simulated ground motions for hypothesized mw 8 subduction earthquakes in washington and oregon the amplitude and duration of strong ground motions from hypothesized mw 8 subduction zone thrust earthquakes in the puget sound portland region were estimated using a semi empirical method the simulation procedure assumes the rupture surface may be represented by a grid of fault elements finite difference wave simulation in a detailed 2 d velocity structure identifies direct s and the moho postcritical reflection as the primary components of the s wave field at horizontal distances of 0 to 150 km green s functions containing these two arrivals are computed with generalized ray theory in an equivalent 1 d structure for each source element receiver propagation path scattering and attenuation structure are empirically modeled by the use of corrected accelerograms from mw 7 michoacan mexico and valparaiso chile aftershocks for the fault element source functions different site conditions are empirically included in the simulation procedure by using empirical source functions recorded on rock and soil sites spatial variations in slip on the fault asperities are introduced by weighting the fault elements the technique has been validated for large subduction zone earthquakes by modeling acceleration time histories and response spectra from the 1985 michoacan mw 8 0 and valparaiso mw 8 0 main shocks somerville et al 1991 fault models for the puget sound and portland regions and seismic velocity structure models have been adapted from published regional studies uncertainty in the location of the asperities on the fault surface results in a large degree of uncertainty in the simulated ground motions at a given site if distance is defined as the distance to the closest asperity then the variability in the ground motions is reduced indicating that this uncertainty can be lessened by constraining the depth of the asperity the ground motion estimates are relatively insensitive to the difference in fault dip between the puget sound and portland fault models the attenuation of peak acceleration with distance r from the fault asperity for a mw 8 subduction earthquake is given by in pga 1 5 3 33 in r 128 0 79 s where s is a site term equal to 0 for rock and 1 for soil when distance r is the distance to the fault plane the attenuation relation is in pga 2 8 1 26 in r 0 79 s formal estimates of uncertainty in the calculated ground motions have been obtained by estimating both parametric uncertainty from the range of source models of hypothesized cascadia subduction earthquakes and modeling and random uncertainty from the misfit between recorded and simulated ground motions of the 1985 michoacan and valparaiso earthquakes for periods less than 1 sec the estimated response spectral velocities on soil sites in the seattle olympia region are about twice those recorded during the 1949 olympia and 1965 seattle wadati benioff zone earthquakes and the durations of strong motion on rock sites are significantly longer 45 to 60 sec versus 10 to 15 sec

prediction of coal seam details and mining safety using multicomponent seismic data a case history from china abstractwith the increasing mining depth of coal mines geologic hazards have become the main source of accidents for this reason coal seam prediction requires more assessment parameters such as the degree of anisotropy vp vs value and elastic modulus these parameters are difficult to determine using only the pp wave we have inverted and interpreted multicomponent seismic data acquired from a 2 56 km2 area in the guqiao mine located on the southern margin of the north china plate under the constraints of drill log and rock physics test data the production coal seam 11 2 of the mine located in the permian formation was our focus of this study and our objective was to determine the structure fracture development thickness surrounding rock lithology roof stability and mining safety of the seam to achieve this we first performed an analysis of s wave splitting and joint pp and ps wave inversions the inversion results were then combined to derive additional parameters such as the vp vs

rupture process of the 1980 izu hanto toho oki earthquake deduced from strong motion seismograms abstract the 1980 izu hanto toho oki earthquake is studied in detail using near field strong motion seismograms recorded at japan meteorological agency stations a seismogram inversion method is applied to deduce the dislocation distribution and the character of rupture propagation during this earthquake this earthquake involves left lateral strike slip motion on the almost vertical plane with a strike of n10 w the fault plane is shallower than about 12 km in depth and the length is about 20 km the large dislocation large seismic moment occurs near the hypocenter and at the southern end of the fault plane the rupture propagates southward from the central part of the fault plane and spreads to the shallow area of the northern part of the fault plane after a delay of about 5 sec relative to the initiation of this earthquake the total seismic moment is about 7 10 25 dyne cm the aftershocks of magnitude equal to or greater than 4 0 take place in the areas where high stresses are expected to remain after this earthquake the mechanical weakness of small submarine monogenetic volcanoes which are located above the source region seems to affect the rupture process of this earthquake

high angle reverse faulting associated with the 1946 nankaido earthquake abstract analysis of vertical crustal deformation data in the southwestern part of shikoku southwest japan suggests that the nankaido earthquake of 1946 m w 8 1 which is a principal interplate thrust earthquake was accompanied by subsidiary faulting on a splay fault adjacent to the coast of shikoku discarding crustal movement resulting from the main thrusting of the nankaido earthquake local leveling data are explained by slip on a simple rectangular thrust fault located just offshore of shikoku although it is difficult to constrain the fault location a possible result is a high angle thrust dipping landward at an angle of about 70 with a dislocation of about 1 5 m and source dimensions of 30 13 km along strike and dip respectively this result indicates that the fault may be one of the steeply dipping subsidiary faults branching from the main low angle thrust as was the case in the alaska earthquake of 1964 although several lines of evidence suggest that this faulting occurred as slow aseismic slip its discrimination from the main seismic event is extremely difficult this kind of high angle thrusting just offshore of the coast would play an important role for the formation of the marine terraces during the late quaternary period

a viscoelastic coupling model for the cyclic deformation due to periodically repeated earthquakes at subduction zones vertical displacements due to periodic reverse faulting events in an elastic plate overlying a viscoelastic maxwell half space are obtained and compared with the observed deformation cycle coseismic strain release postseismic transients interseismic strain accumulation from japan the viscoelastic effects including the influence of buoyant restoring forces are obtained using the method developed by rundle and plate convergence and strain accumulation are incorporated following the procedure suggested by savage the resulting deformation cycle is compared with that of an analogous elastic half space dislocation model in which postearthquake effects are due to transient aseismic slip below the coseismic fault cyclic deformation is similar but not identical for the two models and observations from southwest japan suggest the superiority of the viscoelastic coupling model in particular inclusion of the effects of steady state flow in the asthenosphere overcomes a defect of the elastic half space model and results in agreement with the observed interseismic movement pattern several aspects of the postseismic deformation its landward migration and its transition to the interseismic phase of the cycle are explained as well but the short duration of near trench transients relative to those observed farther inland is not matched the success of a buried slip model in explaining early postseismic near trench movements and asthenospheric flow in accounting for cumulative postearthquake transient motions suggests the existence of a transition zone between lithosphere and asthenosphere whose behavior is brittle elastic in the short term and ductile for longer term deformation and such a modification may reconcile remaining discordant observations however reasonable variations in coupling model parameters cannot account for observed differences in the deformation cycle in other parts of japan and these regional differences remain unexplained

elevation changes and the great 1960 chilean earthquake support for aseismic slip we reanalyse the near field vertical displacement data for the great chilean earthquake of 1960 to provide constraints on the down dip slip distribution a recent study of free oscillation data confirms earlier proposals of a slow precursor comparable in size to the main shock we seek evidence for this event in the displacement data spatial coverage of the data is inadequate to allow determination of a unique fault model but inversions show that thrust slip must extend over a width of about 300 km and to a depth between 60 km and 85 km well below the seismogenic zone for interplate events we propose that this deeper slip is the source of the slow event our preferred class of solutions has moments ranging from 1 6 1023 n m to 2 5 1023 n m only 30 to 45 of the free oscillation value underestimation of the near trench displacements by 50 together with a seismic overestimation of moment by 30 due to tradeoff with rise time would result in satisfactory agreement

the 1960 chile earthquake inversion for slip distribution from surface deformation summary a total of 166 observations of sea level change 130 measurements of elevation difference and 16 determinations of horizontal strain provide an excellent view of the quasi static source process of the great 1960 chilean earthquake these surface deformation data were employed in classical uniform slip fault models as well as more recently developed models that allow spatial variability of slip the best uniform slip planar usp model is 850km long 130km wide and dips 20 seventeen metres of fault displacement contributed to a usp moment of 9 4 times 1022 n m the variable slip planar vsp model concentrates slip on a 900 km long 150 km wide band parallel to the coast several peaks of slip with dimensions of 50 100 km appear in this band and are thought to represent major subduction zone asperities important fractures of the oceanic lithosphere bound the 1960 rupture and are offered as a potential source of fault segmentation within the chilean subduction zone the vsp moment for 1960 earthquake totals 9 5 times 1022 n m about one fifth of the value estimated for the foreshock mainshock sequence from seismic methods except for areas out to sea geodetic resolution on the fault is fairly uniform thus it is unlikely that slip missed by the network could increase the vsp moment much beyond 1 8 times 1023 n m several patches of moment isolated from the main body at 80 110 km depth are found down dip in the vsp model and are presumably indicative of aseismic slip one patch at the northern end of the rupture is probably associated with the initiation phase of the mainshock although the time sequence of the relationship is unknown tide gauge records suggest that another patch between 40 and 43 s responsible for the observed strain and uplifts inland at those latitudes is not of coseismic origin but derives from in place post seismic creep over several years apparently great 1960 type events are not typical members of the 128 yr earthquake cycle in south central chile the nazca south america boundary here is characterized by a variable rupture mode in which major asperities are completely broken by great earthquakes only once in four or five earthquake cycles the more frequent large earthquakes that geographically overlap the great events fill in between the locked zones

a fault model of the 1946 nankaido earthquake derived from tsunami data abstract a fault model of the 1946 nankaido earthquake m 8 2 is determined by the use of tsunami records of uwajima shimotsu and hososhima which were located within or near the area of major coseismic crustal deformation synthetic tsunamis computed for various fault models are matched with the observed tsunamis to determine the fault parameters a low angle thrust model slightly revised from a previous model by ando is consistent with the observed tsunamis the duration of faulting is constrained as less than 10 min based upon the tsunami the fault is divided into an eastern and a western segment corresponding to areas associated with and without aftershocks respectively the fault area and dislocation for the western segment are 150 70 km2 and 6 m and those for the eastern segment are 150 70 km2 and 3 m respectively the total seismic moment is 4 7 1028 dyn cm significantly smaller than that obtained from a geodetic model by fitch and scholz but still larger than that of the seismic model by kanamori the discrepancy in seismic moment between the seismic and the present models ran2 could be interpreted in terms of a slow dislocation on the fault but this interpretation does not match the seismic intensity distribution and damage pattern and the slow slip model for the nankaido earthquake is rejected the discrepancy between the two seismic moments is considered insignificant within error involved in data and modeling assumptions if the revised geodetic model ran2 is modified the seismic moment required to explain the observed tsunamis would be reduced further by 30 if we consider the uncertainties involved in the fault model of kanamori and the fault finiteness effect affecting the amplitude of seismic waves the seismic moment required to interpret the seismic wave data could be increased possibly being more than twice that of kanamori thus the two seismic moments from the different data sets could be close to each other within allowable tolerance this implies that the rise time of the nankaido earthquake was short enough to generate short period seismic waves from both the western and the eastern fault segments

the rupture process and asperity distribution of three great earthquakes from long period diffracted p waves moment tensor source mechanisms for the largest twenty nine shallow earthquakes from 1981 m s 6 5 have been determined using long period rayleigh wave spectra and p wave first motions recorded by the international deployment of accelerograms ida global digital seismic network gdsn and world wide standardised seismic network wwssn these are compared with the centroid moment tensor mechanisms presented by dziewonski and woodhouse which were obtained in a contrasting manner in most cases the source is well represented by a single double couple mechanisms with good agreement of scalar moment and orientation between the two studies apparent mismatches for other cases may be interpreted in terms of source complexity we emphasise the objectivity of comparing moment tensor elements apparent discrepancies in the moment and orientation of double couple fault plane solutions are usually due to differences in the mxz and myz moment tensor elements which are poorly constrained by long period data

inversion of tsunami waveforms for the estimation of heterogeneous fault motion of large submarine earthquakes the 1968 tokachi oki and 1983 japan sea earthquakes waveform inversion of tsunami data provides independent and reliable information on earthquake source processes the propagation effects of tsunamis are accounted for by computing green s functions using a finite difference method and using the known bathymetry which is much better constrained than the seismic velocity structure the observed tsunami waveforms are corrected for the response of the tide gage system by using the results of in situ measurements using green s functions computed from bathymetric data on a 2 5 km grid and the observed waveforms digitized at 1 min intervals the spatial resolution of the faulting heterogeneity by the present method is as small as 30 km this method is applied to the 1968 tokachi oki and 1983 japan sea earthquakes the slip is concentrated at the northwestern part of the fault plane for the 1968 event for the 1983 event the slip on the fault just north of the epicenter is twice that of the other parts these asperity distributions are similar to those inferred from seismic wave analysis however the recovered total seismic moments are smaller the present method is shown to be a useful technique for estimating heterogeneous fault motion especially for historical events with no or poor seismic records and tsunami earthquakes in which the excitations of seismic and tsunami waves are different

depth distribution of coseismic slip along the nankai trough japan from joint inversion of geodetic and tsunami data coseismic slip distribution on the fault plane particularly in the downdip direction associated with large subduction earthquakes can be estimated by joint inversion of geodetic and tsunami data two large earthquakes the 1944 tonankai earthquake mw 8 1 and the 1946 nankaido earthquake mw 8 3 occurred on the nankai trough southwestern japan where the philippine sea plate is subducting beneath the eurasian plate the source areas of these events extended over both land and ocean coseismic crustal movements on land were measured by leveling while those in ocean were recorded as tsunami waveforms on tide gauges the coseismic slip distribution inverted from these data shows that the slip on the shallower part of the fault plane is comparable to that on the deeper parts this indicates that large coseismic slip can occur beneath an accretionary wedge where current seismicity is low the result has implications for other subduction zones having a similar tectonic environment such as for the pacific northwest region of the united states although the possibility of a large earthquake there is still debated should a large subduction earthquake occur in this region coseismic slip on the shallow part could be large and the potential for large tsunamis is high

the earthquake deformation cycle at the nankai trough southwest japan geodetic measurements made landward of the nankai trough site of great subduction zone thrust earthquakes in 1854 and 1946 provide a uniquely detailed picture of the strain buildup process supply constraints on the mechanism of strain accumulation and allow for improved estimates of earthquake recurrence provided the two most recent movement cycles are similar the observations dating from about 1890 may be used to reconstruct a single complete deformation cycle coseismic strain release postseismic transients interseismic strain accumulation very complete leveling and tidal gage data indicate that postseismic deformation extends more than 300 km inland from the plate boundary persists for at least 30 years and shows a clear tendency to become longer wavelength with increasing time the transient movements have two timescales the first of about a year or less corresponds to deformation largely uplift concentrated close to the coseismic fault and is most easily explained by aseismic slip or very localized deformation downdip of the earthquake rupture plane the second longer timescale is associated with a diffusion like spread of the deformation further landward an effect qualitatively similar to that first predicted by elsasser to be an expected consequence of faulting in an elastic plate overlying a viscoelastic asthenosphere cumulative uplift since 1890 correlates well with the distribution of uplifted marine terraces although average post 1890 tilt rates exceed late quaternary and holocene averages by at least a factor of three because of the nonlinearity of strain buildup and the significant permanent deformation simple recurrence calculations typically overestimate the true interval between great earthquakes by a factor of 2 to a factor of 3 strict application of the time predictable model assumed correct overcomes these difficulties provided the cumulative transient deformation and the proportion of permanent deformation per cycle can be estimated

moment tensor summation to derive the active crustal deformation in japan abstract the country of japan and the surrounding area has been divided into 12 seismogenic sources which belong to five belts with an almost uniform orientation of the stress field in each one of these sources the active crustal deformation has been determined using the summation of the seismic moment tensors of mainly post 1964 earthquakes the analysis showed that due to the subduction of the philippine sea plate beneath the eurasian plate the deformation is expressed as compression at a mean direction of n53 w and an average rate of 1 5 cm yr the fault plane solution corresponding to the summed tensor indicates low angle thrust faulting with the following parameters strike 226 dip 21 and rake 98 along the sagami trough dextral strike slip motion is predominant at an average rate of 6 cm yr the summed tensor corresponds to a fault plane solution with strike 291 dip 73 and rake 180 along the districts of fukushima tohoku and the hokkaido island the motion of the pacific plate toward eurasia causes compression at a mean direction of n65 w and an average rate of 4 cm yr the summed tensor corresponds to a fault plane solution with strike 199 dip 18 and rake 81 that is low angle thrust faulting dipping toward the land this pattern resembles the hellenic arc where the deformation is also taken up by motion on low angle 18 thrust faults dipping to the north in the tohoku district the rate of motion reaches a value of 8 cm yr which is the highest calculated in the whole japan area in the inner part of the japanese islands along western honshu and niigata the mean p axis is almost horizontal trending nearly e w in western honshu the deformation is mainly expressed as dextral strike slip motion at an average rate of 5 cm yr in a n59 e direction the representative fault plane solution for this area has strike 240 dip 81 and rake 173 at niigata the western coast of hokkaido and further north at okushiri the deformation is taken up as compression at a mean direction of n96 e at an average rate of 2 cm yr the summed tensor corresponds to a fault plane solution with strike 5 dip 41 and rake 90 the calculated directions and rates of motion along the japanese islands are in good agreement with the expected values from plate motion models

global positioning system measurements of deformations associated with the 1987 superstition hills earthquake evidence for conjugate faulting large station displacements observed from imperial valley global positioning system gps campaigns are attributed to the november 24 1987 superstition hills earthquake sequence thirty sites from a 42 station gps network established in 1986 have been reoccupied during 1988 and or 1990 displacements at three sites within 3 km of the surface rupture approach 0 5 m eight additional stations within 20 km of the seismic zone are displaced at least 10 cm this is the first occurrence of a large earthquake ms 6 6 within a preexisting gps network best fitting uniform slip models of rectangular dislocations in an elastic half space indicate 130 8 cm right lateral displacement along the northwest trending superstition hills fault and 30 10 cm left lateral displacement along the conjugate northeast trending elmore ranch fault the geodetic moments are 9 4 1025 dyn cm and 2 3 1025 dyn cm for the superstition hills and elmore ranch faults respectively consistent with teleseismic source parameters the data also suggest that postseismic slip along the superstition hills fault is concentrated at shallow depths distributed slip solutions using singular value decomposition indicate near uniform displacement along the elmore ranch fault and concentrated slip to the northwest and southeast along the superstition hills fault a significant component of nonseismic secular displacement is observed across the imperial valley which is attributed to interseismic plate boundary deformation

simultaneous inversion of waveform and geodetic data for the rupture process of the 1984 naganoken seibu japan earthquake summary a simultaneous inversion method of strong motion seismograms and geodetic data for inferring earthquake rupture process was developed we assumed a source model composed of many point sources of which slip vectors and rupture times are unknown parameters in addition to observed data we introduced smoothing constraints to determine many parameters without instability since the leastsquares solution depends on the relative weights of the smoothing constraints to a data misfit function we determined their values by minimizing akaike s bayesian information criterion abic by using abic we can objectively choose a unique model this inversion method was applied to the naganoken seibu japan earthquake of 1984 dividing a 15 x 10 km fault into 150 subfaults we obtained the minimum abic model with a seismic moment of 2 7 x los dyne cm this model has a slip peak in the eastern and shallow area of the assumed fault the high slip area is just under the areas where thrown out boulders were found by umeda et al 1987 and iio yoshioka 1988 comparing the aftershock distribution with the estimated slip distribution we found that the high slip area had few aftershocks this suggests that aftershocks are caused by the stress generated in the marginal region around the main shock slip the inversion analysis also revealed that there was a slight change in slip angle during the earthquake the high slip area has some amount of dip slip component while the slips in the other areas are almost purely right lateral

inversion of long period p waves from great earthquakes along subduction zones abstract long period p waves from subduction zone earthquakes in the moment range from 7 1027to 8 1029dyn cm are analyzed using an iterative deconvolution method in addition to discrete subevents which effectively contribute to the observed seismograms the long period component of the moment rate function is derived with the constraint that the total seismic moment is equal to the moment determined from long period surface waves it is shown that great earthquakes can be described by several subevents with a duration of about 10 s which changes little from earthquake to earthquake in the above moment range the long period component of the moment rate function becomes larger for a greater earthquake the greatest earthquakes of mw 9 additionally involve subevents with another duration scale much longer than 10 s it is also shown that the local stress drop is 1 order of magnitude larger than the stress drop averaged over the entire fault plane the constancy of the local stress drop as well as the average stress drop holds among various magnitudes of earthquakes these observations suggest an essential similarity in the mode of dislocation distribution

the tempe volcanic province of mars and comparisons with the snake river plains of idaho abstract the tempe volcanic province of mars is a broad region adjacent to and superimposed upon the western tempe mareotis plateau the volcanic region is a relatively low plain of probable basaltic flood lava affinity superimposed upon this plain are a variety of small constructional and nonconstructional volcanic features all of which appear structurally controlled along an orientation of approximately n60 e constructional features included low shields composite cones and low irregular hills which may be silicic domes the nonconstructional features include linear depressions which may be rift zones fissure flows and pit craters the tempe volcanic province appears comparable in many aspects to features of the snake river plains of idaho while the tempe region may cover a significantly larger area than the snake river plains the same type and scale of features are observed on both areas the tempe region includes three styles of volcanism flood basalts a capping of fissure and vent fed flows with scattered low shields and areas where low shields have coalesced into plains type volcanism the snake river plains are represented by one style plains volcanism this represents one of the few examples where direct comparison can be made between martian and terrestial geologic geomorphic features and where a scale problem is not encountered

seismic potential associated with subduction in the northwestern united states despite good evidence of present day convergence of the juan de fuca and north american plates there has been remarkably little historical seismic activity along the shallow part of the juan de fuca subduction zone although we cannot completely rule out the possibility that the plate motion is being accommodated by aseismic creep we find that the juan de fuca subduction zone shares many features with other subduction zones that have experienced great earthquakes

paleoseismicity of the cascadia subduction zone evidence from turbidites off the oregon washington margin cores from cascadia deep sea channel contain sequences of turbidites that can be correlated and dated by the first occurrence of volcanic glass from the mount mazama eruption 6845 50 radiocarbon yrbp turbidity currents from the tributaries appear to have occurred synchronously to form single deposits in the main channel there being only 13 turbidite deposits in the lower main channel since the mazama eruption instead of the twice as many expected if the tributaries had behaved independently in addition to the cascadia channel 13 post mazama turbidites have been deposited in the astoria canyon and at two sites off cape blanco sample locations that span 580 km of the oregon washington margin pelagic intervals deposited between the turbidites suggest that in each place the turbidity currents occurred fairly regularly every 590 170 years on average the best explanation of the spatial and temporal extent of the data is that the turbidity currents were triggered by 13 great earthquakes on the cascadia subduction zone the variability of turbidite timing is similar to that for great earthquake cycles the thickness of the topmost pelagic layer suggests the last event was 300 60 years ago from three places along the margin but this number may be a biased underestimate it is however consistent with the youngest sudden subsidence event on the washington coast the turbidite data demonstrate that the near term hazard of a great earthquake on the cascadia subduction zone is of the order of 2 10 in the next 50 years

order and diversity in the modes of circum pacific earthquake recurrence recurrence characteristics of great circum pacific earthquakes and determinations of spatial distribution of seismic moment release are surveyed in order to delineate their general features and relate earthquake slip distribution to models of recurrent rupture as noted by others the pattern of moment release is typically very irregular with strong concentrations in a few isolated regions of a much larger aftershock zone despite this complexity rupture nucleation is notably systematic with mainshock epicenter showing a strong tendency to locate in or immediately adjacent to identified regions of high moment release this generalization suggests that earthquake recurrence is more likely to be controlled by maximum rather than average fault slip well documented case histories from twelve plate boundary regions demonstrate that seismic strain release tends to be temporally well ordered while source dimensions slip and cumulative moment release vary considerably from cycle to cycle on none of the segments studied do earthquakes consistently recur in nearly identical events instead individual great earthquakes differ significantly from cycle to cycle or rerupture takes place in a sequence of two or more smaller events despite these differences the duration of the seismic cycle is approximately uniform furthermore when strain release occurs in a sequence of large earthquakes these events take place towards the end of the cycle and occur with increasing rupture length and magnitude these ordered and irregular features of earthquake recurrence argue for the existence of corresponding elements on plate boundary faults the ordered characteristics are identified with zones of concentrated moment release that slip comparable amounts in each cycle and have high shear strength the irregular features are associated with intervening weaker regions that move in response to the stress concentration of dynamic rupture and slip by differing amounts in each event although these mechanistic associations are indirect and tentative the features of recurrent behavior documented here have implications for long term earthquake hazard assessment that are not dependent on the models proposed to explain them chief among these are the cycle to cycle differences among gap filling events and the absence of shocks that fill major slip deficient regions early in the seismic cycle

great earthquakes and subduction along the peru trench subduction along the peru trench between 9 and 15 s involves both large interplate underthrusting earthquakes and intraplate normal fault earthquakes the four largest earthquakes along the peru trench are from north to south the 1970 m 7 9 intraplate normal fault earthquake and the interplate underthrusting earthquakes in 1966 m 8 0 1940 m 8 and 1974 m 8 0 we have studied the rupture process of these earthquakes and can locate spatial concentrations of moment release through directivity analysis of source time functions deconvolved from long period p wave seismograms the 1966 earthquake has a source duration of 45 s with most of the moment release concentrated near the epicenter two intraplate normal fault events occurred in 1963 m 6 7 and 7 0 at the down dip edge of the 1966 dominant asperity the 1940 earthquake is an underthrusting event with a simple source time function of 30 s duration that represents the rupture of a single asperity near the epicenter the 1974 earthquake has a source duration of 45 50s and two pulses of moment release this earthquake has a bilateral rupture with the first pulse of moment release located northwest of the epicenter and the second pulse of moment release located southeast of the epicenter both pulses of moment release occur on the northern half of the aftershock area the 1970 earthquake is one of the largest intraplate normal fault earthquakes to occur in a subduction zone and has a moment release comparable with many large underthrusting events the aftershocks for the 1970 earthquake form two distinct clusters the smaller cluster near the epicenter has focal mechanisms characterized by down dip tension but the second aftershock cluster located 80 km southeast of the epicenter has focal mechanisms characterized by down dip compression the p waves for the main shock can be modeled as two sources with different focal mechanisms and depths similar to the two clusters of aftershocks the first event has a down dip tensional focal mechanism and is followed 40 s later by a distinct second event located 80 km southeast of the epicenter with a down dip compressional focal mechanism and a somewhat shallower depth than the first event the observable directivity indicates that the second source is located at the second cluster of aftershocks that have down dip compressional focal mechanisms the occurrence of both down dip tensional and compressional focal mechanisms may be explained by extreme unbending stresses associated with the anomalous slab geometry the unusually large size of the 1970 earthquake may also be related to the subduction of the mendana fracture zone the historic earthquake record along the peru trench indicates that the previous event in 1746 was much larger than any of the three underthrusting earthquakes this century the 1746 earthquake may have ruptured the entire segment in a multiple asperity earthquake thus the mode of rupture along the peru coast has changed between successive earthquake cycles

seismic moment tensors and source depths determined by the simultaneous inversion of body and surface waves we present a linear moment tensor inversion method that incorporates both body and surface wave data the source coordinates are fixed during the inversion and the source depth is determined by applying the inversion at several depths and taking the depth that produces the minimum residual the inversion is carried out in the time domain for body wave data and in the frequency domain for surface wave data the fundamental rayleigh waves at the period of 256 s from ida stations and the long period p waves from wwssn and gdsn stations whose angular distances are from 30 to 90 are used as the data the simultaneous inversion of both body and surface wave data sets has several merits because of the body wave constraints the present method gives reasonable solutions at shallow depths 30 km where long period surface wave inversions become unstable although the inversion using only body wave data is unstable in the sense that it allows for a variety of solutions with a slight change in the assumed depth probably because of the noise present and poor coverage of body wave data to determine the source mechanisms the inclusion of surface wave data makes the solution stable this is because the surface wave data constrain nodal plane strikes we may also expect better resolution in source depth by including the body wave data because the local crustal structure above the source can be taken into account to generate the green s function we applied our method to two large earthquakes one occurring at the triple junction off boso peninsula central honshu on 18 september 1984 and another at the outer rise area off the chile trench on 16 october 1981 the scalar moments of the boso oki and chile earthquakes are estimated to be 3 2 1026 dyn cm and 6 3 1026 dyn cm respectively the ratio of the minimum absolute eigenvalue of the moment tensor to the maximum which may measure the non double couple component is less than 10 for both earthquakes this suggests that their sources are essentially a double couple the source depths of both events are determined as 30 km the boso oki event is indicative of hinge faulting within the pacific slab at the triple junction the chile event is consistent with compressional bending stress in the deeper part of the lithosphere at the trench outer rise area off chile

slab geometry of the cascadia subduction zone beneath washington from earthquake hypocenters and teleseismic converted waves using earthquake hypocenters obtained from the washington regional seismograph network and analysis of ps converted phases from teleseisms at special temporary stations located south of the olympic peninsula we have developed a preliminary model of the structure of the subducted slab beneath western washington and southern british columbia the slab is smoothly arched beneath puget sound subducting at a dip of about 10 12 at the top of the arch north and south of the arch crest the slab dips 15 20 subcrustal earthquakes are localized near the top of the arch this deformation from planar slab geometry is caused by the change in azimuth of the trench or hinge line of the slab from almost ns south of 47 north latitude to approximately n30 w north of 48 north the slab appears to be easily flexed probably due to small elastic thickness but its geometry reflects conservation of surface area and minimization of in plane strain crustal seismicity in the puget sound basin may result indirectly from the presence of the arch in the underlying juan de fuca plate

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

high precision slip distribution of the 1944 tonankai earthquake inferred from tsunami waveforms possible slip on a splay fault abstract inversion of tsunami waveforms is a well established technique for estimating the slip distributions of subduction zone earthquakes with some of the most detailed results having been obtained for earthquakes in the nankai trough sw japan the present study although it uses a method and tsunami waveform data set almost identical to previous study aims to improve on previous work by using a more precise specification of initial conditions for the calculation of tsunami green s functions specifically we incorporated four improvements in the present study 1 we used a realistic plate model based only on seismic survey results and assumed it to be the fault plane of the 1944 tonankai earthquake 2 the smallest subfaults consistent with the long wavelength approximation were used in the tsunami inversion analysis 3 we included the effect of horizontal displacement of the ocean bottom on tsunami generation and 4 we performed a checkerboard resolution test as obtained in previous studies a zone of high slip 2 0 m was resolved off the shima peninsula however the more precise calculation of tsunami green s functions has revealed additional detail that was not evident in previous studies which we demonstrate is resolvable and correlates with the position of known faults in the accretionary prism while there was little or no slip near the trench axis in the eastern part of the rupture zone there was up to 1 5 m of slip resolved within 30 km of the trough axis in the western part along the coast of the kii peninsula this troughward slip zone coincides with the position of a large splay fault mapped in multichannel reflection surveys furthermore it is also clear that the upper edge of the enshu fault off shima and atsumi peninsulas is consistent with the up dip limit of slip in the eastern part of our model we tested the possibility that slip occurred on the former splay fault instead of on the plate interface during the 1944 tonankai earthquake and find that slip on this splay fault is also consistent with the data although we cannot distinguish whether slip was dominant on the splay fault or on the plate interface we further suggest that the position of the enshu fault may be determined by the subduction of topographic highs and that such faults may have an important influence on the up dip rupture limit of the 1944 tonankai and potentially other subduction zone earthquakes

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

do trench sediments affect great earthquake occurrence in subduction zones seismic energy release is dominated by the underthrusting earthquakes in subduction zones and this energy release is further concentrated in a few subduction zones while some subduction zones are characterized by the occurrence of great earthquakes others are relatively aseismic this variation in maximum earthquake size between subduction zones is one of the most important features of global seismicity previous work has shown that the variation in maximum earthquake size is correlated with the variation in two other subduction zone properties age of the subducting lithosphere and convergence rate these two properties do not explain all the variance in maximum earthquake size i propose that a third subduction zone property trench sediments explains part of the remaining variance in maximum earthquake size subduction zones are divided into two groups 1 those with excess trench sediments and 2 those with horst and graben structure at the trench thirteen of the 19 largest subduction zone events including the three largest occur in zones with excess trench sediments about half the zones with excess trench sediments are characterized by great earthquake occurrence most of the other zones with excess trench sediments but without great earthquakes are predicted to have small earthquakes by the age rate correlation two notable exceptions are the oregon washington and middle america zones overall the presence of excess trench sediments appears to enhance great earthquake occurrence one speculative physical mechanism that connects trench sediments and earthquake size is that excess trench sediments are associated with the subduction of a coherent sedimentary layer which at elevated temperature and pressure forms a homogeneous and strong contact zone between the plates

inversion of strong ground motion and teleseismic waveform data for the fault rupture history of the 1979 imperial valley california earthquake a least squares point by point inversion of strong ground motion and teleseismic body waves is used to infer the fault rupture history of the 1979 imperial valley california earthquake the imperial fault is represented by a plane embedded in a half space where the elastic properties vary with depth the inversion yields both the spatial and temporal variations in dislocation on the fault plane for both right lateral strike slip and normal dip slip components of motion inversions are run for different fault dips and for both constant and variable rupture velocity models effects of different data sets are also investigated inversions are compared which use the strong ground motions alone the teleseismic body waves alone and simultaneously the strong ground motion and teleseismic records the inversions are stabilized by adding both smoothing and positivity constraints the moment is estimated to be 5 0 10 25 dyne cm and the fault dip 90 5 dislocation in the hypocentral region south of the united states mexican border is relatively small and almost dies out near the border dislocation then increases sharply north of the border to a maximum of about 2 m under interstate 8 dipslip motion is minor compared to strike slip motion and is concentrated in the sediments the best fitting constant rupture velocity is 80 per cent of the local shear wave velocity however there is a suggestion that the rupture front accelerated from the hypocenter northward the 1979 imperial valley earthquake can be characterized as a magnitude 5 earthquake at the hypocenter which then grew into or triggered a magnitude 6 earthquake north of the border

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

seismic structure across the active subduction zone of western canada the vancouver island seismic project was conducted in 1980 to study the structure of the subducting oceanic juan de fuca plate and the overriding continental america plate the principal seismic refraction line line i was a 350 km onshore offshore profile perpendicular to the continental margin an array of 32 receivers was located on the america plate on the mainland and across vancouver island and extended offshore with three ocean bottom seismometers obs s two shots were fired at the eastern end of the line and 17 shots were located along the westernmost 100 km of the profile control for the interpretation of the onshore offshore profile was provided by a reversed refraction profile along the length of vancouver island and by a marine refraction profile recorded on the obs s modeling of the seismic structure of this complex region utilized an iterative inversion method for travel times from explosions in which shots at several locations are recorded on the same set of receivers and utilized an algorithim based on asymptotic ray theory for the calculation of synthetic seismograms through two dimensional media the major features of the refraction structural model are that 1 the oceanic lithosphere dips at 3 or less beneath the continental slope so the bend in the subducting slab occurs landward of the foot of the slope 2 the oceanic lithosphere dips at 14 16 beneath the continental shelf until it passes beneath the continental moho at 37 km depth below western vancouver island 3 an upper mantle reflector may correspond to the base of the subducting lithosphere and 4 a segment of high velocity material above the downgoing crust with velocity 7 7 km s and depth range 20 25 km may represent a remnant of subducted lithosphere perhaps detached when the subduction zone jumped westward to its present position

seismicity and focal mechanisms associated with the subduction of the juan de fuca plate beneath the olympic peninsula washington abstract two and a half years of data from an expanded telemetered seismic network on the olympic peninsula reinforce the conclusion that the juan de fuca plate is subducting beneath washington however the rate of seismicity is presently much lower than typical rates at other convergent margins the located earthquakes can be divided into two groups on the basis of depth at the coast a shallow zone extends to a depth of approximately 20 km while a deep zone begins at 25 km on the eastern edge of the peninsula the shallow zone has thickened to 30 km and the depth to the top of the deep zone has increased to 45 km the rate of crustal seismicity is much lower on the peninsula than in the adjacent puget sound region the deep zone is 5 to 10 km thick and dips to the east at an angle of about 11 the deep events appear to be occurring in the upper part of the subducting oceanic lithosphere focal mechanisms were constructed for 16 events in the suite of deep events with magnitudes ranging from 1 6 to 4 3 the solutions show primarily normal faulting with the p axes normal to the slab and the t axes displaying a range of angles within the slab with the average t axis in the downdip direction small events with down dip tension in the downgoing slab are consistent with the slab sinking under its own weight several strike slip events may be caused by a shearing within the slab these features are consistent with ongoing subduction of the juan de fuca plate beneath north america

postseismic coastal uplift in southern chile tide gage records at puerto montt referenced to a mareograph in talcahuano indicate a large at least 75 cm postseismic uplift of the region following the 1960 mw 9 5 event field observations carried out in 1989 at the same locations of previous measurements in 1968 are consistent with tide gage records the postseismic elevation changes are modeled as the result of propagating creep on the downdip extension of the coseismic fault for a 30 e fault dip minimum square error indicates a fault creep velocity of 4 6 km yr and slip amplitude of 3 5 m