controls on back arc basin formation the relationship between subduction and back arc spreading has been well known since the early days of plate tectonics however the reasons why back arc basins are associated with some subduction systems but not all has remained elusive we examine the kinematic controls on subduction and back arc basins for both the present day and cenozoic to differentiate between the major competing hypotheses for back arc basin formation and to explain their temporal and spatial distribution our new data set of subduction and back arc basin parameters uses a new set of paleo oceanic age grids muller et al 2005 associated with a moving atlantic indian ocean hot spot reference frame o neill et al 2005 the plate model includes detailed reconstructed spreading histories of back arc basins based on marine geophysical and satellite gravity data our combined rotation and oceanic paleo age model provides the age distribution of subducting lithosphere through space and time convergence rates and the absolute motion of the downgoing and overriding plates we find that back arc basins develop when the age of subducting normal oceanic lithosphere is greater than 55 million years additionally we establish an age dip relationship showing that the intermediate dip angle of the subducting slab is always greater than 30 with back arc spreading our results suggest that back arc basin formation is always preceded by an absolute motion of the overriding plate away from the subduction hinge thereby creating accommodation space between the overriding and subducting plates once back arc extension is established it continues regardless of overriding plate motion indicating back arc spreading is not a simple consequence of overriding plate behaviour the landward migration of the overriding plate as a precursor to back arc extension may indicate that extension on the overriding plate is influenced by the oceanward lateral flow of the mantle however once back arc extension is established rollback of the subduction hinge appears to be the primary force responsible for the continued creation of accommodation space our analysis indicates the driving mechanism for back arc extension is a combination of surface kinematics properties of the downgoing slab the effect of lateral mantle flow on the slab and mantle wedge dynamics

constraints on the lateral strength of slabs from three dimensional dynamic flow models if the viscosity of subducted lithosphere is described purely by temperature and pressure it should remain considerably more viscous than the surrounding material as it descends through the upper mantle many dynamic models of the long wavelength geoid associated with slabs require an increase in viscosity from the upper to the lower mantle but assume the perturbation to the lateral viscosity structure arising from the slabs themselves can be ignored previous studies indicate that strong and localized viscosity variations should have a dramatic influence on the geoid we present 3d finite element models of the regional geoid of the western pacific subduction zones slab buoyancies and viscosities are defined using the distribution of seismicity the geoid is very sensitive to the lateral strength of the slab very viscous slabs penetrating a low viscosity mantle generate significant 10 50 m long wavelength geoid lows opposite to the geoid high which is observed over slabs to obtain a geoid high comparable to that observed the lower mantle viscosity must be 60 200 times greater than the upper mantle viscosity and the slab must be in contact with the lower mantle these strict requirements suggest that slabs have been weakened and cannot act as stress guides from the deep mantle to the surface

scattering parameters of the lithosphere below the massif central france from teleseismic wavefield records summary teleseismic p wave recordings are analysed in the frequency range 0 3 6 hz to derive structural statistical parameters of the lithosphere underneath the french massif central for this we analyse diverences in frequency dependent intensities of the mean wavefield and the fluctuation wavefield it is possible to discriminate a weak fluctuation regime of the wavefield in the frequency range below 1 hz and a strong fluctuation regime starting above 1 hz and continuing to higher frequencies the observed wavefield fluctuations in the frequency range 0 3 3 hz can be explained by scattering of the teleseismic p wave front at elastic inhomogeneities in the lithosphere a statistical distribution of the inhomogeneities is assumed and the concept of random media is applied the lithospheric structure under the massif central can be described as a 70 km thick heterogeneous layer with velocity fluctuations of 3 7 per cent and correlation lengths of the heterogeneities of 1 16 km

the global short period wavefield modelled with a monte carlo seismic phonon method summary at high frequencies 1 hz much of the seismic energy arriving at teleseismic distances is not found in the main phases e g p pp s etc but is contained in the extended coda that follows these arrivals this coda results from scattering off small scale velocity and density perturbations within the crust and mantle and contains valuable information regarding the depth dependence and strength of this heterogeneity as well as the relative importance of intrinsic versus scattering attenuation most analyses of seismic coda to date have concentrated on s wave coda generated from lithospheric scattering for events recorded at local and regional distances here we examine the globally averaged vertical component 1 hz wavefield 10 range for earthquakes recorded in the iris farm archive from 1990 to 1999 we apply an envelopefunction stacking technique to image the average time distance behavior of the wavefield for both shallow 50 km and deep 500 km earthquakes unlike regional records our images are dominated by p and p coda owing to the large effect of attenuation on pp and s at high frequencies modelling our results is complicated by the need to include a variety of ray paths the likely contributions of multiple scattering and the possible importance of p to s and s to p scattering we adopt a stochastic particle based approach in which millions of seismic phonons are randomly sprayed from the source and tracked through the earth each phonon represents an energy packet that travels along the appropriate ray path until it is affected by a discontinuity or a scatterer discontinuities are modelled by treating the energy normalized reflection and transmission coefficients as probabilities scattering probabilities and scattering angles are computed in a similar fashion assuming random velocity and density perturbations characterized by an exponential autocorrelation function intrinsic attenuation is included by reducing the energy contained in each particle as an appropriate function of traveltime we find that most scattering occurs in the lithosphere and upper mantle as previous results have indicated but that some lower mantle scattering is likely also required a model with 3 to 4 per cent rms velocity heterogeneity at 4 km scale length in the upper mantle and 0 5 per cent rms velocity heterogeneity at 8 km scale length in the lower mantle with intrinsic attenuation of q 450 above 200 km depth and q 2500 below 200 km provides a reasonable fit to both the shallow and deep earthquake observations although many trade offs exist between the scale length depth extent and strength of the heterogeneity

nied seismic moment tensor catalogue for regional earthquakes around japan quality test and application abstract we have examined the quality of the national research institute for earth science and disaster prevention nied seismic moment tensor mt catalogue obtained using a regional broadband seismic network freesia first we examined using synthetic waveforms the robustness of the solutions with regard to data noise as well as to errors in the velocity structure and focal location then to estimate the reliability robustness and validity of the catalogue we compared it with the harvard centroid moment tensor cmt catalogue as well as the japan meteorological agency jma focal mechanism catalogue we found out that the nied catalogue is consistent with harvard and jma catalogues within the uncertainty of 0 1 in moment magnitude 10 km in depth and 15 in direction of the stress axes the nied mt catalogue succeeded in reducing to 3 5 the lower limit of moment magnitude above which the moment tensor could be reliably estimated finally we estimated the stress tensors in several different regions by using the nied mt catalogue this enables us to elucidate the stress deformation field in and around the japanese islands to understand the mode of deformation and applied stress moreover we identified a region of abnormal stress in a swarm area from stress tensor estimates

a regionalized upper mantle rum seismic model seismic velocity heterogeneity in the earth s mantle is strongly concentrated near its top the shallow heterogeneity of the mantle correlates strongly with surface tectonics we use these observations as constraints of a tomographic experiment aimed at building a regionalized upper mantle rum reference model we use a select set of teleseismic travel times to minimize the mapping of mislocation into structure the data selection emphasizes the robustness of individual picks the form of the rum model is a set of velocity profiles as functions of depth through the upper mantle for each of the different tectonic provinces of earth together the profiles constitute a three dimensional model which incorporates considerable structural detail but is described by only 90 parameters and has only about 22 degrees of freedom this is achieved by irregularly sampling a detailed regionalization of the globe by detailed mapping of subducted lithosphere in the mantle as defined by seismicity and by combining these structures in an irregular grid in which bookkeeping is efficiently handled the resulting rum model includes subducting slabs as sharp fast features in the upper mantle old continents are fast young oceans are slow models have been derived for both compressional and shear velocity the rum model is designed to represent as much of upper mantle heterogeneity as seen by body wave travel times as possible with a simple model it can be useful as a reference model for individual tectonic regions travel times are efficiently generated for the rum model mislocations of explosions of known location are significantly reduced when corrections for the rum model are applied to travel time residuals for a spherically symmetrical earth model

significance of non double couple components of deep and intermediate depth earthquakes implications from moment tensor inversions of long period seismic waves abstract analysis of long period seismic waves suggests that non double couple components in source moment tensors of intermediate depth and deep earthquakes are significant and appear to respond to the state of predominant strain release within slabs the strain regime is partially or fully induced by the sources themselves or by slab structures near the sources we observe consistency in the non double couple components from three different seismic wave inversions among 21 earthquakes studied 17 events have the same sign in the three non double couple components and the signs show a correlation with the state of strain release within the slabs the effects of unmodeled propagation structure and instability of the inversion procedure are unlikely to be responsible for the consistent non double couple components because the suites of seismic waves traverse different paths and the various inversion schemes have the different resolvabilities of the moment tensors

multiple scattering from icequakes at erebus volcano antarctica implications for imaging at glaciated volcanoes we examine seismic coda from an unusually dense deployment of over 100 short period and broadband seismographs in the summit region of mount erebus volcano on a network with an aperture of approximately 5 km we investigate the energy partitioning properties of the seismic wavefield generated by thousands of small icequake sources originating on the upper volcano and use them to estimate green s functions via coda cross correlation emergent coda seismograms suggest that this locale should be particularly amenable to such methods using a small aperture subarray we find that modal energy partition between s and p wave energy between 1 and 4 hz occurs in just a few seconds after event onset and persists for tens of seconds spatially averaged correlograms display clear body and surface waves that span the full aperture of the array we test for stable bidirectional green s function recovery and note that good symmetry can be achieved at this site even with a geographically skewed distribution of sources we estimate scattering and absorption mean free path lengths and find a power law decrease in mean free path between 1 5 and 3 3 hz that suggests a quasi rayleigh or rayleigh gans scattering situation finally we demonstrate the existence of coherent backscattering weak localization for this coda wavefield the remarkable properties of scattered seismic wavefields in the vicinity of active volcanoes suggests that the abundant small icequake sources may be used for illumination where temporal monitoring of such dynamic structures is concerned

global slab deformation and centroid moment tensor constraints on viscosity we analyze moment tensor solutions from deep subduction zone earthquakes to determine global slab deformation patterns inferred strain rates are compared to predicted deformation patterns from fluid models to help constrain the first order radial and lateral viscosity structure of the earth while all slabs that reach the lower mantle are compressed at their tip intermediate depth patterns are more complex we compute 3 d spherical flow with various slab rheologies and compare the angular misfit between the compressive eigenvectors of the resultant stress field and global centroid moment tensor gcmt solutions we find that upper mantle slab viscosities of 10 100 and lower mantle viscosities of 30 100 times the upper mantle produce the best match to gcmts a 0 1 viscosity reduction in the asthenosphere seems preferred slab geometry and lower mantle viscosity exert significant control on deformation inclusion of the phase changes at 410 km and 660 km increases extensional deformation at intermediate depth and compressional deformation at the lower mantle improving the match to gcmts for strong slabs our conclusions are fairly insensitive to surface boundary conditions however models which include net rotations of the surface with respect to the lower mantle produce compression at intermediate depths for west directed slabs and extension for east directed slabs without allowing for regional variations these models yield the best match to gcmts while significant deviations between model and seismicity remain our results show that seismicity provides an underutilized constraint for slab dynamics

plate boundary forces at subduction zones and trench arc compression abstract viscoelastic finite element modelling has been used to study the state of stress in the overriding and subducting plates meeting at a subduction zone the subduction fault is included using the dual node technique it is demonstrated that substantial horizontal deviatoric compressive stress occurs in the trench arc region as a result of the downpull of the dense slab and the associated surface depression including the trench and other downflexing of the plates this may be masked at the trench by bending stress it is the lack of significant shearing stress along an unlocked subduction fault in the presence of this compressive stress that gives rise to the slab pull and trench suction plate boundary forces slab pull and trench suction were found to be of comparable magnitude within the range 1 0 to 4 0 10 12 n m in models studied with vertical subduction and there are indications that this may also apply when the slab dips at 45 as a result of viscous flow induced by rollback when the slab dips beneath the arc backarc region it is shown that horizontal deviatoric compression can occur in this region contemporaneous with plate interior tension produced by trench suction this suggests that backarc tension associated with marianas type trench arch systems may be related to the nearly vertical slab whereas backarc compression in the chilean type may result from the low dip and small downpull of the slab it is also shown that successive locking and unlocking of the subduction fault may give rise to large variations of stress in plate interiors

interference of long period seismic wavefield observed by the dense hi net array in japan 1 in order to recover small signals of long period ground motions recorded by a short period seismometer we designed a time domain recursive filter that simulates seismic waves recorded with broadband seismometers we applied the time domain filter to seismograms recorded by the dense hi net array which consists of 800 stations at intervals of approximately 20 km the effectiveness of the time domain filter is demonstrated through the comparison of broadband waveforms obtained by the filter and sts 2 true broadband seismograms from the f net at the same site good agreement is observed between filtered and true broadband waveforms with high correlation coefficients for periods of up to 100 s for waveforms from the 2007 sumatra earthquake m7 9 which occurred approximately 5000 5800 km away from japan the applicability of the time domain filter to the recovery of long period signals from the record of a short period seismometer depends on the signal to noise ratio as a result a limited bandwidth of broadband signal is recovered for smaller earthquake from the spatiotemporal variation of the long period t 20 100 s wavefield obtained by the time domain filter we newly found an anomalous phase pattern of splitting of polarity along the wavefront particularly for the relatively short period band of from 20 50 s array analysis of the dense broadband waveforms revealed multipathing of the rayleigh waves which propagated toward central japan from two directions with slightly different slowness vectors the interference of two rayleigh waves caused such splitting of wavefield

isolated deep earthquakes and the fate of subduction in the mantle the locations of earthquakes with depth in the mantle provide direct evidence of the trajectory and state of stress of subducting lithosphere the occurrence over the last decade of isolated large deep earthquakes outside the main wadati benioff zones in the kurile izu bonin and chile deep seismic areas in addition to the observed flat distributions of deep seismicity beneath the banda sea and fiji plateau areas and the large number of deep earthquakes located away from the main slab in the tonga arc supports recent tomographic and other seismological studies leading to a reconsideration of slabs as simple planar structures sinking undeformed into the lower mantle the focal geometries of these isolated earthquakes which often exhibit near vertical compressional stresses are distinct from adjacent events in the main wadati benioff zone they are also different in orientation from events in the deflecting toe of the slab and from the stress orientation expected if the deflected slab acts as a stress guide these isolated mantle earthquakes are interpreted to take place in subducted lithosphere that has deflected to a horizontal posture at the base of the upper mantle conversely the deep seismicity in the adjacent wadati benioff zone is associated with the deflection of the subducting lithosphere

subduction zones observations and geodynamic models abstract this review of subduction and geodynamic models is organized around three central questions 1 why is subduction asymmetric 2 are subducted slabs strong or weak 3 how do subducted slabs interact with phase transformations changes in mantle rheology and possibly chemical boundaries in the mantle based on laboratory measurements of the temperature dependence of olivine one would conclude that the core of a subducting slab is at least 10 000 times more viscous than ambient mantle however there are a number of complementary but independent observations that suggest that slabs are much weaker than this slabs undergo significant deformation in the upper mantle and may thicken to twice their original width by the time they reach the base of the transition zone the lack of a clear correlation between the observed dip angle of deep slabs and plate velocity rate of trench migration and slab age in modern subduction zones is consistent with hypothesis that subduction is a time dependent phenomenon both tank and numerical convection experiments with plates conclude that subduction is not a steady phenomenon but that slabs bend thicken stretch and change dip through time this is at odds with the assumptions used in steady state slab thermal models where slab deformation is not considered

the super interseismic phase of the megathrust earthquake cycle in chile along a subduction zone great megathrust earthquakes recur either after long seismic gaps lasting several decades to centuries or over much shorter periods lasting hours to a few years when cascading successions of earthquakes rupture nearby segments of the fault we analyze a decade of continuous global positioning system observations along the south american continent to estimate changes in deformation rates between the 2010 maule m8 8 and 2015 illapel m8 3 chilean earthquakes we find that surface velocities increased after the 2010 earthquake in response to continental scale viscoelastic mantle relaxation and to regional scale increased degree of interplate locking we propose that increased locking occurs transiently during a super interseismic phase in segments adjacent to a megathrust rupture responding to bending of both plates caused by coseismic slip and subsequent afterslip enhanced strain rates during a super interseismic phase may therefore bring a megathrust segment closer to failure and possibly triggered the 2015 event

tsunami coda across the pacific ocean following the 2011 tohoku oki earthquake abstract a tsunami coda lasting more than 5 days was clearly recorded across the pacific ocean following the 2011 tohoku oki earthquake we found that the coda energy per unit area elapsed time of 60 96 hr from the earthquake origin time does not show a systematic variation with respect to the travel distance at the stations located at sea depths deeper than 2000 m whereas the maximum amplitude near the source is significantly larger than those of the far field stations this feature is similar to that of a seismic coda in the sense that the coda energy is distributed uniformly in space the average tsunami coda decays with a decay time of 24 hr when we carefully examined the tsunami waveform near japan a large amplitude wave was identified at an elapsed time of 48 hr which corresponds to the round trip travel time for waves reflected from chile to investigate the excitation mechanisms of the tsunami coda we conducted a tsunami simulation using a global model although linear long wave tsunami equations were found to overestimate the observed coda amplitude the equations including an intrinsic energy loss could successfully simulate the observed coda decay if a bottom friction coefficient of 10 5 was assumed this indicates that not only wave scattering but also intrinsic energy loss plays an important role in the tsunami coda excitation and decay process

state of stress in slabs as a function of large scale plate kinematics the state of stress of slabs subducting worldwide as revealed by seismicity is extremely variable both with depth and between different subduction zones although in principle slab pull should enhance downdip extension in the slab the reconstructed stress fields for intermediate depths between 100 and 300 km range from downdip compression to downdip extension using 2 d viscoelastic plane strain models we investigate the dependency of the stress field of slabs on geometry dip of the slab and kinematics velocity of convergence between upper and lower plates and their absolute velocity with respect to the underlying mantle of subduction zones we conclude that although the state of stress in slabs is also controlled by other processes downdip compression in the subducting slab is enhanced by mantle flow opposing the direction of the dip of the slab whereas downdip extension is favored by mantle flow in the same direction of the slab dip i e sustaining it these predictions are in agreement with available geophysical observations although exceptions to this simple pattern are observed worldwide in addition if the slab is decoupled from the upper plate convergence between upper and lower plates induces a downdip compressional component of stress within the slab decreasing the magnitude of extension in models characterized by mantle flow sustaining the slab and increasing compression in models with mantle flow opposing subduction however these are second order variations when compared to the control exerted by absolute plate kinematics and by the magnitude of slab pull sensitivity analysis of rheological parameters allows us to conclude that these results are generally consistent although low values of viscosity of the lithospheric mantle render this prediction less stable

scattering behaviour at merapi volcano java revealed from an active seismic experiment summary the seismic structure of the stratovolcano merapi java indonesia was studied using an active seismic experiment three 3 km long seismic profiles each consisting of up to 30 three component seismometers with an interstation distance of 100 m were built up in an altitude range between 1000 and 2000 m above sea level the detailed study of the seismic properties of the propagation media in active volcanic regions is important to understand the natural seismic signals used for eruption forecasting the seismic experiment at merapi therefore concentrates on the heterogeneous structure within a radius of 5 km from the active dome where the sources of most of the natural volcanic seismic events are located the cone of merapi volcano consists of different materials changing on a small scale due to the layering of eruptive material additionally the topography of the erosion valleys leads to an irregular deposition which cannot be described by a simple 1 d layering these inhomogeneities have a strong influence on seismic signals the direct p and s waves are attenuated quickly and show only small amplitudes on seismograms the energy lost from the direct waves however is not changed into heat but scattered and can be observed as seismic coda following the direct waves the observed seismograms show a spindle like amplitude increase after the direct p phase this shape of the envelope can be explained by the diffusion model according to this model there are so many strong inhomogeneities that the direct wave can be neglected and all energy is concentrated in multiple scattered waves besides the envelope the coherence and polarization properties of the wavefield also indicate strong scattering only the first onset shows coherence over a station spacing of 100 m whereas the late phases carrying the major part of the energy are mainly incoherent the horizontal components of the seismograms have larger amplitudes than the vertical component but within the horizontal plane the polarization is almost arbitrary corresponding to waves arriving from scatterers located arbitrarily in space as a result of the inversion using the diffusion model we obtain values of the s wave scattering attenuation coefficient gs and the s wave intrinsic absorption coefficient gi in the frequency range of 4 20 hz used in this study the scattering attenuation is at least one order of magnitude larger than the intrinsic absorption gs gi the mean free path of s waves is as low as 100 m gs1 100 m the scattering coefficient is independent of frequency gsyf 0 0 whereas the coefficient of intrinsic attenuation increases with increasing frequency giyf 1 6 the natural seismic signals at merapi volcano show similar characteristics to the artificial shots the first onsets have only small amplitudes and the energy maximum arrives delayed compared to the direct waves therefore these signals appear to be strongly affected by multiple scattering also

slab stress and strain rate as constraints on global mantle flow dynamically consistent global models of mantle convection with plates are developed that are consistent with detailed constraints on the state of stress and strain rate from deep focus earthquakes models that best fit plateness criteria and plate motion data have strong slabs that have high stresses the regions containing the m w 8 3 bolivia and m w 7 6 tonga 1994 events are considered in detail modeled stress orientations match stress patterns from earthquake focal mechanisms a yield stress of at least 100 mpa is required to fit plate motions and matches the minimum stress requirement obtained from the stress drop for the bolivia 1994 deep focus event the minimum strain rate determined from seismic moment release in the tonga slab provides an upper limit of 200 mpa on the strength in the slab

evidence for transformational faulting from a deep double seismic zone in tonga double seismic zones planes or earthquakes parallel to the dip of a subducting slab and separated by 20 40 km provide important clues about the earthquake generating mechanisms and strain distribution inside subducting slabs double seismic zones have been found at intermediate depths 70 200 km in many subduction zones1 6 but have not been previously reported in deep slabs here by relocating earthquakes with a hypocentroidal decomposition technique7 and visualizing the earthquake positions and uncertainties in three dimensions we identify a double seismic zone at depths of 350 460 km in the tonga subduction zone source parameters of the earthquakes determined by waveform analysis suggest different stress orientations for the two zones with in plane compression in the lower zone and in plane tension in the upper zone the double zone may be due to transformational faulting as olivine along the edges of a metastable olivine wedge becomes warmer and transforms to spinel8 11

deformation of a weak subducted slab and variation of seismicity with depth oceanic lithosphere is assumed to possess platelike properties whether it is lying at the earth s surface or descending deep into the mantle yet the geometries of subducting slabs indicate significant deformation as they descend through the mantle1 4 suggesting that lithospheric plates might be so weakened during subduction as to act not as a rigid solid but as a viscous fluid numerical and laboratory experiments have shown that fluid slabs can indeed take on realistic profiles5 7 however it has not been clear whether a fluid or weak model of slab dynamics can account for deep earthquakes which are usually ascribed to the deformation of a rigid or strong slab here we show using numerical simulations of slab evolution that a weak slab model is in fact consistent with seismic observations assuming that earthquakes occur at a rate proportional to deformation rate we reproduce the observed variation with depth of seismicity rate and focal mechanisms and the cessation of seismicity at 670 km depth provided that sinking material encounters resistance at depth here modelled as a viscosity jump at 670 km the pattern of seismicity can be explained by any mechanism for deep earthquakes in which the rate of seismicity is proportional to strain rate in the slab

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle in a number of subduction zones earthquakes at 50 200 km depth define two dipping planes separated by 20 40 km that appear to merge downdip upper plane earthquakes are inferred to occur within the subducting oceanic crust whereas lower plane earthquakes occur in the subducting oceanic mantle beneath northeast japan the results of a new finite element heat transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle lower plane earthquakes occur at 550 800 c at 100 km depth and at 350 600 c at 160 km depth these conditions coincide with the dehydration reaction antigorite serpentine forsterite enstatite h 2 o which suggests that lower plane earthquakes may be triggered by dehydration embrittlement which in turn suggests that the subducting oceanic mantle is partially hydrated serpentinization may occur in the trench outer rise region where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere if this hypothesis is correct current subduction zone h 2 o budgets may significantly underestimate the amount of bound h 2 o entering the subduction factory

residence time of diffuse waves in the crust as a physical interpretation of coda q application to seismograms recorded in mexico summary we consider a simple model for elastic wave propagation in the earth s litho sphere consisting of a heterogeneous and scattering crust overlying a homogeneous mantle that is we neglect backscattering from the upper mantle this rough condition is not strictly valid everywhere but this model allows us to investigate the effects of variations of diffracting properties between crust and mantle on the coda decay the free parameters describing the wave propagation are the mean free path of the waves and the intrinsic absorption in the crust in the framework of the radiative transfer theory we treat accurately jumps in s wave speed and the mean free path at the moho without intrinsic absorption this configuration yields a synthetic coda decay 1 texp 2 ft qc a form similar to that proposed by aki chouet 1975 to parametrize the observations the exponential decay in our purely elastic model is due to the partial leakage of diffuse energy into the mantle in order to test the applicability of our model we have determined the coda q parameter qc in the 1 15 hz frequency band from data recorded at four stations in mexico in the low frequency regime around 1 hz we suggest that the energy leakage significantly affects coda q whilst at higher frequencies around 10 hz the anelastic attenuation dominates a weak intrinsic absorption expressed by a frequency independent parameter qi of about 1000 accounts satisfactorily for the observed qc values in the whole frequency range our very simple model has only two free parameters and offers an attractive alternative physical interpretation of coda q in mexico this model is sufficient to interpret the coda observations according to our model in regions where the mean free path is much larger than the crustal thickness weakly heterogeneous crust the leakage effect should be small and the observed qc will mainly be due to the intrinsic absorption on the other hand when the mean free path is of the order of the crustal thickness strongly heterogeneous crust we predict a strong sensitivity of the decay rate of the coda to the crustal thickness this dependence may be important for the interpretation of the regional variations of coda q we propose introducing a new parameter d the residence time of diffuse energy in the crust which is physically more meaningful than a quality factor to quantify the leakage effect

patterns of deep seismicity reflect buoyancy stresses due to phase transitions thermal perturbation of mantle phase relations in subduction zones gives rise to significant buoyancy anomalies finite element modeling of stresses arising from these anomalies reveals transition from principal tension to compression near 400 km depth down dip compression over 400 690 km peaking at 550 km and transition to rapidly fading tension below 690 km such features even when complicated by olivine metastability are consistent with observed patterns of deep seismicity that such a simple model neglecting all effects other than buoyancy anomalies due to temperature and to thermal perturbation of olivine phase relations successfully generates so many observed features of deep seismicity suggests that these buoyancy anomalies are significant contributors to the stress field in subducting slabs it also suggests that the depth distribution of deep seismicity may largely reflect the state of stress in the slab rather than simply a particular mechanism of stress release

distortion of the apparent s wave radiation pattern in the high frequency wavefield tottori ken seibu japan earthquake of 2000 summary the distortion properties of the apparent s wave radiation pattern in the high frequency seismic wavefield of over f 2 hz is investigated using a large number of waveform records of the main shock and 29 aftershocks of the tottori ken seibu japan magnitude mw 6 6 earthquake in 2000 the dense seismic records from the kik net strong motion network show a clear four lobe pattern in the apparent s wave radiation pattern in the low frequency wavefield of f 2 hz and shows an almost isotropic distribution in all directions as the frequency increases above 5 hz the distortion of the apparent s wave radiation pattern in the high frequency wavefield increases as travel distance increases therefore the path effect caused by the scattering of seismic waves due to small scale heterogeneities in the crust is a major cause of distortion of the radiation pattern this hypothesis is examined by a 2 d finite difference method simulation of seismic waves in heterogeneous structure models the results of simulations clearly demonstrate the collapse of the s wave front due to seismic wave scattering in the heterogeneous structure by comparing the observed wavefield and the results of simulations using different sorts of stochastic heterogeneous models the most preferable model that can explain the observation is described by a von karman autocorrelation function with correlation distance of a 3 5 km order of 0 5 and rms value of e 0 07 however our simple stochastic random heterogeneity model proposed herein somewhat overestimates the scattering of low frequency signals below 2 hz

estimation of s wave scattering coefficient in the mantle from envelope characteristics before and after the scs arrival 1 examining seismogram envelopes of regional earthquakes in central asia with focal depths deeper than 150 km in 1s 20s periods for a wide lapse time range up to 2000s we found an offset in each coda envelope before and after the scs arrival at 10s and 15s periods the scs wave has its own tail that is scattered waves of scs waves sometimes dominate over the original coda waves composed of scattered s waves and scattered surface waves for 1s 2s and 4s periods such an offset is not clear however there is a change of coda decay gradient associated with the scs arrival in a layered velocity and attenuation structure according to the prem scattering process of s waves can be simulated by using the monte carlo method based on the radiative transfer theory for isotropic scattering process comparing numerical simulation for a two layered scattering model with observed envelopes we estimated the total scattering coefficients as 1 129 10 3 km 1 and 6 230 10 4 km 1 at 4s and 4 510 10 4 km 1 and 2 710 10 4 km 1 at 10s for the lithosphere and upper mantle and for the lower mantle respectively in the lower mantle scattering loss is dominant in the total attenuation of s waves

rheologic controls on slab dynamics several models have been proposed to relate slab geometry to parameters such as plate velocity or plate age however studies on the observed relationships between slab geometry and a wide range of subduction parameters show that there is not a simple global relationship between slab geometry and any one of these other subduction parameters for all subduction zones numerical and laboratory models of subduction provide a method to explore the relative importance of different physical processes in determining subduction dynamics employing 2 d numerical models with a viscosity structure constrained by laboratory experiments for the deformation of olivine we show that the observed range in slab dip and the observed trends between slab dip and convergence velocity subducting plate age and subduction duration can be reproduced without trench motion i e slab roll back for locations away from slab edges successful models include a stiff slab that is 100 1000 times more viscous than previous estimates from models of plate bending the geoid and global plate motions we find that slab dip in the upper mantle depends primarily on slab strength and plate boundary coupling with a small dependence on subducting plate age once the slab sinks into the lower mantle the primary processes controlling slab evolution are 1 the ability of the stiff slab to transmit stresses up dip 2 resistance to slab descent into the higher viscosity lower mantle and 3 subduction induced flow in the mantle wedge corner

the double seismic zone in kuril kamchatka the tale of two overlapping single zones the classic double seismic zone in the central portion of the kuril kamchatka arc seems to turn into a regular wadati benioff zone with downdip compression and extension in the northeastern and southwestern ends of the arc respectively to delineate this lateral changeover we studied 33 large to moderate sized mb 5 1 earthquakes that occurred at depths shallower than 200 km between 1963 and 1991 these events took place in two regions where the double seismic zone terminates and single seismic zones emerge precise earthquake source parameters including focal depths and mechanisms are determined from a careful analysis of a mixture of analog and digital p and sh waveforms recorded at teleseismic distances our results indicate that the double seismic zone is confined between approximately 46 and 53 n northeast of 53 n intermediate depth earthquakes 70 200 km show only downdip compressional focal mechanisms furthermore the wadati benioff zone there seems to be a continuation of the upper compressional layer of the double seismic zone thus the changeover from a double to a single seismic zone in this case is simply disappearing of the lower extensional sheet of the double seismic zone similarly southwest of 46 n intermediate depth earthquakes show only downdip extensional mechanisms the single seismic zone is unlikely to consist of two layers of extensional seismicity in fact nowhere along the entire arc did we observe two distinct seismogenic layers with a single state of strain in other words these extensional events did not occur in the same seismogenic structure as the upper compressional layer of the double seismic zone however we cannot ascertain whether the single seismic zone in extension is simply a southwestern extension of the lower extensional layer of the double seismic zone lateral variation from a single zone under extension in the southwest to a double seismic zone and finally to a single zone under compression cannot be accounted for by conventional models of a double seismic zone on the other hand the addition of a component of stress transmitted from depths greater than 250 km can explain the observed lateral variation in the state of strain at intermediate depths because the slab between depths of 250 450 km is under extension in the southwestern end of the arc whereas the rest of the arc is under compression vanishing of the lower extensional layer of the double seismic zone toward the northeastern end of the arc is interpreted as a result of compression transmitted from below overshadowing extension caused by effects such as unbending a similar reasoning applies to the southwestern termination of the double seismic zone

systematic biases in subduction zone hypocenters 1 earthquake hypocenters from global datasets commonly have regional biases in their locations due to the use of a one dimensional velocity model in their location to analyze this effect we group intermediate depth earthquakes into forty four 500 km long sections of subduction zone we relocate earthquakes in each group relative to one another and in a three dimensional global velocity model hypocenters shift up to 25 km regionally when these effects are included most earthquakes in circum pacific subduction zones are pulled toward the pacific location uncertainties from the relative relocations are reduced to one fifth of those from single event locations wadati benioff zone thicknesses are reduced to two thirds of their original thickness these biases alter estimates of depth to slab beneath arc volcanoes up to 50 km and alter measurements such as the volume of mantle wedge available for water storage or melt generation and the size of the thrust zone on which large earthquakes can occur