recurrence behavior of short term slow slip and correlated nonvolcanic tremor episodes in western shikoku southwest japan 1 from the national research institute for earth science and disaster prevention hi net tiltmeter data we determine time dependent source processes of seven repeating short term slow slip events sse in the western shikoku region southwest japan over the period from 2002 to 2007 the tiltmeters record clear signals of sses characterized by a slow rise a week long duration and a magnitude on the order of 10 2 10 1 rad we apply a time dependent slip inversion method to the tilt records for the seven sses for each sse a slip area migrates in the strike direction of the subducting slab at a rate of 10 km d 1 comparing the slow slip area with epicenters of accompanying nonvolcanic tremors in subday time resolution we find that the two phenomena are located on a smaller area than previously reported and migrate as a group a moment rate function for each sse exhibits good correlation with the temporal change in tremor activity a comparison of the slip distributions of all of the analyzed sses shows that there is a patch of slow slip area 30 km in diameter that is shared by all of the episodes in addition the location of the slip patch is in good agreement with the epicenters of tremors and very low frequency earthquakes these lines of evidence suggest that the identified slip patch is one of the main slow slip patches in the region and a recurrence of sses may be controlled by the patch and that these slow earthquakes might be different manifestations of a single slip process on the deep plate interface

inverse modelling of elastic thickness by convolution method the eastern alps as a case example an unconventional scheme is used to estimate the flexural rigidity or equivalently the elastic thickness of the lithosphere given the topography and gravity data the flexural rigidity is the parameter that governs the flexural response of the lithosphere in the frame of the thin plate flexure model the scheme is an alternative to the widely used calculation of admittance of topography sea floor or continental topography and gravity bearing some advantages which are explained in the paper the scheme involves the inversion of the gravity data in order to formulate a model of the crust mantle interface cmi undulations in a second step the flexure parameter is then evaluated from the relation between topography and cmi variations instead of calculating the admittance function using a spectral analysis a set of point load response functions are used in order to retrieve the optimal flexure parameter this has two main advantages instabilities of the numerical admittance evaluation at wavenumbers with low spectral energy in the topography are overcome and the analysis can be made over an area which is not necessarily rectangular as required for the spectral analysis the proposed method allows a higher space resolution of elastic thickness than any spectral method for validation the numerical strategy is applied to the situation of a realistic synthetic model where all inputs and outputs are known a priori finally the spatial variations of the elastic thickness are studied in an area across the eastern alps 4 2002 elsevier science b v all rights reserved

very low frequency earthquakes related to small asperities on the plate boundary interface at the locked to aseismic transition 1 very low frequency vlf seismic signals observed in southwestern japan are evidently radiated from shear slips on the upper surface of the subducting philippine sea plate we used grid moment tensor inversion and centroid moment tensor inversion to calculate 242 moment tensor solutions with moment magnitudes between 3 1 and 3 8 from continuous seismograms recorded over a 5 year period by using a very dense broadband seismic network at least 5 10 sequences of repetitive activity were observed during the 5 years the vlf events formed clustered distributions along the 35 km isodepth contour of the subducting plate surface the nodal planes which dip landward of moment tensor solutions of the vlf events reflected the configuration of the subducting plate interface the slip vectors were consistent with the direction of movement of the subducting plate the dip and strike of the slip vectors clearly reflected the configuration of the upper surface of the subducting plate we found that the rates of seismic moment release per unit area associated with five major vlf clusters were very similar although both the seismic magnitudes and sizes of the clusters varied considerably the rate of seismic moment released from detectable vlf sources was 0 1 of the rates of short term slow slip events suggesting that the source areas occupied only 0 1 of the fault segment on which the short term slow slip events occurred

dynamic modeling of slow slip coupled with tremor 1 we theoretically study the generation mechanism of slow slip in a dynamical framework assuming a fault in a fluid saturated thermoporoelastic medium the fluid flow is taken into consideration dynamic slip is assumed to create pores and cause shear heating on the fault key parameters in the modeling are su and su the nondimensional parameter su represents the relative dominance of the effect of pore creation on the fluid pressure change over that of shear heating while su is associated with the dominance of fluid flow effect over the effect of shear heating our calculation shows that we have to assume su 1 and relatively small values for su to simulate rupture speed that is negligibly small compared to the shear wave velocity although this was also found in our former paper our new finding here is the simulation of slow slip coupled with tremor we find that frozen slip is reactivated sporadically because the fluid inflow triggered by the pore creation elevates the fluid pressure on the fault the slip reactivation actually occurs if su is smaller than a threshold such sporadic reactivation of slips can be a model for slow slip coupled with tremor we also show that the moment evolution curves are categorized into two separate groups when su 0 1 one of them represents slow slips while the other represents ordinary earthquakes

tremor genic slow slip regions may be deeper and warmer and may slip slower than non tremor genic regions slow slip events sses are observed worldwide and often coincide with tectonic tremor notable examples of sses lacking observed tectonic tremor however occur beneath k lauea volcano hawaii the boso peninsula japan near san juan bautista on the san andreas fault california and recently in central ecuador these sses are similar to other worldwide sses in many ways e g size or duration but lack the concurrent tectonic tremor observed elsewhere instead they trigger swarms of regular earthquakes we investigate the physical conditions that may distinguish these non tremor genic sses from those associated with tectonic tremor including slip velocity pressure temperature fluids and fault asperities although we cannot eliminate the possibility that tectonic tremor may be obscured in highly attenuating regions slip velocities of sses at k lauea volcano 10 6 m s and boso peninsula 10 7 m s are among the fastest sses worldwide k lauea volcano the boso peninsula and central ecuador are also among the shallowest sses worldwide and thus have lower confining pressures and cooler temperatures in their respective slow slip zones fluids also likely contribute to tremor generation and no corresponding zone of high vp vs has been noted at k lauea or boso we suggest that the relatively faster slip velocities at k lauea volcano and the boso peninsula result from specific physical conditions that may also be responsible for triggering swarms of regular earthquakes adjacent to the slow slip while different conditions produce slower sse velocities elsewhere and trigger tectonic tremor

lithospheric flexure at fracture zones bathymetric profiles across six major fracture zones fz s in the north pacific are used to demonstrate the absence of vertical slip on the fossil fault planes the scarp heights on these fz s are constant with age and equal to the initial vertical offsets at the ridge transform fault fz intersections because of the frozen in scarp and the differential subsidence of lithosphere far from the fz the lithosphere bends in the vicinity of the fz this flexure results in a characteristic ridge trough topographic fz signature the flexural amplitude which is the difference between the scarp height and the overall change in depth across the fz increases with age good fits to the bathymetric profiles across the mendocino and pioneer fz s are obtained by modelling the topography as the flexure of a thin elastic plate with an age dependent effective elastic thickness results of the modelling indicate that the base of the elastic lithosphere is approximately defined by the 450 c isotherm maximum bending stresses at fz s are on the order of 100 mpa substantially less than the stresses encountered at subduction zones because the mendocino and pioneer fz s are separated by less than a flexural wavelength they are elastically coupled

universality of slow earthquakes in the very low frequency band deep tectonic tremors have been observed together with signals in the very low frequency vlf band from 0 02 to 0 05 hz which have been identified as vlf events in limited regions of subduction zones by stacking broadband seismograms relative to the timing of tremors we have detected similar signals in all regions where tremors occur in western japan these signals are inverted to obtain the moment tensor and the fault normal and slip vectors are generally consistent with the geometry of the plate interface and the direction of plate motion therefore these signals are probably radiated by shear slip on the plate interface the ratio between the seismic energy rate estimated from the tremors and seismic moment rate in the vlf band is almost proportional with a proportionality constant i e scaled energy of around 5 10 10 the spatial distribution of scaled energy may reflect spatial variations in the frequency dependent characteristics of slow deformation

episodic slow slip events accompanied by non volcanic tremors in southwest japan subduction zone 1 episodic slow slip events have been recognized by means of tilt changes in the western shikoku area southwest japan the crustal tilt deformation was observed repeatedly with a recurrence interval of approximately six months coincident with the occurrences of major non volcanic deep tremor activities in this area observed tilt changes can be explained by slow slip events occurring around the source area of tremors in each episode the source of the slow slip event and tremor migrate simultaneously the spatial and temporal coincidence of tremors and slow slip events indicates that they both may be coupling phenomena reflecting the stress accumulation process at the subducting plate

toward 1 mgal accuracy in global marine gravity from cryosat 2 envisat and jason 1 more than 60 of the earth s land and shallow marine areas are covered by 2 km of sediments and sedimentary rocks with the thickest accumulations on rifted continental margins figure 1 free air marine gravity anomalies derived from geosat and ers 1 satellite altimetry fairhead et al 2001 sandwell and smith 2009 andersen et al 2009 outline most of these major basins with remarkable precision moreover gravity and bathymetry data derived from altimetry are used to identify current and paleo submarine canyons faults and local recent uplifts these geomorphic features provide clues to where to look for large deposits of sediments while current altimeter data delineate large offshore basins and major structures they do not resolve some of the smaller geomorphic features and basins yale et al 1998 fairhead et al 2001 improved accuracy and resolution is desirable to facilitate comparisons between continental margins as an exploration tool and to permit extrapolation of known structu

a tremor and slip event on the cocos caribbean subduction zone as measured by a global positioning system gps and seismic network on the nicoya peninsula costa rica in may 2007 a network of global positioning systems gps and seismic stations on the nicoya peninsula of northern costa rica recorded a slow slip event accompanied by seismic tremor the close proximity of the nicoya peninsula to the seismogenic part of the cocos caribbean subduction plate boundary makes it a good location to study such events several centimeters of southwest motion were recorded by the gps stations over a period of several days to several weeks and the seismic stations recorded three distinct episodes of tremor during the same time span inversion of the surface displacement data for the depth and pattern of slip on the plate interface shows peak slip at a depth of 25 30 km downdip of the main seismogenic zone estimated temperatures here are 250 300 c lower than in other subduction zones where events of this nature have been previously identified there may also be a shallower patch of slip at 6 km depth these results are significant in that they are the first to suggest that slow slip can occur at the updip transition from stick slip to stable sliding and that a critical temperature threshold is not required for slow slip tremor and low frequency earthquake locations are more difficult to determine our results suggest they occur on or near the plate interface at the same depth range as the deep slow slip but not spatially colocated

characterizing an uncharacteristic ets event in northern cascadia 1 gps and borehole strainmeter data allowed the detection and model characterization of a slow slip event in northern cascadia in november 2006 accompanying a brief episode of seismic tremor the event is much smaller in area and duration than other well known ets events in northern cascadia but is strikingly similar to typical ets events at the nankai subduction zone the 30 45 km depth range and the 2 3 cm slip magnitude as interpreted for this event appear to be common to most ets events in these two subduction zones regardless of their sizes we infer that the nankai type small ets events must be abundant at cascadia and that ets events at the two subduction zones are governed by a similar physical process

along strike variations in short term slow slip events in the southwest japan subduction zone 1 short term slow slip events accompanied by nonvolcanic deep low frequency tremors and deep very low frequency earthquakes in southwest japan were investigated systematically by means of ground tilting studies the change in tilt usually lasts for several days by using a genetic algorithm and a least squares method we inverted the data for tilt steps that were caused by slow slip events and were detected by at least four stations situated near the source of the tremor fault parameters were estimated for 54 slow slip episodes that occurred mainly in the western shikoku northeastern kii and tokai regions from 2001 to 2008 in eastern shikoku two slow slip episodes were detected quantitatively for the first time the fault geometries of all the slow slip events were located within the belt like distribution of tremors in the transition zone between the locked and aseismic slip zones at the plate interface of the subducting philippine sea plate the spatial extent of the fault geometry corresponds roughly to the distribution of clusters of nonvolcanic tremors and very low frequency earthquakes the moment magnitudes ranged from 5 4 to 6 2 and the slip was 1 cm for each slow slip event the rate of moment release by the detected slow slip events was 40 60 of the moment accumulation expected from the relative plate motion and it showed regional differences they may reflect the along strike variations in plate convergence and or the characteristic size of the slow slip fault plane on the plate interface

sedimentary loading lithospheric flexure and subduction initiation at passive margins investigation of lithospheric flexure at continental margins by sedimentary loading has been carried out using elastic plate theory the geometry of mature marginal basins fits the deflection produced by loading of an infinite plate in which the flexural rigidity of the continental lithosphere is larger than that of oceanic lithosphere decoupling of a previously locked passive margin by reactivation of margin faults may cause the lithospheric bending behavior of the margin to change from that of an infinite plate to that of a semi infinite plate with a resultant increase in deflection of the marginal basin and possible subduction initiation

lithospheric flexure modelling seaward of the chile trench implications for oceanic plate weakening in the trench outer rise region summary the chilean subduction zone presents a unique opportunity to study trench outer rise deformation of the subducting oceanic lithosphere at different thermal ages the shape of the outer rise for plate ages ranging from 0 to 50 ma is predicted by using an elastic plate model with variable elastic thickness te x as a function of the distance measured from the trench axis in addition the uncertainties of our results are estimated by performing a monte carlo type analysis and we considered explicitly the sediment loading effect on the lithospheric flexure in regions where the trench is heavily sedimented the results show a systematic reduction in te of up to 50 per cent or reduction in the flexural rigidity d of up to 90 per cent from the peak of the outer rise to the trench axis the reduction in te and d observed in most of the bathymetric profiles is coincident with i high plate curvatures 5 10 7 m 1 ii strong bending moments 10 16 n m iii pervasive fracturing and faulting of the oceanic basement as imaged by high resolution bathymetry data and iv reduction in crustal and mantle seismic velocities the reduction in flexural rigidity towards the trench suggests a weakening of the oceanic lithosphere and is interpreted to be caused partially by fracturing and a likely increase in fluid pore pressure in general our estimates do not show consistent increases in elastic thickness as a function of plate age this result suggests that either te is independent of plate age or te depends strongly on other factors these factors could include lithospheric weakening due to hydro fracturing and the loading history of the plate prior to subduction

interplate coupling in the kanto district central japan and the boso peninsula silent earthquake in may 1996 i studied crustal deformation in the kanto district central japan based on continuous gps data horizontal as well as vertical displacement rate demonstrate significant interaction between the landward kanto block and the philippine sea plate although the subduction effect of the pacific plate is not apparent it is reasonable to consider the entire kanto district is displaced westward due to the interaction with the pacific plate the gps velocity data were inverted to estimate the slip deficit distribution on the sagami trough subduction zone the result delineates a strongly coupled region on the plate interface part of which corresponds to the 1923 kanto earthquake the strongly coupled region is located shallower than 20 km in addition the plate interaction is laterally heterogeneous even in the same depth range implying thermal structure is not the only factor controlling interplate coupling the gps data also detected a silent earthquake event on the interface of the philippine sea slab east of the boso peninsula in the middle of may 1996 the silent rupture propagated over a 50 km 50 km wide area during about a week the maximum slip was approximately 50 mm and the released seismic moment was 4 7 1018nm mw 6 4 there was a small seismicity triggered by this silent event the silent slip was located in the peripheral of the strongly coupled area suggesting that frictional properties and or stress conditions are inhomogeneous on the plate boundary interface

deep low frequency tremors as a proxy for slip monitoring at plate interface 1 we propose a new method to monitor slip at the plate interface using non volcanic deep low frequency dlf tremors we assume that a dlf tremor is the superposition of frequently excited intermittent events meaning that the envelope of the reduced displacement of the dlf tremor provides an apparent moment rate function we estimate a conversion factor from the apparent moment to the seismic moment with an assumption that a total size of dlf tremors of an episode is proportional to the size of corresponding slow slip event sse the cumulative seismic moment estimated by dlf tremors is consistent with that estimated from geodetic methods and provides appropriate slip and slip rate at the plate interface this proves our assumptions and demonstrates that dlf tremors are useful tool for real time monitoring of the slip at the plate interface

tidal response in shallow tectonic tremors various types of slow earthquakes e g tectonic tremors and slow slip events have been reported in tectonic zones especially in the subduction zone the tidal response of a tremor is considered to be strongly related to the weak friction state of the plate interface and many studies have reported observational evidence of such correlation between tides and deep tremor activity here we used the modified frequency scanning method at a single station to detect micro tectonic tremors that have not been previously reported in southern kyushu in the early stage of the tremor activity tremors are mostly modulated by slow slip events in contrast we found a seismic response to ocean tides during the later stage in the shallower part of the subduction zone this might indicate that the tremors are triggered by tidal changes caused by fault weakening due to slow slip events as same as deeper condition

a slow thrust slip event following the two 1996 hyuganada earthquakes beneath the bungo channel southwest japan we report a slow thrust slip event that occurred beneath the bungo channel region southwestern japan on oct and dec 1996 two hyuganada earthquakes both mw 6 7 followed by afterslips occurred in addition a crustal movement characterized by an extremely slow rise was observed around the bungo channel about 200 km north from the epicenters and continued for about 300 days long assuming a slow slip on the plate boundary we estimate its duration and surface displacements from gps time series data by curve fitting and then determine the fault slip distribution we found that a slow slip without any earthquakes continued for nearly one year and released the seismic moment comparable to that of the hyuganada earthquakes occurrence of the slow thrust slip event suggests that this kind of event may be a characteristic mode of stress release at a transition region of interplate coupling

low friction and fault weakening revealed by rising sensitivity of tremor to tidal stress at subduction zones deep parts of the fault can slip slowly generating tremor analysis of tremor in cascadia reveals increasing sensitivity of slip to tidal stresses over several days implying that the fault is weak and weakens as it slips

scaling relationships of source parameters for slow slip events to better understand the physical mechanisms of slow slip events sses detected worldwide we explore the scaling relationships of various source parameters and compare them with similar scaling laws for earthquakes these scaling relationships highlight differences and similarities between slow slip events and earthquakes and hold implications for the degree of heterogeneity and fault healing characteristics the static stress drop remains constant for different sized events as is observed for earthquakes however the static stress drop of slow slip events is within a range of 0 01 1 0 mpa 1 2 orders of magnitude lower than that found for earthquakes which could be related to the low stress state on the fault the average rupture velocity ranging from kilometers per second to kilometers per day decreases linearly with increasing seismic moment in log log space unlike earthquakes that are nearly constant this inverse relationship of rupture velocity with seismic moment could be related to the heterogeneity of fault properties slow slip events typically have ratios of event duration over dislocation rise time less than 3 while earthquakes have ratios greater than 3 this indicates that slow slip events are less pulselike than earthquakes in their mode of propagation and suggests that the healing behind the rupture front is delayed the recurrence statistics of slow slip events on the northern cascadia subduction zone are weakly time predictable and moderately antislip predictable that is the event size and preevent recurrence interval are anticorrelated which may indicate that healing between events strengthens the fault with time online material table of source parameters and data sources

phenomenology of deep slow earthquake family in southwest japan spatiotemporal characteristics and segmentation 1 the phenomenology of deep slow earthquakes including low frequency tremors very low frequency earthquakes and short term slow slip events on the subducting plate interface in southwest japan is investigated on the basis of their spatiotemporal characteristics the belt like distribution of tremors is divided into segments bounded by gaps the repetition of tremor episode depends on the magnitude of the episode which is defined by the number of detected tremors within each episode major tremor episodes with a large number of detected tremors recur at an interval of approximately 6 months accompanying the short term slow slip events in large segments in small segments tremor episodes with small numbers of tremors have recurrence intervals of a few months some of them occasionally accompany the slight ground tilting caused by a small size slow slip event even in the same segment there exist different recurrence intervals according to the magnitude of the tremor episode the migration pattern varies greatly in initiation termination direction and speed however the migration direction has a general tendency for each segment on both sides of the gap the tremor starts next to the gap and propagates to the other side assuming all tremor episodes are caused by slow slip events the equivalent moment and slip are estimated from the number of detected tremor counts and the area of each tremor episode the equivalent slip history estimated from all tremor episodes is almost constant in each segment this indicates that tremor activity is a good proxy for slow slips

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

effect of a compliant fault zone on the inferred earthquake slip distribution 1 we present a new semi analytic method to evaluate the deformation due to a screw dislocation in arbitrarily heterogeneous and or anisotropic elastic half plane the method employs integral transformations to reduce the governing partial differential equations to the integral fredholm equation of the second kind dislocation sources as well as spatial perturbations in the elastic properties are modeled using equivalent body forces the solution to the fredholm equation is obtained in the fourier domain using a method of successive over relaxation and is mapped into the spatial domain using the inverse fast fourier transform we apply this method to investigate the effect of a soft damage zone around an earthquake fault on the co seismic displacement field and on the earthquake slip distribution inferred from inversions of geodetic data in the presence of a kilometer wide damage zone with a reduction of the effective shear modulus of a factor of 2 inversions that assume a laterally homogeneous model tend to underestimate the amount of slip in the middle of the seismogenic layer by as much as 20 this bias may accentuate the inferred maxima in the seismic moment release at depth between 3 6 km suggested by previous studies of large strike slip earthquakes

the frequency size scaling of non volcanic tremors beneath the san andreas fault at parkfield possible implications for seismic energy release abstract we analyse the frequency size distribution of non volcanic tremors observed along the parkfield section of the san andreas fault we suggest that these non volcanic tremors follow a power law scaling typical of scale invariant stick slip tectonic earthquakes but with an unusually high scaling exponent of more than 2 0 and a systematic depth dependency while each individual non volcanic tremor releases only a minuscule amount of energy and slip this is more than compensated by their sheer numbers consequently the integrated contribution of this largely invisible seismicity non volcanic tremors and nano earthquakes is non negligible and could potentially account in selected patches along the san andreas fault for up to 100 of the plate motion

slow earthquakes and nonvolcanic tremor nonvolcanic tremor is observed in close association with geodetically observed slow slip events in subduction zones accumulating evidence points to these events as members of a family of slow earthquakes that occur as shear slip on the downdip extensions of fault zones in a regime that is transitional between a frictionally locked region above and a freely slipping region below by virtue of their locations and their properties slow earthquakes are certain to provide new insights into the behavior of earthquakes and faulting and into the hazard they embody

short term slow slip and correlated tremor episodes in the tokai region central japan 1 we observe the related phenomenon of short term slow slip event sse and low frequency nonvolcanic tremor on the deeper extension of the 1944 tonankai and the anticipated tokai earthquake rupture areas central japan japan nation wide seismometer and tiltmeter array nied hi net has recorded these correlated episodes of sse and tremor known as episodic tremor and slip ets in 2004 and 2005 the events are located on two areas in the tokai region one is near the shima peninsula and the other is around the aichi area both episodes have a typical duration of 2 3 days and released seismic moment corresponding to mw 6 0 moreover these episodes repeatedly occur with a recurrence interval of about six months in both areas this evidence suggests that the coincidence between the sse and the tremor is a characteristic behavior in the whole tremor belt like zone in the nankai trough subduction zone

constraints on 3 d stress in the crust from support of mid ocean ridge topography 1 the direction of crustal stresses acting at mid ocean ridges is well characterized but the magnitude of these stresses is poorly constrained we present a method by which the absolute magnitude of these stresses may be constrained using seafloor topography and gravity the topography is divided into a short wavelength portion created by rifting magmatism and transform faulting and a long wavelength portion associated with the cooling and subsidence of the oceanic lithosphere the short wavelength surface and moho topography are used to calculate the spatially varying 3 d stress tensor in the crust by assuming that in creating this topography the deviatoric stress reached the elastic plastic limiting stress the moho topography is constrained by short wavelength gravity variations under these assumptions an incompressible elastic material gives the smallest plastic failure stress associated with this topography this short wavelength topographic stress generally predicts the wrong style of earthquake focal mechanisms at ridges and transform faults however the addition of an in plane regional stress field is able to reconcile the combined crustal stress with both the ridge and transform focal mechanisms by adjusting the magnitude of the regional stress we determine a lower bound for in situ ridge perpendicular extension of 25 40 mpa along the slow spreading mid atlantic ridge 40 50 mpa along the ultra slow spreading ridges in the western indian ocean and 10 30 mpa along the fast spreading ridges of the southeastern indian and pacific oceans furthermore we constrain the magnitude of ridge parallel extension to be between 4 and 8 mpa in the atlantic ocean between 1 and 7 mpa in the western indian ocean and between 1 and 3 mpa in the southeastern indian and pacific oceans these observations suggest that a deep transform valley is an essential feature of the ridge transform spreading center

state of stress within a bending spherical shell and its implications for subducting lithosphere summary the state of stress within a bending spherical shell has some special features that are caused by sphericity while most lithospheres are more like spherical shells than flat plates our ideas of the state of stress have been dominated by flat plate models as a consequence we might be missing some important aspects of the state of stress within subducting lithospheres in order to examine this problem we analyse spherical shell bending problems from basic equations we present two approaches to solve sphericalshell bending problems one by the variational approach which is suitable for globalscale problems and the other by the asymptotic equation which is valid to first order in h r where h is the thickness of the lithosphere and r is its curvature radius i e under the assumption of small curvature the form of the equation for displacement shows that wavelengths of deformation are determined by the spherical elastic evect and the gravitational buoyancy evect for which only the latter evect is included in the usual flat plate formulations in the case of the earth the buoyancy force is dominant and consequently spherical evects are suppressed to a large extent this explains why flat plate models have been successful for earth s lithospheric problems on the other hand the state of stress shows interesting spherical evects while bending fibre stress along the subduction zone is always important bending stress along the trench strike direction can also be important in particular when the subduction zone arc is small numerical results also indicate that compressive normal stress along the trench strike direction is important when a subduction zone arc is large these two stresses the bending stress and the compressive normal stress both along the trench strike direction may have important implications for intraplate earthquakes at subduction zones

integration of transient strain events with models of plate coupling and areas of great earthquakes in southwest japan we model the crustal deformation caused by two long term subduction slip transients in southwest japan which we refer to as the 2000 2004 tokai and the 2002 2004 bungo channel slow slip events sses we use re analysed geonet position time series and a kalman filter based network inversion method to image the spatiotemporal slip variation of the two events on the plate interface during the period of 1998 2004 67 and 2000 2005 both events are found to have complex slip histories with multiple subevents in addition to a newly identified slip subevent in 2002 2003 we find that the major event in the bungo channel sse initiated in early 2003 beneath the northeastern corner of the region and expanded southwestward in contrast to the slip characteristics suggested by other studies the re analysed gps data in the tokai region shows a renewed slip activity for the tokai sse in early 2003 2004 at a similar location as in the period of 2001 2002 the equivalent m w for both the tokai and bungo channel sses are about 7 0 our results show that the tokai sse appears to start before the miyaki kozu seismovolcanic event integrating plate coupling and sses shows that the transient slip zones are located in a region between the locked zones and the epicentres of the low frequency earthquakes lfes at least part of the interseismic slip deficit is released by episodic sses beneath the bungo channel region we find excellent temporal correspondence between transient slip and adjacent lfes for both sse suggesting that they are closely related and possibly reflect that long term slow slip may modulate the occurrence of lfes

synchronous changes in the seismicity rate and ocean bottom hydrostatic pressures along the nankai trough a possible slow slip event detected by the dense oceanfloor network system for earthquakes and tsunamis donet abstract we detected long term hydrostatic pressure changes at ocean bottom stations of the dense oceanfloor network system for earthquakes and tsunamis donet along the nankai trough off southwestern japan we detected these changes after removing the contributions of ocean mass variations and sensor drift from the records in addition we detected a decrease in the background seismicity rate of a nearby earthquake cluster that was synchronous with the hydrostatic pressure changes we interpreted these observed hydrostatic pressure changes to reflect vertical deformation of the ocean floor of 3 8 cm and we consider the cause of the seafloor crustal deformation to be a slow slip event sse beneath the stations because the pressure changes were observed at stations with distances less than 20 km to each other we inferred that the sse occurred in the shallow part of the sedimentary wedge such as on a splay fault system the synchronous observation of an sse and a seismicity rate change suggests that both were triggered by a change in the regional stress that may be associated with stress accumulation and release processes occurring along the nankai trough these data show that continuous and careful monitoring of crustal activities by donet stations provides an effective way to detect seismic and geodetic signals related to the occurrence of megathrust or other types of large earthquakes

spatial variations of the effective elastic thickness te using multitaper spectral estimation and wavelet methods examples from synthetic data and application to south america 1 improved recovery of the spatial variability in the effective elastic thickness te of the lithosphere is important to interpret earth s structure and geodynamics here we use synthetic topography and gravity data to systematically compare the recovery of te using the multitaper windowing scheme of perez gussinye et al 2004 2007 with the wavelet approach of kirby and swain 2008 we find that spuriously high te estimates previously present in synthetic tests with the multitaper are artifacts arising from fourier transform edge effects introduced by splitting the data into land and ocean areas prior to load deconvolution hence we adopt the mixed land ocean loading scheme of kirby and swain 2008 using this approach we find that underestimation of te when the transitional wavelength approaches the window size is largely related to the limited size of the analysis window and not to random correlations of initial loading surfaces as occurs in the wavelet method to attenuate this bias we apply a correction factor to our resulting multitapered te and find that this technique more accurately recovers steep te gradients and small scale te anomalies than the wavelet method finally we recalculate te of south america using the multitaper and the mixed land ocean approach the new te distribution is very similar to that of perez gussinye et al 2007 2008 suggesting that fourier transform edge effects are less severe in the real earth than in synthetic data this difference arises because coastlines are much less rough in the real earth than in our synthetic data on the basis of the synthetic results we interpret that most of the small scale features present in the multitaper but not in wavelet te estimates of south america presented by tassara et al 2007 are real these include high te over the andean flat subduction zones and low te along the dyke swarms of the parana flood basalts the tacutu graben and amazonian basin

the 2006 aseismic slow slip event in guerrero mexico new results from gps 1 crustal deformation measurements from guerrero mexico were made with continuous gps instrumentation this network spans 75 km along the coast and a 275 km transect from the coast to mexico city which is perpendicular to the middle america trench a large aseismic slip transient occurred from april to december 2006 yielding horizontal displacements of nearly 6 cm in the direction opposite to that recorded interseismically this transient slip episode closely follows a series of m5 deep intraplate events that occurred in the northwestern portion of the guerrero region both horizontal and vertical displacements are inverted for the guerrero gps network to determine slip along the subduction interface using a simple fault model that accommodates changes in the subduction zone geometry it is found that slip is concentrated on interior fault patches and propagated to the southeast this slip event has a minimum equivalent mw of 7 5

lithospheric 3 d flexure modelling of the oceanic plate seaward of the trench using variable elastic thickness summary whendescribingthemechanicalbehaviourofthelithospheremodelledasathinplate themost important parameter corresponds to itsflexural rigidity which is commonly expressed through the effective elastic thickness te this parameter is a measure of the stiffness of the plate and defines the maximum magnitude and wavelength of those surface loads that can be supported without suffering unelastic deformation realistic 3 d models of the flexural response of the lithosphere near the trench are scarce because of the mathematical and computational complexity we present a method for determining the flexure of the lithosphere caused by the combined effect of 3 d seamount loading and bending of the lithosphere near the trench our method consists on solving numerically the flexure equations of the reissner mindlin thin plate theory including variable thickness using the finite element method with mesh adaptation the method was applied to study the flexure of the oceanic nazca lithosphere beneath the o higgins seamount group which lies 70km seaward of the chile trench the results show that an elastic thickness te of 5km under the seamounts a te of 15km far from the trench and a te of 13km near the trench can explain both the down deflection of the oceanic moho and bending of the oceanic lithosphere observed in seismic and gravity profiles in order to study the impact of high trench curvature on the morphology of the outer rise we apply the same methodology to study and model the flexure of the lithosphere in the arica bend region 14 s 23 s results indicate that the te values are overestimated if the 3 d trench curvature is not included in the modelling

detection of short term slow slip events along the nankai trough southwest japan using gnss data 1 we detected short term slow slip events sses previously observable only with tilt and strain data along the nankai trough southwest japan using gnss global navigation satellite system data offsets detected in gnss time series using akaike s information criterion helped automatically identify 207 episodes with a motion direction opposite to that of the relative plate motion from june 1996 to january 2012 by nonlinear inversion of the detected displacement we estimated rectangular fault models for 133 probable and 25 possible short term sses over 15 years the sse moment magnitudes range from 5 5 to 6 3 most sse fault models are located in a narrow band of non volcanic tremors on the interfaces of the subducting philippine sea plate large sses moment magnitude mw 6 often occur in western and central shikoku the cumulative slip is distributed heterogeneously along the strike generally decreasing eastward with the maximum slip 50 cm in western shikoku no definite short term sses were found in the kii channel but several short term sses occurred in ise bay both regions are known as tremor gaps the local maximum of the cumulative slip fills in the tremor gap located in ise bay the long term rate of short term sse cumulative moment increased by threefold around 2003 in eastern shikoku whereas it was almost constant in other regions comparison with short term sse catalogues using tilt data suggests that both this study and previous studies missed some sses along the nankai trough a combination of geodetic data is important in the monitoring of the spatiotemporal distribution of short term sses

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

slow slip and tremor search at kilauea volcano hawaii 1 kilauea volcano hawaii has hosted a long series of slow slip events observed since the installation of the continuous gps network in 1996 kilauea s slow slip events are inferred to occur on the decollement fault at 8 km depth beneath its south flank with a location updip of the epicenters of large regular earthquakes fault slip typically lasts about two days and the events have magnitudes equivalent to mw 5 3 6 0 while slow slip events in subduction zones are commonly accompanied by tectonic tremor also called nonvolcanic tremor no tremor has yet been reported in association with kilauea s slow slip events instead there are swarms of small triggered earthquakes which is a characteristic only seen at select subduction zones e g boso and hikurangi a temporary array of seismometers was installed at kilauea in 2007 in anticipation of a slow slip event here we use several established methods to perform a systematic search for tectonic tremor during geodetically defined slow slip events as well as searching for tremor triggered by teleseismic surface waves we do not detect tectonic tremor using any of these methods although we are able to detect episodes of previously identified deep offshore volcanic tremor at 15 20 km depth and volcanic tremor from kilauea although kilauea s seismic network may not be adequate to observe tectonic tremor because hawaii is seismically noisy and its crust is highly attenuating it is also possible that the specific fault conditions on kilauea s decollement are not conducive to such tremor generation

high vp vs zone accompanying non volcanic tremors and slow slip events beneath southwestern japan abstract in southwestern japan the philippine sea plate subducts beneath the eurasian plate non volcanic tremors and slow slow slip events are observed beneath southwestern japan seismic tomography based on the data obtained from the dense seismic network revealed the velocity structure in and around the subducting philippine sea plate the oceanic crust of the philippine sea plate subducting from the suruga and nankai troughs can be imaged clearly beneath the tokai region and the kii peninsula non volcanic tremors and short term slow slip events may occur in the high vp vs zones owing to the fluid dehydrated from the subducting plate where the oceanic crust of the philippine sea plate encounters the serpentinized wedge mantle of the eurasian plate these tremors and short term slow slip events occur in the high vp vs zone at depths of 30 40 km beneath the tokai region the landward edge of the high vp vs zone beneath the kii peninsula and eastern shikoku region and the seaward edge of the high vp vs zone beneath the western shikoku region long term slow slip events were observed beneath the tokai region and the bungo channel and both these events are located on the seaward side of a high vp vs zone these long term slow slip events are considered to be caused at the plate boundary between the oceanic crust of the philippine sea plate and the lower crust of the eurasian plate by the high pore fluid pressure caused by the fluid dehydrated from the subducting plate the existence of the high vp vs lower crust of the eurasian plate may control the occurrence of the long term slow slip events

kilauea slow slip events identification source inversions and relation to seismicity 1 several slow slip events beneath the south flank ofkilauea volcano hawaii have been inferred from transient displacements in daily gps positions to search for smaller events that may be close to the noise level in the gps time series we compare displacement fields on kilauea s south flank with displacement patterns in previously identified slow slip events matching displacement patterns are found for several new candidate events although displacements are much smaller than previously identified events one of the candidates 29 may 2000 is coincident with a microearthquake swarm as are all of the previously identified slow slip events the microearthquakes follow the onset of slow slip implying that they are triggered by stress changes during slip the new slow slip event brings the total number of events on kilauea between 1997 and 2007 to eight the smallest having m w 5 3 and the largest having m w 6 0 while the recurrence time between the four largest events is 2 11 0 01 years the repeat time for all eight events is 0 9 0 6 years we invert for the fault geometry and distribution of slip during the slow slip events the optimal source depths of 5 km assuming uniform slip dislocations in an elastic half space are considerably shallower than the accompanying swarm earthquakes 6 5 8 5 km which would place the earthquakes in a zone of decreased coulomb stress inversions including the effects of topography and layered elastic structure in the forward models favor depths comparable to microearthquake depths such that the earthquakes are located in a region of increased coulomb stress we also invert for time dependent fault slip directly from the 30 s gps phase observations constraining the source to the optimal uniform slip geometry on the basis of these inversions the larger events last between 1 5 2 2 days the data are unable to resolve migration of slip along the fault the temporal pattern of accompanying microearthquakes is consistent with the fault slip history assuming a seismicity rate theory based on rate and state friction making the swarm earthquakes coshocks and aftershocks of the slow slip events

constraints on yield strength in the oceanic lithosphere derived from observations of flexure summary the wavelength and amplitude of outer rises seaward of sub duction zones and arches surrounding islands and seamounts are used to parameterize flexure profiles in terms of the moment and curvature at the first zero crossing the data show the clear age dependence in the mechanical thickness of the lithosphere up to 60 100myr saturation of moment at large curvature is interpreted in terms of a depth dependent yield strength for the lithosphere using relations adopted from laboratory experiments of rock deformation a comparison of theoretical curves with observed moments indicates that old oceanic lithosphere has no long term strength below about 40 km depth with no difference between 100 and 165 myr old crust moderate axial loading forces 200 mpa can explain most variations in the moment curvature observations except in the case of the kuril trench which appears anomalous given the age of the crust regional tension causes greater variability in moment as compared to regional compression because of the greater slope in the brittle failure envelope under tension the observations point to a lithosphere weaker than the prediction from experimental deformation of rocks of the possible weakening mechanisms elevated pore fluid pressure on faults does not predict the correct age dependence and is incompatible with earthquake focal mechanisms our favoured explanation is that the activation energy q appropriate for ductile flow at geological strain rates is lower than the values derived from laboratory extrapolations of dry olivine data taken at high temperatures if recent oceanic geotherms are reliable q in the lower lithosphere must be lower than 100kcal mol 1 the method used here is most appropriate for trench profiles with curvatures greater than 10 7m 1 for lower curvatures such as along seamount profiles small errors in the curvature estimate cause large changes in rheological parameters

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

spatiotemporal distribution of seismic energy radiation from low frequency tremor in western shikoku japan 1 the spatiotemporal patterns of the radiated seismic energy from low frequency tremor associated with slow slip events in western shikoku japan have been estimated a spatially decaying mean square amplitude and differential traveltime measurement from envelope correlations observed at hi net stations were used to locate tremors and to estimate their seismic energies tremor amplitude was corrected for the site amplification factors estimated using the coda normalization method the location of the tremor is estimated as the position where both the decay of the observed energy and the differential travel times are well explained the temporal pattern of the total energy released agrees very well with the observed tilt records although the spatial patterns of the energy radiation vary strongly among the six slow slip events sses in western shikoku the region where they overlap always radiates relatively high levels of energy the estimated band limited energy rate of the tremor for a 2 to 10 hz bandwidth was typically between 105 and 106 j min though there is temporal variation of the radiation energy the released total energy associated with the sses whose magnitude is 5 9 6 1 is on the order of 108 j within the duration of sses of 8 13 days this is quite small energy radiation compared to the moment release of accompanying sse the scaled energy which is the ratio of the seismic moment and the seismic energy for the slow slip and a very low frequency earthquake was estimated to be 5 orders of magnitude smaller than that of a regular earthquake

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

spatial distribution and energy release of nonvolcanic tremor at parkfield california nonvolcanic tremors nvts are observed in transition zones between freely slipping and locked sections of faults and normally occur below the seismogenic zone based on nvt recordings in the parkfield region of the san andreas fault we provide a novel approach to assess the energy release of these events and assign magnitudes me that are compatible with size estimates of small earthquakes in the same region to assess the energy magnitude of a detected tremor we refine the estimate of its duration and perform a spectral analysis that accounts for local attenuation for the 218 nvts that we were able to process we resolve me values in the range of 0 67 to 0 84 for events which we could not process using the spectral analysis technique we propose a statistical model to estimate me values using observable characteristics such as peak amplitude spectral velocity at the source corner frequency and duration we furthermore provide seismic moment and moment magnitude estimates and calculate stress drops in a range of 3 10 kpa as a result of our spectral analyses we find strong indications regarding the ongoing debate about potential nvt location hypotheses the parkfield nvts have a higher probability to be located in the proposed three dimensional cloud like cluster than in any other suggested location distribution

stress driven relaxation of heterogeneous upper mantle and time dependent afterslip following the 2011 tohoku earthquake stress driven relaxation of heterogeneous upper mantle and time dependent afterslip following the 2011 tohoku earthquake yan hu 1 roland burgmann 1 naoki uchida 2 paramesh banerjee 3 and jeffrey t freymueller 4 university of california berkeley berkeley ca united states graduate school of science tohoku university japan earth observatory of singapore nanyang technological university singapore singapore university of alaska fairbanks fairbanks ak united states corresponding author yan hu earth and planetary science university of california berkeley 307 mccone hall berkeley ca 94720 usa yhu seismo berkeley edu this article has been accepted for publication and undergone full peer review but has not been through the copyediting typesetting pagination and proofreading process which may lead to differences between this version and the version of record please cite this article as doi 10 1002 2015jb012508 2015 american geophysical union all rights reserved

a brownian walk model for slow earthquakes 1 along some subduction plate boundaries slow deformation is observable as seismically detected deep low frequency tremor and geodetically detected slow slip events these phenomena are considered as different manifestations of slow earthquakes characterized by fairly constant seismic moment rate this paper presents a simple model of slow earthquakes that can explain wide variety of observed features including the steady moment rate and scaled energy characteristics of tremor signals both in time and frequency domains and the migration of the source location in this model slow earthquakes are represented as shear slip on circular faults whose radius is a random variable that is governed by a langevin equation and three parameters a diffusion coefficient a damping coefficient and a slip rate coefficient this model expands on a previous scaling law for the slow earthquakes by providing a specific image of kinematics allowing for spatial variations of the parameters could potentially explain differences in behavior of slow slip events worldwide

structural coupling between the qiman tagh and the qaidam basin northern tibetan plateau a perspective from the yingxiong range by integrating field mapping seismic imaging and analogue modeling

thermomechanical evolution of oceanic fracture zones a fracture zone fz model is constructed from existing models of the thermal and mechanical evolution of the oceanic lithosphere as the lithosphere cools by conduction thermal and mechanical boundary layers develop and increase in thickness as age 1 2 surface expressions of this development such as topography and deflection of the vertical i e gravity field are most apparent along major fz s because of the sharp age contrast a simple model including the effects of lateral heat transport but with no elastic layer predicts that variations in seafloor depth and deflection of the vertical will become increasingly smooth and ultimately disappear as the fz ages observations however show that both the fz topography and the deflection of the vertical remain sharp as the fz evolves these two observations as well as the observed asymmetry in deflection of the vertical profiles across the udintsev romanche and mendocino fz s are explained by including a continuous elastic layer in the model the asymmetry in deflection of the vertical is a consequence of elastic thickness variations across the fz modeling also shows that the evolution of the fz topography is extremely sensitive to the initial thermal structure near the ridge transform intersection model geoid steps and their development with age are used to access techniques for measuring geoid offsets across fz s reasonable step estimation techniques will underestimate the overall step amplitude by up to 50 this implies that abnormally thin thermal boundary layers derived from studies of geoid height versus age are not required by the data

an iterative spectral solution method for thin elastic plate flexure with variable rigidity s u m m a r y thin plate flexure theory provides an accurate model for the response of the lithosphere to vertical loads on horizontal length scales ranging from tens to hundreds of kilometres examples include flexure at seamounts fracture zones sedimentary basins and subduction zones when applying this theory to real world situations most studies assume a locally uniform plate thickness to enable simple fourier transform solutions however in cases where the amplitude of the flexure is prominent such as subduction zones or there are rapid variations in seafloor age such as fracture zones these models are inadequate here we present a computationally efficient algorithm for solving the thin plate flexure equation for non uniform plate thickness and arbitrary vertical load the iterative scheme takes advantage of the 2 d fast fourier transform to perform calculations in both the spatial and spectral domains resulting in an accurate and computationally efficient solution we illustrate the accuracy of the method through comparisons with known analytic solutions finally we present results from three simple models demonstrating the differences in trench outer rise flexure when 2 d variations in plate rigidity and applied bending moment are taken into account although we focus our analysis on ocean trench flexure the method is applicable to other 2 d flexure problems having spatial rigidity variations such as seamount loading of a thermally eroded lithosphere or flexure across the continental oceanic crust boundary

flexure modelling at seamounts with dense cores summary the lithospheric response to seamounts and ocean islands has been successfully described by deformation of an elastic plate induced by a given volcanic load if the shape and mass of a seamount are known the lithospheric flexure due to the seamount is determined by the thickness of an elastic plate te which depends on the load density and the age of the plate at the time of seamount construction we can thus infer important thermomechanical properties of the lithosphere from te estimates at seamounts and their correlation with other geophysical inferences such as cooling of the plate whereas the bathymetry i e shape of a seamount is directly observable the total mass often requires an assumption of the internal seamount structure the conventional approach considers the seamount to have a uniform density e g density of the crust this choice however tends to bias the total mass acting on an elastic plate in this study we will explore a simple approximation to the seamount s internal structure that considers a dense core and a less dense outer edifice although the existence of a core is supported by various gravity and seismic studies the role of such volcanic cores in flexure modelling has not been fully addressed here we present new analytic solutions for plate flexure due to axisymmetric dense core loads and use them to examine the effects of dense cores in flexure calculations for a variety of synthetic cases comparing analytic solutions with and without a core indicates that the flexure model with uniform density underestimates te by at least 25 per cent this bias increases when the uniform density is taken to be equal to the crustal density we also propose a practical application of the dense core model by constructing a uniform density load of same mass as the dense core load this approximation allows us to compute the flexural deflection and gravity anomaly of a seamount in the wavenumber domain and minimize the limitations recognized from the analytic tests then the dense core model is applied to predict the lithospheric flexure beneath howland island in the tokelau seamount chain these results are compared with the predictions of the uniform density model based on age dating of howland and the age of the seafloor traditional te versus age curves predict the elastic plate thickness beneath the seamount to be around 20 km which is comparable to the best dense core model of te 26 km however the best uniform density model is found at te 12 km which is significantly less than the predicted from our investigations of synthetic and real seamount cases we conclude that the dense core model approximates the true mass distribution of a seamount better than the uniform density model finally we suggest that the role of underplating in flexure modelling may need to be reexamined because the dense core model predicts substantially less deflections than the uniform density model without requiring additional buoyancy caused by underplated material

a finite difference technique to incorporate spatial variations in rigidity and planar faults into 3 d models for lithospheric flexure summary we present a finite difference formulation for 3 d elastic flexure of the lithosphere which is solved by a direct matrix method to incorporate the effect of spatial variations in rigidity additional terms for the bi harmonic 3 d flexure equation have been derived from a variational displacement formulation as used in finite element methods additionally planar faults are treated as discontinuities these are implemented by an additional degree of freedom for fault heave and a coupled continuum equation for zero differential tilting across the fault the 3 d finite difference results have been tested for line loads point loads and disc loads by analytical solutions and for spatial variation in effective elastic thickness eet by 2 d finite difference solutions fault related flexure patterns are compared to the 2 d analytical broken plate model developed by vening meinesz 1950 we subsequently apply the 3 d fault model to investigate fault controlled 3 d basement geometries in lake tanganyika east africa we show that our model is capable of predicting 3 d basement geometries characteristically observed in rifted basins the modelling results indicate that fault controlled upper crustal flexure patterns are associated with low values for eet a comparison with regional scale model studies showing a superposition of high eet flexure effects supports a multilayered rheological control on continental rifting

depth extent of the long term slow slip event in the tokai district central japan a new insight 1 temporal changes to interplate coupling and long term aseismic slip in the tokai region of central japan from july 1996 to june 2009 are statically inverted using continuous global positioning system gps data and leveling observations and examined using a new interpretation of aseismic slip in this interpretation the interplate coupling and the aseismic slip should be inferred and compared with reference to a state of no coupling on the plate interface rather than assuming steady state interplate coupling before the occurrence of the aseismic event as previous works have done for this purpose we used original time series data without subtracting any linear trends the inferred distribution shows that the entire period can be divided into three subperiods based on the existence of the aseismic slip area from 2000 to 2005 the strength of the interplate coupling does not change throughout the observed period even though the distribution narrows along the dip direction after the aseismic slip terminates the aseismic slip area inferred in this study based on the no coupling reference was determined to be deeper than the area where the long term slow slip event sse defined as the slip corresponding to the residual displacement from the steady state was hypothesized to have occurred in numerous previous studies the total seismic moment released by the aseismic slip is equivalent to a mw 6 6 earthquake while the sse in previous works was considered to be equivalent to a mw 7 0 to 7 1 earthquake this difference can be attributed primarily to the effects on the interplate coupling which previous studies did not take into account when defining the sse the aseismic slip area coincides well with the distribution of other types of plate interactions such as short term slow slip events and low frequency earthquakes

analytical solutions for 3 d flexural deformation of semi infinite elastic plates summary the theory of elastic plate flexure has played a prominent role in isostatic studies and investigations of lithospheric deformation caused by vertical tectonism simple analytical asymmetrical solutions exist when the boundaries of the plate are far removed from the area where the load is applied when deformation is driven by end loads i e bending moments and shear forces the problem is usually simplified to yield a 2 d solution that approximates a cross section of the 3 d solution we present analytical green s functions for the point force response of semi infinite elastic plates resting on an inviscid substratum and being acted upon by constant in plane forces and arbitrary end loads this solution may be used to study deformation and stresses caused by loads close to a free geological boundary e g a trench or weak fracture zone we also present green s functions for the case when two semi infinite plates of different rigidities are mechanically coupled and subject to a point load this solution may be useful when studying deformation near mechanically strong fracture zones or at ocean continent transitions the analytical solutions provide simple alternatives to and calibration of complex numerical finite element or finite difference solutions and furthermore give additional physical insight into the geological process to be examined the analytical methods are used to demonstrate the different stress patterns that may arise near weak and strong geological boundaries we also illustrate how the observed scatter in 2 d determinations of elastic thickness at subduction zones may partly have its origin in 3 d effects

lithospheric bending at subduction zones based on depth soundings and satellite gravity a global study of trench flexure was performed by simultaneously modeling 117 bathymetric profiles original depth soundings and satellite derived gravity profiles a thin elastic plate flexure model was fit to each bathymetry gravity profile by minimization of the l norm the six model parameters were regional depth regional gravity trench axis location flexural wavelength flexural amplitude and lithospheric density a regional tilt parameter was not required after correcting for age related trend using a new high resolution age map estimates of the density parameter confirm that most outer rises are uncompensated we find that flexural wavelength is not an accurate estimate of plate thickness because of the high curvatures observed at a majority of trenches as in previous studies we find that the gravity data favor a longer wavelength flexure than the bathymetry data a joint topography gravity modeling scheme and fit criteria are used to limit acceptable parameter values to models for which topography and gravity yield consistent results even after the elastic thicknesses are converted to mechanical thicknesses using the yield strength envelope model residual scatter obscures the systematic increase of mechanical thickness with age perhaps this reflects the combination of uncertainties inherent in estimating flexural wavelength such as extreme inelastic bending and accumulated thermoelastic stress the bending moment needed to support the trench and outer rise topography increases by a factor of 10 as lithospheric age increases from 20 to 150 ma this reflects the increase in saturation bending moment that the lithosphere can maintain using a stiff dry olivine rheology we find that the lithosphere of the gdh1 thermal model stein and stein 1992 is too hot and thin to maintain the observed bending moments moreover the regional depth seaward of the oldest trenches 150 ma exceeds the gdh1 model depths by about 400 m

long term slow slip events are not necessarily caused by high pore fluid pressure at the plate interface an implication from two dimensional model calculations summary a recent tomographic study proposed that high pore pressure in the deeper portion of the locked zone of a subduction thrust resulting from metamorphic dehydration reactions may cause longterm slow slip events the study used the concept of critical fault stiffness which derives from laboratory derived rate and state dependent friction laws to test the proposition we execute 2 d model calculations using laboratory derived rate and state dependent friction laws our numerical result is against the proposition but it can also be explained by the concept of the critical fault stiffness we agree that metamorphic dehydration reactions definitely produce a bulk property of high fluid saturation but we caution that they do not necessarily lead to high pore pressure in the fault zone

nonvolcanic tremor evolution and the san simeon and parkfield california earthquakes nonvolcanic tremors occur adjacent to locked faults and may be closely related to the generation of earthquakes monitoring of the san andreas fault in the parkfield california region revealed that after two strong earthquakes tremor activity increased in a nearly dormant tremor zone increased and became periodic in a previously active zone and has remained elevated and periodic for over 4 years static shear and coulomb stress increases of 6 to 14 kilopascals from these two earthquakes are coincident with sudden increases in tremor rates the persistent changes in tremor suggest that stress is now accumulating more rapidly beneath this part of the san andreas fault which ruptured in the moment magnitude 7 8 ft tejon earthquake of 1857

three dimensional p and s wave velocity structures beneath the japan islands obtained by high density seismic stations by seismic tomography abstract we construct fine scale 3d p and s wave velocity structures of the crust and upper mantle beneath the whole japan islands with a unified resolution where the pacific pac and philippine sea phs plates subduct beneath the eurasian eur plate we can detect the low velocity low v oceanic crust of the pac and phs plates at their uppermost part beneath almost all the japan islands the depth limit of the imaged oceanic crust varies with the regions high vp vs zones are widely distributed in the lower crust especially beneath the volcanic front and the high strain rate zones are located at the edge of the extremely high vp vs zone however vp vs at the top of the mantle wedge is not so high beneath northern japan we can image the high v subducting pac plate using the tomographic method without any assumption of velocity discontinuities we also imaged the heterogeneous structure in the pac plate such as the low v zone considered as the old seamount or the highly seismic zone within the double seismic zone where the seismic fault ruptured by the earthquake connects the upper and lower layer of the double seismic zone beneath central japan thrust type small repeating earthquakes occur at the boundary between the eur and phs plates and are located at the upper part of the low v layer that is considered to be the oceanic crust of the phs plate in addition to the low v oceanic crust the subducting high v pac plate is clearly imaged to depths of approximately 250 km and the subducting high v phs zone to depths of approximately 180 km is considered to be the phs plate beneath southwestern japan the iso depth lines of the moho discontinuity in the phs plate derived by the receiver function method divide the upper low v layer and lower high v layer of our model at depths of 30 50 km beneath kyushu the steeply subducting phs plate is clearly imaged to depths of approximately 250 km with high velocities the high vp vs zone is considered as the lower crust of the eur plate or the oceanic crust of the phs plate at depths of 25 35 km and the partially serpentinized mantle wedge of the eur plate at depths of 30 45 km beneath southwestern japan the deep low frequency nonvolcanic tremors occur at all parts of the high vp vs zone within the zone the seaward side and the landward side where the phs plate encounters the mantle wedge of the eur plate we prove that we can objectively obtain the fine scale 3d structure with simple constraints such as only 1d initial velocity model with no velocity discontinuity

scaling relationship between the duration and the amplitude of non volcanic deep low frequency tremors 1 we investigate a duration amplitude relation of non volcanic deep low frequency dlf tremors in the tokai region southwest japan to constrain the source process of the tremors we apply two models to the distribution one is an exponential model as a scale bound distribution and the other a power law model as a scale invariant distribution the exponential model shows a better fit to the duration amplitude distribution of the tremors than a power law model implying that the dlf tremors are caused by a scale bound source process the source process of the dlf tremors therefore differs from those for earthquakes we suggest that the non volcanic dlf tremor is possibly caused by a fixed source dimension with variable excess pressure of fluid or variable stress drop

reappraising elastic thickness variation at oceanic trenches we reassess the variation of elastic thickness as a function of lithospheric plate age using a global database of bathymetric and free air gravity profiles which are perpendicular to oceanic trenches as in many previous studies our starting point is the well known floating elastic plate model in order to remove the influence of short wavelength features not associated with lithospheric bending adjacent profiles from 10 myr bins have been stacked together to construct average profiles with standard deviations each average profile was then inverted in a two stage procedure first singular value decomposition was used to determine two unknown flexural parameters together with a regional slope and offset for any given elastic thickness this procedure was repeated for a range of elastic thicknesses second residual misfit was plotted as a function of elastic thickness and the global minimum was identified this two stage procedure makes no prior assumptions about magnitude of the load size of the bending moment or whether the elastic plate is broken continuous we obtained excellent fits between theory and observation for both bathymetric and gravity profiles from lithosphere with an age range of 0 150 ma the shape of the residual misfit function indicates the degree of confidence we have in our elastic thickness estimates the lower limit of elastic thickness is usually well determined but upper limits are often poorly constrained inverse modeling was carried out using a range of profile lengths 250 300 500 and 700 km in general our estimates show no consistent increase of elastic thickness as a function of plate age this surprising result is consistent with recent reassessments of elastic thickness beneath seamounts and implies either that elastic thickness is independent of plate age or that elastic thickness cannot be measured with sufficient accuracy to reveal such a relationship modeling of short free air gravity profiles 250 300 km does tentatively suggest that elastic thickness increases linearly from 5 to 10 km between 0 and 20 ma and from 10 to 15 km between 20 and 150 ma this variation roughly matches the depth to the 200 c isotherm which corresponds to an homologous temperature of 0 4 for wet peridotite unfortunately for longer profile lengths there is no temporal dependence and elastic thicknesses vary considerably for all plate ages bathymetric profile modeling yields similar results although uncertainties are larger

low frequency continuous tremor around the moho discontinuity away from volcanoes in the southwest japan 1 recent enhancement of seismic networks in the japan islands revealed occurrence of low frequency continuous tremors of a beltlike distribution in the southwest japan where the subducting philippine sea plate reaches depths of 30 40 km source depth of the tremor is estimated by selecting tremor segments with relatively clear p wave onsets the source region of the tremor is assumed to correspond to lowermost parts of crust close to a triple boundary of the crust mantle wedge and the subducting slab the long duration of the phenomenon indicates existence of fluid relating to the generation of the tremor the most probable fluid is considered to be water produced by dehydration process of chlorite and amphibole in the slab on the basis of data from high pressure and temperature experiments the northern border of the beltlike distribution is possibly rimmed by the edge of the mantle wedge because serpentine formation absorbs fluid water

deep tremor in new zealand triggered by the 2010 mw8 8 chile earthquake 1 deep non volcanic tremor nvt is usually associated with episodic slow slip events new zealand is one notable exception where numerous slow slip events have been identified yet nvt has remained undetected here we present the first known case of triggered nvt at new zealand s hikurangi subduction margin following the mw8 8 chilean earthquake of february 27 2010 we identify coherent high frequency tremor signals that are in phase with and modulated by the passing rayleigh waves this is consistent with the surface wave triggering potential for strike normal incidence on a low angle thrust fault after constraining the tremor depth on the plate interface we locate the tremor source within 20 km of the source area of episodic slow slip the tremor location is also near the edge of a region with high seismic attenuation that marks the boundary between dehydrated subducted slab and inferred hydrated underplated sediment we speculate that reduced interface friction and high fluid pressures provided by fluid rich underplated sediment facilitates the tremor generation

crustal velocity field of southwest japan subduction and arc arc collision we investigate crustal deformation in southwest japan over a 3 year period revealed by a permanent dense global positioning system gps array southwest japan is a part of the amurian plate a microplate moving about 10 mm yr toward the east with respect to the eurasian plate it overrides the philippine sea plate at the nankai trough and collides with the northeast japan arc in the central part of japan in this paper we first derive gps site velocities relative to the stable part of the amurian plate in order to isolate signals of crustal deformation caused by the subduction and or the collision the velocity field has a conspicuous feature indicating the interseismic elastic loading by the philippine sea plate slab at the nankai trough characterized by the northwestward movements of points throughout the studied area their amplitudes are the largest at the pacific coast and decay toward the japan sea coast with a subtle systematic shift of azimuths a model assuming an elastic half space the convergence rate at the nankai trough based on a refined euler vector and the strength of the coupling inferred from a thermal model could explain the velocity field in the western part of the studied area to a large extent those in the eastern part systematically deviate from them and the residual components there show east west shortening and north south extension this may represent crustal thickening and trenchward extrusion of crustal blocks caused by the collision between southwest and northeast japan this suggests that the collision between southwest and northeast japan gives rise to not only crustal thickening but also trenchward extrusion of crustal block a velocity contrast was found across the median tectonic line the largest inland active fault in japan but the current permanent gps network is not dense enough for us to discuss its coupling depth

contrasting subduction structures within the philippine sea plate hydrous oceanic crust and anhydrous volcanic arc crust we show contrasting subduction structures within the philippine sea plate inferred from active source wide angle reflection data previous studies showed that large amplitude reflections from the slab are observed in southwest japan and indicated that a thin low velocity layer with a high fluid content is formed along the top of the subducting oceanic crust on the contrary we found that the slab reflections have smaller amplitudes in the izu collision zone central japan where the izu bonin volcanic arc has been colliding subducting suggesting that such a low velocity layer does not exist beneath the collision zone this structural difference is also supported by p wave and s wave velocity anomalies by passive source tomography and electrical conductivity and correlates with the regional distribution of deep tremors and intraslab earthquakes both of which are induced by dehydration processes within the downgoing slab based on these comparisons we suggest that the original structure of the incoming plate controls the contrasting subducting systems typical oceanic plate absorbs water by hydrothermal circulation at spreading centers and or seawater infiltration at outer rises whereas volcanic arc crust consumes a large amount of hydrous minerals for melt production and metamorphoses to more stable anhydrous forms before subduction

plate convergence and long term crustal deformation in central japan surveys by continuous global positioning system in and around japan revealed that the amurian plate collides with the north american plate in central japan by 2 cm yr long term crustal deformation seems to be influenced mainly by this collision although subduction of oceanic plates governs short term elastic deformation over the arc here we study the long term deformation field by carefully removing the short term signals inferred from a priori plate convergence vectors and coupling strengths predicted by a thermal model the obtained field shows that the change in velocities occurs along the longitude 135 137 and there exist a relatively rigid block and zones accommodating strains characteristic compressional deformation is found northwest of izu due possibly to the collision of the izu bonin arc with honshu plate convergence rate along the nankai suruga trough is considerably smaller in eastern parts due partly to the transition from the amurian to the north american plate of the landward side and partly to the motion of the izu microplate relative to the philippine sea plate this accounts for longer recurrence intervals of interplate earthquakes in the suruga trough where the tokai earthquake is anticipated to occur

groundwater level anomalies associated with a hypothetical preslip prior to the anticipated tokai earthquake detectability using the groundwater observation network of the geological survey of japan aist we infer the groundwater level anomalies associated with a hypothetical preslip prior to the anticipated m 8 tokai earthquake and evaluate the detectability of the anomalies using data from seven groundwater wells we evaluate the detectability of the anomalies under the following assumptions 1 an mw 5 5 6 5 aseismic preslip event occurs at the plate boundary in and around the hypothetical focal zone of the tokai earthquake 2 the total amount of the strain step at each observation associated with the preslip can be calculated by tensile and shear faulting based on the dislocation model 3 a normalized strain history associated with the preslip is defined from the results of numerical simulations based on rate and state dependent friction laws and 4 the groundwater level anomaly prior to the earthquake is proportional to the estimated history of the strain change associated with the preslip we investigate the detection time of the anomaly at seven wells given an mw 5 5 6 0 or 6 5 aseismic preslip at one of the 272 grid points in and around the area of the hypothetical focal zone of the tokai earthquake as a result over the time interval between 1 and 48 hours prior to the hypothetical tokai earthquake we are able to detect at each of the seven wells a hypothetical mw 6 5 preslip at 10 86 of the 272 grid points an mw 6 preslip at 0 19 grid points and an mw 5 5 preslip at 0 5 grid points

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses