inversion of complex body waves we have developed a method that inverts seismic body waves to determine the mechanism and rupture pattern of earthquakes the rupture pattern is represented as a sequence of subevents distributed on the fault plane this method is an extension of our earlier method in which the subevent mechanisms were fixed in the new method the subevent mechanisms are determined from the data and are allowed to vary during the sequence when subevent mechanisms are allowed to vary however the inversion often becomes unstable because of the complex trade offs between the mechanism the timing and the location of the subevents many different subevent sequences can explain the same data equally well and it is important to determine the range of allowable solutions some constraints must be imposed on the solution to stabilize the inversion we have developed a procedure to explore the range of allowable solutions and appropriate constraints in this procedure a network of grid points is constructed on the i plane where and i are respectively the onset time and the distance from the epicenter of a subevent the best fit subevent is determined at all grid points then the correlation is computed between the synthetic waveform for each subevent and the observed waveform the correlation as a function of and i and the best fit mechanisms computed at each i grid point depict the character of allowable solutions and facilitate a decision on the appropriate constraints to be imposed on the solution the method is illustrated using the data for the 1976 guatemala earthquake

body wave inversion moment tensors and depths of oceanic intraplate bending earthquakes this paper develops a moment tensor inversion technique suited for analysis of long period body waves from distant shallow earthquakes in addition to the moment tensor the procedure resolves precise source depths 5 km and estimates the duration of faulting the technique is applied to digital sro dwwssn recordings of outer rise intraplate earthquakes these earthquakes result from bending of oceanic plates prior to subduction bending induces horizontal tension near the top of the lithosphere and horizontal compression near its base a neutral surface separates compressional from tensional regimes an examination of 11 bending earthquakes in the period january 1981 to july 1982 indicates that the depth to the neutral surface is not geographically uniform although the seismicity sample is very small the depth of the neutral surface appears to correlate with the position of the trench arc in the evolutionary sequence of uyeda and kanamori 1979 in particular neutral surfaces at strongly coupled chilean type plate boundaries are elevated as much as 15 km above those at weakly coupled mariana type boundaries this correlation implies that nonuniformities in neutral surface height are created by variations in regional compressive stress resulting from contrasts in plate plate coupling to account for the observed heights maximum interplate compressive stresses must be comparable with stresses derived from bending

rupture of the 4 february 1976 guatemalan earthquake abstract an expanded body wave data set for the 4 february 1976 guatemalan earthquake is analyzed by iterative multiple station inversion to provide enhanced resolution of the rupture process of this important strike slip event the body wave data set used in an earlier study of this event in which a predominantly unilateral westward propagating rupture was proposed actually does not have sufficient resolution to reliably map the spatial heterogeneity for the source process the expanded azimuthal coverage of the new data set provides better spatial resolution of the primary rupture characteristics the guatemalan earthquake can be modeled as an asymmetric bilateral rupture extending eastward and westward along the motagua fault with the largest episode of moment release occurring 90 km west of the epicenter near a change in strike in the surface faulting the overall rupture velocity is about 3 5 km sec and the total moment is 3 7 10 27 dyne cm inversions with composite fault models having a second focal mechanism appropriate for the western end of the fault are consistent with the data but are not clearly required the characterization of the guatemalan earthquake as an irregular bilateral rupture agrees with the variable co seismic surface displacements measured along the fault outcrop as well as with an earlier surface wave investigation and is consistent in a general sense with previous characterizations of the irregular rupture processes of other large strike slip earthquakes such as the 1972 sitka alaska and the 1980 eureka california events

determinations of focal depths of earthquakes associated with the bending of oceanic plates at trenches abstract the earthquakes examined in this paper are all within the oceanic lithosphere and are associated with the bending of plates before subduction accurate determinations of the depth of these earthquakes are needed to study the stress pattern within a bending plate routinely determined depths of shallow sub oceanic earthquakes published in bulletins are unreliable the depths can be accurately determined to within a few kilometers if the original seismograms from these events are studied in some cases the reflected phases pp and pwp can be clearly identified there exists the possibility that the wave reflected at the water air interface pwp may be misidentified as pp leading to erroneous estimates of depth additional methods of analysis such as surface wave radiation patterns or the apparent frequency dependence of reflection at the crust water interface can remove this possible source of confusion one of the most powerful techniques for depth analysis is the modelling of long period waveforms the pattern of stresses within the bending oceanic lithosphere revealed by the depths and focal mechanisms of these intraplate earthquakes is one of horizontal deviatoric tension down to a depth of about 25 km with horizontal compression at greater depths

seismicity and deformation at convergent margins due to heterogeneous coupling studies of the mechanics of subduction as inferred from earthquake cycle observations suggest that the distribution and style of seismicity in the seafloor between the trench and the outer rise and in the slab at intermediate depth can in some cases serve to identify asperity locations along the thrust interface dmowska and lovison 1992 such asperities identified from seismic wavefield modeling are the zones of highest seismic moment release in large underthrusting events to the extent that asperity locations are relatively stationary from one event to the next their locations provide the zones of highest expected moment release in future large earthquakes and rupture often nucleates at the border of an asperity the region of the thrust interface outside such asperities is apparently less well coupled and releases moment throughout the great earthquake cycle in some combination of aseismic creep and moderate seismicity thus it is reasonable that stress and deformation rates associated with the earthquake cycle should be most pronounced near asperities and that this should have seismic and geodetic consequences three dimensional finite element modeling is used here to understand such stress and deformation patterns and their variation in time in relation to heterogeneity of coupling along thrust interfaces the stress field helps to explain the observed clustering of seafloor seismicity along the strike of the convergent margin in cases of convergence at approximately normal incidence like for the region of the valparaiso chile 1985 thrust event the modeling is consistent with the observation that areas of large earthquakes in the seafloor toward the outer rise and in the slab tend to lie within corridors through thrust zone asperities running perpendicular to the line of the trench we seek to learn if such model stress fields are consistent with observations for the strongly oblique subduction margin of the rat islands western aleutians 1965 event that active areas of the outer rise and slab at intermediate depth are offset along strike from asperity locations modeling results here for the stress in the seafloor raise the possibility that to explain this offset the asperity zones along the thrust interface may have to be strung out along the direction of oblique slip perhaps reflecting the contact path of subducting seamounts or geometric irregularities along the interface shear stress patterns created in the upper plate when there is oblique subduction suggest that favorable areas for back arc strike slip activity following underthrusting as in the adak island central aleutians region of the 1986 andreanof island earthquake ekstrom and engdahl 1989 will also be shifted along strike from asperity locations our analyses show how deformation patterns on the earth s surface above asperities differ from patterns above nonasperities and hence provide tools to identify inhomogeneous coupling from geodetic observations we discuss possible bathymetric topographic and structural signals of strength of coupling and of asperities particularly noting that the density and extent from the trench of seafloor normal faults correlates with seismically inferred zones of strongest coupling in the central aleutians

fault damage zones of the m7 1 darfield and m6 3 christchurch earthquakes characterized by fault zone trapped waves abstract to characterize the subsurface structure of the damage zones caused by the 2010 2011 canterbury earthquake sequence in new zealand s south island we installed two short linear seismic arrays array 1 across the greendale fault gf surface rupture and array 2 over the surface projection of the blind port hills fault phf that ruptured in the 2010 m7 1 darfield and 2011 m6 3 christchurch earthquakes respectively we recorded 853 aftershocks for 4 months after the christchurch earthquake fault zone trapped waves fztws identified at array 1 for aftershocks occurring on both the gf and the phf show that the post s durations of these fztws increase as focal depths and epicentral distances from the array increase suggesting an effective low velocity waveguide formed by severely damaged rocks existing along the gf and phf at seismogenic depths locations of aftershocks generating prominent fztws delineates the subsurface gf rupture extending eastward as bifurcating blind fault segments an additional 5 8 km beyond the mapped 30 km surface rupture into a zone with comparably lower seismic moment release west of the phf rupture which extends westward to within 5 3 1 km of the subsurface gf the propagation of fztw through the intervening gap indicates moderate gf phf structural connectivity we interpret this zone as a fracture mesh reflecting the interplay between basement faults and stress aligned microcracks that enable the propagation of phf sourced fztws into the gf damage zone simulations of observed fztws suggest that the gf rupture zone is 200 250 m wide consistent with the surface deformation width velocities within the zone are reduced by 35 55 with the maximum reduction in the 100 m wide damage core zone corresponding with surface and shallow subsurface evidence for discrete fracturing the damage zone extends down to depths of 8 km or deeper consistent with hypocentral locations and geodetically derived fault models

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

earthquake source parameters and stress distribution in the adak island region of the central aleutian islands alaska source parameters have been systematically determined for all earthquakes with mw 5 that occurred between 172 w and 179 w longitude in the adak island region of the central aleutian islands during 1977 march 1987 we relocate the events using a plate velocity model developed for the region and use two alternative methods of depth determination the first method uses arrival times of direct and prominent reflected phases primarily pwp the second method uses broadband p wave displacement seismograms in an inversion for source depth the analysis of these events provides a well constrained data set for the study of stress release along an active subduction zone three earthquakes which occurred seaward of the trench axis are located just below the crust mantle interface and show extension nearly perpendicular to the trench axis seven events occurred in the wadati benioff zone sixty four events are located in the main thrust zone and except for five unusual events are characterized by thrust mechanisms with one nodal plane dipping north at a shallow angle in cross section the thrust zone appears as a thin 10 15 km thickness interplate region which extends from 15 to 50 km depth a 10 15 average discrepancy between observed slip vector azimuths and the predicted relative motion direction between the pacific and north american plates exists for these events the observed slip vectors are oriented more normal to the trench than is predicted by plate motions five earthquakes which occurred as aftershocks to the may 7 1986 earthquake mw 8 0 in the crust of the overriding plate have strike slip mechanisms consistent with right lateral motion on arc parallel fault planes observations of slip vectors along the whole aleutian arc show a similar trend to that observed in the adak island region the largest differences 30 between observed and predicted slip azimuths occur around 175 e we propose a model of plate interaction in which a portion of the along arc motion occurs along a weak strike slip shear zone in the upper plate near the volcanic line the slip azimuths in the main thrust zone fit this model well if the amount of transcurrent slip occurring in the upper plate is 60 of the arc parallel relative plate motion a consequence of the model is along arc extension of the overriding plate between the accretionary wedge and the volcanic line especially in the western part of the aleutian arc calculations based on a tectonic model by geist et al 1988 for the formation of arc summit basins through block rotation and translation suggest that along arc extension has been significant since late miocene or early pliocene

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

evidence for and implications of a bering plate based on geodetic measurements from the aleutians and western alaska 1 global positioning system gps measurements are used to examine the hypothesis of a clockwise rotating bering plate originally proposed on the basis of seismicity the bering plate encompasses the bering sea western alaska and the aleutian islands gps measurements from the bering plate s interior show south to southwest motions of 3 5 mm a where a is years we construct elastic dislocation models to determine the spatial distribution and intensity of locked patches on the aleutian subduction interface we use these to remove interseismic strain from the gps observation and determine an arc translation velocity for each region of the aleutians revealing south and southwest motions of 4 8 mm a we combine the arc translation rates with measurements from the bering plate s interior sites and estimate the euler pole for the bering plate relative to north america to be located at 42 5 n 121 3 e with an angular speed of 6 0 ma the clockwise rotation of the bering plate may cause left lateral faulting in interior alaska the bering plate s interaction with south central alaska may be responsible for the decreased slip rate on the western denali fault and for contraction across the central alaska range we analyze slip partitioning along the aleutian arc on the basis of both gps measurements and slip azimuths of thrust earthquakes and find a systematic discrepancy between plate convergence direction and slip azimuths we conclude that slip partitioning in the back arc only develops west of amchitka pass whereas slip partitioning in the fore arc is present throughout the arc

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

pb sr nd isotope constraints on the fluid source of the dahu au mo deposit in qinling orogen central china and implication for triassic tectonic setting abstract the dahu au mo deposit is a structure controlled lode system occurring in the northern xiaoqinling terrane huaxiong block qinling orogen this paper reports a new sr nd pb isotope dataset obtained for ore sulfides and the hostrocks within the taihua supergroup in an attempt to constrain the source of the ore forming fluids from a new dimension 16 sulfide samples yield i sr ratios of 0 70470 0 71312 with an average of 0 70854 e nd t values between 13 5 and 18 1 with average of 15 1 and 206 pb 204 pb i 207 pb 204 pb i and 208 pb 204 pb i ratios of 17 033 17 285 15 358 15 438 and 37 307 37 582 with averages of 17 162 15 405 and 37 440 respectively 5 gneiss samples from the taihua supergroup yield i sr ratios of 0 70947 0 73201 averaging 0 72294 e nd t values of 20 0 to 31 1 averaging 25 1 and 206 pb 204 pb i 207 pb 204 pb i 208 pb 204 pb i ratios of 17 127 18 392 15 416 15 604 and 37 498 37 814 with averages of 17 547 15 470 and 37 616 respectively these data show that the ore sulfides have less radiogenic sr nd pb isotope systematics than the hostrocks and suggest that the ore forming fluids which interacted with the wallrocks to form ores must be sourced from a depleted mantle or a depleted subducted oceanic slab in combination with the spatial scenario and geochemical signatures of the triassic magmatites and mineral systems we argue that in the triassic the mianlue ocean was not completely closed and that the northward oceanic plate subduction still survived along the mian lue suture which caused the late triassic magmatism and associated mineralization in qinling orogen including the au mo dahu deposit

analysis of the trade off between hypocentral depth and source time function focal depth determination for shallow 60 km intermediate size ms 6 5 to 7 5 earthquakes continues to be problematic for these events the depth phases i e pp and sp are not distinctly separated in time and thus synthetic modeling of wave shapes becomes necessary unfortunately the uniqueness of the solution is not easily determined because of the trade off between various focal parameters we have investigated the trade off between depth and source function both theoretically and experimentally the changes in the green s function caused by an incorrect assumed depth can almost always be compensated by changes in the source function producing the same observed seismogram thus there is considerable trade off between depth and the source function perturbing the depth from the true depth leads to a more complicated source function the simplest source function then pinpoints the true depth we have used two different measures of simplicity the varimax norm v max and the half absolute moment time t 1 2 both measures successfully characterize the complications that appear in the source functions for incorrect assumed depths the true depth of an event can be interpreted from the resultant v max versus depth and t 1 2 versus depth curves thus we have a technique that determines the depth range of faulting in a systematic quantitative fashion our observations from the synthetic study have been applied to an outer rise event off the coast of central chile 16 october 1981 ms 7 2 to an event in southern mexico 24 october 1980 ms 7 0 and to the borrego mountain event in southern california 9 april 1968 ms 6 4 resulting in faulting depth ranges of 0 to 20 5 km 60 to 70 5 km and 10 to 14 5 km respectively

rupture process of the andreanof islands earthquake of may 7 1986 the andreanof islands earthquake of may 7 1986 ms 7 7 is the largest earthquake to have occurred along the aleutian arc in the past 20 yr source parameters of the mainshock three large aftershocks 6 1 ms 6 7 and a foreshock ms 6 0 are determined by inverting digitally recorded teleseismic waveforms and amplitudes long period p and sh waves are inverted simultaneously to determine the strike dip rake centroid depth source time function and seismic moment of each earthquake complexities in the rupture process of the mainshock are modeled using broadband 5 to 0 01 hz p wave displacement waveforms modeling the mainshock as a propagating line source the long period and broadband waveforms indicate that the locus of largest moment release propagated bilaterally along the arc with a well resolved velocity of 1 5 km sec the source mechanism is consistent with faulting on a low angle 23 1 thrust fault the fault s strike 260 10 and the rake of the slip vector on the fault 93 8 are less well resolved and trade off with each other acceptable centroid depths range between 10 and 25 km within this range of depths the misfit of the synthetic to the observed long period and broadband waveforms varies by only 6 per cent while moment estimates vary by as much as 64 per cent for a centroid depth of 20 km the estimated seismic moment is 1 1 1028 dyne cm 85 per cent of the moment estimated from surface wave analysis ninety per cent of the moment release was generated by three patches on the fault surface all located within 90 km of the nucleation point very little moment however was released near the epicenter of the mainshock the epicentral locations source mechanisms and depths of the foreshock and one of the aftershocks modeled by us are consistent with faulting on the same thrust plane as the mainshock the other two modeled aftershocks are located north of the island arc and have source depths of less than 15 km indicating that they occurred within the overriding plate the focal mechanisms of these events indicate almost pure strike slip faulting but the waveform data do not contain sufficient information to distinguish which of the couples of the double couple mechanism corresponds to the fault plane joint relocations of several of the aftershocks from one of these strike slip earthquakes suggests that right lateral strike slip faulting occurred on a northeasterly trending fault plane

variable rupture mode of the subduction zone along the ecuador colombia coast three large earthquakes occurred within the rupture zone of the 1906 colombia ecuador earthquake m w 8 8 in 1942 m s 7 9 1958 ms 7 8 and 1979 m s 7 7 we compared the size and mechanism of these earthquakes by using long period surface waves tsunami data and macroseismic data the 1979 event is a thrust event with a seismic moment of 2 9 10 28 dyne cm and represents subduction of the nazca plate beneath south america the rupture length and direction are 230 km and n40 e respectively examination of old seismograms indicates that the 1906 event is also a thrust event which ruptured in the northeast direction the seismic moment estimated from the tsunami data and the size of the rupture zone is 2 10 29 dyne cm the 1942 and 1958 events are much smaller about 1 5 to 1 10 of the 1979 event in the seismic moment than the 1979 event we conclude that the sum of the seismic moments of the 1942 1958 and 1979 events is only formula of that of the 1906 event despite the fact that the sequence of the 1942 1958 and 1979 events ruptured approximately the same segment as the 1906 event this difference could be explained by an asperity model in which the fault zone is held by a discrete distribution of asperities with weak zones in between the weak zone normally behaves aseismically but slips abruptly only when it is driven by failure of the asperities a small earthquake represents failure of one asperity and the rupture zone is pinned at both ends by adjacent asperities so that the effective width and the amount of slip are relatively small a great earthquake represents failure of more than one asperity and consequently involves much larger width and slip

a comparison of the spatio temporal distribution of moment release for the 1986 andreanof islands earthquake with relocated seismicity we compare relocated teleseismically recorded seismicity before and after the may 7 1986 andreanof islands earthquake mw 8 to the spatial and temporal distribution of seismic moment release during the mainshock relocated seismicity from 1964 through april 1987 clusters strongly along the main thrust zone in similar locations for the pre and post mainshock periods we performed simultaneous iterative inversions of broadband teleseismic gdsn and nars p waveforms using the method of kikuchi and fukao 1987 fault plane parameters e g strike dip depth and extent of fault plane consistent with regional tectonics and aftershock locations were specified the results of the inversions are spatial and temporal maps of moment release on the fault plane most of the moment found by the inversion is localized in several regions of the fault plane resolution of the location and timing of the major subevents is good the rupture was bilateral with a highly variable rupture velocity a shallow updip region of the fault plane ruptured toward the end of the mainshock process the mainshock moment release tended to occur in regions of the fault plane in which no or few aftershocks or preshocks are located this tendency suggests that regions of high moment release in the 1986 andreanof islands earthquake resulted from rupture of mechanically strong regions i e asperities on the fault plane which exhibited little seismicity before or after the mainshock that ruptured them we infer that the asperity regions were locked and quiescent before the mainshock while surrounding seismically active areas did not store as much strain and slipped mostly aseismically

biological overprint of the geological carbon cycle the oxidation of earth s atmosphere is coupled to the net sequestration of organic matter which is related to the relative fractions of organic carbon forg and carbonate fcarb buried in marine sediments these fractions can be inferred from carbon isotope data we present bulk sediment d 13 c records of carbonate d 13 ccarb and organic carbon d 13 corg with a compilation of evolutionary trajectories of major eucaryotic phytoplankton for the past 205 million years our analysis indicates that changes in phytoplankton community structure coupled with the opening of the atlantic ocean basin and global sea level rise increased the efficiency of organic carbon burial beginning in the early jurassic in turn this organic carbon burial increased the oxidation state of earth s surface while drawing down atmospheric co2 levels assuming no substantial negative feedbacks the net oxidation and co2 drawdown appear to be related to the opening phase of the current wilson cycle where the newly formed passive plate margins store organic matter for hundreds of millions of years this process should reverse during the closing phase of the wilson cycle when the continents reassemble and the atlantic ocean basin closes the associated oxidation and storage of organic matter have contributed to the long term depletion of co2 which was a key factor that selected c4 photosynthetic pathways in marine and terrestrial ecosystems in the latter part of the cenozoic these pathways increasingly influenced d 13 corg and ultimately contributed to the reversal of the long term trend in d 13 ccarb

short and long period subevents of the 4 february 1965 rat islands earthquake short and long period records of the p wave of the 1985 rat islands earthquake were analyzed to locate subevents within the main rupture four subevents were identified on the short period records in the first 100 sec and on the two long period records in the first 30 sec the short period subevents cluster in an area 100 km south of the initial epicenter which appears to be off of the main fault plane an area in which two larger aftershocks have relatively high stress drops the long period subevents are located 90 km west of the initial epicenter the times and locations of the first short and long period subevents indicate they were triggered by a front moving near the p wave velocity 16 references 13 figures 3 tables

directivity analysis of the deep bolivian earthquake of june 9 1994 the june 9 1994 deep bolivian earthquake m w 8 3 d 636 km occurred on a nearly horizontal rupture plane with an approximate area of 2000 km 2 the rupture plane is constrained by directivity analysis of the p wave displacement pulses recorded at local regional and teleseismic distances the fact that there are local recordings of the displacement provides a strong constraint on the vertical extent of faulting which is less than 10 km the moment release had at least four distinct episodes or subevents the best overall explanation for the subevents is rupture which initiates on the western margin of the fault plane propagates to the east and then continues bilaterally along a 350 azimuth with most of the moment release occurring to the north the apparent rupture velocity between subevents is very low approximately 1 2 km s

upper plate stressing and seismicity in the subduction earthquake cycle we investigate upper plate stressing during the earthquake cycle in a subduction segment using three dimensional 3 d elastic models to address the effects of strongly heterogeneous coupling along strike of the interplate interface we show how heterogeneity controls the locations and mechanisms of seismicity in the upper plate oblique subduction segments two from the aleutians andreanof islands 1986 and rat islands 1965 and one from indonesia biak 1996 are studied all examples of upper plate seismicity from the aleutians represent events occurring toward the beginning of a new cycle while in biak indonesia the examined events occur both toward the end of one cycle and the beginning of the next in the majority of cases studied the location and mode of the upper plate seismicity are consistent with space and time dependent stressing as predicted by modeling this confirms earlier observations that seismicity in the vicinity of large great subduction earthquakes toward the outer rise at intermediate depth and now in the upper plate depends in an interpretable manner on the stage in the earthquake cycle as well as on distribution of coupling along the interplate interface

rupture process of an extended earthquake sequence teleseismic analysis of the chilean earthquake of march 3 1985 broadband displacement and velocity records of p waves recorded at teleseismic distances are analyzed to determine the static and dynamic source parameters of the chilean earthquake of march 3 1985 ms 7 8 and seven large mb 5 6 aftershocks besides the usual parameters of depth moment and focal mechanism the analysis provides estimates of radiated energy associated stresses and source dimension for seven of the eight modeled earthquakes to insure that the parameters are describing the same physical aspect of the rupture process the parameters for each shock are computed from the time window in which most of the seismic energy is radiated the hypocenters of an additional 149 small foreshocks and aftershocks of 4 5 mb 5 6 are computed with the method of joint hypocenter determination the spatial and temporal variation of source parameters and earthquake locations are used to infer a description of the rupture process before during and after the main shock the main shock and modeled aftershocks occurred as thrust faulting on the interface between the nazca and south american plates a cluster of foreshocks in the 10 days preceding the main shock also involved thrust faulting on the plate interface the main shock itself was a complex rupture consisting of three events the first two events denoted as ms1 and ms2 released minor amounts of energy and they occurred on the periphery of the small zone defined by the foreshocks the major release of energy occurred with the third event denoted by ms which nucleated downdip of ms1 and ms2 within the time window of major energy release rupture extended approximately 90 km to the south from the point at which ms nucleated the size of the early aftershock zone which far exceeds our inferred dimensions for the major shock and the strong frequency dependence of scalar moment imply that substantial slow slip occurred on the plate interface that was not associated with major energy release the dip of the seismically active interface increases landward from about 15 at 20 km depth to about 35 at 40 km depth where the seismogenic interface reaches its maximum observed depth the vast majority of small and moderate aftershocks occurred in the shallower part of the interface as did the foreshocks ms1 and ms2 it appears however that the asperities that control the rupture of the largest earthquakes are the asperities on the deeper interface ms and the largest 1985 aftershock that of april 9 ms 7 2 occurred on the deeper interface the chilean earthquake of july 9 1971 ms 7 5 which occurred just north of the 1985 earthquake also involved main shock rupture on the deeper interface and was followed by aftershocks on the shallower interface asperities on the deeper interface appear on average to be stronger larger and more uniform in size than asperities on the shallow interface a greater average strength for deeper asperities is suggested by the observation that stress drops and apparent stresses of the main shock and the after shock of april 9 ms 7 2 are higher than those of the large shallow aftershocks and is consistent with arguments that the maximum friction on a fault zone should be near the base of the seismogenic zone one deep interface aftershock with low stress drop and apparent stress may have been situated below the point of maximum friction on the interface the inferred larger and more uniform sizes of the deeper asperities are suggested by the higher proportion of large to small shocks on the deeper interface the downdip change in the asperity size distribution may reflect increased friction on the plate interace or the coalescence of small asperities into large asperities with increased subduction the radiated energies of the main shock and large aftershocks that are computed directly from the broad bandwidth data are smaller than those predicted by standard empirical formulas by factors ranging from 4 to 20

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

iterative deconvolution of complex body waves from great earthquakes the tokachi oki earthquake of 1968 abstract a method of body wave inversion is developed in an attempt to extract the information about asperities or barriers in a fault zone a sequence of point sources each being characterized with the seismic moment the onset time and the location are iteratively derived from observed records at multi stations where the two dimensional extent of the source location is taken into account a modification is made of the iterative method of kikuchi and kanamori on the formulation of inversion procedure to facilitate the computation using this method we analyse long period p waves of the tokachi oki earthquake of 1968 mw 8 2 and obtain several significant subevents with time durations of 10 s their spatio temporal distribution shows that the rupture process consists of three characteristic stages a a stage of introductory rupture b a stage of main rupture and c a stage of aftershocks the main rupture takes place in the form of clustering around a few sites of the fault plane the largest subevent occurs in the northwestern corner the stress drop associated with this event is estimated to be 200 bars one order of magnitude higher than the stress drop averaged over the entire fault plane the sum of the seismic moments of the individual subevents amounts to 2 3 1028 dyn cm which approximately coincides with the one estimated from the analysis of long period surface waves this implies that the source of the tokachi oki earthquake consists of several major subevents with time durations of 10 s in addition to other minor subevents

application of an inhomogeneous stress patch model to complex subduction zone earthquakes a discrete interaction matrix approach in recent years it has been recognized that the level of shear and normal stress along a fault can vary thus the stress is spatially and temporally inhomogeneous moreover it has also been suspected that faults might interact in some way with the result that a variety of earthquake magnitudes might be produced along a given length of fault at varying times in order to explore these ideas we have developed a quantitative formalism which we call the interaction matrix method to express the influence of one fault upon another this matrix is calculated by use of the energy change for a system of interacting cracks or faults and therefore gives energy consistent results specifically the interaction matrix relates the area averaged stress on the fault segment to the area averaged slip state on all the other fault segments in the system since any fault can be subdivided into an arbitrary number of fault segments the interaction matrix can have arbitrary dimension in fact the continuum limit is recovered as the dimension of the matrix approaches infinity we combine this matrix method with a segmentation or patch model for earthquakes in which each discrete segment of a fault has the same coseismic stress change defined as the difference between the driving stress at which healing occurs minus the driving stress at which sliding starts each time it slips we show that slip on a patch during an earthquake can vary substantially depending on how it interacts with other nearby patches in this model it is quite possible for the spatial distribution of stress on the fault following an event to be again in a spatially inhomogeneous state rather than in a uniform state as is often assumed hence the seismic moment produced by an earthquake on a given set of patches can vary substantially depending on the sequence of sliding and healing on the different patches to apply these ideas we devised a means to calculate the interaction matrix elements and used them to quantitatively examine earthquake sequences off the colombia ecuador coast and in the nankai trough near japan

rupture process of the february 4 1965 rat islands earthquake the great rat islands underthrusting earthquake mw 8 7 of february 4 1965 represents subduction of the pacific plate beneath the north american plate along a 600 km segment of the western end of the aleutian islands body wave inversion techniques are used to determine the spatial and temporal heterogeneities associated with the rat islands earthquake we have deconvolved world wide standard seismograph network long period teleseismic p wave seismograms to obtain source time functions directivity associated with the three major pulses of moment release in the source time functions indicates a total source duration of 160 s unilateral rupture in the direction 300 fault length of 420 km and average rupture velocity of 2 5 km s the three pulses of moment release are located along the fault and these regions of high moment release are interpreted as asperities the first asperity extends from the epicenter to 100 km to the wnw this is the largest asperity and corresponds to a smooth pulse of moment release in the source time function with a duration of 50 s the second pulse of moment release is very jagged is less coherent between stations and is centered 200 km wnw of the epicenter the third pulse of moment release extends from 360 to 420 km wnw of the epicenter although the aftershock area is 600 km in length we can not resolve any moment release from the p waves beyond 420 km wnw of the epicenter the rat islands event was closely followed by a large tensional outer rise event on march 30 1965 ms 7 5 which is located oceanward of the largest moment release associated with the rat islands mainshock rupture detailed analysis of the p waves for this large outer rise event confine the depth extent to the upper 30 5 km of the crust the spatial and temporal association between the february 4 mainshock and the march 30 tensional outer rise event suggests the tensional event may have been triggered by the large displacement near the mainshock epicenter the overriding plate along the western aleutian subduction zone is laterally segmented into a series of rigid tectonic blocks separated by fault controlled canyons and extensional basins geist et al 1988 we suggest that the central undeformed parts of the blocks have the strongest coupling with the down going plate and hence are the sites of the largest moment release during an underthrusting earthquake the three asperities determined from the p waves correspond to the rat buldir and near tectonic blocks respectively hence the p wave seismic moment release of the rat islands earthquake is controlled by the lateral segmentation of the overriding plate

asperity distribution and earthquake occurrence in the southern kurile islands arc the seismicity in the southern kurile islands arc is characterized by a complex overlapping of aftershock zones of a few great and many large earthquakes ms 7 0 the spatial relationships of these aftershock zones can be explained in terms of a simple pattern of fault plane heterogeneity the four largest earthquakes in this region are from north to south the 1963 kurile islands mw 8 5 the 1958 etorofu mw 8 3 the 1969 kurile islands mw 8 2 and the 1973 nemuro oki mw 7 8 events detailed body wave studies indicate that the moment release associated with each event is concentrated in a small portion of the aftershock area with scale lengths 100 km these regions of high moment release are the largest asperities the areas that surround the large asperities slip in two modes 1 coseismically during the great events and 2 as distinct large ms 7 0 earthquakes both before and after the great events the second mode involves either rupture of relatively weak regions which results in low stress drop events or rupture of smaller asperities the earthquakes produced by failure of small asperities tend to repeatedly rupture the same regions and they have impulsive source time functions indicating rupture of highly stressed regions of the fault surface by analysis of all ms 7 0 events since 1946 in this region we find a segmentation of the southern kurile islands thrust zone into strongly coupled regions with two scale lengths dominant and secondary asperities and weakly coupled regions the surrounding areas generating the low stress drop events the largest earthquakes and the smaller asperity earthquakes occur at the down dip portion of the lithospheric interface while the low stress drop events occur much closer to the trench axis this implies a difference in the degree of mechanical coupling between the subducting and overriding plates with the deeper portions having greater strength than the shallower parts

determination of magnitude for large shallow earthquakes 1898 1917 abstract in order to arrive at a uniform basis for a study of the global seismicity the surface wave magnitude ms of shallow earthquakes for 1898 1917 is newly determined by using maximum amplitudes of surface waves recorded by undamped milne and omori seismographs the effective gain of these instruments needs to be calibrated with great care because of the lack of a damping device the calibration of milne instruments is based on a comparison of amplitudes measured by milne instruments with those by more modern damped instruments the effective gain varies from 7 to 20 in the range of ms from 6 to 8 5 the reported amplitudes for the years 1910 1912 are twice as large as the standard and need to be corrected by applying these results to a wealth of the milne data ms is determined for a large number of earthquakes between 1898 and 1912 to supplement the data for 1913 1917 records of undamped omori seismographs are used for the entire period from 1898 to 1917 308 shallow earthquakes with m s 7 0 are listed here both this list and the previous catalogue of abe 1981 for 1918 1980 provide a catalogue of large shallow earthquakes much more uniform than any previously established between 1899 and 1910 the annual frequency of shocks with m s 7 0 is 20 which is twice as large as the annual average of 10 during 1911 and 1980

stress transfer and seismic phenomena in coupled subduction zones during the earthquake cycle an elementary mechanical model of a coupled subduction zone is presented it is used to study the transfer of extensional stresses along the underthrusting plate and shear stress interactions between the plate itself and its thrust contact zone with the overriding plate during the earthquake cycle the model assumes an elastic plate which deforms one dimensionally has shear drag interactions treated approximately by the elsasser procedure with a maxwellian viscoelastic mantle and is subject to steady gravitational sinking forces great thrust earthquakes are simulated as sudden stress relieving displacements at the contact zone between the slab and overriding plate the model allows analysis of the space and time variable stresses in the main thrust zone the adjacent oceanic plate and the subducting slab over each earthquake cycle stress variations leading to outer rise events are discussed as well as stress changes responsible for intraplate seismicity downdip from the main thrust zone based on simulations of periodic great earthquakes the results of simulated slow progression of deformation slip from below into the thrust contact zone are also presented such preslip occurring over a time scale that is small in comparison with the whole seismic cycle e g of the order of a tenth or so is shown to transiently slow the rate of buildup of or even relieve extensional stresses in the slab in the absence of preslip those stresses build up steadily in time during the latter part of the cycle the results have implications for induction of sesimic quiescence as well as for explaining patterns of seismicity in relation to spatial position along the underthrusting plate and to time throughout the cycle correlations are reported between the model simulations and seismicity observations from coupled subduction zones around the world the observations include tensional outer rise events after large subduction earthquakes and compressional ones in the latter parts of the cycle normal tensional intraplate earthquakes preceding the main thrust events and positioned downdip from the thrust zones and interlaced periods of seismic activity in the lower part of the main thrust zone and in the descending slab

bathymetric effects on body waveforms from shallow subduction zone earthquakes and application to seismic processes in the kurile trench resolution of earthquake source characteristics is limited by structural complexities in the source region this study explores the effects of dipping near source bathymetry and structure on long period teleseismic body waveforms from shallow subduction zone earthquakes ray theory synthetics calculated for shallow double couple sources and velocity structures incorporating planar dipping interfaces suggest that p waveforms are most sensitive to water depth and sea bottom orientation for sources beneath a substantial water layer the seafloor dip of the inner trench slope can give rise to prominent later arrivals and waveform complexity not found on synthetics calculated for horizontal bathymetry sediment thickness and the dip of the sediment crust and crust mantle interfaces generally have lesser effects the high amplitude arrivals result largely from changes in the source takeoff angles of the reverberations these arrivals present difficulties for source depth and time function determination from p waveforms and may cause spurious complexity in the derived pattern of earthquake moment release sh waveforms show little effect of sea bottom orientation or local structure for shallowly dipping thrust faulting events suggesting that sh waveform analysis can stabilize source parameter studies with respect to unknown or incompletely parameterized bathymetry and structure effects of nonhorizontal structure are observed in the p waveforms from shallow kurile trench events ms 5 8 7 2 which show prominent later arrivals not observed on sh waveforms synthetics calculated for a dipping water crust interface provide a better fit to these arrivals than those calculated for a horizontal interface suggesting that the p waveforms show systematic effects of seafloor dip the synthetics are highly sensitive to the assumed orientation of the seafloor the data are fit best for seafloor orientations consistent with the dip of the inner trench slope the distorting effect of the water layer gives rise to systematic differences in p wave complexity between shallow events near the seismic front which occur beneath a thick water layer and show numerous later arrivals and deeper arcward events which occur beneath a thin water layer and show simple waveforms large reverberations in the p waveforms of the october 20 1963 and june 10 1975 kurile tsunami earthquakes have previously been used as evidence for a long source duration and a change in focal mechanism during rupture however sh waveforms suggest simple source time functions with durations of 25 and 43 s inversion of p and sh waveforms and amplitudes yields shallowly dipping thrust mechanisms with no conclusive evidence for a previously proposed steepening of the fault dip during rupture the large p wave reverberations cannot be fit using a point source and a horizontal seafloor detailed modeling of the 1963 event as an upward propagating rupture extending through low velocity sediments to the ocean bottom combined with the effects of the dipping seafloor allow the reverberation amplitudes to be matched the anomalous tsunami excitation appears to be related to the location of the sources at shallow depths within low velocity sediments

the origin of summit basins of the aleutian ridge implications for block rotation of an arc massif it is proposed that many summit basins along the aleutian arc form from the clockwise rotation of blocks of the arc massif summit basins are arc parallel grabens or half grabens formed within the arc massif and are commonly located near or along the axis of late cenozoic volcanism geomorphically the aleutian arc appears to consist of contiguous rhombic blocks of varying size tens to hundreds of kilometers in length the boundaries between adjacent blocks are delineated by fault controlled canyons that cut the southern slope of the arc transverse to its regional trend evidence that these blocks have rotated clockwise is provided by the triangular shaped summit basins bordering the blocks to the north oblique physiographic trends offsets in the summit platform and broad deflections in the southern slope of the arc we present a model for block rotation that involves translation of blocks parallel to an arc it is suggested that block rotation which appears to have accelerated in late cenozoic time is linked to 1 a shift in the euler pole for the pacific plate 2 the consequential start up of late cenozoic volcanism 3 improved interplate coupling instigated by sediment flooding of the aleutian trench and 4 westward subduction of northeast striking segments of the inactive kula pacific ridge

effects of fault heterogeneity on seismic energy and spectrum abstract we study the effects of friction heterogeneity on the dynamics of a seismogenic fault to this aim we consider a fault model containing two asperities with different static frictions and a rate dependent dynamic friction we consider the seismic events produced by the consecutive failure of the two asperities and study their properties as functions of the ratio between static frictions in particular we calculate the moment rate the stress evolution during fault slip the average stress drop the partitioning of energy release the seismic energy the far field waveforms and the spectrum of seismic waves these quantities depend to various extent on the friction distribution on the fault in particular the stress distribution on the fault is always strongly heterogeneous at the beginning of the seismic event seismic energy and frictional heat decrease with increasing friction heterogeneity while seismic efficiency is constant we obtain an equation relating seismic efficiency to the parameters of the friction law showing that the efficiency is maximum for smaller values of dynamic friction the seismic spectrum depends on the friction distribution as to the positions and the values of the minima however under the model assumption that the slip durations are the same for both asperities the corner frequency is independent of the friction distribution but it depends on the friction law and on the coupling between asperities the model provides a relation between the total radiated energy and the seismic moment that is consistent with the empirical relation between the two quantities the fault model with one asperity is also considered as a particular case the model is applied to the 1965 rat islands alaska earthquake and shows the role of fault heterogeneity in controlling the spatial distribution of stress drop as well as the time dependence and the final amount of radiated energy

spatial distribution of turn of the century seismicity along the alaska aleutian arc abstract epicenters determined for instrumentally observed earthquakes occurring around the turn of the century have figured prominently in estimates of seismic hazard along the alaska aleutian arc unfortunately because of poor timing control and a sparse station distribution the locations quoted in the older catalogs can be very unreliable more reliable epicenter estimates and quantitative assessments of the errors associated with these estimates are needed to address issues related to seismic hazard seventeen earthquakes with surface wave magnitudes above 6 8 that were recorded between 1898 and 1917 are relocated using a modified least squares criterion epicentral estimates are determined using p s and s p arrival times with the a priori constraint that the final location must be near the geographically defined arc although epicenters determined using a standard least squares criterion can be located up to 800 km from the arc the arc proximity constraint provides tectonically more meaningful locations which to a high degree of confidence fit the travel time observations as well as the unconstrained locations epicentral shifts of up to 1 200 km are observed between the relocations and those reported in the earlier catalogs seventy five per cent confidence bounds on the along arc location of each event vary from 30 to 400 km several large events m s 6 9 occurred within or near the rupture zone of the 1965 rat islands earthquake and may indicate that this zone failed in a sequence of events around the turn of the century of the four events previously suggested as being possible candidates for rupturing either the shumagin or unalaska seismic gaps none relocate near these arc segments the relocation of a large m s 7 9 earthquake which occurred in 1917 within the shumagin islands region however provides the first strong evidence of largescale seismic moment release within this intensely studied seismic gap

the rupture process of the great 1979 colombia earthquake evidence for the asperity model we have determined the asperity distribution in the segment of the ecuador colombia subduction zone that was ruptured by the 1979 mw 8 2 and 1958 mw 7 7 earthquakes for the 1979 earthquake we are able to document directivity in the p wave source time functions and consequently have spatially located the moment release by combining results from both p wave and surface wave directivity we have defined three regions 1 from 0 to 56 km northeast of the epicenter with low moment release and a small seismic displacement 2 the asperity from 56 to 116 km northeast of the epicenter with most of the moment release and a displacement of 6 m and 3 a small displacement region from 116 to 180 240 km northeast of the epicenter thus the 1979 rupture zone is composed of an asperity 60 km in length bounded by two weak small displacement regions the displacement at the asperity is equal to the cumulative tectonic displacement between 1979 and 1906 the year of the previous great earthquake in this segment the 1958 earthquake is characterized by the rupture of a small asperity length scale approximately 25 km in the epicentral region the 1958 event continued to rupture past this small epicentral asperity and stopped near the epicenter of the following 1979 event this detailed study of the spatial variation in moment release offers strong support for the asperity model of large earthquake occurrence

seismic potential of the queen charlotte alaska aleutian seismic zone the 5000 km long queen charlotte alaska aleutian seismic zone is subdivided into 17 unequally sized segments their boundaries are delineated based on the prior distribution of large and great earthquakes the 17 segments are chosen to represent areas likely to be ruptured by characteristic earthquakes this term usually implies repeated breakage of a plate boundary segment by either a large or great earthquake whose source dimensions remain consistent from cycle to cycle this definition does not exclude the possibility that occasionally adjacent characteristic earthquake segments may break together in a single giant event that is larger than the characteristic size outlined conversely a segment can also sometimes break in a series of smaller ruptures formal computations of the conditional probabilities for future large and great earthquakes in the 17 segments of the queen charlotte alaska aleutian seismic zone are based on the following data sets and findings 1 recurrence intervals from historic and geologic data 2 direct recurrence time estimates based on rates of relative plate motion and the size or displacement of the most recent characteristic event in each segment and 3 the application of a lognormal distribution of recurrence times for large and great earthquakes results of these computations indicate seven areas that have high i e 60 conditional probabilities for the recurrence of either large or great earthquakes within the next 20 years 1988 2008 these areas include cape st james yakataga the shumagin islands unimak island and the fox delarof and near islands segments of the aleutian arc when a shorter time interval is considered 1988 1998 those segments more likely to rupture in large ms 7 7 7 rather than great earthquakes have a high conditional probability these areas include the unimak fox and delarof islands segments the largest uncertainties in these forecasts stem from the short historic record providing a single recurrence time estimate for some segments or widely varying estimates for others from the unknown importance of aseismic slip and from a vague definition of characteristic earthquake size in fact characteristic earthquake size may not be a time invariant quantity

seismic coupling and outer rise earthquakes variation of interplate seismic coupling at subduction zones is a major factor controlling the size of the largest underthrusting events this variation also has a profound effect on the regional intraplate stresses in the vicinity of the subduction zone outer rise seismicity is strongly correlated with variations in interplate coupling reflecting the stress state of the interplate coupled zone over 200 outer rise earthquakes with known focal mechanisms are used to investigate the relationship between stresses in the outer rise and interplate seismic coupling these events occur within the downgoing i e oceanic plate near the bathymetric trench axes and generally fall into the categories of tensional normal or compressional thrust with their tensional or compressional stress axes oriented approximately horizontal and perpendicular to the trench in uncoupled subduction zones only tensional outer rise earthquakes occur which indicates that the outer rise is dominated by tensional stresses associated with plate bending and or slab pull forces in strongly coupled subduction zones both tensional and compressional outer rise events are found these events are related both spatially and temporally to the distribution of large underthrusting earthquakes and are thus an integral part of the earthquake cycle in the strongly coupled regions tensional outer rise events follow large underthrusting events as the outer rise is temporarily in tension due to the underthrusting motion compressional outer rise events take place as compressional stress slowly accumulates oceanward of locked sections of the interplate zone in four instances compressional outer rise earthquakes have been followed by large underthrusting events which have occurred 2 4 7 and 19 years after the associated outer rise event the remaining compressional outer rise events are located in regions that are either known seismic gaps or in regions where the seismic potential is unknown the occurrence of compressional outer rise earthquakes suggests that compressional stress is accumulating in the adjacent interplate region and that there is the potential for a future large underthrusting event in the region thirty compressional outer rise events have been located in trench segments of middle and south america the kurile islands the tonga and kermadec islands the new hebrides arc and the solomon islands regions in both the southern kamchatka and northern new hebrides regions the outer rise seismicity indicates that the stress regimes in the outer rise have changed with time from tensional following a previous large underthrusting event to compressional at present thus three stages of the cycle from underthrusting to tensional outer rise regime to compressional outer rise regime are present requiring only the occurrence of the next underthrusting event to complete the cycle the occurrence of compressional outer rise events is useful for assessing the seismic potential of a region on an intermediate time scale

multi trace deconvolution with unknown trace scale factors omnilinear inversion of p and s waves for source time functions seismic source and wave propagation theories allow seismologists to compute long period synthetic seismograms that commonly match even subtle details of observed waveshapes but the overall trace amplitudes must be scaled by arbitrary factors to emphasize the excellent waveshape correspondence this unexplained amplitude scatter causes considerable difficulties when formally inverting observed waveforms for seismic source parameters omnilinear inversion simultaneously determines the linear model parameters and trace scaling factors to minimize the mismatch between observed and synthetic seismograms omnilinear inversion is applied to source time function deconvolution from long period p and sh waves a simulation with synthetic data shows that omnilinear inversion finds the proper scaling factors and the source time function and focal depth are better determined than for standard linear inversion omnilinear inversion is then applied to a data set of seven p waves and one sh wave from the underthrusting earthquake of july 5 1968 ms 6 6 in northern honshu japan omnilinear inversion produces a best fit focal depth of 36 km with a single pulse time function of 6 s duration and seismic moment of 4 1018 n m mw 6 4 this focal depth supports the notion that the seismically coupled plate interface extends no deeper than 40 km in northern honshu

intermediate term seismic precursors for some coupled subduction zones the paper discusses model results and then reviews observational data concerning some aspects of the mechanics of mature seismic gaps in coupled subduction zones the concern is with space and time varying stresses as signalled by the presence and mechanisms of earthquakes in the outer rise zones adjacent to main thrust areas of large subduction events and down dip from such areas in the downgoing slab observations are shown to be consistent with the expectation that in mature seismic gaps as a result of interplate boundary locking in presence of sustained gravitational driving forces at least the deeper portions of the ocean plate in the outer rise zones are under increased compression and the downgoing slab is under increased tension the observational data cover two cases of closed seismic gaps namely the region of the chilean valparaiso earthquake of march 3 1985 and the earthquake of october 4 1983 four other cases concern still to be closed gaps in northern chile and along the coast of guatemala and also the kurile islands trench gap and the northern new hebrides gap it is concluded that the intermediate term precursor consisting of a combination of compressional outer rise earthquake s and tensional intermediate depth intra plate events in the downgoing slab which mechanically signals the latter part of the earthquake cycle could be useful in evaluating the maturity and hence great earthquake potential of a seismic gap

aftershock patterns and main shock faulting we have compared aftershock patterns following several moderate to large earthquakes with the corresponding distributions of coseismic slip obtained from previous analyses of the recorded strong ground motion and teleseismic waveforms well located aftershock hypocenters are projected onto the main shock fault plane and their positions are examined relative to the zones of coseismic displacement indicated by the estimated distributions of main shock slip we also examine the aftershock focal mechanisms when these data are available in an attempt to identify possible patterns of secondary faulting within the aftershock zone our results are consistent with a hypothesis of aftershock occurrence that requires a secondary redistribution of stress following primary failure on the earthquake fault aftershocks following the earthquakes examined in this study occur mostly outside of or near the edges of the source areas indicated by the patterns of main shock slip the spatial distribution of aftershocks reflects either a continuation of slip in the outer regions of the areas of maximum coseismic displacement or the activation of subsidiary faults within the volume surrounding the boundaries of main shock rupture

effects of fault interaction on moment stress drop and strain energy release solutions for collinear shear cracks are used to examine quantitatively the effects of fault slip zone interaction on determinations of moment stress drop and static energy release two models the barrier model and the asperity model are considered in the asperity model the actual distribution of strengths on a fault plane is idealized as a combination of two limiting cases areas which slip freely at a uniform value of a residual friction stress and unbroken ligaments or asperities across which slip occurs only at the time of a seismic event in the barrier model slip zones separated by unbroken ligaments barriers are introduced into a uniformly stressed medium to approximate the nonuniform fault propagation proposed by das and aki the strain energy change due to introducing collinear slip zones or due to breaking the asperities between them is shown to be given by the usual formula for an isolated slip zone with the stress drop replaced by the effective stress significant interaction between slip zones occurs only if the length of the asperity is less than half the length of the slip zones for the case of two collinear slip zones fracture of the asperity between them is shown to cause a large moment primarily because of the additional displacement which is induced on the adjacent slip zones for example if the asperity length is 0 05l where l is the length of each adjacent slip zone then fracture of the asperity causes a moment almost 1 8 times the moment caused by introducing a slip zone of length l for two collinear slip zones the local stress drop due to fracture of the separating asperity is shown to become unbounded as the asperity length goes to zero but in the same limit the stress drop averaged over the entire fault length is approximately equal to the apparent stress drop inferred for an isolated fault of the same moment and total fault length this apparent stress drop is approximately equal within a factor of 2 or 3 to the effective stress and hence can be used in the usual formula to give a good estimate of the strain energy change for the barrier model numerical results are given for the ratio of the stress drop calculated on the assumption of an isolated slip zone to the true stress drop for example in the case of two collinear slip zones of length l separated by a barrier of length 0 2l this ratio is 0 5 whereas for a barrier length equal to that of the adjacent slip zones the ratio is 0 24 stress drop estimates become worse with increasing number of fault segments