stress evolution in southern california and triggering of moderate small and micro size earthquakes we calculate the evolution of stresses in southern california extending the study of deng and sykes 1997 by increasing from 6 to 36 the number of earthquakes for which coseismic changes in stress are computed and by expanding from m 6 to m 1 8 the range of magnitudes m of events whose focal mechanism solutions are examined in the context of the evolving stress field the cumulative stress on a given date is calculated with respect to an arbitrary zero baseline just before the 1812 wrightwood earthquake by taking into account the long term stress loading associated with 98 fault segments and coseismic stress changes for 36 significant earthquakes our calculations indicate that more than 85 of m 5 earthquakes from 1932 1995 occurred in regions of positive change in coulomb failure function cff most of the remaining about 15 earthquakes that occurred in areas of negative cff fall very close to boundaries between positive and negative cff some of which are sensitive to the less well controlled slip distributions of the earliest historic events calculations also show that from 1981 until just before the 1992 landers earthquake more than 85 of small m 3 and micro size m 1 8 shocks in the seeber and armbruster 1995 catalog with mechanisms involving either nw trending right lateral or ne trending left lateral strike slip faulting occurred in regions of positive cff the ratio of encouraged to all small and micro size events reaches a high value of about 88 if an apparent coefficient of friction between 0 0 and 0 6 is used the highest percentage of earthquakes occurred in areas where stress is about 1 mpa above the 1812 baseline most 66 events occurred in regions of cff between 0 0 and 2 0 mpa the upper limit indicates that the approximate range of stress variation in the earthquake cycle is of the order of 2 0 mpa the fact that the locations of most moderate small and micro size earthquakes are still related to stress changes remaining from large historical events might be used to constrain slip distribution of some of those earthquakes and to constrain the locations of future significant events

determination of fault planes at coalinga california by analysis of patterns in aftershock locations the may 2 1983 coalinga california earthquake was not anticipated because it took place along a previously unrecognized fault concealed beneath an anticline in a region that had little historic seismicity the main shock hypocenter was at 10 km depth and faulting did not break the surface there has been considerable controversy about which of the nodal planes from the main shock fault plane solution was the slip plane conflicting results from geodetic geologic and aftershock data imply that slip occurred along one or the other or both nodal planes the aftershock zone is large several planar features within the aftershock zone indicate that faulting was complex to delineate the planes of slip we applied the three point method a statistical technique by which event planes can be determined from aftershock locations we obtained several important results from our study first we identified a number of planes using locations of aftershocks some of which have not been previously identified from visual inspection of the aftershock locations second the method allows us to choose slip planes by associating three point planes with fault plane solutions based on proximity and similar orientation finally three of the planes identified by the method intersect near themore main shock hypocenter making it difficult to determine which plane is the main shock slip plane nevertheless aftershocks during the first 24 hours after the main shock clearly define the high angle ne dipping nodal plane whereas few aftershocks were located along the conjugate plane assuming that the aftershocks took place along the rupture zone we conclude that slip during the main shock occurred predominantly along the high angle ne dipping plane from the fault plane solution for this event copyright american geophysical union 1989 less

seismic and aseismic deformation associated with the 1952 kern county california earthquake and relationship to the quaternary history of the white wolf fault synthesis of geodetic geologic and seismic data from the white wolf fault california indicates that the fault separates an area of late quaternary and continuing rapid uplift in the tehachapi mountains and transverse ranges from even more rapid subsidence in the southern san joaquin valley on july 21 1952 rupture of the white wolf fault produced the ml 7 2 kern county earthquake we used the aftershock zone to delimit the size of the faulted slip surface and applied constraints imposed by the known 1952 1953 horizontal shear strains to model the measured coseismic vertical displacements with an elastic dislocation model a curved fault trace with decreasing fault depth 27 to 10 km from the surface vertically to the base slip 3 to 1 m and dip 75 to 20 from the 1952 epicenter at the southwest end of the fault toward the northeast provides the fit most consistent with the geodetic record the measured seismic moment the fault plane solution and the pattern of surface rupture two short releveled lines near the 1952 epicenter tilted 4 and 17 rad down to the north from 5 10 years before the earthquake the preseismic tilts differ significantly from ten other surveys of these lines left lateral fault crossing shear strain from 0 2 20 years before the quake was two times greater than both preseismic off fault strains and the post seismic fault crossing strains during the first seven years after the earthquake aseismic deformation was negligible from 1959 to 1972 uplift reached 160 mm over an area larger than the aftershock zone rising first in the epicentral region and then at the northeast end of the fault this was unaccompanied by any surface fault slip reconstruction of the vertical separation on the white wolf fault from late quaternary and late miocene stratigraphic marker beds shows that the rate of reverse fault slip increased forty fold from 0 1 0 2 mm yr to 3 9 mm yr between the past 10 15 m y and the most recent 0 6 1 2 m y we estimate a 170 to 450 yr average recurrence interval for earthquakes on the white wolf fault with slip equivalent to that in 1952 the 1952 earthquake appears to be characteristic of the quaternary record of fault displacement in the increase in white wolf slip toward the san andreas fault the ratio of reverse to lateral slip 1 3 1 and the ratio of vertical fault slip to emergence of the hanging wall block 3 1 the 8500 m deep sedimentary basin on the down thrown block cannot be explained by repeated slip of the white wolf fault in an elastic medium

the 1930 santa monica and the 1979 malibu california earthquakes the malibu earthquake m l 5 0 that occurred 1 january 1979 in santa monica bay was located at 33 56 9 n and 118 41 3 w and at a depth of 10 km the focal mechanism of the main shock as determined from first motions recorded by short period high gain seismic stations in southern california shows reverse faulting with one plane dipping 55 to 60 to the north striking 275 to 290 and the second plane dipping 30 to 38 to the south and striking 100 to 120 more than 400 aftershocks were recorded during 1979 focal mechanisms for 20 aftershocks of m 3 0 show similar reverse mechanisms as the main shock the main shock and aftershocks m 2 5 were relocated using a local velocity model and a set of corresponding station delays the aftershocks define a volumetric hypocentral zone 4 km 6 km west trending with depths ranging from 6 to 11 km this 1979 malibu main shock aftershock sequence probably occurred on the eastern end of the anacapa dume fault the santa monica earthquake m l 5 2 that occurred 31 august 1930 was relocated using station delays calculated from the 1979 date set the relocated 1930 epicenter is located near the western end of the santa monica fault the 1979 and 1930 main shocks can be interpreted to define a 8 to 10 km north south sidestep between the anacapa dume and santa monica faults possible segmentation of these two fault systems suggests that they are unlikely to rupture simultaneously in one large earthquake

source complexity of the october 1 1987 whittier narrows earthquake long and short period body waveform data recorded at regional and teleseismic distances are modeled to obtain the source parameters of the whittier narrows earthquake of october 1 1987 although the long period data can be satisfied by a simple thrust earthquake on a plane striking 280 at a depth of 14 km we require a double source solution to satisfy both the long and short period teleseismic data the first subevent has a focal mechanism of strike 250 dip 30 n and a slip vector 90 the second is 280 40 n and 98 the second event occurred 1 0 s after the first was 5 times larger 1 2 1025 dyn cm for the second 0 23 1025 for the first in terms of seismic moment and was also shallower 12 km whereas the first subevent was at 15 km the two subevents probably occurred on a north dipping listric fault but may have occurred on two subparallel faults these features were determined by studying a variety of phases including teleseismic p and sh and regional pnl the whittier narrows earthquake was a high stress drop event but a single value for stress drop cannot be obtained because of the complex nature of the source if we define the fault area as the aftershock area we obtain a stress drop of about 750 bars

seismicity and geometry of a 110 km long blind thrust fault 1 the 1985 kettleman hills california earthquake the august 4 1985 kettleman hills earthquake was the third in a sequence of moderate shocks to occur beneath the northern half of a 110 km long fold chain bounding the eastern california coast ranges the 1982 mw 5 4 new idria 1983 mw 6 5 coalinga and 1985 mw 6 1 kettleman hills events define a southward progression of seismic activity beneath the fold we use teleseismic waveforms geodetic modeling hypocenters relocated in a three dimensional velocity model and subsurface structural data to investigate the kettleman hills earthquake the main shock results from motion on a shallowly dipping thrust fault buried at 10 km depth aftershocks and coseismic fault slip extend 20 km along the fold axis nearly the full extent of the kettleman hills north dome anticline aftershocks occur primarily several kilometers in front of the fault tip and in the core of the anticline the main shock and several foreshocks occurred at a 2 km right step in the quaternary fold axis which also corresponds to the southern end of the 1983 coalinga and northern end of the 1985 kettleman hills aftershock zones from this we infer that the step in the fold is caused by an offset or tear in the underlying fault the scalar seismic moment is 1 6 1018 n m consistent with the geodetic deformation and the duration of rupture is 16 s 3 4 times greater than for the average earthquake with this scalar moment the slow rate of moment release provides an explanation for the low level of ground shaking and low local magnitude reported for the event the peak of the geodetic uplift is located 5 km perpendicular to the quaternary fold axis we argue that the fault is propagating northeast into the undeformed san joaquin valley sediments and that the overlying fold is growing at about 0 5 mm yr

changes in static stress on southern california faults after the 1992 landers earthquake the magnitude 7 5 landers earthquake of 28 june 1992 was the largest earthquake to strike california in 40 years the slip that occurs in such an earthquake would be expected to induce large changes in the static stress on neighbouring faults these changes in stress should in turn affect the likelihood of future earthquakes stress changes that load faults towards failure have been cited as the cause of small1 5 moderate6 and large7 earthquakes conversely those that relax neighbouring faults have been related to a decrease in seismicity5 here we use an elastic half space model8 to estimate the stress changes produced by the landers earthquake on selected southern california faults including the san andreas we find that the estimated stress changes are consistent with the triggering of four out of the five aftershocks with magnitude greater than 4 5 and that the largest changes 1 10 bar occurring on part of the san bernardino segment of the san andreas fault may have decreased the time to the next magnitude 8 earthquake by about 14 years

a source extent analysis of the imperial valley earthquake of october 15 1979 and the victoria earthquake of june 9 1980 a new method for estimating source extent parameters has been applied to the imperial valley earthquake of october 15 1979 and the victoria earthquake of june 9 1980 from the stations of the world wide standard seismograph network and canadian seismic network estimates of the duration of far field long period sh waves were made by computing the variance or second central moment 2 of instrument corrected attenuation corrected pulses seventeen measurements for the imperial valley event and 25 for the victoria event were made we inverted for the source duration t the fault length l and the directivity parameter d by least squares we find that for the imperial valley t 9 0 0 5 s d 130 43 km s and l 46 13 km and for the victoria event t 10 9 0 6 s and d 251 71 km s l is not well constrained for this second event the inversion gives 23 km the lower bound obtained by constraining 2 to be positive everywhere on the focal sphere although a value as high as 40 km would still be within the uncertainties the positive values of d for both events indicate that rupture propagation was predominantly to the northwest in order to estimate the degree of unilateralness we have computed the rupture mode index defined by r d lt which is 1 for a unilateral rupture and zero for a bilateral rupture we find a value of r 0 31 0 14 for the imperial valley event significantly less than unity which indicates that a southern component of rupture is also present assuming uniform slip we estimate the northern and southern fault segments to be 26 and 20 km respectively the presence of a bilateral component is supported by near field studies which have made use of stations along the southern half of the imperial fault for the victoria event due primarily to the uncertainty in l the value of r can vary from 0 6 to 1 with the value of 1 being most probable assuming that the rupture is unilateral we can obtain an estimate of fault length from the relation l d t which yields l 23 6 km geodetic data and the distribution of aftershocks are consistent with a unilateral rupture mode and the fault length that we have obtained values of seismic moment from measurements of the zeroth moment of the displacement pulse are found to be 5 8 0 4 for imperial valley and 4 8 0 4 for victoria in units of 1018 n m the presence of a southern component of rupture for the imperial valley event would suggest a closer and perhaps causal relationship to the victoria event which occurred 8 months later and about 60 km to the south in a tectonic framework we interpret these two earthquakes as the failure of adjacent transform faults and consider the possibility that a concurrent spreading event may have occurred in the vicinity of cerro prieto

how similar were the 1934 and 1966 parkfield earthquakes bakun and mcevilly 1984 showed that surface waves from the 1934 and 1966 parkfield earthquakes are remarkably similar this observation together with other seismic and geologic data led to the forecast of a repeat of a characteristic parkfield earthquake by 1993 in this paper we model near field geodetic measurements collected before and after the 1934 and 1966 parkfield earthquakes the slip in 1934 on the northwestern half of the rupture is poorly constrained due to the limited station coverage in this part of the rupture near the hypocenter the geodetic measurements permit the two earthquakes to have the same slip distribution on the other hand the data clearly show that the 1966 earthquake ruptured through the right step in the surface trace of the fault while the 1934 earthquake stopped northwest of the step if we assume only that the slip distribution is to some degree smooth we find that the centroid of the 1934 slip distribution could be as much as 10 km northwest of the centroid of the 1966 slip the data are consistent with but do not demand the largest slip in 1934 occurring in a region of low slip in 1966 the geodetically determined moments of the two earthquakes from smooth inversions are indistinguishable m0 4 4 0 4 1018 n m we also examine inversions in which we assume that the two earthquakes start identically and estimate how far down the fault the two earthquakes could have remained the same we find that the 1934 event could have had the same slip as in 1966 up to the fault step the 1934 earthquake must have stopped at the step whereas the 1966 event slipped southeast of the step in this quasi characteristic model m0 of the 1934 quake is only 60 of that in 1966 the difference presumably occurring as afterslip in 1966 if the 1934 geodetic moment was indeed less than in 1966 and the loading rate was constant this observation could explain the early occurrence of the 1934 earthquake

moment tensor solutions for the 24 november 1987 superstition hills california earthquakes the teleseismic long period waveforms recorded by the global digital seismograph network from the two largest superstition hills earthquakes are inverted using an algorithm based on optimal filter theory these solutions differ slightly from those published in the preliminary determination of epicenters monthly listing because a somewhat different improved data set was used in the inversions and a time dependent moment tensor algorithm was used to investigate the complexity of the main shock the foreshock origin time 01 54 14 5 m b 5 7 m s 6 2 had a scalar moment of 2 3 1025 dyne cm a depth of 8 km and a mechanism of strike 217 dip 79 rake 4 the main shock origin time 13 15 56 4 m b 6 0 m s 6 6 was a complex event consisting of at least two subevents with a combined scalar moment of 1 0 1026 dyne cm a depth of 10 km and a mechanism of strike 303 dip 89 rake 180

comparison of seismic waveform inversion results for the rupture history of a finite fault application to the 1986 north palm springs california earthquake the july 8 1986 north palm springs earthquake is used as a basis for comparison of several different approaches to the solution for the rupture history of a finite fault the inversion of different waveform data is considered both teleseismic p waveforms and local strong ground motion records linear parametrizations for slip amplitude are compared with nonlinear parametrizations for both slip amplitude and rupture time inversions using both synthetic and empirical green s functions are considered in general accurate green s functions are more readily calculable for the teleseismic problem where simple ray theory and flat layered velocity structures are usually sufficient however uncertainties in the variation in t with frequency most limit the resolution of teleseismic inversions a set of empirical green s functions that are well recorded at teleseismic distances could avoid the uncertainties in attenuation in the inversion of strong motion data the accurate calculation of propagation path effects other than attenuation effects is the limiting factor in the resolution of source parameters the assumption of a laterally homogeneous velocity structure is usually not a good one and the use of empirical green s functions is desirable considering the parametrization of the problem any degree of fault rupture complexity can be described in terms of a linear parametrization for slip amplitudes however a nonlinear parametrization for rupture times and slip amplitudes can have a distinct advantage over a simple linear one by limiting the number of unknown parameters regardless of the choice of data or the type of parametrization the model or solution will be affected by the choice of minimization norm and the type of stabilization used

influence of static stress changes on earthquake locations in southern california earthquakes induce changes in static stress on neighbouring faults that may delay hasten or even trigger subsequent earthquakes1 10 the length of time over which such effects persist has a bearing on the potential contribution of stress analyses to earthquake hazard assessment but is presently unknown here we use an elastic half space model11 to estimate the static stress changes generated by damaging magnitude m 5 earthquakes in southern california over the past 26 years and to investigate the influence of these changes on subsequent earthquake activity we find that in the 1 5 year period following a m 5 earthquake any subsequent nearby m 5 earthquake almost always ruptures a fault that is loaded towards failure by the first earthquake after this period damaging earthquakes are equally likely to rupture loaded and relaxed faults our results suggest that there is a short period of time following a damaging earthquake in southern california in which simple coulomb failure stress models could be used to identify regions of increased seismic hazard

dislocation model for aseismic crustal deformation at hollister california geodetic observations at many active plate margins reveal relatively steady aseismic motion during the time between major earthquakes we model the aseismic motion by the relative motion between several blocks that suffer frictional resistance in the upper crust frictional drag is represented by uniform dislocation on a fault surface we assume that friction occurs only on the upper brittle zone of each block boundary and that frictionless sliding occurs below this zone using a bayesian inversion procedure we invert geodetic data to determine the parameters of the block and dislocation model parameters to be estimated include the two horizontal components of velocity for each block and the dislocation rate dip angle slip angle and width of each rectangular fault patch for hollister we assume five blocks and nine rectangular fault patches rates of length change on 92 lines observed by trilateration make up the data triangulation leveling very long baseline interferometry satellite ranging and global positioning system data can also be easily included in our model our inversion procedure uses prior estimates of all block and fault parameters based on geological and seismological data the block parameters are estimated from geological observations of fault displacement rates while the widths of the fault patches are estimated from earthquake hypocentral depths we choose prior estimates of the dislocation rate to match observed creep rates the estimated block motion is well resolved by the geodetic data and agrees well with the geological estimates this suggests that the block motion is steady on time scales from 10 years to 1 m y the net motion across the san andreas calaveras fault system is 38 3 mm yr oriented s38 e thus some additional displacement possibly offshore is needed to match the rate of 56 mm yr predicted by plate tectonic models the depth of the transition from frictional to free sliding varies considerably from 0 4 km on the southern calaveras to 11 km on the central san andreas fault two segments of the san andreas north of 36 50 and south of 36 40 have the greatest potential for moderate to large earthquakes

images of buried graves in bayat afyon turkey from high resolution magnetic data and their comparison with preliminary excavations dedemezari necropolis the site of our study is located in the west of turkey near afyon archaeologists think that it belongs to the middle bronze age a high resolution magnetic survey identified buried material highly accurately in particular anomalies which are transformed by the analytic signal method clarify the observed magnetic anomalies magnetic surveying was applied in three different areas in this study the map of analytic signal transformed anomalies presented good results on preliminary excavation in trenches a and b two types of graves are found in the study area one is made of clay and the second is carved in the rocks shapes of the buried graves are mainly cylindrical and elliptical most of the magnetic anomalies are elliptical and reflect the shape of the buried materials future excavations in dedemezari necropolis will be able to be carried out more easily due to the known locations of the buried graves

initial investigation of the landers california earthquake of 28 june 1992 using terrascope the 1992 landers earthquake m s 7 5 m w 7 3 was recorded at six terrascope stations in southern california peak accelerations ranged from 0 16 g at svd 63 km to 0 0092 g at isa 245 km decreasing with distance away from the fault zone the peak velocity showed a different pattern reflecting the rupture directivity from south to north the largest peak velocity 19 cm sec was observed at gsc 125 km moment tensor inversion of long period surface waves yielded a mechanism with m 0 1 1 10 27 dyne cm m w 7 3 dip 74 rake 176 and strike 340 inversion of teleseismic p and s waves revealed two distinct sub events of 6 and 8 sec duration and about 10 sec apart the source parameters for the first and second events are m 0 1 9 10 26 dyne cm dip 83 rake 179 strike 359 and m 0 6 1 10 26 dyne cm dip 87 rake 178 strike 333 respectively the radiated wave energy e s was estimated as 4 3 10 23 ergs the ratio e s m 0 3 9 10 4 corresponds to a stress drop of 280 bars and suggests that the landers earthquake belongs to the group of high stress drop earthquakes and occurred on a fault with a long recurrence time the rupture directivity can be seen clearly in the records from pfo 68 km located to the south and gsc located to the north of the epicenter the maximum displacement at pfo is only 13 of that at gsc despite the shorter epicentral distance to pfo than to gsc the slip distribution determined with the empirical green s function method indicates that the landers earthquake consists of two distinct sub events about 30 km apart with the second sub event to the north being about twice as large as the first one this slip distribution is consistent with the teleseismic data and the surface offsets mapped in the field

the san andreas fault system through the transverse ranges as illuminated by earthquakes the pattern of seismogenic faulting in the san gorgonio pass san bernardino basin area of the san andreas fault saf zone is mapped from about 7000 quality selected focal mechanisms 1981 nov 92 they are derived from phase data of the southern california network by a relocation procedure based on location dependent station corrections and by a grid search procedure about 3 4 of the mechanisms have been interpreted as planes of rupture or slip planes that delineate many distinct faults one of these faults is right lateral dips steeply northeast and is continuous trough the san gorgonio pass area we interpret it as the main branch of the saf a very large earthquake seems possible on this portion of the saf because of its continuity and because of the unusually deep reach of the seismicity 23 km southwest of this fault a volume of diffuse and persistent seismicity has a sharp downward cut off which may reflect a basal detachment dipping 20 northeast and intersecting the saf along the deepest seismicity northeast of the saf the floor of the seismicity is shallower by as much as 10 km stress characteristics are derived from slip planes in 10 selected subregions the regime is transpressional near the constricting bend of the saf at san gorgonio pass it is dominated by horizontal extension in the san bernardino basin area and it is again transpressional in the eastern san gorgonio mountains north of the cucamonga thrust the broad features of this zonation may be related to the intersection of the saf with the pinto mountain fault and with the san jacinto fault a change of fault kinematics in the yucaipa cluster coincides with the 1992 landers and big bear main shocks and may be a manifestation of static stress change

source mechanisms of three moderate california earthquakes of july 1986 abstract we studied the source processes of three moderate size earthquakes in california the m s 6 0 event of july 8 1986 near palm springs the m s 5 8 shock of july 13 1986 offshore of oceanside and the m s 6 2 event of july 21 1986 in the chalfant valley on the east side of the sierra nevada the centroidal source parameters are estimated by inversion of long period p and sh waves recorded at teleseismic distances by seismographs of worldwide networks a more detailed description of the source is then obtained by analyzing broadband seismograms the north palm springs earthquake had a simple rupture process most of the seismic moment was released in the first 3 sec in a single step the strike of the source fault plane follows the average trend of the banning fault and its dip agrees with the dip of the fault plane delineated by the aftershocks the faulting mechanism of the main shock is right lateral strike slip with a sizable reverse component that is consistent with uplift north of the banning fault the earthquake offshore of oceanside had a slightly more complex rupture process the far field source time function consists of two pulses that represent the release of most of the moment in the first 3 sec the source mechanism indicates reverse faulting with the compressional axis trending northeast almost perpendicular to the trend of mapped faults offshore the event does not appear to be associated with major mapped faults in the area but the strike of one of the nodal planes agrees with the trend of the major mapped faults offshore and its mechanism is consistent with the missing component of plate convergence not taken up by the onshore strike slip fault systems and the idea of a weak san andreas fault system the rupture process of the chalfant valley earthquake was yet more complicated the source time function indicates that in a 4 sec time window three asperities were broken the source mechanism indicates right lateral strike slip faulting with a small normal faulting component on a fault that dips approximately 50 southwest earthquake activity suggests that presently the deformation in the northern part of the owens valley which includes the chalfant valley is dominated by conjugate strike slip faults which strike obliquely to the main range bounding normal faults the waveforms indicate the presence of a low velocity zone at a depth of about 20 km in the crust in the chalfant valley a synthesis of information from waveform modeling first motion fault plane solutions and aftershock distributions indicates that the nucleation and or termination of rupture for the north palm springs and the chalfant valley events were controlled by preexisting fault geometry

recurrence models and parkfield california earthquakes main shocks of the earthquake sequences that occurred on the parkfield section of the san andreas fault in central california in 1922 1934 and 1966 are characterized by southeast rupture expansion over the same 20 to 30 km long section of the fault whereas the seismic moments for the 1922 and 1934 events are identical to within a precision of 10 the seismic moment for 1966 is 20 greater than for the earlier events to within a precision of 20 the parkfield area seismicity in general seems well described by a recurring moderate size characteristic earthquake repeating the same epicenter magnitude seismic moment rupture area and southeast direction of rupture expansion an unexplained 10 year advance of the 1934 event is the only discrepancy in the hypothesis that the parkfield earthquakes in 1857 1881 1901 1922 1934 and 1966 represent a strictly periodic process assuming the strictly periodic model and the absence since 1966 of the perturbations hypothesized for the 1922 to 1934 period the next characteristic parkfield earthquake should occur between 1983 and 1993

centroid moment tensor solutions for 35 earthquakes in western north america 1977 1983 abstract seismic records from the gdsn sro asro dwwssn rstn and ida networks were used to obtain long period t 45 sec focal mechanisms for 35 earthquakes in western north america longitude 140 to 110 latitude 30 to 60 employing the centroid moment tensor method of dziewonski et al 1981 and dziewonski and woodhouse 1983 the same algorithm was applied to all events the smallest m 0 6 4 10 23 dyne cm being an aftershock to the 1979 imperial valley california earthquake and the largest m 0 8 5 10 26 dyne cm the 1980 eureka california earthquake all events were analyzed with and without the constraint m r m r 0 on the moment tensor an ad hoc criterion on the relative variance reduction was employed to determine a favored solution our solutions show general agreement with other published results as well as with tectonic structures agreement with results obtained using the abundant local and regional data confirms the adequacy of the method in determining long period source parameters and verifies its applicability in regions where local and regional data are scarce or nonexistent in some cases disagreement between our solution and results from first motion studies appears to be related to source complexities our moment tensor represents the total stress release of the composite event a consistent westward movement of the centroid locations reflects lateral heterogeneities in the crust and mantle

strain accumulation along the laguna salada fault baja california mexico strain accumulation observed over the 1978 1991 interval in a 30 100 km aperture trilateration network spanning the laguna salada fault is described by the principal strain rates 0 101 0 012 strain yr n80 e 2 and 0 021 0 012 strain yr n10 w 2 extension reckoned positive these strain accumulation rates have been corrected to remove coseismic effects of the nearby 1979 imperial valley m 6 5 1980 victoria baja california m 6 4 1987 superstition hills m 6 5 and 1987 elmore ranch m 5 9 earthquakes the observed strain rates indicate extension at a rate of about 0 08 strain yr perpendicular to the trend n35 w of the salton trough as well as a right lateral tensor shear strain rate 0 05 strain yr across it the extension perpendicular to the trough is observed neither farther north near the salton sea nor farther south across the gulf of california however holocene slip on the laguna salada fault about equal parts right lateral and normal slip is consistent with the observed strain accumulation a simple dislocation model intended to explain the observed strain accumulation as a product of slip at depth on the laguna salada fault would require that the fault be listric

late quaternary activity of the laguna salada fault in northern baja california mexico faulted alluvial fans and bajadas along the central laguna salada fault zone in northernbajacaliforniarecordarecurrent history of oblique slip holocene earthquakes alluvialsurfaces whichrangefrom late pleistocene to historic in age have been progressively displaced along the base of the crystalline rangefront as well as along more basinward fault splays in alluvium the recurrence interval determined from displacedalluvialdepositsthataredatedby soil profile development is in the range of 1 2 ka with a corresponding right lateral slip rate of 2 3 mm yr similar to that of thesouthernelsinorefaultinsoutherncalifornia the most recent event along the fault zone is probably the widely felt earthquake of february 23 1892 at least 22 km ofthefaultzonerupturedduringthisevent along both an oblique dextral section of the northwest strikinglagunasaladafaultand

complex faulting deduced from broadband modeling of the 28 february 1990 upland earthquake ml 5 2 the 1990 upland earthquake was one of the first sizable local events to be recorded broadband at pasadena where the green s functions appropriate for the path are known from a previous study the synthetics developed in modeling the 1988 upland sequence were available for use in rapid assessment of the activity first motion studies from the caltech usgs array data gave two solutions for the 1990 main shock based on the choice of regional velocity models although these focal mechanisms differ by less than 5 in strike and 20 rake it proved possible to further constrain the solution using these derived green s functions and a three component waveform inversion scheme we obtain from long period waves a fault plane solution of 216 77 5 0 m 0 2 5 10 24 dyne cm depth 6 km and a source duration of 1 2 sec for which the orientation and source depth are in good agreement with the first motion results of hauksson and jones 1991 comparisons of the broadband displacement records with the high pass wood anderson simulations suggests the 1990 earthquake was a complicated event with a strong asperity at depth double point source models indicate that about 30 per cent of the moment was released from a 9 km deep asperity following the initial source by 0 0 to 0 5 sec our best fitting distributed fault model indicates that the timing of our point source results is feasible assuming a reasonable rupture velocity the rupture initiated at a depth of about 6 km and propagated downward on a 3 5 by 3 5 km length by width fault both the inversion of long period waves and the distributed fault modeling indicate that the main shock did not rupture the entire depth extent of the fault defined by the aftershock zone a relatively small asperity about 1 0 km 2 with a greater than 1 kbar stress drop controls the short period wood anderson waveforms this asperity appears to be located in a region where seismicity shows a bend in the fault plane

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

state of stress from focal mechanisms before and after the 1992 landers earthquake sequence the state of stress in the eastern california shear zone ecsz changed significantly because of the occurrence of the 1992 m w 6 1 joshua tree and the m w 7 3 landers earthquakes to quantify this change focal mechanisms from the 1975 galway lake sequence the 1979 homestead valley sequence background seismicity from 1981 to 1991 and the 1992 landers sequence are inverted for the state of stress in all cases the intermediate principal stress axis s2 remained vertical and changes in the state of stress consisted of variations in the trend of maximum and minimum principal stress axes s 1 and s 3 and small variations in the value of the relative stress magnitudes in general the stress state in the ecsz has s 1 trending east of north and 0 43 to 0 65 suggesting that the ecsz is a moderate stress refractor and the style of faulting is transtensional south of the pinto mountain fault in the region of the 1992 joshua tree earthquake the stress state determined from the 1981 to 1991 background seismicity changed on 23 april and again on 28 june 1992 in the central zone s 1 rotated from n14 5 e to n28 5 e on 23 april and back again to n16 5 e on 28 june thus the landers mainshock in effect recharged some of the shear stress in the region of the m w 6 1 joshua tree earthquake comparison of the state of stress before and after 28 june 1992 along the landers mainshock rupture zone showed that the mainshock changed the stress orientation the s1 trend rotated 7 to 20 clockwise and became progressively more fault normal from south to north along the emerson camp rock faults the variation was so prominent that the focal mechanisms of aftershocks could not be fit by a single deviatoric stress tensor the complex distribution of p and t axes suggests that most of the uniform component of the applied shear stress along the northern part of the rupture zone was released in the mainshock the san bernardino mountains region of the m w 6 2 big bear earthquake has a distinctively different state of stress as compared to the landers region with s 1 trending n3 5 w this region did not show any significant change in the state of stress following the 1992 m w 6 2 big bear sequence

depth of seismicity in the imperial valley region 1977 1983 and its relationship to heat flow crustal structure and the october 15 1979 earthquake focal depths from over 1000 earthquakes occuring between 1977 and 1983 in the imperial valley southern peninsular ranges are used to study relationships between the depth of seismicity heat flow and crustal structure this study used relocated a and b quality events from the california institute of technology catalog that were carefully selected to insure focal depth precision of 2 km regional variations in focal depth appear to be related to regional heat flow variation whereas local variations in focal depth especially in the central imperial valley may be related to crustal structure these variations are studied by rheologic modeling a comparison of focal depths of earthquakes occurring before and after the october 15 1979 m 6 6 earthquake indicates that aftershocks during the first 2 months of the sequence were 2 3 km deeper than earthquakes occurring in other time periods the deepest earthquakes in imperial valley are spatially associated with a subbasement dome near the northern end of the imperial fault this dome coincides with the region where the imperial fault undergoes a transition from stick slip behavior to aseismic fault creep models of slip during the 1979 mainshock are also compared with premainshock and postmainshock seismicity a relocation of the 1940 m 7 1 mainshock suggests that this sequence began by rupturing the same portion of the fault that experienced maximum slip during the 1979 mainshock

source parameters of the 1933 long beach earthquake regional seismographic network and teleseismic data for the 1933 m l 6 3 long beach earthquake sequence have been analyzed both the teleseismic focal mechanism of the main shock and the distribution of the aftershocks are consistent with the event having occurred on the newport inglewood fault the focal mechanism had a strike of 315 dip of 80 to the northeast and rake of 170 relocation of the foreshock main shock aftershock sequence using modern events as fixed reference events shows that the rupture initiated near the huntington beach newport beach city boundary and extended unilaterally to the northwest to a distance of 13 to 16 km the centroidal depth was 10 2 km the total source duration was 5 sec and the seismic moment was 5 10 25 dyne cm which corresponds to an energy magnitude of m w 6 4 the source radius is estimated to have been 6 6 to 7 9 km which corresponds to a brune stress drop of 44 to 76 bars both the spatial distribution of aftershocks and inversion for the source time function suggest that the earthquake may have consisted of at least two subevents when the slip estimate from the seismic moment of 85 to 120 cm is compared with the long term geological slip rate of 0 1 to 1 0 mm yr along the newport inglewood fault the 1933 earthquake has a repeat time on the order of a few thousand years

seismicity and fault interaction southern san jacinto fault zone and adjacent faults southern california implications for seismic hazard the southern san jacinto fault zone is characterized by high seismicity and a complex fault pattern that offers an excellent setting for investigating interactions between distinct faults this fault zone is roughly outlined by two subparallel master fault strands the coyote creek and clark san felipe hills faults that are located 2 to 10 km apart and are intersected by a series of secondary cross faults seismicity is intense on both master faults and secondary cross faults in the southern san jacinto fault zone the seismicity on the two master strands occurs primarily below 10 km the upper 10 km of the master faults are now mostly quiescent and appear to rupture mainly or solely in large earthquakes our results also indicate that a considerable portion of recent background activity near the april 9 1968 borrego mountain rupture zone m l 6 4 is located on secondary faults outside the fault zone we name and describe the palm wash fault a very active secondary structure located about 25 km northeast of borrego mountain that is oriented subparallel to the san jacinto fault system dips approximately 70 to the northeast and accommodates right lateral shear motion the vallecito mountain cluster is another secondary feature delineated by the recent seismicity and is characterized by swarming activity prior to nearby large events on the master strand the 1968 borrego mountain and the april 28 1969 coyote mountain m l 5 8 events are examples of earthquakes with aftershocks and subevents on these secondary and master faults mechanisms from those earthquakes and recent seismic data for the period 1981 to 1986 are not simply restricted to strike slip motion dipslip motion is also indicated teleseismic body waves long period p and sh of the 1968 and 1969 earthquakes were inverted simultaneously for source mechanism seismic moment rupture history and centroid depth the complicated waveforms of the 1968 event m o 1 2 10 19 nm are interpreted in terms of two subevents the first caused by right lateral strike slip motion in the mainshock along the coyote creek fault and the second by a rupture located about 25 km away from the master fault our waveform inversion of the 1969 event indicates that strike slip motion predominated releasing a seismic moment of 2 5 10 17 nm nevertheless the right lateral nodal plane of the focal mechanism is significantly misoriented 20 with respect to the master fault and hence the event is not likely to be associated with a rupture on that fault from this and other examples in southern california we conclude that cross faults may contribute significantly to seismic hazard and that interaction between faults has important implications for earthquake prediction

in the shadow of 1857 the effect of the great ft tejon earthquake on subsequent earthquakes in southern california the great 1857 fort tejon earthquake is the largest earthquake to have hit southern california during the historic period we investigated if seismicity patterns following 1857 could be due to static stress changes generated by the 1857 earthquake when post 1857 earthquakes with unknown focal mechanisms were assigned strike slip mechanisms with strike and rake determined by the nearest active fault 13 of the 13 southern california m 5 5 earthquakes between 1857 and 1907 were encouraged by the 1857 rupture when post 1857 earthquakes in the transverse ranges with unknown focal mechanisms were assigned reverse mechanisms and all other events were assumed strike slip 11 of the 13 earthquakes were encouraged by the 1857 earthquake these results show significant correlations between static stress changes and seismicity patterns the correlation disappears around 1907 suggesting that tectonic loading began to overwhelm the effect of the 1857 earthquake early in the 20th century

the 1971 san fernando earthquake a double event evidence is presented which suggest that the 1971 san fernando earthquake may have been a double event that occurred on two separate subparallel thrust faults it is postulated that the initial event took place at depth on the sierra madre fault zone which runs along the base of the san gabriel mountains rupture is postulated to have occurred from a depth of about 15 km to a depth of about 3 km a second event is thought to have initiated about 4 sec later on another steeply dipping thrust fault which is located about 4 km south of the sierra madre fault zone the surface trace of this fault coincides with the san fernando fault zone which was the principal fault associated with surface rupture it is postulated that rupture propagated from a depth of 8 km to the free surface the moments of the first and second events are approximately 0 7 10 26 dyne cm and 1 0 10 26 dyne cm respectively this model is found to explain the combined data sets of strong ground motions teleseismic p and s waveforms and static offsets better than previous models which consist of either a single fault plane or a plane having a dip angle which shallows with decreasing depth nevertheless many features of the observed motions remain unexplained and considerable uncertainty still exists regarding the faulting history of the san fernando earthquake

the july 1986 oceanside ml 5 3 earthquake sequence in the continental borderland southern california an earthquake of m l 5 3 occurred at 32 58 7 n 117 51 5 w southwest of oceanside in san diego county at 13 47 13 july 1986 ut this main shock was followed by an extensive aftershock sequence with 55 events of m l 3 0 during july 1986 the epicenters of the main shock and aftershocks are located at the northern end of the san diego trough bahia soledad fault zone sdt bs where it changes strike from northwest to a more westerly direction through a left offset or a bend in the fault the northwest striking sdt bs is one of three strike slip fault systems that constitute the offshore agua blanca fault system the spatial distribution of the aftershocks indicates a unilateral 7 to 9 km long rupture to the east southeast away from the epicenter of the main shock the focal mechanism of the main shock also has an east southeast striking and south dipping plane with mostly reverse movement on it focal mechanisms of the m l 3 0 aftershocks show both reverse and strike slip movement the reverse focal mechanisms indicate that this sequence may have occurred on a thrust fault that provides for a left stepping offset or a bend in the san diego trough fault as movement is transferred to the west along the santa cruz catalina island escarpment some of the aftershocks that are located to the southeast of the main shock and have strike slip focal mechanisms suggest activation of the northwest trending san diego trough fault a stress inversion of the focal mechanism data shows that the maximum principal stress determined from the focal mechanisms of the main shock and 22 aftershocks that occurred within 36 hours of the main shock has an azimuth of s30 w plunging 18 the maximum principal stress determined from 30 aftershocks that occurred from 15 july to 2 october 1986 has an azimuth of s20 w plunging 18 the values the measure of the relative sizes of the principal stresses are approximately 0 07 and 0 1 respectively indicating that the intermediate and minimum principal stress are of similar magnitude the results of the stress inversion and the focal mechanisms that showed reverse faulting suggest that the inner continental borderland offshore from oceanside is not currently a pure strike slip tectonic regime but rather a strike slip mixed with reverse faulting regime when the 52 aftershock focal mechanisms are divided into four groups and the stress inversion is repeated the change in stress can be described as a progressive counter clockwise rotation of 14 of the orientation of the maximum principal stress to a more southerly direction with the greatest change in stress orientation observed shortly after the main shock the abundance of aftershocks may be related to the large temporal variation in stress orientation that in turn may have resulted from the small stress drop of the main shock

aftershock sequence of the 3 december 1988 pasadena earthquake the pasadena earthquake m l 4 9 of 3 december 1988 occurred at a depth of 16 km probably on the santa monica raymond fault which is recognized as one of the most important faults in the los angeles basin for its potential seismic hazard prior to this event no earthquake larger than magnitude 4 had been recorded since 1930 in this area high quality seismograms were recorded with the pasadena very broadband vbb system iris terrascope station not only for the mainshock but also for the aftershocks at epicentral distances of 3 to 4 km we determined the focal mechanisms of 9 aftershocks using these data for most of the aftershocks the first motion data are too sparse to determine the mechanism we combined the first motion data and the waveform data of p sv and sh waves recorded with the vbb instrument to determine the mechanism and seismic moment of nine aftershocks the average orientations of the p and t axes of the aftershocks are consistent with the strike of the raymond fault the ratio of the logarithm of cumulative seismic moment of aftershocks to that of the seismic moment of the mainshock is significantly smaller than commonly observed

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

evolution of the stress field in southern california and triggering of moderate size earthquakes a 200 year perspective changes in stress in southern california are modeled from 1812 to 2025 using as input 1 stress drops associated with six large 7 0 7 5 earthquakes through 1995 and 2 stress buildup associated with major faults with slip rates 3 mm yr as constrained by geodetic paleoseismic and seismic measurements evolution of stress and the triggering of moderate to large earthquakes are treated in a tensoffal rather than a scalar manner we present snapshots of the cumulative coulomb failure function acff as a function of time for faults of various strike dip and rake throughout southern california we take acff to be zero everywhere just prior to the great shock of 1812 we find that about 95 of those well located m 6 earthquakes whose mechanisms involve either strike slip or reverse faulting are consistent with the coulomb stress evolutionary model that is they occurred in areas of positive acff the interaction between slow moving faults and stresses generated by faster moving faults significantly advanced the occurrence of the 1933 long beach and 1992 landers events in their earthquake cycles coulomb stresses near major thrust faults of the western and central transverse ranges have been accumulating for a long time future great earthquakes along the san andreas fault especially if the san bernardino and coachella valley segments rupture together can trigger moderate to large earthquakes in the transverse ranges as appears to have happened in the santa barbara earthquake that occurred 13 days after the great san andreas shock of 1812 maps of current acff provide additional guides to long term earthquake prediction

slip along the san andreas fault associated with the great 1857 earthquake historical records indicate that several meters of lateral slip along the san andreas fault accompanied the great 1857 earthquake in central and southern california these records together with dendrochronological evidence suggest that the rupture occurred along 360 to 400 km of the fault including several tens of kilometers of the currently creeping reach in central california geomorphic expressions of late holocene right lateral offsets are abundant along the 1857 rupture along 300 kilometers of the 1857 rupture between cholame and wrightwood the youngest discernible offset ranges from 3 to 9 1 2 meters dormancy of the fault since 1857 almost certainly indicates that this latest offset was created in 1857 fault slip apparently associated with the 1857 earthquake varies in a broadly systematic way along the trace of the fault it is relatively uniform along each of several long segments but changes rather abruptly in value between these segments this nonuniform displacement pattern may imply that some segments of the fault rupture more frequently or experience a slower long term slip rate than others the 1857 offsets indicate a seismic moment m o between 5 3 and 8 7 10 27 dyne cm assuming a 10 to 15 km depth of rupture and relatively uniform slip as a function of depth a comparison with the rupture length average slip value and tectonic setting of the california earthquake of 1906 m s 8 1 4 indicates a value of m 8 1 4 for the 1857 event

centroid moment tensor solutions for april june 1992 abstract centroid moment tensor solutions are presented for 245 earthquakes that occurred during the second quarter of 1992 the solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model sh8 u4l8 of dziewonski and woodward the solution for an earthquake under mindanao philippines on 9 may represents the 10 000th event in the cmt catalogue

broadband waveform observation of the 28 june 1991 sierra madre earthquake sequence ml 5 8 the sierra madre earthquake m i 5 8 of 28 june 1991 occurred at a depth of about 12 km on the clamshell sawpit fault in the san gabriel mountains high quality seismograms were recorded with terrascope not only for the mainshock but also for the aftershocks at epicentral distances of about 16 km we determined the focal mechanisms and seismic moments of the mainshock and 21 aftershocks by combining the waveform and first motion data we classified the events into five groups according to the location and waveforms recorded at pas most events located within 5 km west of the mainshock are similar to the mainshock in waveform the mechanisms thus determined are thrust mechanisms a few events located east of the mainshock have waveforms different from the mainshock and have strike slip mechanisms the average q values along the paths from the hypocenters of the sierra madre and the 3 december 1988 pasadena earthquake m l 4 9 to pas are about 130 and 80 respectively the stress drop of the mainshock is about 500 bars most of the aftershocks have stress drops between 10 and 100 bars

strain localization along the san andreas fault consequences for loading mechanisms the orientation of the near fault strain accumulation associated with the relative motion between the pacific and north american plates is better predicted by the orientation of the local faulting than by the orientation of the regional plate motion this observation follows from consideration of shear strain results from geodetic studies ranging from the imperial valley to shelter cove along the san andreas fault system this set of results includes calculations from regions where the faulting and plate motion are considerably oblique to each other both the localization of the strain associated with pacific north american motion and the orientation of the associated strain can be simply caused by failure of the lithosphere as opposed to localization of plate driving forces two types of models for the rheology and geometry of lithospheric failure have been prevalent in the geodetic literature 1 viscoelastic models which are dominated by horizontal layering of the crust and 2 elastic half space models with geometry which is dominantly vertical horizontal models can accommodate the observed strain results only if postseismic relaxation times are long compared to the repeat times of large earthquakes vertical models produce strains which are parallel to the local structure at all times if the horizontal detachments of the viscoelastic models exist they must be at depths 2 3 times the seismogenic thickness geologic studies also indicate that major transcurrent features extend as ductile shear zones to depths much larger than would be expected in horizontal models these considerations lead us to favor models with vertical geometries the vertical structure of these models can be reconciled with laboratory measurements of rock strength through the strain softening processes associated with flow of composite materials such as quartzo feldspathic rocks independent of the horizontal rheological layering in the starting low strain material progressive deformation will lead to preferential weakening in the plane of shear and to the development of a shear zone which will always be weaker than the surrounding material thus shear zones such as the san andreas system develop vertical tabular geometries which extend far below the brittle plastic transition and which are mechanically persistent

the 1987 whittier narrows earthquake sequence in los angeles southern california seismological and tectonic analysis the october 1 1987 whittier narrows earthquake m l 5 9 was located at 34 2 96 n 118 4 86 w at a depth of 14 6 0 5 km in the northeastern los angeles basin the focal mechanism of the mainshock derived from first motion polarities shows pure thrust motion on west striking nodal planes with dips of 25 5 and 65 5 respectively the aftershocks define an approximately circular surface that dips gently to the north centered at the hypocenter of the mainshock with a diameter of 4 6 km hence the spatial distribution of the mainshock and aftershocks as well as the focal mechanisms of the mainshock indicate that the causative fault was a 25 north dipping thrust fault striking west and is confined to depths from 10 to 16 km although most of the 59 aftershock focal mechanisms presented here document a complex sequence of faulting they are consistent with deformation of the hanging wall caused by the thrust faulting observed in the mainshock a cluster of reverse faulting events on north striking planes occurred within hours after the mainshock 2 km to the west of the mainshock the largest aftershock m l 5 3 occurred on october 4 and showed mostly right lateral faulting on the same north northwest striking plane within the hanging wall similarly several left lateral focal mechanisms are observed near the eastern edge of the mainshock rupture the earthquake and calibration blast arrival time data were inverted to obtain two refined crustal velocity models and a set of station delays when relocating the blast using the new models and delays the absolute hypocentral location bias is less than 0 5 km the mainshock was followed by nearly 500 locatable aftershocks which is a small number of aftershocks for this magnitude mainshock the decay rate of aftershock occurrences with time was fast while the b value was low 0 67 0 05 for a los angeles basin sequence

climate forcing due to the 8200 cal yr bp event observed at early neolithic sites in the eastern mediterranean we explore the hypothesis that the abrupt drainage of laurentide lakes and associated rapid switch of the north atlantic thermohaline circulation 8200 yr ago had a catastrophic influence on neolithic civilisation in large parts of southeastern europe anatolia cyprus and the near east the event at 8200 cal yr bp is observed in a large number of high resolution climate proxies in the northern hemisphere and in many cases corresponds to markedly cold and arid conditions we identify the relevant archaeological levels of major neolithic settlements in central anatolia cyprus greece and bulgaria and examine published stratigraphic architectural cultural and geoarchaeological studies for these sites the specific archaeological events and processes we observe at a number of these sites during the study interval 8400 8000 cal yr bp lead us to refine some previously established neolithisation models the introduction of farming to south east europe occurs in all study regions thrace macedonia thessaly bulgaria near 8200 cal yr bp we observe major disruptions of neolithic cultures in the levant north syria south east anatolia central anatolia and cyprus at the same time we conclude that the 8200 cal yr bp aridity event triggered the spread of early farmers by different routes out of west asia and the near east into greece and bulgaria

a faulting model for the 1979 imperial valley earthquake by comparing synthetic particle velocities with the near source strong motion data we have constructed by trial and error a faulting model for the 1979 imperial valley earthquake the calculation of the synthetic seismograms takes into account the vertical inhomogeneity of the elastic parameters in the imperial valley and the spatial variation of the slip rate parameters on the fault plane the independent slip rate parameters are 1 the strike slip rate amplitude 2 the dip slip rate amplitude 3 the duration that slip rate is nonzero the rise time of the slip function and 4 the rupture time which determines when the slip rate is initiated our faulting model has the following principal features 1 faulting occurred on the imperial fault and on the brawley fault rupture on the brawley fault being triggered by rupture on the imperial fault 2 the imperial fault is a plane 35 km long and 13 km wide with a strike of 323 measured clockwise from north and a dip of 80 ne the brawley fault is a 10 km long and 8 km wide plane with a strike of 360 and a dip of 90 3 faulting on the imperial fault is primarily right lateral strike slip with a small component of normal dip slip in the sediments at its northern end the larger strike slip rates are generally confined between depths of 5 and 13 km with maximum values of about 1 0 m s the duration varies on the fault with a maximum of 1 9 s which is considerably shorter than the total time for the rupture to take place 4 the rupture velocity on the imperial fault is highly variable locally it exceeds the shear wave velocity and in one instance the compressional wave velocity the average rupture velocity though is less than the shear wave velocity 5 although the slip on the brawley fault contributes only about 4 of the total moment it greatly affects the ground motion at nearby stations 6 the total seismic moment is 6 7 1018 n m where the imperial fault contributes 6 4 1018 n m and the brawley fault contributes 2 7 1017 n m in the process of trying almost 300 faulting models we found that given the elastic parameters of the medium the synthetic seismograms were most sensitive to the specification of the rupture time although the slip rate amplitudes are linearly related to the data the rupture time and the duration are not the parameterization of the nonlinear variables has a strong effect on the generation of synthetic seismograms from a finite fault

a re examination of historic earthquakes in the san jacinto fault zone california the high level of seismic activity and the potential for large earthquakes in the san jacinto fault zone southern california make it desirable to have accurate locations and source parameters for as many previous events as possible prior to the installation of a dense seismic network in this region earthquakes were located using only a few stations with generally poor azimuthal coverage resulting in considerable uncertainty in the locations we relocate and obtain moment estimates for historic pre wwssn earthquakes in the western imperial valley by comparing the waveforms and travel times with recent earthquakes in the region all the events are in the m l 5 5 to 6 5 range the historic earthquakes of interest occurred in 1937 1942 and 1954 we use the 1968 borrego mountain 1969 coyote mountain and 1987 elmore ranch earthquakes as calibration events we employ regional and teleseismic data from continuously operating stations with pasadena debilt berkeley ottawa and st louis recording most of the events the waveforms imply that all the events are almost pure strike slip events on vertical or near vertical faults approximate values for the strikes were obtained and are within the range of observed strikes for well studied earthquakes in this region the earthquakes are relocated by comparing s p and surface wave s travel times of historic events with the presumably well located recent events the relocations require only a small change in location for the 1954 event and a larger adjustment in the 1942 epicenter it also appears that the 1969 earthquake may have been mislocated the moment estimates are obtained by direct comparison of the maximum amplitudes the moment estimates imply that the 1968 and not the 1942 earthquake is the largest to have occurred in the region this century previous magnitude estimates suggested the 1942 event was larger

contrasts between source parameters of m 5 5 earthquakes in northern baja california and southern california summary earthquake source processes are determined for four historic events that occurred in baja california between 1915 and 1954 these results are combined with previous studies of recent and historic seismicity to compare earthquake processes along the san jacinto imperial cerro prieto and northern baja fault systems of southern california and northern mexico observed regional differences in rupture complexity fault rupture lengths and aftershock foreshock activity appear to be related to variations in heat flow and total fault offset there appear to be no systematic variations in stress drop as measured from far field observations between earthquakes along the imperial cerro prieto fault zone and those within the peninsular ranges of northern baja california slip rates estimated from historical seismicity between 1915 and 1992 fall 1 5 cm yr 1 short of the estimated relative plate motion between the pacific plate and north america for this region

evolution of moderate seismicity in the san francisco bay region 1850 to 1993 seismicity changes related to the occurrence of large and great earthquakes the rate of seismic activity of moderate size m 5 5 earthquakes in the san francisco bay sfb region has varied considerably during the past 150 years as measured by the rate of seismic moment release seismic activity in the sfb region is observed to accelerate prior to m 7 0 earthquakes in 1868 1906 and 1989 and then to decelerate following them we examine these seismicity changes in the context of the evolution of the stress field in the sfb region as a result of strain accumulation and release using a model of dislocations in an elastic halfspace we use a coulomb failure function cff to take into account changes in both shear and normal stresses on potential failure planes of varying strike and dip in the sfb region we find that the occurrence of a large or great earthquake creates a stress shadow a region where the stress driving earthquake deformation is decreased interseismic strain accumulation acts to reverse this process gradually bringing faults in the sfb region out of the stress shadow of a previous large or great earthquake and back into a state where earthquake failure is possible as the stress shadow generated by a large or great earthquake disappears it migrates inward toward the fault associated with that large or great event the observed changes in the rate of occurrence of moderate earthquakes in the sfb region are broadly consistent with this model in detail the decrease in seismicity throughout most of the sfb region and a localized increase in the monterey bay region following the great 1906 earthquake is consistent with our predicted stress changes the timing and location of moderate size earthquakes when the rate of seismicity increases again in the 1950s is consistent with areas in which the 1906 stress shadow had been eliminated by strain accumulation in the sfb region those earthquakes that are most inconsistent with our stress evolution model including the 1911 earthquake southeast of san jose are found to occur in regions where dip slip faulting is common in addition to strike slip the 1906 earthquake brought that zone of dip slip faulting closer to failure suggesting that the 1911 event may have been a reverse faulting earthquake rather than a strike slip one similar to the 1984 morgan hill earthquake the occurrence of activity on faults very close to the san andreas such as the lake elsman earthquakes of 1988 and 1989 appear to be associated with the last disappearence of the stress shadow on the loma prieta segment of the san andreas fault thus events of that type may represent an intermediate term precursor to a large earthquake such as the 1989 loma prieta event much of the moderate size earthquake activity in the sfb region appears to be modulated in time by the buildup and release of stress in large and great earthquakes a tensorial approach to earthquake prediction i e taking into account changes in the components of the stress tensor has several advantages over examining scalar changes such as those in seismic activity and moment release rates this tensorial approach allows for both activation and quiescence but in different subregions prior to as well as after large earthquakes

surface faulting along the superstition hills fault zone and nearby faults associated with the earthquakes of 24 november 1987 the m 6 2 elmore desert ranch earthquake of 24 november 1987 was associated spatially and probably temporally with left lateral surface rupture on many northeast trending faults in and near the superstition hills in western imperial valley three curving discontinuous principal zones of rupture among these breaks extended northeastward from near the superstition hills fault zone as far as 9 km the maximum observed surface slip 12 5 cm was on the northern of the three the elmore ranch fault at a point near the epicenter twelve hours after the elmore ranch earthquake the m 6 6 superstition hills earthquake occurred near the northwest end of the right lateral superstition hills fault zone surface rupture associated with the second event occurred along three strands of the zone here named north and south strands of the superstition hills fault and the wienert fault for 27 km southeastward from the epicenter in contrast to the left lateral faulting which remained unchanged throughout the period of investigation the right lateral movement on the superstition hills fault zone continued to increase with time a behavior that was similar to other recent historical surface ruptures on northwest trending faults in the imperial valley region we measured displacements over 339 days at as many as 296 sites along the superstition hills fault zone and repeated measurements at 49 sites provided sufficient data to fit with a simple power law data for each of the 49 sites were used to compute longitudinal displacement profiles for 1 day and to estimate the final displacement that measured slips will approach asymptotically several years after the earthquakes the maximum right lateral slip at 1 day was about 50 cm near the south central part of the north strand of superstition hills fault and the predicted maximum final displacement is probably about 112 cm at imler road near the center of the south strand of the superstition hills fault the overall distributions of right lateral displacement at 1 day and the estimated final slip are nearly symmetrical about the midpoint of the surface rupture the average estimated final right lateral slip for the superstition hills fault zone is about 54 cm the average left lateral slip for the conjugate faults trending northeastward is about 23 cm the southernmost ruptured member of the superstition hills fault zone newly named the wienert fault extends the known length of the zone by about 4 km the southern half of this fault south of new river expressed only vertical displacement on a sinuous trace the maximum vertical slip by the end of the observation period there was about 25 cm but its growth had not ceased photolineaments southeast of the end of new surface rupture suggest continuation of the superstition hills fault zone in farmland toward mexico

rupture patterns and preshocks of large earthquakes in the southern san jacinto fault zone we relocated the large 1937 1942 and 1954 earthquakes in the san jacinto fault zone the epicenters of the main shocks aftershocks and some preshocks were determined using empirical station corrections from recent small events in the study areas the 1937 ml 5 9 earthquake has an epicenter between the surface traces of the san jacinto and buck ridge faults and aftershocks suggest about 7 km of rupture predominantly to the northwest a significant increase in small earthquake activity occurred about formula yr before this event the 1954 m l 6 2 earthquake is located at the southeast end of the mapped trace of the san jacinto fault and aftershocks suggest about 15 km of rupture further southeast into an area of folded young sediments with no surface fault trace this event was preceded by a cluster of small earthquakes which occurred within an 8 hr period 10 weeks before the main event and in the eventual rupture zone the 1942 m l 6 3 earthquake is located southwest of the southeast end of the coyote creek fault large aftershocks of this event are spread over a 15 by 18 km area southwest of the coyote creek fault and are not associated with any one fault the relation of the 1942 event to the san jacinto fault zone is not simple

changes in state of stress on the southern san andreas fault resulting from the california earthquake sequence of april to june 1992 the april to june 1992 landers earthquake sequence in southern california modified the state of stress along nearby segments of the san andreas fault causing a 50 kilometer segment of the fault to move significantly closer to failure where it passes through a compressional bend near san gorgonio pass the decrease in compressive normal stress may also have reduced fluid pressures along that fault segment as pressures are reequilibrated by diffusion that fault segment should move closer to failure with time that fault segment and another to the southeast probably have not ruptured in a great earthquake in about 300 years