an elastic plate model for interseismic deformation in subduction zones geodetic observations of interseismic surface deformation in the vicinity of subduction zones are frequently interpreted using simple kinematic elastic dislocation models edm in this theoretical study we develop a kinematic edm that simulates plate subduction over the interseismic period the elastic subducting plate model espm having only 2 more degrees of freedom than the well established back slip model bsm an elastic plate thickness and the fraction of flexural stresses due to bending at the trench that are released continuously unlike the bsm in which steady state deformation in both plates is assumed to be negligible the espm includes deformation in the subducting and overriding plates owing to plate thickness while still preserving the correct sense of convergence velocity between the subducting and overriding plates as well as zero net steady state vertical offset between the two plates when integrated over many seismic cycles the espm links elastic plate flexure processes to interseismic deformation and helps clarify under what conditions the bsm is appropriate for fitting interseismic geodetic data at convergent margins we show that the espm is identical to the bsm in the limiting case of zero plate thickness thereby providing an alternative motivation for the bsm the espm also provides a consistent convention for applying the bsm to any megathrust interface geometry even in the case of nonnegligible plate thickness the deformation field predicted by the espm reduces to that of the bsm if stresses related to plate flexure at the trench are released either continuously and completely at shallow depths during the interseismic period or deep in the subduction zone below 100 km however if at least a portion of these stresses are not continuously released in the shallow portion of the subduction zone via seismic or aseismic events then the predicted surface velocities of these two models can differ significantly at horizontal distances from the trench equivalent to a few times the effective interseismic locking depth

temporal change of the vertical crustal movement on the western coast of suruga bay central japan abstract we investigated the spatial pattern of vertical crustal movement and its temporal change in the eastern tokai region central japan on the basis of leveling measurements by the geographical survey institute of japan in order to analyze the data along the leveling route between kakegawa bm140 1 and hamaoka bm25951 we used the program baysea to see how the trend of subsidence at bm25951 relative to bm140 1 has changed since 1982 we found that the subsidence accelerated in the period of 1987 1989 and has been decelerating since then the average spatial pattern of vertical displacement in the eastern tokai region for the last 18 years is characterized by conspicuous subsidence along the western coast of suruga bay with a gradual decrease of the subsidence rate in the inland direction the spatial pattern however was noticeably different during the period when the subsidence at hamaoka relative to kakegawa was accelerated the subsidence rate at that time was larger in the northern part of the western coast of suruga bay compared to that in the southern part on the other hand it seems that subsidence on the western coast of suruga bay has been decelerating in late 1990s

coseismic and postseismic stress changes in a subducting plate possible stress interactions between large interplate thrust and intraplate normal faulting earthquakes 1 a large intraplate normal faulting earthquake mw 7 5 occurred in 1999 in the subducting cocos plate below the downdip edge of the ruptured thrust fault of the 1978 oaxaca mexico earthquake mw 7 8 this situation is similar to the previous case of the 1997 normal faulting event mw 7 1 that occurred beneath the rupture area of the 1985 michoacan mexico earthquake mw 8 1 we investigate the possibility of any stress interactions between the preceding 1978 thrust and the following 1999 normal faulting earthquakes for this purpose we estimate the temporal change of the stress state in the subducting cocos plate by calculating the slip distribution during the 1978 earthquake through teleseismic waveform inversion the dynamic rupture process and the resultant coseismic stress change together with the postseismic stress variations due to plate convergence and the viscoelastic relaxation process to do this we calculate the coseismic and postseismic changes of all stress components in a three dimensional space incorporating the subducting slab the overlying crust and uppermost mantle and the asthenosphere for the coseismic stress change we solve elastodynamic equations incorporating the kinematic fault slip as an observational constraint under appropriate boundary conditions to estimate postseismic stress accumulations due to plate convergence a virtual backward slip is imposed to lock the main thrust zone the effects of viscoelastic stress relaxations of the coseismic change and the back slip are also included the maximum coseismic increase in the shear stress and the coulomb failure stress below the downdip edge of the 1978 thrust fault is estimated to be in the range between 0 5 and 1 5 mpa and the 1999 normal faulting earthquake was found to take place in this zone of stress increase the postseismic variations during the 21 years after the 1978 event modify the magnitude and patterns of the coseismic stress change to some extent but are not large enough to overcome the coseismic change these results suggest that the coseismic stress increase due to the 1978 thrust earthquake may have enhanced the chance of occurrence of the 1999 normal faulting event in the subducting plate if this is the case one of the possible mechanisms could be static fatigue of rock materials around preexisting weak planes involved in the subducting plate and it is speculated that that one of these planes might have been reactivated and fractured because of stress corrosion cracking under the applied stress there for 21 years

recognition of a locked state in plate subduction from microearthquake seismicity a tectonic state of a locked subduction is considered to be a possible source of a future interplate earthquake discriminating an actually locked state to verify its extent is therefore essential in constructing an accurate prospect against the forthcoming earthquake micorearthquake seismicity is an effective tool for such an analysis because it is considered to be a faithful indicator of the stress state and is expected to exhibit a characteristic pattern in the area where the locked state in the subduction appears with a certain stress concentration focusing on the microearthquake seismicity around the tokai district in central japan where a large interplate earthquake is feared to occur we tried to identify such an area of locked subduction on the philippine sea plate possibly related to the future earthquake we investigated the microearthquake seismicity from various perspectives first the hypocenter distribution was analyzed to identify the extent of the locked area the characteristic profile of the distribution was presumed to represent a stress concentrated area induced from the mechanical contact between both plates the second approach is to interpret stress patterns reflected in focal mechanisms the locked state was recognized and verified by a comparison of the p axis distribution pattern with that expected from a model imaging a partially locked subduction the third approach is to monitor the temporal change of the seismic wave spectrum analyzing predominant frequencies of p and s waves and monitoring their changes for a period of 10 years we found a trend of gradual increase common to both waves this means an increase of stress drop in microfracturings and in its turn implies accumulation of stress around the focus area the rate of the stress change converted from the frequency change was compared with the result derived from a numerical simulation the simulation performed on the basis of a constitutive friction law for a stick sliding on the plate interface computed a changing rate of the maximum shear stress around the locked zone and showed its spatial variation along the subduction axis thus the simulated result indicated a certain compatibility with the observed one although ambiguities and uncertainties still exist in the study all the results derived here seem to indicate an identical conclusion that the plate subduction is actually locked in this region at present

estimation of slip distribution using an inverse method based on spectral decomposition of green s function utilizing global positioning system gps data 1 an inverse method based on the spectral decomposition of the green s function was employed for estimating a slip distribution we conducted numerical simulations along the philippine sea plate ph boundary in southwest japan using this method to examine how to determine the essential parameters which are the number of deformation function modes and their coefficients japanese gps earth observation network geonet global positioning system gps data were used for three years covering 1997 1999 to estimate interseismic back slip distribution in this region the estimated maximum back slip rate is about 7 cm yr which is consistent with the philippine sea plate convergence rate areas of strong coupling are confined between depths of 10 and 30 km and three areas of strong coupling were delineated these results are consistent with other studies that have estimated locations of coupling distribution

a model for possible crustal deformation prior to a coming large interplate earthquake in the tokai district central japan we perform numerical simulations of a seismic cycle by applying a laboratory derived friction law to evaluate possible crustal deformations prior to the hypothesized tokai earthquake which is expected to occur in the near future along the suruga trough central japan the frictional force obeying a rate and state dependent friction law is assumed to act on the plate interface in a 2d model of the subduction zone we test two versions of the friction law the so called slowness version and the slip version we select model parameters on the basis of historical documents about past large earthquakes in the tokai district and the present coupling state of the plate interface beneath the tokai district estimated by gps observations simulation results for both the slowness and the slip version are as follows 1 the coupling region of the plate interface becomes narrower with time in the interseismic period 2 the inland crust contracts at a nearly constant rate for most of the interseismic period the contraction rate decreases or a change from contraction to extension takes place before the occurrence of a large interplate earthquake 3 significant abnormal crustal deformation precedes the earthquake and maximum amplitudes about one day before the earthquake exceed the noise levels in the case of clear weather of borehole strainmeters installed in the tokai district 4 the spatiotemporal variation of aseismic sliding on the plate interface perturbs the regional stress field around the source volume of the coming earthquake leading possibly to precursory seismic quiescence in the overriding plate and a change in focal mechanisms of small earthquakes in the subducting slab the most significant difference in the results between the slowness version and the slip version of the friction law is that amplitudes of preslip and crustal deformation just before earthquakes are several times larger for the slowness version than for the slip version although some ambiguity remains in the present model mainly because of the incompleteness of the friction law and the deficiency of observational data constraining model parameters the physical model can be improved through the comparison of predicted crustal deformation with observations continuous and concentrated observations of crustal deformation and seismic activity are thus important in the tokai district to understand quantitatively the sliding process preceding a large interplate earthquake there

interplate coupling in northeast japan deduced from inversion analysis of gps data abstract the spatial distribution of the strength of interplate coupling between the subducting pacific plate and the overlying continental plate in northeast japan was investigated through an inversion analysis of gps data employing akaike s bayesian information criterion the gps data used for the inversion analysis are rates of baseline length changes and vertical displacement rates of crustal movements during the period from april 6 1996 to march 20 1998 which were obtained at 161 continuous gps observation stations operated by the geographical survey institute of japan the result shows the average drag rates of the overriding plate on the model source regions off the tohoku and hokkaido districts are 6 7 cm yr and 6 8 cm yr respectively which are 0 85 and 0 87 as interplate coupling ratio these values are much higher than those in previous studies the average directions of drag rates are oriented n73 w 15 and n78 w 13 on the model source regions off the tohoku and hokkaido districts respectively these directions agree well with the direction of coseismic slip of the foot wall side n75 w of the 1978 miyagiken oki earthquake m 7 4 but differ slightly from the direction estimated from the plate motion model n65 w seno et al j geophys res 101 1996 11305 11315 we also found afterslip of 5 5 cm yr associated with the 1994 sanriku haruka oki earthquake m 7 5 off aomori prefecture interestingly enough large afterslip occurred on the plate boundary even when more than a year had passed after the sanriku haruka oki earthquake the amount of drag rate becomes dramatically small at depths greater than about 50 km on the model source regions indicating weak interplate coupling there

the block structure and quaternary strike slip block rotation of central japan central japan is situated on the inflection point of the bow shaped japanese islands numerous nw se trending active faults arranged in parallel at intervals of 20 to 80 km are found in this area these active faults are more than 30 km long with shattered zones from 30 to 300 m wide several active faults constitute a given block boundary which serves as the dividing line for one of the four blocks that make up central japan the block boundaries require careful study since numerous historical earth quakes have occurred along these lines offset measurements of basement rocks created during the quaternary period due to left lateral faulting amount to 1 to 7 km gravity lineaments which link points of sudden change and saddles of bouguer anomalies are clearly found along the block boundaries the nw se trending active faults appearing on the ground surface are associated with motions of the block boundaries block rotational movement caused by left lateral faulting plays an important role in the crustal deformation of central japan rotational angles of the blocks calculated from the amount of displacement of basement rocks initiated during the quaternary period are estimated to be 3 to 7 in a clockwise manner

earthquake source kinematics of moderate earthquakes from the inversion of apparent source time functions summary we present an approach to infer the slip and rupture velocity distributions on the fault plane fromthenon linearinversionoftheapparentsourcetimefunctions obtainedfromtheempirical green s function deconvolution method the main advantage of this technique is that it allows overcoming in the forward modelling the limitations related to the computation of the green s function as the choice of a correct and reliable earth propagation model we perform a parameter resolution and uncertainty study which is based on the analysis of the misfit function in the neighbourhood of the best fitting model in this paper we present the results obtained by applying the technique to synthetic and real records from an mw 4 event which occurred during the 2009 l aquila central italy aftershock sequence results show a heterogeneous slip distribution characterized by two main high slip patches located nw of the hypocentre and an average slip of 3 7 cm corresponding to a seismic model of about 0 82 10 15 nm

intraplate deformation in the japanese islands a kinematic study of intraplate deformation at a convergent plate margin relative motions within the deforming japanese islands with respect to the sea of japan are determined using earthquake records over the last 414 years slip rates on quaternary faults and angular change rates obtained from triangulation in the last century we use a least squares inversion method to match the strain rates on the surface of the earth with continuous spline functions to recover the velocity gradient tensors the directions of the principal strain axes obtained from seismic geological and geodetic data are in general agreement with each other with the maximum shortening axis oriented in a wnw direction principal strain rate magnitudes obtained from the geodetic data are about 3 5 times larger than the principal strain rates obtained from the seismic and geological data intraplate deformation in southwestern japan determined from the seismic data accommodates a velocity of5 5 2 l mm yr in a direction parallel to the nankai trough which is about 25 of the plate motion velocity component parallel to the nankai trough between the philippine sea and eurasian plates in northern honshu the velocity vectors from the seismic data are nearly parallel to the plate motion vector at the japan trench and the intraplate deformation accommodates about 6 of the total pacific eurasian or north american plate motion velocity the velocity vector at kashima northern honshu calculated from the geodetic data is about 9 12 mm yr in a direction of n55 w n76 w which is about half of the very long baseline interferometry velocity calculated in the eurasian reference frame suggesting that the sea of japan may not be rigidly attached to the assumed rigid eurasian plate

crustal block kinematics and seismic potential of the northernmost philippine sea plate and izu microplate central japan inferred from gps and leveling data 1 we clarify the contemporary deformation observed by gps and leveling for the greater tokyo kanto eastern tokai and the izu islands region where the izu ogasawara bonin arc is subducting and colliding with the central part of the japan arc from these data we develop a kinematic model of fault sources with variable components of seismic and aseismic slip under the assumption that the contemporary deformation during 1995 2000 is representative of the long term interseismic strain field the geodetic data are inverted to estimate the rotation poles of the crustal blocks the degree of elastic strain accumulation on faults and the volumetric inflation sources beneath volcanoes the present crustal movements are explained by four crustal blocks the izu microplate the central japan block the pacific plate and the philippine sea plate along the suruga and sagami margins of the philippine sea plate lie strongly coupled faults which include sites of the 1854 m 8 4 tokai 1923 m 7 9 kanto and 1703 m 8 2 genroku kanto earthquakes in contrast the philippine sea plate boundary immediately north of the izu peninsula site of m 7 5 collisional earthquakes is only weakly coupled the boundary between the izu microplate and the philippine sea plate experiences left lateral motion with the rate of 30 mm yr most of this boundary is locked and thus has a large potential of future earthquakes the izu microplate is found to rotate rapidly clockwise at 10 myr with a rotation pole relative to the central japan block located just north of its northern boundary the pole and angular velocity of the izu microplate depends however on the assumed location of the boundary with the exception of the site of the 1938 m 7 earthquake swarm which we infer has the current potential to produce a m 8 1 earthquake the pacific megathrust has a very low seismic potential although the geodetic data have little resolving power for much of the pacific plate interface east of tokyo

lateral stretching of the philippine sea plate subducting along the nankai suruga trough focal mechanisms are investigated for shallow earthquakes h 60 km of central japan by using telemetered seismic data of microearthquakes shallow earthquakes in central and southwestern japan can be distinguished into three groups upper crustal earthquakes subcrustal earthquakes and interplate thrust earthquakes the first and the second ones being subjects of the present study most of the upper crustal earthquakes are of strike slip type or reverse faulting with e w to se nw compression whereas the subcrustal earthquakes are characterized by strike slip or normal faulting with e w to ne sw extension the subcrustal seismic zone subparallel to the nankai trough becomes progressively deeper landward to the north and progressively shallower to the east so that it emerges into a very shallow seismic activity in the izu peninsula which has been interpreted as a microcontinent of the philippine sea plate colliding against the eurasian plate on the basis of hypocentral distribution three dimensional seismic velocity structure and focal mechanisms subcrustal earthquakes are considered to occur within the subducting philippine sea plate that extends down to a depth of at least 60 km on the basis of this and other focal mechanism studies stress trajectories are drawn for the subducting philippine sea plate and for the overriding eurasian plate the overriding plate is on the whole under the e w compression which coincides with the moving direction of the pacific plate relative to the eurasian plate the subducting plate on the other hand is under lateral extension along its strike we interpret that the latter is a stress regime unique to an incipient subduction zone where the subducting plate at subcrustal depths is forced to warp downward being constrained laterally at the arc arc junctions and thereby comes under lateral stretching the lateral constraints arise either from apparent increase in flexural strength of the plate at the arc arc junction or from buoyancy of a microcontinent that resists subduction

interseismic locking of the plate interface in the northern cascadia subduction zone inferred from inversion of gps data abstract we inverted gps velocities from 20 continuous and 53 campaign sites in the northern cascadia subduction zone using a bayesian inverse method to estimate the locking state of the plate interface the results are consistent with previous estimates based on thermal arguments and forward modeling they suggest that the completely locked segment of the plate interface is offshore and that the degree of locking gradually decreases landward the very gradual transition from full locking to full slip approximates the effect of stress relaxation that is not included by the elastic model assumed for the inversion

the plate coupling in the tokai district the central japan inferred from the different data using triangular dislocation elements abstract we estimate interseismic coupling on the subducting plate interface in the tokai area central japan by inverting two geodetic data sets the data record surface motion between march 1996 to may 2000 one represents vertical motion deduced from the leveling observations and the other is the horizontal velocity field deduced from gps observations in the inversion we employed the analytical solutions of surface displacement due to a triangular dislocation element embedded in a homogeneous elastic half space in order to represent the curved plate interface the vertical data show that the most strongly coupled portion of the subduction interface is concentrated beneath omaezaki cape while the horizontal data show strongest coupling in the shallower region of the subducting plate interface the estimated maximum value of coupling from the horizontal data is 40 mm year while that from vertical data is 25 mm year

a dislocation model for the earthquake cycle at convergent plate boundaries summary a kinematic model for the earthquake cycle at convergent plate boundaries has been constructed on the basis of dislocation theory we model the lithosphere asthenosphere system by a stratified semi infinite medium under gravity consisting of an elastic surface layer an intervening layer with maxwell viscoelasticity and an elastic substratum the steady motion of plate convergence is naturally represented by uniform slip at a constant rate on the upper boundary of the descending oceanic plate the occurrence of interplate earthquakes is regarded as a perturbation of the steady state plate motion and represented by a periodic sequence of step slips on a finite seismic zone of the plate interface based on dislocation theory we can show that the steady slip on the interface deeper than the lithosphereasthenosphere boundary does not contribute to surface deformation in the steady state accordingly the surface deformation associated with the earthquake cycle is given by the superposition of viscoelastic responses to the steady slip on the interface shallower than the lithosphere asthenosphere boundary the steady backslip on the seismic zone and the periodic sequence of seismic slips we have computed cyclic patterns of vertical displacements at the free surface for two representative cases taking account of gravity effects the patterns of vertical displacements differ notably in the latter stage of the cycle depending on whether or not the seismic zone extends through the entire thickness of the lithosphere after the completion of one earthquake cycle our model yields a certain amount of permanent deformation resulting from the steady plate convergence the pattern of the permanent deformation that is characterized by steep uplift on the continental side sharp subsidence at the plate boundary and gentle uplift on the oceanic side is irrespective of the cyclic process of stress accumulation and release repeated on the seismic zone the earthquake cycle model developed here provides a possible explanation for the formation of earthquake origin marine terraces

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

interplate coupling in the kanto district central japan deduced from geodetic data inversion and its tectonic implications abstract interplate coupling between the philippine sea phs the north american nam and the pacific pac plates in the kanto district central japan has been investigated through the inversion analysis of geodetic data using akaike s bayesian information criterion abic the data used for the analysis are annual rates of level changes 1972 1985 and horizontal length changes 1973 1990 which presumably represent average crustal movements during the interseismic period disregarding effects of steady plate subduction we may ascribe the observed crustal movements to the effects of locking or faulting on some parts of the plate boundaries in our model the effect of locking is represented by back slip on the plate boundary and that of faulting by forward slip the results of the inversion analysis show the existence of a strongly coupled area on the southwestern shallower part of the nam phs plate boundary which almost coincides with the faulting area of the 1923 kanto earthquake m 7 9 and a notable faulting area on the northeastern deeper part of the phs pac plate boundary in the southwestern strongly coupled area the rate of back slip reaches 3 5 cm yr and its direction is oriented n33 w which is almost opposite to the direction of fault slip s29 e at the time of the 1923 event this suggests that tectonic stress accumulation for the next large event is effectively proceeding there and its recurrence time is roughly estimated as 245 years the strongly coupled region is also recognized as corresponding to a recent seismic gap the inverted forward slip distribution on the phs pac plate boundary which has the average rate of 1 9 cm yr and the average direction of n56 e may be explained by aseismic northeastward slip in relation to the process to form a peculiar configuration of downward bend of the phs plate beneath the kanto district

immediate foreshocks time variation of the b value abstract we compare b values in the frequency magnitude law for the foreshocks and long term seismicity foreshocks are considered in two non overlapping time intervals preceding a main shock i last hours ii last days statistical analysis of the regional and global earthquake catalogs shows that in either of these intervals b value drops by a half compared to long term seismicity this extends the previous results by molchan and dmitrieva molchan g dmitrieva o 1990 dynamics of the magnitude frequency relation for foreshocks phys earth planet inter 61 99 112 who have found such a drop a few hours before a mainshock our results concern a statistical regularity not necessarily seen for each individual sequence of foreshocks

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

the seismically quiescent boundary between the philippine sea plate and the eurasian plate in central japan abstract the boundary between the philippine sea phs and eurasian eur plates which is presently seismically quiescent is delineated on the basis of seismological data from the kanto tokai k t district during the period from january 1980 to april 1993 most of the earthquakes in the k t district occur either in the overriding plate the eur plate or within the subducting slab the phs plate at depths of less than around 30 km the plate boundary between the phs and eur plates is characterized by a very low level of seismic activity i e the plate boundary itself is presently seismically quiescent and the seismicity consists of intra plate events above or below the plate boundaries in the tokai district earthquakes in the overriding eur plate are mostly characterized by strike slip faulting with an e w directed compressional axis while those within the subducting phs slab are characterized by strike slip faulting with a n s directed compressional axis in the kanto district the earthquakes are mostly characterized by faults with nw se directed compressional axes the above difference between the tokai and kanto districts is thought to be due to the difference in the slab configuration and or regional stress conditions considering the results of the present paper together with geodetic data the seismically quiescent plate boundary at depths from 10 to 30 km suggests strong coupling between the phs and eur plates and is consistent with strain accumulation during an inter seismic period

response of the water level in a well to earth tides and atmospheric loading under unconfined conditions the response of the water level in a well to earth tides and atmospheric loading under unconfined conditions can be explained if the water level is controlled by the aquifer response averaged over the saturated depth of the well because vertical averaging tends to diminish the influence of the water table the response is qualitatively similar to the response of a well under partially confined conditions when the influence of well bore storage can be ignored the response to earth tides is strongly governed by a dimensionless aquifer frequency q u the response to atmospheric loading is strongly governed by two dimensionless vertical fluid flow parameters a dimensionless unsaturated zone frequency r and a dimensionless aquifer frequency qu the differences between q u and qu are generally small for aquifers which are highly sensitive to earth tides when q u and qu are large the response of the well to earth tides and atmospheric loading approaches the static response of the aquifer under confined conditions at small values of q u and qu well response to earth tides and atmospheric loading is strongly influenced by water table drainage when r is large relative to qu the response to atmospheric loading is strongly influenced by attenuation and phase shift of the pneumatic pressure signal in the unsaturated zone the presence of partial penetration retards phase advance in well response to earth tides and atmospheric loading when the theoretical response of a phreatic well to earth tides and atmospheric loading is fit to the well response inferred from cross spectral estimation it is possible to obtain estimates of the pneumatic diffusivity of the unsaturated zone and the vertical hydraulic conductivity of the aquifer

the interplate coupling and stress accumulation process of large earthquakes along the nankai trough southwest japan derived from geodetic and seismic data abstract the interplate coupling and stress accumulation process associated with the subduction of the philippine sea plate beneath the eurasian plate along the nankai trough where large earthquakes have occurred periodically have been investigated in detail taking into account the three dimensional complicated structure of the philippine sea plate and its motion relative to the eurasian plate for this purpose we mainly used extensive geodetic data in southwest japan such as levelling tide gauge and trilateration which presumably are effective means of revealing coupling properties and applied a three dimensional finite element method here two seismic periods associated with the 1944 tonankai and 1946 nankaido earthquakes were considered interseismic and pre seismic stages comparing the coupled regions obtained for the two cases a possible explanation of its temporal change was proposed it might gradually spread from a shallower to deeper portion with transition from the interseismic to pre seismic stage a comparison of the coupling properties between southwest japan and northeast japan reveals the following properties in southwest japan the coupling is generally strong and its region is limited in a relatively shallow portion the pre seismic coupled regions coincide with the co seismic and post seismic fault zone suggesting that the accumulated stress is released at the time of a large earthquake in northeast japan on the other hand the coupling is weak and its extent appears to reach a depth of about 100 km and the co seismic and post seismic regions are limited to the upper half of the coupled region the weakly coupled region in southwest japan seems to correspond well to the region with high seismic activity a high dip angle subducting slab and the possible existence of seamounts in contrast to the strongly coupled region correlated with low seismicity low dip angle subduction and no seamounts on the western coupled region off shikoku an anomalously strongly coupled region corresponding to a convergence rate of 7 8 cm year 1 was identified this may be caused by geomorphological properties such as the low dip angle of the subducting plate and smooth plate boundary resulting in a large amount of stress concentration there the stress field associated with the interplate coupling was also calculated there is fair agreement between the observations and calculations near the plate boundary beneath the southern kii peninsula and shikoku although the effects of the interplate coupling may have some influence on strike slip faulting with the n s compressive axes beneath shikoku other earthquake generating stress such as a two layered double buckling would be dominant there

coseismic slip resolution along a plate boundary megathrust the nankai trough southwest japan geodetic survey measurements are used to estimate the coseismic slip distribution in the 1944 tonankai mw 8 1 and 1946 nankaido mw 8 3 earthquakes and to assess quantitatively the degree to which this slip is resolved on the plate boundary megathrust data used include 798 angle changes from triangulation surveys 328 leveling section differences and 5 coseismic tidal gage offsets many of the nominally coseismic triangulation data span 50 years nearly half the earthquake cycle and correction for interseismic deformation using post 1950 observations is applied microseismicity is used to define the configuration of the plate boundary interface and approximate it with a continuous multisegment fault model because the onshore geodetic data have very limited resolving power for offshore fault segments offshore coseismic slip was constrained by satake s 1993 estimation based on tsunami data the majority of the coseismic slip occurs between 15 and 25 km depth although resolution declines toward the trench axis it is sufficiently good to define two distinct high slip regions one off southeastern shikoku island 11 m maximum and the other offshore of kii peninsula 3 m maximum the slip magnitude off southeastern shikoku coupled with the plate convergence rate would imply an recurrence interval of about 270 years much longer than the average repeat time of 120 years for historical great earthquakes on the nankai trough however the maximum coseismic slip is sensitive to the assumed fault geometry and slippage on trough parallel splay faults could significantly decrease the maximum slip to about 6 m

interplate coupling and a recent aseismic slow slip event in the guerrero seismic gap of the mexican subduction zone as deduced from gps data inversion using a bayesian information criterion a large scale slow slip event with a magnitude equivalent to mw 7 4 has been observed at seven continuous gps stations located in the guerrero oaxaca region southern mexico for several months from late 2001 to middle 2002 geophys res lett 2003 30 doi 10 1029 2003gl017219 we re analyzed the gps time series data to estimate the displacements associated with the slow slip event and also the displacement rates during the interseismic period prior to this event by applying an inversion technique using a bayesian information criterion abic we then carried out gps data inversion again with abic and some detailed forward modeling to obtain spatial distributions of the back slip rates and the slow slip on a 3 d curved plate interface between the subducting cocos and the overriding north america plates the results show that the average direction of the back slip rates is n31 3 e 5 6 which is congruent with that of plate convergence in this region interplate coupling is found to be very strong down to a depth of about 45 km indicating a coupling ratio of 0 83 0 86 except for a couple of segments on the model region and decreases dramatically down below the average direction of the slow slip is oriented about 20 35 counterclockwise from the opposite direction of the plate convergence the slow slip detected on the mid depth segment to the upper rim of the model region reaches about 9 18 cm in the landward region of the guerrero seismic gap from inversion and forward modeling from these results we conclude it is possible that the slow slip may have invaded a deeper part of the strongly coupled seismogenic zone at least up to a depth of about 25 km we also estimate stress changes due to the slow slip event some decrease in shear stress on the plate interface at these depths suggests that if such a slow slip event occurred episodically it is possible that the time of occurrence of a forthcoming large earthquake in the guerrero gap would be delayed to some extent 2004 elsevier b v all rights reserved

is northern honshu a microplate abstract seismic slip vectors along the japan trench the eastern margin of the japan sea and the sagami trough are compared with global relative plate motions rm2 minster and jordan 1978 to test a new hypothesis that northern honshu japan is part of the north american plate this hypothesis also claims that the eastern margin of the japan sea is a nascent convergent plate boundary kobayashi 1983 nakamura 1983 seismic slip vectors along the japan trench are more parallel to the direction of the pacific north american relative motion than that of the pacific eurasian relative motion however the difference in calculated relative motions is too small avoid to the possibility that a systematic bias in seismic slip vectors due to anomalous velocity structure beneath island arcs causes this apparent coincidence seismic slip vectors and rates of shortening along the eastern margin of the japan sea for the past 400 years are also consistent with the relative motion between the north american and eurasian plates calculated there seismic slip vectors and horizontal crustal strain patterns revealed by geodetic surveys in south kanto beneath which the philippine sea plate is subducting indicate two major directions one is the relative motion between the north american and philippine sea plates and the other that between the eurasian and philippine sea plates one possible interpretation of this is that the eastern margin of the japan sea may be in an embryonic stage of plate convergence and the jump of the north american eurasian plate boundary from sakhalin central hokkaido to the eastern margin of the japan sea has not yet been accomplished in this case northern honshu is a microplate which does not have a driving force itself and its motion is affected by the surrounding major plates behaving as part of either the eurasian or north american plate another possibility is that the seismic slip vectors and crustal deformations in south kanto do not correctly represent the relative motion between plates but represent the stresses due to non rigid behaviors of part of northern honshu

interplate coupling in the tokai district central japan deduced from continuous gps data i invert gps displacement rates in the tokai district to investigate strain accumulation associated with the anticipated tokai earthquake i apply a representation of interseismic strain accumulation at subduction zones by savage 1983 and estimate a distribution of virtual normal fault slip back slip on the plate boundary surface estimated back slip takes its maximum value of 30 40mm year under the sea of enshu this value is nearly equal to the relative plate motion there implying almost complete coupling back slip is smaller under suruga bay and its direction is different from the relative motion between the eurasian and the philippine sea plates localized deformation at the northern tip of the philippine sea plate may be responsible for this discrepancy partitioning tectonic strain due to the plate convergence into faults on both sides of the izu peninsula and also distributed strain inside the peninsula

shear partitioning in the eastern nankai trough evidence from submersible dives abstract submersible dives over the wedge of the easternmost nankai trough demonstrate the existence of a left lateral strike slip system parallel to the subduction zone this system develops as the trench turning northward toward land becomes highly oblique to the motion although shear partitioning is well known in other subduction zones the process here has the peculiarity of continuing northward into land

a non linear geodetic data inversion using abic for slip distribution on a fault with an unknown dip angle summary we have developed a method of geodetic data inversion for slip distribution on a fault with an unknown dip angle a common strategy for obtaining slip distribution in previous studies is to first determine the fault geometry by minimizing the square misfit under the assumption of a uniform slip on a rectangular fault and then apply the usual linear inversion technique to estimate a slip distribution on the determined fault it is not guaranteed however that the fault determined under the assumption of a uniform slip gives the best fault geometry for a spatially variable slip distribution the inverse problem is non linear for cases with unknown fault geometries but the non linearity of the problems is actually weak when we can assume the fault surface to be a flat plane in particular when a clear trace of coseismic faults is observed on the earth s surface only the dip angle is an unknown parameter to determine the fault geometry then we regarded the dip angle as an hyperparameter that prescribed the structure of parametric models and obtained the best estimate of the dip angle using akaike s bayesian information criterion abic with the best estimate of the dip angle we can obtain the slip distribution on the fault based on the maximum likelihood principle we applied the method to the insar data of the 1995 dinar turkey earthquake and obtained a much lower dip angle than the previous analyses

crustal deformation and stress localization in kanto tokai central japan revealed by gps summary horizontal deformation associated with plate interaction in the kanto tokai district central japan has been determined using the japanese permanent global positioning system gps array with an accuracy of better than 2 5 mm yr 1 previous analysis of the gps observations mainly focused on the pattern of deformation or strain and did not attempt to assess stress change associated with plate subduction in this study using the gps observations and the geometry of subducting slabs as constraints we constructed a 3 d kinematic model to simulate the subduction of the philippine sea plate at the suruga and sagami troughs and the pacific plate at the japan trench the model incorporating the effects of major plate driving forces ridge push slab pull and drag force associated with plate subduction provides an overall fit to the horizontal deformation observed by gps during 1996 2001 in the kanto tokai region the horizontal motion of the philippine sea plate is estimated to be 41 5m m yr 1 60 5 nw consistent with that ca 43 50 mm yr 1 inferred from earthquake slip vectors and geological data the horizontal motion of the pacific plate is estimated to be 78 6m m yr 1 84 5 nw close to that ca 73 79 mm yr 1 deduced from earthquake slip vectors after extracting the regional deformation associated with subduction of the philippine sea and pacific plates from the gps observations two types of motion trends are revealed for the northern part of the kanto tokai region the southeastward motion of the gps stations revealed by the residual displacements in northern tokai indicates a possible resistance force from the north in northern kanto the west southwestward residual motion of the gps stations indicates the motion of the north american plate relative to the eurasian plate the distinct boundary between these two different types of motion trends provides strong support for the presence of the north american plate in the region which demonstrates that the model has delineated the main characteristics of plate interaction further a stress analysis indicates that stress is accumulating on the suruga trough at a rate of approximately 0 7 bar yr 1 at a depth of 15 km our results indicate that the pattern of stress localization is mainly controlled by the kinematic characteristics of the subducted slabs geometry and slip distribution and the persistent stress concentration is responsible for repeated large interplate earthquakes in the region

geometry and relative motion of the philippine sea plate and pacific plate beneath the kanto tokai district japan the recent acquisition of high density and improved seismic data by the kanto tokai k t observational network of the national research institute of earth science and disaster prevention requires the revision of conventional plate configuration models for the k t district we propose a new interpretation of the configuration and relative motion of the philippine sea phs plate pacific pac plate and eurasian eur plate on the basis of the distribution of hypocenters velocity structure and focal mechanisms in the k t district the new model for the configuration of the phs and pac slabs clearly delineates the phs slab subducting northwestward beneath the k t district from the sagami and suruga troughs and the pac slab subducting westward beneath the phs slab from the japan trench the thickness of the descending phs slab is estimated to be 30 5 km the proposed configuration of the phs slab is significantly different from those obtained by other researchers including our own previous study the new model is fully consistent with global plate motion data the new model leads to an interpretation of the tectonic processes taking place beneath the k t district for the sagami trough wing of the phs slab the northeastern portion of the slab is dipping northwestward in direct contact with the upper surface of the pac slab the southwestern portion carrying the izu peninsula has been bent upward because of its buoyancy producing the observed contorted shape for the suruga trough wing the dip direction of the phs slab gradually changes from northwestward at the eastern portion to northward at the western portion the new model also indicates the presence of offset within the phs slab between the sagami and suruga trough wings this area is characterized by a low level of seismic activity the absence of a high velocity structure and the presence of quaternary volcanoes it seems reasonable to speculate that the distribution of quaternary volcanic edifice construction is controlled by the location of the offset within the phs slab in view of this new model the tectonic processes of large earthquakes which occurred recently in the studied area are also reexamined

interplate coupling and relative plate motion in the tokai district central japan deduced from geodetic data inversion using abic summary the spatial distribution of the strength of interplate coupling between the subducting philippine sea and overlying continental plates in the tokai district central japan was investigated in detail through the inversion analysis of geodetic data using akaike s bayesian information criterion abic the geodetic data used for the analysis are annual rates of level changes 1972 1984 and horizontal length changes 1977 1988 which presumably represent average crustal movements during the interseismic period the result of the inversion analysis shows the existence of a strongly coupled region extending from 10 to 30 km in depth the total seismic moment accumulated in this area since the last event the 1854 ansei earthquake is roughly estimated to be 5 5 1027 dyne cm which corresponds to ms 7 8 the interplate coupling becomes weaker toward the shallower and deeper portions this is consistent in general tendency with a rheological model inferred from petrological viewpoints the strength of coupling also tends to decrease toward northeast over the west coast of suruga bay the direction of plate convergence inferred from the inversion analysis is oriented n30 w this is significantly different from the direction of relative plate motion between the philippine sea and eurasian plates but concordant with that between the philippine sea and north american plates

interplate coupling in southwest japan deduced from inversion analysis of gps data abstract recently the geographical survey institute of japan completed the installation of a gps continuous observation network in japan which has enabled us to investigate real time crustal movements in this study we attempt to obtain spatial distribution of interplate coupling and relative plate motion between subducting and overriding plates in southwest japan using horizontal and vertical deformation rates which were observed at 247 gps observation stations during the period from april 6 1996 to march 20 1998 for this purpose we carried out an inversion analysis of geodetic data incorporating akaike s bayesian information criterion abic as a result strong interplate coupling was found off shikoku and kumanonada regions which corresponds well with the fault regions of the 1946 nankai m 8 1 and the 1944 tonankai m 8 0 earthquakes respectively we also found that interplate coupling becomes weak at depths deeper than about 30 to 40 km beneath the shikoku and kii peninsula the recurrence time of great trench type earthquakes was roughly estimated as 107 years which is consistent with previous research the direction of relative plate motion is oriented n53 w which is close to the direction predicted from the plate motion model on the other hand a large forward slip was found in the hyuganada region off southeast of kyushu since the coseismic displacements associated with the two 1996 hyuganada earthquakes m 6 6 m 6 6 are removed from the gps data this suggests that after slip occurred near the source region and or that kyushu moves southeastward stationarily due to other tectonic forces

geodetic data inversion using a bayesian information criterion for spatial distribution of fault slip summary we developed a new inversion method to reconstruct static images of seismic sources from geodetic data using akaike s bayesian information criterion abic coseismic surface displacements are generally related with a slip distribution on a fault surface by linear integral equations parametric expansion of the fault slip distribution by a finite number of known basis functions yields a set of observation equations expressed in a simple vector form incorporating prior constraints on the smoothness of slip distribution with the observation equations we construct a bayesian model with unknown hyperparameters the optimal values of the hyperparameters which control the structure of the bayesian model are objectively determined from observed data by using abic once the values of hyperparameters are determined we can use the maximum likelihood method to find the optimal distribution of fault slip we examined the validity of this method through a numerical experiment using theoretical data with random noise we analysed geodetic data associated with the 1946 nankaido earthquake ms 8 2 by using this method the result shows that the fault slip distribution of this earthquake has two main peaks of 4 and 6 m located off kii peninsula and muroto promontory these two high slip areas are clearly separated by a low slip zone extending along kii strait such a slip distribution corresponds with the fact that the rupture process of this earthquake in the western part is notably different from that in the eastern part

geodetic data inversion using abic to estimate slip history during one earthquake cycle with viscoelastic slip response functions summary we developed a new method of geodetic data inversion to estimate slip history at a plate interface by using akaike s bayesian information criterion abic in this method we considered the effects of viscoelastic stress relaxation in the asthenosphere which cannot be neglected to estimate slip history at a plate interface during one earthquake cycle we also introduced a proper formulation to incorporate two sorts of partially dependent prior information into observed data by bayes rule by applying the new inversion method to levelling data for 1893 1983 in shikoku southwestern japan we reconstructed the pattern of space time variation in slip motion during one earthquake cycle including the 1946 nankai earthquake at the interface between the eurasian and the philippine sea plates the result shows that a steady slip motion at a plate convergence rate 40 mm yr 1 proceeds in the shallow and the deep regions through the entire earthquake cycle in the intermediate depth range 10 30 km on the other hand an instantaneous slip of approximately 4 m occurs at the time of the nankai earthquake after that this portion keeps in stationary contact until the occurrence of the next nankai earthquake if we neglect the effects of viscoelastic stress relaxation the inversion analysis gives geophysically unrealistic results