spatial distribution of intermediate depth earthquakes with horizontal or vertical nodal planes beneath northeastern japan abstract the 1993 kushiro oki off kushiro earthquake m jma 7 8 is one of the largest intermediate depth earthquakes that have occurred in northeastern japan this earthquake is a notable event that has a nearly horizontal fault plane extended from the lower seismic plane of the double planed seismic zone toward the upper seismic plane we have examined the spatial distribution of focal mechanism solutions of deep and intermediate depth earthquakes beneath northeastern japan concerning this peculiar event analyses of 127 new solutions along with existing solutions confirm the previous results on essential features of their spatial distribution the predominant types of solutions are down dip compression in the upper plane except beneath eastern hokkaido and down dip extension in the lower plane p axes in the upper seismic plane and t axes in the lower plane are in the direction of plate convergence beneath hokkaido however t axes show significant deviation from the direction of plate convergence the stress axes beneath the junction between the northeastern japan arc and the kuril arc exhibit slight deviation from those in the adjacent areas suggesting slab distortion beneath the junction we next investigate the spatial distribution of events with horizontal nodal planes as candidates for events with similar focal mechanisms to the kushiro oki earthquake the activity of earthquakes with horizontal nodal planes is definitely high in the lower plane especially beneath hokkaido many of them have similar focal mechanisms to the kushiro oki earthquake indicating that the kushiro oki earthquake is not exceptional but is a typical event in the region however the fault planes of these events are variable in space some events are known to have vertical fault planes there are shallow and deep depth bounds and a southwestern boundary for the events with horizontal fault planes these facts suggest that the horizontal faulting is probably caused by some regional stress concentration owing to the unbending and the distortion of the segmented slab at the junction

oblique convergence and deformation along the kuril and japan trenches the hypothesis that present day deformation within the southern kuril forearc is driven by oblique subduction of the pacific plate is tested using 397 horizontal slip directions derived from shallow thrust earthquakes from the kuril and japan trenches for the period 1963 1991 a simple two plate model fits the 397 slip vectors significantly worse than a model that permits strike slip motion of the southern kuril forearc relative to the overlying plate weighted mean slip directions along the southern kuril trench are systematically rotated toward the direction orthogonal to the trench which implies that the net convergence is partitioned into less oblique subduction and trench parallel displacement of the southern kuril forearc the angular discrepancy between the observed slip direction and the direction predicted by the nuvel 1 pacific north america euler vector implies that the southern kuril forearc translates 6 11 mm yr 1 to the southwest relative to the overlying north american plate these results are consistent with geologically geodetically and seismologically observed convergence at the leading edge of the forearc sliver in southern hokkaido and with previously inferred extension at the trailing edge of the sliver which is located at the bussol strait at 46 n the percentage of the oblique component of the rigid plate convergence vector that is transferred into forearc strike slip motion is only 30 which may be evidence that the hokkaido honshu bend in the trench hinders southwest forearc motion or that the greater strength of oceanic forearcs impedes motion along forearc strike slip faults

inversion of complex body waves we have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes the rupture pattern is represented as a sequence of subevents distributed on the fault plane this method is an extension of our earlier method in which the subevent mechanisms were fixed in the new method the subevent mechanisms are determined from the data and are allowed to vary during the sequence when subevent mechanisms are allowed to vary however the inversion often becomes unstable because of the complex trade offs between the mechanism the timing and the location of the subevents many different subevent sequences can explain the same data equally well and it is important to determine the range of allowable solutions some constraints must be imposed on the solution to stabilize the inversion we have developed a procedure to explore the range of allowable solutions and appropriate constraints in this procedure a network of grid points is constructed on the i plane where and i are respectively the onset time and the distance from the epicenter of a subevent the best fit subevent is determined at all grid points then the correlation is computed between the synthetic waveform for each subevent and the observed waveform the correlation as a function of and i and the best fit mechanisms computed at each i grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution the method is illustrated using the data for the 1976 guatemala earthquake

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

joint focal mechanism determination with source region station corrections using short period body wave amplitude data abstract we have developed a method that simultaneously determines focal mechanism solutions of many small earthquakes and source region station corrections for short period body wave amplitudes by inverting amplitude data of p sh and sv waves together with p wave polarity data the observed seismic waveform includes the effects of site amplification and attenuation along its ray path in addition to the radiation pattern of earthquake source the amplitude of seismograms at frequencies higher than a few hertz is extremely sensitive to heterogeneous structure near the ground surface consequently we need to know in detail the effects of site amplification and attenuation in order to estimate focal mechanisms by using short period waveforms however at present we do not know the detailed crustal structure with a resolution necessary for this estimation in the present study we assume that p and s wave attenuation factors along ray paths from hypocenters to each station can be expressed as a function of hypocentral distance backazimuth and incident angle based on this assumption we determined focal mechanism solutions of many earthquakes and the coefficients in the function for each station simultaneously by using p sh and sv wave amplitudes and p wave polarities we applied the present method to 170 aftershocks of the 1996 onikobe earthquake m 5 9 which occurred in the central part of northeastern japan we obtained focal mechanism solutions of many microearthquakes whose mechanism solutions could not be determined by using p wave polarity data alone p axes of almost all the obtained focal mechanism solutions are horizontal and oriented in the east west direction t axes are on average near vertical at the shallowest depth as the depth approaches 5 km the t axes become horizontal and then gradually become near vertical again

earthquake relocation and focal mechanism determination using waveform cross correlation nicoya peninsula costa rica the nicoya peninsula in costa rica directly overlies the seismogenic zone of the middle america trench making it an ideal location for geophysical investigations of shallow subduction zone earthquake processes as part of the collaborative costa rica seismogenic zone experiment crseize a seismic network consisting of 20 land and 14 ocean bottom seismometers recorded small magnitude local earthquakes along the nicoya peninsula from december 1999 to june 2001 previous studies have used these data to compute local earthquake locations and 3d velocity structure to identify plate boundary seismicity and to investigate seismogenic behavior here we utilize waveform cross correlation and clustering techniques in an attempt to improve earthquake relocations and determine first motion focal mechanisms to validate refine and expand on existing models due to the high quality of the original locations and the small cross correlation p wave arrival time adjustments large differences between the previously determined and the cross correlated earthquake locations are not observed however focal mechanism determinations using cross correlated p waveforms are significantly enhanced approximately 90 of the focal mechanisms computed for events previously identified as interplate earthquakes are consistent with underthrusting focal mechanisms for continental intraplate events indicate dextral strike slip motion in the central region and normal faulting at the southern tip of the peninsula these motions may be associated with oblique convergence and seamount subduction respectively within the subducting plate steep p and t axes of earthquakes below 50 km depth are consistent with unbending of the slab

a study of clast size distribution ash deposition and fragmentation in a hawaiian style volcanic eruption ash deposits from hawaiian style eruptions are generally of small volume but are important in understanding the mechanisms of these eruptions an analysis of the ash deposit from the 1959 kilauea iki eruption of kilauea volcano is presented and the whole deposit grain size distribution for the eruption is calculated the grain size distribution is found to be heavily skewed towards the coarsest clast sizes and to differ considerably from size distributions for plinian deposits

focal mechanisms of intermediate depth earthquakes beneath southeastern hokkaido japan implications of the double seismic zone in this study the focal mechanism solutions and spatial distributions of 100 intermediate depth earthquakes are examined and discussed these earthquakes occurred from 1985 to 1993 mostly in the upper and lower planes of the double seismic zone beneath the southeastern part of hokkaido the focal mechanism solutions along with the spatial distribution of their epicenters as a function of depth and the minimum and maximum compressive stress directions confirm the following essential features a double planed deep seismic zone between the depths from 60 to 120 km has been clearly revealed in this region and the number of earthquakes located in the upper plane are less than those for the lower one low angle thrust fault mechanism is dominant to a depth of about 55 60 km in the zone at depths deeper than about 60 km focal mechanism solutions are mostly characterized by down dip extension type or down dip compression type in both lower and upper planes of the zone most of the earthquakes deeper than 60 km have dip slip components and some of the solutions have significant strike slip components as well although the down dip extensional type events are dominant seven events that are located on the lower plane are constrained as down dip compression mechanism the t axes of earthquake focal mechanisms on the lower plane show extension at about n40 e and n35 w and the dip angle between 25 and 70 almost coincide with the dip of the subducting pacific plate in this region we also investigate the statistical features of the stress directions of the earthquakes occurring both on the lower and upper plane after converting all the events onto the plane having the same strike and dip angle as the subducting lithosphere in the region seventy two percent of the minimum compressive stress directions are oriented mainly in the ns or nne ssw directions within 45 of the dip direction at the plate surface of the lower plane and are almost parallel to the dip of the local slab in this region the number of earthquakes located in the upper plane is less than that for the lower one those which occurred on the upper plane show various types of mechanism solutions weak systematic orientation is found in the direction distribution of stress in the upper plane

postseismic crustal deformation following the 1993 hokkaido nansei oki earthquake northern japan evidence for a low viscosity zone in the uppermost mantle 1 postseismic crustal deformation following the 1993 hokkaido nansei oki earthquake m 7 8 northern japan was observed by gps tide gauge and leveling measurements in southwestern hokkaido as a result of comprehensively analyzing these three kinds of geodetic data we found the chief cause of the postseismic deformation to be viscoelastic relaxation of the coseismic stress change in the uppermost mantle afterslip on the main shock fault and its extension by contrast cannot explain the deformation without unrealistic assumptions the viscoelastic structure which we estimated from the postseismic deformation consists of three layers an elastic first layer with thickness of 40 km a viscoelastic second layer with thickness of 50 km and viscosity of 4 1018 pa s and an elastic half space this model suggests the presence of an anomalously low viscosity zone in the uppermost mantle the depth of the viscoelastic layer roughly agrees with that of the high temperature portion and the low vp zones of the mantle wedge beneath the coast of the japan sea in northern honshu these correlations suggest that the low viscosity primarily results from high temperature of the mantle material combined with partial melt and the presence of h2o these correlations also suggest that the low viscosity zone in the uppermost mantle is widely distributed in the back arc side of the volcanic front and that postseismic deformation induced by viscoelastic relaxation may be frequently observed after large earthquakes in this area

upper mantle velocity structure estimated from ps converted wave beneath the north eastern japan arc summary the upper boundary of the descending oceanic plate is located by using ps waves converted from p to s at the boundary in the tohoku district the north eastern part of honshu japan the observed ps p time data are well explained by a two layered oceanic plate model composed of a thin low velocity upper layer whose thickness is less than 10 km and a thick high velocity lower layer the upper and lower layers respectively have 6 per cent lower and 6 per cent higher velocity than the overriding mantle the estimated location of the upper boundary is just above the upper seismic plane of the double planed deep seismic zone this result indicates that events in the upper seismic plane at least in the depth range from 60 to 150 km occur within the thin low velocity layer on the surface of the oceanic plate

depth of seismic coupling along subduction zones underthrusting at subduction zones can cause large earthquakes at shallow depths but it is always accommodated by aseismic deformation below a certain depth the maximum depth of the seismically coupled zone or seismogenic zone is a transition from unstable to stable sliding along the plate interface we have determined the depth of this stability transition for the circurn pacific subduction zones of honshu kuriles kamchatka aleutians alaska mexico and chile these subduction zones have experienced great interplate earthquakes and the aftershock regions are well located depth estimates of interplate events that are located at the downdip edge of the aftershock regions are used to determine the maximum depth of seismic coupling for an average p wave velocity of 6 7 km s 1 above the plate interface we find that for most subduction zones the stability transition occurs at 40 5 km depth there are however several exceptions at the hokkaido trench junction where the japan trench and the kurile trench intersect seismic coupling is deep and extends down to 52 55 km deep coupling was also found in the coquimbo region in central chile the mexico subduction zone has shallow coupling the transition occurs at 20 30 km depth previous studies of micro earthquakes in honshu hokkaido the aleutians and alaska show that earthquakes within the upper plate extend no deeper than the downdip edge of the coupled zone that we find given our measurements of seismic coupling depth we then explore the mechanism that may determine coupling depth the concept of critical temperature has been used to explain the depth of seismic coupling in other tectonic environments thus we first test whether a critical temperature can explain our results temperatures at the plate interface are dependent on many variables but two that are poorly determined are shear stress and radiogenic heat generation shear stress has been constrained by inversion of heat flow data assuming a crustal radiogenic heat production rate of 3 1 exp z 8 5 wm 3 and a constant coefficient of friction we find two critical temperatures of about 400 c and 550 c the lower critical temperature may be characteristic of regions with a relatively thick continental crust and the higher temperature of regions with a relatively thin continental crust on the other hand one single critical temperature of about 250 c can explain the coupling depths if shear stresses are constant with depth

source inversion of the 1988 upland california earthquake determination of a fault plane for a small event abstract we examined short period p waves to investigate if waveform data could be used to determine which of two nodal planes was the actual fault plane for a small m l 4 6 earthquake near upland california we removed path and site complications by choosing a small aftershock m l 2 7 as an empirical green function the main shock p waves were deconvolved by using the empirical green function to produce simple far field displacement pulses we used a least squares method to invert these pulses for the slip distribution on a finite fault both nodal planes strike 125 dip 85 and strike 221 dip 40 of the first motion focal mechanism were tested at various rupture velocities the southwest trending fault plane consistently gave better fitting solutions than the southeast trending plane we determined a moment of 4 2 10 22 dyne cm the rupture velocity and thus the source area could not be well resolved but if we assume a reasonable rupture velocity of 0 87 times the shear wave velocity we obtain a source area of 0 97 km 2 and a stress drop of 38 bars choice of a southwest trending fault plane is consistent with the trend of the nearby portion of the transverse ranges frontal fault zone and indicates left lateral motion this method provides a way to determine the fault plane for small earthquakes that have no surface rupture and no obvious trend in aftershock locations

different stress directions in the aftershock focal mechanisms of the kushiro oki earthquake of jan 15 1993 se hokkaido japan and horizontal rupture in the double seismic zone abstract on 15 january 1993 a large intermediate depth earthquake with a magnitude of m 7 8 shook the eastern part of the island of hokkaido japan close to the northeastern japan trench and to the kuril trench this earthquake is now referred to as the kushiro oki earthquake the main shock of the kushiro oki earthquake showed a down dip extension mechanism having horizontal and vertical nodal planes the hypocenter was located at a depth of 107 km in the lower plane of the double seismic zone in the subducting pacific plate after the mainshock more than 300 aftershocks were recorded by three seismograph networks from these data we were able to determine the aftershock hypocenters and 63 focal mechanism solutions for the aftershocks the aftershock region extended horizontally in a northerly direction from the lower plane towards the upper plane of the double seismic zone in the subducting plate it did not however reach the upper plane we estimated the maximum and minimum compressional stress directions along the deep seismic zone by converting the mechanism solutions from horizontal plane projections to an oblique plane parallel to the deep seismic zone thirty eight mechanisms were found to have maximum and minimum compressional stress directions very similar to those of the mainshock but 25 aftershock mechanisms had almost opposite directions to those of the mainshock the hypocenters of these aftershocks were spatially mixed this suggests that the stress state is strongly inhomogeneous in the aftershock region and that horizontal movement occurs by the down dip extensional stress which is stored in the region between two planes of the deep seismic zone the remanent down dip extensional stress was released as the consistent group of aftershocks high aftershock activity may be attributed to the inhomogeneity of the material in the region the previous four intermediate depth earthquakes beneath hokkaido island as well as aftershock distributions of two of them have been examined for comparison with the kushiro oki earthquake the mainshocks of the four large intermediate depth earthquakes are also located on the lower plane with a similar mechanism to that of the kushiro oki earthquake the aftershock regions of two of these earthquakes are also distributed horizontally they indicate dominantly horizontal movement with down dip extension mechanisms at an intermediate depth in the subducting pacific plate beneath hokkaido island

global seismicity of 1981 centroid moment tensor solutions for 542 earthquakes abstract centroid moment tensor solutions are presented for 542 earthquakes that occurred during 1981

high resolution seismic velocity structure beneath the hokkaido corner northern japan arc arc collision and origins of the 1970 m 6 7 hidaka and 1982 m 7 1 urakawa oki earthquakes 1 using travel time data from both a nationwide dense seismic network and a dense temporary seismic network we obtain a high resolution three dimensional seismic velocity structure beneath the hokkaido corner considerable inhomogeneity in the seismic velocity structure is clearly imaged above the subducting pacific slab our results indicate that a broad low velocity zone of p and s waves with velocities consistent for crustal rocks is observed west of the hidaka main thrust at depths of 35 90 km the images also indicate that several smaller scale high velocity zones are located at depths of 0 35 km striking approximately north south and inclined to the east northeastward at 40 60 all of these anomalous high velocity zones are located at the deeper extension of neogene thrust faults the clearest high velocity zone is located beneath the hidaka metamorphic belt and is in contact with the eastern edge of the broad low velocity zone moreover the boundary between the clearest high velocity and the broad low velocity zones corresponds to the fault plane of the 1970 mj magnitude determined by the japan meteorological agency 6 7 hidaka earthquake the western boundary of another small high velocity zone at depths of 20 to 30 km within the broad low velocity zone corresponds to the fault plane of the 1982 mj 7 1 urakawa oki earthquake these observations suggest that these two large and anomalously deep inland earthquakes occurred at sharp material boundaries under a northeast southwest compressional stress field caused by ongoing arc arc collision process

double seismic zone beneath the middle of hokkaido japan in the southwestern side of the kurile arc abstract the vertical section of microearthquakes determined accurately by using the hokkaido university network shows two dipping zones the double seismic zone 25 30 km apart in the depth range of 80 150 km beneath the middle of hokkaido in the southwestern side of the kurile arc hypocentral distribution of large earthquakes m b 4 based on the isc international seismological centre bulletin also shows the double seismic zone beneath the same region the hypocentral distribution indicates that the frequency of events occurring in the lower zone is four times greater than that in the upper zone the difference in seismic activity between the two zones beneath hokkaido is in contrast with the region beneath northeastern honshu in the northeastern japan arc composite focal mechanisms of microearthquakes and individual mechanisms of large events mainly characterize the down dip extension for the lower zone as is observed beneath northeastern honshu for the upper zone however the stress field is rather complex and not necessarily similar to that beneath northeastern honshu this may be considered to indicate the influence of slab contortion or transformation in the hokkaido corner between the kurile and the northeastern japan arcs

stopping phases in seismograms and the spatiotemporal extent of earthquakes on the basis of long period wwssn and some broadband seismograms from grafenberg we investigate the following earthquakes view this table p wave seismograms or true ground displacements show clearly the directivity effect of rupture propagation and the multiple event nature of these earthquakes fault plane solutions of individual source shocks and their relative localization by the master event technique yield fixpoints of rupture history the most striking feature is a strong stopping phase which is interpreted kinematically by abrupt termination of rupture expansion at the end of the fault we use the stopping phase to determine total source extent in space and time as well as the mean rupture velocity the results of this first stage of investigation are subsequently used for modeling the rupture according to the dislocation theory of shear faulting wave propagation through the earth s mantle is treated by ray theory this forward modeling approach yields seismic moment source dimensions mean dislocation and stress drop source complexities are related to unbroken fault gaps bending of the fault plane acceleration of rupture propagation in asperities and most impressive in its radiation effect to the sudden termination of rupture in front of a barrier source complexities are also discussed with respect to tectonic implications

the 1993 kushiro oki japan earthquake a high stress drop event in a subducting slab a spatial slip distribution of the 1993 kushiro oki earthquake mw 7 6 is undoubtedly constrained using near field data and the detailed aftershock distribution this earthquake has peculiar features in that the slip on the fault plane is restricted within an extremely small area about 40 km 20 km between the upper and lower planes of the double seismic zone in the subducting slab beneath hokkaido the total seismic moment averaged slip and stress drop in this area are 3 3 1020 nm 5 5 m and 4 2 107 pa respectively this earthquake indicates that the region of the slab between the planes of the double seismic zone beneath hokkaido is strong enough to sustain a stress of about 4 107 pa the slip motion of this event is consistent with retrograde slab migration along the southern kuril trench and may represent a process by which retrograde motion of subducting slabs could occur

magnetic analyses of soils from the wind river range wyoming constrain rates and pathways of magnetic enhancement for soils from semiarid climates in order to constrain the rate of magnetic enhancement in soils we investigated modern soils from five fluvial terraces in the eastern wind river range wyoming profiles up to 1 2 m deep were sampled in 5 cm intervals from hand dug pits or natural riverbank exposures soils formed in fluvial terraces correlated to the sacajawea ridge 730 610 ka bp bull lake 130 100 ka bp and pinedale age 20 ka bp glacial advances one soil profile formed in holocene age sediment abundance mineralogy and grain size of magnetic minerals were estimated through magnetic measurements magnetic enhancement of the a horizon as well as an increase in fine grained magnetic minerals occurred mostly in bull lake profiles but was absent from the older profile such low rates of magnetic enhancement may limit the temporal resolution of paleosol based paleoclimate reconstructions in semiarid regions even where high sedimentation rates result in multiple paleosols a loss of ferrimagnetic and an increase in antiferromagnetic minerals occurred with age our findings suggest either the conversion of ferrimagnetic minerals to weakly magnetic hematite with progressing soil age or the presence of ferrimagnetic minerals as an intermediate product of pedogenesis absolute and relative hematite abundance increase with age making both useful proxies for soil age and the dating of regional glacial deposits all coercivity proxies are consistent with each other which suggests that observed changes in hirm and s ratio are representative of real changes in hematite abundance rather than shifts in coercivity distributions even though the modified l ratio varies widely

the 2000 nemuro hanto oki earthquake off eastern hokkaido japan and the high intraslab seismic activity in the southwestern kuril trench 1 the 2000 nemuro hanto oki earthquake mw6 8 occurred in the southwestern part of the kuril trench the hypocenter was located close to the aftershock region of the great 1994 kuril earthquake mw8 3 named the 1994 hokkaido toho oki earthquake by the japan meteorological agency for which the fault plane is still in debate analysis of the 2000 event provides a clue to resolve the fault plane issue for the 1994 event the hypocenters of the 2000 main shock and aftershocks are determined using arrival times from a combination of nearby inland and submarine seismic networks with an improved azimuthal coverage they clearly show that the 2000 event was an intraslab event occurring on a shallow dipping fault plane between 55 and 65 km in depth the well focused aftershock distribution of the 2000 event the relative location of the 1994 event with respect to the 2000 event and the similarity between their focal mechanisms strongly suggest that the faulting of the great 1994 earthquake also occurred on a shallow dipping fault plane in the subducting slab the recent hypocenter distribution around the 1994 aftershock region also supports this result large intraslab earthquakes occurring to the southeast of hokkaido may occur due to a strong coupling on the plate boundary which generates relatively large stress field within the subducting pacific plate

earthquakes and plastic deformation of anhydrous slab mantle in double wadati benioff zones double wadati benioff seismic zones dsz with two parallel planes of seismicity separated by 15 30 km are a global feature of subduction zones in the 50 200 km depth range upper plane seismicity is generally attributed to dehydration of the oceanic crust but the origin of the lower seismicity plane is debated serpentine or hydrous phase dehydration embrittlement is a commonly advocated mechanism that implies significant slab mantle hydration high resolution seismic tomography revealed low seismic velocities in the lower seismicity plane that are better explained by seismic anisotropy of anhydrous deformed peridotites than by serpentinization earthquakes correlate with anisotropic planar shear zones and favor a shear instability mechanism as the cause of lower plane seismicity without requiring the presence of water in the center of subducting slabs the contribution of the subducted lithospheric mantle to the water budget of subduction zones is thus likely limited to the first 2 3 kilometers beneath oceanic crust

determination of stresses from heterogeneous focal mechanism data an adaptation of the multiple inverse method abstract a new method is proposed to separate stresses from earthquake focal mechanism data double couple component of focal mechanisms from spatially or temporarily varying state of stress the underlying multiple inverse method was originally devised for geological fault slip data but we have adapted its resampling scheme to focal mechanism data to cope with the nodal planes ambiguity problem the stresses compatible with a dataset are recognized as the clusters of reduced stress tensors the clusters are identified with the k means clustering the uncertainty of the stresses are evaluated from the spread of the clusters the technique is shown to successfully determine valid stresses from simulated data we have also applied the technique to natural data from southwestern japan

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