out of sequence thrust faults developed in the coseismic slip zone of the 1946 nankai earthquake mw 8 2 off shikoku southwest japan a multi channel seismic mcs reflection survey was conducted to study the subsurface structure of the coseismic slip zone of the 1946 nankai earthquake mw 8 2 off shikoku island in 1997 we could identify a splay fault system consisting of several sigmoid out of sequence thrust ost faults dipping landward with slope 10 25 on the poststack depth migrated mcs profiles most of the osts are apparently developed from the subducting oceanic basement to the seafloor in the forearc region cutting both underthrust sediments and the overriding accretionary prism in addition most of the osts are within the coseismic zone of the 1946 nankai earthquake and the interseismic locked zone the osts are considered to be related to large interplate earthquakes including the 1946 nankai earthquake these osts may be responsible for tsunami generation following deformation of forearc accretionary wedge

geodetic and seismic constraints on some seismogenic zone processes in costa rica new seismic and geodetic data from costa rica provide insight into seismogenic zone processes in central america where the cocos and caribbean plates converge seismic data are from combined land and ocean bottom deployments in the nicoya peninsula in northern costa rica and near the osa peninsula in southern costa rica in nicoya inversion of gps data suggests two locked patches centered at 14 2 and 39 6 km depth interplate microseismicity is concentrated in the more freely slipping intermediate zone suggesting that small interseismic earthquakes may not accurately outline the updip limit of the seismogenic zone the rupture zone for future large earthquakes at least over the short 1 year observation period we also estimate northwest motion of a coastal sliver block at 8 3 mm yr probably related to oblique convergence in the osa region to the south convergence is orthogonal to the trench cocos caribbean relative motion is partitioned here with 8 cm yr on the cocos panama block boundary including a component of permanent shortening across the fila costena fold and thrust belt and 1 cm yr on the panama block caribbean boundary the gps data suggest that the cocos plate panama block boundary is completely locked from 10 50 km depth this large locked zone as well as associated forearc and back arc deformation may be related to subduction of the shallow cocos ridge and or younger lithosphere compared to nicoya with consequent higher coupling and compressive stress in the direction of plate convergence

full interseismic locking of the nankai and japan west kurile subduction zones an analysis of uniform elastic strain accumulation in japan constrained by permanent gps we analyze permanent global positioning system gps data obtained over japan between 1995 and 1997 to estimate the instantaneous interseismic coupling ratio of the seismogenic zones due to the subduction of the pacific and philippine sea plates below the japanese islands we first derive the gps strain rate fields that characterize the crustal deformation of southern and northern japan and invert them to determine the effective subduction velocity along the central nankai trough on one side and the japan west kurile trench on the other these reference free velocities are close to those predicted by plate motion models with respect to eurasia we conclude that the eurasian reference frame gives a good approximation to the subduction motion and that to first order both subduction zones were fully locked during the period of measurements we then test whether the coupling ratio shows local variations within the seismogenic zones to do this we divide the subduction interface into 35 km 30 km elements that we model by point source groups and we invert the gps velocity field referenced to eurasia to derive the coupling ratio between 0 and 1 on each fault element the results are coherent over the 3 years and confirm that both the central nankai and the japan west kurile seismogenic zones are homogeneously fully locked most of the coupling ratios are close to 1 and a few are close to 0 intermediate values are rare the zones of decoupling correspond either to strong postseismic afterslip associated with the 1994 sanriku oki interplate earthquake japan trench or to a small overestimation of the actual lower limit of the locked zone we conclude that within the resolution of the gps data and the model 1 partial coupling did not exist during these 3 years along the nankai and japan west kurile trenches 2 the small seismic coupling ratio previously derived from earthquakes analysis for the japan and kurile trenches may indicate that a significant part of the elastic energy is dissipated silently through slow earthquakes and postseismic afterslip and 3 the heterogeneous coseismic slip pattern observed for the large and great earthquakes that rupture both subduction zones is in great contrast to the homogeneous loading finally we discuss the nonelastic residual deformation within the frame of the long term deformation of the japanese islands

micro seismicity around the seaward updip limit of the 1946 nankai earthquake dislocation area we observed micro seismicity around the seaward updip limit of the 1946 nankai earthquake dislocation area by using pop up type ocean bottom seismographs deployed for three months in 1998 at the subduction zone we must consider strong lateral heterogeneity of the seismic velocity structure to locate earthquakes precisely we constructed 3 d p and s wave velocity structure models based on the results of airgun obs seismic surveys along the nankai trough we located 83 earthquakes using these 3 d models these hypocenters show two groups of seismicity one seismicity group locates in oceanic layer 3 within a subducting seamount these earthquakes may reflect the deformation of the seamount caused by its subduction the other seismicity group locates near the updip limit of the thermally modeled locked zone these hypocenters are determined to be near the boundary between the accretionary prism and the subducting oceanic crust

a cause of rupture segmentation and synchronization in the nankai trough revealed by seismic imaging and numerical simulation 1 a giant earthquake occasionally occurs in a subduction zone owing to a simultaneous rupture in adjacent segments which have been previously ruptured by large earthquakes however it is still unknown if a giant earthquake coincidentally occurs or if there is a causal factor to control its generation in this study we show a cause and a growth process of a giant earthquake which may occur along southwestern japan on the basis of seismic images obtained from wide angle seismic data and a numerical simulation incorporating the structural images the wide angle seismic data were acquired along three trough parallel profiles crossing the rupture segmentation boundary between the 1944 tonankai moment magnitude mw 8 1 and the 1946 nankai mw 8 4 earthquakes the seismic imaging detected a high seismic velocity body forming a strongly coupled patch at the segmentation boundary the numerical simulation explained the historic rupture patterns and shows the occurrence of a giant earthquake along the entire nankai trough a distance of over 600 km long mw 8 7 the growth process revealed from the simulated slip history in and around the strongly coupled patch is 1 prior to the giant earthquake a small slow event or earthquake occurs near the segmentation boundary 2 this accelerates a very slow slip slower than the plate convergent rate at the strong patch which reduces a degree of coupling and 3 then a rupture easily propagates through the strong patch when the next earthquake is nucleated near the segmentation boundary consequently growing into a giant earthquake

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

thermal constraints on the seismogenic portion of the southwestern japan subduction thrust for coastal cities an important factor in earthquake hazard from subduction zone earthquakes is the landward extent of the seismogenic portion of the subduction thrust fault in this study we test the hypothesis that the maximum downdip extent is defined by a critical temperature we have developed a transient thermal model for the nankai subduction zone of southwest japan to allow comparison of the thermally estimated downdip extent of the seismogenic zone with that from 1 seismicity and tsunami data for two great subduction earthquakes 2 the coseismic faulting extent of these events estimated from geodetic deformation data and 3 the interseismic locked zone determined from interseismic geodetic data the nankai margin has extensive heat flow and heat production data to control thermal models and thus crustal temperatures it has earthquake tsunami and geodetic data that constrain the coseismic rupture portion of the subduction thrust fault for past great earthquakes and the portion of the thrust fault that is locked and storing interseismic elastic strain on the nankaido margin off shikoku island the thermal model indicates that a temperature of 350 c taken to be the limit for seismic initiation from laboratory and field data is reached on the subduction thrust fault 150 km from the trench a transition zone into which rupture may extend with decreasing offset taken to be 450 c extends an additional 45 km downdip the thermal model results are in excellent agreement with the maximum downdip extent of coseismic displacement for the 1946 nankaido ms 8 2 earthquake off shikoku island and with the downdip extent of the present locked zone in the region of the 1944 tonankai ms 8 2 earthquake to the northeast the subduction angle is much steeper and the thermal models indicate a narrower downdip seismogenic extent the seismogenic locked zone from earthquake and geodetic data is also narrower thus our analysis of the southwest japan margin indicates that all three constraints on the downdip extent of the seismogenic zone thermal coseismic and interseismic geodetic data are in general agreement the study also supports the hypothesis that the seismogenic portion of subduction thrust faults is limited primarily by temperature the thermal control implies that subduction thrust faults with shallow dip have wider seismogenic zones compared to those with steep dip the subducting plate age and thus heat flow and the thickness of the insulating sediments on the incoming plate are also very important to the thermal regime and thus to the seismogenic width the relation of the maximum seismic rupture area to the interseismic locked zone is particularly important for earthquake hazard estimation on subduction margins such as cascadia where there have been no great historical events

crustal structure across the coseismic rupture zone of the 1944 tonankai earthquake the central nankai trough seismogenic zone 1 differences in the coseismic rupture process between the 1944 tonankai and the 1946 nankaido earthquakes have been studied by many fault models to understand what factors control coseismic rupture zones it is important to investigate differences in deep crustal structures of the rupture zones between the 1944 and 1946 earthquakes the previously published crustal structure of the rupture zone of the 1946 earthquake shows that the coseismic rupture extends to the neogene quaternary accretionary prism however little is known about the structure of the rupture zone of the 1944 earthquake to obtain a complete image of the seismogenic zone of the 1944 earthquake a wide angle seismic survey was performed across the presumed coseismic rupture zone of the 1944 earthquake from ocean to land our model for the crustal structure is based on ocean bottom seismographic data the crustal structure appears characteristic for subducting oceanic crust and a neogene quaternary accretionary prism bounded by an island arc crust the neogene quaternary accretionary prism reaches a maximum thickness of 7 km at 50 km distance landward from the deformation front the subducting oceanic crust can be traced down to 35 km the subduction angle becomes steeper landward reaching up to 11 beneath the island arc crust the depth of the top of subducting oceanic crust at the downdip limit of the rupture zone is 23 km while the updip limit is located beneath the island arc upper crust similar structures of the updip and downdip limits are also published for several other subduction zones

subducted seamount imaged in the rupture zone of the 1946 nankaido earthquake the nankai trough is a vigorous subduction zone where large earthquakes have been recorded since the seventh century with a recurrence time of 100 to 200 years the 1946 nankaido earthquake was unusual with a rupture zone estimated from long period geodetic data that was more than twice as large as that derived from shorter period seismic data in the center of this earthquake rupture zone we used densely deployed ocean bottom seismographs to detect a subducted seamount 13 kilometers thick by 50 kilometers wide at a depth of 10 kilometers we propose that this seamount might work as a barrier inhibiting brittle seismogenic rupture

geological record of fluid flow and seismogenesis along an erosive subducting plate boundary a fossil erosive subduction channel the shear zone marking the plate boundary preserved in the northern apennines of italy is identified the fossil zone records the presence of two decollements which seem to have been simultaneously active at the top and base of the subduction channel in modern subduction zones the onset of seismic activity is believed to occur at 150 c however in the fossil channel the onset seems to have occurred at colder temperatures tectonic erosion of the overriding plate by the downgoing slab is believed to occur at half the earth s subduction zones1 2 in situ investigation of the geological processes at active erosive margins is extremely difficult owing to the deep marine environment and the net loss of forearc crust to deeper levels in the subduction zone until now a fossil erosive subduction channel the shear zone marking the plate boundary3 has not been recognized in the field so that seismic observations have provided the only information on plate boundary processes at erosive margins here we show that a fossil erosive margin is preserved in the northern apennines of italy it formed during the tertiary transition from oceanic subduction to continental collision and was preserved by the late deactivation and fossilization of the plate boundary the outcropping erosive subduction channel is 500 m thick it is representative of the first 5 km of depth with its deeper portions reaching 150 c the fossil zone records several surprises two decollements were simultaneously active at the top and base of the subduction channel both deeper basal erosion and near surface frontal erosion occurred at shallow depths extension was a key deformation component within this erosive convergent plate boundary and slip occurred without an observable fluid pressure cycle at depths greater than about 3 km a fluid cycle is clearly shown by the development of veins and the alternation of fast co seismic and slow inter seismic slip in the deepest portions of the outcropping subduction channel extension is finally overprinted by compressional structures in modern subduction zones the onset of seismic activity is believed to occur at 150 c but in the fossil channel the onset occurred at cooler palaeo temperatures

western nankai trough seismogenic zone results from a wide angle ocean bottom seismic survey the nankai trough southwestern japan is recognized as a vigorous seismogenic zone with well studied historic earthquakes this paper presents results of a wide angle ocean bottom seismographs obs study at the western nankai trough seismogenic zone the obs data used were acquired on a profile 250 km long across the presumed coseismic slip zone of the 1946 nankaido earthquake ms 8 2 the main purpose of the seismic study is to obtain an entire crustal cross section of the seismogenic zone for the 1946 earthquake the crustal model is characterized by a gentle sloping of subducting oceanic crust and thick overlying sedimentary wedge p wave seismic velocities of the subducting oceanic crust show normal oceanic crustal velocities vp 5 0 5 6 km s and 6 6 6 8 km s in oceanic layers 2 and 3 respectively the maximum thickness of the sedimentary wedge is 9 km at 70 km from the trough axis with vp 3 4 4 6 km s in the deeper part the subducting oceanic crust traced down to 25 km depth shows that the subduction angle becomes steeper landward 3 2 and 7 2 at 0 50 km and 50 100 km from the trough axis respectively the oceanic crust is smooth to the hypocenter zone down to 40 km depth beneath shikoku island our crustal model shows that the downdip limit of the coseismic slip area does not extend to the deep end of the oceanic crust island arc crust contact zone even though there is large uncertainty about the seaward limit of the coseismic slip zone the crustal model clearly indicates that the updip limit of the coseismic slip zone extends beneath the young accretionary prism

clustering and periodic recurrence of microearthquakes on the san andreas fault at parkfield california the san andreas fault at parkfield california apparently late in an interval between repeating magnitude 6 earthquakes is yielding to tectonic loading partly by seismic slip concentrated in a relatively sparse distribution of small clusters 20 meter radius of microearthquakes within these clusters which account for 63 of the earthquakes in a 1987 92 study interval virtually identical small earthquakes occurred with a regularity that can be described by the statistical model used previously in forecasting large characteristic earthquakes sympathetic occurrence of microearthquakes in nearby clusters was observed within a range of about 200 meters at communication speeds of 10 to 100 centimeters per second the rate of earthquake occurrence particularly at depth increased significantly during the study period but the fraction of earthquakes that were cluster members decreased

along strike variability in the seismogenic zone below nicoya peninsula costa rica 1 at the subduction zone in northwestern costa rica the seismogenic zone lies directly beneath the nicoya peninsula allowing for near source seismic studies of earthquake activity we located 650 earthquakes along the seismogenic plate interface using a dense seismic network in the vicinity of the nicoya peninsula using these data we constrained the updip limit of the seismogenic zone there and found a transition in depth 10 km in the south to 20 km in the north that occurs where the subducting oceanic crust changes from warmer cocos nazca spreading center cns origin to colder east pacific rise epr origin we argue that the temperature of the incoming oceanic crust controls the seismogenic updip limit beneath nicoya costa rica subducting colder oceanic crust deepens the seismogenic updip limit

accurate hypocentre determination in the seismogenic zone of the subducting nazca plate in northern chile using a combined on offshore network the coupled plate interface of subduction zones commonly called the seismogenic zone has been recognized as the origin of fatal earthquakes a subset of the after shock series of the great antofagasta thrust type event 1995 july 30 mw 8 0 has been used to study the extent of the seismogenic zone in northern chile to achieve reliable and precise hypocentre locations we applied the concept of the minimum 1 d model which incorporates iterative simultaneous inversion of velocity and hypocentre parameters the minimum 1 d model is complemented by station corrections which are influenced by near surface velocity heterogeneity and by the individual station elevations by relocating mine blasts which were not included in the inversion we obtain absolute location errors of 1 km in epicentre and 2 km in focal depth a study of the resolution parameters ale and dspr documents the importance of offshore stations on location accuracy for offshore events based on precisely determined hypo centres we calculate a depth of 46 km for the lower limit of the seismogenic zone which is in good agreement with previous studies for this area for the upper limit we found a depth of 20 km our results of an aseismic zone between the upper limit of the seismogenic zone and the surface correlates with a detachment zone proposed by other studies the results are also in agreement with thermal studies for the antofagasta forearc region

petroleum and aqueous inclusions from deeply buried reservoirs experimental simulations and consequences for overpressure estimates abstract synthetic hydrocarbon and aqueous inclusions have been created in the laboratory batch reactors in order to mimic inclusion formation or re equilibration in deeply buried reservoirs inclusions were synthesized in quartz and calcite using pure water and mexican dead oil or n tetradecane c14h30 at a temperature and pressure of 150 c and 1 kbar one phase hydrocarbon inclusions are frequently observed at standard laboratory conditions leading to homogenization temperatures between 0 and 60 c uv epifluorescence of mexican oil inclusions is not uniform blue and green yellow colored inclusions coexist however no clear evidence of variations in fluid chemistry were observed homogenization temperatures were recorded and the maxima of th plotted on histograms are in good agreement with expected th in a range of 6 c broad histograms were reconstructed showing non symmetrical th distributions over a 20 c temperature range centered on the expected th this histogram broadening is due to the fragility of the fluid inclusions that were created by re filling of pre existing microcavities such th histograms are similar to th histograms recorded on natural samples from deeply buried carbonate reservoirs th values lower than those expected were measured for hydrocarbon inclusions in quartz and calcite and for aqueous inclusions in calcite however the results confirm the ability of fluid inclusions containing two immiscible fluids to lead to pt reconstructions even in overpressured environments

three dimensional seismic refraction tomography a comparison of two methods applied to data from the faeroe basin this paper presents a comparison of two tomographic methods for determining three dimensional 3 d velocity structure from first arrival travel time data the first method is backprojection in which travel time residuals are distributed along their ray paths independently of all other rays the second method is regularized inversion in which a combination of data misfit and model roughness is minimized to provide the smoothest model appropriate for the data errors both methods are nonlinear in that a starting model is required and new ray paths are calculated at each iteration travel times are calculated using an efficient implementation of an existing method for solving the eikonal equation by finite differencing both inverse methods are applied to 3 d ocean bottom seismometer obs data collected in 1993 over the faeroe basin consisting of 53 479 travel times recorded at 29 obss this is one of the most densely spaced large scale 3 d seismic refraction experiments to date different starting models and values for the free parameters of each tomographic method are tested a new form of backprojection that converges more rapidly than similar methods compares favorably with regularized inversion but the latter method provides a simpler model for little additional computational expense when applied to the faeroe basin data bounds on two model features are assessed using regularized inversion with combined smoothness and flatness constraints an inversion of synthetic data corresponding to 100 data recovery from the real experiment shows a marked improvement in lateral resolution at deeper depths and demonstrates the potential of currently feasible 3 d refraction experiments to provide well resolved long wavelength velocity models the similarity of the final models derived from the two tomographic methods suggests that the results from the new form of backprojection can be relied on when limited computational resources rule out regularized inversion

deformable backstop as seaward end of coseismic slip in the nankai trough seismogenic zone abstract wide angle seismic surveys performed in the last decade have clarified the 3 d crustal structure along the nankai trough the geometry and velocity structure of the southwestern japan subduction zone are now well constrained comparing these observations with the rupture distribution of historic great thrust earthquakes it appears that the coseismic rupture occurred along plate boundaries deeper than the wedge backstop boundary the boundary between the neogene quaternary accretionary wedge and the crust forming the backstop from the view of spatial relationship both rupture distributions of the last two large events and the crust forming the backstop are considerably retreated from the trough axis in the west and east off the kii peninsula in both areas seamount or ridge subduction is apparent in seismic results geomorphological data and geomagnetic data the landward indentation of the deformable backstop which corresponds to the crustal block of old accreted sediments may be formed by seamount subduction according to published results of sandbox modeling in particular the subducted seamount may be a structural factor affecting the recession of the crustal block forming the backstop

microseismicity at the seaward updip limit of the western nankai trough seismogenic zone 1 the seismogenic zone at subduction zones does not normally extend to the trench axis the shallowest part of the plate interface is considered to be seismically decoupled the location of the seaward updip limit of the seismogenic zone and its relation to the crustal structure are important for understanding the transition process from aseismic slip to the seismic rupture at the plate interface the nankai trough seismogenic zone southwestern japan is one of the most well studied subduction seismogenic zones in the world however the offshore seismicity around the nankai trough is very low and hypocenters are not determined accurately by the on land seismic network we performed microseismicity observations using ocean bottom seismographs off cape muroto hypocenters were determined by using a three dimensional vp and vs structure based on seismic survey results the observed microseismicity seems to be classifiable into two types the first is earthquakes that occur around the plate interface the second is a group of earthquakes that occur in the uppermost mantle of the subducting oceanic plate the seismicity around the plate interface forms several clusters the seaward limit of the clusters is characterized by earthquakes with a similar waveform these similar waveform earthquakes are considered to occur at small asperities in the aseismic seismogenic transition zone of the plate interface the seismicity in the uppermost mantle may be related to dehydration embrittlement of subducting serpentinized mantle

seismological studies at parkfield vi moment release rates and estimates of source parameters for small repeating earthquakes abstract waveform data from a borehole network of broadband seismographic stations have been used to study microearthquakes along the parkfield segment of the san andreas fault saf analysis of almost 10 years of such data demonstrates that much of the seismicity in this region consists of repeating sequences quasiperiodic sequences of earthquakes that are essentially identical in terms of waveform size and location scalar seismic moments have been estimated for 53 of these repeating sequences and combined with equivalent estimates from 8 similar but larger event sequences from the stone canyon section of the fault and the main parkfield sequence these estimates show that seismic moment is being released as a function of time in a very regular manner measurements of the moment release rate combined with an assumed tectonic loading rate lead to estimates of the seismic parameters source area slip and recurrence interval such parameters exhibit a systematic dependence upon source size over a range of 10 10 in seismic moment that can be described by three simple scaling relationships several implications of these scaling relationships are explored including the repeat time of earthquakes average stress drop strength of the fault and heat generated by earthquakes what emerges from this analysis of moment release rates is a quantitative description of an earthquake process that is controlled by small strong asperities that occupy less than 1 of the fault area this means that the fault is highly heterogeneous with respect to stress strength and heat generation such heterogeneity helps to explain many of the apparent contradictions that are encountered in the study of earthquakes such as why faults appear weak why significant heat flow is not observed how significant high frequencies can be generated by large earthquakes and how various geologic features such as pseudotachylytes might form

updip limit of the seismogenic zone beneath the accretionary prism of southwest japan an effect of diagenetic to low grade metamorphic processes and increasing effective stress off southwest japan the seaward limit of coseismic displacement or updip limit of the seismogenic zone of the 1946 mw 8 3 thrust earthquake reaches to 4 km depth and 40 km landward of the trench this limit coincides with the estimated location of the 150 c isotherm and has been linked to changes in physical properties associated with the smectite to illite clay mineral transition here we show that this limit correlates with a suite of diagenetic to low grade metamorphic processes characterized by 1 declining fluid production and decreasing fluid pressure ratio and 2 active clay carbonate and zeolite cementation and the transition to pressure solution and quartz cementation these diagenetic to low grade metamorphic changes cause the onset of velocity weakening during thrust faulting an increase in effective stress and strengthening of the hanging wall which together combine to produce recordable earthquakes

pseudotachylyte from an ancient accretionary complex evidence for melt generation during seismic slip along a master decollement pseudotachylyte is present in the shimanto accretionary complex in shikoku japan the pseudotachylyte occurs as a vein in a fault zone and displays flow structure tio 2 microlites and other melting related textures the mineral assemblage of the pseudotachylyte suggests that the temperature was raised at least 450 c above the background levels in the host rocks this is the first direct evidence of seismogenic fault rock from an accretionary complex the pseudotachylyte bearing fault is a roof thrust above underthrusted melange which is overlain by an offscraped package of coherent units the geologic setting of the pseudotachylyte is consistent with melt generation during seismic slip along the master decollement in the subduction zone

thermal stresses in subducting lithosphere can explain double seismic zones the discovery of double planed zones of intermediate depth seismicity associated with subducting oceanic lithosphere refs 1 7 and l s h and k h j in preparation has generated much interest in the origin and distribution of stresses in descending plates several mechanisms have been proposed to explain why double zones occur8 here we investigate the role of thermal stresses for the state of stress in a descending plate and test whether they can account for the observed characteristics of double seismic zones by modelling the descending slab as a thin elastic plate and using the temperatures in the slab estimated by toksoz et al 9 we find that the polarities of thermally generated stresses agree with those obtained from the focal mechanisms of earthquakes in the two zones thermal stresses also can explain the separation between the two zones of seismicity at shallow depths and their gradual merging at greater depth

subducting seamounts and deformation of overriding forearc wedges around japan abstract two subducting seamounts under inner trench slopes have been identified around japan on the basis of magnetic anomalies morphology and geological structure the first one is located under the foot of the inner trench slope at the junction between the japan trench and the kuril trench another one occurs beneath the slope slightly seaward of the tosabae the basement high at the trench slope break along the nankai trough off shikoku the magnetic anomalies of seamount origin are accompanied by the characteristic morphology of a forearc wedge i e a swell landward and a depression seaward the seamounts beneath the inner trench slopes have preserved magnetization showing reasonably consistent directions which suggests that the subducting seamounts have kept roughly their original shapes the morphology of the forearc wedge can be explained by a subducting seamount on the oceanic crust pushing the forearc material forward and upward deformation of the forearc wedge by the subducting seamount extends to the forearc basin the seamounts are stronger and less deformable than the inner slope material and are not offscraped onto inner trench slopes two other examples of deformed inner trench slopes around japan which can be explained by subduction of topographic highs are presented one example is a depression on the foot of the inner trench slope northeast of the junction between the kyushu palau ridge and the nankai trough another one is an area of complex morphology of the inner trench slope along the japan trench around the daiichi kashima seamount

detection and location of earthquakes in the central aleutian subduction zone using island and ocean bottom seismograph stations a network of eight university of texas ocean bottom seismographs obs operated for 6 weeks in 1978 about 50 km offshore of adak island alaska and nearby islands in 1979 a similar network of nine instruments was deployed for 7 weeks farther offshore within and up to 100 km seaward of the aleutian trench for shallow earthquakes on the outer trench slope for shallow earthquakes in the thrust zone and for intermediate depth events we have analyzed the obs and island based network data and evaluated the island network s capabilities for earthquake detection and location and for focal mechanism determination our three major conclusions are presented the first concerns shallow earthquakes on the outer trench slope in 1979 about 30 earthquakes occurred within the aleutian trench and up to 60 km seaward of the trench axis the island network located none of these events and detected p phases for only three of them ray tracing shows that the islands lie in a geometric shadow zone for events on the outer trench slope the best located events are shallower than 20 km and exhibit first motions consistent with normal faulting several authors have suggested that these events are caused by bending of the oceanic lithosphere at the outer rise prior to subduction if so then the event locations reported here show that the bending stresses exceed the strength of lithosphere only in a narrow zone extending about 10 km landward and 60 km seaward of the trench axis the second conclusion concerns shallow earthquakes in the thrust zone epicenters determined by island stations alone are virtually identical to epicenters determined using data from both island and obs stations the locations are similar whether they are determined using flat layered velocity models or a more realistic model and a ray tracing scheme nearly all the earthquakes detected by the obs network are also recorded by island stations three composite focal mechanism solutions that use data from both island and obs stations have p axes parallel to the direction of plate convergence the thrust zone events are separated from events on the outer trench slope by a gap in seismicity about 50 km wide situated on the inner trench slope the third conclusion concerns earthquakes deeper than 70 km epicenters determined using island network stations alone lie 10 to 80 km south of those determined using obs and island stations with the differences between epicenters depending both on event depth and on the velocity model used ray tracing with a realistic velocity model shows that for events at 200 km depth locations are about 80 km north of locations determined from island data alone these results do not support the observed increase in the dip of the benioff zone beneath 100 km depth that is suggested by locations determined from island network data alone using a flat layered velocity model

loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones earthquakes do not extend updip along the plate interface at most subduction zones all of the way to the plate boundary at the trench axis or deformation front rather the shallowest part of that interface moves mainly through stable aseismic slip that part of the plate boundary referred to here as the aseismic zone occurs along the base of an accreted wedge of young sediments the probable primary cause for the existence of this aseismic zone is the stable slip properties of the unconsolidated and semiconsolidated sediments in that zone subducted sediment is progressively dewatered and underplated to the base of the overriding plate through this process more consolidated rocks eventually come into contact at depth across both sides of the plate boundary from the point where sufficiently hard rock is found across the plate interface that interface will change its slip behavior to unstable stick slip sliding which is characteristic of consolidated material under most conditions this type of motion is accommodated seismically as episodic slip in large earthquakes the location of this transition to seismic behavior referred to as the seismic front marks the deep end of the aseismic zone and the top of the seismogenic zone i e that part of the plate interface that moves primarily in thrust earthquakes several convergent plate margins are discussed to illustrate the seismic front and the aseismic zone we find that the seismic front defined by smaller earthquakes that occur during the interval between large events is nearly the same as that for large events as inferred from the locations of their aftershocks the location of the seismic front is important for making estimates of the maximum possible size of thrust earthquakes along the plate boundary because it delimits the trenchward limit of the potential rupture area of these events the size of large thrust earthquakes is proportional to rupture area and to at least the cube of the downdip width w of that area thus by defining the location of the seismic front for convergent margins it is possible to make better estimates of w and hence to deduce the maximum size of future interplate earthquakes the average repeat time of such events is also related to w and so better estimates of average repeat time are possible from improved knowledge of w the hypothesis that the location of the seismic front is related to the maximum depth of subduction of unconsolidated sediment has implications for forearc mechanics we use a mechanical analysis laboratory modeling and multichannel seismic information to develop a simple model explaining the growth of forearcs the outer arc high or trench slope break that occurs arcward of most accretionary wedges represents an abrupt change in the critical taper of the accretionary wedge we argue that that change is caused by a large arcward increase in the strength of the material within the overriding plate that stronger material is called the backstop our laboratory modeling experiments indicate that a backstop with a trenchward dipping upper surface results in the development of an overlying structure that includes all of the primary morphologic features observed in modern forearcs in our modeling an accretionary wedge develops trenchward of the backstop an outer arc high develops above the trenchward toe of the backstop and farther arcward a passive forearc basin forms above the stronger material of the backstop this model is consistent with the observation that earthquakes do not extend updip along the plate interface all the way to the trench axis we hypothesize that plate motion is accommodated seismically along the base of the consolidated backstop and mostly or entirely aseismically trenchward of the toe of the backstop along the base of the accretionary wedge at several margins the seismic front is approximately coincident with the outer arc high supporting this interpretation

are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle in a number of subduction zones earthquakes at 50 200 km depth define two dipping planes separated by 20 40 km that appear to merge downdip upper plane earthquakes are inferred to occur within the subducting oceanic crust whereas lower plane earthquakes occur in the subducting oceanic mantle beneath northeast japan the results of a new finite element heat transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle lower plane earthquakes occur at 550 800 c at 100 km depth and at 350 600 c at 160 km depth these conditions coincide with the dehydration reaction antigorite serpentine forsterite enstatite h 2 o which suggests that lower plane earthquakes may be triggered by dehydration embrittlement which in turn suggests that the subducting oceanic mantle is partially hydrated serpentinization may occur in the trench outer rise region where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere if this hypothesis is correct current subduction zone h 2 o budgets may significantly underestimate the amount of bound h 2 o entering the subduction factory

seismic structure of western end of the nankai trough seismogenic zone 1 in 1998 a seismic experiment along a 320 km long profile using an air gun array and ocean bottom seismographs obss was collected in the on shore offshore forearc region of the western nankai trough seismogenic zone southwestern japan this area is outside the coseismic rupture area of the 1946 nankaido earthquake ms 8 2 the main objectives of this paper are to clarify the p wave and the poisson s ratio profile around the forearc region of southwestern japan and estimate differences between this area and the nankaido coseismic rupture area the p wave velocity model estimated by two dimensional 2 d ray tracing and travel time inversion indicates three units with distinctive velocity patterns the lens shaped unit has a maximum thickness of 5 km and velocity of 3 4 4 0 km s the underlying unit has velocity of 5 4 5 6 km s and its p wave velocity increases on land vp 6 2 6 4 km s the relative homogeneous unit with p wave velocity of 6 6 6 9 km s exists just beneath secondary unit with p velocity of 5 4 5 6 km s the angle of subduction is 7 we also obtained a poisson s ratio of the crustal structure of the forearc region the poisson s ratio of the southern part of secondary unit is larger than that of the northern part we conclude that these data indicate clay rich metamorphic rocks plus oceanic basaltic rocks rather than high porosity materials comprise the majority of the layer we suggest that the region of high poisson s ratio probably corresponds to underplating and the metamorphism of the clay rich sediments and possibly the oceanic basalt just below the decollement

structural factors controlling the rupture process of a megathrust earthquake at the nankai trough seismogenic zone summary the nankai trough is a vigorous subduction zone where large earthquakes have been recorded with a recurrence time of 100 200 yr the 1946 nankaido earthquake is well known as an unusual event among these earthquakes because the rupture zone estimated from long period geodetic data is more than twice as large as that derived from seismic wave data in the summer of 1999 an onshore offshore deep seismic survey was performed along a 355 km long profile in the western nankai trough seismogenic zone seismic signals both from an airgun array 207 l and land explosions maximum of 500 kg were recorded simultaneously by 98 oceanbottom seismographs and 93 land seismic stations conventional 2 d seismic reflection data were also acquired along part of the offshore profile from the wide angle seismic data we found a subducting seamount at the centre of the proposed rupture zone with dimensions of 13 km thick by 50 km wide at 10 km depth the seismic velocity image also shows that the seamount is now colliding with the japanese island arc crust from this significant structure this paper proposes that the subducted seamount functioned as a barrier at least during the 1946 earthquake i e the rupture of the 1946 earthquake extended over the entire locked zone to the east of the subducted seamount and then the rupture was deflected around the subducted seamount at the down dip end of the locked zone between cape muroto and cape ashizuri another significant structure a highly reflective layer is obtained beneath shikoku island a very slow p wave velocity 3 km s 1 is necessary in a thin layer at the base of the island arc crust in order to explain the observed high amplitude reflection phases an area of low resistivity obtained by a previous magnetotelluric study corresponds to the highly reflective layer this suggests a possible water layer at the base of the island arc crust the water may be generated by dehydration of the downgoing probably partially serpentinized mantle which is implied by a low p wave velocity 7 5 km s 1 beneath the subducted seamount a locally observed non slip region during the 1946 earthquake at the eastern part of shikoku island is interpreted as a result of weak coupling at the possible water layer

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

upper plate deformation associated with seamount subduction in many active margins severe deformation is observed at the front of the overriding plate where seamounts or aseismic ridges subduct such deformation appears to be a main tectonic feature of these areas which influences the morphology and the seismicity of the margin to better understand the different stages of seamount subduction we have performed sandbox experiments to study in detail the evolution of deformation both in space and time and thus complement seismic images and bathymetry interpretation we focus in this paper on the surface deformation directly comparable with seafloor morphology two types of subducting seamounts were modelled relatively small conical seamounts and larger flat topped seamounts the indentation of the margin by the seamount inhibits frontal accretion and produces a re entrant the margin uplift includes displacement along backthrusts which propagate from the base of the seamount and out of sequence forethrusts which define a shadow zone located on the landward flank of the seamount when the seamount is totally buried beneath the margin this landward shielded zone disappears and a larger one is created in the wake of the asperity due to

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