splitting of the philippine sea plate and a magma chamber beneath mt fuji 1 mt fuji is located in a tectonically unique area but various aspects of the structure have not been fully explained here we show the results from a magnetotelluric survey across mt fuji along a 70 km observation line the profile shows that a conductive body is located between two resistive bodies at depths greater than 15 km low frequency earthquakes occur above the conductor we interpret these results in a model where beneath mt fuji the subducting philippine sea plate is split into two parts and a magma chamber is located in the gap due to this unique structure mt fuji may be able to sustain a high basalt magma flux throughout its entire history

central japan triple junction revisited we show that the two dimensional kinematic discussion of mckenzie and morgan s ttt type triple junction is not sufficient as it ignores the interaction of the sinking plates we derive a relation for the three dimensional existence of ttt triple junctions and show that it is satisfied at the unstable central japan triple junction we then use the geometry of the subducting pacific plate beneath the philippine sea and eurasian plates to show that the present situation did not hold before about 3 ma before that time the triple junction was probably located north of the present one and very close to the pole of relative motion finally we discuss the geological implications of this model

strain partitioning in the zone of transition from nankai subduction to izu bonin collision central japan implications for an extensional tear within the subducting slab abstract the izu bonin ridge collision with central japan is a type example of collision of two island arcs in a subduction context based on the analysis of active faults paleomagnetic data and recent seismicity in the northern tip of the izu bonin arc and the adjacent oceanic plate we propose a tectonic schema of the izu collision system and its transition to the nankai subduction to the west the dense gps network deployed in central japan allows us to constrain the kinematics of the different blocks identified this new kinematic model demonstrates the essential role of strain partitioning there as the northward component of the izu bonin arc motion is locked in the collision zone 30 of the convergence between the philippine sea plate and central japan is accommodated within the philippine sea slab along the zenisu east izu deformation system we show that this pattern of deformation is related to an extensional tear of the philippine sea slab propagating southward along the volcanic arc we discuss the implications of this kinematic model on the seismogenic potential of the area and the dynamics of the central japan convergence zone

tectonic evolution of the japanese island arc system abstract growth of the japanese arc system which has mainly taken place along the continental margin of asia since the permian is the result of subduction of the ancient pacific ocean floor backarc basin formation in the tertiary shaped the present day arc configuration the neotectonic regime which is characterized by strong east west compression has been triggered by the eastward motion of the amur plate in the quaternary the tectonic evolution of the japanese arc system includes formation of rock assemblages common in most orogenic belts because the origin and present day tectonics of these assemblages are better defined in the case of the japanese arc system study of the system provides useful insight into orogenesis and continental crust evolution

analytical method for evaluating superficial fault displacements in volcanic air fall deposits case of the hirayama fault south of tanzawa mountains central japan since 21 500 years b p the establishment of an analytical method for evaluating superficial fault displacements must contribute greatly to the improvement of methods for earthquake prediction in the quaternary volcanic areas where many volcanic air fall deposits of known age are widely traceable however because the air fall deposits of uniform thickness cover many surfaces of blocks separated by faults cumulative displacements along faults cannot always be detected in the deposits then the study of geomorphological developments along a fault becomes important in analysis of superficial faults in air fall deposits the hirayama fault south of the tanzawa mountains central japan is analyzed as an optimal example for studying the methodology in air fall deposits the most important result of the detailed analysis is that the displacement rate of the hirayama fault has been decreasing since 21 500 years b p that is the hirayama fault has nonuniform history of movement the result consequently indicates that the assumption of uniform history in fault movement is not always safe in earthquake prediction

internal tide energy over topography 1 the method used to separate surface and internal tides ultimately defines properties such as internal tide generation and the depth structure of internal tide energy flux here we provide a detailed analysis of several surface internal tide decompositions over arbitrary topography in all decompositions surface tide velocity is expressed as the depth average of total velocity analysis indicates that surface tide pressure is best expressed as the depth average of total pressure plus a new depth dependent profile of pressure which is due to isopycnal heaving by movement of the free surface internal tide velocity and pressure are defined as total variables minus the surface tide components corresponding surface and internal tide energy equations are derived that contain energy conversion solely through topographic internal tide generation the depth structure of internal tide energy flux produced by the new decomposition is unambiguous and differs from that of past decompositions numerical simulations over steep topography reveal that the decomposition is self consistent and physically relevant analysis of observations over kaena ridge hawaii and the oregon continental slope indicate o 50 w m 1 error in depth integrated energy fluxes when internal tide pressure is computed as the residual of pressure from its depth average while these errors are small at major internal tide generation sites they may be significant where surface tides are larger and depth integrated fluxes are weaker e g over continental shelves

a trench fan in the izu ogasawara trench on the boso trench triple junction japan soh w taira a and tokuyama h 1988 a trench fan in the izu ogasawara trench on the boso trench triple junction japan mar geol 82 235 249 the mogi trench fan 18 km in diameter is located in the 9 1 km deep izu ogasawara trench analysis of the morphology and internal structure of the mogi fan was based on 3 5 khz records seismic reflection profiles and seabeam bathymetry the mogi fan was fed from a point source and displays an even shaped partial cone morphology which can be divided into upper middle and lower fans the upper fan is defined as an apparent topographic mound having a large scale single channel with well defined levees that appear to be composed mainly of coarse grained turbidites the middle fan is characterized by divergent channels and lobes the lower fan is a smooth mound with no channel features it is postulated that the lower fan is constructed chiefly by turbidity currents that reflected back from the higher outer slope seismic reflection records across the fan show deformation resulting from plate subduction in the upper fan this deformation can be traced laterally to the lower bulge of the inner slope neighbouring the mogi fan several characteristics of the morphology of the mogi fan indicate that it is presently inactive the fan development is interpreted to have formed and prograded in the trench during a period of lowering sea level during rising sea level the sediment supply to the trench abruptly decreased and the tectonic deformation of the fan morphology was correspondingly enhanced

central japan triple junction a three dimensional compression model abstract we present a new interpretation of the tectonics of the trench trench trench triple junction off central japan using more specifically data obtained during the french japanese kaiko cruise in 1984 data analysis and consideration on the three dimensional geometry of the junction and its kinematics lead to the conclusion that the whole area undergoes large scale compressional intraplate deformation this is caused by the subduction of the philippine sea plate in the restricted space between the pacific and northern honshu plates we then apply the same analysis to the whole central japan area for which we consider the role of the interaction of the three plates at depth as a basis for tectonic reconstructions since the miocene

how the continents deform the evidence from tectonic geodesy space geodesy now provides quantitative maps of the surface velocity field within tectonically active regions supplying constraints on the spatial distribution of deformation the forces that drive it and the brittle and ductile properties of continental lithosphere deformation is usefully described as relative motions among elastic blocks and is block like because major faults are weaker than adjacent intact crust despite similarities continental block kinematics differs from global plate tectonics blocks are much smaller typically 100 1000 km in size departures from block rigidity are sometimes measurable and blocks evolve over 1 10 ma timescales particularly near their often geometrically irregular boundaries quantitatively relating deformation to the forces that drive it requires simplifying assumptions about the strength distribution in the lithosphere if brittle elastic crust is strongest interactions among blocks control the deformation if ductile lithosphere is the stronger its fl

shear partitioning near the central japan triple junction the 1923 great kanto earthquake revisited i summary we propose the existence of a major right lateral transform fault which we call the boso transform fault it is related to the sagami trough a portion of the philippine sea plate boundary south of the kanto area central japan this boso transform fault is the result of shear partitioning due to oblique subduction and has delimited a boso sliver for 2 myr the rate of motion is estimated at 16 mm yr and the total offset at 30 km the fault cuts through the miura and boso peninsulas onland where it has a multiple surface expression roughly along the limit of a steeply dipping miocene ophiolitic body these subaerial faults have been identified as active and their cumulated rate of slip across the miura peninsula can be estimated to be greater than 12 mm yr in reasonable agreement with the above estimate we propose that the slip on the boso transform fault was responsible for two large m 7 0 and 7 5 aftershocks which occurred on the second day after the 1923 great kanto earthquake this explains the unusual duration of the aftershock sequence and the large magnitudes of some of the aftershocks

oblique and near collision subduction sagami and suruga troughs preliminary results of the french japanese 1984 kaiko cruise leg 2 abstract leg 2 of the french japanese 1984 kaiko cruise has surveyed the suruga and the sagami troughs which lie on both sides of the northwestward moving and colliding izu bonin ridge the northernmost part of the philippine sea plate the transition from the nankai trough to the suruga trough is characterized by northward decrease in width of the accretionary prism in good agreement with the increasing obliquity between the through axis and the direction of the convergence as the strike of the convergent boundary changes from ene nne to south north south of the area the southern margin of the zenisu ridge shows contractional deformations this supports the interpretation made by the team of leg 1 who studied the western extension of the area we studied that it is an intra oceanic thrusting of the ridge over the shikoku basin in the sagami trough where the relative motion is highly oblique to the plate boundary active subduction is mostly confined in the east west trending portions of the trough located south of the boso peninsula and along the lower boso canyon near the ttt triple junction in between the present motion is mainly right lateral along the northwest trending boso escarpment however an inactive but recent pliocene to lower pleistocene accretionary prism exists south of the boso escarpment which suggests that the relative motion was more northerly than at present before about 1 ma ago

postcollisional exhumation history of the tanzawa tonalite complex inferred from u th he thermochronology and fission track analysis 1 the tanzawa mountains in the south fossa magna region in the middle of the japan arc were formed due to collision of the izu bonin arc against the japan arc during the last several million years the tanzawa tonalite complex is exposed in the tanzawa mountains and is inferred to represent middle crust of the izu bonin arc we performed new thermochronometry consisting of u th he dating fission track dating and fission track length analysis for the complex to investigate the exhumation history of the tanzawa tonalite complex caused by arc arc collision typical u th he ages were 2 0 0 2 ma on apatite 3 3 0 2 ma on zircon and 4 5 to 6 9 ma on zircon using the fission track method the mean fission track length was 9 7 0 6 m 2 se the cooling rate of the tonalite has been calculated to be in the range of 20 60 c ma 1 from 7 to 3 3 ma 80 c ma 1 from 3 3 to 2 0 ma and 30 c ma 1 from 2 0 to 0 ma assuming a surface temperature of 10 c the exhumation rate has been calculated to be 0 5 to 1 5 mm a 1 from 7 to 3 3 ma 2 mm a 1 from 3 3 to 2 0 ma and 0 8 mm a 1 from 2 0 to 0 ma with an assumed geothermal gradient of 40 c km 1 the exhumation rate has not changed significantly despite the simultaneous tectonic changes around the tanzawa mountains after 7 ma

effects of oceanic ridge subduction on accretionary wedges experimental modeling and marine observations sandbox modeling is used to study the deformation of accretionary wedges caused by the subduction of oceanic ridges the first experiment incorporates a massive ridge within a sand wedge the wedge thickens and shortens when the forward propagation of the basal decollement ceases the wedge thickening results in taper change reactivation of preexisting thrusts and retreat of the frontal part of the sand wedge similar mechanisms may have affected some margins that have undergone ridge subduction such as the tonga margin after the subduction of the oblique louisville oceanic ridge the second experiment shows the effects of an active basement thrust slice as it enters a subduction zone this process may have happened in the eastern nankai accretionary wedge initially the wedge in this experiment behaved similarly to that of the first experiment rapidly the topographic slope changed the wedge thickened above the basement slice generating a slope break in the topography a deeper propagating accretionary wedge again characterized by a small taper developed these results when compared with observations made in the eastern nankai trench are in agreement with the past subduction of a basement thrust slice in this area

unstable forearc stress in the eastern nankai subduction zone for the last 2 million years abstract transpressional tectonics characterizes the sw japan arc however we will show in this article that offshore seismic profiles and onshore mesoscale faults indicate that the eastern part of the forearc was subject to transtensional tectonics since ca 2 0 ma offshore normal faults imaged on the profiles run parallel to the nankai trough and started activity at 1 0 ma but transtensional tectonics commenced the onshore area earlier in order to understand the stress history in the forearc region we collected fault slip data from onshore mesoscale faults in plio pleistocene sedimentary rocks in the kakegawa area at the northeastern extension of the offshore normal faults most of the mesoscale faults are oblique normal indicating that the area was subject to transtensional tectonics the faults suggest that the compressional tectonic regime was followed by the transtensional one at 2 0 ma in agreement with regional tectonostratigraphic data which indicate that folding ceased at that time present compressional stress followed the transtensional tectonic regime sometime in the late pleistocene transtensional or extensional tectonic zone shifted from the kakegawa area to the offshore region these observations indicate that the state of stress just behind the accretionary prism of the eastern nankai subduction zone has been unstable in the last 2 million years suggesting that the forearc wedge has been at critical state in that gravitational force and basal shear traction on the wedge have been balanced but the forearc tectonics has been susceptible to small perturbations possible factors compatible with the observed stress history include the change of subduction direction of the plate at 1 0 ma and the rapid uplift of central japan thereafter

simulation of tectonic evolution of the kanto basin of japan since 1 ma due to subduction of the pacific and philippine sea plates and the collision of the izu bonin arc abstract the kanto basin the largest lowland in japan developed by flexure as a result of 1 the subduction of the philippine sea phs and the pacific pac plates and 2 the repeated collision of the izu bonin arc fragments with the japanese island arc geomorphological geological and thermochronological data on vertical movements over the last 1 my suggest that subsidence initially affected the entire basin after which the area of subsidence gradually narrowed until finally the basin began to experience uplift in this study we modeled the tectonic evolution of the kanto basin following the method of matsu ura and sato 1989 for a kinematic subduction model with dislocations in order to quantitatively assess the effects of phs and pac subduction we include the steady slip rate deficit permanent locking rate at the plate interface in our model to account for collision process we explore how the latest collision of the izu peninsula block has been affected by a westerly shift in the phs plate motion vector with respect to the eurasian plate thought to have occurred between 1 0 0 5 ma using long term vertical deformation data to constrain extent of the locked zone on the plate interface we evaluated the change in vertical deformation rate for two scenarios 1 a synchronous shift in the orientation of the locked zone as phs plate motion shifts and 2 a delayed shift in the orientation of the locked zone following the shift in plate motion observed changes in the uplift subsidence pattern are better explained by scenario 2 suggesting that recent

neogene stress field in sw japan and mechanism of deformation during the sea of japan opening we present new structural data from sw japan and discuss the mechanism of deformation during the opening of the sea of japan we studied the miocene basins at the southern margin of the sea of japan northern coast of sw japan and of the median tectonic line mtl by means of fault slip analysis to determine stress field directions during the opening of the sea of japan in early and middle miocene time on the southern margin the stress field was extensional or transtensional with a nw se trending extension direction the mtl was a normal fault in shikoku during early middle miocene and probably a normal fault with a left lateral component in central japan before having been strained by the collisions of the tanzawa and izu blocks with central japan we combine our results with published data for the sea of japan area and show that early and middle miocene stress field directions are remarkably consistent on the eastern southern and western margins of the sea of japan with hmax trending ne sw and hmin trending nw se the stress regime is transpressional or transtensional on the eastern and western margin and almost purely extensional on the southern margin this stress field distribution is in agreement with the model of the opening of the sea of japan in an extensional transfer zone between two n s right lateral strike slip zones proposed by jolivet et al 1991 analogue modelings and kinematic reconstructions showed that this model accounts for 20 to 30 of clockwise rotation in sw japan while paleomagnetic rotations reached 40 to 50 during the opening we present structural data showing that sw japan was strained during the rotation and did not behave as a rigid block our observations suggest that rotations of small blocks may have occurred we propose that sw japan has been sheared between the right lateral strike slip zones which bound the sea of japan and that the missing 10 to 30 of rotation are to be found in its internal deformation in our model the mtl is a normal fault with a strike slip component rotating in a right lateral shear zone

semi periodic sequences and extraneous events in earthquake forecasting ii application forecasts for japan and venezuela in order to analyze observed seismicity in central japan and venezuela we applied a new method to identify semi periodic sequences in the occurrence times of large earthquakes which allows for the presence of multiple periodic sequences and or events not belonging to any sequence in the time series we also explored a scheme for diminishing the effects of a sharp cutoff magnitude threshold in selecting the events to analyze a main four event sequence with probability p c 0 991 of not having occurred by chance was identified for earthquakes with m 8 0 in central japan venezuela is divided from west to east into four regions for each of these the magnitude ranges and identified sequences are as follows region 1 m 6 0 a six event sequence with p c 0 923 and a four event sequence with p c 0 706 region 2 m 5 6 a five event sequence with p c 0 942 region 3 m 5 6 a four event sequence with p c 0 882 region 4 m 6 0 a five event sequence with p c 0 891 forecasts are made and evaluated for all identified sequences having four or more events and probabilities 0 5 the last event of all these sequences was satisfactorily aftcast by previous events whether the identified sequences do in fact correspond to physical processes resulting in semi periodic seismicity is of course an open question but the forecasts properly used may be useful as a factor in seismic hazard estimation

nankai trough and the fossil shikoku ridge results of box 6 kaiko survey abstract seabeam mapping and detailed geophysical surveying have been conducted over the nankai trough where the fossil shikoku ridge is subducted below southwest japan the geometry of the oceanic lithosphere bending under the margin as well as the three dimensional structure of the accretionary prism have thus been determined in detail three 350 trending probably transform faults have been identified in the area of the survey they do not extend further south and appear to be limited to the last phase of spreading within the shikoku basin probably between 15 and 12 ma this last phase of spreading would then have been accompanied by a sharp change in spreading direction from east west to n 350 the two eastern transform faults limit a zone of reduced nankai trench fill of turbidites opposite to the tosa bae embayment this observation suggests that the tosa bae embayment actually results from this reduced supply of trench fill to the imbricate thrusting process the accretionary prism can be divided into three different tectonic provinces separated by continuous mappable thrusts the lower and upper main thrusts surface shortening is limited to the lower accretionary prism south of the upper main thrust umt whereas uplift with possible extension characterizes the prism above the umt deformation due to the relative plate motion mostly affects the lower accretionary prism south of the umt

distribution of shortening landward and oceanward of the eastern nankai trough due to the izu ogasawara ridge collision abstract the convergence of the philippine sea and eurasia plates is accommodated primarily by plate subduction along the nankai trough west of about 137 3 e but additionally by widely distributed onshore and offshore deformation to the east this zone with a total dimension of 200 km parallel to the nankai trough by 350 km has developed as one highly coupled region since the collision of the izu peninsula with mainland japan about 1 2 m y ago the onshore faulting in central japan is consistent with a wnw ese direction of maximum horizontal stress while the oceanward deformation is driven by diffuse nw se shortening of the izu ogasawara volcanic ridge which produces a kinematic discontinuity with respect to the philippine sea oceanic crust to its west the strain release associated with the 1891 nobi earthquake and the onshore deformation as a whole suggests that the philippine sea eurasia convergence is partially accommodated by onshore shortening between the tsuruga bay ise bay tectonic line and the izu peninsula

submarine eruption fed and resedimented pumice rich facies the dogashima formation izu peninsula japan in the izu peninsula japan the pliocene pumice rich dogashima formation 4 55 0 87 ma displays exceptional preservation of volcaniclastic facies that were erupted and deposited in a below wave base marine setting it includes high concentration density current deposits that contain clasts that were emplaced hot indicating an eruption fed origin the lower part of the dogashima 2 unit consists of a very thick sequence 1 m pumice and ash rich bed overlying the breccias may be derived from delayed settling of pyroclasts from suspension in dogashima 1 and 2 thick cross and planar bedded facies composed of sub rounded pumice clasts are intercalated with eruption fed facies implying inter eruptive mass wasting on the flank of a submarine volcano and reworking and resedimentation by high energy tractional currents in a below wave base environment

some remarks on the stability of sphene in diagenetic environments abstract equilibrium diagrams showing the chemical stability of sphene and the assemblage rutile quartz calcite in the range 25 300 c and 1 2000 bar were constructed from mineral reactions that likely occur in diagenetic environments the results reveal that stability of sphene depends on activities of hco 3 ca 2 sio 2 aq and h given appropriate conditions sphene is predicted to be stable even in surficial environments

tectonic evolution along the northernmost border of the philippine sea plate since about 1 ma abstract recent structural tephrochronologic and magnetostratigraphic studies conducted along the northernmost border of the philippine sea phs plate enable us to reconstruct the precise tectonic evolution along the convergent boundary between the phs plate and the northeast japan nej plate or the north american nam plate since about 1 ma the authors of the present study split the tectonic evolution into five stages and present the characteristics of each stage a plate tectonic interpretation is proposed based upon the tectonic evolution with special reference to the mode of convergent motion in brief our interpretations are as follows the relative motion between the phs and the nej plates was not recognized geologically within the area studied from about 1 0 to 0 9 ma stage 1 suggesting either none or small influence from the coupling between the two plates during that period of time convergence between the phs and the nej plates was possibly in n s direction from 0 9 to 0 5 ma stage ii and probably north northwestward since 0 5 ma stages iii to v the mode of the convergent motion was that of buoyant subduction in stages ii and iii the mode changed gradually from buoyant subduction during stage iv to collision in stage v 0 07 ma to the present

tectonic record of convergence changes in a collision area the boso and miura peninsulas central japan abstract late cenozoic formations in boso and miura have been affected by several tectonic events tectonic analysis enables us to reconstruct six different paleostress types 1 early extension affecting the oligocene early miocene mineoka group 2 and 3 relatively minor compressional and extensional events probably early pliocene in age 4 major nne ssw compression dominating prior to 2 or 3 ma ago and 5 and 6 more recent major nnw sse compression to the west and wnw ese extension to the east both types affecting the pleistocene and prevailing since 1 2 ma ago the counterclockwise change from nne ssw to nnw sse compression is not accurately dated but very likely occurred between 2 and 3 ma ago it is compared to similar evolutions in other areas of the izu collision zone we conclude that it corresponds to a major counterclockwise change in the direction of plate convergence from sse nnw to se nw the relationships between the directions of convergence and the distributions of plio quaternary compressional paleostresses in and around the collision zone are described through a simple analogy for the two stages of plio quaternary collision this counterclockwise change in stress fields and relative motions also described in the taiwan collision zone along the same philippine sea plate eurasia boundary is interpreted as a major event at the scale of the plate the possible significances of the other paleostress types identified in boso are discussed we conclude that tectonic analysis in and along collision boundaries provides a key for understanding kinematic evolution

strain modes within the forearc arc and back arc domains in the izu japan and taiwan arc continent collisional settings in this study i examine the strain modes of the forearc arc and back arc domains in arc continent collisional settings leading to arc material subduction delamination and or accretion the study focusses on two well documented colliding island arcs the izu bonin mariana ibm arc in japan and the luzon arc in taiwan both carried by the philippine sea plate firstly there is a body of evidence that both the ibm and the luzon arcs were built on the same late jurassic to early cretaceous proto philippine sea plate crust their internal structure is thus more heterogeneous than expected from paleogene or neogene supposedly intra oceanic island arcs secondly those arc systems and proximal back arcs have similar seismic characteristics attesting either for the presence of a middle crust with continental velocities and or serpentinized uppermost mantle that facilitate crustal shortening slivering and subsequent decoupling from the rest of the subducting plate it is shown that the proximal back arc domain called rear arc in case of paleoarc activity overlying the mantle wedge and the subducting slab may lose its strength if slab derived hydration occur decoupling then occurs below the moho arc delamination likely occurs in mid crustal levels because middle crust heated by nearby magmatism becomes weak accretion of arc material onto the upper plate depends on the characteristics of the arc itself and the geodynamic configuration most of the accreted material is probably underplated rather than frontally accreted

collision of the izu block with central japan during the quaternary and geological evolution of the ashigara area abstract the ashigara area is located in the northern part of the izu collision zone and has been suspected as a possible location of the boundary between the eurasian and the philippine sea plates in order to analyse its evolution during the collision we have determined the age of the ashigara group and studied its depositional environments and its deformation the age of the ashigara group is early to middle pleistocene the lower part was deposited in a 1000 2000 m deep trough which lay between the izu peninsula and central japan at that time the sedimentary facies show subsequent upward coarsening and shallowing the beginning of the collision resulted in the uplift of the tanzawa mountains which are the source of the voluminous conglomerates deposited in this area soon after the deposition of the upper part of the ashigara group this area was over thrust by the tunzawa mountains along the kannawa fault and strongly folded under a nw se compressional stress field then about 0 3 ma ago the direction of compression drastically changed to n s or ne sw when the colliding izu peninsula was finally locked against central japan

long distance magma transport from arc volcanoes inferred from the submarine eruptive fissures offshore izu oshima volcano izu bonin arc abstract long distance lateral magma transport away from volcanic centers in island arcs is emerging as a common phenomenon where the regional stress regime is favorable it should also be recognized as an important factor in the construction and growth of island arcs and a potential trigger for devastating eruptions in this contribution we report on recent investigations into the magma dynamics of izu oshima volcano an active basaltic volcano with an extensive fissure system izu oshima is flanked by numerous subparallel nw se trending submarine ridges extending up to 22 km to the nw and the se from the central vent during a recent submersible survey we have identified that these ridges are fissures which erupted basaltic spatter and lava flows furthermore lavas are petrographically similar along each ridge while there are noticeable differences between ridges the subparallel ridges are observed to transect a series of seamounts the izu tobu monogenetic volcanoes which are dispersed across this area of the rear arc however there are consistent petrographic and chemical differences between these seamounts and the ridges indicating that they have different magma sources yet they are essentially bounding each other in dive tracks the most appropriate scenario for their development is one where the izu tobu volcanoes are fed by an in situ underlying source while the nw se ridges are fed by lateral magma transport from izu oshima magma erupted from each ridge is of a consistent geochemistry along its length but has experienced crystal fractionation and some plagioclase accumulation compositions of the ridges are also very similar to lavas from the subaerial cones that can be traced down the flanks of izu oshima this implies that pairs of subaerial cones and submarine ridges represent the locus of magma transport events away from the storage system beneath izu oshima hence magma from this crustal reservoir moved upward to feed the on edifice cones as well as laterally to supply submarine fissures and dykes to a distance of 22 km

the 1923 kanto earthquake reevaluated using a newly augmented geodetic data set 1 this study revisits the mechanism of the 1923 ms 7 9 kanto earthquake in japan we derive a new source model and use it to assess quantitative and qualitative aspects of the accommodation of plate motion in the kanto region we use a new geodetic data set that consists of displacements from leveling and angle changes from triangulation measurements obtained in surveys between 1883 and 1927 two unique aspects of our analysis are the inclusion of a large number of second order triangulation measurements and the application of a correction to remove interseismic deformation the geometry of the fault planes is adopted from a recent seismic reflection study of the kanto region we evaluate the minimum complexity necessary in the model to fit the data optimally our final uniform slip elastic dislocation model consists of two adjacent 20 dipping low angle planes accommodating reverse dextral slip of 6 0 m on the larger eastern plane and 9 5 m on the smaller western plane with azimuths of 163 and 121 respectively the earthquake was located in the sagami trough where the philippine sea plate subducts under honshu compared to the highly oblique angle of plate convergence the coseismic slip on the large fault plane has a more orthogonal orientation to the strike of the plate boundary suggesting that slip partitioning plays a role in accommodation of plate motion what other structure is involved in the partitioning is unclear uplift records of marine coastal terraces in sagami bay document 7500 years of earthquake activity and predict average recurrence intervals of 400 years for events with vertical displacement profiles similar to those of the 1923 earthquake this means that the average slip deficit per recurrence interval is 50 of the relative plate convergence these findings of plate motion partitioning and slip deficit lead us to suggest that instead of a simple recurrence model with characteristic earthquakes additional mechanisms are necessary to describe the accommodation of deformation in the kanto region so far obvious candidates for these alternative mechanisms have not been discovered

collision and subduction structure of the izu bonin arc central japan revealed by refraction wide angle reflection analysis abstract since the middle miocene the izu bonin arc iba has been colliding from the south with the honshu arc in central japan associated with subduction of the philippine sea plate psp this process is responsible for forming a complex crustal structure called the izu collision zone icz to obtain direct evidence of the deep structure dominated by collision and subduction an intensive seismic experiment using explosive and vibroseis sources was conducted in the eastern part of the icz cmp reflection and refraction wide angle reflection data were acquired on a 130 km long seismic line crossing the kanto mountains km the tonoki aikawa tectonic line tatl and the tanzawa mountains tm from north to south the tatl is considered to be a collision boundary separating the honshu arc km from the iba tm the structural model constructed by refraction tomography and forward ray tracing shows remarkable lateral velocity variation across the tatl and some clear reflectors in the deep crust a north dipping reflector beneath the km was interpreted to be the deeper extension of the tatl from the geometry of reflectors we interpret the tanzawa block is delaminated from the subducting slab due to the collision to form a wedge like body thrusting between the upper and lower crust of honshu the tm itself shows a velocity structure almost consistent with the upper part of the crust of the izu intraoceanic arc south of our study area the relocated hypocenter distribution using our velocity model shows high seismic activity concentrated around the collision boundary which is in a marked contrast of low seismicity within the tanzawa block these features of seismicity are strongly dominated by the collision of the iba with the honshu arc

zenisu ridge a deep intraoceanic thrust related to subduction off southwest japan abstract during the french japanese kaiko project seabeam seismic and submersible observations were made in the eastern part of the nankai subduction zone close to the area of collision between the izu bonin island arc and the japan margin the most prominent feature is the zenisu ridge an elongated relief of the philippine sea plate running parallel to the trench magnetic anomalies indicate that the crust of the zenisu ridge is a part of the shikoku oceanic basin formed in the early miocene 23 ma ago and presumably uplifted at a later stage structural analysis of seismic data and diving observations lead us to interpret the superficial structure as being due to compressive tectonics mapping the acoustic basement reveals that the southeastern flank of the ridge is bounded by a double thrust both segments being of equal magnitude vertical offset about 1 to 1 5 km geophysical data support the hypothesis of a main thrust cutting through most of the lithosphere and flattening at depth the overall structure of the surrounding area reveals a compressive deformation zone widening toward the east the magnitude of the compressive deformation decreasing westward as well as southward of the zenisu ridge

accretion of japanese island arcs and implications for the origin of archean greenstone belts the present day region of arc arc collision between the izu bonin arc and mainland japan honshu arc in the western pacific called the izu collision zone or icz provides a useful kinematic model for the development of many archean greenstone belts the compositional assemblage thermal structure and structural style of the crust in the icz are very similar to those inferred for many archean greenstone belts in both cases thick young and flaky arc crust with a high heat flux is in collision with another crustal block the icz has resulted in the imbrication of very thick slices of crust and associated sediments juxtaposed against older accretionary prisms many archean greenstone belts show similar geological features and we suggest that the incremental imbrication of the crust was a common tectonic style in some archean convergent margins

deep seismic imaging of the eastern nankai trough japan from multifold ocean bottom seismometer data by combined travel time tomography and prestack depth migration 1 the easternmost segment of the nankai trough remained unruptured during the 1944 and 1946 earthquakes that affected the rest of the trench it is therefore a zone of seismic gap that also undergoes a peculiar tectonic regime due to its proximity with the izu collision zone we investigate this area with densely sampled active seismic data one hundred ocean bottom seismometers spaced only 1 km apart were deployed along a line perpendicular to the trough a two step tomographic approach designed for this type of acquisition is applied first a long wavelength velocity model is built by a linearized inversion of first arrival travel times its resolution is assessed and its reliability is confirmed through a semiglobal approach in which a large range of starting models is tested second this tomographic model is used to exploit late reflected arrivals with a prestack depth migration the result of which is validated by classically modeling refraction and reflection travel times in the derived structural model thus both velocities and discontinuities are mapped in our model with a minimum of data interpretation and a priori knowledge our results bring new insights on the deep crustal zones of the margin and its evolution the existence of presently inactivated major thrusts is very clearly evidenced in the backstop a general timing for its evolution and deformation is proposed farther seaward the presence of a subducted ridge beneath the accretionary wedge is confirmed its compressive structure and origin are demonstrated it exhibits a thickened lower crust which nature is discussed besides strong analogies between this paleo zenisu subducting ridge and the zenisu ridge observed farther south are also emphasized bringing evidences for a steady state crustal deformation pattern