comparison of smectite and illite rich gouge frictional properties application to the updip limit of the seismogenic zone along subduction megathrusts along plate boundary subduction thrusts the transformation of smectite to illite within fault gouge at temperatures of 150 c is one of the key mineralogical changes thought to control the updip limit of seismicity if correct this hypothesis requires illite rich gouges to exhibit frictionally unstable velocity weakening behavior here we report on laboratory experiments designed to investigate the frictional behavior of natural and synthetic clay rich gouges we sheared 5 mm thick layers of commercially obtained pure ca smectite a suite of smectite quartz mixtures and natural illite shale grain size ranging from 2 to 500 m in the double direct shear geometry to shear strains of 7 30 at room humidity and temperature xrd analyses show that the illite shale contains dominantly clay minerals and quartz within the clay sized fraction 40 mpa our data specifically the velocity strengthening behavior of illite shale under a wide range of conditions do not support the hypothesis that the smectite illite transition is responsible for the seismic aseismic transition in subduction zones we suggest that other depth and temperature dependent processes such as cementation consolidation and slip localization with increased shearing may play an important role in changing the frictional properties of subduction zone faults and that these processes in addition to clay mineralogy should be the focus of future investigation

influence of pre existing pervasive fabrics on fault patterns during orthogonal and oblique rifting an experimental approach abstract pervasive mechanical anisotropy e g metamorphic foliation in the pre rift basement rocks often appears to control the orientation of new faults in natural rift systems and thus tend to influence formation of fault bounded sedimentary basins however the extent to which this influence works in nature has remained debatable and contrasting ideas exist regarding the issue of structural inheritance in natural rifts like the east african rift system earlier experimental works have investigated different kinematic aspects of faulting in simulated orthogonal and oblique rift systems including the role of a crustal scale pre existing zone of weakness on continent scale rifting these experiments have tested the influence of discrete pre existing weak zones e g pre existing faults or an ancient orogenic belt on localization of subsequent rifting but have not considered the control of pervasive fabric anisotropy e g metamorphic foliation etc in pre rift basement rocks on the fault patterns during rifting here we present a series of simple roughly scaled analog model experiments where pervasive strength anisotropy in the basement was created by brushing plaster of paris on a viscous pitch substratum this way a transverse anisotropy of tensile strength was generated in the plaster of paris layer due to thickness variation across the brush marks the basement was overlain by a dry sand pack simulating weak sedimentary rocks overlying a foliated crystalline basement orthogonal and oblique extension of the models at various angles and with differently oriented strength anisotropy exhibited the control of the anisotropy on the orientations of new faults in the sand layer in general anisotropy could significantly control the fault pattern only when it was at a high angle 45 with respect to the maximum instantaneous horizontal stretching direction e h1 otherwise they largely disregarded the anisotropy and formed orthogonal to e h1 faults never formed orthogonal to the displacement vector d in oblique rifting unless the fault controlling anisotropy was perpendicular to d faults strongly following the anisotropy were much longer than those not following that trend faults oblique to the anisotropy were short segmented and tortuous in nature and mainly grew by segment linkage the experimental results support many field observations made in the east african rift system and are likely to be applicable to other rift systems like gondwana basins of india where field data on such structural inheritance are meager

basin centered asperities in great subduction zone earthquakes a link between slip subsidence and subduction erosion 1 published areas of high coseismic slip or asperities for 29 of the largest circum pacific megathrust earthquakes are compared to forearc structure revealed by satellite free air gravity bathymetry and seismic profiling on average 71 of an earthquake s seismic moment and 79 of its asperity area occur beneath the prominent gravity low outlining the deep sea terrace 57 of an earthquake s asperity area on average occurs beneath the forearc basins that lie within the deep sea terrace in sw japan slip in the 1923 1944 1946 and 1968 earthquakes was largely centered beneath five forearc basins whose landward edge overlies the 350 c isotherm on the plate boundary the inferred downdip limit of the locked zone basin centered coseismic slip also occurred along the aleutian mexico peru and chile subduction zones but was ambiguous for the great 1964 alaska earthquake beneath intrabasin structural highs seismic slip tends to be lower possibly due to higher temperatures and fluid pressures kilometers of late cenozoic subsidence and crustal thinning above some of the source zones are indicated by seismic profiling and drilling and are thought to be caused by basal subduction erosion the deep sea terraces and basins may evolve not just by growth of the outer arc high but also by interseismic subsidence not recovered during earthquakes basin centered asperities could indicate a link between subsidence subduction erosion and seismogenesis whatever the cause forearc basins may be useful indicators of long term seismic moment release the source zone for cascadia s 1700 a d earthquake contains five large basin centered gravity lows that may indicate potential asperities at depth the gravity gradient marking the inferred downdip limit to large coseismic slip lies offshore except in northwestern washington where the low extends landward beneath the coast transverse gravity highs between the basins suggest that the margin is seismically segmented and could produce a variety of large earthquakes

controls on the shape and kinematics of the central andean plateau flanks insights from numerical modeling abstract the flat surface of orogenic plateaus requires a weak detachment at the base of the crust that prevents the formation of large wavelength topography in the centre of the orogen and decouples the deformation of crust and mantle this study uses numerical distinct element simulations of plateau orogeny to show that the mass flux across the lateral tips of this detachment controls the shape and kinematics of the plateau flanks if the mass flux is directed towards the plateau e g foreland crust moves into the detached section it forms a low slope small taper pro flank if the mass flux is directed outward thrusting towards the foreland generates a steep slope large taper retro flank the mass flux itself is controlled by the shear strength distribution at the base of the crust the two general types of orogenic flanks from three combinations that can be used to classify the evolution of plateau orogens in general application of this concept to the andes allows to decode the lower crustal shear strength distribution during the formation of the altiplano puna plateau their western slope is a retro flank localized by the underlying oceanic subduction zone on the eastern side a pro flank accretes foreland crust which is progressively decoupled from the moving mantle below

slip weakening distance of faults during frictional melting as inferred from experimental and natural pseudotachylytes the slip weakening distance d c over which a fault weakens during seismogenic fault motion is one of most important parameters controlling fault instability a paradox over the last decade has been the source of the large difference between laboratory measured values of d c using conventional friction experiments 10 5 10 3 m and seismically determined d c values for natural earthquakes 10 1 10 0 m d c is a difficult quantity to estimate for natural faults but this study attempts to determine the order of d c values for natural faults containing pseudotachylytes based on a comparison between experimental and natural pseudotachylytes experiments at a constant slip velocity of 0 85 m sec and a normal stress of 1 4 mpa on gabbro have revealed that the progressive growth of a molten layer causes marked slip weakening slip weakening a reduction in frictional strength with displacement and the growth of the molten layer an increase in the thickness of the molten layer with displacement are both characterized by exponential changes with nearly the same characteristic distances the comparison suggests that the order of d c can be estimated from the relationship between pseudotachylyte thickness along its generation fault and fault displacement sibson s 1975 data from the outer hebrides exhibit a similar pseudotachylyte thickness versus fault displacement curve with a characteristic distance of 0 38 m this is the same order of magnitude as seismically inferred values of d c the effect of frictional melting is a possible solution for the d c paradox although it is not the only possible solution since frictional melting along large faults is rather uncommon

response of the san andreas fault to the 1983 coalinga nunez earthquakes an application of interaction based probabilities for parkfield 1 the parkfield cholame section of the san andreas fault site of an unfulfilled earthquake forecast in 1985 is the best monitored section of the world s most closely watched fault in 1983 the m 6 5 coalinga and m 6 0 nunez events struck 25 km northeast of parkfield seismicity rates climbed for 18 months along the creeping section of the san andreas north of parkfield and dropped for 6 years along the locked section to the south right lateral creep also slowed or reversed from parkfield south here we calculate that the coalinga sequence increased the shear and coulomb stress on the creeping section causing the rate of small shocks to rise until the added stress was shed by additional slip however the 1983 events decreased the shear and coulomb stress on the parkfield segment causing surface creep and seismicity rates to drop we use these observations to cast the likelihood of a parkfield earthquake into an interaction based probability which includes both the renewal of stress following the 1966 parkfield earthquake and the stress transfer from the 1983 coalinga events we calculate that the 1983 shocks dropped the 10 year probability of a m 6 parkfield earthquake by 22 from 54 22 to 42 23 and that the probability did not recover until about 1991 when seismicity and creep resumed our analysis may thus explain why the parkfield earthquake did not strike in the 1980s but not why it was absent in the 1990s we calculate a 58 17 probability of a m 6 parkfield earthquake during 2001 2011 index terms 7223 seismology seismic hazard assessment and prediction 7230 seismology seismicity and seismotectonics 7260 seismology theory and modeling keywords coalinga earthquake parkfield stress change earthquake probability seismicity rate

global frequency of magnitude 9 earthquakes for decades seismologists have sought causal relationships between maximum earthquake sizes and other properties of subduction zones with the underlying notion that some subduction zones may never produce a magnitude 9 or larger event the 2004 andaman m w 9 2 earthquake called into question such ideas given multicentury return times of the greatest earthquakes ignorance of those return times and our very limited observation span i suggest that we cannot yet make such determinations present evidence cannot rule out that any subduction zone may produce a magnitude 9 or larger earthquake based on theoretical recurrence times i estimate that one to three m9 earthquakes should occur globally per century and the past half century with five m9 events reflects temporal clustering and not the long term average

detection and monitoring of ongoing aseismic slip in the tokai region central japan analysis of global positioning system data shows that the rate of crustal deformations in the tokai region of japan a seismic gap area changed over the past 18 months kalman filtering analysis shows aseismic slip on the plate boundary in the western tokai region centered on lake hamana adjacent to the anticipated tokai earthquake source area the cumulative moment magnitude reaches 6 7 in june 2002 with a relative slip increase northeast of lake haman from january 2002 an existence of aseismic slip in the western tokai supports the hypothesis of a silent event as the cause of uplifting several days before the 1944 tonankai earthquake

why large earthquakes do not nucleate at shallow depths although by definition tectonic earthquakes may nucleate anywhere within the seismogenic layer in almost all cases those earthquakes that become large nucleate near the base of the seismogenic layer because frictional strength and stress drop should increase with depth and in situ stress measurements suggest that the ambient stress increases with depth we have examined spontaneous rupture models with gradients of both average stress drop and average strength relative to ambient stress to determine whether this explains the observed phenomena we report here that ruptures that are nucleated with in the low stress drop shallow regions of the model are inhibited from propagating but those that are nucleated within the high stress drop regions can propagate over the entire fault plane

modeling earthquake cycles in the shumagin subduction segment alaska with seismic and geodetic constraints deformation associated with the earthquake cycle in the shumagin islands segment of the alaska aleutian subduction zone is analyzed with the use of a two dimensional finite element model the model consists of an oceanic plate dipping under an upper plate both of which respond elastically to stress fluctuations in the earthquake cycle and these are underlain by asthenospheric mantle and mantle wedge regions which respond viscoelastically it is tailored to the geometry of the shumagin islands region by using seismicity to define the position of the interplate interface and partially coupled region along it the model is preconditioned by forcing this interface to undergo periodically repeated slips up to and including the time of the may 31 1917 event ms 7 4 in that region with each chosen to be consistent with the moment and estimated rupture area of that event we investigated the dependence of model results for geodetic signals on the strength of seismic coupling between the plates and viscoelastic relaxation of deviatoric stresses in the mantle including in the mantle wedge close to the plate junction and along the aseismic downdip continuation of the thrust interface in models with significant relaxation in the wedge or downdip thrust zone results show that as the intraseismic stage matures there is a region of diminished compressional strain rates and even of locally extensional rates on the earth s surface above the downdip end of the seismically coupled zone based on the seismic estimates of the location of the coupled zone this region is in the area of the shumagin islands we find that if approximately 20 of the convergence takes place seismically compatible with the previous seismic history and if an extensive region of relaxed deviatoric stress is assumed to be present in the wedge and or along the downdip interface then deformations predicted by the model can be made consistent with the measured strain data from the shumagin islands geodetic network lisowski et al 1988 larson and lisowski 1994 as well as uplift and tilt data savage and plafker 1991 beavan 1992 our model simulations here suggest that the shumagin segment is capable of large earthquakes the hypothesis of totally aseismic subduction is not similarly consistent with all geodetic constraints

opposing gradients of permanent strain in the aseismic zone and elastic strain across the seismogenic zone of the kodiak shelf and slope alaska an industry acquired seismic reflection line from the aleutian arc to the alaska trench was prestack depth migrated and further constrained with wide angle ocean bottom hydrophone seismic velocity data and swath mapping from the seismic reflection image individual thrust slices the backstop and the forearc basin were resolved sufficiently well to balance the section tectonic shortening and age estimates provided rates of permanent contraction in five discrete segments over 150 km during the past 3 myr about 70 of the plate convergence is measured as contraction across the 40 km wide neogene accretionary prism which composes less than 10 of the forearc permanent contraction is minimal above the 1964 alaska earthquake aftershock zone indicating dominently elastic strain since elastic strain is poorly recorded in the geological record it was approximated from the historical record of the 1964 alaska earthquake the 1964 coseismic vertical deformation displays an inverse pattern to the permanent strain consistent with the earthquake cycle concept elastic deformation is dominant beneath the shelf and upper slope rapid permanent deformation is concentrated beneath the lower slope and a transition between them occurs beneath the middle slope

tsunami earthquakes possibly widespread manifestations of frictional conditional stability 1 tsunami earthquakes shallow events that produce larger tsunamis than expected given their surface wave magnitudes ms typically have long durations and a source spectrum depleted in short period energy seven cases of underthrusting tsunami earthquakes provide information on the rupture processes but little constraint on geographic distribution or frequency we compare their rupture characteristics with smaller magnitude earthquakes on circum pacific interplate thrust faults comparable moment release time histories are found for large tsunami earthquakes and for many smaller shallow subduction zone earthquakes with significantly longer durations and additional source complexity than for events deeper than 15 km thus very shallow interplate earthquake ruptures are scale invariant with variable frictional properties on the plate interface controlling the depth dependent rupture process widespread occurrence of small shallow interplate earthquakes with long durations suggests that many subduction faults have frictional properties that may enable large tsunami generating earthquakes to occur fortunately large shallow ruptures are infrequent

geological record of fluid flow and seismogenesis along an erosive subducting plate boundary a fossil erosive subduction channel the shear zone marking the plate boundary preserved in the northern apennines of italy is identified the fossil zone records the presence of two decollements which seem to have been simultaneously active at the top and base of the subduction channel in modern subduction zones the onset of seismic activity is believed to occur at 150 c however in the fossil channel the onset seems to have occurred at colder temperatures tectonic erosion of the overriding plate by the downgoing slab is believed to occur at half the earth s subduction zones1 2 in situ investigation of the geological processes at active erosive margins is extremely difficult owing to the deep marine environment and the net loss of forearc crust to deeper levels in the subduction zone until now a fossil erosive subduction channel the shear zone marking the plate boundary3 has not been recognized in the field so that seismic observations have provided the only information on plate boundary processes at erosive margins here we show that a fossil erosive margin is preserved in the northern apennines of italy it formed during the tertiary transition from oceanic subduction to continental collision and was preserved by the late deactivation and fossilization of the plate boundary the outcropping erosive subduction channel is 500 m thick it is representative of the first 5 km of depth with its deeper portions reaching 150 c the fossil zone records several surprises two decollements were simultaneously active at the top and base of the subduction channel both deeper basal erosion and near surface frontal erosion occurred at shallow depths extension was a key deformation component within this erosive convergent plate boundary and slip occurred without an observable fluid pressure cycle at depths greater than about 3 km a fluid cycle is clearly shown by the development of veins and the alternation of fast co seismic and slow inter seismic slip in the deepest portions of the outcropping subduction channel extension is finally overprinted by compressional structures in modern subduction zones the onset of seismic activity is believed to occur at 150 c but in the fossil channel the onset occurred at cooler palaeo temperatures

a discrete episode of seismic and aseismic deformation of the nankai trough subduction zone accretionary prism and incoming philippine sea plate new insights into episodic deformation at the nankai trough subduction zone are provided by data from two ocean drilling program borehole hydrologic observatories drilled into the philippine sea plate site 1173 and the seaward part of the nankai accretionary prism off southwestern japan site 808 and from an array of high sensitivity borehole accelerometer and velocity seismometers on shikoku island hi net fluid pressures monitored at multiple levels in each of the offshore boreholes document steady secular trends that indicate contraction of the crust and sedimentary section at the incoming plate site and relaxation of the accretionary prism toe the rate of strain ca 10 6 yr 1 inferred from the rates of pressure rise at site 1173 up to 5 kpa yr 1 is similar to that which would be produced by plate convergence if convergent elastic strain were distributed over a region a few tens of km wide opposing these trends are transient pressure anomalies observed in late june early july 2003 that indicate an episode of rapid relaxation of the incoming plate and contraction of the outer prism concurrent with these transients a swarm of very low frequency earthquakes occurred farther landward in the prism above the estimated seaward limit of the currently locked seismogenic portion of the subduction thrust reverse fault mechanisms determined for one event of this swarm and several of another also indicate transient contraction of the prism we suggest that both the earthquakes and the pressure transients are the consequences of an aseismic slip dislocation that initiated on about june 26 2003 at or near the up dip limit of the locked portion of the subduction thrust and propagated seaward over the course of about 10 days to the accretionary prism toe along the decollement separating the prism and underthrust section deformational events like this may serve incrementally to relieve stress locally along the subduction thrust and to load neighboring areas in addition to demonstrating that the prism is far from inactive during interseismic intervals the observations may also provide a small amplitude analog for strain and hydrologic response at the time of great subduction earthquakes

saline groundwater and brines in crystalline rocks the contributions of john andrews and jean charles fontes to the solution of a hydrogeological and geochemical problem abstract investigations into the suitability of crystalline rock for the long term containment of high level nuclear waste were initiated in the early 1970s a significant task was thereby to assess fluid movements to and from potential repositories this paper focuses on the contributions of j n andrews and j ch fontes to this problem and presents some aspects of their work on fluids in crystalline rock environments waste repository studies saw major progress with the initiation of the stripa project in 1977 andrews joined this project during the first year and participated until its closure in the late 1980s fontes involvement began during the second phase of the project after the first ideas about the geochemistry and evolution of groundwater encountered in the stripa granite had been developed the problems associated with dating of such hydrogeological systems their geochemical evolution and the question of origin of dissolved salts occupied both of them and their co workers those privileged to participate in the long and lively discussions will remember these times as extremely fruitful the results are documented in a multitude of international studies where the stripa ideas were further developed and tested not only in sweden based on the data from swedish and other environments with crystalline rock brines andrews fontes and others had recognised the need to develop at least one additional project designed to clarify major points of uncertainty regarding the age and evolution as well as the origin of saline fluids found in most crystalline rock environments the untimely death of these two applied geochemists is the reason why these discussions have not yet been concluded

stress triggering in thrust and subduction earthquakes and stress interaction between the southern san andreas and nearby thrust and strike slip faults 1 we argue that key features of thrust earthquake triggering inhibition and clustering can be explained by coulomb stress changes which we illustrate by a suite of representative models and by detailed examples whereas slip on surface cutting thrust faults drops the stress in most of the adjacent crust slip on blind thrust faults increases the stress on some nearby zones particularly above the source fault blind thrusts can thus trigger slip on secondary faults at shallow depth and typically produce broadly distributed aftershocks short thrust ruptures are particularly efficient at triggering earthquakes of similar size on adjacent thrust faults we calculate that during a progressive thrust sequence in central california the 1983 mw 6 7 coalinga earthquake brought the subsequent 1983 mw 6 0 nunez and 1985 mw 6 0 kettleman hills ruptures 10 bars and 1 bar closer to coulomb failure the idealized stress change calculations also reconcile the distribution of seismicity accompanying large subduction events in agreement with findings of prior investigations subduction zone ruptures are calculated to promote normal faulting events in the outer rise and to promote thrust faulting events on the periphery of the seismic rupture and its downdip extension these features are evident in aftershocks of the 1957 mw 9 1 aleutian and other large subduction earthquakes we further examine stress changes on the rupture surface imparted by the 1960 mw 9 5 and 1995 mw 8 1 chile earthquakes for which detailed slip models are available calculated coulomb stress increases of 2 20 bars correspond closely to sites of aftershocks and postseismic slip whereas aftershocks are absent where the stress drops by more than 10 bars we also argue that slip on major strike slip systems modulates the stress acting on nearby thrust and strike slip faults we calculate that the 1857 mw 7 9 fort tejon earthquake on the san andreas fault and subsequent interseismic slip brought the coalinga fault 1 bar closer to failure but inhibited failure elsewhere on the coast ranges thrust faults the 1857 earthquake also promoted failure on the white wolf reverse fault by 8 bars which ruptured in the 1952 mw 7 3 kern county shock but inhibited slip on the left lateral garlock fault which has not ruptured since 1857 we thus contend that stress transfer exerts a control on the seismicity of thrust faults across a broad spectrum of spatial and temporal scales

dynamic deformation of the accretionary prism excites very low frequency earthquakes 1 we have detected anomalous very low frequency earthquakes within the accretionary prism along the nankai trough southwestern japan centroid moment tensor inversion analysis reveals that the earthquake hypocenters are distributed at 10 km depth above the upper surface of the subducting philippine sea plate and within 50 70 km landward of the trough axis the focal mechanisms indicate reverse faulting their hypocenters are distributed beneath a deformation zone of an accretionary prism in sea floor topography these observations suggest that the occurrence of very low frequency earthquakes is related to numerous reverse fault systems within the accretionary prism and that the earthquakes reflect the dynamics of deformation within this accretionary prism

paleogeodetic records of seismic and aseismic subduction from central sumatran microatolls indonesia we utilize coral microatolls in western sumatra to document vertical deformation associated with subduction microatolls are very sensitive to fluctuations in sea level and thus act as natural tide gauges they record not only the magnitude of vertical deformation associated with earthquakes paleoseismic data but also continuously track the long term aseismic deformation that occurs during the intervals between earthquakes paleogeodetic data this paper focuses on the twentieth century paleogeodetic history of the equatorial region our coral paleogeodetic record of the 1935 event reveals a classical example of deformations produced by seismic rupture of a shallow subduction interface the site closest to the trench rose 90 cm whereas sites further east sank by as much as 35 cm our model reproduces these paleogeodetic data with a 2 3 m slip event on the interface 88 to 125 km from the trench axis our coral paleogeodetic data reveal slow submergence during the decades before and after the event in the areas of coseismic emergence likewise interseismic emergence occurred before and after the 1935 event in areas of coseismic submergence among the interesting phenomenon we have discovered in the coral record is evidence of a large aseismic slip or silent event in 1962 27 years after the 1935 event paleogeodetic deformation rates in the decades before after and between the 1935 and 1962 events have varied both temporally and spatially during the 25 years following the 1935 event submergence rates were dramatically greater than in prior decades during the past four decades however rates have been lower than in the preceding decades but are still higher than they were prior to 1935 these paleogeodetic records enable us to model the kinematics of the subduction interface throughout the twentieth century

laboratory results indicating complex and potentially unstable frictional behavior of smectite clay a central problem in explaining the apparent weakness of the san andreas and other plate boundary faults has been identifying candidate fault zone materials that are both weak and capable of hosting earthquake like unstable rupture our results demonstrate that smectite clay can be both weak and velocity weakening at low normal stress 30 mpa our data are consistent with previous work which has focused on higher normal stress conditions 50 mpa and greater and found only velocity strengthening if natural fault zones contain significant smectite one key implication of our results is that localized zones of high pore pressure which reduce effective normal stress could be important in controlling potential sites of earthquake nucleation our experiments indicate that friction of smectite is complex and depends upon both sliding velocity and normal stress this complexity highlights the need for detailed experiments that reflect in situ conditions for fault gouges

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

splay fault branching along the nankai subduction zone seismic reflection profiles reveal steeply landward dipping splay faults in the rupture area of the magnitude m 8 1 tonankai earthquake in the nankai subduction zone these splay faults branch upward from the plate boundary interface that is the subduction zone at a depth of approximately 10 kilometers approximately 50 to 55 kilometers landward of the trough axis breaking through the upper crustal plate slip on the active splay fault may be an important mechanism that accommodates the elastic strain caused by relative plate motion

probable low angle thrust earthquakes on the juan de fuca north america plate boundary in 2004 two clusters of earthquakes occurred in the central part of the cascadia forearc which displays several characteristics indicative of along strike and downdip variations in plate coupling moment tensor analysis for the main shock in each cluster indicates that both events which had magnitudes of 4 9 and 4 8 were compatible with low angle thrust motion on fault planes dipping 6 15 to the east consistent with the plate boundary dip of 12 by tracing rays through a high resolution two dimensional crustal velocity model to match arrival times of secondary arrivals pp and pmp we estimate that the source depth was 9 11 km for the m4 9 event and 15 17 km for the m4 8 event we conclude that these earthquakes probably represent seismogenic thrust motion on the nominally locked or transitional part of the cascadia megathrust updip of where episodic tremor and slip ets have been documented continued high resolution observations of seismicity and improved crustal models are needed to confirm whether apparent temporal correlations between ets and continued seismic activity in these clusters indicate stress transfer along the megathrust

shocks in a box an analogue model of subduction earthquake cycles with application to seismotectonic forearc evolution 1 we introduce and test an experimental approach to simulate elastoplastic megathrust earthquake cycles using an analogue model and apply it to study the seismotectonic evolution of subduction zones the quasi two dimensional analogue model features rate and state dependent elastic frictional plastic and viscoelastic material properties and is scaled for gravity inertia elasticity friction and viscosity the experiments are monitored with a high resolution strain analysis tool based on digital image correlation particle imaging velocimetry piv providing deformation time series comparable to seismologic geodetic and geologic observations in order to separate elastic and nonelastic effects inherent the experimental deformation patterns we integrate elastic dislocation modeling edm into a hybrid approach we use the analogue earthquake slip and interseismic locking distribution as edm dislocation input and forward model the coseismic and interseismic elastic response the residual which remains when the edm prediction is subtracted from the experimental deformation pattern highlights the accumulation of permanent deformation in the model the setup generates analogue earthquake sequences with realistic source mechanisms and elastic forearc response and recurrence patterns and reproduces principal earthquake scaling relations by applying the model to an accretionary type plate margin we demonstrate how strain localization at the rupture peripheries may lead to a seismotectonically segmented forearc including a tectonically stable shelf and coastal high 20 plate convergence accommodated by internal shortening overlying the area of large megathrust earthquake slip fifty to 75 of plate convergence is accommodated by internal shortening in the slope region where earthquake slip tapers out toward the trench the inner forearc region remains undeformed and represents a basin

loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones earthquakes do not extend updip along the plate interface at most subduction zones all of the way to the plate boundary at the trench axis or deformation front rather the shallowest part of that interface moves mainly through stable aseismic slip that part of the plate boundary referred to here as the aseismic zone occurs along the base of an accreted wedge of young sediments the probable primary cause for the existence of this aseismic zone is the stable slip properties of the unconsolidated and semiconsolidated sediments in that zone subducted sediment is progressively dewatered and underplated to the base of the overriding plate through this process more consolidated rocks eventually come into contact at depth across both sides of the plate boundary from the point where sufficiently hard rock is found across the plate interface that interface will change its slip behavior to unstable stick slip sliding which is characteristic of consolidated material under most conditions this type of motion is accommodated seismically as episodic slip in large earthquakes the location of this transition to seismic behavior referred to as the seismic front marks the deep end of the aseismic zone and the top of the seismogenic zone i e that part of the plate interface that moves primarily in thrust earthquakes several convergent plate margins are discussed to illustrate the seismic front and the aseismic zone we find that the seismic front defined by smaller earthquakes that occur during the interval between large events is nearly the same as that for large events as inferred from the locations of their aftershocks the location of the seismic front is important for making estimates of the maximum possible size of thrust earthquakes along the plate boundary because it delimits the trenchward limit of the potential rupture area of these events the size of large thrust earthquakes is proportional to rupture area and to at least the cube of the downdip width w of that area thus by defining the location of the seismic front for convergent margins it is possible to make better estimates of w and hence to deduce the maximum size of future interplate earthquakes the average repeat time of such events is also related to w and so better estimates of average repeat time are possible from improved knowledge of w the hypothesis that the location of the seismic front is related to the maximum depth of subduction of unconsolidated sediment has implications for forearc mechanics we use a mechanical analysis laboratory modeling and multichannel seismic information to develop a simple model explaining the growth of forearcs the outer arc high or trench slope break that occurs arcward of most accretionary wedges represents an abrupt change in the critical taper of the accretionary wedge we argue that that change is caused by a large arcward increase in the strength of the material within the overriding plate that stronger material is called the backstop our laboratory modeling experiments indicate that a backstop with a trenchward dipping upper surface results in the development of an overlying structure that includes all of the primary morphologic features observed in modern forearcs in our modeling an accretionary wedge develops trenchward of the backstop an outer arc high develops above the trenchward toe of the backstop and farther arcward a passive forearc basin forms above the stronger material of the backstop this model is consistent with the observation that earthquakes do not extend updip along the plate interface all the way to the trench axis we hypothesize that plate motion is accommodated seismically along the base of the consolidated backstop and mostly or entirely aseismically trenchward of the toe of the backstop along the base of the accretionary wedge at several margins the seismic front is approximately coincident with the outer arc high supporting this interpretation

interaction between forearc and oceanic plate at the south central chilean margin as seen in local seismic data we installed a dense amphibious temporary seismological network to study the seismicity and structure of the seismogenic zone in southern chile between 37 and 39 s the nucleation area of the great 1960 chile earthquake 213 local earthquakes with 14 754 onset times were used for a simultaneous inversion for the 1 d velocity model and precise earthquake locations relocated artificial shots suggest an accuracy of the earthquake hypocenter of about 1 km horizontally and 500 m vertically crustal events along trench parallel and transverse deep reaching faults reflect the interseismic transpressional deformation of the forearc crust due to the subduction of the nazca plate the transverse faults seems to accomplish differential lateral stresses between subduction zone segments many events situated in an internally structured planar seismicity patch at 20 to 40 km depth near the coast indicate a stress concentration at the plate s interface at 38 s which might in part be induced by the fragmented forearc structure

earthquake rupture stalled by a subducting fracture zone we showed that the rupture produced by the great peru earthquake moment magnitude 8 4 on 23 june 2001 propagated for 70 kilometers before encountering a 6000 square kilometer area of fault that acted as a barrier the rupture continued around this barrier which remained unbroken for 30 seconds and then began to break when the main rupture front was 200 kilometers from the epicenter the barrier had relatively low rupture speed slip and aftershock density as compared to its surroundings and the time of the main energy release in the earthquake coincided with the barrier s rupture we associate this barrier with a fracture zone feature on the subducting oceanic plate

crustal motion in the zone of the 1960 chile earthquake detangling earthquake cycle deformation and forearc sliver translation temporary deformation in great earthquake cycles and permanent shear deformation associated with oblique plate convergence both provide critical clues for understanding geodynamics and earthquake hazard at subduction zones in the region affected by the mw 9 5 great chile earthquake of 1960 we have obtained gps observations that provide information on both types of deformation our velocity solutions for the first time span the entire latitudinal range of the 1960 earthquake the new observations revealed a pattern of opposing roughly arc normal motion of coastal and inland sites consistent with what was reported earlier for the northern part of this region this finding supports the model of prolonged postseismic deformation as a result of viscoelastic stress relaxation in the mantle the new observations also provide the first geodetic evidence for the dextral motion of an intravolcanic arc fault system and the consequent northward translation of a forearc sliver the sliver motion can be modeled using a rate of 6 5 mm a accommodating about 75 of the margin parallel component of nazca south america relative plate motion with the rate diminishing to the north furthermore the new gps observations show a southward decrease in margin normal velocities of the coastal area we prefer explaining the southward decrease in terms of changes in the width or frictional properties of the megathrust seismogenic zone because of the much younger age of the subducting plate and warmer thermal regime in the south the currently locked portion of the plate interface may be narrower using a three dimensional viscoelastic finite element model of postseismic and interseismic deformation following the 1960 earthquake we demonstrate that this explanation although not unique is consistent with the gps observations to the first order

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

slow slip and frictional transition at low temperature at the hikurangi subduction zone temperature changes with depth do not appear to be a primary control for either slow slip or fault locking processes at the hikurangi margin north island new zealand both slow slip events and the geodetically observed transition from fault locking to free slip at depth occur at temperatures as low as 100 c shallow portions of faults exhibit brittle stick slip behaviour that gives way to more stable sliding with increasing depth limiting the depths to which earthquake inducing slip can occur the increase of temperature with depth is often assumed to transform friction in the fault from velocity weakening unstable to velocity strengthening stable and to change conditions from brittle to ductile1 2 3 4 5 a temperature of 350 c has been suggested to mark the base of the frictionally unstable portion of the fault which becomes free slipping at depth where temperatures rise above 450 c refs 2 4 6 here we show that both slow slip events and the geodetically observed transition from fault locking to free slip at the hikurangi subduction zone east of the north island new zealand occur at temperatures as low as 100 c suggesting that temperature is not a primary control on either slow slip or fault locking processes at the hikurangi margin although globally shallow slow slip events are rare five out of seven events observed so far at the hikurangi margin are less than 15 km deep

age estimates of coastal terraces in the andaman and nicobar islands and their tectonic implications the great indian ocean earthquake of december 26 2004 caused significant vertical changes in its rupture zone about 800 km of the rupture is along the andaman and nicobar islands which forms the outer arc ridge of the subduction zone coseismic deformation along the exposed land could be observed as uplift subsidence here we analyze the morphological features along the coast of the andaman and nicobar islands in an effort to reconstruct the past tectonics taking cues from the coseismic effects we obtained radiocarbon dates from coastal terraces of the island belt and used them to compute uplift rates which vary from 1 33 mm yr 1 in the little andaman to 2 80 mm yr 1 in south andaman and 2 45 mm yr 1 in the north andaman our radiocarbon dates converge on sim 600 yr and sim 1000 yr old coastal uplifts which we attribute to the level changes due to two major previous subduction earthquakes in the region

seismicity and geometry of the south chilean subduction zone 41 5 s 43 5 s implications for controlling parameters in 2005 an amphibious seismic network was deployed on the chilean forearc between 41 75 s and 43 25 s 364 local events were observed in a 11 month period a subset of the p and s arrival times were inverted for hypocentral coordinates 1 d velocity structure and station delays main seismic activity occurred predominantly in a belt parallel to the coast of chiloe island in a depth range of 12 30 km presumably related to the plate interface the 30 inclination of the shallow part of the wadati benioff zone is similar to observations further north indicating that oceanic plate age is not controlling the subduction angle of the shallower part for the chilean subduction zone the down dip termination of abundant intermediate depth seismicity at approximately 70 km depth seems to be related to the young age and high temperature of the oceanic plate crustal seismicity is associated with the liquine ofqui fault zone and active volcanoes