gps constraints on the 2011 2012 oaxaca slow slip event that preceded the 2012 march 20 ometepec earthquake southern mexico

finite fault analysis of the 1979 march 14 petatlan mexico earthquake using teleseismic p waveforms summary vertical teleseismic p waves recorded for the 1979 march 14 petatlan mexico earthquake were used to derive the distribution of coseismic slip using a linear finite fault inversion scheme that solves for the amount of slip in each of a series of consecutive time windows data recorded by six stations of the global digital seismograph network were inverted in addition to digitized analogue long period recordings available from nine worldwide standardized seismograph network stations the digital data include four broad band and short period velocity waveforms reconstructed from the short and long period components the time window approach allows for a variable rise time on the fault and accounts for the source multiplicity evident in the recorded p waveforms synthetic tests conducted using the inversion method on the limited data set however reveal that the data are insufficient to identify the exact dislocation duration on the fault the method is thus implemented by prescribing the fault rise time using five consecutive 1 s time windows the coseismic slip inferred from the p waves shows a small 70 cm peak near the earthquake hypocentre and a large zone of dislocation 1 2 m maximum further south east the slip pattern covers depths from 3 to 25 km and is located south east of other recent large interplate ruptures on the michoacan segment of the mexican subduction zone this result indicates that the 1979 petatlan earthquake broke an independent adjacent portion of the cocos north america plate boundary the seismic moment of 1 5 1027 dyn cm inferred from the p waves is approximately one half the long period moment estimated by other investigators from the observed surface waves although the discrepancy is within the uncertainty of the seismic moment estimates it may suggest the presence of a component of slow interplate motion that did not radiate significant p wave energy

a large silent earthquake in the guerrero seismic gap mexico 1 geodetic measurements from a network of permanent gps stations along the pacific coast of mexico reveal a large silent earthquake along the segment of the cocos north american plate interface identified as the guerrero seismic gap the event began in october of 2001 and lasted for 6 7 months average slip of 10 cm produced measurable displacements over an area of 550 250 km2 the equivalent moment magnitude of the event was mw 7 5 recognition of this and previous slow event here indicate that the seismogenic portion of the plate interface is not loading steadily as hitherto believed but is rather partitioning the stress buildup with episodic as opposed to steady state or periodic slip downdip of the seismogenic zone this process increases the stress at the base of the seismogenic zone bringing it closer to failure these results call for a reassessment of the seismic potential of guerrero and other seismic gaps in mexico

initiation of changjiang yangtze delta implication from sedimentary facies radiocarbon dates and fossils information of the cores pk01 and xj03

a slow thrust slip event following the two 1996 hyuganada earthquakes beneath the bungo channel southwest japan we report a slow thrust slip event that occurred beneath the bungo channel region southwestern japan on oct and dec 1996 two hyuganada earthquakes both mw 6 7 followed by afterslips occurred in addition a crustal movement characterized by an extremely slow rise was observed around the bungo channel about 200 km north from the epicenters and continued for about 300 days long assuming a slow slip on the plate boundary we estimate its duration and surface displacements from gps time series data by curve fitting and then determine the fault slip distribution we found that a slow slip without any earthquakes continued for nearly one year and released the seismic moment comparable to that of the hyuganada earthquakes occurrence of the slow thrust slip event suggests that this kind of event may be a characteristic mode of stress release at a transition region of interplate coupling

thermal models of the mexico subduction zone implications for the megathrust seismogenic zone 1 it has been proposed that the seismogenic zone of subduction thrust faults is primarily controlled by temperature or rock composition changes we have developed numerical models of the thermal structure of the mexico subduction zone to examine the factors that affect the temperature of the subduction thrust fault although the oceanic plates subducting beneath mexico are young the top of the oceanic plate at the trench is cool because of the lack of a thick cover of insulating sediments marine heat flow observations suggest that hydrothermal circulation may further cool the oceanic plate this results in a cool subduction thrust fault where the brittle part of the fault extends to depths of over 40 km at these depths even slight frictional heating may have significant effects on temperature along the thrust fault particularly for regions with a high convergence rate and shallow plate dip with the addition of a small amount of frictional heating the temperatures of the deep 30 40 km thrust fault are increased by over 200 c as the observed downdip limit of rupture in recent well constrained megathrust earthquakes is confined to depths above the intersection of the thrust fault and the continental moho a temperature of 350 c may control the downdip extent of the seismogenic zone thus in order to be consistent with the observed shallow rupture areas it is necessary to include a small amount of frictional heating corresponding to an average shear stress of 15 mpa

crustal deformation measurements in guerrero mexico 1 gps measurements of crustal deformation in guerrero southern mexico include surveys collected between 1992 and 2001 as well as continuous gps measurements at a few sites these geodetic observations are used to calculate interseismic deformation rates and assess the presence and possible location of transient deformation during the period encompassing 1992 25 to 2001 75 the data are used to examine transient deformation in 1998 previously described from data at a single site by lowry et al 2001 survey measurements and continuous data from a site near popocatepetl volcano confirm the 1998 transient and survey data also suggest another transient occurred following the 14 september 1995 mw 7 3 copala earthquake all of the available gps position estimates have been inverted for a combined model of slip during each event plus the steady state slip on the plate interface modeling of the steady state deformation rates confirms that the guerrero seismic gap is partially frictionally locked at depths shallower than about 25 km and accumulating strain that may eventually be released in a great earthquake the data also suggest that there is frictional coupling to much greater 40 km depths which releases more frequently in aseismic slip events the locations and sizes of the transient events are only partially constrained by the available data however the transient models which best fit the gps coordinate time series suggest that aseismic slip was centered downdip of the seismogenic portion of the plate bounding thrust in both events and the moment release had equivalent magnitudes mw 7 1 1 3 1 0 in 1995 1996 and 7 1 0 4 0 1 in 1998

late quaternary sedimentary evolution of the outer shelf of the east china sea abstract this study investigated the late quaternary sedimentary evolution of the outer shelf of the east china sea ecs using borehole core data dh02 three unconformity bounded sedimentary facies namely prodelta facies of marine isotopic stage mis 3 intertidal to subtidal flat facies of the deglacial period and offshore tidal sand ridge facies of the holocene were identified from the base upward according to combined evidence of lithology microfossil assemblages and geochronology through comparative analysis with other neighboring borehole cores these three facies along with the mis 3 nearshore facies and the last glacial maximum lgm river facies were found to constitute the late quaternary sediment strata sedimentary processes of these five facies were interpreted based on a synthetic consideration of sea level change climate variation paleotopographic setting and paleoriver sea interactions in particular the development of the delta was attributable to integration of the slow sea level decline during the later stage of the mis 3 and abundant sediment supply resulting from the warm wet climate the gentle shelf topography suggests the presence of shallow and wide rivers in the delta and the lateral shift of river channels might have induced the local formation of nearshore facies at the abandoned river mouth the lgm river facies which was rarely observed presumably due to marine erosion with deglacial transgression was inferred to have been deposited during the latter half of the lgm in response to the rise in river base level occurrences of tidal flat facies imply the formation of a tide dominated estuary with the deglacial transgression furthermore tidal flat sediments were reworked by enhanced tidal currents in the estuary front environment forming mud ridges during subsequent deglacial transgression formation of tidal sand ridges in offshore environments of the holocene was attributable to the uneven base caused by the underlying estuarine erosion mud ridges and this promoted the occurrence of tidal depositional processes the sedimentary evolution in association with sea level change was constructed using the above mentioned results overall this study systematically elucidates the late quaternary sedimentary processes and evolution of the outer ecs shelf and provides an insight into the sedimentary development of analogous shelves around the world

detection and monitoring of ongoing aseismic slip in the tokai region central japan analysis of global positioning system data shows that the rate of crustal deformations in the tokai region of japan a seismic gap area changed over the past 18 months kalman filtering analysis shows aseismic slip on the plate boundary in the western tokai region centered on lake hamana adjacent to the anticipated tokai earthquake source area the cumulative moment magnitude reaches 6 7 in june 2002 with a relative slip increase northeast of lake haman from january 2002 an existence of aseismic slip in the western tokai supports the hypothesis of a silent event as the cause of uplifting several days before the 1944 tonankai earthquake

formation processes and evolution of holocene sediments in the yangtze offshore area a review and new interpretation on the response to sea level rise and deltaic progradation abstract seismic profiles integrated with borehole cores from offshore of the yangtze river delta characterize three distinctive holocene deposition units in correspondence to estuarine shallow marine and prodelta facies respectively units to sedimentary processes and evolution of three units in response to sea level rise and yangtze delta development was studied estuarine facies occurred as a cluster of mud ridges unit was considered to be formed during the later stage of the period 10 8 cal ka bp with the prevalence of tidal erosion processes at the same time however the area south of the ridge field experienced rapid deposition possibly due to shelter effects of the zhoushan archipelago unit characterized by homogeneous mud composition rapid accumulation rates and a seaward dipping structure indicated a gravity flow deposition most likely related to the wave and or tidal current induced fluid mudflows during the period of 8 5 cal ka bp intrusion of a mud diapir during 5 3 cal ka bp related to gravity subsidence localized near the river mouth lead to deformation of unit during the 3 ka to present the delta advanced to the study area depositing subaqueous clinoform unit meanwhile erosional processes dominated by tidal currents were promoted by the southeastward progradation of the delta sediments deposited in the north of the area were transformed into tidal sand ridges we predict that the delta will likely reduce in area with a further progradation in the recent future

strong seismic coupling underneath garhwal kumaun region nw himalaya india abstract no great earthquake has occurred in the garhwal kumaun region of nw himalaya in the past 500 years or more we report results of continuous gps measurements from 28 sites from the region to suggest that the convergence rate in this part of the himalaya is about 18 mm yr which is leading to strain accumulation in the region the main himalayan thrust mht in the frontal part of the himalaya under the outer and southern lesser himalaya is strongly coupled for a width of about 85 km the midcrustal ramp where earthquakes of himalayan seismic belt occur exhibits low coupling strong coupling on the mht beneath the outer and lesser himalaya is homogeneous except in the very shallow updip part of the mht subduction of sediments of the indo gangetic plains or the delhi hardwar ridge does not seem to influence coupling a high rate of strain accumulation which has continued for more than 500 years on a strongly coupled mht makes this one of the most earthquake vulnerable segments of the himalayan arc

slow slip event in the mexican subduction zone evidence of shallower slip in the guerrero seismic gap for the 2006 event revealed by the joint inversion of insar and gps data slow slip events sses in subduction zones have been observed in the last decade with continuous gps stations some of them could be related to the lateral segmentation of subduction interface that seems to be a critical parameter for the propagation of large subduction earthquakes in 2006 one of the largest sses recorded so far was captured by a dozen continuous gps stations in the guerrero area mexico along the mexican subduction zone previous studies based on these data suggested a lateral variation of the updip depth of the see at the guerrero seismic gap but suffered from a lack of resolution east of the gap here we show the ability of insar technique to capture a part of the 2006 sse cumulative displacement east of the guerrero gap by a stacking approach we processed long strip envisat interferograms corrected for orbital errors and interseismic signal using gps data we first use a forward modelling approach to test insar sensitivity to the amount of slip depth and width of the slipping area on the subduction interface due to its high spatial resolution insar allows one to comprehensively sample the north south spatial wavelength of the sse deformation complementing the sparse gps network insar locates the maximum of uplift and subsidence caused by the sse more precisely than the gps data giving better constraints on the updip slip limit of the sse we then inverted the insar and gps data separately to understand how each inversion resolves the slip at depth finally we performed a joint inversion of insar and gps data which constrained the sse slip and its location on the plate interface over the entire guerrero area the joint inversion shows significant lateral variation of the sse slip distribution along the trench with a shallower updip edge in the guerrero seismic gap west of acapulco and a deeper slip edge further east

geometry of subduction and depth of the seismogenic zone in the guerrero gap mexico the western coast of guerrero in southern mexico has been identified as a seismic gap on the middle american trench in which no large earthquakes have occurred at least since 1908 it has been suggested that the seismic energy accumulated since 1908 will eventually be released by a large earthquake a permanent seismic network was installed to monitor the seismicity of this mature seismic gap and to understand the geometry of the subducted slab beneath this region the seismicity defines an unusual distribution along two bands of seismic activity parallel to the coast the resulting geometry of the subduction zone shows that the cocos plate dips at a shallow angle beneath the north american plate to a depth of 40 km from there the subducted slab is bent upward following a subhorizontal trajectory extending inland at a depth of 50 km this plate geometry is reminiscent of that found in peru and central argentina two other regions where a young oceanic plate is being subducted in mexico however the slab underplates an overriding plate which is only half as thick as that observed in south america a possible oceanic origin of the allochtonous terranes comprising southern mexico may explain the presence of the anomalously thin lithosphere in this region

interplate coupling in the kanto district central japan deduced from geodetic data inversion and its tectonic implications abstract interplate coupling between the philippine sea phs the north american nam and the pacific pac plates in the kanto district central japan has been investigated through the inversion analysis of geodetic data using akaike s bayesian information criterion abic the data used for the analysis are annual rates of level changes 1972 1985 and horizontal length changes 1973 1990 which presumably represent average crustal movements during the interseismic period disregarding effects of steady plate subduction we may ascribe the observed crustal movements to the effects of locking or faulting on some parts of the plate boundaries in our model the effect of locking is represented by back slip on the plate boundary and that of faulting by forward slip the results of the inversion analysis show the existence of a strongly coupled area on the southwestern shallower part of the nam phs plate boundary which almost coincides with the faulting area of the 1923 kanto earthquake m 7 9 and a notable faulting area on the northeastern deeper part of the phs pac plate boundary in the southwestern strongly coupled area the rate of back slip reaches 3 5 cm yr and its direction is oriented n33 w which is almost opposite to the direction of fault slip s29 e at the time of the 1923 event this suggests that tectonic stress accumulation for the next large event is effectively proceeding there and its recurrence time is roughly estimated as 245 years the strongly coupled region is also recognized as corresponding to a recent seismic gap the inverted forward slip distribution on the phs pac plate boundary which has the average rate of 1 9 cm yr and the average direction of n56 e may be explained by aseismic northeastward slip in relation to the process to form a peculiar configuration of downward bend of the phs plate beneath the kanto district

distribution of seismic coupling on the subducting plate boundary in northeastern japan inferred from gps observations abstract many global positioning system gps stations of the geographical survey institute and tohoku university in northeastern japan observed clear coseismic and postseismic deformations associated with the m7 5 1994 far off sanriku earthquake applying the geodetic inversion algorithm given by yabuki and matsu ura geophys j int 1992 109 363 376 to the observed gps data we estimate in this paper the spatial distributions of the coseismic and postseismic slip on the boundary between the subducting pacific plate and the overriding continental plate it is noted that the coseismic slip estimated in this paper denotes the slip for a period between the day before and the day after the main shock the inversion for coseismic slip shows that the main shock was thrust faulting typical for interplate earthquakes which is consistent with the results of other seismological studies the short term postseismic deformation std observed for a period of ca 10 days between the main shock and its largest aftershock shows that the initially high deformation rate was lowered rather rapidly in comparison with the subsequent long term deformations ltd lasting longer than 1 year which decreased at an almost constant low rate the observed data of std can be explained by postseismic slip distributed approximately on the same region as the faulting area of the main shock where the moment release during this period is ca 16 of that estimated for the coseismic slip the region of the sliding causing ltd is found to have significantly expanded southward and downward or westward on the plate boundary the moment released by the postseismic sliding for nearly 1 year amounts approximately to 80 of that released by the coseismic slip in addition to the contribution from uniform steady sliding due to the relative plate motion furthermore we find from the inversion analysis a locked region to the south of the long term postseismic sliding region on the plate boundary the amount of nominal back slip estimated is so large that the locked region of the plate boundary is considered to have been tightly coupled for the year of 1995 this study demonstrates that the gps observation only for ca 1 year can distinguish the decoupled region due to the postseismic sliding around the source area of the main shock from the coupled one of the plate boundary

a revised dislocation model of interseismic deformation of the cascadia subduction zone 1 cas3d 2 a new three dimensional 3 d dislocation model is developed to model interseismic deformation rates at the cascadia subduction zone the model is considered a snapshot description of the deformation field that changes with time the effect of northward secular motion of the central and southern cascadia forearc sliver is subtracted to obtain the effective convergence between the subducting plate and the forearc horizontal deformation data including strain rates and surface velocities from global positioning system gps measurements provide primary geodetic constraints but uplift rate data from tide gauges and leveling also provide important validations for the model a locked zone based on the results of previous thermal models constrained by heat flow observations is located entirely offshore beneath the continental slope similar to previous dislocation models an effective zone of downdip transition from locking to full slip is used but the slip deficit rate is assumed to decrease exponentially with downdip distance the exponential function resolves the problem of overpredicting coastal gps velocities and underpredicting inland velocities by previous models that used a linear downdip transition a wide effective transition zone etz partially accounts for stress relaxation in the mantle wedge that cannot be simulated by the elastic model the pattern of coseismic deformation is expected to be different from that of interseismic deformation at present 300 years after the last great subduction earthquake the downdip transition from full rupture to no slip should take place over a much narrower zone

characteristic silent earthquakes in the eastern part of the boso peninsula central japan 1 analysis of global positioning system gps data shows transient crustal deformation in the boso peninsula central japan for about fifty days from october 2002 the southeastward horizontal displacements detected suggest the occurrence of interplate aseismic slip between the philippine sea plate and the north american plate six years after a similar event in 1996 we estimate an area of aseismic slip with moment magnitude mw 6 6 centered off the boso peninsula adjacent to associated seismic activity slip evolution of the 2002 aeismic slip started on october 4 and migrated from north to south for about ten days followed by gradual subsidence and restarting of slip which lasted until december 2 propagating more southward similarities in focal area and slip process between the 1996 and the 2002 events suggest the boso aseismic slips as characteristic silent earthquakes together with the repetitive occurrence of seismic swarm in a similar region every six to seven years a more tightly coupled southern area that did not release enough energy in the 1996 event may have caused the differences in magnitude central slip area and duration between the 1996 and the 2002 events if the hypothesized recurrence interval for strain accumulation is true based on swarm seismicity

coseismic slip of two large mexican earthquakes from teleseismic body waveforms implications for asperity interaction in the michoacan plate boundary segment teleseismic body waves recorded for the october 25 1981 playa azul and september 21 1985 zihuatanejo earthquakes in western mexico were inverted to derive the distributions and depths of coseismic slip broadband p wave displacements and intermediate period records were included to incorporate a wide frequency band in the analysis for the zihuatanejo earthquake digital sh records located away from the nodal directions were used in the inversion process nonnodal sh waveforms were not available for the playa azul earthquake but additional azimuthal coverage was obtained by including selected worldwide analog p wave records the results for the playa azul earthquake indicate that rupture occurred in two separate zones both updip and downdip of the point of initial nucleation with the majority of the slip concentrated in a circular region 15 km radius downdip from the hypocenter the maximum slip in this downdip region exceeds 3 6 m and is at a depth of about 16 km the computed seismic moment is 7 14 1026 dyn cm coseismic slip is observed to occur entirely within the area of reduced slip separating the two main shallow sources of the michoacan earthquake that occurred almost 4 years later on september 19 1985 for the zihuatanejo earthquake the p and sh data suggest rupture over a larger area 30 km radius with a lower peak slip 2 m and encompassing depths between 12 and 26 km slip is concentrated in an area adjacent to one of the major sources of the michoacan earthquake and represents the southeastern continuation of rupture along the cocos north america plate boundary the corresponding seismic moment is 1 35 1027 dyn cm the zones of peak slip observed for the playa azul zihuatanejo and michoacan earthquakes are interpreted as asperity regions that control the cessation and generation of large earthquakes within the michoacan segment of the plate boundary the stress drops within each of these asperities are observed to be very similar and may reflect a characteristic asperity strength for this portion of the subduction zone

a dislocation model of strain accumulation and release at a subduction zone strain accumulation and release at a subduction zone are attributed to stick slip on the main thrust zone and steady aseismic slip on the remainder of the plate interface this process can be described as a superposition of steady state subduction and a repetitive cycle of slip on the main thrust zone consisting of steady normal slip at the plate convergence rate plus occasional thrust events that recover the accumulated normal slip because steady state subduction does not contribute to the deformation at the free surface deformation observed there is completely equivalent to that produced by the slip cycle alone the response to that slip is simply the response of a particular earth model to embedded dislocations for a purely elastic earth model the deformation cycle consists of a coseismic offset followed by a linear in time recovery to the initial value during the interval between earthquakes for an elastic viscoelastic earth model elastic lithosphere over a viscoelastic asthenosphere the postearthquake recovery is not linear in time records of local uplift as a function of time indicate that the long term postseimic recovery is approximately linear suggesting that elastic earth models are adequate to describe the deformation cycle however the deformation predicted for amore simple elastic half space earth does not reproduce the deformation observed along the subduction zones in japna at all well if stick slip is restricted to the main thrust zone as recognized earlier by shimazaki seno and kato the uplift profiles could be explained if stick slips were postulated to extend along the plate interface beyond the main thrust zone to a depth of perhaps 100 km but independent evidence suggests that stick slip at such depths if unlikely less

interplate coupling and a recent aseismic slow slip event in the guerrero seismic gap of the mexican subduction zone as deduced from gps data inversion using a bayesian information criterion a large scale slow slip event with a magnitude equivalent to mw 7 4 has been observed at seven continuous gps stations located in the guerrero oaxaca region southern mexico for several months from late 2001 to middle 2002 geophys res lett 2003 30 doi 10 1029 2003gl017219 we re analyzed the gps time series data to estimate the displacements associated with the slow slip event and also the displacement rates during the interseismic period prior to this event by applying an inversion technique using a bayesian information criterion abic we then carried out gps data inversion again with abic and some detailed forward modeling to obtain spatial distributions of the back slip rates and the slow slip on a 3 d curved plate interface between the subducting cocos and the overriding north america plates the results show that the average direction of the back slip rates is n31 3 e 5 6 which is congruent with that of plate convergence in this region interplate coupling is found to be very strong down to a depth of about 45 km indicating a coupling ratio of 0 83 0 86 except for a couple of segments on the model region and decreases dramatically down below the average direction of the slow slip is oriented about 20 35 counterclockwise from the opposite direction of the plate convergence the slow slip detected on the mid depth segment to the upper rim of the model region reaches about 9 18 cm in the landward region of the guerrero seismic gap from inversion and forward modeling from these results we conclude it is possible that the slow slip may have invaded a deeper part of the strongly coupled seismogenic zone at least up to a depth of about 25 km we also estimate stress changes due to the slow slip event some decrease in shear stress on the plate interface at these depths suggests that if such a slow slip event occurred episodically it is possible that the time of occurrence of a forthcoming large earthquake in the guerrero gap would be delayed to some extent 2004 elsevier b v all rights reserved

shape of the subducted rivera and cocos plates in southern mexico seismic and tectonic implications the geometry of the subducted rivera and cocos plates beneath the north american plate in southern mexico was determined based on the accurately located hypocenters of local and teleseismic earthquakes the hypocenters of the teleseisms were relocated and the focal depths of 21 events were constrained using a body wave inversion scheme the suduction in southern mexico may be approximated as a subhorizontal slab bounded at the edges by the steep subduction geometry of the cocos plate beneath the caribbean plate to the east and of the rivera plate beneath north america to the west the dip of the interplate contact geometry is constant to a depth of 30 km and lateral changes in the dip of the subducted plate are only observed once it is decoupled from the overriding plate on the basis of the seismicity the focal mechanisms and the geometry of the downgoing slab southern mexico may be segmented into four regions 1 the jalisco region to the west where the rivera plate subducts at a steep angle that resembles the geometry of the cocos plate beneath the caribbean plate in central america 2 the michoacan region where the dip angle of the cocos plate decreases gradually toward the southeast 3 the guerrero oaxaca region bounded approximately by the onshore projection of the orozco and o gorman fracture zones where the subducted slab is almost subhorizontal and underplates the upper continental plate for about 250 km and 4 the southern oaxaca and chiapas region in southeastern mexico where the dip of the subduction gradually increases to a steeper subduction in central america these drastic changes in dip do not appear to take place on tear faults suggesting that smooth contortions accommodate these changes in geometry the inferred 80 and 100 km depth contours of the subducted slab lie beneath the southern front of the trans mexican volcanic belt suggesting it is directly related to the subduction thus the observed nonparallelism with the middle american trench is apparently due to the changing geometry of the rivera and cocos plates beneath the north american plate in southern mexico and not to zones of weakness in the crust of the north american plate as some authors have suggested

interplate coupling in the tokai district central japan deduced from continuous gps data i invert gps displacement rates in the tokai district to investigate strain accumulation associated with the anticipated tokai earthquake i apply a representation of interseismic strain accumulation at subduction zones by savage 1983 and estimate a distribution of virtual normal fault slip back slip on the plate boundary surface estimated back slip takes its maximum value of 30 40mm year under the sea of enshu this value is nearly equal to the relative plate motion there implying almost complete coupling back slip is smaller under suruga bay and its direction is different from the relative motion between the eurasian and the philippine sea plates localized deformation at the northern tip of the philippine sea plate may be responsible for this discrepancy partitioning tectonic strain due to the plate convergence into faults on both sides of the izu peninsula and also distributed strain inside the peninsula

slip distribution of the 19 september 1985 michoacan mexico earthquake near source and teleseismic constraints we simultaneously invert the strong motion velocity records and the long and intermediate period teleseismic p waveforms of the 19 september 1985 michoacan mexico earthquake to recover the distribution of slip on the fault using a point by point constrained and stabilized least squares inversion method a fault plane with strike fixed at 300 and dip fixed at 14 is placed in the region of the earthquake hypocenter and divided into 120 subfaults rupture is assumed to propagate at a velocity of 2 6 km sec away from the hypocenter synthetic near source ground motions and teleseismic p waveforms for pure strike slip and dipslip dislocations are calculated for each subfault the observed data are then inverted to obtain the amount of strike slip and dip slip displacement required of each subfault we also invert the data sets using a time window procedure where the subfaults are allowed to slip up to three times this approach relaxes the constraint of fixed subfault rupture time imposed by a constant rupture velocity inversion of the strong motion data alone yields a slip model similar to the solution previously obtained using only teleseismic waveforms this result supports the use of teleseismic waveform data for the derivation of fault dislocation models in the absence of strong motion recordings our simultaneous inversion of both data sets suggests that rupture during the michoacan earthquake was controlled largely by the failure of three major asperities located along the length and down the dip of a 150 km segment of the cocos north america plate boundary the solution contains three major source regions including an 80 km by 55 km source near the hypocenter with a peak slip of 6 5 meters two additional sources are present on the southeast portion of the fault about 70 km away from the hypocenter one of these sources with a peak slip of 5 meters covers a 45 km by 60 km area and extends downdip from a depth of about 10 km to 24 km the third source region is somewhat smaller 30 km by 60 km area 3 1 meters peak slip and extends further downdip at depths between 27 km and 39 km aftershock activity following the earthquake was associated mainly with the two shallow sources these two sources are separated by the aftershock zone of the 1981 playa azul earthquake

transient fault slip in guerrero southern mexico the guerrero region of southern mexico has ac cumulated more than 5 m of relative plate motion since the last major earthquake in early 1998 a continuous gps site in guerrero recorded a transient displacement modeling indicates that anomalous fault slip propagated from east to west along strike of the subduction megathrust campaign gps and leveling data corroborate the model the moment release was equivalent to an mw 6 5 earthquake no m 5 earthquakes accompanied the event indicating the frictional regime is velocity strengthening at the location of slip

time dependent inversion study of the slow thrust event in the nankai trough subduction zone southwestern japan we estimate time variation of interplate coupling between the subducting philippine sea and the overriding continental plates along the western part of the nankai trough japan during the period between 1996 and 1999 after subtracting annual components and average annual velocities deviation from steady crustal deformation in the shikoku and kyushu islands is examined around 15 gps stations facing the bungo channel detected the following three stage ground displacements of up to 3 cm in 1997 with a duration of 1 year 1 increased displacement rate associated with the 1996 hyuga nada earthquakes 2 an increasingly rapid anomalous period of deformation from june 1997 to the end of 1997 and 3 a slowdown period and return to normal rates of deformation after the end of 1997 linearized least squares locate the slip area of this slow event near the plate boundary beneath the bungo channel the estimated slip is opposite the motion of the philippine sea plate these results support the hypothesis that the observed anomalous caistal deformation is due to a rebound process of the philippine sea plate time dependent inversion analysis shows the following slip history during the initial period of increased displacement rate a visible slip area is resolved beneath the coastal area of southwestern shikoku in the next stage of accelerated displacements the slip area expanded southwestward to the bungo channel beginning around june 1997 when postseismic slip of the 1996 hyuga nada earthquakes almost ended after this accelerated period the estimated thrust slip ends with time

interplate coupling in southwest japan deduced from inversion analysis of gps data abstract recently the geographical survey institute of japan completed the installation of a gps continuous observation network in japan which has enabled us to investigate real time crustal movements in this study we attempt to obtain spatial distribution of interplate coupling and relative plate motion between subducting and overriding plates in southwest japan using horizontal and vertical deformation rates which were observed at 247 gps observation stations during the period from april 6 1996 to march 20 1998 for this purpose we carried out an inversion analysis of geodetic data incorporating akaike s bayesian information criterion abic as a result strong interplate coupling was found off shikoku and kumanonada regions which corresponds well with the fault regions of the 1946 nankai m 8 1 and the 1944 tonankai m 8 0 earthquakes respectively we also found that interplate coupling becomes weak at depths deeper than about 30 to 40 km beneath the shikoku and kii peninsula the recurrence time of great trench type earthquakes was roughly estimated as 107 years which is consistent with previous research the direction of relative plate motion is oriented n53 w which is close to the direction predicted from the plate motion model on the other hand a large forward slip was found in the hyuganada region off southeast of kyushu since the coseismic displacements associated with the two 1996 hyuganada earthquakes m 6 6 m 6 6 are removed from the gps data this suggests that after slip occurred near the source region and or that kyushu moves southeastward stationarily due to other tectonic forces

interplate coupling and relative plate motion in the tokai district central japan deduced from geodetic data inversion using abic summary the spatial distribution of the strength of interplate coupling between the subducting philippine sea and overlying continental plates in the tokai district central japan was investigated in detail through the inversion analysis of geodetic data using akaike s bayesian information criterion abic the geodetic data used for the analysis are annual rates of level changes 1972 1984 and horizontal length changes 1977 1988 which presumably represent average crustal movements during the interseismic period the result of the inversion analysis shows the existence of a strongly coupled region extending from 10 to 30 km in depth the total seismic moment accumulated in this area since the last event the 1854 ansei earthquake is roughly estimated to be 5 5 1027 dyne cm which corresponds to ms 7 8 the interplate coupling becomes weaker toward the shallower and deeper portions this is consistent in general tendency with a rheological model inferred from petrological viewpoints the strength of coupling also tends to decrease toward northeast over the west coast of suruga bay the direction of plate convergence inferred from the inversion analysis is oriented n30 w this is significantly different from the direction of relative plate motion between the philippine sea and eurasian plates but concordant with that between the philippine sea and north american plates

geodetic data inversion using a bayesian information criterion for spatial distribution of fault slip summary we developed a new inversion method to reconstruct static images of seismic sources from geodetic data using akaike s bayesian information criterion abic coseismic surface displacements are generally related with a slip distribution on a fault surface by linear integral equations parametric expansion of the fault slip distribution by a finite number of known basis functions yields a set of observation equations expressed in a simple vector form incorporating prior constraints on the smoothness of slip distribution with the observation equations we construct a bayesian model with unknown hyperparameters the optimal values of the hyperparameters which control the structure of the bayesian model are objectively determined from observed data by using abic once the values of hyperparameters are determined we can use the maximum likelihood method to find the optimal distribution of fault slip we examined the validity of this method through a numerical experiment using theoretical data with random noise we analysed geodetic data associated with the 1946 nankaido earthquake ms 8 2 by using this method the result shows that the fault slip distribution of this earthquake has two main peaks of 4 and 6 m located off kii peninsula and muroto promontory these two high slip areas are clearly separated by a low slip zone extending along kii strait such a slip distribution corresponds with the fact that the rupture process of this earthquake in the western part is notably different from that in the eastern part

interplate coupling in northeast japan deduced from inversion analysis of gps data abstract the spatial distribution of the strength of interplate coupling between the subducting pacific plate and the overlying continental plate in northeast japan was investigated through an inversion analysis of gps data employing akaike s bayesian information criterion the gps data used for the inversion analysis are rates of baseline length changes and vertical displacement rates of crustal movements during the period from april 6 1996 to march 20 1998 which were obtained at 161 continuous gps observation stations operated by the geographical survey institute of japan the result shows the average drag rates of the overriding plate on the model source regions off the tohoku and hokkaido districts are 6 7 cm yr and 6 8 cm yr respectively which are 0 85 and 0 87 as interplate coupling ratio these values are much higher than those in previous studies the average directions of drag rates are oriented n73 w 15 and n78 w 13 on the model source regions off the tohoku and hokkaido districts respectively these directions agree well with the direction of coseismic slip of the foot wall side n75 w of the 1978 miyagiken oki earthquake m 7 4 but differ slightly from the direction estimated from the plate motion model n65 w seno et al j geophys res 101 1996 11305 11315 we also found afterslip of 5 5 cm yr associated with the 1994 sanriku haruka oki earthquake m 7 5 off aomori prefecture interestingly enough large afterslip occurred on the plate boundary even when more than a year had passed after the sanriku haruka oki earthquake the amount of drag rate becomes dramatically small at depths greater than about 50 km on the model source regions indicating weak interplate coupling there

source rupture process of the tecoman colima mexico earthquake of 22 january 2003 determined by joint inversion of teleseismic body wave and near source data the spatial and temporal slip distribution of the tecoman colima mexico earthquake is estimated from near source strong motion and teleseismic body wave data to perform a stable inversion we applied smoothing constraints and determined their optimal relative weights on the observed data using an optimized akaike s bayesian information criterion abic the source parameters are as follows strike dip slip 300 20 93 seismic moment m 2 3 1020 n m source duration 30 sec along strike distance 35 km along dip distance 70 km we found that the rupture process can be divided into three stages the rupture nucleated near the hypocenter stage i then it broke the first asperity centering about 15 km southwest from the epicenter stage ii and the rupture propagated to the northeast and the second asperity was broken stage iii we also estimated the shear stress change due to the rupture process of the mainshock on and around the major fault zone it appears that one cluster of aftershocks for the first 5 days which took place in and adjacent to the zones of stress increased due to the fault rupture during the mainshock but overall correlation between the aftershock location and the stress pattern is not clear manuscript received 14 may 2003

source time functions of large mexican subduction earthquakes morphology of the benioff zone age of the plate and their tectonic implications we study source parameters of large shallow mexican subduction zone earthquakes 95 w to 106 w which occurred between 1928 and 1986 by modeling the p waves recorded on galitzin wilip seismograph in debilt dbn holland for post 1962 events the source parameters retrieved from dbn seismograms alone agree well with those obtained from long period world wide standardized seismograph network records giving us confidence in our results for pre 1962 events all earthquakes are shallow h 10 to 20 km with few exceptions the sources in oaxaca 95 w to 99 w are very simple to the northwest of 99 w they are simple as well as complex the ratio of surface wave to body wave seismic moment mos mop which is a measure of long to short period radiation is smaller in oaxaca 1 5 0 5 than in the regions northwest of 99 w 3 1 1 3 these results suggest a change in the plate interface characteristics near 99 w the sharp change in the rupture mode and the intersection of the o gorman fracture zone ofz with the trench occur near 99 w two strike slip events offshore close to ofz suggest a segmentation of the subducting plate near 99 w the age of the plate near the trench in oaxaca is not well known it is possible that it does not increase continuously from northwest to southeast in the region but jumps across 99 w if so then the older age of the subducted plate southeast of 99 w may be the cause of the distinct rupture mode of the oaxaca earthquakes the length of the benioff zone which is greatest below oaxaca 400 km and decreases toward the northwest can be explained by the correlation between the length of the subducted slab and the product of the lithosphere age and convergence rate the relative complexity of sources the weaker background seismicity and the lesser number of aftershocks northwest of oaxaca may be explained by a stronger interface coupling resulting from subduction of younger oceanic slabs 5 to 13 m y old in this region this however explains neither larger mos mop values northwest of oaxaca nor the low stress drop estimates obtained from the analysis of near field strong motion data for the michoacan earthquake of 1985 both of which indicate weaker coupling of the interface thus the issue of whether subduction of very young plates 10 m y old results in strong or weak coupling remains unsolved from the presently available mexican data

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses