spatial and temporal variation of coda wave attenuation in the faryab region southeast of the sanandaj sirjan zone using aftershocks of the tiab earthquake of 28 february 2006 coda quality factor q c estimates for the faryb region in the southeast zagros have been obtained by using aftershock data of the tiab earthquake of 28 february 2006 recorded within an epicentral distance of 100 km more than 394 earthquakes were recorded by a local temporary network consisting of 9 short period stations installed for one month after 28 february 2006 q c was estimated using the single backscattering model in frequency bands of 0 5 24 hz in this research the spatial and temporal variation of q c in the sanandaj sirjan zone is studied to explore the lateral variation q c is calculated for nine stations and in different parts of the covered area the lowest values are derived in kali station in the southern part and the higher value in baga station in the northern part but generally there is an absence of significant lateral variation in coda q c in the faryab region to investigate the attenuation variation with depth q c value was calculated for eight lapse times the average frequency relation for this region is q c 28 1 1 f 1 14 0 01 at 5 s lapse time window length and q c 105 1 3 f 0 98 0 047 at 30 s lapse time window length q c increases with depth however the rate of increase of q c is not uniform with depth in the study area beneath the faryab region the rate of increase of q c is greater at depths less than 49 km compared to larger depths indicating a low attenuation structure at less than 49 km depth which is the moho depth in this region

temporal variations in coda duration of local earthquakes in the wellington region new zealand temporal variations of coda duration relative to event magnitude for local earthquakes near wellington have been investigated the region is one of plate convergence and subduction the data consist of routinely made observations for events from 1978 through 1985 1552 events magnitude 1 6 to 5 2 depth 0 5 to 90 0 km the observed average over time correspondence between duration as measured from the origin time and magnitude is reasonably well predicted by the single backscattering theory of coda formation for events of magnitude 4 or less for larger events the observed durations are longer than predicted this theory predicts that a temporal increase in scattering attenuation will reduce the coda duration relative to magnitude thus any temporal changes in the duration magnitude relation can be interpreted in terms of changes inq however it is necessary to consider spatial biases since the observed durations are relatively long for shallow events and for events of all depths at stations situated in the south eastern half of the region usually believed part of the accretionary border for both these situations other evidence would suggest that the scattering coefficient would be relatively high lowq these observations may be due to a relatively high component of surface wave scattering and the importance of multiple scattering in the later part of the coda in regions of lowq as suggested by finite element studies of coda formation despite the lack of any significant earthquakes during the 1978 1985 period there is nevertheless a significant temporal change in duration observed in the wellington region a change from relatively long to relatively short codas occurred in mid 1981 this change correlates well with changes in the rate of activity b value radon emission and ground tilt as derived from lake levels it is not yet clear how all these parameters are related physically but an episode of aseismic slip or creep along the plate interface below the region may have been the cause

latent heat effects of the major mantle phase transitions on low angle subduction abstract very low to zero shallow dip angles are observed at several moderately young subduction zones with an active trenchward moving overriding plate we have investigated the effects of latent heat for this situation where mantle material is pushed through the major mantle phase transitions during shallow low angle subduction below the overriding plate the significance of the buoyancy forces arising from the latent heat effects on the dynamics of the shallowly subducting slab is examined by numerical modeling when a 32 ma old slab is overridden with 2 5 cm yr by a continent flat subduction occurs with a 4 5 cm yr convergence rate when latent heat is included in the model forced downwellings cause a thermal anomaly and consequently thermal and phase buoyancy forces under these circumstances the flat slab segment subducts horizontally about 350 km further and for about 11 ma longer than in the case without latent heat before it breaks through the 400 km phase transition the style of subduction strongly depends on the mantle rheology increasing the mantle viscosity by one order of magnitude can change the style of subduction from steep to shallow similarly an overriding velocity of less than 1 cm yr leads to steep subduction which gradually changes to flat subduction when increasing the overriding velocity however these model parameters do not change the aforementioned effect of the latent heat provided that low angle subduction occurs in all models latent heat resulted in a substantial increase of the flat slab length by 300 400 km varying the olivine spinel transition clapeyron slope from 1 to 6 mpa k reveals a roughly linear relation between and the horizontal length of the slab based on these results we conclude that buoyancy forces due to latent heat of phase transitions play an important role in low angle subduction below an overriding plate

deformation of subducted oceanic lithosphere summary the deformation of subducted oceanic lithosphere is revealed by detailed studies of the distribution of seismicity in benioff zones and by the increasingly well resolved images of the upper mantle obtained by seismic p wave tomography by using numerical experiments based on a simplified dynamical model of a subducted lithospheric slab we test the idea that the observed deformation of the subducted slab results from a balance between internal buoyancy forces and the viscous stresses associated with deformation the simplified model assumes that the lithosphere has uniform physical properties it is denser and much more viscous than the upper mantle and its penetration below 670 km depth is resisted by a step like density increase at that level the primary determinant of the style of deformation is the buoyancy number f a ratio of buoyancy stress generated by the mass anomaly in the slab to the stress associated with viscous deformation at a strain rate u l defined in terms of subduction rate u and slab thickness l for a small buoyancy number deformation of the slab is dominated by viscous flexure for large f down dip extension of the slab appears to dominate and a buckling instability of the slab is observed if there is resistance to penetration of the 670 km level the transition value of f at which this change in behaviour is observed is about 0 05 for constant viscosity and about 0 2 for stressdependent viscosity with a stress versus strain rate exponent of n 3 with n 3 a boudinage type instability of the slab is observed for large f penetration of the slab below the 670 km level depends on the density contrast between slab and lower mantle resisted only by the density difference the slab may initially penetrate several hundred kilometres below the 670 km level for plausible density parameters before a buckling instability causes this part of the slab to rotate and ascend comparison of the deforming slab geometry and stress field with seismicity distribution and tomographic images from the tonga subduction zone suggests that the effective buoyancy number f for this slab is approximately equal to the transition value that we determined experimentally we use this constraint to estimate average rheological parameters for the tonga slab which when compared with published creep deformation laws for olivine are consistent with the average temperature of the slab being about 0 4 to 0 45 times the melting temperature based on constitutive laws for olivine at shallow depths 100 km these temperatures correspond to around 590 c for wet olivine or around 700 c for dry olivine

thermal structure beneath tohoku northeast japan combined with geophysical observations and petrological considerations we have studied the thermal models of the tohoku northeast japan subduction zone the temperature of the deeper part of the mantle wedge is constrained by the petrological requirements that demand a high temperature region 1400 c beneath the volcanic zone to satisfy this condition namely to explain the arc volcanism in the tohoku subduction zone a high temperature upper mantle beneath the japan sea possibly 1400 c at the depth shallower than 100 km is required heat flow distribution which constrains the temperature structure at shallower depth shows that low heat flow values ranging from 20 to 50 mw m2 are distributed between the trench and aseismic front and consistently high heat flow values 80 120 mw m2 prevail in the area landward of the volcanic front including the japan sea recent heat flow measurements indicate that the transition from low to high heat flow occurs between the aseismic front and volcanic front this heat flow distribution restricts the form of the flow in the mantle wedge and shear stress acting at the plate boundary a rigid non moving triangular region bounded by the subducting slab and the vertical plane beneath the aseismic front is necessary to explain the low heat flow zone between the trench and aseismic front shear stress of about 500 to 1000 bar is supposed to act at the interface between the subducting slab and rigid triangular mantle we also suggest that to explain the peak of heat flow near the volcanic front 120 mw m2 magma migration rate of about 0 05 cm3 cm 2 year 1 is necessary a comparison of the thermal structure of the tohoku subduction zone with that of the central south america which was preliminary calculated by the similar scheme shows that the temperature in the mantle wedge of the central south america is probably lower than that of the tohoku subduction zone this difference may result in that of the nature of volcanic rocks

temporal change in coda q before the tangshan earthquake of 1976 and the haicheng earthquake of 1975 a new earthquake precursor called coda q is demonstrated for the tangshan earthquake of 1976 and for the haicheng earthquake of 1975 the precursor is based on the measurement of coda decay rate for small local earthquakes in the general area that is preparing for a major earthquake during the 3 year period preceding the tangshan earthquake the coda q was about 3 times lower than it was before or after this period a comparable change also occured at the time of the haicheng earthquake the observed change cannot be due to the change in the source parameters such as the focal depth or the source spectrum the coda q precursor appears to detect the same physical process detected by other precursors such as b value quiescence radon water quality vp vs and animal behavior but has advantages over them in its clearer physical mechanism easier sampling and better defined anomalous region

radial models of mantle viscosity results from a genetic algorithm summary the long wavelength component of the geoid has been used to constrain radial variations in mantle viscosity using 3 d velocity anomaly models determined from seismic tomography as the buoyancy forces driving viscous flow using a genetic algorithm ga the robustness of mantle viscosity models obtained by calculusbased optimization methods is investigated gas are a relatively new class of optimization techniques which are generating a growing interest in the geophysics community these methods can be particularly useful for solving highly non linear optimization problems unlike traditional techniques gas do not require derivative information to form a jacobian matrix or sensitivity kernels models are constructed from pieces of successful models following a simple set of rules these simple rules are driven by stochastic rather than deterministic means because of the efficiency of the ga for the mantle viscosity problem it is possible to explore a greater range of potential solutions than with a traditional optimization method it is found that while both calculus based and ga optimization methods find viscosity models with a low viscosity transition zone the ga finds another class of models with a high viscosity in the transition zone that also satisfies the geoid data the geoid alone is unable to resolve low viscosity transition zone models from high viscosity transitionzone models the effect of the scaling between seismic velocity and density on the preferred viscosity model is also explored while a velocity to density scaling with a pronounced increase in the transition zone provides the best fit to the geoid the preferred vertical viscosity profile is insensitive to the velocity to density scaling

numerical simulations of deformation and dynamics of horizontally lying slabs summary recent results of seismic tomography have revealed that configurations of subducted slabs are varied some lie horizontally on the boundary between the upper and the lower mantle and others penetrate into the lower mantle in this study to understand the physical mechanism of deformation and dynamics of subducted slabs in the mantle a 2 d numerical model was constructed for this purpose the slabs were treated so as to move freely in an incompressible fluid model by removing a guide right after its bending at a depth of 660 km and at a depth of 400 km in the upper mantle incorporating density and viscosity jumps and a clapeyron slope for a pv mw phase transformation of olivine obtained from high pressure and hightemperature experiments at a depth of 660 km into the model we investigate temperature and velocity distributions state of stress and viscosity distribution in relation to the formation of horizontally lying slabs at a depth of 660 km when the slabs are treated to move freely just after their bending at the 660 km phase boundary the slabs temporarily appear to lie horizontally on the phase boundary however the slabs penetrate the phase boundary and eventually sink into the lower mantle the length of the horizontally lying portion of the slab becomes longer as descending velocity increases the lengths are about 1500 km 1200 km and 800 km for subducting velocities of 8 cm yr 1 4 cm yr 1 a nd 2c m yr 1 respectively when a viscosity jump of 10 to 100 times the viscosity for the upper mantle is given at the 660 km phase boundary the length of the horizontally lying portion of the slab becomes shorter because of friction between the slab and the underlying lower mantle for all the models in which the guide was removed at a depth of 400 km the slabs penetrate into the lower mantle without lying horizontally on the phase boundary these results indicate that it is difficult for the slabs to bend at the pv mw phase boundary and to form the configuration of the horizontally lying slabs even if the slabs descend with a small dip angle the calculated maximum principal compressive stress axes are oriented horizontally within the horizontally lying portion of the slabs which is inconsistent with seismologically observed vertical directions of the p axes there in order to explain the observed configurations and state of stress within the horizontally lying slabs it would be necessary to investigate the effects of a complicated viscosity and composition structure in the mantle in more detail

grain size evolution in subducted oceanic lithosphere associated with the olivine spinel transformation and its effects on rheology we investigate the role of grain size reduction during the olivine spinel transformation on rheological properties of subducting slabs on the basis of a scaling model for microstructural development during nucleation and growth in this model the size of spinel grains nucleating at olivine grain boundaries is controlled by the relative rates of nucleation and growth taking into account the impingement through the collision of grains due to growth when the volume fraction of spinel reaches a certain threshold value critical volume fraction 1 10 depending on the p t conditions in the slab the new phase will form a continuous film and will significantly reduce the strength of the two phase aggregate if spinel grain size is small the size of spinel grains d at this stage is calculated and is shown to be highly sensitive to 0 size and 2 a subducting slab has a complicated rheological structure containing a weak region below the tip of a metastable olivine bearing wedge in a cold slab possible implications of these anomalous rheological structures of slabs on the dynamics of subduction are discussed including the mechanisms of deep earthquakes

impact of phase change kinetics on the mariana slab within the framework of 2 d mantle convection abstract recent high pressure and high temperature experiments indicate that metastable olivine might persist in the cold core of a slab due to the low reaction rate of the olivine wadsleyite phase transformation recent seismological observations detected a metastable olivine wedge that survives to a depth of 630 km in the mariana slab to consider the problem of non equilibrium phase transformation we developed a two dimensional 2 d cartesian numerical code that incorporates the effects of kinetics into a thermal convection model we consider the kinetics of the 410 km olivine wadsleyite and the 660 km ringwoodite pv mw phase transformations including the effects of water content at the 410 km phase boundary the latent heat release of the 410 km non equilibrium phase transformations inside the slab is also considered the results show positive correlations between some of the controlling parameters and the length of the metastable olivine wedge the faster the subducting velocity and the lower the water content the deeper is the metastable olivine wedge with increasing depth of phase transformation the effect of latent heat release is enhanced heating of at most 100 c occurs if olivine transforms into wadsleyite at a depth of approximately 570 km in our model setting temperature increase due to the latent heat released stimulates further phase transformation resulting in further temperature increase acting as a positive feedback effect we also attempt to explain the seismological observations by calculating the temperature and phase structures in the mariana slab if we assume that the age of the mariana slab is 150 myr the subduction velocity is 9 5 cm yr phase transformation occurs from the grain boundary of the parental phase and the water content is 250 wt ppm for a grain size of 1 mm 300 wt ppm for one of 5 mm and 100 wt ppm for intracrystalline transformation then the metastable olivine wedge survives to a depth of 630 km which is in good agreement with the seismological observations this suggests that the deeper portion of the mariana slab is relatively dry assuming that the depression of the 660 km discontinuity by 20 30 km within the mariana slab as is indicated by seismological observation is explained by a combination of the depression caused by a negative clapeyron slope in the cold slab and that due to the kinetics of the 660 km phase transformation we obtain a gentle clapeyron slope of 0 9 mpa k for the phase transformation from ringwoodite to pv mw

physical conditions producing slab stagnation constraints of the clapeyron slope mantle viscosity trench retreat and dip angles abstract recent seismic tomography has revealed various morphologies in the subducted lithosphere in particular significant flattening and stagnation of slabs around the 660 km boundary are seen in some areas beneath the northwestern pacific subduction zones we examined the cause of slab stagnation in terms of the clapeyron slope of the phase transformation from ringwoodite to perovskite magnesiowustite trench retreat velocity dip angles and high viscosity of the lower mantle based on two dimensional 2 d numerical simulations of thermal convection in particular we examined the conditions necessary for slab stagnation assuming a very small absolute value of the clapeyron slope which were proposed based on recent high pressure high temperature high p t experiments our calculations show that slabs tend to stagnate above the 660 km boundary with an increasing absolute value of the clapeyron slope viscosity jump at the boundary and trench retreat velocity and a decreasing initial dip angle stagnant slabs could be obtained numerically for a realistic range of parameters obtained from high p t experiments and other geophysical observations combining buoyancy high lower mantle viscosity and trench retreat we found that a low dip angle of a descending slab at the bottom of the upper mantle plays an important role in slab stagnation two main regimes underlie slab stagnation buoyancy dominated and viscosity dominated regimes in the viscosity dominated regime it is possible for slabs to stagnate above the 660 km boundary even when the value of the clapeyron slope is 0 mpa k

generation of ascending flows in the big mantle wedge bmw beneath northeast asia induced by retreat and stagnation of subducted slab 1 we conducted two dimensional numerical experiments of mantle convection with imposed kinematic motions of cold slabs in order to study the mechanism for the generation of ascending flows in the big mantle wedge bmw which has been recently proposed in order to relate the stagnant pacific slab with the intraplate volcanism in northeast asia our calculations demonstrated that the bmw is expanded oceanward in response to the retreating motion of trench and slab which strongly affects the flows in the region in particular the subducting and retreating motion of slab induces a local but strong circulation near the oceanward end or a hinge of the stagnant slab in the bmw our findings suggest that ascending flows in the bmw can be triggered most easily near the hinge of the stagnant slab which is in good agreement with the occurrence of several active intraplate volcanoes above the stagnant pacific slab

wet plume atop of the flattening slab insight into intraplate volcanism in east asia abstract geophysical observations imply the intraplate volcanism in east asia is related to dehydration of slab stagnating in the transition zone to better understand the dynamics of such process a thermochemical mantle convection model is constructed to simulate numerically the thermal evolution of slab and the transportation of water in the process of slab downgoing flattening and stagnation equation of water transfer is included and water effects on density and viscosity are considered model results indicate the warming of slab by surrounding mantle is rather slow water could be successfully dragged into the transition zone if the reference viscosity of the hydrous layer with initial water of 2 wt is higher than 1017 pa s and that of mantle is 1021 pa s wet plumes could then originate in the flat lying part of the slab relatively far from the trench generally the viscosity of the hydrous layer governs the initiation of wet plume whereas the viscosity of the overlying mantle wedge controls the activity of the ascending wet plumes they are more active in the weaker wedge the complex fluid flow superposed by corner flow and free thermal convection influences greatly the water transport pattern in the upper mantle modeling results together with previous modeling infer three stages of water circulation in the big mantle wedge 1 water is brought into the mantle transition zone by downward subducting slab under some specific thermo rheological conditions otherwise water is released at shallow depth near wedge tip 2 wet plume generates from surface of the flattening slab warmed by surrounding mantle and 3 water spreads over the big mantle wedge wet plume from the flattening pacific plate arrives at the lithospheric base and induces melting which can explain the intraplate cenozoic volcanoes in east asia

seismological and experimental constraints on metastable phase transformations and rheology of the mariana slab abstract a wedge with a low seismic wave velocity implying the presence of metastable olivine and the depression of the 660 km seismic discontinuity have been observed inside the mariana slab based on these seismic observations and numerical calculation of thermal structures of the slab we suggest that the phase transformations from olivine to ringwoodite and from ringwoodite to perovskite ferropericlase the post spinel transformation occur at the depth of 630 km and 550 c and at 690 km and 700 c respectively in the central coldest part of the mariana slab combining this information with recent experimental kinetic data we constrain the details of non equilibrium phase transformations and rheology of the mariana slab the observed depth of the metastable olivine inside the mariana slab can be explained by growth controlled olivine ringwoodite transformation under relatively dry condition such as the water content of about 150 wt ppm h2o on the other hand nucleation process controls the depth of the post spinel transformation in the mariana slab and therefore the observed depressions of the 660 km discontinuity should depend on both the overpressure needed for the nucleation and the negative clapeyron slope for the transformation when the overpressure for the nucleation is 0 5 gpa the observed depressions can be explained by the clapeyron slope of 0 7 mpa k grain size evolution and viscosity structures in the mariana slab are estimated based on these metastable phase transformations the slab weakening due to the grain size reduction is limited in about 40 km in width below the metastable olivine wedge at the depths from 630 to 690 km whereas the width of the weakened portion of the slab increases to more than 120 km at deeper than the 690 km depth after the post spinel transformation these viscosity structures are consistent with the behaviors of the mariana slab estimated from seismic tomography the slab vertically descends to the bottom of the transition zone and substantially deforms and thickens at the top of the lower mantle

effects of trench migration on fall of stagnant slabs into the lower mantle abstract global seismic tomography has recently revealed horizontally lying slabs near the upper and lower mantle boundary beneath the northwestern pacific region although physical mechanisms that could produce such slab stagnation have been proposed based on numerical simulations there has been little research into what occurs after slab stagnation we proposed trench advance and trench jumps as effective mechanisms related to the fall of stagnant slabs into the lower mantle and our numerical simulations of temperature and fluid flow associated with slab subduction in a 2 d box model confirmed these mechanisms our results indicate that a supply of slab material associated with further slab subduction after slab stagnation plays an important role in differentiating further slab stagnation from the falling of slabs into the lower mantle a shortage of material supply would produce extended slab stagnation near the 660 km boundary for ringwoodite to perovskite magnesiowustite phase transformation whereas downward force due to further slab subduction on a stagnant slab would enhance its fall into the lower mantle the behaviors of falling stagnant slabs were not affected by clapeyron slope values associated with phase equilibrium transformation within the range from 3 0 to 0 0 mpa k compared with models of normal mantle viscosity a high viscosity lower mantle played a role in hindering the fall of slabs into the lower mantle resulting in complicated shapes and slow falling velocities lower mantle viscosity structure also affected slab behavior slabs tended to stagnate when a low viscosity zone lvz existed just below a depth of 660 km because friction between the slab and the lvz was weak there slab stagnation around a depth of 660 km also occurred when a high viscosity zone existed below a depth of 1200 km and acted as a resistive force against a slab even if the slab existed in the lower part of the upper mantle

the horizontally lying slab an unusual earthquake mb 6 2 occurred in the izu bonin island region in july 1982 the event occurring at a depth of 545 km was clearly separated from the main inclined seismic zone by a distance of about 200 km to diagnose the situation we have considered two possibilities 1 the event might be an unusual event in the sense that its location is unrelated to the position of the subducted slab and that there is probably no slab around it or 2 the slab at this depth might be lying horizontally on top of the 650 km discontinuity and the unusual event occurred in this horizontal part both possibilities have been modeled structurally we have computed the p wave travel times for the present event as well as for two other events located unambiguously in the inclined part of the seismic zone by employing a 3 d ray tracing program the computed travel time residuals have been compared with the observations at stations in western japan the observations are found to be consistent with the model of the horizontally lying slab our result thus suggests that the slab might be horizontal at this depth in this part of the izu bonin islands

trench dynamics effects of dynamically migrating trench on subducting slab morphology and characteristics of subduction zones systems abstract understanding the mechanisms of trench migration retreat or advance is crucial to characterizing the driving forces of earth s tectonics plates the origins of subducting slab morphologies in the deep mantle and identifying the characteristics of subduction zones systems which are among the fundamental issues of solid earth science a series of numerical simulations of mantle convection focusing on plate subduction in a three dimensional 3 d regional spherical shell coordinate system was performed to examine subduction zone characteristics including geodynamic relationships among trench migration back arc stress and slab morphology the results show that a subducting slab tends to deflect around the base of the mantle transition zone and form a sub horizontal slab because its front edge its toe is subject to resistance from the highly viscous lower mantle as the sub horizontal slab starts to penetrate into the lower mantle from its heel the toe of the slab is drawn into the lower mantle the results for models with dynamically migrating trenches suggest that trench retreat is the dynamically self consistent phenomenon in trench migration the reason for this is that the strong lateral mantle flow that is generated as a sequence of events leading from corner flow at the subduction initiation to return flow of the formation of a sub horizontal slab in the shallower part of mantle wedge produces the retreat of the subducting slab in fact a mantle suction force which is generated in the mantle wedge to fill space left by the retreating subducting plate is enhanced by the subsequent trench retreat even when upwelling flow with significant positive buoyancy originates just above a mantle phase boundary at a depth of 410 km as inferred from independent seismic tomographic geodynamic geochemical and mineral physics reaches the base of the overriding plate and the overriding plate is slightly thinned lithospheric stress tends to be compressed above the upwelling flow the reason for this is that the strong lateral mantle flow originating from the upwelling flow generates resistance drag force at the base of the overriding plates this situation may apply to a case of east asia under which the typical morphology of sub horizontal slabs can be seen by seismic tomography the strong lateral velocity observed in the shallower mantle wedge in the present numerical simulation may account for both the compressional subduction tectonics and back arc compression in the japan kuril kamchatka aleutian and south chile trenches as well as for weak plate slab coupling strong seismic coupling and the possibility of great earthquakes along these trenches

reverse tracing of short term earthquake precursors abstract we introduce a new approach to short term earthquake prediction named reverse tracing of precursors rtp since it considers precursors in reverse order of their appearance first we detect the candidates for the short term precursors in our case these are newly introduced chains of earthquakes reflecting the rise of an earthquake correlation range then we consider each chain one by one checking whether it was preceded by an intermediate term precursor in its vicinity if yes we regard this chain as a precursor in prediction it would start a short term alarm the chain indicates the narrow area of possibly complex shape where an intermediate term precursor should be looked for this makes possible to detect precursors undetectable by the direct analysis rtp can best be described on an example of its application we describe retrospective prediction of two prominent californian earthquakes landers 1992 m 7 6 and hector mine 1999 m 7 3 and suggest a hypothetical prediction algorithm this paper descripes the rtp methodology which has potentially important applications to many other data and to prediction of other critical phenomena besides earthquakes in particular it might vindicate some short term precursors previously rejected as giving too many false alarms validation of the algorithm per se requires its application in different regions with a substantial number of strong earthquakes first and positive retrospective results are obtained for 21 more strong earthquakes in california m 6 4 japan m 7 0 and the eastern mediterranean m 6 5 these results are described elsewhere the final validation requires as always prediction in advance for which this study sets up a base we have the first case of a precursory chain reported in advance of a subsequent strong earthquake tokachi oki japan 25 september 2003 m 8 1 possible mechanisms underlying rtp are outlined

plastic deformation of garnets systematics and implications for the rheology of the mantle transition zone abstract plastic properties of materials with garnet structure have been studied under wide temperature conditions ranging from room temperature to 95 of the melting temperatures using uniaxial compression and hot microhardness tests garnets studied include single crystals of oxide garnets y 3 al 5 o 12 gd 3 ga 5 o 12 and y 3 fe 5 o 12 and silicate garnets various solid solutions including grossular almandine andradite pyrope spessartine and uvarovite both uniaxial compression and hot hardness tests indicate that there is a general trend in the plasticity of garnets when the data are compared at normalized conditions t t m and and that the resistance to plastic deformation in garnets is significantly higher than most of the other minerals in the earth s mantle based on both stress dip tests and microstructural observations it is proposed that the creep strength of garnet is largely controlled by the resistance to dislocation glide rather than by recovery processes this conclusion is consistent with the high peierls stress inferred from the hot hardness tests the high peierls stress in garnets is presumably due to the large unit cell i e long burgers vectors and or the bcc packing which are common to all garnets we postulate therefore that the present results can be applied to the strength of high pressure garnet majorite and suggest that garnet rich layers in the earth such as subducted oceanic crust in the transition zone or a possibly garnet rich bottom part of the transition zone will be considerably stronger than surrounding regions

influence of temperature and depth dependent viscosity structures on postseismic deformation predictions for the large 1946 nankai subduction zone earthquake abstract to investigate the influence of spatial change of viscosity on postseismic deformation associated with the interplate 1946 nankai earthquake m 8 0 at the nankai trough southwest japan we newly constructed a realistic viscoelastic structure model taking into account temperature and depth dependent viscosity of materials for this purpose we first compiled leveling and triangulation data during postseismic periods and clarified characteristics of the amount and spatial patterns of postseismic vertical displacement and principal strain fields then we calculated the spatial distributions of viscosity from temperature and flow fields which were obtained from 2d subduction models by incorporating the obtained viscosity structure into 3d viscoelastic finite element models we constructed a temperature and depth dependent viscosity structure model model p2 based on model p2 we constructed a viscoelastic structure model taking into account poisson s ratio for the oceanic plate and low velocity regions and the existence of low viscosity materials beneath the shikoku and chugoku districts model p3 which were revealed from seismic tomography we also constructed a conventional layered viscoelastic structure model model l1 and plate subduction model model p1 with constant viscosity for each region and evaluated the effects of different viscoelastic structures on postseismic surface deformations using the coseismic slip distribution obtained by inversion analyses of geodetic data we also compared the calculated surface deformations with the observed postseismic crustal deformations in and around shikoku the results show that postseismic surface deformation fields for the newly constructed model p2 are rather different from those for models l1 and p1 landward horizontal displacements for model p2 are smaller than those for models l1 and p1 seaward horizontal displacements are negligible and vertical displacement is characterized by small subsidence over shikoku the postseismic horizontal principal strain field for model p2 is characterized by contractions in the n s to nw se directions at amounts smaller than those for models l1 and p1 postseismic surface deformations for model p3 are almost the same as those for model p2 the observed postseismic vertical displacement and horizontal principal strain fields could not be explained by the viscoelastic response for the realistic viscoelastic structure models p2 and p3 this indicates that the effects of elastic and viscoelastic responses due to interplate coupling on the plate interface after slip at the extension of the coseismic slipped region and poroelasticity should be taken into account to precisely estimate postseismic surface deformation this also suggests that in order to evaluate postseismic crustal deformations derived from a large interplate subduction zone earthquake it is essential to use realistic temperature and depth dependent viscoelastic structure models

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

spatial and temporal correlation between coda q 1 and seismicity and its physical mechanism from the analysis of local earthquake coda waves recorded by the benioff short period vertical seismograph at riverside california we found a surprisingly large and systematic temporal variation in coda q 1 in the southern california during the 55 year period from 1933 to 1987 comparison of the coda q 1 variation with the b value obtained for earthquakes with m 3 occurred in the area within 180 km from riverside corresponding to the area for which q 1 is estimated by the coda method led to an extraordinary result that coda q 1 correlates positively with b value with the correlation coefficient of 0 79 the significance of correlation cannot be rejected at the confidence level of 0 974 since the data used for measuring q 1 namely the coda part of seismograms and the data used for measuring b namely the seismicity catalog are totally independent the observed correlation supports strongly the physical reality of temporal change in both coda q 1 and b value the correlation between coda q 1 and b value has been reported for several other cases but the correlation was positive in some cases and negative in others our current preferred model for explaining the observed correlation is the following aseismic creep is going on from time to time over various sizes of cracks in the ductile part and in some cases in the brittle part of lithosphere at and near the plate boundary creep activities tend to increase the crack density and coda q 1 observed in the region if the creek occurs over distributed cracks with a certain predominant scale length spatial stress concentration may enhance seismicity for earthquakes with a similar size if this characteristic size is in the lower part of the magnitude range for which the b value is estimated the enhanced seismicity will increase b on the other hand if it is in the upper part of the magnitude range it will decrease b thus we can explain the observed strong correlation between coda q 1 and b value for both positive and negative cases the existence of such a characteristic crack size in a seismic region has been supported by various observations on earthquake scaling

relationships among temperature dehydration of the subducting philippine sea plate and the occurrence of a megathrust earthquake low frequency earthquakes and a slow slip event in the tokai district central japan abstract in this study we performed two dimensional numerical simulations of temperature distribution associated with subduction of the philippine sea phs plate and estimated the dehydration process of hydrous mid ocean ridge basalt morb in the oceanic crust in the tokai district central japan we discuss the relationship among temperature dehydration and a seismogenic zone of an expected megathrust tokai earthquake and the occurrence of a slow slip event sse and deep low frequency earthquakes lfes the depth range of the seismogenic zone for the megathrust earthquake was estimated to be 8 22 km narrowing toward the east the most suitable value of the pore pressure ratio on the plate interface was estimated to be 0 97 indicating minimal frictional heating there the temperatures of the upper surface of the phs plate where the tokai sse occurred from 2000 to 2005 were estimated to be 350 450 c therefore the tokai sse is considered to have occurred at the transition zone between unstable and stable sliding in addition hydrous morb was transformed from blueschist into greenschist near the region where the tokai sse occurred the temperatures of the upper surface of the phs plate where lfes occur were estimated to be 450 500 c therefore lfes are considered to occur near the down dip limit of the transition zone the amount of dehydration from the oceanic crust of the subducting phs plate near the region where lfes are distributed in a belt like form decreases toward the east with deepening of the hypocenters of the lfes thus the mechanisms of generation of lfes in the tokai district might differ from those in southwest and central japan where lfes generally occur at depths between 27 and 35 km in summary we consider that the occurrence of lfes is related to both temperature conditions and dehydration process however there is an exceptional area in the eastern part of the tokai district where the occurrence of lfes is restricted by temperature conditions rather than dehydration from the oceanic crust because the hypocenter depth is greater than that in southwest and central japan

temporal un correlations between coda q and seismicity multiscale trend analysis this paper introduces a statistical technique based on the recently developed multiscale trend analysis mta for quantifying correlations between non stationary processes observed at irregular non coincident time grids we apply this technique to studying the temporal correlation between the dynamics of the ductile and brittle layers in the lithosphere our results confirm the previously reported strong positive correlation between the coda q 1 and seismicity and its drop before major earthquakes observed in california the proposed technique has significant advantages over the conventional correlation analysis 1 mta allows one to work directly with non coincident time series without preliminary resampling the data 2 the correlation is defined via the stable objects trends rather than noisy individual observations hence it is highly robust 3 the correlations are quantified at different time scales the suggested technique seems promising for the wide range of applied problems dealing with coupled time series

remote surface mapping using orthophotos and geologic maps draped over digital elevation models application to the sheep mountain anticline wyoming remote mapping of surface structures can be conducted by draping digital orthophotos and geologic maps over digital elevation models in geographic information systems formational contacts can be mapped by viewing the intersections of these contacts with the topography on a true to scale three dimensional image bedding orientations can be determined from 1 the trends and slopes of dip slopes or 2 best fit planes to multiple points of intersection of a bedding surface with the topography the methods are tested against mapped formation contacts and bedding attitudes for the sheep mountain anticline in the bighorn basin wyoming detailed mapping of formation contacts is improved by careful observations of the contacts on three dimensional images interpreted bedding attitudes from slope and multiple point solutions show strong positive correlations with field measurements this mapping method provides an efficient and accurate alternative to stereoscopic mapping using aerial photographs and satellite images particularly for remote and inaccessible areas

inferences of viscosity from the oceanic geoid indication of a low viscosity zone below the 660 km discontinuity abstract we have attempted to infer details of the viscosity structure in the top 1000 km of the mantle from the geoid and tomographic structure beneath the oceans in order to eliminate the gravity signal from problematic masses located below the subduction zones and the continents we have considered only the intermediate degrees of the oceanic geoid l 12 25 a genetic algorithm has been used to determine the family of viscosity models which give the best correlation with the observed geoid our inversion clearly identifies the asthenosphere just below the lithosphere and also confirms the viscosity increase in the lower mantle predicted by previous inferences but suggests that the main viscosity jump occurs at a depth of about 1000 km and not at the usually stated 660 km boundary somewhere in the depth range of 400 1000 km a low viscosity zone may exist where the viscosity decreases to a value comparable with the asthenosphere existence of such a low viscosity zone is supported by recent analysis of deep mantle anisotropy which favours a flow pattern with a strong horizontal component in the top part of the lower mantle unfortunately the resolution of the inversion as well as the quality of recent seismic tomographic models are not sufficient to localize the depth and to come up with a higher accuracy for the viscosity of this low viscosity channel

heat sources and melting in subduction zones in order to understand the mechanism of melt generation and the origin of high heat flow in subduction zones a series of numerical models of the thermal and flow structures in the mantle have been tested in two dimensional boxes with an inclined subducting slab of constant velocity in contrast to previous models 1 large convecting cells are used without imposing a high temperature profile on the backarc boundary which enables us to discuss the global heat balance and to seek for the heat sources responsible for the melting and high heat flow observed in subduction zones and 2 various mechanical conditions e g coupling between the slab and overlying mantle wedge buoyancy associated with melting and heat sources e g heat flux from below internal heat generation viscous heating are tested in varying proportions in all the calculations steady state or near steady state with a small instability periodically occurring at the upper thermal boundary layer is achieved under these conditions the global heat balance can be described by a simple boundary layer argument the results show that in order to attain a high enough temperature for melting and a high heat flux a large amount of internal heating i e more than 2 5 10 7 w m3 is required if the convection is limited within the upper mantle the high internal heating required may be explained if the radioactive nuclides in fluids expelled from the subducting slab are added to the wedge and circulate in the convection cell for a sufficient time another possible explanation for melting and the high heat flux is that the hot material is supplied from the lower mantle based on the thermal and flow structures obtained melting regimes in subduction zones are discussed in which the following key processes take place 1 melting associated with pressure change of a rock packet with its movement especially compression melting in the downward flow of the mantle wedge along the slab and 2 melting due to compositional change of the rock packet associated with migration of h2o and melt to solve these problems in detail further studies on distribution and migration of the fluids will be required

laboratory experiments on the structure of subducted lithosphere abstract mantle seismic tomography images indicate a variety of shapes for slabs of oceanic lithosphere subducted into the transition zone in the vicinity of the 660 km discontinuity both horizontally deflected slabs and nearly vertical slabs have been inferred at subduction zones on the pacific and indian plates here we determine experimentally the behavior of a cold negatively buoyant and highly viscous slab incident on a fluid interface with a viscosity and a density increase the experiments are scaled to mantle conditions with realistic subduction peclet number ratio of trench migration to subduction velocities and physical properties of the interface a wide range of slab deformation styles is observed when slight changes in the parameters are imposed sinking slab stagnant slab spreading slab sinking pile and stagnant pile represent the five different modes identified we show that these deformation modes are governed by two velocity ratios which characterize the horizontal and the vertical components of the slab velocity near the interface slab penetration into the lower layer takes the form of folded slab piles for large values of the velocity ratios and stagnant behavior is promoted by rapid retrograde trench motion the sensitivity of slab behavior to transition zone density and viscosity changes and to trench migration can account for the coexistence of different modes of deformation in subducted lithosphere

regionality of deep low frequency earthquakes associated with subduction of the philippine sea plate along the nankai trough southwest japan abstract the fukuoka district meteorological observatory recently logged three possible deep low frequency earthquakes lfes beneath eastern kyushu japan a region in which lfes and low frequency tremors have never before been identified to assess these data we analyzed band pass filtered velocity seismograms and relocated lfes and regular earthquakes using the double difference method the results strongly suggest that the three events were authentic lfes each at a depth of about 50 km we also performed relocation analysis on lfes recorded beneath the kii peninsula and found that these lfes occurred near the northwest dipping plate interface at depths of approximately 29 38 km these results indicate that lfes in southwest japan occur near the upper surface of the subducting philippine sea phs plate to investigate the origin of regional differences in the occurrence frequency of lfes in western shikoku the kii peninsula and eastern kyushu we calculated temperature distributions associated with phs plate subduction then using the calculated thermal structures and a phase diagram of water dehydration for oceanic basalt the water dehydration rate wt km which was newly defined in this study was determined to be 0 19 0 12 and 0 08 in western shikoku the kii peninsula and eastern kyushu respectively that is the region beneath eastern kyushu has the lowest water dehydration rate value considering that the kyushu palau ridge that is subducting beneath eastern kyushu is composed of tonalite which is low in hydrous minerals this finding suggests that the regionality may be related to the amount of water dehydration associated with subduction of the phs plate and or differences in lfe depths notable dehydration reactions take place beneath western shikoku and the kii peninsula where the depth ranges for dehydration estimated by thermal modeling agree well with those for the relocated lfes the temperature range in which lfes occur in these regions is estimated to be 400 500 c

scale dependence in earthquake phenomena and its relevance to earthquake prediction abstract the recent discovery of a low velocity low q zone with a width of 50 200 m reaching to the top of the ductile part of the crust by observations on seismic guided waves trapped in the fault zone of the landers earthquake of 1992 and its identification with the shear zone inferred from the distribution of tension cracks observed on the surface support the existence of a characteristic scale length of the order of 100 m affecting various earthquake phenomena in southern california as evidenced earlier by the kink in the magnitude frequency relation at about m3 the constant corner frequency for earthquakes with m below about 3 and the sourcecontrolled fmax of 5 10 hz for major earthquakes the temporal correlation between coda q 1 and the fractional rate of occurrence of earthquakes in the magnitude range 3 3 5 the geographical similarity of coda q 1 and seismic velocity at a depth of 20 km and the simultaneous change of coda q 1 and conductivity at the lower crust support the hypotheses that coda q 1 may represent the activity of creep fracture in the ductile part of the lithosphere occurring over cracks with a characteristic size of the order of 100 m the existence of such a characteristic scale length cannot be consistent with the overall self similarity of earthquakes unless we postulate a discrete hierarchy of such characteristic scale lengths the discrete hierarchy of characteristic scale lengths is consistent with recently observed logarithmic periodicity in precursory seismicity

coda q before and after the 1983 misasa earthquake of m 6 2 tottori prefecture japan to detect any temporal change in coda q associated with an earthquake of m 6 2 digitized data from analog magnetic records of microearthquakes for a small confined region just around its hypocenter have been processed with special attention to measuring errors the m 6 2 earthquake took place at the misasa town tottori prefecture in southwest japan with preceding anomalous seismicity changes theq values were elevated 20 around three years before the m 6 2 event for frequencies of 5 to 20 hz and tended to decrease around two years before data with high quality show undulated temporal variation with a period of 5 6 years before the earthquake which is correlated with the regional seismic activity there is a possibility that the observedq change stands for precursory effect of the medium scale earthquake the aftershock sequence also shows an undulated temporal variation ofq with a period of 150 days for around 10hz correlating also with the seismic activity the fracturing processes by numerous microearthquakes may be responsible for the attenuation property of randomly scattered seismic waves

geophysical and mineralogical constraints on the post spinel transformation for the tonga slab abstract the depth of the post spinel transformation is investigated for the tonga slab by using data from short period seismic networks at western united states and japan for about 100 deep and intermediate depth earthquakes within a small 200 km by 200 km area near 20 s later phases in a time window 3 to 20 s after direct p waves are analyzed to search for s to p converted waves at the 660 km discontinuity which represents the post spinel transformation we find that immediately beneath the foci of the deepest earthquakes the discontinuity is depressed down to the depths of 685 5 km on average and that it dips towards wnw by 10 3 km within 70 km laterally we constrain the thermal structure near the s to p conversion points based on a plausible assumption that the deepest earthquakes occur around the coldest core of the tonga slab the distribution of the hypocenters relocated in this study as well as previously published tomographic images of the same region suggest that the tonga slab bends upward when approaching the 660 km discontinuity and transiently stagnates around the discontinuity with these observations as the constraints we numerically model the thermal structure of the tonga slab and estimate the temperature around the conversion points as 1200 100 c which is 300 100 k colder than the surrounding mantle as the average depression of the discontinuity down to 685 5 km corresponds to an pressure excess over the global average 660 km by 1 0 0 2 gpa the assumption of equilibrium post spinel transformation results in an estimate of the clapeyron slope c1 of 3 3 2 7 1 3 mpa k we also obtain an independent estimate of the clapeyron slope c2 of 2 0 1 0 mpa k based on the observation of the dip of the discontinuity and the computed temperature variation by about 200 k the discrepancy between c1 and c2 is marginally significant and can be diminished by considering that the slab materials at the conversion points are currently descending across the phase boundary fast enough and thus the depth of the post spinel transformation is controlled by nucleation kinetics as well as by the temperature

quantitative structural analysis with stereoscopic remote sensing imagery we employ powerful stereoscopic methods with satellite imagery to obtain surface bedding attitude measurements these data are used to constrain geologic structures in three dimensions using quantitative structural models we document a numerical method for measuring the strike and dip of bedding using stereoscopic pairs of air photos as well as landsat thematic mapper and spot images in examples from the north american and andean cordilleras these remote measurements prove consistent with direct surface control and subsurface structures imaged in seismic reflection profiles remotely derived measurements are combined with subsurface data to generate balanced structural interpretations that define complex structural traps using fault related folding theory these remote sensing methods can provide a low cost and rapid means of delineating prospects and leads

stagnant slab a review a stagnant slab is a subducted slab of oceanic lithosphere subhorizontally deflected above across or below the 660 km discontinuity this phenomenon has now been widely recognized beneath subduction zones around the circum pacific and in the mediterranean collaboration of seismic and electromagnetic observations mineral physics measurements and geodynamic modeling has begun to provide a consistent picture of stagnant slab

rheological structure and deformation of subducted slabs in the mantle transition zone implications for mantle circulation and deep earthquakes abstract rheological structure of subducted slabs of oceanic lithosphere in the mantle transition zone is investigated based on mineral physics observations incorporating grain size stress temperature and pressure dependence of rheology it is shown that the rheological structure of slabs depends strongly on subduction parameters through temperature that controls the grain size of spinel ringwoodite and the magnitude of forces acting on a slab we use a theoretical model of grain size evolution associated with the olivine spinel transformation plastic flow laws of olivine and spinel combined with a thermal model of subducting slab in which the effect of latent heat release is incorporated three deformation mechanisms for olivine and spinel diffusional creep power law dislocation creep and the peierls mechanism are considered due to the large variation in temperature stress and grain size a subducting slab is shown to have a complicated rheological structure which varies both laterally and with depth a cold slab in the deep transition zone is characterized by a weak fine grained spinel region surrounded by narrow but strong regions the flexural rigidity and the curvature of a slab are calculated using a new formulation in which the effects of stress dependent rheology is incorporated in a self consistent fashion although uncertainties in both the transformation kinetics and the rheology of high pressure phases are still large the general trend of dependence of slab flexural rigidity and the curvature on the subduction parameters is well constrained slabs with very low thermal parameters warm slabs are weak but slabs with large thermal parameters cold slabs are also weak due to small spinel grain size and large external force bending moment slabs with intermediate thermal parameters will have a relatively large flexural rigidity and could penetrate into the lower mantle without much deformation thus the 660 km discontinuity may work as a rheological filter for mantle convection this prediction provides a natural explanation for a paradoxical observation that significant deformation of slabs is observed exclusively in the western pacific where temperatures of the slabs are considered to be low our slab rheology models also have important implications for deep earthquakes overall rheological weakening of slabs in the deep transition zone results in high rates of deformation under relatively low temperatures providing a favorable environment for thermal runaway instability adiabatic shear instability our model predicts heterogeneous energy dissipation as a result of heterogeneous rheology energy dissipation in deep cold slabs is concentrated in high strength regions surrounding a weak fine grained spinel core the regions of high energy dissipation are prone to thermal runaway instability and are likely to be seismogenic the width of this seismogenic region is well constrained by our model and is predicted to be 40 60 km which is in excellent agreement with seismological observations other features of deep earthquakes including low seismic efficiency and low aftershock activities can also be explained by the thermal instability model

temperature distribution of the upper surface of the subducted philippine sea plate along the nankai trough southwest japan from a three dimensional subduction model relation to large interplate and low frequency earthquakes summary the nankai subduction fault in southwest japan is a plate boundary along which large megathrust earthquakes have repeatedly occurred a previous 2 d thermal model suggested that the updip and downdip limits of the seismogenic zone are closely related to the temperature of the plate interface we estimated a new temperature distribution for the plate interface based on a 3 d thermal convection model that incorporates revised governing equations temporal changes in relative plate motion the shape of the subducted slab temperature variations related to basin evolution and more heat flow data and model regions than have been used in previous models the margin parallel spatial variation in the temperature of the plate interface was caused by spatio temporal changes associated with the cooling of the shikoku basin the updip limit of the thermally delineated seismogenic zone thermal seismogenic zone was approximately parallel to the trough axis due to a spreading rate that increased with distance from a fossil ridge although the margin parallel age dependency of the subducting plate was taken into account the downdip limit of the thermal seismogenic zone was determined by temperature except in eastern kyushu where it was determined by a boundary between the continental moho depth and the upper surface of the subducting plate the maximum coseismic slip associated with the 1944 tonankai m 7 9 and the 1946 nankai m 8 0 earthquakes and a strongly coupled region estimated from gps data inversion obtained by previous studies were both located in the thermal seismogenic zone on the plate interface low frequency earthquakes located at the downdip of the thermal seismogenic zone took place at temperatures of about 400 500 c this may be related to the phase transformation of hydrous minerals in oceanic crust from lawsonite blueschist jadeite to amphibole eclogite and the associated decreases in water content of approximately 2 4 wt the thermal seismogenic zone is narrowest off the kii peninsula where the hypocentres of the tonankai and nankai earthquakes were located the narrow seismogenic zone may have determined the rupture initiation points for these earthquakes as was previously demonstrated by numerical simulations of the earthquake cycle

difference in the maximum magnitude of interplate earthquakes off shikoku and in the hyuganada region southwest japan inferred from the temperature distribution obtained from numerical modeling the proposed hyuganada triangle abstract to clarify differences in the maximum magnitude of interplate earthquakes offshore of shikoku m 8 and in the western adjacent hyuganada region m 7 5 southwest japan where the oceanic philippine sea phs plate subducts beneath the continental amuria plate along the nankai trough we calculated the temperature distribution for the plate interface using a three dimensional thermal convection model we attribute the observed very low heat flow values less than approximately 45 mw m 2 in the southern part of the hyuganada region to the subduction of the kyushu palau ridge which is presumably older than the adjacent shikoku basin to the east trial and error methods suggest an age difference of about 20 myr between these two areas the heat flow calculated from the temperature distribution fits the observed heat flow in the hyuganada region well the results show that hyuganada earthquakes may take place in the newly proposed hyuganada triangle bounded by a southeastern isotherm of about 200 c on the plate interface a northwestern boundary between the upper surface of the subducting phs plate and the continental moho discontinuity a northern isotherm of about 350 c and a northeastern margin normal zone with a possible barrier or large friction parameter l interplate earthquakes in such a limited seismogenic zone may reach a maximum magnitude of m 7 5 our results also suggest that a seismic gap seaward of the hyuganada region may be caused by stable sliding arising from low temperature resulting in a deeper updip limit than that offshore of shikoku the seismic gap offshore southwest of southern kyushu along the ryukyu trench may result from the overlapping of two stable sliding regimes on the plate interface one with low temperature at shallower depths caused by the subduction of the old phs plate and the other a serpentinized mantle wedge at deeper depths

the interaction of a subducting lithospheric slab with a chemical or phase boundary we have used a finite element model of time dependent convection to determine the conditions for penetration of the subducted plate into the lower mantle a temperature dependent and non newtonian rheology is applied to achieve platelike behavior of the upper and sinking thermal boundary layer of convection the 650 km discontinuity is taken as either a chemical or phase boundary or as a combination of both it is represented by a marker chain which effects additional buoyancy when distorted out of its equilibrium position when the compositional density contrast is greater than about 5 the descending slab is deflected sidewards at the boundary and two layer convection prevails a resulting depression of the boundary in the range of 50 200 km should be detectable with seismic methods below 5 density difference the slab plunges several hundred kilometers into the lower mantle and below 2 it will probably not stop before reaching the core mantle boundary and extensive mixing would be expected with a pure phase change a negative clapeyron slope of about 6 mpa k 60 bar k is required to establish a type of leaky double layer convection a more moderate slope can aid a small compositional density difference to prevent slab penetration into the lower mantle with the present uncertainties about the physical nature of the 650 km discontinuity a variety of convective styles appears possible on dynamical grounds

the influence of trench migration on slab penetration into the lower mantle abstract a two dimensional numerical convection model in cartesian geometry is used to study the influence of trench migration on the ability of subducted slabs to penetrate an endothermic phase boundary at 660 km depth the transient subduction history of an oceanic plate is modelled by imposing plate and trench motion at the surface the viscosity depends on temperature and depth a variety of styles of slab behaviour is found depending predominantly on the trench velocity when trench retreat is faster than 2 4 cm a the descending slab flattens above the phase boundary at slower rates it penetrates straight into the lower mantle although flattening in the transition zone may occur later leading to a complex slab morphology the slab can buckle independent of whether it penetrates or not especially when there is a localised increase in viscosity at the phase boundary flattened slabs are only temporarily arrested in the transition zone and sink ultimately into the lower mantle the results offer a framework for understanding the variety in slab geometry revealed by seismic tomography

an experimental study of subduction and slab migration results of fluid dynamical experiments on the behavior of subducted slabs are presented that address two important characteristics of subduction slab penetration through the transition zone and horizontal slab migration cold negatively buoyant molded slabs of concentrated sucrose solution with viscosities of 3 5 106 p were introduced into a more dilute two layered sucrose solution representing the upper and lower mantle the transition zone was modeled by a step increase in both density and viscosity ratios of slab thickness to upper layer depth the relative viscosities in each layer and the effective rayleigh number were close to mantle values the initial configuration consisted of two plates separated by a trench gap with one plate attached to a dipping slab simulating a developing subduction zone with an overriding plate two different boundary conditions were used at the trailing ridge end of the subducting plate 1 fixed end corresponding to zero ridge motion and 2 free end corresponding to zero ridge resistance based on results from 15 experiments with various combinations of upper layer lower layer and slab densities u l s and viscosities we find that slab penetration depth is sensitive to both the normalized slab density anomaly r s l s u and the dip angle three penetration regimes are recognized r 0 5 in which the slab sinks into the lower layer without distortion 0 2 r 0 5 which permits limited penetration the amount dependent on dip angle in this regime the slab develops a thick root just beneath the transition zone and r 0 2 in which the slab is deflected by the transition and little or no penetration occurs retrograde slab motion and trench migration occurred in nearly every case with the overriding plate moving parallel to but faster than the trench causing the gap between subducting and overriding plates to close with time prograde trench motion was never observed and the trench remained stationary only in the extreme case free ridge condition with equal upper and lower layer densities these results suggest that the transient extension found in many back arc basins may be a direct consequence of the tendency for retrograde subduction and the amount of extension may be governed in part by the degree of slab penetration through the transition zone

the effects of metastable olivine α wedge in subducted slabs on theoretical seismic waveforms of deep earthquakes 1 assuming three kinds of materials 100 olivine mg0 89fe0 11 2sio4 pyrolite and piclogite as composition of the upper mantle we constructed pressure temperature dependent seismic velocity structure models in the upper mantle beneath a subduction zone for this purpose we employed values of physical properties of composite minerals newly determined by using a genetic algorithm ga and obtained the thermal structure beneath the subduction zone as a numerical solution of a two dimensional 2 d thermal fluid coupled problem furthermore we investigated the effects of a metastable olivine wedge in subducted slabs on theoretical seismic waveforms of deep earthquakes which propagate through the velocity structure by solving the p sv problem numerically the effects of center frequency of the excited waves at hypocenters focal mechanism location of hypocenter upper mantle compositions values of physical properties a dip angle of subducting slabs anelastic attenuation subducting velocity and depth of deep earthquakes on seismic waveforms were investigated in detail theoretical seismograms at the earth s surface indicate that coda waves with small amplitudes of duration time of 20 30 s appear following direct p and s waves for models with the metastable olivine wedge inside the slabs the coda waves can be explained by the excitation by reflections and conversions at a large seismic velocity discontinuity between the metastable olivine wedge and the surrounding slab for models with center frequency of 0 25 hz purer olivine composition a faster subducting velocity and focal depth of 500 km waves with larger amplitudes were excited at the discontinuity the results of this study may give important clues to detect the existence of the metastable olivine wedge from seismological observations

two dimensional thermal modeling associated with subduction of the philippine sea plate in southern kyushu japan abstract in hyuga nada southern kyushu in southwest japan afterslip events were found in association with the two large interplate earthquakes which occurred on october 19 and december 3 1996 in kyushu low frequency earthquakes lfes and tectonic tremors are not common but a considerable concentration of tectonic tremors is observed beneath the pacific coast of the miyazaki prefecture to investigate the generation mechanisms of these seismic events we performed 2 d box type time dependent thermal modeling in southern kyushu as a result the temperature range of the upper surface of the subducting philippine sea phs plate where the afterslip occurred reached approximately 300 to 350 c the temperatures where the tectonic tremors occurred ranged from 450 to 650 c in the mantle wedge corner we also estimated the spatial distribution of water content within the subducting phs plate using phase diagrams of hydrous mid ocean ridge basalt morb and ultramafic rock then we found that no characteristic phase transformations accompany the dehydration of the subducting phs plate in the afterslip region but phase transformation from lawsonite blueschist to lawsonite eclogite is expected within the oceanic crust of the phs plate just below the active region of the tectonic tremors our estimated water content distribution is consistent with the vp vs ratio calculated from the seismic tomography therefore we conclude that the occurrence of the afterslip is controlled by the temperature condition at the plate boundary and occurs near the down dip limit of the seismogenic zone on the other hand determining the major factors leading to the occurrence of the tectonic tremors is difficult we estimated the temperature in the mantle wedge is ranging from 450 c to 650 c and dehydration of 1 0 wt would be expected from the subducting phs plate near the active region of the tectonic tremors

temporal change in scattering and attenuation associated with the earthquake occurrence a review of recent studies on coda waves the study of coda waves has recently attracted increasing attention from seismologists this is due to the fact that it is viewed as a new means by which the stress accumulation stage preceding a large earthquake can be measured since the scattering paths nearly uniformly cover a fairly large region around the focus and observation stations compared with the direct ray paths to date we have had many reports on the temporal variation of the relation between coda duration and amplitude magnitude and that of the coda attenuationq 1 which is estimated from coda amplitude decay some of these have shown a precursor like behavior however others seem to have shown a coseismic change we have critically reviewed these reports and discussed what these observational facts tell us about the change in the heterogeneous crust we found significant temporal variations not only in the mean but also in the scatter ofq 1 associated with the mainshock occurrence the formation of new cracks the reopening and growing of existing cracks the interaction of these cracks and the pore water movement through these cracks might correspond to such variations in addition we may expect an inhomogeneous distribution of crack clusters in a fairly large region compared with the aftershock region the gradual appearance of such crack clusters seems to be the most plausible mechanism by which coda decay gradients are caused to largely scatter in the stress accumulation stage

magmatic processes under arcs and formation of the volcanic front temperature structure and stress under arcs are simulated in a two dimensional cross section taking into account the flow induced by the subducting slab in the mantle wedge results of the calculations show three important features with respect to magmatic processes under arcs 1 temperature structure in the crust and the mantle wedge under arcs is insensitive to the angle and velocity of slab subduction the temperature structure of the slab and that of the back arc region this indicates that physical conditions such as temperature and pressure are similar under various arcs it is thus inferred that primary magmas generated under various arcs should have similar chemical compositions if chemical composition and the flux rate of fluid from the slab are similar and the chemical compositions of mantle wedge materials are the same 2 calculated deviatoric stress magnitude is relatively large more than a few tens of megapascals in the partially molten mantle cracks may open under high differential stress and magma can easily segregate and accumulate through interconnected cracks while the buoyancy driven compaction of partially molten mantle proceeds 3 the deviatoric stress values in the region over the partially molten mantle are relatively large and the direction of the principal stress changes horizontally the direction of the maximum compressional stress is nearly vertical under the volcanic zone and is nearly horizontal under the fore arc region it is considered that magma segregated in partially molten mantle migrates upward through the brittle mantle and crust by the magma fracturing mechanism the propagation direction of magma filled fissures is controlled by the stress field in the crust and mantle and is parallel to the maximum principal stress the calculated stress is highly compressive horizontally on the trench side while it becomes tensile on the back arc side the location of this stress transition coincides with that of the volcanic front the location of this transition indicates that the volcanic front marks a change in the ease of upward migration of the magma filled cracks under relatively high differential stress field

subducting slabs stagnant in the mantle transition zone the p wave velocity structure beneath the western pacific is found from the international seismological center first arrival data special attention was paid to the deep structure beneath the wadati benioff zone we discretized the whole mantle into blocks with finer blocks in the region of interest to obtain the velocities of all the blocks this way of discretization minimizes a problem with tomographic studies of regional scale difficulty in making corrections for the effects outside the region of interest our solution is iterative with the alternate step of the relocation of earthquakes using the whole mantle model of inoue et al 1990 as a starting model a first order smoothness constraint was imposed to suppress the possible fluctuation of the solution around the initial model the essential result depends little on whether the reference spherical model is smooth or discontinuous near 400 and 670 km depths we examined the resolution by calculating the resolving kernels for selected blocks and by reconstructing the checkerboard test patterns of velocity perturbation and the test structures of subducting lithosphere the resolution is depth dependent but in general good enough to see the slab configuration beneath the southern kurile japan izu bonin arcs and the java arc it is relatively poor beneath the northern kurile and mariana arcs the seismic image of subducting slab beneath the southern kurile to bonin arcs bends to subhorizontal near the leading edge of the wadati benioff zone and extends continentward over a distance of more than 1000 km the subhorizontal portion of the slab connects a high velocity blob to the bottom that reaches a depth of at least 800 km across the 670 km discontinuity under the japan arc although the image of the java slab directly penetrates the 670 km discontinuity it then bends to a shallow dip with a considerable spread reaching a depth of about 1200 km these results suggest that descending slabs of lithosphere in the western pacific tend to be stagnant in the transition zone under a subtle control of the 670 km discontinuity although stagnant slab materials eventually descend into the lower mantle they no longer maintain their original configuration below the 670 km discontinuity

two dimensional thermal modeling of subduction of the philippine sea plate beneath southwest japan abstract in this study we constructed a two dimensional thermal convection model associated with subduction of the philippine sea phs plate beneath southwest japan to estimate the thermal state to evaluate the validity of the calculated temperature distribution we compared the calculated heat flows with observed heat flow data such as the high sensitivity seismograph network japan hi net borehole land borehole marine probe and bottom simulating reflectors hi net heat flow data which have never been used for thermal modeling have spatially high resolutions on land and enable detailed estimation of the thermal state beneath the japanese islands we considered the spatio temporal change in the age of the phs plate change in its plate motion direction at 3 ma and the geometry of its upper surface calculated heat flows associated solely with subduction of the phs plate passing through central and eastern shikoku and the kii peninsula were not consistent with observations they tended to be lower on the seaward side of the corner of the mantle wedge and to have much longer spatial wavelengths on the continental side to explain heat flow data in southwest japan frictional heating at the plate interface on the seaward side of the corner of the mantle wedge and temperature changes due to surface erosion and sedimentation on land associated with crustal deformation during the quaternary must be incorporated into the model the most suitable pore pressure ratio to explain the heat flow data is estimated to be 0 95 temperatures on the upper surface of the phs plate obtained in this study are lower at shallow depths and higher at greater depths than those obtained by hyndman et al 1995 indicating that the seismogenic zone inferred from this study is narrower than that of their model microearthquakes within the subducting phs plate occur at temperatures below 700 800 c

velocity anomalies and spatial distributions of physical properties in horizontally lying slabs beneath the northwestern pacific region abstract we calculated velocity anomalies by constructing thermal structural models of horizontally lying slabs beneath kuriles central japan northern izu bonin and central izu bonin arcs the values of v p and v s anomalies become maximum in the depth range of 100 to 200 km within the slabs reaching 5 to 6 4 for v p anomalies and 7 to 8 5 for v s anomalies the calculated v s anomalies are larger than the v p anomalies and the ratio of the former to the latter is 1 2 to 1 5 within the slab this suggests that v s anomalies are more sensitive to temperature variation than v p anomalies indicating that the existence of the horizontally lying slabs is detectable in more detail by using s wave we also calculated spatial distributions of physical properties in the horizontally lying slabs beneath the four regions in the calculation we assumed that the material of the slabs consists of olivine mg 0 89 fe 0 11 2 sio 4 the estimated values of physical properties within the slabs are larger than those in the surrounding upper mantle except for poisson s ratio while the values within the metastable wedge of olivine become smaller this is most remarkable within the slab beneath the central izu bonin arc due to the lowest temperature among the four regions the horizontally lying slab above the 660 km seismic discontinuity beneath the central izu bonin arc is denser up to 60 kg m 3 and the slab within the metastable wedge of olivine is lighter up to 120 kg m 3 the values of the physical properties except for poisson s ratio decrease with increasing the horizontal distances of the horizontally lying portions of the slabs this is due to thermal assimilation which is largely dependent on moving velocity of the slabs

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle

physical properties of subducted slab and surrounding mantle in the izu bonin subduction zone based on broadband ocean bottom seismometer data 1 seismic attenuation and traveltime anomalies in the vicinity of the izu bonin subduction zone are inferred from waveform data recorded by broadband ocean bottom seismometers bbobss deployed as part of the stagnant slab project we measure the path averaged attenuation of p waves and traveltime anomalies of both p and s waves using waveform data from 10 regional earthquakes within the izu bonin slab the observed relation between the attenuation and traveltime anomalies is quantitatively evaluated on the basis of mineral physics for the data that sample the slab and the pacific mantle beneath the slab the relation between the attenuation and traveltime anomalies is consistent with predictions based on the thermal effect the data that sample the mantle wedge show pronounced positive traveltime anomalies unaccompanied by comparable high attenuation the traveltime anomalies are too intense to be explained solely in terms of the temperature effect the temperature anomalies estimated from attenuation suggest that the temperature in the mantle wedge reaches the dry solidus of peridotite the relative intensity of s to p wave traveltime anomalies of mantle wedge data after correction for the thermal effect is relatively small rs p 0 5 1 2 similarly low velocity anomalies without comparable high attenuation and with relatively small rs p 1 5 have been reported in previous studies of subduction zones at tonga in the southwest pacific and tohoku japan we discuss to what extent the present seismological observations can be explained by partial melt on the basis of recent theoretical and experimental studies on partially molten rocks