addition we model the stress distribution within the pacific plate along two bathymetric profiles extending seaward from the mariana subduction trench axis to better understand whether our earthquake depth solutions match modeled scenarios for plate bending under applied external forces results from our flexure models match the locations of extensional and compressional earthquakes and suggest that the pacific plate at southern mariana is experiencing larger compressional stresses possibly due to greater interplate coupling additionally we conclude that if extensional faulting promotes the infiltration of water into the subducting plate mantle then the top 5 15 km of the pacific plate mantle are partially serpentinized and a higher percentage of serpentinization is located near the central mariana trench where extensional events extend deeper

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

a reassessment of outer rise seismicity and its implications for the mechanics of oceanic lithosphere we use body waveform modelling to constrain the source parameters of earthquakes occurring globally in oceanic lithosphere beneath the subduction zone outer rise and outer trench slope these data are then used to map the stress state in the lithosphere of the downgoing plate as it bends into the subduction zone our results provide new constraints on the faulting of oceanic lithosphere at the outer rise which is important for understanding the transmission of plate driving forces through the subduction system in all cases shallow normal faulting earthquakes are observed at the top of the plate and are separated in depth from any deeper thrust faulting earthquakes no temporal variation associated with large thrust faulting earthquakes on the subduction interface is seen in the depth extent of each type of faulting at the outer rise the transition depth from trench normal extension to compression is found to vary in agreement with models in which deformation is driven by the combination of in plane stresses and bending stresses resulting principally from slab pull combining the seismologically derived constraints on the thickness of the elastic core of the plate with estimates of the plate curvature we place upper bounds on the strength of the lithosphere at the outer rise which is required to be 300 mpa for a constant yield stress model or governed by an effective coefficient of friction of 0 3

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

seismicity in the outer rise forearc region and configuration of the subducting lithosphere with special reference to the japan trench along nine subduction zones where the largest mw or ms 7 6 normal faulting earthquakes have occurred beneath the outer rise there is an apparent correlation between a large curvature of the subducting slab and regions of low background seismicity in the forearc in six cases a gentle inner trench is also observed taking advantage of the excellent coverage of teleseismic stations and extensive previous studies using local data we investigate the nature of this spatial correlation between seismicity and bathymetry along the honshu forearc in some detail an aseismic region in the forearc lies directly west of the rupture zone of the 1933 sanriku earthquake mw 8 4 a great normal faulting earthquake that most likely has ruptured the entire brittle portion of the lithosphere in the outer rise along this particular cross section of the japan trench large to moderate sized interplate earthquakes span a short interval of depths between approximately 14 and 31 km about half the interval observed along an adjacent profile to the south 12 45 km the aseismic portion of the plate interface between depths of approximately 30 and 45 km has persisted for over 100 years the occurrence of lithospheric normal faulting earthquakes in the outer rise region indicates that bending moment within the subducting slab limited by yield strength of the lithosphere has nearly reached an asymptotic value moment saturation at this state the curvature of the plate can increase with little increase in bending moment as such any stresses transmitted across the plate interface will be relaxed by a permanent increase in the plate s curvature and little elastic strain can accumulate across the interplate thrust zone to produce large earthquakes thus the saturation of bending moment in the subducting lithosphere offers a straightforward interpretation for the observed correlation between seismicity in the outer rise forearc region and configuration of the subducting lithosphere

serpentinization in the trench outer rise region offshore of nicaragua constraints from seismic refraction and wide angle data summary recent seismic evidence suggested that most oceanic plate hydration is associated with trenchouter rise faulting prior to subduction hydration at trenches may have a significant impact on the subduction zone water cycle previous seismic experiments conducted to the northwest of nicoya peninsula northern costa rica have shown that the subducting cocos lithosphere is pervasively altered which was interpreted to be due to both hydration serpentinization and fracturing of the crustal and upper mantle rocks new seismic wide angle reflection and refraction data were collected along two profiles running parallel to the middle american trench axis offshore of central nicaragua revealing lateral changes of the seismic properties of the subducting lithosphere seismic structure along both profiles is characterized by low velocities both in the crust and upper mantle velocities in the uppermost mantle are found to be in the range 7 3 7 5 km s 1 thus are 8 10 per cent lower than velocities typical for unaltered peridotites and hence confirm the assumption that serpentinization is a common process at the trench outer rise area offshore of nicaragua in addition a prominent velocity anomaly occurred within the crust beneath two seamounts here velocity reduction may indicate increased porosity and perhaps permeability supporting the idea that seamounts serve as sites for water percolation and circulation

seismic constraints on the water flux delivered to the deep earth by subduction for the first time large quantities of water are identified throughout the length of a subducting plate and an efficient pathway is defined for injecting water into the deep 600 km earth by the incorporation of water into the mantle of an oceanic plate as serpentine water is thought to facilitate plate tectonics but the quantities of water involved and thus its effect are poorly known taking advantage of an anomalous seismic arrival recorded in the tonga fiji subduction zone the presence volume water content and flux of serpentine and water associated with the subducting oceanic mantle at tonga is quantified at 2 10 8 tg ma h 2 o many times larger than previous inferences based on surface observations water fluxes of this magnitude into the deep earth will substantially increase the role of water in hypotheses of plate tectonic movements mantle plumes and flood basalt activity

stress transfer and seismic phenomena in coupled subduction zones during the earthquake cycle an elementary mechanical model of a coupled subduction zone is presented it is used to study the transfer of extensional stresses along the underthrusting plate and shear stress interactions between the plate itself and its thrust contact zone with the overriding plate during the earthquake cycle the model assumes an elastic plate which deforms one dimensionally has shear drag interactions treated approximately by the elsasser procedure with a maxwellian viscoelastic mantle and is subject to steady gravitational sinking forces great thrust earthquakes are simulated as sudden stress relieving displacements at the contact zone between the slab and overriding plate the model allows analysis of the space and time variable stresses in the main thrust zone the adjacent oceanic plate and the subducting slab over each earthquake cycle stress variations leading to outer rise events are discussed as well as stress changes responsible for intraplate seismicity downdip from the main thrust zone based on simulations of periodic great earthquakes the results of simulated slow progression of deformation slip from below into the thrust contact zone are also presented such preslip occurring over a time scale that is small in comparison with the whole seismic cycle e g of the order of a tenth or so is shown to transiently slow the rate of buildup of or even relieve extensional stresses in the slab in the absence of preslip those stresses build up steadily in time during the latter part of the cycle the results have implications for induction of sesimic quiescence as well as for explaining patterns of seismicity in relation to spatial position along the underthrusting plate and to time throughout the cycle correlations are reported between the model simulations and seismicity observations from coupled subduction zones around the world the observations include tensional outer rise events after large subduction earthquakes and compressional ones in the latter parts of the cycle normal tensional intraplate earthquakes preceding the main thrust events and positioned downdip from the thrust zones and interlaced periods of seismic activity in the lower part of the main thrust zone and in the descending slab

rate of serpentinization in seafloor environments the permeability of partially serpentinized peridotites has been measured near room temperature and found to be in the range of 10 11 to 10 12 darcys or 10 7 to 10 8 cm2 sec 1 comparable to shaley materials serpentinites act as semipermeable membranes to salt ions in ocean crust and deep groundwater environments serpentinization must be a continuous process controlled only by water access to ultramafic rocks through their cover given this it will generate high local strains and stresses with episodic cracking the serpentinization process may also contribute to heat flow and ocean floor topography

insight into complex rupturing of the immature bending normal fault in the outer slope of the japan trench from aftershocks of the 2005 sanriku earthquake mw 7 0 located by ocean bottom seismometry 1 the distribution of aftershocks of a large mw 7 0 normal faulting earthquake beneath the outer slope of the japan trench in 2005 measured in 2007 using ocean bottom seismographs indicates that the earthquake was involved with a set of conjugate normal faults although the faults reach to the upper mantle the estimated vp and vp vs show no remarkable changes that can be related to extensive hydration in the crust or uppermost mantle the absence of horst graben topographic structure in the rupture area suggests that immaturity of the bending fault system is responsible for the relatively unhydrated lithosphere several earthquakes below the aftershock zone may belong to the lower plane seismicity of the shallow double seismic zone because no earthquakes were recorded in the area for more than 80 years before the 2005 event shallow extensional and deep compressive earthquakes may be activated concurrently in the focal area of this earthquake

structure and composition of the aleutian island arc and implications for continental crustal growth we present results of a seismic reflection and refraction investigation of the aleutian island arc designed to test the hypothesis that volcanic arcs constitute the building blocks of continental crust the aleutian arc has the requisite thickness 30 km to build continental crust but it differs strongly from continental crust in its composition and reflectivity structure seismic velocities and the compositions of erupted lavas suggest that the aleutian crust has a mafic bulk composition in contrast to the andesitic bulk composition of continents the silicic upper crust and reflective lower crust that are characteristic of continental crust are conspicuously lacking in the aleutian intraoceanic arc therefore if island arcs form a significant source of continental crust the bulk properties of arc crust must be substantially modified during or after accretion to a continental margin the pervasive deformation intracrustal melting and delamination of mafic to ultramafic residuum necessary to transform arc crust into mature continental crust probably occur during arc continent collision or through subsequent establishment of a continental arc the volume of crust created along the arc exceeds that estimated by previous workers by about a factor of two

seismic coupling and outer rise earthquakes variation of interplate seismic coupling at subduction zones is a major factor controlling the size of the largest underthrusting events this variation also has a profound effect on the regional intraplate stresses in the vicinity of the subduction zone outer rise seismicity is strongly correlated with variations in interplate coupling reflecting the stress state of the interplate coupled zone over 200 outer rise earthquakes with known focal mechanisms are used to investigate the relationship between stresses in the outer rise and interplate seismic coupling these events occur within the downgoing i e oceanic plate near the bathymetric trench axes and generally fall into the categories of tensional normal or compressional thrust with their tensional or compressional stress axes oriented approximately horizontal and perpendicular to the trench in uncoupled subduction zones only tensional outer rise earthquakes occur which indicates that the outer rise is dominated by tensional stresses associated with plate bending and or slab pull forces in strongly coupled subduction zones both tensional and compressional outer rise events are found these events are related both spatially and temporally to the distribution of large underthrusting earthquakes and are thus an integral part of the earthquake cycle in the strongly coupled regions tensional outer rise events follow large underthrusting events as the outer rise is temporarily in tension due to the underthrusting motion compressional outer rise events take place as compressional stress slowly accumulates oceanward of locked sections of the interplate zone in four instances compressional outer rise earthquakes have been followed by large underthrusting events which have occurred 2 4 7 and 19 years after the associated outer rise event the remaining compressional outer rise events are located in regions that are either known seismic gaps or in regions where the seismic potential is unknown the occurrence of compressional outer rise earthquakes suggests that compressional stress is accumulating in the adjacent interplate region and that there is the potential for a future large underthrusting event in the region thirty compressional outer rise events have been located in trench segments of middle and south america the kurile islands the tonga and kermadec islands the new hebrides arc and the solomon islands regions in both the southern kamchatka and northern new hebrides regions the outer rise seismicity indicates that the stress regimes in the outer rise have changed with time from tensional following a previous large underthrusting event to compressional at present thus three stages of the cycle from underthrusting to tensional outer rise regime to compressional outer rise regime are present requiring only the occurrence of the next underthrusting event to complete the cycle the occurrence of compressional outer rise events is useful for assessing the seismic potential of a region on an intermediate time scale

thermal cracking and the deep hydration of oceanic lithosphere a key to the generation of plate tectonics 1 the development of transient thermal stress in suboceanic mantle is investigated on the basis of two dimensional thermoviscoelastic models incorporating composite rheology appropriate for dry oceanic lithosphere thermal stress is shown to be sufficiently high to deeply fracture the coldest part of lithosphere e g to the depth of at least 30 km and possibly down to 50 km in 100 ma old lithosphere the release of thermal stress by tension cracking is limited to the vicinity of cracks and the cascade crack system is suggested to be required given the finite fracture strength of mantle materials possible physical and chemical consequences of deep thermal cracking are also discussed the rheological evolution of oceanic lithosphere is likely to be affected by thermal cracking and subsequent serpentinization which introduces the localized zones of weakness in the otherwise stiffest part of lithosphere this localized weakening may help to explain why plate tectonic convection not stagnant lid convection operates in earth s mantle

mantle wedge water contents estimated from seismic velocities in partially serpentinized peridotites 1 water carried into subduction zones with the down going plate and subsequently released by dehydration reactions at depth affects the composition of the mantle wedge triggers partial melting and affects subduction zone seismicity partially serpentinized peridotite may be a significant reservoir for water in the subducted plate the mantle wedge and the overriding plate here we develop a model that relates the degree of serpentinization and water content of partially serpentinized peridotites to their seismic p wave velocities in partially serpentinized ultramafic rocks a 1 decrease in p wave velocity corresponds to a 2 4 increase in serpentine content and a 0 3 0 18 moles m3 increase in h2o content up to a maximum of 13 where there is evidence of serpentinization mantle serpentine content is typically 15 corresponding to 4 5 wt h2o 6 10 moles m3

the updip seismic aseismic transition of the sumatra megathrust illuminated by aftershocks of the 2004 aceh andaman and 2005 nias events great subduction earthquakes exhibit segmentation both within the rupture of individual events and in the long term history of the margin the 2004 december 26 aceh andaman and 2005 march 28 nias event in northern sumatra are two of the largest earthquakes in recent years with both co and post seismic displacements constrained in unprecedented detail using aftershock locations from a temporary seismic array in the boundary region between both events and waveform modelling of large aftershocks we demonstrate that the vast majority of aftershocks in the study region occur on the plate interface within a narrow band 20 km seaward of the outer arc high comparing the seismicity distribution to the co and post seismic displacements we infer that the seismic band marks the transition between the seismogenic zone and stable sliding the location of the band and therefore the transition appears to be correlated with the 500 m bathymetry contour this close correspondence is disrupted at the boundary between the two great earthquakes where the transition to seismogenic behaviour occurs further landward by 25 km to the west of simeulue where seafloor bathymetry throughout the forearc is deeper than 500 m the seismic band terminates abruptly and the focus of aftershock activity is found near the trench the seismic efficiency of afterslip varies dramatically along strike the segment below the banyak islands in the gap between the two main asperities of the nias earthquake accommodates a much larger proportion of afterslip seismically