earthquakes faulting and stress in the los angeles basin since 1920 fourteen moderate sized m l 4 9 6 4 earthquakes have been reported in the los angeles basin these events are associated with both mappable surficial faults and concealed faults beneath the basin sediments to determine the style of faulting and state of stress in the basin single event focal mechanisms for 244 earthquakes of m 2 5 that have occurred during 1977 1989 have been calculated fifty nine percent of the events are strike slip and are mostly located near two of the major northwest striking right lateral strike slip faults in the basin the newport inglewood fault and the palos verdes fault the 1988 pasadena and the 1988 upland earthquakes showed left lateral strike slip on northeast striking faults numerous small earthquakes in the eastern part of the basin show left lateral strike slip faulting and form a northeast trend near yorba linda thirty two percent of the events have reverse mechanisms and are distributed along two broad zones the first the elysian park fold and thrust belt coincides with anticlines along the eastern and northern flank of the los angeles basin extending into santa monica bay the second the torrance wilmington fold and thrust belt coincides with anticlines mapped on the southwest flank of the basin and extends from offshore newport beach to the northwest into santa monica bay oblique faulting that could be inferred by the merging of strike slip and compressional tectonics does not occur in the basin instead the coexistence of zones of thrusting and large strike slip faults in the basin suggests that the thrust and strike slip movements are mostly decoupled a few normal faulting mechanisms appear to be related to faulting orthogonal to the axes of plunging anticlines the trend of the maximum horizontal stress varies from n1 w to n31 e across the basin and consistently forms high angles with the fold axes the stress field that exists along the flanks of the basin has a vertical minimum stress axis this stress field and ongoing folding and thrusting suggest that tectonic deformation is concentrated along the flanks of the deep central basin today the deformation of the basin consists of uplift and crustal thickening and lateral block movement to accommodate the north south compression across the basin

evidence for prehistoric earthquakes on the superstition hills fault from offset geomorphic features offset geomorphic features along the superstition hills fault show evidence for at least one slip event prior to the 1987 surface rupture and possibly as many as four to five earlier prehistoric earthquakes we documented several geomorphic features that appeared offset by multiple events by making detailed topographic maps offset features were abundant along reaches of the fault with high topographic relief and large displacement slip distribution for the penultimate event as recorded by offset rills streams and shrub coppice dunes is very similar to the slip distribution from the 1987 earthquake through april 1988 this similarity may prove to be fortuitous if afterslip from the 1987 event continues to increase the total slip for this earthquake but if afterslip associated with the 1987 event ceases in the near future then the past two earthquakes were nearly identical in slip and the superstition hills fault may be expected to produce characteristic earthquakes of roughly magnitude 6

chronology and rates of faulting of ventura river terraces california new evidence concerning the chronology of four late pleistocene terraces of the ventura river near oak view california that are vertically offset and tilted by reverse and reverse flexural slip faults provides a means of estimating rates of fault movement and downcutting by the ventura river radiocarbon ages of charcoal contained within terrace deposits date two of the terraces correlation of soils that have developed on terrace deposits and extrapolation of the rate of vertical displacement of the arroyo parida santa ana fault not a flexural slip fault date the others resulting age estimates for the four main late pleistocene terraces are qt5b 30 000 qt6a 38 000 qt6b 54 000 and qt6c 92 000 yr b p vertical slip rates on flexural slip faults range from 0 3 to 1 1 mm yr and apparently are related to the rate form and mechanics of folding of the north limb of the ayers creek canada larga syncline average rates of downcutting of the ventura river for several intervals during the late pleistocene estimated from the chronology and relative elevation of river terraces north of the arroyo panda santa ana fault upstream from the zone of faulting vary from 0 5 to 1 3 mm yr the range in rates probably reflects variations in local uplift as well as adjustments to changing eustatic sea level climatic conditions and or regional deformation of the western transverse ranges the average rate of downcutting of the ventura river north of the zone of flexural slip faulting is 0 8 mm yr compared with 1 2 to 2 2 mm yr in the deformation zone immediately to the south this apparently indicates that during tectonic deformation there is an approximate balance between the rate of uplift due to faulting and folding and the rate of downcutting by the fluvial system

variable rates of late quaternary strike slip on the san jacinto fault zone southern california three strike slip displacements of strata with known approximate ages have been measured at two locations on the san jacinto fault zone minimum horizontal offset between 5 7 and 8 6 km in no more than 0 73 m y northeast of anza indicates 8 12 mm yr average slip rate since late pleistocene time two measures of more recent displacement are based on trenching studies of stratigraphic offsets on the coyote creek fault in western imperial valley horizontal slip of 1 7 m has been calculated for the youngest sediment of lake cahuilla since its deposition 275 510 yr b p the corresponding slip rate is 2 8 5 0 mm yr right lateral offset of 10 9 m measured on a buried stream channel older than 5060 yr b p but younger than 6820 yr b p yields average slip rates for the intermediate time periods 400 to 6000 yr b p of 1 2 mm yr the average rates of slip for these three time intervals suggest a relatively quiescent period for the san jacinto fault zone from about 4000 b c to about 1600 a d to the extent that long term variations in seismic activity of major strike slip faults elsewhere are known fluctuations in slip rate for the san jacinto fault would not appear to be abnormal if the san jacinto and adjacent segments of the san andreas fault alternately assume dominant roles in absorbing motion between the pacific and american plates perhaps even more recently than 400 yr ago the san andreas fault south of the transverse ranges expressed most of the motion but has since become relatively inactive

seismicity and geometry of a 110 km long blind thrust fault 2 synthesis of the 1982 1985 california earthquake sequence during 1982 1985 three 5 4 mw 6 5 earthquakes migrated 65 km along the north half of a string of quaternary folds at the east front of the california coast ranges there is no surface fault associated with these earthquakes but the fold chain oriented parallel to the san andreas fault 30 km to the west takes up contraction normal to the san andreas fault identification of active blind thrust faults is hindered by the absence of a fault trace but may be revealed by the presence of growing folds at the surface and by earthquakes at depth because of excellent seismic geodetic and geologic data this earthquake sequence illuminates the geometry of the blind faults and enables us to probe their mechanics we relocate seismicity and compute focal mechanisms in a three dimensional velocity space and model the vertical deformation associated with the 1983 m 6 5 coalinga earthquake the aftershock zones abut at echelon offsets in the fold axes and the mainshocks display reverse slip perpendicular to the axes suggesting that the folds conceal a contiguous segmented thrust fault background seismicity concentrates at bends and breaks in the fold chain sites that may correspond to tears and ramps in the thrust fault at depth seismic reflection profiles reveal thrust and reverse faults dipping toward the san andreas fault at depths of 5 10 km with several kilometers of cumulative slip and high angle reverse faults in the anticlines with several hundred meters cumulative slip coseismic fold uplift accompanied the coalinga and kettleman hills north dome earthquakes suggesting that fold growth is episodic and coupled closely to repeated earthquakes on the underlying thrusts the north half of the fold chain has been the site of several 6 m 6 5 earthquakes since 1885 thus kettleman hills middle dome the next fold segment to the south has an elevated seismic potential the aftershock zones of the three main shocks are diffuse and occupy a region much larger than the site of seismic slip aftershocks occur 5 7 km from the fault regions where on the basis of a boundary element model the shear strain increase caused by the fault slip exceeds 20 ppm equivalent to about 0 7 mpa we argue that the broad aftershock zone is a product of high sustained off fault stress caused by repeated displacement on faults that do not cut the earth s surface the fault tip stresses lead to the formation of secondary faults which can become sites of aftershocks and postseismic creep

strain accumulation in southern california 1973 1980 frequent surveys of seven trilateration networks in southern california over the interval 1973 1980 suggest that a regional increment in strain may have occurred in 1978 1979 prior to 1978 and after late 1979 the strain accumulation has been predominantly a uniaxial north south compression this secular trend was interrupted sometime in 1978 1979 by an increment in both north south and east west extension in five of the seven networks the onset of this change appears to have occurred first in the networks farthest south the changes occurred without any unusual seismicity within the networks but the overall seismicity in southern california was unusually low prior to and has been unusually high since the occurrence the average principal strain rates for the seven networks in the 1973 1980 interval are 0 17 mu strain yr north south contraction and 0 08 mu strain yr east west extension although the observed increment in strain could be related to unidentifed systematic error in the measuring system a careful review of the measurements and comparison with three other measuring systems reveal no appreciable cumulative systematic error

fault slip rates and earthquake histories for active faults in southern california abstract within the context of the historical record of seismicity we review the geological data bearing on the quaternary slip rates and paleoearthquake histories of active faults in southern california

late quaternary rate of slip along the san jacinto fault zone near anza southern california the anza section of the san jacinto fault southern california has not experienced a groundbreaking earthquake since at least 1918 and probably since before 1899 leading previous workers to designate this 20 km long fault segment as a seismic gap study of displaced pleistocene and holocene alluvial fan and fluvial deposits across the san jacinto fault near anza california dated using 14c and soil development age control indicates a minimum right lateral slip rate of 9 2 2 mm yr since 9 5 ka and a slip rate of 11 9 5 mm yr since 14 ka 12 9 5 mm yr since 17 ka and 13 10 6 mm yr since 50 ka these estimates agree with a previously determined 700 ka to present minimum slip rate of 10 2 mm yr based on an estimated average slip rate of 12 mm yr from the longer term slip rate estimates about 0 8 m of potential slip has accumulated if the 1918 earthquake broke the anza segment over 1 1 m of potential slip has accumulated if the last earthquake occurred prior to 1899 these data attest to the potential for an earthquake m6 5 for this section of the fault

structural transect of the western transverse ranges california implications for lithospheric kinematics and seismic risk evaluation a retrodeformable cross section that integrates surface and subsurface data across the western transverse ranges california illustrates an actively developing fold and thrust belt that began forming at 2 3 ma high level thrusts are interpreted to root in a mid crustal detachment at 12 15 km depth which coincides with the maximum depth of earthquakes the cross section documents 53 km of convergence above the mid crustal detachment dividing this by the time since onset of deformation yields convergence rates of 17 6 26 5 mm yr the high density lithospheric anomaly beneath the transverse ranges is related to subduction of lower crust and lithosphere below the mid crustal detachment to balance the shallow crustal shortening thrust ramps coincide with zones of high seismicity in the transverse ranges this suggests that ramp regions have the highest potential for compressive earthquake events e g the recent whittier narrows earthquake of october 1 1987 m sub l 5 9 occurred along the eastern part of a ramp zone

crustal strain near the big bend of the san andreas fault analysis of the los padres tehachapi trilateration networks california in the region of the los padres tehachapi geodetic network the san andreas fault saf changes its orientation by over 30 degree from n 40 degree w close to that predicted by plate motion for a transform boundary to n 73 degree w the strain orientation near the saf is consistent with right lateral shear along the fault with maximum shear rate of 0 38 plus minus 0 01 mu rad yr at n 63 degree w in contrast away from the saf the strain orientations on both sides of the fault are consistent with the plate motion direction with maximum shear rate of 0 19 plus minus 0 01 mu rad yr at n 44 degree w the strain rate does not drop off rapidly away from the fault and thus the area is fit by either a broad shear zone below the saf or a single fault with a relatively deep locking depth the fit to the line length data is poor for locking depth d less than 25 km for d of 25 km a buried slip rate of 30 plus minus 6 mm yr is estimated the authors also estimated buried slip for models that include the garlock and big pine faults in addition to the saf slip rates on other faults are poorly constrained bymore the los padres tehachapi network the best fitting garlock fault model had computed left lateral slip rate of 11 plus minus 2 mm yr below 10 km buried left lateral slip of 15 plus minus 6 mm yr on the big pine fault within the western transverse ranges provides significant reduction in line length residuals however deformation there may be more complicated than a single vertical fault a subhorizontal detachment on the southern side of the saf cannot be well constrained by these data the authors investigated the location of the saf and found that a vertical fault below the surface trace fits the data much better than either a dipping fault zone located south of the surface trace less

an offset holocene stream channel and the rate of slip along the northern reach of the san jacinto fault zone san bernardino valley california the three dimensional geometry of a buried channel fill deposit that crosses and is offset by a strand of the san jacinto fault zone in san bernardino california is reconstructed from an extensive suite of trench logs the reconstruction shows that the original course of the channel was deflected for a short distance along strike of the fault before it was offset as a result apparent offset of the channel deposit across the fault zone is about 12 m in a right lateral sense but only 3 5 to 6 m of the apparent offset is due to fault slip the age of the deposit is 1931 109 yr b p based on radiometric dating of detrital charcoal collected from the unit our excavations do not encompass all active strands of the fault at this site hence our study points to a minimum slip rate of 1 7 to 3 3 mm yr along the northern san jacinto fault zone historical records show that it has been at least 64 to 90 yr since the last large earthquake occurred along the northern san jacinto fault it can thus be inferred from our estimate of fault slip rate that at least 10 to 30 cm of slip is currently stored along the northern segment of the san jacinto fault because prior studies along the same fault zone to the southeast indicate much higher slip rates on the order of 10 mm yr and coseismic offsets for the largest historical earthquakes along the san jacinto fault zone have been on the order of 30 cm it is inferred that the northern reach of the san jacinto fault is now in the later stages of the strain accumulation cycle prior to a moderate to large m 6 7 earthquake additionally the complex geometry of the reconstructed channel serves to illustrate the caution that must be exercised when determining the true offset of buried channel deposits

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

dynamics of the itcz boundary layer abstractthis paper presents high resolution numerical solutions of a nonlinear zonally symmetric slab model of the intertropical convergence zone itcz boundary layer the boundary layer zonal and meridional flows are forced by a specified pressure field which can also be interpreted as a specified geostrophically balanced zonal wind field ug y one narrow on equatorial peak in boundary layer pumping is produced when the forcing is easterly geostrophic flow along the equator and two narrow peaks in boundary layer pumping are produced on opposite sides of the equator a double itcz when the forcing is westerly geostrophic flow along the equator in the case when easterlies are surrounding a westerly wind burst once again a double itcz is produced but the itczs have significantly more intense boundary layer pumping than the case of only westerly geostrophic flow a comparison of the numerical solutions to those of classical ekman theory suggests that the meridional advection term y plays a vita

the elmore ranch and superstition hills earthquakes of 24 november 1987 introduction to the special issue on 24 november 1987 two significant earthquakes occurred along the southern san jacinto fault zone and related structural elements in southern california not far from the international border these two events the elmore ranch earthquake m 6 2 at 0154 gmt and the superstition hills earthquake m 6 6 at 1315 gmt both moment magnitudes from sipkin 1989 and their aftershocks have yielded a rich harvest of geological seismological and engineering data pertinent to the cause and effect of earthquakes in this region where the southern san jacinto fault zone enters the salton depression from the peninsula ranges bordering it on the southwest fig 1 this special issue of the bulletin presents 18 geologic and seismologic investigations of these earthquakes a collection of papers born in el centro california on 8 and 9 february 1988 at a meeting attended by approximately 60 scientists interested in these earthquakes for one reason or another

age constraints for the present fault configuration in the imperial valley california evidence for northwestward propagation of the gulf of california rift system releveling and other geophysical data for the imperial valley of southern california suggest the northern section of the imperial brawley fault system which includes the mesquite basin and brawley seismic zone is much younger than the age of the valley itself a minimum age of 3000 years is calculated for the northern segment of the imperial fault from correlations between surface topography and geodetically observed seismic interseismic vertical movements calculation of a maximum age of 100 000 years is based upon displacements in the crystalline basement along the imperial fault inferred from seismic refraction surveys this young age supports recent interpretations of heat flow measurements and the evolution of geothermal systems which also suggest that the current patterns of seismicity and faulting in the imperial valley are not long lived the current fault geometry and basement morphology suggest a northwestward growth of the imperial fault and a northwestward migration of the brawley seismic zone if this localized process is representative of more regional tectonic processes along the extent of the salton trough we suggest that this migration is a manifestation of the propagation of the gulf of california rift system into the north american continent

earthquakes quaternary faults and seismic hazard in california data describing the locations slip rates and lengths of quaternary faults are the primary basis in this work for constructing maps that characterize seismic hazard in california the expected seismic moment m0e and the strength of ground shaking resulting from the entire rupture of each mapped fault or fault segment are estimated using empirical relations between seismic moment m0 rupture length source to site distance and strong ground motions assuming a fault model whereby the repeat time t of earthquakes on each fault equals m0e m0g where the moment rate m0g is proportional to fault slip rate it is observed that the moment frequency distribution of earthquakes predicted from the geologic data agrees well with the distribution determined from a 150 year historical record the agreement is consistent with the argument that the geologic record of quaternary fault offsets contains information sufficient to predict the average spatial and size distribution of earthquakes through time in california the estimates of t for each fault are the foundation for constructing maps that depict the average return period of 0 1g peak horizontal ground accelerations and the horizontal components of peak acceleration peak velocity and the pseudovelocity response at 1 period and 5 damping expected to occur at the level of 0 1 probability during a 50 year period of time a map is also formulated to show the probability that 0 1g horizontal ground accelerations will occur during the next 50 years the maps serve to illustrate the potential value of quaternary fault studies for assessing seismic hazard interpretation of available slip rates indicates that the largest and most frequent occurrence of potentially destructive strong ground motions are associated principally with the san andreas san jacinto calaveras hayward and ventura basin fault zones other regions of similarly high hazard may yet remain unrecognized this inadequacy results primarily from an incomplete data set numerous faults for example are mapped along the coastal region of northern california and within the modoc plateau but relatively few studies relating to fault slip rate are reported a similar problem exists for other stretches of coastal california where marine reflection studies provide evidence of active faulting offshore yet yield little or no information of fault slip rate geological and geophysical field studies can work to remove these deficiencies a concerted effort to locate and define rates of activity on all faults in california is the most promising means to further quantify present levels of seismic hazard in california

surficial offsets on the central and eastern garlock fault associated with prehistoric earthquakes geomorphic features offset along the central and eastern garlock fault record the amount of surface slip associated with prehistoric earthquakes along the easternmost 90 km of the fault the smallest offsets cluster around 2 3 m of left lateral slip apparently associated with the most recent earthquake on this portion of the fault larger offsets along this part of the fault especially in pilot knob valley cluster around values consistent with 2 4 m of slip in each of the past several events farther west south of el paso mountains offset geomorphic features suggest that each of the past two earthquakes on this stretch of the garlock fault was produced by about 7 m of slip whereas the third event back was produced by about 4 m of slip vertical displacements of geomorphic features range from 0 to 30 of the left lateral offsets within pilot knob valley along the southern side of the slate range vertical displacements are consistently up on the northern side whereas within the avawatz mountains both north and south side up vertical displacements are present on the basis of the geomorphic offsets the geometry of the garlock fault and the precedents set by historical strike slip earthquakes elsewhere a number of different rupture patterns are plausible these range from rupture of the entire garlock fault in a single event with a maximum magnitude of about mw 7 8 to separate rupture of the western segment and of the central and eastern segments combined with approximate magnitudes mw 1 1 and mw 7 5 respectively to separate rupture of even shorter segments producing earthquakes of magnitudes mw 6 6 to mw 7 5 in conjunction with available slip rates for the garlock fault the geomorphic offsets suggest that average recurrence intervals are probably within the range of 600 1200 years south of el paso mountains about 200 750 years in searles valley about 200 1300 years in pilot knob valley and about 200 3000 years near leach lake and in the avawatz mountains

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

seismicity of the garlock fault california the garlock fault is a 265 km long left slip fault striking northeastward from the san andreas fault in southern california relocations of earthquakes that occurred from 1932 to 1981 on and near this major fault were made using the master event technique the spatial distribution of seismicity along the fault is different west and east of its midpoint near rand where the largest en echelon offset and a marked change in strike occur these two segments also display distinct geologic features and different seismic and aseismic behavior the 150 km long segment west of rand has shown continuous low seismic activity during the past 50 yr well documented aseismic creep and has a relatively complex fault trace in contrast the 155 km long segment east of rand has very few small earthquakes no demonstrable creep and a simpler fault trace p wave first motion studies substantiate predominantly left slip motion along the garlock fault overall energy release during this 50 yr period gives a seismic moment rate of 2 75 10 21 dyne cm yr much lower than that inferred from holocene geologic offsets thus indicating that the garlock fault currently represents a temporal seismic gap and that the potential exists for large earthquakes if behavior of the garlock fault is similar to that of the san andreas the western segment of the fault can be compared with the central creeping segment of the san andreas and the eastern segment with those segments broken by the 1857 and 1906 earthquakes thus larger events might be expected on the eastern segment than on the western segment or if the entire fault breaks during a single event larger displacements toward the east

irregular recurrence of large earthquakes along the san andreas fault evidence from trees old trees growing along the san andreas fault near wrightwood california record in their annual ring width patterns the effects of a major earthquake in the fall or winter of 1812 to 1813 paleoseismic data and historical information indicate that this event was the san juan capistrano earthquake of 8 december 1812 with a magnitude of 7 5 the discovery that at least 12 kilometers of the mojave segment of the san andreas fault ruptured in 1812 only 44 years before the great january 1857 rupture demonstrates that intervals between large earthquakes on this part of the fault are highly variable this variability increases the uncertainty of forecasting destructive earthquakes on the basis of past behavior and accentuates the need for a more fundamental knowledge of san andreas fault dynamics

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

a 100 year average recurrence interval for the san andreas fault at wrightwood california evidence for five large earthquakes during the past five centuries along the san andreas fault zone 70 kilometers northeast of los angeles california indicates that the average recurrence interval and the temporal variability are significantly smaller than previously thought rapid sedimentation during the past 5000 years in a 150 meter wide structural depression has produced a greater than 21 meter thick sequence of debris flow and stream deposits interbedded with more than 50 datable peat layers fault scarps colluvial wedges fissure infills upward termination of ruptures and tilted and folded deposits above listric faults provide evidence for large earthquakes that occurred in a d 1857 1812 and about 1700 1610 and 1470

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

strain rate profile across the elsinore san jacinto and san andreas faults near palm springs california 1973 81 a profile of the strain accumulation rate along a line trending n50 e across the subparallel elsinore san jacinto and san andreas faults near palm springs california has been constructed from trilateration surveys in the 1973 81 interval the strain accumulation is principally right lateral shear across a vertical plane parallel to fault strike n40 w the strain rate profile for that component exhibits two clearly resolved maxima one centered on the san jacinto fault max 0 35 0 02 rad a and the other on the san andreas fault max 0 40 0 02 rad a no maximum is associated with the elsinore fault this result clearly implies that slip at depth on both the san andreas and san jacinto faults is loading the shallower sections of these faults and that eventually rupture can be expected on both

a single deformed bow shock for titan saturn system ssduring periods of high solar wind pressure saturn s bow shock is pushed inside titan s orbit exposing the moon and its ionosphere to the solar wind the cassini spacecraft s t96 encounter with titan occurred during such a period and showed evidence for shocks associated with saturn and titan it also revealed the presence of two foreshocks one prior to the closest approach foreshock 1 and one after foreshock 2 using electromagnetic hybrid kinetic ions fluid electrons simulations and cassini observations we show that the origin of foreshock 1 is tied to the formation of a single deformed bow shock for the titan saturn system we also report the observations of a structure in foreshock 1 with properties consistent with those of spontaneous hot flow anomalies shfas formed in the simulations and previously observed at earth venus and mars the results of hybrid simulations also show the generation of oblique fast magnetosonic waves upstream of the outbound titan bow shock in agreement with the observations of large amplitude magnetosonic pulsations in foreshock 2 we also discuss the implications of a single deformed bow shock for new particle acceleration mechanisms and also saturn s magnetopause and magnetosphere

a cross section of the los angeles area seismically active fold and thrust belt the 1987 whittier narrows earthquake and earthquake hazard retrodeformable cross sections across the los angeles area interpret the pliocene to quaternary deformation to be a developing basement involved fold and thrust belt the fold and thrust belt is seismically active as evidenced by the 1987 whittier narrows earthquake ml 5 9 and the 1971 san fernando earthquake mw 6 6 the structural geology of the los angeles area is dominated by three major compressional uplift trends 1 the palos verdes anticlinorium and western shelf 2 the santa monica mountains anticlinorium and 3 the verdugo mountains san rafael hills and the san gabriel mountains these trends result from major thrust ramps off a detachment s at 10 15 km depth thrusts of the verdugo mountains san rafael hills and the san gabriel mountains reach the surface the other two uplifts are associated with blind thrusts compressional seismicity is concentrated along these thrust ramps the 1987 whittier narrows earthquake probably occurred on the elysian park thrust which underlies the santa monica mountains anticlinorium the thrust interpretation accounts for the geometry of the anticlinorium the seismological characteristics of the earthquake and the geometry of coseismic uplift the earthquake and aftershocks occurred within a structurally complex narrow zone of miocene and pliocene northwest trending faults that cross the anticlinorium at a high angle these northwest trending faults are interpreted to be reactivated faults now behaving as tears in the elysian park thrust and not the result of active right lateral deformation extending into the whittier narrows area our analysis suggests the whittier narrows earthquake sequence occurred within a structurally weakened zone along the elysian park thrust we also suggest that the whittier fault is not an important quaternary structure and may not be seismogenic the regional cross section is a nonunique solution and other possible solutions are considered multiple solutions arise from the presence of two intersecting compressional belts in the los angeles area the transverse ranges and the northern peninsular ranges the belts may be due to one or more regional detachments and the northern peninsular ranges may be northeast or southwest vergent the deformed top of the crystalline basement along the regional cross section requires a minimum of 15 0 km of north south convergence between the palos verdes hills and the san andreas fault regardless of the structural solution restoration of our cross section solution requires 21 4 km of north south convergence on top of the crystalline basement including 6 4 km of slip continuing offshore to make structures of the continental borderland and 29 7 km of convergence on the basal detachment geologic relationships suggest major shortening started between early and late pliocene time 2 2 4 0 ma which yields a minimum convergence rate of 3 8 6 8 mm yr between the palos verdes hills and the san andreas fault convergence rates for our solution range from 5 4 to 13 5 mm yr between the edge of the continental borderland and the san andreas fault slip rate estimates for the elysian park thrust along the eastern portion of the santa monica mountains anticlinorium are 2 5 5 2 mm yr if the 1987 whittier narrows earthquake is the characteristic earthquake along the 170 km long anticlinorium then recurrence intervals are estimated to be 5 6 12 1 years this recurrence is higher than the historic seismic record along the santa monica mountains anticlinorium and suggests that 1 our slip rates are too high 2 some crustal shortening is taken up aseismically and or 3 earthquakes larger than the 1987 whittier narrows earthquake will occur along the anticlinorium but less frequently

behavior of the superstition hills fault during the past 330 years we have investigated the recent prehistoric behavior of the superstition hills fault by examining its effect on the beach deposits of ancient lake cahuilla excavation of these sediments in three dimensions where they are cut by the fault has enabled determination of total offset since the latest highstand of the lake about 330 years ago as of 3 march 1988 total dextral offset was 1106 50 mm about 609 mm of this amount can be attributed to one or more slip events before 1987 the remaining slip occurred during the moderate earthquake of 24 november 1987 and as subsequent aftercreep additional aftercreep here might produce a total of 1210 100 mm for that event thus if afterslip continues according to our prediction slip associated with the penultimate seismic slip event on the superstition hills fault was only about half of the co seismic slip and predicted afterslip associated with the 1987 earthquake this difference established at a single site on the fault may reflect a larger size for the 1987 event than for its predecessor we calculate that the slip rate of the superstition hills fault zone averaged over the past 330 years is between about 2 and at least 6 mm yr at this site during this time the average interval between large surface slip events on the superstition hills fault has been between about 150 and 300 years the pre 1987 slip event documented in our excavations could have occurred at any time between a d 1660 and about a d 1915 these results are significant for understanding earthquake recurrence and patterns of earthquakes in the imperial and coachella valleys

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

seismicity and fault kinematics through the eastern transverse ranges california block rotation strike slip faulting and low angle thrusts hypocentral locations and first motion data from the southern california seismic network are used to infer the present pattern of deformation along the southern san andreas fault the study area lies between the san jacinto fault and desert hot springs and includes the san bernardino mountains and san gorgonio pass this region has some of the deepest earthquakes observed anywhere along the entire san andreas system exhibits a complex surface geology characterized by both right and left lateral faults has high topographic relief as a result of recent uplift and is a potential site for the nucleation of a great earthquake although this area is unusually seismogenic little activity can be directly associated with major throughgoing faults seismicity is also generally absent in the upper 5 km the predominant style of faulting above 10 12 km is oblique slip with a large reverse component the spatial distribution of relocated hypocenters and first motion data suggests the presence of a system of left slip faults striking northeast this pattern of faulting in conjunction with an unusual set of normal and reverse focal mechanisms is interpreted as the clockwise rotation of a small set of crustal blocks subject to regional right lateral shear at depths greater than about 10 km seismicity defines a wedge shaped volume undergoing pervasive internal deformation on a combination of strike slip and low angle thrust faults velocity structures determined from earthquake arrival times suggest a low velocity zone at about 10 km below the san bernardino mountains but not below the san jacinto mountains this is nearly the same depth as the transition between the two layers of different kinematic behavior seen south of the mill creek mission creek branch of the san andreas fault and the maximum depth of seismicity seen north of that fault the low velocity zone and the transition between block rotations and the deeper deformation may thus correspond to a detachment under much of this region and would imply that the overthrust san bernardino mountains are allochthonous the present pattern of seismic deformation in shocks of small to moderate size may only characterize the interval between large earthquakes and may change systematically as the region prepares to accommodate large right lateral displacements

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

the 1988 and 1990 upland earthquakes left lateral faulting adjacent to the central transverse ranges two earthquakes m l 4 6 and m l 5 2 occurred at almost the same location in upland southern california in june 1988 and february 1990 and had similar strike slip focal mechanisms with left lateral motion on a northeast striking plane the focal mechanisms and aftershock locations showed that the causative fault was the san jose fault an 18 km long concealed fault that splays west southwest from the frontal fault of the central transverse ranges left lateral strike slip faults adjacent to the frontal faults may play an important role in the deformation of the transverse ranges and the los angeles basin as suggested by these upland earthquakes the left lateral strike slip 1988 m l 4 9 pasadena earthquake on the raymond fault 30 km to the west of upland and scattered background seismicity along other active left lateral faults these faults may transfer slip away from part of the frontal fault toward the south alternatively these faults could represent secondary faulting related to the termination of the northwest striking right lateral strike slip faults to the south of the range front the 1988 and 1990 upland earthquakes ruptured abutting or possibly overlapping segments of the san jose fault the edges of the overlapping aftershock zones which are sharply defined together with background seismicity outline a 14 km long aseismic segment of the san jose fault the 1988 mainshock originated at 9 5 km depth and caused aftershocks between 5 and 12 km in contrast the 1990 mainshock focus occurred at the top of its aftershock zone at 5 km and caused aftershocks down to 13 km depth these deep aftershocks tapered off within 2 weeks the rate of occurrence of aftershocks in magnitude time space was the same for both sequences the state of stress reflected in the focal mechanisms of the aftershocks is identical to that determined from background activity and did not change with time during the aftershock sequence the constant stress state suggests that the 1988 and 1990 events did not completely release all the stored slip on that segment of the fault the presence of 14 km of unbroken fault the abrupt temporal termination of deep aftershocks and the constant stress state all suggest that a future moderate sized earthquake m l 6 0 6 5 on the san jose fault is possible with a rupture length of at least 14 km and possibly 18 km