seismic evidence for an earthquake nucleation phase near source observations show that earthquakes initiate with a distinctive seismic nucleation phase that is characterized by a low rate of moment release relative to the rest of the event this phase was observed for the 30 earthquakes having moment magnitudes 2 6 to 8 1 and the size and duration of this phase scale with the eventual size of the earthquake during the nucleation phase moment release was irregular and appears to have been confined to a limited region of the fault it was characteristically followed by quadratic growth in the moment rate as rupture began to propagate away from the nucleation zone these observations suggest that the nucleation process exerts a strong influence on the size of the eventual earthquake

seismic guided waves trapped in the fault zone of the landers california earthquake of 1992 a mobile seismic array of seven stations was deployed at 11 sites along the fault trace of the m7 4 landers earthquake of june 28 1992 with a maximum offset of 1 km from the trace we found a distinct wave train with a relatively long period following the s waves that shows up only when both the stations and the events are close to the fault trace this wave train is interpreted as a seismic guided wave trapped in a low velocity fault zone to study the distribution of amplitude of the guided waves with distance from the fault trace and also their attenuation with travel distance along the fault zone we eliminated source and recording site effects by the coda normalization method the normalized amplitudes of guided waves show a spectral peak at 3 4 hz which decays sharply with distance from the fault trace spectral amplitudes at high frequencies 8 15 hz show an opposite trend increasing with distance from the fault trace the normalized amplitudes of guided waves at 3 4 hz also show a systematic decrease with hypocentral distance along the fault zone from which we infer an apparent q of 50 in order to confirm the existence of the guided waves a dense array of 31 stations was deployed at one of the 11 sites the resultant records revealed unequivocal evidence for the existence of guided waves associated with the fault zone by modeling the waveforms as s waves trapped in a low velocity waveguide sandwiched between two homogeneous half spaces with velocity vs 3 0 km s we infer a waveguide width of about 180 m a shear velocity of 2 0 2 2 km s and a q of 50 hypocenters of aftershocks with clear guided waves show a systematic distribution both laterally and with depth delineating the extent of the low velocity fault zone in three dimensions we find that the zone extends to a depth of at least 10 km this zone apparently continues to the south across the pinto mountain fault because guided waves are observed at stations north of the pinto mountain fault for earthquakes with epicenters south of it on the other hand the zone appears to be discontinuous at the fault bend located about 20 km north of the mainshock epicenter guided waves were observed for stations and epicenters which are located on the same sides of the fault bend but not for those on the opposite sides

seismic moment catalog of large shallow earthquakes 1900 to 1989 we compile a worldwide catalog of shallow depth 70 km and large ms 7 earthquakes recorded between 1900 and 1989 the catalog is shown to be complete and uniform at the 20 sec surface wave magnitude ms 7 0 we base our catalog on those of abe 1981 1984 and abe and noguchi 1983a b for events with ms 7 0 those catalogs however are not homogeneous in seismicity rates for the entire 90 year period we assume that global rates of seismicity are constant on a time scale of decades and most inhomogeneities arise from changes in instrumentation and or reporting we correct the magnitudes to produce a homogeneous catalog the catalog is accompanied by a reference list for all the events with seismic moment determined at periods longer than 20 sec using these seismic moments for great and giant earthquakes and a moment magnitude relationship for smaller events we produce a seismic moment catalog for large earthquakes from 1900 to 1989 the catalog is used to study the distribution of moment released worldwide although we assumed a constant rate of seismicity on a global basis the rate of moment release has not been constant for the 90 year period because the latter is dominated by the few largest earthquakes we find that the seismic moment released at subduction zones during this century constitutes 90 of all the moment released by large shallow earthquakes on a global basis the seismic moment released in the largest event that occurred during this century the 1960 southern chile earthquake represents about 30 to 45 of the total moment released from 1900 through 1989 a frequency size distribution of earthquakes with seismic moment yields an average slope b value that changes from 1 04 for magnitudes between 7 0 and 7 5 to b 1 51 for magnitudes between 7 6 and 8 0 this change in the b value is attributed to different scaling relationships between bounded large and unbounded small earthquakes thus the earthquake process does have a characteristic length scale that is set by the downdip width over which rupture in earthquakes can occur that width is typically greater for thrust events at subduction zones than for earthquakes along transform faults and other tectonic environments

geological reactive systems from the mantle to the abyssal sub seafloor preface the formation and alteration of the oceanic lithosphere represent one of the main processes for energy and chemical exchanges between the deep earth and its outer envelopes however the steep thermal gradients characterizing these environments especially at the main thermal and lithological interfaces along mid ocean accretion zones fig 1 mean that the physical and chemical mechanisms controlling these exchanges remain poorly understood yet these interfaces are the main transitions for the physical and rheological properties of rocks such as permeability and viscosity that control melt focussing and transport from the partially molten mantle to the surface as well as deformation mechanisms and the influx of seawater into the cooling oceanic lithosphere these processes also give rise to hydrothermal systems that produce economically valuable ore deposits and play a major role in the global chemical budget some hydrothermal reactions produce hydrogen and abiotic hydrocarbon hence these extreme environments sustain life and they are potentially implicated in its origin finally these processes determine the architecture and composition of the lithosphere plunging into the deep earth along subduction zones and contribute to a broad range of mechanisms driving arc magmatism and localization of earthquakes in these regions

fine structure of the landers fault zone segmentation and the rupture process observations and modeling of 3 to 6 hertz seismic shear waves trapped within the fault zone of the 1992 landers earthquake series allow the fine structure and continuity of the zone to be evaluated the fault to a depth of at least 12 kilometers is marked by a zone 100 to 200 meters wide where shear velocity is reduced by 30 to 50 percent this zone forms a seismic waveguide that extends along the southern 30 kilometers of the landers rupture surface and ends at the fault bend about 18 kilometers north of the main shock epicenter another fault plane waveguide disconnected from the first exists along the northern rupture surface these observations in conjunction with surface slip detailed seismicity patterns and the progression of rupture along the fault suggest that several simple rupture planes were involved in the landers earthquake and that the inferred rupture front hesitated or slowed at the location where the rupture jumped from one to the next plane reduction in rupture velocity can tentatively be attributed to fault plane complexity and variations in moment release can be attributed to variations in available energy

reflections from a distant subduction zone accurate models of the distribution of heterogeneities in the earth s mantle especially of subduction zones are crucial for our understanding of the composition and dynamics of the earth here we present broad band bb seismograms with p waves reflected at the subduction zone under southern japan at about 84 distance from german regional seismic network the earthquakes occurred in the mariana islands region at 100 to 105 distance array techniques allow the separation of this wave with an asymmetric path p1p2 from reflections and conversions under the source the receivers and the midpoint between the source and the receivers our observation demonstrates that subduction zones can act as strong seismic reflectors and can be mapped with a depth resolution of 20km with bb arrays at teleseismic distances the analysis of such data will help to derive a more detailed picture of the structure of the earth s mantle

sources of tsunami and tsunamigenic earthquakes in subduction zones we classified tsunamigenic earthquakes in subduction zones into three types earth quakes at the plate interface typical interplate events earthquakes at the outer rise within the subducting slab or overlying crust intraplate events and tsunami earthquakes that generate considerably larger tsunamis than expected from seismic waves the depth range of a typical interplate earthquake source is 10 40km controlled by temperature and other geological parameters the slip distribution varies both with depth and along strike recent examples show very different temporal change of slip distribution in the aleutians and the japan trench the tsunamigenic coseismic slip of the 1957 aleutian earthquake was concentrated on an asperity located in the western half of an aftershock zone 1200km long this asperity ruptured again in the 1986 andreanof islands and 1996 delarof islands earthquakes by contrast the source of the 1994 sanriku oki earthquake corresponds to the low slip region of the previous interplate event the 1968 tokachi oki earthquake tsunamis from intraplate earthquakes within the subducting slab can be at least as large as those from interplate earthquakes tsunami hazard assessments must include such events similarity in macroseismic data from two southern kuril earthquakes illustrates difficulty in distinguishing interplate and slab events on the basis of historical data such as felt reports and tsunami heights most moment release of tsunami earthquakes occurs in a narrow region near the trench and the concentrated slip is responsible for the large tsunami numerical modeling of the 1996 peru earthquake confirms this model which has been proposed for other tsunami earthquakes including 1896 sanriku 1946 aleutian and 1992 nicaragua

on the reported magnetic precursor of the 1993 guam earthquake 1 using 1 second magnetometer data recorded 67 km from the epicenter of the 1993 mw 7 7 guam earthquake hayakawa et al 1996 and miyahara et al 1999 identify anomalous precursory changes in ultra low frequency magnetic polarization the ratio of vertical to horizontal field components in a check of their results we compare their data gam with 1 second data from the kakioka observatory kak in japan and the global magnetic activity index kp we also examine log books kept by usgs staff working on the guam magnetic observatory we find 1 analysis problems with both hayakawa et al and miyahara et al 2 significant correlation between the gam kak and kp data and 3 an absence of identifiable localized anomalous signals occurring prior to the earthquake the changes we do find in polarization are part of normal global magnetic activity they are unrelated to the earthquake

subduction zones an introduction to comparative subductology abstract the subduction model for trench arc back arc systems was deduced more or less as a logical consequence of sea floor spreading many features in these systems such as thrust type inter plate earthquakes and deep earthquakes along wadati benioff zones are readily explained by the subduction model but other features such as extensional spreading and high heat flow in back arc regions and arc volcanism are difficult to explain an approach to solve these problems may be to recognize the existence of the two basically different modes of subduction one chilean type causing a compressional stress regime in the arc and back arc regions the other mariana type causing a tensional stress regime implications of the two types of the mode of subduction for some tectonic problems such as vertical movement and sediment accretion at trenches are discussed with special reference to the results of recent dsdp active margin drilling in discussing the possible causes of the two modes it is suggested that the large scale thermal and stress regimes in the back arc areas may not be the direct products of simple subduction but of some additional factors involving much greater energy such as mantle flow associated with the motion of major plates finally the possible great importance of various types of collision accretion and erosion at subduction zones is emphasized

the 1995 kobe earthquake seismic image of the source zone and its implications for the rupture nucleation in order to understand what may have triggered the 1995 kobe japan earthquake and how the rupture proceeded after initiation we determined high resolution three dimensional 3 d p and s wave velocity and poisson s ratio structures in the kobe epicentral area and relocated the aftershocks with the obtained 3 d velocity model we used 64 337 p and 49 200 s wave high quality arrival times from 3634 kobe aftershocks and local microearthquakes recorded by both permanent seismic networks and portable stations that were set up following the kobe mainshock significant velocity variations of up to 6 are revealed in the aftershock area we found that areas with high aftershock activity are generally associated with low poisson s ratio which may be the strong and competent parts of the fault zone and were apt to generate aftershocks the kobe mainshock hypocenter is located in a distinctive zone characterized by low p and s wave velocities and a high poisson s ratio this anomaly exists in the depth range of 16 to 21 km and extends 15 to 20 km laterally this anomaly may be due to a fluid filled fractured rock matrix that contributed to the initiation of the kobe earthquake our interpretation has been supported by many pieces of evidence from hydrological geochemical seismological and geophysical investigations conducted at the kobe earthquake region

intermediate and deep seismicity and lateral structure of subducted lithosphere in the circum pacific region in this paper we present a region by region review of the wadati benioff zone structure of most of the world s seismically active subduction zones focusing primarily on the intermediate and deep seismicity lateral changes in wadati benioff zone structure are common in every major subduction zone in this study we use these changes to define possible boundaries between portions or segments of lithosphere with differing subduction geometries although earthquake data seldom have the resolution to show conclusively whether these boundaries separate independent blocks of lithosphere the available data indicate that the active process at most of these segment boundaries is ductile deformation of the subducting plate rather than tearing we found the strongest evidence for the existence of tears where tears are geometrically necessary such as where a transform boundary terminates a trench as at the new hebrides tonga and south sandwich trenches weak evidence suggesting other tears does exist in some regions such as taiwan the japan izu bonin corner and the philippines the causes of these changes in structure are in most cases unclear only about 34 of the possible segment boundaries coincide with subducting bathymetric features some boundaries occur where there is apparent lateral strain caused by anomalous trench geometry we have designed a simple modeling procedure which incorporates published plate motion and the observed geometry of trenches and wadati benioff zones to estimate the lateral strain in subduction zones throughout the circum pacific region although no observed subduction zone has a perfectly strain free geometry there is a broad range of geometries for which the lateral strain is small indeed the observed geometry of most subduction zones involve relatively little lateral strain comparison with centroid moment tensor focal mechanisms indicates that in zones where the modeling predicted little lateral strain the mechanisms of intermediate and deep earthquakes show no effects of lateral stress and downdip stresses are clearly dominant in regions such as the mariana arc where the model predicts very large lateral extension lateral tension is very evident in the focal mechanisms in regions such as the hokkaido corner where the modeling predicts large compressional strains the plate appears to buckle and bending stresses parallel to the trench are evident in general this study finds that subducted lithosphere is remarkably cohesive and rigid and only rarely deforms by breaking or stretching

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

faulting process of the august 8 1993 guam earthquake a thrust event in an otherwise weakly coupled subduction zone we study a large mw 7 7 earthquake that occurred on june 8 1993 slightly offshore and under the island of guam in the southern mariana island arc from a complete study of p and sh body waves a relocation of the aftershocks and the subevents of the main shock we propose a relatively simple model of the rupture process of this event we propose that this earthquake ruptured a shallow dipping thrust fault that corresponds to the subduction interface under guam like many other earthquakes this event started with a small foreshock and was followed by two large energy release events located to the northeast of the epicenter along the subduction zone the rupture process had a relatively short duration of about 32 s with a weak starting phase that lasted about 8 s seismic moments estimated from body waves surface waves and global positioning system gps are very similar of the order of 4 5 1020 n m the displacement field produced by our best model was compared to the gps measurements of coseismic slip obtained by beavan et al 1994 we find an excellent agreement both in displacement direction and magnitude between the predicted and observed gps displacements this appears to be then the largest earthquake to have occurred on a shallow dipping thrust fault in the mariana subduction zone during this century its occurrence requires a reassessment of the concept of seismic coupling in this subduction zone

faulting process of the 1990 june 20 iran earthquake from broadband records summary we study the rupture process of the iranian earthquake of 1990 june 20 from broad band data we redetermined its moment tensor and source mechanism from long period surface waves then we studied the p and sh broad band recordings from the geoscope and iris seismic networks we inverted the body waves using a full waveform modelling for the inversion we used the gradient method of nab lek mixed with a limited exploration of the parameter space in order to control the resolution of inversion we proceeded by a stepwise procedure increasing the model complexity with every new inversion we used the reduction of cost functional as a criterion for the validity of the inverted model we first inverted the body waves considering a single point source model we found that body waves are dominated by a large energy release about 20 s from the onset of the signals in the next step fixing fault plane solution we inverted for the directivity of the source we found clear evidence of propagation towards the east of the epicentre we determined a rupture velocity of 2 5km s 1 the preceding source inversion reduced the variance of the residuals by about 30 per cent further improvement of the fit of the body wave signals was obtained using an extended line source with constant fault plane solution we obtained a very good fit with an asymmetrical model initially rupture is bilateral but north western propagation stops after 10 s later rupture continues unilaterally in the south eastern direction with a rupture velocity of the order of 2 5 km s 1 finally an inversion was attempted with a line source with varying fault mechanism the final solution obtained by this procedure reduces the variance of the body wave residuals by 60 per cent with respect to the variance of the observed body wave signals variations in fault plane solution along the fault are considered to be well resolved

structural geology of the pelona schist and vincent thrust san gabriel mountains california the vincent thrust of the san gabriel mountains southern california separates an upper plate that contains abundant granitic rocks from a lower plate composed of metagraywackes and metabasites pelona schist the vincent thrust is part of a regional thrust system of late cretaceous early tertiary age vincent chocolate mountain thrust several earlier workers have concluded that the upper plate was derived from the southwest implying the former presence of continental rocks separated from north america by a marginal oceanic basin other workers consider that regional correlations indicate thrusting of the opposite sense the pelona schist of the san gabriel mountains has undergone a complex structural history the earliest recognized structures are tight synmetamorphic isoclinal folds style 1 folds these folds are present throughout the entire 4 km of presently exposed section and do not become more appressed or more abundant near the thrust isoclinally refolded folds and sheath folds were observed at several localities textural observations indicate that the present schistosity of the pelona schist formed by the extreme flattening of crenulations developed in an earlier schistosity this is evidence that the isoclinally refolded folds although only infrequently noticed in the field are actually widespread the isoclinal folding was followed by the development of a macroscopic synform narrows synform and associated minor folds in the 700 m of pelona schist closest to the upper plate these folds style 2 folds are more open than the style 1 folds and do not have a well developed axial planar schistosity they are generally parallel in orientation to the style 1 folds both style 1 and style 2 folds are in turn parallel to tight folds in mylonitic rocks present at the base of the upper plate it is believed that the three groups of folds are cogenetic and possibly were produced in a subduction zone the sheath folds and other evidence of stretching parallel to fold axes might indicate that fold axes were rotated into the direction of thrusting on the other hand the parallelism of the early isoclinal folds and the later relatively open style 2 folds is evidence against rotation thus the orientation of fold axes relative to the thrusting direction is not clear subsequent to metamorphism the pelona schist and the mylonites were folded into a broad arch style 3 folding uncertainty as to the orientation of the axes of style 1 and 2 folds relative to the thrusting direction the presence of the style 3 folding and recently described paleomagnetic evidence of large scale rotation of crustal blocks in southern california make determination of thrusting direction impossible at the present time thus no evidence was found either to support or to invalidate the marginal basin model for the origin of the pelona schist

fracturing and hydrothermal alteration in normal fault zones large normal fault zones are characterized by intense fracturing and hydrothermal alteration displacement is localized in a slip zone of cataclasite breccia and phyllonite surrounding corrugated and striated fault surfaces slip zone rock grades into fractured but less comminuted and hydrothermally altered rock in the transition zone which in turn grades abruptly into the wall rock fracturing and fluid flow is episodic because permeability generated during earthquakes is destroyed by hydrothermal processes during the time between earthquakes

magnetotelluric survey of an active fault system in the northern part of kinki district southwest japan abstract combined geological and geophysical surveys are effective in finding hidden active faults and estimating the dimensions of their fracture zones in the northern part of the kinki district southwest japan the magnetotelluric method using electromagnetic energy in the elf frequency band can define the dimensions of a fault and suggest whether it is active or not the results of the survey show that individual active strike slip faults may be linked through new faults found in unexposed areas the distribution of major active faults suggest that they form conjugate sets defining boundaries to tectonic blocks at least seven tectonic blocks are recognized with different patterns of occurrence of microearthquakes and distributions of active faults historically large earthquakes magnitude 5 as determined by maximum velocity amplitude occur at the boundaries of the blocks these patterns are caused by the different stress fields applied to each block

seismic moment distribution revisited ii moment conservation principle summary we compare the tectonic rate with earthquake data using several theoretical distributions to approximate the seismic moment frequency relation we derive formulae to estimate parameters of the seismic moment distributions by comparing the earthquake occurrence rate with tectonic strain we analyse the tectonic moment rate and the earthquake moment distribution for several types of tectonic environments including subduction zones plate bounding transform faults and deforming continental regions in this analysis we use finer subdivisions of tectonic regions to infer whether earthquake size distribution properties established previously for large provinces flinn engdahl seismic regions could be applied to smaller zones extending over a few hundred kilometres although the shortness of the available earthquake catalogues makes our results less reliable several conclusions can be drawn from the analysis the universality of the parameter the slope of the moment frequency relation is reasonably well confirmed for all of these tectonic regions and their subdivisions in each region we estimate the strain rate or relative plate motion from a plate tectonic model active fault data or geodetic data according to the moment frequency relation the ratio of the tectonic moment rate to the earthquake rate depends on the value the corner magnitude mc and the effective seismogenic thickness we find that this ratio does not vary systematically with the relative plate velocity the ratio of parallel to perpendicular plate motion time since the last major earthquake and the length of fault zone segments established by previous earthquake history we obtain similar results for the western united states and southeast asia the corner magnitude is approximately equal to that for the circum pacific rim and there is no systematic dependence of the mc parameter on a geographic region or strain rate assuming the commonly accepted values for seismogenic thickness elastic modulus and 100 per cent seismic coupling the corner magnitude values for plate boundary zones and continental areas are 8 3 8 8 i e similar to those values obtained by statistical analysis

faulting within the pacific plate at the mariana trench implications for plate interface coupling and subduction of hydrous minerals we investigate faulting within the incoming pacific plate at the mariana subduction trench to understand stresses within the bending plate regional stresses acting upon the plate interface and the extent of possible faulting induced mantle serpentinization we determine accurate depths by inverting teleseismic p and sh waveforms for earthquakes occurring during 1990 2011 with global centroid moment tensor gcmt solutions for earthquakes with mw 5 0 we determine centroid depths and source time functions and refine the fault parameters results from central mariana indicate that all earthquakes are extensional and occur at centroid depths down to 11 km below the moho at the southern mariana trench extensional earthquakes continue to 5 km below the moho one compressional earthquake at 34 km below the seafloor suggests stronger plate interface coupling here in addition we model the stress distribution within the pacific plate along two bathymetric profiles extending seaward from the mariana subduction trench axis to better understand whether our earthquake depth solutions match modeled scenarios for plate bending under applied external forces results from our flexure models match the locations of extensional and compressional earthquakes and suggest that the pacific plate at southern mariana is experiencing larger compressional stresses possibly due to greater interplate coupling additionally we conclude that if extensional faulting promotes the infiltration of water into the subducting plate mantle then the top 5 15 km of the pacific plate mantle are partially serpentinized and a higher percentage of serpentinization is located near the central mariana trench where extensional events extend deeper

mesoscopic structure of the punchbowl fault southern california and the geologic and geophysical structure of active strike slip faults we examine the distribution density and orientation of outcrop scale structures related to the punchbowl fault an exhumed ancient trace of the san andreas fault southern california in order to determine the structure of the fault zone the punchbowl fault has 44 km of right lateral slip and cuts the cretaceous pelona schist in the study area the mesoscopic structures examined include fractures small faults and veins they were inventoried using scan lines at closely spaced stations along three strike perpendicular traverses 200 250 m long across the fault the fault zone thickness is a function of the type of structure measured slip along narrow 2 m wide ultracataclasite cores of the faults results in foliation reorientation over a distance of 50 m from the cores fracture and fault densities appear to increase 50 80 m from the fault cores and vein densities are highly variable across the fault zone fractures and faults in the damaged zone have a variety of orientations but most are at high angles to the main fault zone when coupled with previous geochemical and microstructural data these data show that large displacement faults of the san andreas system are up to 200 250 m thick and enclose zones of mineralogic and geochemical alteration that are 20 30 m thick extreme slip localization occurs over zones 1 5 m thick when reconciled with geophysical imaging our data suggest that trapped headwaves travel in the damaged zone and that some aftershock events produce slip on faults and fractures which often have orientations very diaerent from the principal slip surfaces 7 2000 elsevier science ltd all rights reserved

the 40ar 39ar geochronology of the pelona schist and related rocks southern california seventeen 40ar 39ar ages for hornblende celadonitic muscovite and biotite from the pelona orocopia rand and portal ridge por schists range from 39 to 85 ma the 85 ma is from portal ridge consistent with the proposed correlation of that body with the schist of sierra de salinas two muscovites and one hornblende from the rand schist have ages of 72 to 74 ma indistinguishable from the k ar age of 74 ma for hornblende from a posttectonic granodiorite that intrudes the schist but younger than the 79 ma u pb age of the intrusion four muscovite and two hornblende ages for schist and mylonite from the east fork area of the san gabriel mountains range from 55 to 61 ma similar to k ar ages from nearby upper plate rocks a third hornblende has an age of 73 ma but is not well constrained concordance of schist and upper plate ages confirms structural and metamorphic evidence that the vincent thrust in the san gabriel mountains has not undergone significant postmetamorphic disruption ages from the orocopia mountains are 75 ma for hornblende from nonmylonitic upper plate 52 ma for muscovite from structurally high orocopia schist that is mylonitic and 41 ma for muscovite from nonmylonitic orocopia schist these are consistent with field evidence that the orocopia thrust is a postmetamorphic normal fault muscovite and hornblende from the gavilan hills have ages of 48 to 50 ma younger than ages from the san gabriel mountains but similar to schist ages from the orocopia mountains the geochronologic and structural complexities of the vincent chocolate mountains orocopia and rand thrusts imply that previously cited northeastward vergence may not relate to prograde metamorphism subduction of the por schists the 40ar 39ar data indicate substantial uplift of the por schists prior to middle tertiary detachment faulting which confirms other geochronologic evidence of uplift in southern california and southern arizona during the late cretaceous early tertiary

intraplate stresses and plate driving forces in the philippine sea plate we use a spherical shell elastic finite element analysis to characterize intraplate stresses and plate driving forces in the philippine sea plate our finite element mesh is comprised of 489 nodes and 914 elements and provide a spatial resolution of about 1 the philippine sea plate is unusual in that it does not have large numbers of seismic or tectonic stress indicators and there are no midplate borehole stress data which can be used as constraints in this finite element study however it does possess a unique boundary configuration which potentially can lend more insight to plate forces and stresses than can similar studies on plates with more seismically active interiors because the plate s boundary consists primarily of subduction zones with roughly equal proportions of boundary on the overlying and downgoing sides of trench the resultant plate stress field is very sensitive to subduction related forces as a result borehole stress measurements in certain parts of the plate would be useful in further constraining these forces we find that trench pull forces can explain a large component of the velocity of the philippine sea plate however trench pull has somewhat less overall effect on the stresses across the plate than do the lateral density and topographic variations within the plate i e gravitational or potential energy force and trench suction forces are probably about an order of magnitude smaller than the net trench pull forces collisional forces may dominate the stress fields near taiwan and the izu peninsula but they are of only secondary importance in determining the motion and deformation of the philippine sea plate the forces that generate stress orientations most closely matching the observed stress indicators produce a lithospheric stress field across the plate that averages about 15 mpa over a 70 km thick lithosphere scaling inversely with lithosphere thickness values up to four times as great are produced locally

san jacinto fault zone guided waves a discrimination for recently active fault strands near anza california we deployed three 350 m long eight element linear seismic arrays in the san jacinto fault zone sjfz near anza california to record microearthquakes starting in august through december 1995 two arrays were deployed 18 km northwest of anza across the casa loma fault clf and the hot springs fault hsf strands of the sjfz the third array was deployed across the san jacinto fault sjf in the anza slip gap we observed fault zone guided waves characterized by low frequency large amplitudes following s waves at the clf array and the sjf array for earthquakes occurring within the fault zone however we did not observe guided waves at the hsf array for any events the amplitude spectra of these guided waves showed peaks at 4 hz at the clf and 6 hz at the sjf which decreased sharply with the distance from the fault trace in contrast no spectral peaks at frequency lower than 6 hz were registered at the hsf array we used a finite difference method to simulate these guided modes as 5 waves trapped in a low velocity waveguide sandwiched between high velocity wall rocks the guided mode data are adequately fit by a waveguide on the clf with the average width of 120 m and s velocity of 2 5 km s about 25 reduced from the s velocity of the surrounding rock this waveguide becomes 40 to 60 m wide with the 5 velocity of 2 8 km s in the anza slip gap on the other hand there is not a continuous waveguide on the hsf at depth locations of the events with guided modes suggest that the fault plane waveguide extends along the clf between the towns of san jacinto and anza dipping northeastward at 75 80 to a depth of about 18 km it becomes nearly vertical in the anza gap we speculate that the existence of a continuous low velocity waveguide on the clf can be caused by the rupture of the magnitude 6 9 earthquake on april 21 1918 occurring near the towns of san jacinto and hemet further the lack of a clear waveguide on the hsf suggests that it was not ruptured in this event

rupture process of large earthquakes in the northern mexico subduction zone the cocos plate subducts beneath north america at the mexico trench the northernmost segment of this trench between the orozco and rivera fracture zones has ruptured in a sequence of five large earthquakes from 1973 to 1985 the jan 30 1973 colima event m s 7 5 at the northern end of the segment near rivera fracture zone the mar 14 1979 petatlan event m s 7 6 at the southern end of the segment on the orozco fracture zone the oct 25 1981 playa azul event m s 7 3 in the middle of the michoacan gap the sept 19 1985 michoacan mainshock m s 8 1 and the sept 21 1985 michoacan aftershock m s 7 6 that reruptured part of the petatlan zone body wave inversion for the rupture process of these earthquakes finds the best earthquake depth focal mechanism overall source time function and seismic moment for each earthquake in addition we have determined spatial concentrations of seismic moment release for the colima earthquake and the michoacan mainshock and aftershock these spatial concentrations of slip are interpreted as asperities and the resultant asperity distribution for mexico is compared to other subduction zones the body wave inversion technique also determines the moment tensor rate functions but there is no evidence for statistically significant changes in the moment tensor during rupture for any of the five earthquakes an appendix describes the moment tensor rate functions methodology in detail

factors affecting seismic moment release rates in subduction zones the amount of shallow seismic activity in subduction zones varies greatly from region to region we quantify this seismicity by calculating seismic moment release rates and seismic slip rates for 24 subduction zones to calculate the moment release we sum the seismic moment for all interplate thrust type events with surface wave magnitude ms 7 0 occurring from 1904 to 1980 we present a time history plot of the seismic moment release for each subduction zone these exhibit the differences in the seismic release patterns for subduction zones where the time window of our study is not representative the total moment release is corrected using information on repeat times the moment release rates are compared with various subduction parameters in order to determine which factors influence the degree of coupling these parameters include the age of the subducting lithosphere absolute velocities of the upper and subducting plates convergence velocity and length maximum depth and dip of the wadati benioff zone the moment release rate decreases as the age of the subducting lithosphere increases when the zones belonging to a single subducting plate are considered this age versus moment release relation is consistent for the zones in which the pacific cocos philippine sea and indian plates are subducting the moment release rates for the subduction zones in which the pacific plate is subducting are much higher than for zones of other plates with similar age the age versus moment release relation holds among the zones which belong to one subducting plate however zones with similar ages but belonging to different plates do not have the same moment release rates this suggests that within a single plate the age is the dominating factor affecting the strength of seismic coupling but that each plate as a whole has a characteristic moment release budget zones with retreating upper plates tend to have lower moment release rates the moment release rate does not increase with convergence velocity no simple relationship was found between these two parameters the moment release rate depends most clearly on the age of the subducting lithosphere and the absolute velocity of the upper plate these are the two independent subduction zone parameters among the variables considered the other variables depend on these two parameters

3 d configuration of subducted lithosphere in the western pacific summary computer graphics techniques are used to portray the shape of the subducted lithosphere as seen from different views the configuration of the subducted lithosphere is represented by a surface that approximately fits the upper envelope of the slab like spatial distribution of earthquake hypocentres in the benioff wadati zone regions considered include tonga kermadec izu bonin mariana japankuril kamchatka and sunda banda in the western pacific these regions include all known zones of deep focus earthquakes except the tectonically complex regions of new hebrides solomon new britain molucca sea phillipines the mediterranean and south america shown at the same scale and with the effect of earth s sphericity included the views are very useful for illustrating variations in the configuration of the subducted lithosphere between different regions and within the same region the results illustrate the segmentation of the upper several hundred kilometres of subducted lithosphere into major units of relatively uniform configuration these segments are also clearly defined by major features of surface morphology of the trench and upper plate the distribution of subduction related volcanism and by the intersection with the subduction zone of major bathymetric features of the subducting plate in contrast the deeper portions of the slabs have a more complex and variable configuration discordances between the deep and shallow structures are often important on a regional scale and suggest that there are relative horizontal movements between the surface trace of the subduction zone and the material at depth into which the subducted lithosphere is sinking these relative movements appear to be intimately associated with the tectonic evolution of the region and indicate a regional aspect of subduction that is often overlooked in an attempt to explain the global variability of the shapes of subducted lithosphere although earthquake activity ceases at 680 km possibly because of a phase transition the geometry of the deep portion of subducted lithosphere suggests that subducted lithosphere does penetrate to greater depth

controls on back arc crustal accretion insights from the lau manus and mariana basins abstract together the lau manus and mariana basins encompass a broad range of conditions of back arc basin development marine surveys have determined the tectonic setting and reconnaissance scale geophysical and geochemical properties of the extension axes in these basins we review these data to examine crustal accretion characteristics in the back arc setting in each basin magmatism is enhanced in spreading centres near the arc volcanic front but decreases becoming deficient in axes further from the arc in the lau and manus basins the axes extend far behind the arc and develop typical mid ocean ridge mor like characteristics we propose that these variations are controlled by the subducting slab which shapes composition and flow within the mantle wedge water released from the slab lowers the mantle solidus and increases in concentration toward the volcanic arc spreading centres near the arc advect the hydrated mantle material enhancing melt production slab induced flow carries melt depleted mantle material back beneath the basin where it mixes with ambient mantle spreading centres further from the arc advect this mantle mixture and produce thinner than normal crust far from the arc spreading centres advect essentially mid ocean ridge basalt morb source mantle and their characteristics are mor like thus in addition to spreading rate effects on mantle melting that predominate at mors in back arc basins the subducting slab introduces additional systematic effects

initial results from wing the continuous gps network in the western pacific area to investigate tectonic deformation in the western pacific a continuous gps tracking network has been established and named the western pacific integrated network of gps wing between 1995 and march 1997 we establised ten new sites data for the period july 1995 to october 1996 were analyzed together with data from international gps service for geodynamics igs global sites to estimate daily coordinates a fiducial free approach was used to obtain the most accurate baseline estimates to fix the estimated coordinates to the terrestrial reference frame the tsukuba igs site is assumed to be moving westward relative to the stable eurasian continent at 2 cm yr according to heki s 1996 estimate we find that 1 velocities of sites well within oceanic plates are in good agreement with rigid plate motion models 2 sites close to plate boundaries are all affected by the deformation at those boundaries among which back arc rifting spreading is clearly visible at the mariana and okinawa troughs 3 sites in eastern asia are moving east to east southeast relative to the stable eurasian continent suggesting long distance effects of the northward collision of india with asia

seismogenic characteristics of the northern mariana shallow thrust zone from local array data 1 the northern mariana seismogenic zone has no shallow thrust earthquakes larger than ms 7 4 in the historical seismological record and is traditionally considered decoupled or aseismic during the 2003 2004 mariana subduction factory imaging experiment we recorded local shallow earthquakes throughout the central and northern regions of the mariana forearc using an array of terrestrial broadband and ocean bottom seismographs accurate locations for both the 2003 2004 local seismicity as well as earthquakes with global centroid moment tensor gcmt solutions from 1976 to 2008 were obtained using the hypocentroidal decomposition relocation method and a local velocity model additionally focal mechanisms for the largest 2003 2004 earthquakes were determined using regional waveform inversion thrust faulting earthquakes occur along the mariana megathrust between depths of 20 60 km showing that the lack of great shallow thrust earthquakes does not result from a narrow seismogenic zone and that most seismicity occurs where the downgoing plate contacts the overriding mantle wedge clusters of small plate interface earthquakes with ml 1 6 4 7 occur within patches 100 120 km west of the trench at depths of 30 45 km furthermore the larger gcmt earthquakes mw 4 9 5 8 occur mostly updip and downdip of the patches of smaller earthquakes recorded by our local array and is suggestive of changes in the fault properties with depth clusters of small forearc earthquakes occur discontinuously along the length of the mariana subduction zone showing that northern mariana is variable both along the strike of the margin and with depth along the seismogenic zone we propose that the lack of great mw 8 thrust faulting earthquakes is due in part to the variable frictional heterogeneity along the megathrust

rupture processes of the 1987 1988 gulf of alaska earthquake sequence three large earthquakes occurred in the gulf of alaska during 1987 1988 one on november 17 1987 m w 7 2 the second one on november 30 1987 m w 7 8 and the third one on march 6 1988 mw 7 7 these major intraplate strike slip oceanic events occurred along conjugate trends in a region of no previous seismicity body wave modeling places their centroid depths in the upper mantle the depth of the first two events is approximately 20 km and of the third is 15 km as compared to other large events the body wave models for the two largest earthquakes have short rupture lengths 110 km and 40 km respectively and short duration times 36 and 20 s respectively for their magnitude the latter of the two events has a rupture length much shorter than that inferred from aftershock seismicity the short rupture lengths and source durations may reflect differences between the strength of oceanic and continental lithosphere and suggest that events in oceanic lithosphere have a higher moment release per unit area than similar continental events most of the moment release occurred near the epicenter and or regions of apparent structural complexities where seismicity trends intersect

fault zone guided waves from explosions in the san andreas fault at parkfield and cienega valley california fault zone guided waves were successfully excited by near surface explosions in the san andreas fault zone both at parkfield and cienega valley central california the guided waves were observed on linear three component seismic arrays deployed across the fault trace these waves were not excited by explosions located outside the fault zone the amplitude spectra of guided waves show a maximum peak at 2 hz at parkfield and 3 hz at cienega valley the guided wave amplitude decays sharply with observation distance from the fault trace the explosion excited fault zone guided waves are similar to those generated by earthquakes at parkfield but have lower frequencies and travel more slowly these observations suggest that the fault zone wave guide has lower seismic velocities as it approaches the surface at parkfield we have modeled the waveforms as s waves trapped in a low velocity wave guide sandwiched between high velocity wall rocks resulting in love type fault zone guided waves while the results are nonunique the parkfield data are adequately fit by a shallow wave guide 170 m wide with an s velocity 0 85 km sec and an apparent q 30 to 40 at cienega valley the fault zone wave guide appears to be about 120 m wide with an s velocity 0 7 km sec and a q 30

continental subduction and three dimensional crustal structure the northern south island new zealand the three dimensional vp and vp vs structure of a region where subduction transitions to oblique transform faulting has been determined using arrival times from 579 local earthquakes recorded during a temporary deployment and 3146 earthquakes have been relocated between 40 km and 100 km depth the subducted plate is imaged as a relatively low velocity feature in the uppermost mantle reflecting the continental nature of the subducted crust in this region an increase in amplitude of this low velocity feature from northeast to southwest can be related to an increase in the thickness of the crust of the subducted plate in this direction velocity variations within the subducted and overlying plates show some spatial correlation this suggests an interaction between the plates which extends well beyond the plate interface and is consistent with other geophysical and geological evidence that the plate interface beneath marlborough is currently not accommodating much active subduction in the overlying plate the awatere fault is a major structural feature associated with a low velocity zone extending to 23 km depth there is a marked change in structure near this fault with seismic velocities being lower to the southeast a relatively high level of seismicity occurs in this region of lower seismic velocities suggesting a relationship between the two a possible explanation for this is elevated pore pressures caused by fluids derived from dehydration of the continental subducted crust the low velocity region in the overlying plate coincides with the region of most intense active deformation suggesting it is relatively weak

on the mechanism of seismic decoupling and back arc spreading at subduction zones to address the problem of the great variability of the mechanical state of subduction zones we investigate the mechanics of back arc spreading and seismic decoupling back arc spreading is assumed to be due to rifting of the upper plate and hence occurs when trench normal tension reaches a critical value seismic decoupling is assumed to occur when the normal stress at the frictional interface is decreased by an amount sufficient to cross the friction stability transition two forces are important in this problem the first is a small component of the slab pull force which remains unbalanced by the subduction resistance and exerts a vertical suction force at the trench the second is a sea anchor force exerted on the slab that resists its lateral motion assumed to occur at the upper plate velocity both forces contribute to the coupling problem only the sea anchor force is responsible for back arc spreading the unbalanced slab pull force is determined from a force balance for subduction the sea anchor force is computed as the hydrodynamic resistance to the facewise translation of an elliptical disc through a viscous fluid the model predicts three regimes seismically coupled compressional arcs with advancing upper plates seismically decoupled extensional arcs with retreating upper plates and strongly extensional arcs which also have back arc spreading this model is applied in detail to the izu bonin marianas system it shows that back arc spreading occurs when the integrated tension in the upper plate exceeds a value of about 1 1013 n m 1 and requires a residual tension of about a third that to drive the back arc spreading once rifting is completed it shows why the plate interface near guam is seismically coupled while the plate boundary everywhere farther north is decoupled when applied globally it successfully predicts the state of seismic coupling and back arc spreading in more than 80 of the world s subduction zones of the remaining half can be seen to contain additional complications not included in the model about 10 of cases remain unexplained but some of these may have incorrectly determined seismic coupling coefficients

circum pacific seismic potential 1989 1999 the seismic potential for 96 segments of simple plate boundaries around the circum pacific region is presented in terms of the conditional probability for the occurrence of either large or great interplate earthquakes during the next 5 10 and 20 years i e 1989 1994 1989 1999 and 1989 2009 this study represents the first probabilistic summary of seismic potential on this scale and involves the comparison of plate boundary segments that exhibit varying recurrence times magnitudes and tectonic regimes presenting these data in a probabilistic framework provides a basis for the uniform comparison of seismic hazard between these differing fault segments as well as accounting for individual variations in recurrence time along a specific fault segment and uncertainties in the determination of the average recurrence time

a land ocean bottom seismograph experiment in the mariana forearc abstract a network of land and ocean bottom seismographs in the north central mariana islands located 20 earthquakes during a 22 day recording period the locations of 14 subcrustal events define an inclined seismic zone extending to approximately 280 km depth with one isolated event at 580 km depth the geometry of this zone coincides with that defined by teleseismic earthquake locations no reliably located earthquakes occurred within the accretionary prism consistent with the hypothesis that deformation within the accretionary prism takes place aseismically

the april 5 1990 mariana islands earthquake and subduction zone stresses abstract the april 5 1990 mariana islands earthquake m w 7 5 involved normal faulting at shallow depth in the pacific plate under the mariana trench about 200 km east of the central mariana islands the earthquake is significant for its tectonic setting focal mechanism and large size the event occurred in the vicinity of a proposed slab segment boundary and may in part owe its unusually large size to lateral variation in intraplate strain as appears to be the case for other large normal faulting events near trenches detailed analysis of the seismic source parameters of the earthquake and of the relationship between seismicity and tectonics in the mariana islands is conducted to improve our understanding of the mechanical behavior and seismotectonics in island arcs with aseismic convergence the 1990 mariana earthquake rupture initiated with very low levels of energy release for an interval of about 7 2 s followed by a large single impulsive event with a duration of about 6 s body wave inversion yields a strike 215 dip 49 rake 95 and a moment 1 2 10 20 nm with a centroid time of 11 12 s the source mechanism is further constrained by the long period rayleigh and love wave data with inversions yielding a strike 184 5 dip 67 rake 90 5 and moment 2 2 0 5 10 20 nm the long period centroid depth is 23 km with a centroid time of 20 3 s

present day tectonic plate motions and crustal deformations from the doris space system doppler orbitography and radiopositioning integrated by satellite doris data acquired between january 1993 and december 1996 from the spot 2 spot 3 and topex poseidon satellites have been analyzed to determine velocities for 45 sites on eight major tectonic plates for 28 sites far from deformation zones the velocity estimates agree with plate model predictions least squares computation of poles of rotation which model the plate motions shows that for eurasia africa pacific and south america plates the agreement is better with nuvel 1 while for australia antarctica nazca and north america plates the doris euler vectors are closer to ntjvel 1a in general doris results do not differ significantly from other space geodetic techniques determinations but provide better estimates for plates poorly or inhomogeneously covered by global positioning system gps very long baseline interferometry vlbi and satellite laser ranging slr networks such as africa the doris coverage of this plate allows discussion of intraplate deformations due to the motion of the eastern africa part which constitues the somalia plate sites located in deformation zones such as western eurasia boundaries central asia southwestern america coast south east asia show motion with respect to their own plates comparisons with other geodetic measurements for colocated stations or with regional geodynamical models show the interest of doris in active zones where global plate models are not valid

source parameters of the 1 october 1995 dinar turkey earthquake from sar interferometry and seismic bodywave modelling abstract the 1 october 1995 m s 6 1 dinar earthquake ruptured a 10 km section of the nw se dinar civril fault there are discrepancies between the published source parameters from seismic data with seismic moments in disagreement by over a factor of two we use both sar interferometry and seismic bodywave modelling to determine earthquake source parameters an interferogram generated from ers 1 2 sar imagery spanning the event and separated by 5 months is used to derive source parameters by a downhill simplex inversion with multiple monte carlo restarts we model the displacements in the satellite line of sight initially using uniform slip on a rectangular dislocation in an elastic half space the resultant model fault plane agrees in strike and location with the observed surface break but systematic residuals exist in the line of sight deformation field resulting in a r m s residual of 20 mm in the interferogram the residuals are reduced if the depth distribution of slip is allowed to vary spatially in four segments along a continuous fault plane our best fitting solution with a r m s misfit of 8 mm reveals two distinct areas of slip on the fault plane strike 145 dip 49 rake 270 a main rupture slipping by 1 44 m between depths of 1 and 8 km becoming deeper to the se and matching the observed surface rupture and an along strike continuation to the nw of the same fault plane but between depths of 8 and 13 km and not associated with a surface break the total geodetic moment 4 5 0 1 10 18 nm is more than twice as large as published seismic moments based on the inversion of p waveforms alone but close to the harvard cmt moment 4 7 10 18 nm we use sh waveforms in addition to the p waves used previously to determine an alternative seismic source mechanism sh waves constrain the depth to be shallower than solutions based on p waves alone agreeing with the depths from the interferometric inversion and resulting in a larger moment 3 1 0 4 10 18 nm than the previous bodywave estimates 2 2 1 10 18 nm the cmt moment reduces in magnitude to a similar size 3 3 10 18 nm if the centroid depth and fault dip are constrained to the values determined from bodywave modelling and interferometry thus the geodetic moment is 40 bigger than the moment determined from seismology

complex refolding history of the pelona orocopia and rand schists southern california previous workers studying the structural geology of the pelona and related schists of southern california and southwestern arizona have generally reported two important sets of folds an early group of isoclinal folds and a set of later more open folds field and petrographic evidence presented here indicates the existence of additional isoclinal folds predating the ones already described all folds are thought to have formed during underthrusting of the pelona schist in a subduction zone

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

geologic structure near the cajon pass scientific drill hole the geologic structure near the cajon pass drill site is controversial the squaw peak fault was previously interpreted as the edge of a thrust plate of large displacement this may be incorrect based on the style of deformation and the presence of similar granodiorite on both sides of the fault alternatively mid miocene crustal extension on a west facing listric normal fault may have formed the basin in which the cajon formation was deposited post miocene crustal shortening normal to the san andreas fault may have folded the upper plate of this fault and thrust it northeastward by this interpretation steeply inclined beds at the drill site terminate on a fault ramp and squaw peak is the corner of the initially stable block that was broken off and ramped upward

implications for mechanical properties of brittle faults from observations of the punchbowl fault zone california field observations of the punchbowl fault zone an inactive trace of the san andreas are integrated with results from experimental deformation of naturally deformed punchbowl fault rocks for a qualitative description of the mechanical properties of the fault and additional information for conceptual models of crustal faulting the punchbowl fault zone consists of a single continuous gouge layer bounded by zones of extensively damaged host rock fault displacements were not only localized to the gouge layer but also to discrete shear surfaces within the gouge deformation in the exposure studied probably occurred at depths of 2 to 4 km and was dominated by cataclastic mechanisms textural data also suggest that significant amounts of pore fluids were present during faulting and that fluid assisted mechanisms such as dissolution diffusion and precipitation were operative the experimental data on specimens collected from the fault zone suggest that there is a gradual decrease in strength and elastic modulus and an increase in relative ductility and permeability toward the main gouge zone the gouge layer has fairly uniform mechanical properites and it has significantly lower strength elastic modulus and permeability than both the damaged and the undeformed host rock for the punchbowl fault and possibly other brittle faults the variations in loading of the gouge zone with time are primarily governed by the morphology of the fault and the mechanical properties of the damaged host rock in addition the damaged zone acts as the permeable unit of the fault zone and surrounding rock it appears that the gouge primarily governs whether displacements are localized and it therefore may have a significant influence on the mode of slip

interpretation of the 1993 2001 and 2002 guam earthquakes as intraslab events by a simultaneous relocation of the mainshocks aftershocks and background earthquakes it has been unclear whether the 1993 2001 and 2002 guam earthquakes m w 7 8 7 0 and 7 1 respectively in the southern mariana subduction zone were interplate events or intraplate ones within the pacific slab we relocated their mainshocks aftershocks and numerous long term background earthquakes simultaneously with high relative accuracy using the modified joint hypocenter determination mjhd method as a result their mainshocks and three day aftershocks are clearly distributed within the background slab seismicity therefore we conclude that the 1993 2001 and 2002 guam earthquakes were not interplate events which scarcely occur in the mariana subduction zone but intraslab earthquakes within the subducted pacific plate the 1993 fault plane is inferred to be the near horizontal nodal plane of the mechanism solution though the 2001 and 2002 fault planes remain unknown the relocation of target events simultaneously with background earthquakes has turned out very effective for discriminating between large interplate and intraslab earthquakes in subduction zones

seismotectonics around the mariana trough abstract strike slip type mechanism solutions with approximately east west striking tension axes were obtained for large shallow earthquakes under the central topographic high of the northern mariana trough these solutions are consistent with the model that spreading is in progress in the direction perpendicular to the structural lineations trending n s in most of the trough and nnw sse in the northern part weak coupling at the subduction zone between the pacific plate and the landward plate mariana platelet is suggested by the occurrence of few thrust type and many normal fault type earthquakes near the mariana trench spreading striking nearly e w with an approximate total rate of 6 cm yr in the widest section of the trough is speculatively inferred from the plate motion model of the philippine sea plate with respect to the surrounding plates and from the anchored slab model and the collision effect of the caroline ridge with the philippine sea plate

global omori law decay of triggered earthquakes large aftershocks outside the classical aftershock zone 1 triggered earthquakes can be large damaging and lethal as evidenced by the1999 shocks in turkey and the 2001 earthquakes in el salvador in this study earthquakes with ms 7 0 from the harvard centroid moment tensor cmt catalog are modeled as dislocations to calculate shear stress changes on subsequent earthquake rupture planes near enough to be affected about 61 of earthquakes that occurred near defined as having shear stress change 0 01 mpa the ms 7 0 shocks are associated with calculated shear stress increases while 39 are associated with shear stress decreases if earthquakes associated with calculated shear stress increases are interpreted as triggered then such events make up at least 8 of the cmt catalog globally these triggered earthquakes obey an omori law rate decay that lasts between 7 11 years after the main shock earthquakes associated with calculated shear stress increases occur at higher rates than background up to 240 km away from the main shock centroid omori s law is one of the few time predictable patterns evident in the global occurrence of earthquakes if large triggered earthquakes habitually obey omori s law then their hazard can be more readily assessed the characteristic rate change with time and spatial distribution can be used to rapidly assess the likelihood of triggered earthquakes following events of ms 7 0 i show an example application to the m 7 7 13 january 2001 el salvador earthquake where use of global statistics appears to provide a better rapid hazard estimate than coulomb stress change calculations

geodetic evidence of back arc spreading in the mariana trough 1 repeated gps surveys in the mariana islands show that the mariana block is moving apart from the philippine sea plate the velocities of the islands relative to the philippine sea plate range from about 15 mm yr in the northern islands to about 45 mm yr near guam the data also suggest convergence rates for the mariana forearc with respect to the pacific plate of 35 45 mm yr at 19 n increasing to 55 70 mm yr at 13 5 n in addition the velocity vectors show a slight north south expansion of the arc the estimated location of the euler pole of the mariana forearc with respect to the philippine sea plate is well south of the geographical point where the back arc basin narrows to zero width

body waveform modeling of east mediterranean earthquakes at intermediate distances 17 30 with a gaussian beam summation method we study eastern mediterranean earthquakes in the magnitude range 5 5 6 5 by body waveform modeling at distances between 17 and 30 we compute synthetics at these distances using the gaussian beam summation gbs method taking into account all waves refracted and reflected by the upper mantle discontinuities modeling depends crucially on the choice of an appropriate upper mantle model using the well known may 13 1995 kozani grevena event in greece we choose a representative column of the 3smac upper mantle model as an average velocity structure for modeling eastern mediterranean earthquakes recorded in europe we studied three events with different tectonic settings location and magnitude for the july 20 1996 dodecanese earthquake we determined a depth of 12 5 km a source duration of 6 s and a seismic moment of 1 20 1018 nm most of the seismograms for this event are very well modeled using only seismograms recorded at the epicentral range 17 30 we modeled the october 1 1995 turkey event we find a depth of 9 km and a normal fault mechanism in agreement with the regional tectonic setting finally we studied the february 23 1995 earthquake in cyprus for which we retrieved depth source time function and a seismic moment of 2 04 1018 nm the upper mantle phases for all these events are well identified and modeled we conclude that gbs can be used to model seismograms at intermediate distances retrieving information about both fault plane solution depth and source time functions propagation effects are also observed but they can not be inverted with this limited data set

3 d velocity structure of the san jacinto fault zone near anza california i p waves summary seismic arrival times from microearthquakes ml 4 on the san jacinto fault near anza california are used to find spatial variations in the seismic velocity that are related to the crustal structure of the fault zone preliminary modelling of the 1 d p wave velocity structure of the upper 25 km of crust reveals that most of the variation in velocity is lateral rather than depth dependent the traveltime anomalies due to lateral structure can be partially compensated for by applying station corrections however the variance of the traveltime residuals is still 2 25 times larger than the variance of the picking error the spatially correlated residuals show that this variance is due to localized velocity anomalies and that the data require further modelling using a 3 d velocity structure because the 3 d inverse problem is non unique smoothness constraints are applied to find the model that has the minimum structure required to fit the data to the picking error where a smooth model is defined such that the gradient of the velocity perturbation from the original 1 d model is small with small non zero station corrections a 3 d velocity model can be found that fits the data well the structure is well resolved from 3 to 9 km depth where lateral perturbations of up to 7 per cent are determined independently of the trade off between station corrections and poorly resolved near surface structure the model shows a horizontal gradient with overall faster velocities in the north east side of the fault zone at 3 6 km depth the signature of the fault zone is evident in the lower velocities beneath the surface trace of the fault however at 9 km depth higher seismic velocities are found extending into the fault zone from the north east block this higher velocity region occurs where there is a distinct lack of seismicity on the fault there is also a localized feature in the south west of the modelled region that is more than 10 km from the main trace of the fault with velocities 3 per cent slower than average

centroid moment tensor solutions for july september 1993 abstract centroid moment tensor solutions are presented for 186 earthquakes that occurred during the third quarter of 1993 the solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model sh8 u4l8 of dziewonski and woodward

seismological studies at parkfield i simultaneous inversion for velocity structure and hypocenters using cubic b splines parameterization a model for the 3d p and s wave velocity structure at parkfield has been developed by the systematic inversion of arrival times for microearthquakes and surface vibroseis sources recorded by nearby calnet stations and by the local borehole installed 10 station high resolution seismic network there a modification of the linear b splines model parameterization method of thurber 1983 to a cubic b splines formulation through tests inverting synthetic data sets for the parkfield configuration is found to produce smooth velocity distributions that still retain some small scale features application of the method to 953 p wave and 288 s wave arrivals from 111 microearthquakes and 8 vibroseis locations throughout the study area yields a velocity structure having variable resolution a low velocity zone appears to coincide with the creeping segment of the san andreas fault zone northwest of the 1966 parkfield earthquake epicenter an increase of about 15 per cent in seismic velocities is seen from the northeast to southwest across this segment of the fault zone the higher velocities to the southwest apparently correspond to the granitic basement the contrast across the fault is subdued to the southeast along the 1966 rupture zone a high vp vs ratio of 1 9 is seen in a 2 km wide volume near the location of the 1966 main shock hypocenter presumably the nucleation zone for the expected m 6 parkfield earthquake