structure and holocene rupture of the morelia fault trans mexican volcanic belt and their significance for seismic hazard assessment abstract the trans mexican volcanic belt is an active volcanic arc being deformed by an intra arc extensional fault network the morelia fault with a length of 14 km and a maximum throw of 260 m is a major normal fault in the central part of this volcanic arc and forms part of a 36 km long fault array slickenlines measured on the primary fault surface indicate north south extensional dip slip with a minor left lateral component in a well exposed cross section in the hanging wall of the morelia fault some few meters from the master fault trace two buried soil layers containing ceramic shards and stone tools yielded radiocarbon ages of a d 420 660 prehispanic classic period chupicuaro culture and a d 1290 1435 postclassic period respectively the underlying bedrock atecuaro ignimbrite is cut in a staircase pattern by three synthetic secondary normal faults that delimit two colluvial wedges three slip events with displacements ranging between 46 and 185 cm average 109 cm can be inferred from this section the latest of these events must have occurred after a d 1290 1435 during the postclassic period or possibly during early novohispanic times the morelia fault which is capable of generating m w 6 7 earthquakes poses a major seismic ground shaking hazard and locally a surface fault rupture hazard within the metropolitan morelia region with a population of more than 900 000 habitants the foci of a sequence of seven small 2007 earthquakes on the morelia fault were located below the center of town at a depth of 11 km and the fault trace crosses urban residential areas moreover the town repeatedly experienced severe ground shaking during historical earthquakes the best documented is the 19 june 1858 earthquake which caused morelia extensive damage corresponding to a macroseismic intensity of viii ix on the modified mercalli scale online material radiocarbon analysis reports local 1894 1904 and 1909 1911 earthquake catalogs

new insights on 3 d plates interaction near taiwan from tomography and tectonic implications abstract recent tomographic results are used to trace the south china sea and philippine sea subducting slabs south and northeast of taiwan along the manila and ryukyu trenches respectively in particular the 3 d plates interaction beneath taiwan is discussed based on a close up view of the tomographic sections together with earthquake hypocenters distribution our study indicates that 1 the east dipping south china sea slab can be followed to the north up to the latitude of hualien 2 the eurasian plate subducts beneath most part of the taiwan island down to the 670 km depth discontinuity 3 the north dipping philippine sea slab can be followed slightly west of the longitude of hualien both plates thus interact beneath northern taiwan where the arc continent collision is paroxysmal 4 slab detachment is envisaged at the northern edge of the subducted eurasian plate beneath the coastal range of taiwan which may facilitate the northwestward motion of the philippine sea plate with respect to eurasia slabs geometries obtained from tomographic sections allow us to reconstruct the late neogene plate kinematics and dynamics in this region our main conclusions are 1 the size of the original south china sea was about twice its present size 2 the subducted part of the west philippine basin i e the largest oceanic basin of the philippine sea plate extends only 400 km north of the ryukyu trench 3 slab detachment might have occurred 3 5 my ago beneath the central and northern ryukyu arc along a weak zone that is aligned with the gagua ridge an ancient plate boundary 4 the ryukyu trench has propagated westward from 126 e of longitude southeast of miyako island to the locus of the present arc continent collision along a major lithospheric tear that cut through the continent ocean boundary first and then through the continental lithosphere as a consequence the southern ryukyu margin developed progressively from east to west as a subduction zone during the last 8 my it has been elaborated onto a passive margin which corresponded to the edge of the eurasian continental shelf bordering the south china sea prior to pliocene 5 the southern okinawa trough rifted during the last 2 1 ma following the propagation of the subduction zone 6 subduction of the eurasian margin probably stopped recently beneath northern taiwan and a flip of subduction from west to east is being initiated near hualien

crustal deformation at the southernmost part of the ryukyu subduction east taiwan as revealed by new marine seismic experiments abstract the southernmost part of the ryukyu subduction where the philippine sea plate is subducting under the eurasian plate is known to be a very seismically active region of transition from a north dipping subduction along the ryukyu subduction to an se nw collision along the taiwanese orogenic wedge in this paper we will focus on the ryukyu forearc area close to taiwan where the deformation is paroxysmal in order to decipher the nature of the seismic deformation in this region a three month passive experiment combining 22 ocean bottom seismometers and 51 onland stations has been led starting from an a priori heterogeneous model we have obtained 801 well located earthquake hypocenters a precise p wave tomography model and 14 focal mechanisms the seismicity along the ryukyu forearc is mainly located not only in the vicinity of the interplate seismogenic zone isz but also within both the subducting psp and the overriding plate seismicity within the upper plate is essentially localized east of nanao basin where nw se extension occurs and northwest of the hoping basin where strike slip dominates as revealed by both the p wave velocity structure and the newly derived seismicity we argue that a sub vertical step offsetting the subducting psp around 10 km may support the presence of a trench parallel tear the psp also undergoes extension in its upper part that is probably caused by buckling and slab pull the p wave velocity structure reveals three other major features 1 a continuity between the central range and the ryukyu arc with a shallower moho 30 km depth between 122 3 n and 122 5 n along the ryukyu arc 2 high p wave velocities along the eastern side of the central range and 3 two bodies with similar high crustal velocities 6 5 7 0 km s at 12 18 km depths embedded within the ryukyu arc basement just north of hoping basin and north of the nanao basin

three dimensional seismic tomography from p wave and s wave microearthquake travel times and rock physics characterization of the campi flegrei caldera 1 the campi flegrei cf caldera experiences dramatic ground deformations unsurpassed anywhere in the world the source responsible for this phenomenon is still debated with the aim of exploring the structure of the caldera as well as the role of hydrothermal fluids on velocity changes a multidisciplinary approach dealing with three dimensional delay time tomography and rock physics characterization has been followed selected seismic data were modeled by using a tomographic method based on an accurate finite difference travel time computation which simultaneously inverts p wave and s wave first arrival times for both velocity model parameters and hypocenter locations the retrieved p wave and s wave velocity images as well as the deduced v p v s images were interpreted by using experimental measurements of rock physical properties on cf samples to take into account steam water phase transition mechanisms affecting p wave and s wave velocities also modeling of petrophysical properties for site relevant rocks constrains the role of overpressured fluids on velocity a flat and low v p v s anomaly lies at 4 km depth under the city of pozzuoli earthquakes are located at the top of this anomaly this anomaly implies the presence of fractured overpressured gas bearing formations and excludes the presence of melted rocks at shallow depth a high v p v s anomaly located at 1 km suggests the presence of rocks containing fluids in the liquid phase finally maps of the v p v s product show a high v p v s horseshoe shaped anomaly located at 2 km depth it is consistent with gravity data and well data and might constitute the on land remainder of the caldera rim detected below sea level by tomography using active source seismic data

the largest instrumentally recorded earthquake in taiwan revised location and magnitude and tectonic significance of the 1920 event summary the ryukyu subduction is known to generate very few earthquakes in its central segment contrarily to its two extremities we focus in this study on the southernmost part of the ryukyu subduction zone offshore east taiwan our first task was to build a homogeneous earthquake cataloguefortheperiod1900 2007 thenewcatalogueprovideshomogenized m

seismotectonics of northeastern taiwan kinematics of the transition from waning collision to subduction and postcollisional extension 1 the northeastern region of taiwan is one of the few places on earth that experienced a transition from collision to subduction and back arc opening in a young orogenic belt this provides us a rare opportunity to study the structural characteristics in such a transition we attempt to analyze the seismicity and stress patterns in this transitional region using data obtained from a dense and high quality seismic network our results show that from south to north fault types change from thrust to strike slip then to normal faulting the transitional domain between ilan and hualien is dominated by left lateral strike slip faulting with a nnw sse minimum principal stress axis consistent with the opening of the okinawa trough to the north the ilan plain is formed by structural activities of a series of normal fault systems to the south the spatial distribution of an earthquake cluster north of taiwan s coastal range appears to parallel the subducting interface between the philippine sea and eurasian plates this implies the northern extension of the coastal range has subducted consequently the opening of the okinawa trough becomes an important tectonic driving force in northeastern taiwan the westernmost ryukyu arc is pushed by the opening okinawa trough and moves southward the northern part of taiwan s central range moves with the arc to bend southeastward and the ilan plain forms in the space north of the range the bending of the central range belt results in bedding plane slip with left lateral strike slip earthquakes in the transitional domain to accommodate the deformation

the taiwan ryukyu subduction collision complex folding of a viscoelastic slab and the double seismic zone 1 the termination of the ryukyu trench against eurasia and the oblique subduction of the philippine sea plate create a subduction collision complex offshore taiwan which has not previously been elucidated in detail we combine traveltime data from the seismic networks in taiwan and japan to better illuminate how the subducting ryukyu slab deforms in this subduction collision zone more than 5000 events recorded by both networks were relocated with the double difference method using an optimal regional one dimensional velocity model the offshore seismicity indicates that the double seismic zone with a gap of 15 20 km exists in the subducting slab in the depth range of 40 80 km focal mechanisms suggest that the double seismic zone is caused by east west compression resulting from oblique convergence the improved hypocentral locations for the first time reveal folding of the slab into a horizontal curvature larger in magnitude than and opposite in sign to that of the ryukyu trench in the depth range 50 100 km the anomalous curvature together with the focal mechanisms suggests that the slab folds against the eurasian lithosphere and that this deformation cannot be fully elastic we model this deformation mode as the developing instability of a viscoelastic maxwell layer embedded in a viscous medium the characteristic wavelength of the instability i e 250 km is consistent with folding of a slab whose viscosity is 100 times higher than that of the surrounding mantle for an along strike elastic membrane strain as small as 0 01 or more than 3 orders of magnitude higher if 5 elastic strain is allowed

seismogenic patterns in the taiwan region insights from source parameter inversion of bats data abstract we systematically invert source parameters of 96 earthquakes that occurred in the taiwan region between july 1995 and june 1998 using waveforms recorded by the newly established broadband array in taiwan for seismology bats the results are utilized to delineate the seismogenic patterns associated with the regional tectonic processes between the eurasia and philippine sea plates in general high seismicity is observed at five locations including the nan ao basin 24 n 122 4 e near the hualien area 24 2 n 121 7 e east of the longitudinal valley and within the philippine sea plate the forearc region to the west of lanshu and the okinawa trough events with focal depths less than 25 km scatter across a wide region from the taiwan strait to the ryukyu and luzon arcs whereas events between 25 and 50 km concentrate offshore east of taiwan most earthquakes that occurred beneath the nan ao basin are consistent with the northward subduction of the philippine sea plate along the plate interface events offshore east of the longitudinal valley and within the philippine sea plate are dominated by a compressional regime along the nw nww directions whereas the lanshu lutao forearc is characterized by e w compression normal faults showing n s extension are observed in the okinawa trough where we observed a successive rotation of t axes from n s to nw se as the epicenters approach taiwan indicating a possible interaction between the extensional and compressional strain regimes there most events between 50 and 65 km depth within the subducted philippine sea plate beneath ryukyu are in downdip extension with lateral compression approximately parallel to the strike of the slab the depth where the state of strain switches from downdip extension to downdip compression is constrained at 110 125 km for the southernmost ryukyu slab based on our results we propose that the subducted philippine sea plate beneath ne taiwan plays a significant role in the overall orogenic process of taiwan the lithospheric collision in taiwan should consist of two major components one is the well recognized arc continent collision that dominates the central and southern taiwan and another the slab continent collision that reigns in northern taiwan a cluster of earthquakes near hualien suggests that an incipient west or northwestward subduction of the philippine sea plate is currently taking place there alternatively they can be interpreted to suggest that the steeply east dipping longitudinal valley fault may have transformed near hualien into a diffuse thrust zone which eventually connects itself to the plate interplate beneath the southernmost ryukyu arc earthquakes beneath taiwan and the taiwan strait are compatible with the overall collision convergence and lateral extrusion nonetheless the lateral extrusion is probably only a secondary feature in the tectonic processes of taiwan

numerical study on signatures of atmospheric convective cells in radar images of the ocean current and wind variations at the ocean surface can give rise to a modulation of the sea surface roughness and thus become visible in radar images the discrimination between radar signatures of oceanic and atmospheric phenomena can be quite difficult since signatures of different origin can have very similar shapes and magnitudes and are often superimposed upon each other in this work we employ a numerical radar imaging model for an investigation of typical properties of radar signatures of atmospheric convective cells and of theoretical differences between such atmospherically induced radar signatures and those of oceanic phenomena we show that main characteristics of observed multifrequency multipolarization radar signatures of atmospheric convective cells over the gulf stream are reproduced quite well by the proposed model this encourages us to vary wind and radar parameters systematically in order to get a general overview of the dependency of atmospherically induced radar signatures on these parameters finally we compare typical characteristics of radar signatures of atmospheric and oceanic phenomena and we present simulated radar images of a scenario of superimposed atmospheric convective cells and oceanic internal waves we show that the proposed model supports the experimental finding that radar signatures of oceanic phenomena are stronger at horizontal hh than at vertical vv polarization while atmospherically induced radar signatures are better visible at vv polarization

seismogenic faults along the major suture of the plate boundary deduced by dislocation modeling of coseismic displacements of the 1951 m7 3 hualien taitung earthquake sequence in eastern taiwan abstract research at plate boundaries generally focuses on the kinematics of plate motions the genesis of large earthquake and natural resources however often the present state of knowledge is still insufficient to reveal the details of such dynamic systems to define in more detail the dynamics of a major plate boundary fault the coastal range fault eastern taiwan we use seismologic and geodetic data to unravel its subsurface geometry and relevant fault behavior by recomputing the archive triangulation datasets 1917 1978 and using a dislocation algorithm we determine that it consists of three segments our model results are consistent with the observation of the 1951 m7 3 hualien taitung earthquake sequence that the southern two segments are both oblique slip faults with significant thrust components while the northern segment is a pure strike slip fault according to our model each segment has the potential to produce an earthquake larger than m7 however based on gps observations and seismic records slip on the southern segment may take the form of aseismic creep and multiple moderate earthquakes rather than large events with long recurrence interval as occur on the two more northerly segments with regard to the central segment the epicenter distribution of catalogued earthquakes occurring after 1951 reveals the presence of a seismic gap indicating that slip on this segment is predominantly coseismic with a correspondingly higher potential for large earthquakes in the future

back arc extension in the okinawa trough the okinawa trough lying to the east of china is a back arc basin formed by extension within continental lithosphere behind the ryukyu trench arc system middle to late miocene uplift associated with normal faulting of the initially adjacent ryukyu nonvolcanic arc and the taiwan sinzi folded belt corresponds to the first rifting phase the timing of rifting is supported by the presence of marine sediments of corresponding age drilled in the northern okinawa trough the rifting occurred after a major early miocene change in the motion of the philippine plate with respect to eurasia and ceased during the pliocene a second rifting phase started about 2 m y ago at the plio pleistocene boundary and has continued until the present time it has proceeded to a more advanced stage in the middle and southern okinawa trough than it has farther north detailed bathymetric sea beam seismic reflection and magnetics data collected during the pop 1 cruise of the r v jean charcot reveal the principal features of the extensional processes the back arc spreading phase started very recently in the southern and middle okinawa trough as exemplified by several en echelon and in some cases overlapping active central graben oriented n70 e n80 e some of these depressions are intruded by volcanic ridges of fresh back arc basalt with associated large magnetic anomalies transform faults between these en echelon active rifts are not obvious we suggest that the major part of the southern okinawa trough is underlain by a thinned continental crust and that except for the system of en echelon rifts of the southern okinawa trough the back arc basin oceanic domain is limited to a width of a few tens of kilometers or less in the axial portion of the trough the system of axial back arc volcanic ridges that occur in the rifts ends at the latitude of okinawa island whereas active volcanoes in the ryukyu arc occur only north of okinawa island we refer to this transition between active arc and back arc volcanism as the volcanic arc rift migration phenomenon vamp globally back arc volcanism propagated from the southern okinawa trough to the okinawa vamp area rifting continues to occur in the northern okinawa trough but is not yet accompanied by associated volcanism the okinawa vamp area is characterized by a series of parallel basaltic ridges oriented n75 e with associated linear magnetic anomalies characteristic of dyke intrusions we suggest that the formation of the back arc oceanic domain took place along the axial back arc extensional zone trending n75 e and that this zone presently ends at the southern extremity of the active volcanic chain the initial phase of formation of back arc basin oceanic crust is non steady state and is characterized by the lack of a developed fracture zone pattern the termination of the vamp area in the direction of the volcanic zone of the arc is consistent with the suggestion of molnar and atwater that the volcanic arc is a fundamental line of weakness which determines where initial back arc spreading occurs apparently back arc extension initially occurred within the continental lithosphere located westward of the ryukyu arc along its whole length but the subsequent back arc volcanism was initiated in the southernmost portion of the region and then moved northward this migration was accompanied by the shutting down of volcanic activity along the abandoned portions of the arc it is this transfer of volcanism that we call the vamp process thus arc and back arc basin volcanism seem to be associated in such a manner that spreading tends to migrate simultaneously with a cessation in volcanic activity along the arc this interplay of arc and back arc activity is probably linked to changes in the parameters of plate convergence since the plate motion in the phillippine sea is oblique to the trench at least in the southern part of the okinawa trough we suggest that the oblique resisting force applied to the edge of the overriding plate engenders the development of en echelon extensional features within and behind the arc the motion of the ryukyu platelet with respect to eurasia is consequently an extensional strike slip motion trending roughly parallel to the okinawa trough

new insights on the interseismic active deformation along the north ecuadorian south colombian nesc margin 1 the north ecuadorian south colombian subduction zone was the site of the 1906 mw 8 8 megathrust earthquake this main shock was followed by three large events in 1942 1958 and 1979 whose rupture zones were located within the 500 km long 1906 rupture area acombined onshore andoffshore temporary seismic network covering from thetrench to the andes was deployed during 3 months in the area of large earthquakes in order to obtain a detailed knowledge of the seismic background activity resulting earthquakes location and mechanisms bring new insights on interseismic active deformation distribution in the three main tectonic units of the margin namely the interplate seismogenic zone the fore arc region which is part of the north andean block and the downgoing oceanic nazca plate the interplate seismic activity presents along strike variations suggesting that the seismicity and the associated stress buildup along the plate interface depend on the time elapsed since the last large earthquakes according to our results the updip and downdip limits of the seismogenic zone appear to be located at 12 and 30 km depth respectively shallow to intermediate depth seismicity indicates a slab dip angle of 25 north of the carnegie ridge the wadati benioff plane is defined beneath the fore arc down to 100 km depth facing the ridge the wadati benioff plane extends beneath the andes down to 140kmdepth thisobservationconflictswiththehypothesisofthepresenceofaflatslabat a depth of 100 km facing the ridge in the overlying fore arc region the crustal seismicity occurs down to 40 km depth and is mainly concentrated in a roughly nw se 100 km wide stripe stretching from the coast at about 1 n to the andes the location of this active deformation stripe coincides with observed tectonic segmentation of the coastal domain as evidenced by the presence of an uplifting segment to the south and a subsiding segment to the north of the stripe it also corresponds to a 30 change in the trend of the andes suggesting that the curvature of the volcanic arc might play an important role in the deformation of the fore arc region

improved seismic velocity reference model from local earthquake data in northwestern italy resolution and reliability estimates of results obtained by seismic tomography strongly depend on the reference model inadequate initial reference models may severely distort tomographic images or introduce artefacts that lead to misinterpretations of the results reference models are usually obtained by means of a priori near surface geological information or by geophysical information derived by controlled source seismology starting from the idea that a reference model must approximate the weighted average of data selected for the three dimensional 3d inversion one dimensional 1d model for northwestern italy is derived that is able to minimize mean of rms of a set of well locatable earthquakes by computing a solution of the coupled hypocentre 1d velocity problem such a model termed the minimum 1d model can be used both as an initial reference model for 3d inversion and as a reference velocity model for high quality routine earthquake location

tropical pacific baroclinic mode contribution and associated long waves for the 1994 1999 period from an assimilation experiment with altimetric data an ocean general circulation model ogcm of the tropical pacific in which combined topex poseidon and ers sea level anomalies are assimilated over january 1994 through july 1999 is used to investigate equatorial wave characteristics during the intense 1997 1999 el nino la nina event near the equator the linear vertical modes are estimated at each grid point of the ogcm simulation with and without assimilation consistently with an increase of the vertical gradient within the thermocline and a rise of the thermocline depth in the eastern basin the assimilation results in an increased contribution of the higher order baroclinic modes in the eastern basin and a decreased contribution of the first baroclinic mode in the western pacific for the zonal current variability for pressure the first baroclinic mode contribution is reduced whereas the higher order modes contribution is weakly impacted kelvin and first meridional rossby waves are then derived for the first two more energetic baroclinic modes in the simulation with assimilation kelvin waves of both modes constructively contribute to the strong warming observed in 1997 with the first second baroclinic mode being more energetic than the second first baroclinic mode in the early mature stage of the warming kelvin waves of both modes reflect as first meridional rossby waves at the eastern boundary reflection efficiency of 95 and contribute to push back the warm pool westward the reversal of the warming is apparently initiated by the second baroclinic mode contribution which controls the position of the 28 c isotherm in the surface layer in the far eastern pacific from january 1998 at the western boundary reflections of rossby waves take place for both modes with an estimated total efficiency of 50 at 165 e this suggests that in our model the delayed oscillator theory is not applicable for explaining the reversal from warm to cold conditions in 1998 while the zonal advective feedback was at work more generally the study suggests that it is necessary to take into account the vertical structure of the ocean when interpreting altimetric data which can be done through an assimilation experiment

subducting oceanic high causes compressional faulting in southernmost ryukyu forearc as revealed by hypocentral determinations of earthquakes and reflection refraction seismic data absolute earthquake hypocenter locations have been determined in the area offshore eastern taiwan at the southernmost ryukyu subduction zone location process is run within a 3d velocity model by combining the taiwanese and neighboring japanese networks and using the 3d maxi technique the study focuses on the most active seismic cluster in the taiwan region that occurs in the forearc domain offshore eastern taiwan earthquakes distribute mainly along 2 active planes the first one aligns along the subduction interface and the second one shallower affects the overriding margin focal mechanisms within the shallow group indicate that nodal planes are either compatible with high angle back thrusts or low angle thrusts the active seismic deformation exclusively indicates reverse faulting revealing that the forearc basement undergoes trench perpendicular strong compression by integrating the seismological image into the regional context we favor the hypothesis in which the dense seismicity occurring offshore marks the activity of en echelon high angle reverse faults accommodating the uplift of a broken piece of ryukyu arc basement called hoping basement rise the uplift is inferred to be caused by the subduction of an oceanic relief either exotic block seamount or oceanic crust sliver our favored solution satisfies the narrowness of epicenter s cluster along the hoping canyon and the observation of high angle active faults on seismic lines crossing the area furthermore this solution is compatible with the active uplift of the hoping rise demonstrated from morphological and sedimentological data we do not exclude the branching of the high angle reverse faults system onto a splay fault connected with the subduction interface but further investigations are needed to map precisely the 3d distribution of active faults that break the margin

effects of three dimensional velocity structure on the seismicity of the 1984 morgan hill california aftershock sequence abstract a three dimensional velocity model for the area surrounding the 24 april 1984 morgan hill earthquake has been developed by simultaneously inverting local earthquake and refraction arrival time data this velocity model corresponds well to the surface geology of the region predominantly showing a low velocity region associated with the sedimentary sequence to the southwest of the madrone springs fault this low velocity sequence extends to a depth of approximately 5 km previously constructed geologic cross sections for this region had suggested that these sediments extended to a depth of less than 2 km relocation of 2 179 hypocenters with the three dimensional velocity model did not show great differences when compared to locations done with a one dimensional velocity model with station corrections in both cases the calaveras fault is steep but has a noticeable dip to the northeast and the seismicity does not intersect the surface along the trace of the calaveras fault in the southern half of the rupture zone the greatest differences between the two sets of locations was that the locations using the three dimensional velocity model were deeper and fewer earthquakes were located within the low velocity region the focal mechanisms were also determined for 946 earthquakes using both the one dimensional and three dimensional earth models both earth models yield similar focal mechanisms for these earthquakes

detailed structures of the subducted philippine sea plate beneath northeast taiwan a new type of double seismic zone we studied the detailed structure of the subducted philippine sea plate beneath northeast taiwan where oblique subduction regional collision and back arc opening are all actively occurring simultaneous inversion for velocity structure and earthquake hypocenters are performed using the vast high quality data recorded by the taiwan seismic network we further supplement the inversion results with earthquake source parameters determined from inversion of teleseismic p and sh waveforms a critical step to define the position of plate interface and the state of strain within the subducted slab the most interesting feature is that relocated hypocenters tend to occur along a two layered structure the upper layer is located immediately below the plate interface and extends down to 70 80 km at a dip of 40 50 below approximately 100 km the dip increases dramatically to 70 80 the lower layer commences at 45 50 km and stays approximately parallel to the upper layer with a separation of 15 5 km in between down to 70 80 km below that the separation decreases and the two layers seem to gradually merge into one wadati benioff zone we propose to term the classic double seismic zones observed beneath japan and kuril as type i and that we observed as type ii respectively a global survey indicates that type ii double seismic zones are also observed in new zealand near the southernmost north island cascadia just north of the mendocino triple junction and the cook inlet area of alaska all of them are located near the termini of subducted slabs in a tectonic setting of oblique subduction we interpret the seismogenesis of type ii double seismic zones as reflecting the lateral compressive stress between the subducted plate and the adjacent lithosphere originating from oblique subduction and the downdip extension from slab pulling force the upper seismic layer represents seismicity occurring in the upper crust of a subducted plate and or along the plate interface whereas the lower layer is associated with events in the uppermost mantle

three dimensional p wave velocity structure on the shallow part of the central costa rican pacific margin from local earthquake tomography using off and onshore networks summary the central costa rican pacific margin is characterized by a high seismicity rate coincident with the subduction of rough relief ocean floor and has generated earthquakes with magnitude up to seven in the past we inverted selected p wave traveltimes from earthquakes recorded by a combined on and offshore seismological array deployed during 6 months in the area simultaneously determining hypocentres and the 3 d tomographic velocity structure on the shallow part of the subduction zone 70 km the results reflect the complexity associated to subduction of ocean floor morphology and the transition from normal to thickened subducting oceanic crust the subducting slab is imaged as a high velocity perturbation with a band of low velocities lvb on top encompassing the intraslab seismicity deeper than 30 km the lvb is locally thickened by the presence of at least two subducted seamounts beneath the margin wedge there is a general eastward widening of the lvb over a relatively short distance closely coinciding with the onset of an inverted forearc basin onshore and the appearance of an aseismic low velocity anomaly beneath the inner forearc the latter coincides spatially with an area of the subaerial forearc where differential uplift of blocks has been described suggesting tectonic underplating of eroded material against the base of the upper plate crust alternatively the low velocities could be induced by an accumulation of upward migrating fluids other observed velocity perturbations are attributed to several processes taking place at different depths such as slab hydration through outer rise faulting tectonic erosion and slab dehydration

taiwan orogeny thin skinned or lithospheric collision abstract the taiwan orogeny is young and presently very active it provides an excellent environment for studying ongoing orogenic processes especially since the region is monitored intensively with dense seismological and geodetic networks and new studies aiming at deciphering shallow and deep structures in and around taiwan have been recently conducted or are being planned the available data can be used continually to test critically hypotheses of the taiwan orogeny hypotheses dealing with the mechanics of mountain building are basic to the understanding of taiwan orogeny and are particularly amenable to testing the widely cited thin skinned tectonics hypothesis was formulated to explain mainly the geologic and relatively shallow

active faulting and deformation of the coalinga anticline as interpreted from three dimensional velocity structure and seismicity this work gives a clear picture of the geometry of aftershock seismicity in a large thrust earthquake interpretation of hypocenters and fault plane solutions from the 1983 coalinga coast range california earthquake sequence in combination with the three dimensional velocity structure shows that the active faulting beneath the fold primarily consists of a set of southwest dipping thrusts uplifting blocks of higher velocity material above the main listric blind thrust there is a conjugate fault steeply northeast dipping that provides the western limit of the aftershocks within the coalinga anticline and that corresponds in location and spatial extent with the adjacent pleasant valley syncline the character of the seismicity varies with the degree of previous deformation on each section of the anticline where the previous uplift was largest the shallow seismicity shows secondary faulting on either side of the fold with orientations that correspond to the preexisting geologic structure diffuse seismicity characterizes the area with the least previous deformation the mainshock rupture terminated where the fold trend was no longer uniform but had competing north and west trending features the upward extent of the mainshock rupture ended at the approximate boundary between franciscan and great valley sequence rocks above that depth the main thrust appears to splay into a steeper segment and a near horizontal segment thus the extent of rupture area is limited by the area of uniform structural orientation and by the variation in the type of material with the three dimensional velocity model each individual hypocenter moved slightly 0 2 km in accord with the details of the surrounding velocity structure so that secondary features in the seismicity pattern are more detailed than with a local one dimensional model and station corrections the overall character of the fault plane solutions was not altered by the three dimensional model but the more accurate ray paths did result in distinct changes in particular the mainshock has a fault plane dipping 30 southwest instead of the 23 obtained with the one dimensional model

the 2005 ilan earthquake doublet and seismic crisis in northeastern taiwan evidence for dyke intrusion associated with on land propagation of the okinawa trough summary northern taiwan underwent mountain building in the early stage of the taiwan orogeny but is currently subjected to post collisional crustal extension it may be related to gravitational collapse or to the rifting of the okinawa trough which lies offshore northeastern taiwan the ilan plain northeastern taiwan which is bounded by the normal fault systems and filled up with thick pliocene pleistocene sedimentary sequences formed under such an extension environment over there on 2005 march 5 two earthquakes with about the same magnitude m l 5 9 occurred within 68 s and produced intense aftershocks activity according to the records of central weather bureau seismic network of taiwan we relocated the earthquake sequence by the three dimension earthquake location algorithm with the newly published 3d vp and vp vs velocity model and determined the first polarity focal mechanisms of the earthquake doublet one major cluster of aftershocks which trends e w and dips steeply to the south can be identified and picked up as a potential fault plane the focal mechanisms of the two main shocks are both classified as normal type by first polarity but strike slip by centroid moment tensor inversion however two methods both yield consistent e w strike static coseismic deformation was additionally determined from global positioning system gps daily solutions at a set of continuous gps stations and from strong motion seismographs these data show nw se extension at high angle to the fault plane which cannot be explained from a simple strike slip double couple mechanism on the other hand the small vertical displacements and steep fault plane cannot be explained from a simple normal event as well we present from elastic dislocation modelling that the geodetic data are best explained by significant component of tensile source with centimetre scale of opening on a 15 km long fault extending from 1 to 13 km depth we therefore interpret the crisis as the result of dyke intrusion at the very tip of the okinawa trough which is reasonably driven by backarc spreading

magnitudes of large shallow earthquakes from 1904 to 1980 abstract in order to obtain a uniform magnitude catalogue surface wave magnitudes ms and broad band body wave magnitudes mb have been determined for large shallow earthquakes from 1904 to 1980 in making the catalogue homogeneous the author consistently adheres to the original definitions of ms and mb given by gutenberg 1945 and gutenberg and richter 1956 the determinations of ms and mb are all based on the amplitude and period data listed in gutenberg and richter s unpublished notes bulletins from stations worldwide and other basic information mb is measured on broad band instruments in periods of 8 s consistency of the magnitude determinations from these different sources is carefully checked in detail more than 900 shallow shocks of magnitude 7 and over are catalogued the meaning of the magnitude scales in various catalogues is examined in terms of ms and mb most of the magnitudes listed by gutenberg and richter 1954 in their seismicity of the earth are basically ms for large shocks shallower than 40 km but are basically mb for large shocks at depths of 40 60 km the surface wave magnitudes given by earthquake data reports are higher than ms by 0 2 unit whether the combined horizontal amplitude or the vertical amplitude is used mb and the currently used 1 s body wave magnitude are measured at different periods and should not be directly compared

microseismicity and faulting in the southwestern okinawa trough abstract in november 2003 15 ocean bottom seismometers were deployed in the southwestern okinawa trough more than 3300 microearthquakes were located during the 10 days passive seismic experiment the earthquake activity is characterized by the ceaseless occurrence of small earthquakes in the vicinity of all the instruments the seismicity is essentially restricted to the central part of the southwestern okinawa trough except for one cluster of events situated in the southern part of it cluster 2 the seismic activity terminates abruptly against the ne sw trending prolongation of the lishan fault most of the microearthquakes are aligned along the e w trending normal faults showing where the present day active normal faulting occurs and how it accounts for the n s extension in the okinawa trough according to the p wave velocity spectra estimated from some deep earthquakes located beneath the cross backarc volcanic trail area the existence of a lower crustal upper mantle magma chamber is confirmed by the presence of low frequency earthquakes in the 3 10 hz bandwidth

local tomography and focal mechanisms in the south western alps comparison of methods and tectonic implications abstract we investigate how focal solutions and hypocenter locations may depend on the ray tracing algorithm and the strategy of velocity inversion using arrival times from a temporary seismological network in the south western alps a local earthquake tomography has been performed by paul et al paul a cattaneo m thouvenot f spallarossa d bethoux n and frechet j 2001 a three dimensional crustal velocity model of the south western alps from local earthquake tomography j geophys res 106 19367 19390 with the method developed by thurber thurber c h 1993 local earthquake tomography velocity and vp vs theory in seismic tomography theory and practice iyer h m and irahara eds chapman and hall new york 563 583 another inversion of the same data set is performed here using a different tomography code relying on a shooting paraxial method and cubic interpolation of velocities the resulting images display the same main features although thurber s code appears to be more robust in regions with scarce ray coverage and strong velocity contrasts concerning hypocenter location in piemont units one major result is the concentration of hypocenters at the boundary between the mantle wedge of the ivrea body and the european crust forty six focal mechanisms are shown that were computed using both the take off angles in the minimum 1 d model and in the 3 d velocity structures resulting from the two inversions the sets of focal solutions are very similar proving the reliability and the coherency of the focal solutions the widespread extension in the core of the western alps is confirmed whereas a few compressive solutions are found east of the piemont units these results constrain the sharp change of stress tensor and evidence a decoupling of strain beneath the east of dora maira massif up to beneath the north of argentera massif on a geodynamical point of view seismicity and focal mechanism distribution are compatible with the present day models published for the western alps where the major feature is the lithospheric thickening schmid s m and kissling e 2000 the arc of the western alps in the light of geophysical data on deep crustal structure tectonics 19 62 85 implying widespread extension in the core of the western alps sue c thouvenot f frechet j and tricart p 1999 widespread extension in the core of the western alps revealed by earthquake analysis j geophys res 104 25611 25622 however the existence of compressive events dealing at depth with the boundary of ivrea body allows to postulate that this geological structure is still tectonically active even if field work has not shown this so far the insubric line appears to extend toward the south at depth as a blind fault and to play a key role in the dynamics of the south western alps

three dimensional p and s wave velocity structure of redoubt volcano alaska the three dimensional p and s wave structure of redoubt volcano alaska and the underlying crust to depths of 7 8 km is determined from 6219 p wave and 4008 s wave first arrival times recorded by a 30 station seismograph network deployed on and around the volcano first arrival times are calculated using a finite difference technique which allows for flexible parameterization of the slowness model and easy inclusion of topography and source receiver geometry the three dimensional p wave velocity structure and hypocenters are determined simultaneously while the three dimensional s wave velocity model is determined using the relocated seismicity and an initial s wave velocity model derived from the p wave velocity model assuming an average vp vs ratio of 1 78 convergence is steady with approximately 73 and 52 reduction in p and s wave arrival time rms respectively after 10 iterations the most prominent feature observed in the three dimensional velocity models derived for both p and s waves is a relative low velocity near vertical pipelike structure approximately 1 km in diameter that extends from 1 to 6 km beneath sea level this feature aligns axially with the bulk of seismicity and is interpreted as a highly fractured and altered zone encompassing a magma conduit the velocity structure beneath the north flank of the volcano between depths of 1 and 6 km is characterized by large lateral velocity variations high velocities within this region are interpreted as remnant dikes and sills and low velocities as regions along which magma migrates no large low velocity body suggestive of a magma chamber is resolved in the upper 7 8 km of the crust

new results from physical modelling of arc continent collision in taiwan evolutionary model abstract both the analysis of the geological data and the results from 2d and 3d experimental modelling of arc continent collision in taiwan reveal the following major stages in this process 1 failure and thickening of the subducting crust of the chinese continental margin under the frontal part accretionary prism of the philippine sea plate psp since ca 6 7 ma 2 uplift squeezing and exhumation of the subducted crustal slice of the chinese margin and initiation of the central range about 5 ma ago 3 flexural buckling subsidence of the luzon arc fore arc under strong horizontal compression and sedimentary infilling of this area by the material shed from the growing and exhuming central range 4 failure of the psp along the west vergent longitudinal valley fault dipping beneath the arc and closure of the fore arc 5 subduction reversal and stoppage of continental subduction which has already occurred in northern taiwan and propagates to the south the southward propagation of the collision is not stationary currently occurring initial collision in southern taiwan and offshore to the south does not follow the same described above evolution as in central and northern taiwan the evolution of the collision in time and space should be considered in the framework of a 3d model where the psp rupture is preparing in the central and northern taiwan during a long time passing through a number of evolutionary stages once it has occurred the rupture longitudinal valley fault rapidly propagates to the south into the domain which did not undergo the pre rupture evolution the rupture front is currently located near 21 30 n

probabilistic earthquake location in complex three dimensional velocity models application to switzerland 1 we present a new approach to determine precise and reliable hypocenter locations in the tectonically complex region of switzerland a three dimensional 3 d p wave velocity model to be used for earthquake relocation is obtained by simultaneously inverting arrival times of local earthquakes for hypocenter locations and 3 d p wave velocity structure a 3 d p wave velocity model derived from controlled source seismology css is used as an initial reference model the final 3 d model thus combines all available information from both css and local earthquake data the probabilistic nonlinear formulation of the earthquake location problem includes a complete description of location uncertainties and can be used with any kind of velocity model in particular the combination of nonlinear global search algorithms such as the oct tree importance sampling with probabilistic earthquake location provides a fast and reliable tool for earthquake location the comparison of hypocenter locations obtained routinely by the swiss seismological service sed to those relocated in the new 3 d velocity model using a probabilistic approach reveals no systematic shifts but does exhibit large individual shifts in some epicenter locations and focal depths we can attribute these large shifts in part to large uncertainties in the hypocenter location events with a low number of observations 8 and no observation within the critical focal depth distance typically show large location uncertainties improved hypocentral locations particularly for mine blasts and earthquakes whose routine hypocenter locations had been questionable confirm that improved velocity model and probabilistic earthquake location yield more precise and reliable hypocenter locations and associated location uncertainties for switzerland

surface deformation and tectonic setting of taiwan inferred from a gps velocity field 1 we have determined the present day surface deformation of taiwan by computing the velocity gradient field and fault slip from 143 gps velocity vectors in southern taiwan the derived strain and rotation rates and fault slips are indicative of lateral extrusion toward the south in northern taiwan we infer the onset of gravitational collapse which is induced by the on land extension of the okinawa trough in the eastern central range the observed inverted nw se extension is consistent with geological observations and high heat flow measurements this could be the result of exhumation of crustal material the model further shows a significant decrease in slip rate northward along the longitudinal valley fault at 23 7 n the northern coastal range shows high strain rates and two oppositely rotating blocks by combining the surface deformation model with seismicity data and seismic tomography we are able to propose a coherent model for the present day tectonic activity both seismicity and tomography show further evidence for active southward propagating exhumation of a crustal slice in the eastern central range offshore east taiwan we deduce strong evidence of a southward propagating crustal tear fault accommodating most of the philippine sea plate eurasian plate convergence the tear is the crustal response to incipient northwestward subduction of the philippine sea plate thus the ryukyu trench is bending southward becoming almost perpendicular to the convergence direction while subduction of the philippine sea plate continues in this setting a sudden rapid southward propagation of the afore mentioned tear is conceivable

repeating earthquakes and seismic potential along the northern longitudinal valley fault of eastern taiwan 1 the northern longitudinal valley fault in eastern taiwan creeps at the surface with a small rate of 1 cm yr but slips in large earthquakes to improve seismic hazard assessment it is important to comprehend the slip deficit rate distribution at depth we discovered 25 ml 2 1 4 6 repeating earthquakes in this area and inverted gps measurements for producing an image of the along strike spatial distribution of deep fault slip rates the repeating events are located at the depths of 10 22 km with 24 9 77 5 mm yr slip rates which are comparable with the gps derived slip rates of 47 5 5 8 mm yr at similar depth ranges based on distribution of gps derived slip deficits since 1951 the northern longitudinal valley fault has become capable of releasing stored strain in a future mw 7 3 earthquake

three dimensional patterns of seismic deformation in the taiwan region with special implication from the 1999 chi chi earthquake sequence abstract we compile a comprehensive list of hypocentral locations and source parameters for earthquakes occurred in the taiwan region the collected database is used to construct 3d patterns of seismic deformation that are associated with various tectonic processes such as subduction and collision between the philippine sea plate and eurasia specifically we divide the entire taiwan region into a 3d grid of 10km interval and the amount of seismic moment released at each point is the sum of all individual events in the vicinity in case when the physical dimension of an earthquake is larger than 10km the corresponding seismic moment is distributed across multiple grid points this quantitative approach gives a better depiction on many first order features in the region the most intriguing one is that a significant deficit in the total amount of released seismic moment can be clearly identified around the chi chi source region before the big earthquake occurred in 1999 the deficit trough is then filled by the seismic moment of the chi chi earthquake sequence this deficit then fill pattern in a region s seismic moment distribution can be used as an indicator to pinpoint the locations of large earthquakes in the foreseeable future following this argument we suggest that disastrous earthquakes with magnitudes comparable to that of the chi chi earthquake are due for the two regions adjacent to the chi chi source area i e the miaoli hsinchu domain to the north and the chiayi domain to the south

crustal deformation in the central and southern ryukyu arc estimated from gps data abstract deformation of the ryukyu arc in the period 1999 2001 was investigated on the basis of gps data for horizontal velocity the results show that deformation of the ryukyu arc is divided into three tectonic blocks with all counter clockwise rotation rates of between 18 and 195 nrad yr extensional strain along the ryukyu arc is observed in all three blocks being largest in the southwestern ryukyu arc this suggests that southward bending of the arc occurs in the southwest associated with the arc continent collision in taiwan resulting in clockwise block rotation of the taiwan ryukyu junction thus bending of the ryukyu arc appears to be caused by collision of the luzon arc with the eurasian plate and is likely to contribute to rifting of the okinawa trough

aseismic crustal movement in southern ryukyu trench southwest japan 1 anomalous crustal deformation following the mw 7 1 march 31 2002 hualien earthquake was observed over 5 years using the global positioning system gps network in the south ryukyu islands the analysis showed an afterslip event at a depth of 30 km on the subducting philippine sea plate the magnitude of the cumulative moment reached 7 4 the released moment of the afterslip exceeded the mainshock itself the afterslip promoted coulomb failure stress on the fault of the repeating slow slip events sses in the vicinity of the afterslip the increase in the slip rate of the sses predicted through the change in the coulomb failure stress is 13 this is close to the observed increase in slip rate 19 after the earthquake this suggests that the afterslip had accelerated the slip rate of the sses after the earthquake

a synthesis of the geologic evolution of taiwan abstract the island arc of taiwan is composed of cenozoic geosynclinal sediments more than 10 000 m thick lying on a pre tertiary metamorphic basement pleistocene to miocene andesitic islands surround the main island and are related mostly to arc magmatism the penghu island group in the taiwan strait is covered with pleistocene flood basalt neogene shallow marine clastic sediments are exposed mainly in the western foothills with pleistocene andesitic extrusives at the northern tip and the northeastern offshore islands a thick sequence of paleogene to miocene argillitic to slaty metaclastic rocks underlies the western central range and forms the immediate sedimentary cover on the pre tertiary metamorphic complex to the east which represents an older mesozoic arc trench system the coastal range in eastern taiwan is a neogene andesitic magmatic arc including also a large variety of volcaniclastic and turbiditic sediments cenozoic taiwan is the site of arc continent collision where the luzon arc on the philippine sea plate overrides the chinese continental margin on the eurasian plate east and northeast of taiwan the polarity of subduction changes whereby the oceanic philippine sea plate is subducting beneath the ryukyu arc system on the eurasian plate continent arc collision in taiwan island is anomalous and may occur in a broad belt of deformation rather than along a well defined plate boundary or subduction zone

evolutionary model for the taiwan collision based on physical modelling abstract 2 d and 3 d physical modelling of lithospheric convergence in the luzon taiwan ryukyu region is performed with properly scaled laboratory models the lithospheric model consists of two parts continental the asian plate ap and oceanic the philippine sea plate psp the oceanic lithosphere has one layer while the continental lithosphere includes both mantle and crustal layers the continental margin is covered by sediments a low viscosity asthenosphere underlies the lithosphere the opposing luzon and ryukyu subduction zones are initiated by inclined cuts made within the psp the subduction collision is driven by a piston pre collisional intraoceanic subduction along the luzon and ryukyu boundaries results in the formation of a transform zone between them with two tear faults at the ends the psp undergoes strong compression along this zone subduction of the chinese margin under the luzon boundary further increases the compression compressive stresses reach the yield limit of the psp in the arc area which is a weak zone in the experiments the plate fails at the western side of the arc along an eastward dipping fault the longitudinal valley fault underthrusting of the frontal wedge of the psp along this fault results in the closure of the fore arc basin and is then blocked the psp fails at the opposite side of the luzon arc along the westward dipping fault the failure releases lithospheric compression in this region and results in the initiation of southward propagating subduction of the psp under northeastern taiwan the incipient subduction zone becomes part of the southeastward retreating ryukyu subduction zone which allows the okinawa back arc rift to propagate into taiwan the taiwan collision thus includes the following succession of major processes over time or from south to north 1 an e w shortening of the psp in the luzon arc 2 a failure of this plate at the western side of the arc and the formation of the eastward dipping longitudinal valley fault the transient plate boundary 3 a closure of the fore arc basin and a rapid uplift of the orogen 4 a failure of the psp at the eastern side of the luzon arc partly overthrusting the orogen and the initiation of westward wn ward subduction of the psp 5 and finally back arc rifting in the rear of this incipient subduction zone i e in northern taiwan all these processes commence with some delay with respect to the preceding ones and propagate southwards

origin of the southern okinawa trough volcanism from detailed seismic tomography 1 magmatism associated with subducting plate edges or slab tears has been suggested in the southern okinawa trough the cross back arc volcanic trail which consists of a cluster of about 70 seamounts is located above a ryukyu slab tear lying along the 123 3 e meridian in november 2003 more than 3300 earthquakes recorded in this area by 15 ocean bottom seismometers and surrounding land stations during a period of 12 days were used to determine the three dimensional vp and vs velocity structures and vp vs ratios a mantle inflow characterized by low vp and vs and high vp vs passing through the slab tear is imaged the fluid and or melt component is rising obliquely from the slab tear in the directions of the cross back arc volcanic trail the northern slope of the southern okinawa trough and to north of iriomote island the asthenospheric intake is also imaged by an inclined chip like high vp vs and low vp and vs body dipping northerly which might be linked to the slab retreat west of the slab tear most of the earthquakes are located around low vp and vs and high vp vs bodies which suggests that the seismicity is related to magmatic and or fluid activities east of it earthquakes are concentrated in an area characterized by high vp and vs velocities and low vp vs suggesting that the magma chamber is absent beneath the axial part of the trough and that normal faulting is the main factor controlling the seismicity

hypocenter velocity structure coupling in local earthquake tomography abstract local earthquake tomography involves the use of seismic wave arrival times from earthquake sources with uncertain locations for the inference of three dimensional variations in seismic velocity formally this inverse problem contains explicit coupling between the earthquake hypocenters and the velocity structure however many studies fail to treat the hypocenter velocity structure coupling explicitly an examination of previous studies provides evidence that this failure can lead to significant bias in the resulting velocity model a set of simple hypothetical test cases clearly demonstrates the seriousness of the potential bias when coupling is neglected producing models that underestimate the actual velocity perturbations by about a factor of four under the same conditions strict simultaneous inversion and simultaneous inversion with parameter separation both produce accurate inverted models the use of appropriate station corrections and iteration of the solution can improve the results when the coupling is ignored but velocity anomalies will still be underestimated by about a factor of two at best

tomography of the westernmost ryukyu subduction zone and the serpentinization of the fore arc mantle 1 three dimensional tomographic images of the subducting philippine sea slab and the fore arc mantle beneath ne taiwan and the westernmost ryukyu were generated in this study more than 5600 events recorded simultaneously by seismic networks in taiwan and japan were relocated for the inversion for variations in vp vs and vp vs analysis of the trade off between the data variance reduction and model variance helps to determine the appropriate strictness for regularization to avoid either overinterpretatioin or underinterpretation of data the regularization parameters were also chosen to ensure suppression of artificial vp vs anomalies the subducting slab is characterized by high vp high vs and intermediate to low vp vs notable in the mantle wedge is the high vp vs anomalies that abut the surface of the subducting slab at depths of 30 80 km the previously identified positive vp vs channel connecting the slab and the arc volcano interpreted to be a melt pathway is not reproduced in this study we convert vs and vp vs to perturbations of temperature and serpentinization of the mantle the slab is cooled by 200 c 400 c relative to the mantle in accord with the estimates from theoretical modeling of subduction the serpentinization reaches 15 or 2 water content at 50 km depth in the fore arc mantle we interpret the peak serpentinization as hydrated by the water released from the basalt eclogite metamorphic reaction in the oceanic crust of the subducting philippine sea plate the spatial limitation of the present seismic networks in this region with respect to subduction zone events hinders a full description of the pattern of melting in much of the mantle wedge resolution tests of the tomographic inversion provide a basis to guide our interpretation to better resolved regions

a new seismic tomography of aigion area gulf of corinth greece from the 1991 data set summary an ew three dimensional delay traveltime tomography is performed to image the intermediate structure of the western gulf of corinth a large data set collected in 1991 during a two month passive tomographic experiment has been reanalysed for the reconstruction of detailed vp and vs images an improved tomography method based on an accurate traveltime computation is applied to invert simultaneously delayed p and s first arrival traveltimes for both velocity and hypocentre parameters we perform different synthetic tests to analyse the sensitivity of tomography results to the model parametrization and to the starting 1 d model selection the analysis of the retrieved vp and vs models as well as deduced vp vs and vp vs images allows us to interpret and delineate the distribution of lithological variation porosity crack content and fluid saturation in the upper 9 11 km of the crust beneath the gulf the tomographic models image a rather complex crustal structure which is characterized by a vertical change in both velocity features and seismicity distribution we identify a shallower zone of the crust 0 5 km depth in which velocity distributions seem to be controlled by the still active n s extensional regime and a deeper zone 7 11 km depth which matches the seismogenic zone the correlation between this latter and a specific unit of the hellenic mountain structure the pyllite quartzite series allows us to suggest a possible explanation for seismicity concentration in a narrow band at 7 9 km depth finally the occurrence of clusters showing low angle normal fault mechanisms in areas characterized by high vp vs values indicates a possible role of fluids in triggering brittle creep along the identified low angle normal faults

constraints on backstop geometry of the southwest ryukyu subduction based on reflection seismic data abstract based on the analysis of 45 seismic reflection profiles the top of the ryukyu arc basement is mapped in the forearc region east of taiwan the ryukyu arc slope dips 10 trenchward while beneath the forearc basins the arc basement shows a series of morphologic along arc highs and lows one of the basement highs the nanao basement rise has been described as an area uplifted by the subduction of the gagua ridge the other one the hoping basement rise is suggested in this study to be an uplifted feature caused by the subduction or the underplating of some local asperity the nature of this asperity could either be a detached block of the luzon arc an isolated seamount or some material removed from the frontal part southernmost extremity of the ryukyu margin from seismic reflection profiles we infer that the right lateral transcurrent fault system located at the rear of the accretionary wedge defines the surface projection of the southern termination of the ryukyu arc basement two crustal sections across the ryukyu subduction system demonstrate that the backstop terminates in the form of a near vertical wall with its height over 5 km such an unusual geometry could be produced by crustal truncation at the toe of the backstop

a seismic refraction study of the crustal structure in the active seismic zone east of taiwan an array of 10 hawaii institute of geophysics hig ocean bottom seismometers obss was emplaced off the east coast of taiwan for a cooperative study conducted by scientists from hig and institutions in taiwan as part of this experiment three overlapping seismic refraction profiles were shot across the array parallel to the coast of taiwan the results of the ray trace modeling of these data indicate that the crust off southern and central taiwan is about 8 km thick in this area and can be modeled by several layers with velocities and thicknesses that lie within the range associated with normal oceanic crust near 23 5 n a downwarping and thickening of the crustal layers occurs in the model this downwarped trough of low velocity materials may represent the sediment filled axis of the ryukyu trench if this is true it indicates a more southerly trench position in this area than that previously published by other investigators to the north of this downwarping the bottom shoals rapidly and the velocity structure undergoes a transition that may indicate a change to arc or continental type crust

a comprehensive relocation of earthquakes in taiwan from 1991 to 2005 we have carried out a comprehensive relocation of a total of 267 210 earthquakes in taiwan that occurred during the past 15 yr we based our relocation process on the earthquake catalog of the taiwan central weather bureau seismic network cwbsn and made improvements in three aspects first we incorporated a large dataset of the s p times from 680 taiwan strong motion instrumentation pro gram tsmip stations distributed throughout the island of taiwan to improve the coverage of earthquakes on the island secondly we added 18 japan meteorological agency jma stations in the southern ryukyu island chain to enhance the station coverage for eastern offshore events especially around the subduction zone northeast of taiwan thirdly we adopted 3d vp and vp vs models in predicting the travel times of p and s waves the effectiveness of these improvements in earthquake re location can be seen in three aspects 1 the reduction in the residuals of p wave arrival times and s p times 2 a better understanding of the attenuation relationship between the peak ground acceleration and epicentral distance and 3 the geologically meaningful patterns of station corrections to p wave arrival times and s p times online material catalog of relocated earthquakes in taiwan from january 1991 to

an alternative interpretation for slip vector residuals of subduction interface earthquakes a case study in the westernmost ryukyu slab abstract slip partitioning along oblique subduction zones has conventionally been examined by slip vector residuals determined from interface earthquakes it is measured through contrasting the slip vector with respect to the local plate convergence direction defined on the surface of the earth by the euler vector interpretation of regional plate kinematics based on slip vector residuals thus defined may be misleading besides the apparent discrepancy attributable to the surface projection of vectors on an obliquely plunging surface a more fundamental problem is the construction of the reference subduction flow field for the subducted slab based upon the known plate convergence it can be shown that for complicated slab geometry the conventional practice of intuitively rotating the euler kinematics onto the subducted slab leads to unrealistic intraplate deformation that invalidates the mechanical integrity of the oceanic lithosphere alternatively we propose to calculate the velocity field for the specific slab geometry based on the rationale that the subduction flow field should be the one that endures the least amount of intraplate deformation the calculated flow field for the westernmost ryukyu slab that is in transition to the philippine sea plate psp eurasian plate ep collision near taiwan reveals quite distinct subduction flow field the particle paths associated with the calculated flow field are in better accordance with the observed slip vectors while the corresponding strain rate field is consistent with the previously reported lateral compression seismogenic zone furthermore it is speculated that the plate kinematics of the northwestern corner of psp that is impinging the vicinity of the collision might have also undergone significant readjustment to accommodate the potential intraplate deformation it is noted that the westernmost subducted ryukyu slab and the northwestern corner of psp belong to a mechanically coherent tectonic unit the 3d geometric constraints set up by the subducted slab and the unfriendly kinematic constraints imposed by the collision boundary both play important roles in configuring the intraplate deformation within psp manifestation in the local kinematics might indicate deviation from the general psp ep convergence and is thus of crucial importance to tectonic interpretations of geophysical observations in this area

does extrusion occur at both tips of the taiwan collision belt insights from active deformation studies in the ilan plain and pingtung plain regions abstract we analyse the present day deformation in two key areas of the taiwan collision belt the ilan plain to the nw and the pingtung plain to the sw our approach is mainly based on consideration of horizontal displacement revealed by recent geodetic gps surveys derived strain rate tensors indicating horizontal deformation and three dimensional seismotectonic stress regimes issued from inversion of focal mechanisms of earthquakes we reconstruct a consistent albeit complex tectonic pattern involving non rigid rotations clockwise in ne taiwan anticlockwise in sw taiwan transitions from pure compression near the mountains to transtension near the sea and simple shear affecting the deforming domain nw se left lateral in ne taiwan nne ssw right lateral in sw taiwan these tectonic patterns reflect lateral extrusion processes towards mechanically weak domains with respect to collision zone i e adjacent subduction zones ryukyu to the ne manila to the sw the extrusion of the ilan plain area occurs towards the se whereas that of the pingtung plain area occurs towards the sw the extrusion in ne taiwan is facilitated by the opening of the okinawa trough so that velocity and deformation rates are higher than in sw taiwan despite less active collision in this northern area the symmetry of the extrusion patterns is altered by differences in relative positions and orientations of collision belt and adjacent subduction zones which depend on the plate tectonic configuration and tectonic history of the taiwan region

regional travel time uncertainty and seismic location improvement using a three dimensional a priori velocity model we demonstrate our ability to improve regional travel time prediction and seismic event location accuracy using an a priori 3d velocity model of western eurasia and north africa wena1 0 travel time residuals are assessed relative to the iasp91 model for approximately 6000 pg pn and p arrivals from seismic events having 2 epicenter accuracy between 1 km and 25 km ground truth 1 gt1 and gt25 respectively recorded at 39 stations throughout the model region ray paths range in length between 0 and 40 local regional and near teleseismic providing depth sounding that spans the crust and upper mantle the dataset also provides representative geographic sampling across eurasia and north africa including aseismic areas the wena1 0 model markedly improves travel time predictions for most stations with an average variance reduction of 29 for all ray paths from the gt25 events when we consider gt5 and better events alone the variance reduction is 49 for location tests we use 196 geographically distributed gt5 and better events in 134 cases 68 of the events locations are improved and average mislocation is reduced from 24 9 km to 17 7 km we develop a travel time uncertainty model that is used to calculate location coverage ellipses the coverage ellipses for wena1 0 are validated to be representative of epicenter error and are smaller than those for iasp91 by 37 we conclude that a priori models are directly applicable where data coverage limits tomographic and empirical approaches and the development of the uncertainty model enables merging of a priori and data driven approaches using bayesian techniques online material correction surfaces and histograms of travel time residuals for 40 stations

anomalous pn waves observed in eastern taiwan implications of a thin crust and elevated oceanic upper mantle beneath the active collision zone suture normal pn waves are commonly observed in taiwan from shallow regional earthquakes at epicentral distances larger than 120 km similar to the observations in many other continental regions however the critical distances to observe pn waves for shallow eastern taiwan earthquakes vary with azimuth corresponding to a significant variation of crustal thickness in particular anomalous pn waves are commonly observed for shallow eastern taiwan earthquakes recorded on seismic stations at epicentral distances as small as 60 km along the collision zone suture the longitudinal valley for the same event normal pn waves are observed at other seismic stations elsewhere on the island the apparent velocity of the anomalous and normal pn waves from the same event is 7 8 0 15 km sec which is consistent with the average pn velocity in the taiwan area thus the unusually short critical distance for pn waves in eastern taiwan suggests that the crust beneath the collision zone suture must be very thin and the upper mantle beneath the longitudinal valley must be relatively elevated compared with that beneath the other parts of taiwan assuming a simple 1d layered velocity model the moho depth beneath the suture zone can thus be estimated at 23 2 km this observation is consistent with the recent report from a high resolution 3d tomographic inversion that a narrowly confined anomalously elevated and north northeast south southwest elongated oceanic upper mantle was imaged beneath the longitudinal valley from hualien in the north to taitung in the south kim et al 2005 2006 furthermore the preceding observations may also support the interpretation that the conduction of excess heat supply from the elevated hot oceanic upper mantle into the adjacent mid to lower continental crust over a long period of geological time may play an important role in the crustal deformation beneath the continent including metamorphism thickening and uplifting

rupture at the flank of the subducted gagua ridge the 18 december 2001 earthquake mw 6 8 offshore eastern taiwan abstract the southern ryukyus represents an area where different tectonic stress regimes result in high seismicity and increased seismic hazard for nearby areas such as taiwan on 18 december 2001 at 04 03 gmt a strong earthquake mw 6 8 occurred in the forearc area of the southern ryukyu subduction zone revised moment tensor solutions published by gcmt and bats groups show a normal faulting mechanism with some strike slip component and also point to a shallow focal depth 12 km we use arrival times picked at both taiwanese and japanese stations along with a 3d geo realistic a priori velocity model in order to obtain accurate absolute locations for the mainshock and 153 of its aftershocks locations are derived by using the maximum intersection maxi algorithm which has been used in many previous seismicity studies in the southern ryukyus these improved locations indicate that the mainshock was caused by the failure of a ne sw oriented fault that extends from the edge of the nanao forearc sedimentary basin to the ryukyu arc basement far field p and sh waveforms of the mainshock recorded at stations surrounding the source and at distances 30 100 were inverted for the purpose of investigating its rupture process a non negative least squares inversion technique utilizing multiple time windows was used to derive the spatio temporal slip distribution the preferred slip distribution model shows that there is one large area of high slip 0 9 m at 5 15 km depth that essentially represents the crystalline rocks of the ryukyu arc basement another smaller area with lower slip 0 4 m extends at 10 15 km depth beneath the nanao basin most aftershocks are located in areas of low slip

okinawa trough backarc basin early tectonic and magmatic evolution the okinawa trough lying between japan and taiwan is a backarc basin formed by extension within the continental lithosphere behind the ryukyu trench arc system stress directions associated with the two last extensional phases in the southwestern okinawa trough have been deduced from a comparison with analog modeling the direction of extension is n 150 o for the pleistocene phase of extension 2 0 1 ma and n 170 o for the late pleistocene holocene phase of extension 0 1 0 ma the present day ryukyu volcanic arc a narrow continuous feature extending from japan to taiwan is located on the eastern side of the okinawa trough 80 100 km above the wadati benioff zone the minimum depth for emplacement of arc magmatism scarce present day backarc volcanism appears in the middle and southern okinawa trough within linear en echelon bathymetric depressions a n045 o oriented seamount volcanic chain cuts across obliquely the southwestern okinawa trough and lies in the direct line of extension of the gagua ridge a n s linear volcanic feature of the philippine sea plate associated with this extension of the gagua ridge a large reentrant located at the base of the ryukyu prism the uplift of part of the nanao forearc basin and the deformation of the sedimentary arc suggest that the voluminous cross backarc volcanism could be tied to the subduction of the gagua ridge located there at a depth of 80 100 km beneath the backarc basin a second area of anomalous volcanism has been identified in the middle okinawa trough in the ene extension of the daito ridge a wnw ese 400 km long volcanic feature of the philippine sea plate we suggest that the gagua and daito ridges initially induced stress at the base of the arc which is still brittle and cracks propagated through the overlying brittle lithosphere allowing magmas with arc affinities to erupt at the seafloor this excessive magmatism reaches the seafloor through conduits which preferentially follow in their shallowest portion the crustal normal faults of the backarc rifts the okinawa trough is consequently still in an early stage of evolving from arc type to backarc activity

moment tensor inversion for offshore earthquakes east of taiwan and their implications to regional collision reliable determination of source parameters for offshore earthquakes east of taiwan with mb 5 5 was a difficult task because of the poor azimuthal coverage by local network and the lack of signals at teleseismic distances we take advantage of the recently established broadband array in taiwan for seismology bats to invert seismic moment tensors for 7 such events occurred in 1996 to cope with different patterns of background noise and unknown structural details we utilize variable frequency bands in the inversion and adapt a two step procedure to select best velocity models for individual epicenter station paths our results are consistent with the overall patterns of regional collision and indicate that the resulting compressive stress has caused significant intraplate deformation within the philippine sea plate simulation of the region s geological evolution and orogenic processes should take this factor into account and allow the philippine sea plate to deform internally

a rigid block rotation model for the gps derived velocity field along the ryukyu arc abstract using a gps derived velocity field we try to estimate the kinematic features of the ryukyu arc and discuss its implications to the spreading of the okinawa trough the gps derived strain rate field implies that internal deformation within the ryukyu arc is not significant therefore we adopt a rigid block rotation model for the motion of the ryukyu arc the velocity field in the ryukyu arc is best explained by a model with three blocks rather than one with only one block the estimated block rotations predict the spreading along the okinawa trough the predicted spreading is slow 10 mm per year in the north and fast 50 mm per year in the southwest the direction of the predicted spreading is normal to the okinawa trough n170 e n180 e in the southwest while in the north the direction of the spreading is oblique to the okinawa trough n150 e n180 e

earthquakes along the ryukyu kyushu arc strain segmentation lateral compression and the thermomechanical state of the plate interface we systematically determined the focal depths and mechanisms of 49 large to moderate sized earthquakes mb 5 4 that occurred along the ryukyu kyushu arc since 1963 by inverting the waveforms and amplitudes of p and sh wave trains at teleseismic distances the results are sufficiently precise to delineate seismogenic structures near the plate interface in detail revealing features not predicted by plate kinematics in contrast to previous studies of this arc and those along other subduction zones shallow seismicity along the plate interface is systematic showing two distinct layers of activities the second layer of seismicity is delineated by a few earthquakes that occurred at depths between 50 and 65 km some 10 20 km directly beneath the seismogenic portion of the interplate thrust zone these earthquakes indicate lateral compressional strain within the subducted slab as their p axes are subparallel to the local strike of the arc not perpendicular to the arc as one would expect from the direction of plate convergence the occurrence of these events cannot be accounted for by membrane stress due to the geometry of the subducted slab to our knowledge similar earthquakes occurred only beneath the northern indoburman ranges and along the banda arc where subducted slabs as part of the indian plate are being dragged northward with their northern edges bumping into east west trending collision zones nearby by drawing an analogy between the tectonic settings of these three regions we interpret events beneath the plate interface along the ryukyu kyushu arc as a consequence of ongoing collision between the philippine sea plate and eurasia near taiwan the interplate thrust zone is largely aseismic down to a depth of approximately 30 km a large number of earthquakes showing low angle thrust faulting commence at this depth and are accompanied by two events that show antithetic thrust faulting at a slightly shallower depth of 20 25 km these observations suggest that the strength of the plate interface increases significantly below the depth of proximately 25 5 km based upon available heat flow measurements between the trench axis and the volcanic arc we estimated the temperature field and magnitude of shear traction along the interplate thrust zone the deepest interplate earthquakes 40 50 km correspond to temperatures of approximately 730 980 c comparable to the limiting temperature of intraplate mantle earthquakes 800 c and that of intermediate and deep focus earthquakes at wadati benioff zones potential temperature 900 k therefore the cessation of seismicity in general is probably controlled by similar temperature conditions regardless of the tectonic settings of source regions the magnitude of shear traction along the seismogenic portion 40 50 km depth of the interplate thrust zone seems to be of the order of 100 mpa which in turn implies a low average coefficient of friction of only 0 10 0 05 considerably less than those observed for laboratory specimens the inception of intermediate focus earthquakes within the subducted slab is at a depth of about 100 km whereas shallow earthquakes show no apparent variations along the entire arc intermediate depth earthquakes indicate downdip extension along the northern end of the arc near kyushu but abruptly change to downdip compression along the rest of the arc all available evidence indicates that this is a spatial pattern unrelated to earthquake cycles at the moment tectonic interpretation of such a distinct strain segmentation along the arc is enigmatic without the presence of clear discontinuities in the age of the subducted slab the sudden switch in the strain field of the slab is difficult to explain by the continuously varying rate of subduction along the arc in any case this pattern of strain segmentation requires the presence of a major accommodation structure such as a tear fault in the subducted slab near the tokara channel appendix is available with entire article on microfiche order from american geophysical union 2000 florida ave n w washington dc 20009 document b91 004 2 50 payment must accompany order

melting features along the western ryukyu slab edge northeast taiwan tomographic evidence 1 behind the sedimentary ryukyu arc lies the okinawa trough whose termination is located at the tip of the ilan plain northern taiwan just above the ryukyu slab edge the present day active volcanic front is located 80 100 km above the ryukyu slab and extends from japan to kueishantao island an islet situated 10 km offshore the ilan plain between december 1990 and may 1999 3370 earthquakes recorded in northern taiwan by 65 seismic land stations were used to determine the three dimensional vp and vs velocity structures and vp vs ratios a low vs but high vp vs sausage like body 30 km in diameter lies within the eurasian mantle wedge on top of the western ryukyu slab extremity at depths ranging between 20 and 100 km we suggest that the h2o rich component formed by dehydration processes from subducting sediments oceanic crust and serpentinized mantle above the ryukyu slab and along the vertical portion of the slab edge might explain the presence of the sausage like body a low vs but high vp vs channel rises obliquely from the sausage like body at a depth of 40 km in direction of the andesitic kueishantao island we propose that the h2o rich component and or melt rise up from the sausage like body and interfere with the okinawa trough back arc basin magmas formed in the upper mantle lower crust then magmas propagate upward within the upper brittle crust through veins and or narrow conduits

rapid three dimensional hypocentral determination using a master station method a master station ms method is presented in this paper to rapidly determine hypocenters in three dimensional 3 d heterogeneous velocities an equal differential time edt surface is defined as the collection of all spatial points that satisfy the time difference between two arrivals which can be two picks at two stations or two different phase picks at one station the edt surface is independent of the origin time and will contain the hypocenter for an event with j arrivals there are j 1 independent edt surfaces the ms method determines the hypocenter that satisfies two types of constraints to be traversed by most edt surfaces and to yield minimum travel time residual statistics the statistics include both the residual variance and the amplitude of the origin time error the combined use of the edt surfaces and residual statistics allows for a unique determination of the hypocenter and origin time using different types of phase arrivals in principle only three arrivals are minimally required to constrain a unique solution if three different stations are used for a 3 d velocity model the edt surfaces and the residual statistics can be computed efficiently using a reference file created by moser s 1991 ray tracing method an illustration of the ms method is given for 27 small events that occurred in southern california using a p wave velocity model modified from that of magistrale et al 1992 the average misfit between the bulletin hypocenters and the new solutions is 3 8 km if a 3 d velocity model is accurate the ms method can be a viable means of hypocenter determination

crustal scale seismic profiles across taiwan and the western philippine sea abstract we have used combined onshore and offshore wide angle seismic data sets to model the velocity structure of the taiwan arc continent collision along three cross island transects although taiwan is well known as a collisional orogen relatively few data have been collected that reveal the deeper structure resulting from this lithospheric scale process our southern transect crosses the hengchun peninsula of southernmost taiwan and demonstrates characteristics of incipient collision here 11 km thick transitional crust of the eurasian plate eup subducts beneath a large rapidly growing accretionary prism this prism also overrides the n luzon forearc to the east as it grows just west of the arc axis there is an abrupt discontinuity in the forearc velocity structure because this break is accompanied by intense seismicity we interpret that the forearc block is being detached from the n luzon arc and philippine sea plate psp at this point our middle transect illustrates the structure of the developing collision steep and overturned velocity contours indicate probable large scale thrust boundaries across the orogen the leading edge of the coherent psp appears to extend to beneath the east coast of taiwan deformation of the psp is largely limited to the remnant n luzon arc with no evidence of crustal thickening to the east in the huatung basin our northern transect illustrates slab continent collision the continuing collision of the psp and eup as the psp subducts the collisional contact is below 20 km depths along this transect ne of hualien this transect shows elements of the transition from arc continent collision to ryukyu arc subduction both of our models across the central range suggest that the paleozoic to mesozoic basement rocks there may have been emplaced as thick coherent thrust sheets this suggests a process of partial continental subduction followed by intra crustal detachment and buoyancy aided exhumation although our models provide previously unknown structural information about the taiwan orogen our data do not define the deepest orogenic structure nor the structure of western taiwan additional seismic active and passive geologic and geodynamic modeling work must be done to fully define the structure the active deformation zones and the key geodynamic process of the taiwan arc continent collision

inverse ray imaging from triangulation of zero offset reflection times summary imaging of subsurfaces based on 3 d inverse rays shooting from coincident sources and receivers along oblique profiles is presented two take off angles of a 3 d inverse ray one along the seismic profile and another orthogonal to the profile are determined based on the common direction of inverse rays shooting from three non collinear sources receivers inverse ray tracing from the triangulation of sources receivers to the reflection points provides more objective imaging than does map migration for which traveltimes along the oblique and or sparse profiles must be interpolated into rectangular grids error analysis of 3 d inverse ray imaging over a 2 d subduction model with variation in spacing and orientation of profiles indicates that the variation of the structural interface can be well resolved when spacing of the traveltime grids is less than the lateral scale of structural variation all possible triangulations of reflection times along two neighbouring profiles are also implemented in inverse ray imaging of a seg eaeg over thrust model the result shows that the coverage of inverse rays from triangulation of all reflection times is much denser than that from triangulation based on the three closest spaced stations furthermore imaging errors of the interface depth from most of the seg eaeg over thrust model where the imaging density is higher than 2000 points km 2 are less than 0 1 km even when the profile spacing is relatively large in this study migration velocity analysis of 2 d multi channel seismic data and all possible triangulations of picked reflection times along oblique seismic profiles in the southernmost ryukyu subduction zone are conducted application of 3 d inverse rays to a forearc basin in this subduction zone shows that the sediment thickness near the western end of the ryukyu forearc region varies strongly with a maximum thickness greater than 6 km furthermore a sediment subsidence greater than 1 km is found in the se portion of the basin slab buckling and basement rise due to the oblique subduction of the philippine sea plate may have resulted in this subsidence and induced strong and shallow earthquakes below the forearc basin

three dimensional elastic wave velocity structure of the hualien region of taiwan evidence of active crustal exhumation the hualien region of taiwan is located at a complex transition of the boundary between the eurasian and philippine sea plates to the southwest the mountains of taiwan are uplifting rapidly as a consequence of an ongoing arc continent collision while to the east the oceanic philippine sea plate is subducting northward beneath eurasia we investigated the structure and dynamics of this region by analyzing seismograms of local earthquakes recorded during a deployment of the portable array for numerical data acquisition ii network p and s wave velocity structures deduced from travel time tomography analysis show that the collisional suture to the south of hualien is characterized by a narrow 10 km width near vertically dipping zone of low velocities that extends to depths in excess of 20 km velocities in the eastern central range west of the suture zone are significantly higher and define a feature 10 15 km wide that appears to be continuous from the near surface to depths as great as 40 km farther to the west beneath the western central range the velocities again decrease focal mechanisms of local earthquakes show that while thrust faulting is the predominate mode of deformation throughout the region normal faulting occurs as well beneath the eastern central range thus the rapid uplift of the mountains of taiwan may be a result not only of compressional shortening but also of an excess of positive buoyancy we suggest that the higher velocities and extensional mechanisms in the eastern central range are caused by the ongoing exhumation of previously subducted continental crust while the lower velocities to the west reflect continued underthrusting of the crust beneath the eastern central range

spatial changes of inter plate coupling inferred from sequences of small repeating earthquakes in japan 1 we extract sequences of small repeating earthquakes to clarify inter plate coupling of subducting plates over a large area of the japanese islands as a result many sequences arc detected at the philippine sea plate subducting from the ryukyu trench and pacific plate subducting from the kuril japan trench the average slip rates and standard deviations estimated from the sequences show substantial spatial changes of inter plate coupling the large deviations of slip rates correspond to the occurrence of episodic slips such as after slips following large earthquakes constant slip rates approaching the relative plate motion indicate weak coupling areas slip deficits and sparse distributions of repeating groups suggest locked areas in the nankai trough deep low frequency earthquakes in the transition zone and burst type repeating sequences within plates have not been located in the downdip direction of groups with slow slip rates this suggests that the space time characteristics of inter plate coupling affected these seismic events

crustal structure of the southernmost ryukyu subduction zone obs mcs and gravity modelling summary in 1995 a combined ocean bottom seismometric obs and multichannel seismic mcs survey with strong air gun shots was carried out in the southernmost ryukyu subduction zone a crustal velocity structure constructed from the layer stripping monte carlo inversion of three obs mcs profiles and the associated density models inverted from gravity data in the sw end of the ryukyu arc trench system are presented parallel to the arc in the southernmost ryukyu subduction system the obs mcs profiles show sedimentary layers of the hoping nanao and east nanao forearc basins from west to east warping of the arc basement and buckling of the subducted slab beneath the hoping basement rise the arc parallel variation of the crustal structure may result from increasing lateral compression westward due to oblique subduction of the philippine sea plate and collision with the luzon arc near the northwestern edge of the forearc region northward subduction and arc parallel compression of the slab also have generated thrust faulting along the subduction interface and strike slip faulting within the subducted slab respectively on 2002 march 31 an earthquake with a moment magnitude of 6 84 was induced by buckling of the subducted slab and strongly affected cities within an epicentral distance of 100 km the velocity interface models the density models and the focal mechanisms presented in this paper therefore suggest that earthquakes induced by slab buckling or arc parallel compression have been stronger but less frequent than those generated by northward subduction in the ryukyu seismogenic zone off taiwan

contrasting styles and rates of tectonic uplift of coral reef terraces in the ryukyu and daito islands southwestern japan abstract coral reef terraces on kikai hateruma and yonaguni in the ryukyu islands on the eurasian lithospheric plate and those of kita daito and minami daito islands on the philippine sea plate indicate three distinct patterns of coral reef terrace formation and preservation primarily reflecting different uplift rates the kikai type which includes many terraces from oxygen isotope stages 5e to 1 is a result of high uplift rate of ca 1 8 m kyr on the fore arc rise of a chilean type subduction zone arcward westward tilting and faulting are also characteristic of the kikai type the hateruma type is characterized by two coral reef terraces younger than the stage 5e terrace and they are arranged in a concentric annular pattern slow uplift rate of ca 0 3 m kyr adjacent to a mariana type subduction margin is typical of the hateruma type the emergence of kikai and hateruma islands is recent younger than isotope stage 5e and 7 respectively reflecting the active deformations of the leading edge at the overriding eurasian plate in the daito type atolls surrounded by fringing reefs have been uplifted very slowly less than 0 05 m kyr but over a time period of at least several million years slow uplift and long history of daito type islands is the result of up bulging of the philippine sea plate as it migrates toward the ryukyu trench

three dimensional velocity structure seismicity and fault structure in the parkfield region central california this study examines the three dimensional velocity structure in a 60 by 80 km region containing the parkfield segment of the san andreas fault we use local earthquake and shot p arrival times in an iterative simultaneous inversion for velocity and hypocentral parameters using the three dimensional model we relocated 5251 events that occurred from 1969 to 1991 as well as the 1966 aftershocks and computed 664 fault plane solutions the san andreas fault saf characterized by a sharp across fault velocity gradient is the primary feature in the velocity solution there is a 5 20 lateral change in velocity over a 4 km width the contrast being sharper where there is better resolution the model also shows significant variations in the velocity and in the complexity of the velocity patterns along the saf the largest across fault velocity difference is below middle mountain where a large volume of low velocity material impinges on the saf from the northeast this material is inferred to be overpressured and may be key to understanding the unusual behavior in the parkfield preparation zone a 20 km long high velocity slice is imaged northeast of the saf near gold hill its along fault length corresponds to the length of the maximum slip in 1966 the relocated seismicity shows that the san andreas fault is a planar vertical fault zone at seismogenic depths ninety percent of the fault plane solutions that are on or near the saf were right lateral strike slip on subvertical fault planes that parallel the saf thus the surface fault complexities do not appear to extend to depth and therefore do not explain the rupture character at parkfield at parkfield variations in material properties play a key role in fault segmentation and deformation style our observations suggest that there may be a general relation between increasing velocity and increasing ability of the rocks to store strain energy and release it as brittle failure

seismic coupling and uncoupling at subduction zones seismic coupling has been used as a qualitative measure of the interaction between the two plates at subduction zones kanamori 1971 introduced seismic coupling after noting that the characteristic size of earthquakes varies systematically for the northern pacific subduction zones a quantitative global comparison of many subduction zones reveals a strong correlation of earthquake size with two other variables age of the subducting lithosphere and convergence rate the largest earthquakes occur in zones with young lithosphere and fast convergence rates while zones with old lithosphere and slow rates are relatively aseismic for large earthquakes results from a study of the rupture process of three great earthquakes indicate that maximum earthquake size is directly related to the asperity distribution on the fault plane asperities are strong regions that resist the motion between the two plates the zones with the largest earthquakes have very large asperities while the zones with smaller earthquakes have small scattered asperities this observation can be translated into a simple model of seismic coupling where the horizontal compressive stress between the two plates is proportional to the ratio of the summed asperity area to the total area of the contact surface while the variation in asperity size is used to establish a connection between earthquake size and tectonic stress it also implies that plate age and rate affect the asperity distribution plate age and rate can control asperity distribution directly by use of the horizontal compressive stress associated with the preferred trajectory i e the vertical and horizontal velocities of subducting slabs are determined by the plate age and convergence velocity indirect influences are many including oceanic plate topography and the amount of subducted sediments all subduction zones are apparently uncoupled below a depth of about 40 km and we propose that the basalt to eclogite phase change in the down going oceanic crust may be largely responsible this phase change should start at a depth of 30 35 km and could at least partially uncouple the plates by superplastic deformation throughout the oceanic crust during the phase change

a model for the termination of the ryukyu subduction zone against taiwan a junction of collision subduction separation and subduction boundaries 1 the nw moving philippine sea plate psp collides with the eurasian plate eup in the vicinity of taiwan and at the same time it subducts toward the north along sw ryukyu the ryukyu subduction zone terminates against eastern taiwan while the ryukyu trench is a linear bathymetric low about 100 km east of taiwan closer to taiwan it cannot be clearly identified bathymetrically owing to the deformation related to the collision making the location of the intersection of the ryukyu with taiwan difficult to decipher we propose a model for this complex of boundaries on the basis of seismicity and 3 d velocity structures in this model the intersection is placed at the latitude of about 23 7 n placing the northern part of the coastal range on eup as psp gets deeper along the subduction zone it collides with eup on the taiwan side only where they are in direct contact thus the eurasian plate on the taiwan side is being pushed and compressed by the nw moving philippine sea plate at increasing depth toward the north offshore of northeastern taiwan the wedge shaped eup on top of the ryukyu subducting plate is connected to the eup on the ryukyu side and coupled to the nw moving psp by friction at the plate interface the two sides of the eup above the western end of the subduction zone are not subjected to the same forces and a difference in motions can be expected the deformation of taiwan as revealed by continuous gps measurements geodetic movement along the east coast of taiwan and the formation of the hoping basin can be understood in terms of the proposed model

holocene sea level change and tectonic uplift deduced from raised reef terraces kikai jima ryukyu islands japan abstract kikai jima kikai island is surrounded by four holocene raised coral reef terraces which are thought to be an offlapping sequence of reef deposits caused by combined effects of seismic uplift and holocene sea level change many studies in this region have investigated holocene sea level changes and reef growth but there are relatively few in which reliable sea level indicators are given we have found that pocillopora verrucosa one of the most abundant coral species on the upper reef slopes of fringing reefs in the ryukyus has its peak abundance at a depth of 1 5 m therefore this species is considered ideal for the analysis of relative sea level change and can be used as a dipstick for the holocene reef deposits in this area based on the distribution of p verrucosa on the four holocene raised terraces we calculate relative paleo mean sea levels to be 10 8 11 1 and 8 5 8 9 m for terrace i 5 0 5 3 m for terrace ii 4 0 4 3 m for terrace iii and 1 9 2 5 m for terrace iv these results combined with hitherto known and newly measured radiometric dates 103 total clearly show that the four terraces formed in response to repeated seismic uplifts at 6 3 4 1 3 1 and 1 4 ka and that sea level was higher than present between 7 0 and 6 3 ka

strain partitioning and interplate friction in oblique subduction zones constraints provided by experimental modeling physical modeling of oblique subduction is performed to study the mechanism of strain partitioning the model is two layer and includes the elasto plastic lithosphere the overriding and subducting plates and the low viscosity liquid asthenosphere the subduction is driven by a push force from a piston and a pull force when the density contrast ap between the subducting plate and the asthenosphere is positive we vary both ap and the interplate friction frictional stresses slip partitioning is obtained only in the models with high interplate friction and only when the overrid ing plate contains a weak zone this zone in the models corresponds either to locally thinned litho sphere or to cut fault the horizontal trench normal component of the interplate friction force ffh can be comparable with the absolute value of the horizontal component of the nonhydrostatic interplate pressure force fph in the subduction zone ffh is always negative compression while fph can be either negative compressional subduction regime or positive extensional regime high friction which promotes partitioning can completely cancel the extensional suction force fph back arc tension and strike slip faulting appear thus as conflicting processes although they can coexist in the same subduction zone depending on the relative values of relevant forces it ap pears that high friction can exist only in compressional subduction zones where partitioning should develop more easily this conclusion is supported by the comparison of two oblique subduction zones having similar geometry the compressional southern kurile zone strong partitioning and extensional southern ryukyu zone no lithospheric scale partitioning other factors controlling the strain partitioning are the length of the oblique subductlon zone the boundary conditions at the transverse limits of the forearc sliver and of course the obliquity of subduction

local earthquake tomography of shallow subduction in north chile a combined onshore and offshore study selected travel time data from the aftershock series of the great antofagasta earthquake m w 8 0 have been inverted simultaneously for both hypocenter locations and three dimensional v p and v p v s struc ture the data were collected with a dense 44 station seismic network including ocean bottom hydrophones we performed a series of inversions with increasing complexity 1 d 2 d and 3 d this approach was cho sen to account for the heterogeneous seismicity distribution and to obtain a smooth regional model in areas of low resolution special efforts were made to assess the solution quality including standard resolution esti mates and tests with synthetic travel times the subducted plate is imaged between 20 and 50 km in depth as an eastward dipping high v p feature high v p v s ratios within the oceanic crust possibly indicate elevated fluid content underplating of material eroded close to the trench is found beneath the mejillones peninsula the lower crust of the overlying plate is characterized by an average v p of 6 8 6 9 km s and an average to low v p v s ratio large areas of anomalously high v p are found in the lower crust south of the city of antofagasta they are interpreted as remants of magmatic intrusions a zone of high v p v s ratios is found within the rup ture area of the antofagasta main shock just above the subducted slab its location within the region of high est stress release from the main shock suggests that the main shock rupture causes the high v p v s ratio the high v p v s ratio could indicate postseismic fluid migration from the subducted oceanic crust into the overly ing lower crust

p wave velocity structure of the southern ryukyu margin east of taiwan results from the acts wide angle seismic experiment abstract an active seismic experiment has been conducted across the southern ryukyu margin east of taiwan over the whole trench arc backarc system in may 2009 twenty four ocean bottom seismometers obs were deployed from the ryukyu trench to the southern okinawa trough over the ryukyu arc and forearc wide angle seismic data were recorded by the obs array while coincident reflection seismic data were acquired using a 6 km long streamer and a 6600 cubic inch seismic airgun array results from tomographic inversion of 21091 travel time picks along this line allowed us to image crustal structures of the ryukyu margin down to a depth of 25 km the transect has been designed to provide a better seismic velocity structure of the subduction zone in a highly deformed area that has produced an m8 earthquake in 1920 the line crosses a seismic cluster of earthquakes which source mechanisms are still poorly understood the subducting oceanic crust of the huatung basin is about 5 6 km thick the underlying mantle exhibits low seismic velocities around 7 8 km s suggesting some hydrothermal alterations or alteration of the upper mantle through faults generated by the flexure of the subducting plate as it enters the subduction low velocities up to 4 5 km s associated with the accretionary wedge are well imaged from the trench back to the nanao forearc a major result concerns the abrupt termination of the buttress at the rear of the accretionary wedge despite the low resolution of the tomographic inversion near the subduction interface several lines of evidence supporting the presence of a low velocity zone beneath the toe of the forearc buttress could be established the moho beneath the ryukyu non volcanic arc is located at a depth around 25 km depth

transition from oblique subduction to collision earthquakes in the southernmost ryukyu arc taiwan region tectonic characteristics of the region between taiwan and the southernmost ryukyu arc are inferred from a detailed analysis of local seismicity and source parameters of 62 recent earthquakes of 5 5 mb 6 6 five major seismogenic structures can be delineated the collision seismic zone csz the interface seismic zone isz the wadati benioff seismic zone wbsz the lateral compression seismic zone lcsz and the okinawa seismic zone osz in the csz located along the east coast of taiwan and offshore earthquake focal mechanisms show horizontal p axes distributed in two directions 287 10 and 333 16 possibly reflecting a strain partition associated with the relative plate convergence between the eurasia plate and the philippine sea plate the corresponding seismic strain tensor indicates a maximum compressive strain rate of 1 2 10 7 yr 1 along 293 and a comparable extension in vertical direction presumably resulted from plate collision in the region the geometry of the isz which is distorted significantly at its westernmost end can be approximated by a north dipping plane that is gradually pushed northward with increasing dip the seismogenic portion of the interface spans a short depth range from 10 km to 35 km a clear pattern of earthquake slip partition is observed the average slip vector residual is as large as 35 seismic strain patterns within the subducted philippine sea slab show predominantly downdip extension between 80 and 120 km and downdip compression at 270 km different from the pattern of strain segmentation observed for the rest of the ryukyu arc where the northern and southern portions are dominated by downdip extension and compression respectively owing to the large convergence obliquity the slab is descending at a rate significantly slower near taiwan than in the southern ryukyu thus we interpret the appearance of downdip extension within the subducted lithosphere as a combined result of oblique subduction and the slab s negative buoyancy a number of thrust or oblique strike slip earthquakes between isz and wbsz show a consistent pattern of lateral compression with p axes oriented roughly parallel to the local strike of the trench arc system they are probably due to the compressive strain originated from the collision and transmitted laterally within the lithosphere shallow normal faulting earthquakes show successive rotation of t axes from approximately n s in the okinawa trough to approximately e w in northeast taiwan possibly as a result of interaction between the extension from the opening of the okinawa trough and the compression from collision one normal event january 18 1991 mb 5 9 occurred in the central range with t axis roughly parallel to the structural trend of taiwan implying that the nature of the orogeny in taiwan has changed from thin skinned deformation to lithospheric collision involving the whole crust and uppermost mantle

on the biannually repeating slow slip events at the ryukyu trench southwestern japan 1 global positioning system data show that about 20 slow slip events occurred during 1997 2007 in the southwestern part of the ryukyu arc japan where large interplate thrust earthquakes are not known to have occurred in spite of relatively fast plate convergence they recur fairly regularly on one patch of the subduction fault which is as deep as 20 40 km and mechanically isolated in an uncoupled subduction zone they have time constants of a month or so and release seismic moment equivalent of mw 6 6 on average this is one of the best recorded sequences of repeating fault slips with enough events for statistical analyses they have fairly constant recurrence interval of 0 5 year but do not show preferred months of occurrence in a year suggesting that they are not controlled by seasonal forcing the correlation between intervals and amounts of fault slip show statistically significant time predictable recurrences there is one significant disruption in the rhythm but the stress perturbations due to nearby earthquakes do not appear to play a significant role this part of the fault might be an altered asperity that used to rupture fast but now slips only slowly because of the increased ambient temperature

improved seismic tomography offshore northeastern taiwan implications for subduction and collision processes between taiwan and the southernmost ryukyu summary to improve the resolution in imaging the 3 d vp and vp vs structures in the taiwan region especially offshore eastern taiwan we combine the arrival times from eleven ocean bottom seismometers obss and from seismic stations of the japan meteorological agency jma with those from the permanent stations of taiwan central weather bureau seismic network cwbsn and taiwan strong motion instrumentation program tsmip by doing so we have obtained a new 3 d tomographic model for taiwan and its surrounding regions with a better resolution especially in the area offshore northeastern taiwan we also used this new tomography model to relocate the hypocentres of the earthquakes in northeastern taiwan and determined the focal mechanisms of relatively large events our tomography results indicate that in the region northeast of taiwan the subducting oceanic philippine sea plate beneath the eurasian continental lithosphere is characterized by a high vp layer surrounded by lower vp areas this p wave velocity characteristics of the subducting slab provides a better constraint on the geometry of the subduction interface especially in its shallower portion in the hangingwall eurasian block above the subduction interface a vertically elongated high vp vs body appears to originate from the interface at depths between 100 and 140 km we suggest that it represents the partially melted materials that are related to the magmatic activity of the ryukyu volcanic arc

physical characteristics of subduction interface type seismogenic zones revisited 1 based on global earthquake catalogs the hypocenters nodal planes and seismic moments of worldwide subduction plate interface earthquakes were extracted for the period between 1900 and 2007 assuming that the seismogenic zone coincides with the distribution of 5 5 m 7 earthquakes the subduction interface seismogenic zones were mapped for 80 of the trench systems and characterized with geometrical and mechanical parameters using this database correlations were isolated between significant parameters to identify cause effect relationships empirical laws obtained in previous studies were revisited in light of this more complete accurate and uniform description of the subduction interface seismogenic zone the seismogenic zone was usually found to end in a fore arc mantle rather than at a moho depth the subduction velocity was the first order controlling parameter for variations in the physical characteristics of plate interfaces determining both the geometry and mechanical behavior as such the fast subduction zones and cold slabs were associated with large and steep plate interfaces which in turn had large seismic rates the subduction velocity could not account for the potential earthquake magnitude diversity that was observed along the trenches events with mw 8 5 preferentially occurred in the vicinity of slab edges where the upper plate was continental and the back arc strain was neutral this observation was interpreted in terms of compressive normal stresses along the plate interface large lateral ruptures should be promoted in neutral subduction zones due to moderate compressive stresses along the plate interface that allow the rupture to propagate laterally

seismotectonics of taiwan abstract high quality seismicity data and focal mechanism solutions obtained during 1973 1983 by the permanent taiwan telemetered seismographic network and several temporary local seismographic networks are used for a detailed study of the seismotectonics of the taiwan area seismicity distribution in southern taiwan clearly reveals an east dipping benioff zone which has a thickness of about 30 km and begins to deepen along 121 e at a dip angle of 55 60 the leading edge of this benioff zone reaches a depth of about 180 km between 21 n and 22 n but tapers off to a shallower depth of about 100 km from 22 n to 23 n the presence of this seismic zone implies that subduction of the south china sea plate under the philippine sea plate extends from luzon northward to about 23 n the position of the northern boundary of the south china sea plate as tentatively defined according to the seismicity distribution passes through southern taiwan from the offshore area in the taiwan strait west of kaohsiung in an east northeast direction to the taitung area where a triple junction probably lies seismicity is found to disappear abruptly below a certain depth in many parts of taiwan this phenomenon may be attributed to the frictional to quasiplastic transition in the crust or upper mantle comparison of shallow seismicity with surface faults and fractures shows that all areas of active shallow seismicity are marked by densely developed faults and fractures however the converse is not necessarily true this may be partly due to the relatively short duration of seismicity data and partly due to excessive weakening of some of the severely faulted and fractured areas finally focal mechanism solutions for west central taiwan and the kuangfu fuli area in eastern taiwan predominantly show a maximum horizontal compression in the se nw direction which can be related to collision between the eurasian and philippine sea plates however focal mechanism solutions for both the hualien area in eastern taiwan and the tainan area in southwestern taiwan show remarkable irregularities which may result from local tectonic complexities

neotectonic architecture of taiwan and its implications for future large earthquakes the disastrous effects of the 1999 chi chi earthquake in taiwan demonstrated an urgent need for better knowledge of the island s potential earthquake sources toward this end we have prepared a neotectonic map of taiwan the map and related cross sections are based upon structural and geomorphic expression of active faults and folds both in the field and on shaded relief maps prepared from a 40 m resolution digital elevation model augmented by geodetic and seismologic data the active tandem suturing and tandem disengagement of a volcanic arc and a continental sliver to and from the eurasian continental margin have created two neotectonic belts in taiwan in the southern part of the orogen both belts are in the final stage of consuming oceanic crust collision and suturing occur in the middle part of both belts and postcollisional collapse and extension dominate the island s northern and northeastern flanks both belts consist of several distinct neotectonic domains seven domains kaoping chiayi taichung miaoli hsinchu ilan and taipei constitute the western belt and four domains lutao lanyu taitung hualien and ryukyu make up the eastern belt each domain is defined by a distinct suite of active structures for example the chelungpu fault source of the 1999 earthquake and its western neighbor the changhua fault are the principal components of the taichung domain whereas both its neighboring domains the chiayi and miaoli domains are dominated by major blind faults in most of the domains the size of the principal active fault is large enough to produce future earthquakes with magnitudes in the mid 7 values