poisson s ratio and porosity at coso geothermal area california high resolution three dimensional compressional and shear wave velocity models derived from microearthquake traveltimes are used to map the distribution of poisson s ratio and porosity at coso geothermal area inyo county california spatial resolution of the three dimensional poisson s ratio and porosity distributions is estimated to be 0 5 km horizontally and 0 8 km vertically model uncertainties 1 in the interior and 2 3 around the edge of the model are estimated by a jackknife method we use perturbations of r vp vs ratio and vp vs product to derive distributions of poisson s ratio and porosity which are then used to constrain and delineate possible zones of intense heat fracture accumulation and fluid saturation poisson s ratio at coso ranges from 0 15 to 0 35 with an average of 0 224 lower than the crustal average of 0 25 high poisson s ratios are more extensive in shallower depths 1 5 km while lower poisson s ratios are found in the deeper section 1 5 3 0 km of the target area two major features with low poisson s ratio are identified at geothermal production depth 1 3 km around stations s2 s6 and s1 s3 s4 the two low features are separated by a northwest southeast trending high belt with variable width of 1 3 km a high body is found around s2 and s6 and extends down in depth a circular low belt corresponding to the high belt is located around s2 s6 and is linked to a previously reported structure in vs tomography this low highly porous belt is probably a horizontal conduit reservoir of geothermal fluid a vertical low and high channel beneath triangle s1 s3 s4 corresponds to a high attenuation dome like feature we propose an upwell and spread magma intrusion model for the last major magmatism in the coso region the magmatic upwelling is centered in the s1 s3 s4 area the model predicts potential geothermal resources to the south and west of triangle s1 s3 s4 based on local faulting patterns

episodic slow slip events accompanied by non volcanic tremors in southwest japan subduction zone 1 episodic slow slip events have been recognized by means of tilt changes in the western shikoku area southwest japan the crustal tilt deformation was observed repeatedly with a recurrence interval of approximately six months coincident with the occurrences of major non volcanic deep tremor activities in this area observed tilt changes can be explained by slow slip events occurring around the source area of tremors in each episode the source of the slow slip event and tremor migrate simultaneously the spatial and temporal coincidence of tremors and slow slip events indicates that they both may be coupling phenomena reflecting the stress accumulation process at the subducting plate

suboceanic earthquake location and seismic structure in the kanto district central japan abstract we present a combined method using sp depth phase data and double difference arrival times to determine the precise hypocenter locations of earthquakes that occur under the pacific ocean outside of the area covered by the land based seismic network we assess the effectiveness of the combined method using a data set of p and s wave arrival times and sp depth phase from suboceanic earthquakes recorded by both land based seismic stations and offshore seismic stations ofs the hypocenters of the offshore earthquakes relocated using the combined method are consistent with those determined using the standard location method and ofs data the differences in the hypocenters relocated by the two methods are less than 4 km we applied the method to the subduction region that underlies the kanto district central japan and located a large number of earthquakes that occurred beneath the pacific ocean we then determined the detailed 3d seismic velocity structure by inverting a large number of arrival times of p and s waves and sp depth phase from the relocated earthquakes in the study region high velocity anomalies related to the cold subducting pacific slab and low velocity anomalies related to the hot mantle wedge are clearly imaged beneath active volcanoes low velocity zones are visible from the surface to a depth of 100 km reflecting fluids released by dehydration of the subducting pacific slab strong lateral heterogeneities are revealed on the upper boundary of the pacific slab beneath the forearc region the low velocity areas under the offshore region are associated with low seismicity and weak interplate coupling a low velocity layer is imaged along the upper boundary of the philippine sea slab in the northern part of kanto district which may reflect dehydration of the slab our tomographic images indicate that the overlaying philippine sea plate has effects on the spatial distribution of active volcanoes related to the subducting pacific slab in the study region

3 d seismic structure of kii peninsula in southwest japan evidence for slab dehydration in the forearc abstract kii peninsula is located in the forearc region of southwest japan and has distinct structural and tectonic features characterized by high seismicity in the crust and the subducting philippine sea slab high surface heat flow high 3he 4he isotopic ratio and a local change in the geometry of the subducting slab we have tried to determine detailed 3 d p and s wave velocity structures of this region using a large number of high quality arrival time data recorded by dense seismic networks on the japan islands from the obtained seismic velocities we further estimated 3 d distributions of poisson ratio crack density saturation rate and porosity parameter in the study area our results show significant heterogeneities in the crust and upper mantle wedge characterized by low seismic velocities high poisson ratio high values of crack density saturation rate and porosity these results suggest the existence of fluids in the crust and mantle wedge resulting from the dehydration of the subducting philippine sea slab which can explain the observed geophysical and geochemical features in kii peninsula

influence of fluids and magma on earthquakes seismological evidence abstract in this paper we present seismological evidence for the influence of fluids and magma on the generation of large earthquakes in the crust and the subducting oceanic slabs under the japan islands the relationship between seismic tomography and large crustal earthquakes m 5 7 8 0 in japan during a period of 116 years from 1885 to 2000 is investigated and it is found that most of the large crustal earthquakes occurred in or around the areas of low seismic velocity the low velocity zones represent weak sections of the seismogenic crust the crustal weakening is closely related to the subduction process in this region along the volcanic front and in back arc areas the crustal weakening is caused by active volcanoes and arc magma resulting from the convective circulation process in the mantle wedge and dehydration reactions in the subducting slab in the forearc region of southwest japan fluids are suggested in the 1995 kobe earthquake source zone which have contributed to the rupture nucleation the fluids originate from the dehydration of the subducting philippine sea slab the recent 2001 geiyo earthquake m 6 8 occurred at 50 km depth within the subducting philippine sea slab and it was also related to the slab dehydration process a detailed 3d velocity structure is determined for the northeast japan forearc region using data from 598 earthquakes that occurred under the pacific ocean with hypocenters well located with sp depth phases the results show that strong lateral heterogeneities exist along the slab boundary which represent asperities and results of slab dehydration and affect the degree and extent of the interplate seismic coupling these results indicate that large earthquakes do not strike anywhere but only anomalous areas which can be detected with geophysical methods the generation of a large earthquake is not a pure mechanical process but is closely related to physical and chemical properties of materials in the crust and upper mantle such as magma fluids etc

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

high pore fluid pressure may cause silent slip in the nankai trough silent slip events have been detected at several subduction zones but the cause of these events is unknown using seismic imaging we detected a cause of the tokai silent slip which occurred at a presumed fault zone of a great earthquake the seismic image that we obtained shows a zone of high pore fluid pressure in the subducted oceanic crust located down dip of a subducted ridge we propose that these structures effectively extend a region of conditionally stable slips and consequently generate the silent slip

spatial and temporal diagenetic evolution of syntectonic sediments in a pulsatory uplifted coastal escarpment evidenced from the plio pleistocene makran subduction zone iran

double seismic zone and dehydration embrittlement of the subducting slab 1 dehydration embrittlement of metamorphosed oceanic crust and mantle in the subducting slab may be responsible for the occurrence of intermediate depth earthquakes we explore the possibility that this hypothesis can explain the morphology of the double seismic zones observed in northeast japan southwest japan northeast taiwan northern chile cape mendocino and eastern aleutians we calculate transient temperature structures of slabs based on geologically estimated subduction histories of these regions we then determine dehydration loci of metamorphosed oceanic crust and serpentinized mantle using experimentally derived phase diagrams the depth range of the dehydration loci of metamorphosed oceanic crust and serpentine is dependent on slab age the dehydration loci of serpentine produce a double layered structure because the upper dehydration loci of serpentine are mostly located in the wedge mantle above the slab we regard the upper plane seismicity representing dehydration embrittlement in the oceanic crust and we fix the slab geometry so that the upper plane seismicity is just below the upper surface of the slab we find that the lower plane seismicity is located at the lower dehydration loci of serpentine which indicates that the morphology of the double seismic zones is consistent with the dehydration embrittlement

structure of the nankai trough accretionary zone from multichannel seismic reflection data new multichannel seismic reflection data collected over the nankai trough image the accretionary complex in two areas the international program of ocean drilling leg 87 transect area western area and the region of upcoming ocean drilling program leg 131 eastern area the incoming shikoku basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites the basin section is overlain by a trench wedge that is 12 16 km wide and 350 750 m thick at the thrust front accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge the zone in the western area is 4 5 km wide and is characterized by kink folds the zone in the eastern area is only 2 5 km wide and does not exhibit such folds the frontal thrusts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section the overthrusting sediments form fault bend folds over these ramps thrust spacing at the toe of the slope is 1 5 2 5 km the second thrust cuts up from an inferred decollement within the shikoku basin sedimentary section in the eastern area a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6 3 km seaward of the thrust front and underlies the entire protothrust zone this reflector continues with reversed polarity under the accretionary complex and is at the level of the basal decollement the underlying basin pelagic section is apparently thrust undisturbed beneath the accretionary prism the reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface this decrease in velocity and density may indicate that the decollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism the reflections from the first and second thrusts are also reversed polarity possibly indicating that they also are pathways of fluid expulsion the critical wedge taper of the western area is greater than that of the eastern area an observation that is consistent with the existence of an overpressured decollement in the eastern area

periodic slow earthquakes from the cascadia subduction zone continuous geodetic measurements from convergent margins have shown that deep transient creep events can release large amounts of strain energy without detectable seismic shaking and they are thus known as slow or silent earthquakes because subduction zones generate the largest earthquakes

friction experiments on saturated sediments and their implications for the stress state of the nankai and barbados subduction thrusts the nankai and barbados forearcs have low stress subduction thrusts the sediments entering the subduction zone and namely the material in the decollement zones have been well characterized by numerous deep sea drilling legs and studies of the recovered cores nankai has high heat flow and significant amounts of illite while barbados is a smectite dominated system based on results from ring shear 2 mpa normal stress and direct shear 30 mpa tests on marine sediments and mineral standards this translates into a residual frictional resistance of r 0 25 and r 0 11 in clay horizons respectively such values agree with theoretical estimates from critical wedge theory nankai b 0 16 0 26 and barbados b 0 06 0 09 and fault spacing geometries from seismic profiles nankai b 0 12 0 23 and barbados b 0 11 0 19 maximum pore pressure ratios of 0 85 and 0 73 for nankai and barbados respectively allow us to estimate effective shear stresses as a function of friction coefficient and density of the sediment gouge to reach only 10 mpa or less in the frontal 50 km of the decollement zone respectively our data support the contention that fluid pressure transients and sediment composition contribute equally to the weakness along plate boundary faults down to the seismogenic zone with the first probably dominating the shallow decollement shear velocity stepping tests show that the clay dominated gouges strengthen velocity irrespective of the clay mineralogy and hence suggest that clay transformation does not affect the onset of seismogenesis

the higher grade metamorphic zonation of the sambagawa metamorphic belt in central shikoku japan the higher grade metamorphic zonation of the sambagawa sanbagawa belt is established for the first time for the whole area of central shikoku as discontinuous reactions to define the isograd are absent the metamorphic grade is primarily determined by the mg fe partitioning between garnet and chlorite along representative traverses however for regional mapping mineralogical features of the pelitic schists such as using mineral assemblages of more than divariant equilibrium the modal garnet to chlorite ratio and the optical properties of chlorite are employed as auxiliary criteria the presence of the highest grade mineral zone in the middle of the structural level is confirmed but its spatial distribution is far more complex than hitherto accepted thermal axes are now confirmed at three different structural levels a model is presented in which the stacking of thrust sheets of different grade took place while metamorphic reactions were in progress thermal readjustment brought a continuous metamorphic temperature gradient across and within the thrust sheets tectonic blocks of metagabbro and ultramafic rock were emplaced synchronously with thinning and subsequently also re equilibrated local anomalies of metamorphic grade represented by mixing of schists of different metamorphic grade exist but they are due to a later stage event

experimental sytectonic dehydration of serpentinite under conditions of controlled pore water pressure we have studied the mechanical behavior of serpentinite under conditions favouring dehydration to olivine talc water at temperatures ranging between 300 and 600 and at total pressures from 100 to 300 mpa constant stress stress relaxation and constant displacement rate testing techniques permitted a wide range of deformation rates and paths to be accessed effective confining pressure was held constant during the dehydration process by means of a controlled pore water pressure system stress supported by the samples was almost independent of deformation rate at temperatures below those required for the onset of the breakdown reaction at 500 and 600 c however the strength was linearly dependent on the inverse of the deformation rate and independent of effective confining pressure over the range investigated microstructural studies revealed that the weakening was associated with sliding in narrow shear zones containing ultrafine grained 0 1 m olivine produced in the breakdown reaction the enhanced deformability is interpreted in terms of flow of the olivine in these shear zones by diffusion accommodated grain boundary sliding these experimental results suggest that metamorphic transformations affect the deformability of rocks and demonstrate enhancement of deformability through the transient existence of fine grained reaction products the results may also be applicable to the mechanics of deformation processes in oceanic transform faults and give some indication of the degree of weakening that might be expected from grain size reduction by dynamic recrystallization in plastically deforming rocks

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

a deep strong reflector in the nankai accretionary wedge from multichannel seismic data implications for underplating and interseismic shear stress release 1 we present the results of structural and stratigraphic interpretations for the multichannel seismic profiles to reveal the structure of the nankai accretionary wedge off shikoku island on the basis of reflection characteristics we identify three major seismic units that correspond to a unit consisting of both oligocene miocene accretionary prism and the sedimentary cover unit a a unit of underthrust miocene pliocene hemipelagic sediments unit b and a unit of subducting pre miocene oceanic crustal rocks unit c according to relative strength of the topmost reflector of unit b the nankai accretionary wedge can be divided into three different structural zones along the cross strike direction apparent decollement reflector zone low amplitude reflector lar zone and deep strong reflector dsr zone despite the velocity uncertainty involved in the depth conversion and therefore thickness estimation error the decollement step down and the possible duplex structures in the lar and dsr zones support that the landward thickness variation of unit b is caused by the structural thickening such as the underplating process structural features and reverse polarity suggest that the dsr as an upper interface of the underplated layer unit b corresponds to a shear plane or a fault zone with high pore fluid pressures we found that most of the dsr was observed in a region which has not suffered from the 1946 coseismic dislocation even though it was locked during the interseismic period the dsr as a shear plane or a fault zone constrains a regional variety of the coseismic dislocation around the updip limit of the nankai seismogenic zone considering all of the dsr characteristics we propose a hypothesis for the dsr s development and role the dsr may play an important role as a possible releaser of the shear stress energy stored at the plate boundary around the updip limit of the nankai seismogenic zone a good correlation between the theoretical vertical displacement and seafloor topographic feature support the possible slip along the dsr as a shear plane

seismological detection of slab metamorphism the occurrence of more or less continuous ground vibrations volcanic tremor is an important indicator of volcanic activity but results from the hi net seismic network in japan reported by obara 1 show that continuous ground vibrations can occur far away from any volcanic activity in his perspective julian 2 discusses the idea that this tremor is excited by flow of metamorphic fluids he also identifies other possible locations where such a tremor may be detected and explains what may be learnt from measuring it 1 http www sciencemag org cgi content short 296 5573 1679 2 http www sciencemag org cgi content full 296 5573 1625

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

structural heterogeneity and its implications for the low frequency tremors in southwest japan abstract to better understand the generation of low frequency tremors lfts that occurred in the nankai subduction zone detailed 3 d seismic velocity vp and vs and poisson s ratio structures are determined by inverting 181 576 p wave and 159 517 s wave arrival time data from 7572 earthquakes we also estimated the crack density e saturation rate and porosity structures along the lft belt using the inverted velocity data our results provide geophysical evidence for that the long duration tremors are closely related to the dehydration reactions of the subducting philippine sea slab and in the lowermost crust low vp low vs high high e high and high porosity anomalies are imaged at depths of 25 45 km along the lft belt which may indicate the existence of fluids liberated by the dehydration of the descending slab and or by the metamorphism in the lowermost crust the interpretation accords well with the behavior of the subducted hydrous minerals which are expected to dehydrate at the temperature of 350 500 c at depths of 35 50 km in southwest japan two prominent lft gaps between shikoku and kii peninsula and between kii peninsula and tokai along the tremor belt are imaged with low and low anomalies due to the lack of dehydration reactions within those gaps we conclude that the lfts are caused by the dehydrated fluids migrating in the faults and or by crack opening closing and extending in the corner of mantle wedge along the subducting philippine sea slab possibly due to the high pore fluid pressure and high crack density under the forearc region of the nankai subduction zone

pressure and porosity influences on vp vs ratio in unconsolidated sands elevated pore pressures commonly encountered in the shallow unconsolidated section of the sedimentary column present a significant hazard during the drilling and completion of offshore wells the porosity of the ocean floor sediments is high and a cover of low permeability clay can prevent the underlying sediments from draining even at very shallow depths below the seafloor as further deposition loads the sediment the entrapped fluids impede normal compaction by becoming pressurized the lowered effective stress that results from the higher pore fluid pressure produces a proportional drop in the strength of non cohesive sediments resulting in very weak shallow sands this weakness can result in washouts or fracturing in the shallow strata potentially leading to the loss of the well and of neighboring wells

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