stress strain paradox plate coupling and forearc seismicity at the cascadia and nankai subduction zones abstract in the continental forearcs of the cascadia and nankai subduction zones geodetic strain measurements indicate crustal contraction in the direction of plate convergence nearly margin normal but earthquake focal mechanisms and other stress indicators show maximum compression along strike margin parallel it is evident that the geodetic strain signals reflect the temporal fluctuations of elastic stress associated with subduction earthquake cycles not the absolute stresses the absolute margin normal stress is not only much less than the margin parallel stress but also no greater than the lithostatic value indicating a very weak subduction thrust fault weak subduction faults are also consistent with geothermal data which require very low frictional heating along the faults great subduction earthquakes occur at very low shear stresses and cause small perturbations to the forearc stress regime because these small elastic stress perturbations are relatively fast they give large strain rates that are detected by geodetic measurements the present nearly margin normal contraction in both places is due to the locking of the subduction fault and the consequent increase in elastic stress in the direction of plate convergence with the margin parallel compression being dominant the small increase in margin normal stress may change the forearc seismicity in cascadia from a mixture of thrust and strike slip types into mainly thrust before the next great subduction earthquake in nankai the change should be from strike slip to quiescence as observed in the nankai forearc prior to the 1944 1946 great subduction earthquakes

development and applications of double difference seismic tomography double difference dd tomography is a generalization of dd location it simultaneously solves for the three dimensional velocity structure and seismic event locations dd tomography uses a combination of absolute and more accurate differential arrival times and hierarchically determines the velocity structure from larger scale to smaller scale this method is able to produce more accurate event locations and velocity structure near the source region than standard tomography which uses only absolute arrival times we conduct a stability and uncertainty analysis of dd tomography based on a synthetic data set currently three versions of the dd tomography algorithms exist tomodd tomofdd and tomoadd tomodd assumes a flat earth model and uses a pseudo bending ray tracing algorithm to find rays between events and stations while tomofdd uses a finite difference travel time algorithm and the curvature of the earth is considered both codes are based on a regularly distributed inversion grid with the former for a local scale and the latter for a regional scale in contrast tomoadd adapts the inversion mesh to match with the data distribution based on tetrahedral and voronoi diagrams we discuss examples of applying dd tomography to characterize fault zone structure image high resolution structure of subduction zones and determine the velocity structure of volcanoes

mechanism of deep low frequency earthquakes further evidence that deep non volcanic tremor is generated by shear slip on the plate interface 1 we study the mechanism of low frequency earthquakes lfes in the nankai trough in western shikoku japan precise locations have previously suggested that they represent shear slip on the plate boundary in this paper we examine the mechanism of these events due to the low signal to noise ratio we analyze stacked lfe waveforms and compare them with the waveforms of nearby earthquakes of known mechanism within the subducting philippine sea plate analysis of both the focal mechanism using p wave first motions and the moment tensor using s waveforms indicates that lfes represent shear slip on a low angle thrust fault dipping to the northwest namely the plate interface together with reports that deep tremor consists of a swarm of lfes our results suggest that deep tremor is generated directly by shear slip on the plate interface and as such represents a seismic signature of the accompanying slow slip events

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

thermal regime of the southwest japan subduction zone effects of age history of the subducting plate abstract during a mid miocene at about 15 ma tectonic reorganization an ocean spreading ridge perpendicular to the strike of the nankai trough subduction zone stopped spreading since that time there has been subduction of the cooling fossil spreading ridge in the present study we have developed a time dependent thermal subduction model for this region using the finite element method there are good seismological and geological constraints for the plate geometry and for the subduction history of the past 15 ma boundary conditions are specified so that the oceanic plate subducting at the nankai trough becomes cooler as it gets older the model results agree with the present heat flow trend that decreases landward and explain the paleothermal regime of high mid miocene temperatures inferred from land geological studies the tectonics of the region prior to 15 ma has some uncertainties assuming subduction of an active spreading ridge or an 80 ma old lithosphere prior to 15 ma give the same results for the present thermal regime of the seaward portion of the forearc but different results for the most landward region 250 km the thermal history of the forearc since 15 ma can be summarized as a rapid warming period as a consequence of ridge subduction followed by a cooling trend to the present as a result of the aging of the subducting plate the results illustrate the thermal consequences of one type of ridge subduction they also demonstrate that the thermal regime of a subduction zone depends critically on the age history of the subducting oceanic lithosphere especially if it is young as well as such parameters as the subducting plate dip angle and thickness of insulating sediments on the incoming oceanic crust this dependence is especially important when the thermal regime is used to constrain the seismogenic behaviour of the subduction thrust fault

thin tephra layers in peat revealed by x radiography x radiography has been found to provide a rapid economical non destructive method of locating very thin fine grained tephra of icelandic origin in scottish peats tephra layers form time parallel marker horizons or isochrones that are of great archaeological significance because they cover large areas offer precise chronological control for associated material and occur at times of significant cultural and environmental changes not all of the discrete deposits of recent volcanic fallout in scotland are sufficiently dense to show clearly on x radiographs but those which do provide particularly clear easily located marker horizons with many applications in palaeoenvironmental studies the tephra layer in caithness produced by fallout from a c 4000 by eruption of hekla provides a particularly good example of the use of x radiography to locate volcanic ash the method also reveals other structures of archaeological and palaeoenvironmental significance butler 1992 journal of archaeological science 19 151 161

thermal constraints on the seismogenic portion of the southwestern japan subduction thrust for coastal cities an important factor in earthquake hazard from subduction zone earthquakes is the landward extent of the seismogenic portion of the subduction thrust fault in this study we test the hypothesis that the maximum downdip extent is defined by a critical temperature we have developed a transient thermal model for the nankai subduction zone of southwest japan to allow comparison of the thermally estimated downdip extent of the seismogenic zone with that from 1 seismicity and tsunami data for two great subduction earthquakes 2 the coseismic faulting extent of these events estimated from geodetic deformation data and 3 the interseismic locked zone determined from interseismic geodetic data the nankai margin has extensive heat flow and heat production data to control thermal models and thus crustal temperatures it has earthquake tsunami and geodetic data that constrain the coseismic rupture portion of the subduction thrust fault for past great earthquakes and the portion of the thrust fault that is locked and storing interseismic elastic strain on the nankaido margin off shikoku island the thermal model indicates that a temperature of 350 c taken to be the limit for seismic initiation from laboratory and field data is reached on the subduction thrust fault 150 km from the trench a transition zone into which rupture may extend with decreasing offset taken to be 450 c extends an additional 45 km downdip the thermal model results are in excellent agreement with the maximum downdip extent of coseismic displacement for the 1946 nankaido ms 8 2 earthquake off shikoku island and with the downdip extent of the present locked zone in the region of the 1944 tonankai ms 8 2 earthquake to the northeast the subduction angle is much steeper and the thermal models indicate a narrower downdip seismogenic extent the seismogenic locked zone from earthquake and geodetic data is also narrower thus our analysis of the southwest japan margin indicates that all three constraints on the downdip extent of the seismogenic zone thermal coseismic and interseismic geodetic data are in general agreement the study also supports the hypothesis that the seismogenic portion of subduction thrust faults is limited primarily by temperature the thermal control implies that subduction thrust faults with shallow dip have wider seismogenic zones compared to those with steep dip the subducting plate age and thus heat flow and the thickness of the insulating sediments on the incoming plate are also very important to the thermal regime and thus to the seismogenic width the relation of the maximum seismic rupture area to the interseismic locked zone is particularly important for earthquake hazard estimation on subduction margins such as cascadia where there have been no great historical events

double difference tomography the method and its application to the hayward fault california we have developed a double difference dd seismic tomography method that makes use of both absolute and relative arrival times by reducing systematic errors using the more accurate relative arrival times the method produces an improved velocity model simultaneously it yields event locations of a quality equivalent to those of the dd location method while avoiding simplifying assumptions of that method we test this method on a synthetic dataset and find that it produces a more accurate velocity model and event locations than standard tomography we also test this method on a hayward fault california earthquake dataset spanning 1984 1998 the earthquakes relocated by this method collapse to a thin line along the fault trace consistent with previous results the dd velocity model has sharper velocity contrasts near the source region than the standard tomography model

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

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

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

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

p wave velocity and anisotropy of lawsonite and epidote blueschists constraints on water transportation along subducting oceanic crust p wave velocity vp and the anisotropy of lawsonite and epidote blueschists were measured up to 1 0 gpa and 400 c using the ultrasonic pulse transmission technique the slowest vp in the direction normal to foliation is similar between lawsonite and epidote blueschists 7 0 7 2 km s at 1 0 gpa and room temperature while the fastest vp in the direction parallel to lineation markedly differs between lawsonite blueschists 7 4 7 6 km s at 1 0 gpa and room temperature and epidote blueschist 7 9 km s at 1 0 gpa and room temperature crystallographic orientation measurements for main constituent min erals revealed that both epidote 0 1 0 axes fastest vp direction in epidote single crystal and amphibole 0 0 1 axes fastest vp direction in amphibole single crystal are preferentially oriented parallel to lin eation to enhance vp anisotropy of the epidote blueschist in contrast lawsonite 0 0 1 axes fastest vp direction in lawsonite single crystal are oriented subnormal to foliation whereas amphibole 0 0 1 axes are oriented subparallel to lineation so that relatively weak vp anisotropy was observed in the lawsonite blueschist our experimental results in conjunction with recent seismological observations suggest that the vp of the subducting oceanic crust at 50 km has been observed in several subducting slabs however the high vp values at 50 km depth are difficult to be explained by blueschists this indicates that the blueschist would be at least partially transformed to hydrous mineral bearing eclogite at 50 km depth in subducting oceanic crusts

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 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

low frequency tremors intraslab and interplate earthquakes in southwest japan from a viewpoint of slab dehydration 1 low frequency tremors lft recently found in the southwest japan fore arc likely occur due to hydro fracturing from fluids leaving the dehydrating philippine sea slab however there are some places without such lft they are n izu kanto e shikoku and s kyushu these are the places where island arc type crust is subducting we propose that dehydration of the subducted crust does not occur beneath these regions because it is composed mainly of tonalite lacking the quantity of hydrous minerals seen in normal subducting oceanic crust we show that almost no earthquake occurs within the subducted crust in such regions consistent with dehydration embrittlement hypothesis for intraslab seismicity the lack of dehydration from the crust would also affect the mode of occurrence of interplate earthquakes because pore fluid pressure is more difficult to rise at the thrust zone this might be reflected in the irregular occurrence of great earthquakes in these regions

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

subduction of the philippine sea plate beneath southwestern japan slab geometry and its relationship to arc magmatism 1 we carry out high resolution three dimensional seismic tomography of the crust and upper mantle beneath southwestern sw japan using arrival time data obtained from the nationwide seismograph network the tomographic images provide new insights into the configuration of the philippine sea slab and arc magmatism the results confirm the existence of an aseismic portion of the philippine sea slab at greater depths beyond the seismic portion the philippine sea slab is subducting aseismically down to at least a depth of 200 km in chubu 60 80 km in kinki and 60 km in chugoku and is subducting seismically to depths of 150 200 km in kyushu in the chubu district it is subducting seismically at a shallow dip and then bends downward beneath the volcanic area whereas in the kinki district it is subducting subhorizontally as far as the japan sea the differences in the slab geometry between adjacent regions correlate with the differences in volcanic activity suggesting the importance of the geometry of the philippine sea slab on arc magmatism our tomographic images further imply subduction of the philippine sea slab down to a depth of around 70 km northwest of the izu peninsula the philippine sea slab in that region might have split into western and eastern parts separated by a slab tear and collided with the pacific slab at depths of 150 200 km

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

the slip distribution of the 1946 nankai earthquake estimated from tsunami inversion using a new plate model abstract a new model for the shape of the philippine sea plate psp subducting beneath southwest japan was constructed by combining marine seismic survey results and seismicity data this resulted in a plate model that is generally 3 5 km shallower than previous estimates and contains new details of plate boundary structure near the nankai trough axis we estimated the slip distribution of the 1946 nankai earthquake by inverting tsunami waveforms using the new plate boundary model and found that the large slip off cape muroto and seismic moment were reduced by about 10 from the previous estimate of tanioka and satake 2001a however we also found reason to suspect the tide gauge closest to the epicenter at shimotsu and this resulted in a much more substantial reduction in the amount of slip in the eastern segment of the rupture area we also find that our new rupture model correlates well with features of the subducting plate boundary such as a large subducting seamount off cape muroto kodaira et al 2000a this suggests that such features have an important effect on either earthquake rupture propagation or the interseismic accumulation of elastic strain energy or both

revel a model for recent plate velocities from space geodesy 1 we present a new global model for recent plate velocities revel describing the relative velocities of 19 plates and continental blocks the model is derived from publicly available space geodetic primarily gps data for the period 1993 2000 we include an independent and rigorous estimate for gps velocity uncertainties to assess plate rigidity and propagate these uncertainties to the velocity estimates the velocity fields for north america eurasia and antarctica clearly show the effects of glacial isostatic adjustment and australia appears to depart from rigid plate behavior in a manner consistent with the mapped intraplate stress field two thirds of tested plate pairs agree with the nuvel 1a geologic 3 myr average velocities within uncertainties three plate pairs caribbean north america caribbean south america and north america pacific exhibit significant differences between the geodetic and geologic model that may reflect systematic errors in nuvel 1a due to the use of seafloor magnetic rate data that do not reflect the full plate rate because of tectonic complexities most other differences probably reflect real velocity changes over the last few million years several plate pairs arabia eurasia arabia nubia eurasia india move more slowly than the 3 myr nuvel 1a average perhaps reflecting long term deceleration associated with continental collision several other plate pairs including nazca pacific nazca south america and nubia south america are experiencing slowing that began 25 ma the beginning of the current phase of andean crustal shortening

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

seismic consequences of warm versus cool subduction metamorphism examples from southwest and northeast japan warm and cool subduction zones exhibit differences in seismicity seismic structure and arc magmatism which reflect differences in metamorphic reactions occurring in subducting oceanic crust in southwest japan arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth in northeast japan arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth thermal petrologic models predict that oceanic crust subducting beneath southwest japan is 300 to 500 c warmer than beneath northeast japan resulting in shallower eclogite transformation and slab dehydration reactions and possible slab melting