the palos verdes anticlinorium along the los angeles california coast implications for underlying thrust faulting 1 the palos verdes anticlinorium pva is a 70 km wide nw trending pliocene quaternary transpressional high that separates the onshore los angeles basin from the offshore san pedro basin the pva southwest limb underlies a 70 km long sw facing seafloor escarpment this escarpment is separated into two segments by a 6 km right step it overlies two levels of ne dipping low angle thrust faults these faults project beneath los angeles toward known sw verging blind thrust faults and may be the same regional faults these conclusions are based on a detailed geometric representation constructed from a regional analysis of 5000 km of multichannel seismic reflection profiles logs and paleontology from 19 wells published seafloor geology and multibeam bathymetric data patterns of sedimentation and erosion indicate that folding of pva initiated during pliocene time and propagated southward with folding of part of the southern pva occurring during the quaternary structural relief continues to grow with regional long term rock uplift of the pva crest and ongoing subsidence in the adjacent basins the low angle ne dipping faults beneath the pva must be late quaternary active in order to maintain hanging wall rock uplift above subsiding footwalls structural geometry requires that the underlying faults have slipped at 1 1 1 6 mm yr over quaternary time

empirical observations regarding reverse earthquakes blind thrust faults and quaternary deformation are blind thrust faults truly blind active thrust faults pose a significant seismic hazard worldwide many of these faults include blind thrusts where the propagating fault tip does not reach the earth s surface and buried faults where the geomorphic expression of the fault is obscured by subsequent sedimentation and or erosion this raises the issue of whether conventional geologic geomorphic and paleoseismic methods can be used to identify and characterize thrust faults for the assessment of seismic hazards or whether these faults sometimes are truly blind we compiled a data base of 148 worldwide moderate to large magnitude thrust reverse earthquakes to evaluate whether or not the event occurred on a fault that could have been identified prior to the earthquake on the basis of recognizable quaternary surface deformation i e a pre existing fault or fold analysis of the data shows that interplate reverse earthquakes almost always are associated with pre existing quaternary deformation that was or could have been recognized prior to the earthquake in particular most interplate reverse earthquakes are associated with an active reverse fault at the surface and or an active anticline in contrast intraplate reverse earthquakes seldom occur on faults associated with pre existing recognizable surface deformation we conclude that thrust faults can be detected in interplate regions with careful quaternary geologic and geomorphic mapping furthermore the absence of quaternary surface deformation can be used to infer the absence of an underlying active blind thrust fault in interplate tectonic settings however the data show that quaternary geologic mapping techniques alone likely are insufficient to characterize blind thrusts in intraplate regions in these areas inclusion of a floating or random earthquake may be necessary to assess earthquake hazards

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

seismic potential revealed by surface folding 1983 coalinga california earthquake the 2 may 1983 coalinga california earthquake magnitude 6 5 failed to rupture through surface deposits and instead elastically folded the top few kilometers of the crust the subsurface rate of fault slip and the earthquake repeat time are estimated from seismic geodetic and geologic data three larger earthquakes up to magnitude 7 5 during the past 20 years are also shown to have struck on reverse faults concealed beneath active folds

seismically active fold and thrust belt in the san joaquin valley central california structural analysis of late cenozoic folds along the western and southern margins of the san joaquin basin suggests that the folds are related to development of a fold and thrust belt rather than to wrench tectonics the folds have formed by the processes of fault bend folding and fault propagation folding which commonly occur in fold and thrust belts our structural interpretation attributes the seismically active coalinga and kettleman hills north dome anticlines to fault bend folding above a thrust fault s that steps up from a detachment within the franciscan assemblage to a detachment at the base of the great valley group this thrust does not reach the surface blind thrust and its slip is consumed in backthrusting and formation of subsurface folds under the san joaquin valley movement on the postulated thrust s would explain the cause of the may 2 1983 coalinga earthquake and the august 15 1985 avenal earthquake and would account for the lack of significant surface rupture or shallow subsurface faulting during both earthquakes well documented examples of fault bend and fault propagation folding also occur at wheeler ridge in the san emigdio mountains and at kettleman hills south dome deformation associated with fold and thrust belts can be extremely complicated considerable fault movement can occur without accompanying surface rupture or shallow level faulting conversely surface rupture can occur that has no direct relationship to fault slip at depth an example would be flexural slip folding wherein the slip planes reach the surface but do not root into any significant faults at depth consequently traditional geologic approaches to seismic risk evaluation which rely largely on surface data are subject to numerous pitfalls when applied to fold and thrust belts our interpretation of the structural style that developed in central california during the late cenozoic requires that the strain associated with the transpressive motion between the pacific and north american plates be resolved into normal and tangential components thrust faulting and folding normal to the plate boundary and strike slip faulting parallel to the plate boundary san andreas fault the thrust faults root in a deecollement at the brittle ductile transition zone above which shortening is associated with folding and thrust faulting and beneath which shortening in the lower crust is accommodated by ductile processes of tectonic thickening or incipient subduction

phytoliths indicators of grassland dynamics during the late holocene in intertropical africa abstract the reconstruction of african tropical grassland history during the late holocene can be carried out using phytolith analysis fossil phytolith assemblages from lake guiers in the sahelian region of senegal and from lake sinnda in the guineo congolian region of congo were investigated the results are interpreted on the basis of modern phytolith assemblages from the same regions and compared to pollen data previously obtained tall or short grass associations are discriminated by their phytolith index iph saddle cross dumbel saddle while the density of shrubs and trees is indicated by relative proportions of the dicotyledon phytoliths the phytolith data emphasize that in the guineo congolian region around lake sinnda the driest phase of the late holocene occurred between 4000 and 1200 yr b p commencing with the opening of the dense forest and its replacement by a short grass savanna from ca 1000 yr b p wetter climatic conditions developed as represented by the setting up of a tall grass savanna woodland the modern shrub and tall grass savanna was developed ca 700 yr b p in the sahelian region around lake guiers the driest phase occurred after about 2000 yr b p and has not been followed by moister conditions a tall grass savanna woodland was gradually replaced by a shrub and short grass savanna which still occurs a short period of development of swampy vegetation which can be related to a lake level change interrupted the semi arid adaptation of the vegetation between about 2000 yr b p and the present

imaging topographic growth by long lived postseismic afterslip at sefidabeh east iran this paper describes observations and models of the postseismic deformation following the 1994 sefidabeh earthquake sequence in east iran which shed light on the nature of the earthquake cycle and the mechanisms of topographic growth in the region interferometric synthetic aperture radar observations show creeping fault motion postseismic afterslip on an array of faults some of these faults probably represent the extensions of those that ruptured in the blind thrust faulting earthquakes in 1994 and cut through the entire seismogenic layer while others are shallow and break up the hanging walls of the coseismic faults the postseismic slip accommodates at least part of the vertical displacement gradient resulting from the buried coseismic slip which was concentrated at depths of greater than 5 km the postseismic afterslip is visible for over 16 years following the earthquakes agreement between the areas of postseismic uplift and indications of long term motion preserved in the geomorphology suggest that shallow fault slip during seismic cycles similar to the one we have observed governs the development of the landscape in the region slip on an array of shallow faults provides a mechanism for the development of short wavelength topography and geological structures above active thrust faults and has important implications for the interpretation of shallow geological features produced in regions experiencing similar seismic cycles to that at sefidabeh

modeling of wave propagation in northern los angeles basin abstract the los angeles basin is characterized by deep structural complexity and is the site of numerous earthquakes we analyze the amplitude and waveform data of earthquakes recorded in the los angeles basin in order to identify the structurally controlled path effects this has been done in order to determine which features of the structure affect the observations and to generate improved models in different parts of the basin increasing our capabilities of predicting seismic hazard several structural models of the l a basin that are based on regional subsurface studies exposed sections scattered oil wells and other limited geophysical data have recently become available in this study we have used seismic modeling techniques together with geologic models to attempt to explain site variations in some important data bases we have modeled waveforms of 1 the 19 january 1989 malibu earthquake data recorded in pasadena 2 an m 2 8 earthquake recorded by the usc downhole array in the baldwin hills southern california and 3 two aftershocks of the whittier narrows earthquake recorded at five stations along a profile the modeling of the malibu earthquake results in a layered one dimensional model representing 6 5 km of layered sediment underlain by a high velocity basement direct arrivals basin reflected phases and multiple reverberations are modeled fairly well by this one dimensional model the downhole array data in the baldwin hills show a significant near surface amplification that can be modeled by very low velocity near surface materials a series of aftershocks of the whittier narrows earthquake recorded at close distances at temporary recording sites shows a wide variation in peak accelerations due both to radiation pattern and propagation effects the effect of lateral heterogeneity is evident in the travel time where stations within the basin show later arrival times than those away from the basin for the same epicentral distance seismograms recorded at different sites show variation in waveforms due to multi pathing of rays we use such triplicated arrivals to constrain the structure of different interfaces using these criteria and the synthetic seismograms calculated by ray theory and finite difference methods we have been able to model the tangential seismograms due to two of these events recorded at five stations along a two dimensional profile all three modeling exercises helped us understand wave propagation in the basin environment a basin reflected phase was identified in the seismogram from the malibu earthquake which was used to derive an effective depth of the basement near surface amplification of the waves observed in the downhole data could be explained by constructive interference of the multiply reflected waves through the thin near surface low velocity layer similarly site dependent variation of waveform and travel time shown by the seismograms from the aftershocks of the whittier narrows california earthquake could be accounted for when a laterally varying structure was used

modeling of energy amplification recorded within greater los angeles using irregular structure we have investigated energy amplification observed within greater los angeles basin by analyzing regional waveforms recorded from several nevada test site nts nuclear explosions although the stations are located nearly at the same azimuth distances ranging from 350 to 400 km the seismograms recorded in compton the central part of the basin long beach the southern edge of the basin and downtown los angeles are remarkably different even for a common explosion following the onset of l g waves the long beach sites have recorded surface waves for more than 100 sec from one explosion the sites within downtown los angeles have recorded seismograms with strong 3 sec surface waves these waves are not observed on the seismograms recorded in the neighboring hard rock site california institute of technology cit station thus they must have been generated by local wave guides numerically we modeled these 3 sec waves by convolving the cit seismogram with the response of a sedimentary strata dipping gently about 6 from cit toward downtown we also examined the irregular basin effect by analyzing the variation of cumulative temporal energy across the basin relative to the energy recorded at cit from the same explosion variation up to a factor of 30 was observed to model the energy variation that is caused by extended surface waves in the long beach area we used numerically simulated site transfer functions stf from a nne ssw oriented two dimensional basin structure extending from montebello to palos verdes that included low velocity sedimentary material in the uppermost layers these stfs were convolved with the cit seismogram recorded from the mast explosion to simulate elongated duration of surface waves we introduced in the upper sedimentary structure some discontinuous microbasin structures of varying size each microbasin delaying the seismic waves propagating through them consequently the surface reflected phases through these structures are delayed and reflected into the upper medium by the underlying interfaces this mechanism helps delayed energy to appear at a later time and result in a longer time duration at sites located at southern edge of the basin

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

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

an application of seismic tomography to basin focusing of seismic waves and northridge earthquake damage 1 damage during the 1994 northridge earthquake was anomalously concentrated in santa monica california aftershock studies suggest that the basin substructure caused localized amplification of seismic waves by focusing the energy however the geometry and velocities of the geological structure are uncertain relying on extrapolation from widely separated boreholes we present results of a tomographic study which includes inverting over 30 000 travel times from a seismic array experiment on a 194 station array and a 10 km long vibroseis survey we find that the damage zone is located above a subbasin several kilometers deep where low velocity sediments of the northeastern los angeles basin are separated from higher velocity rocks of the santa monica mountains by the steeply dipping santa monica potrero canyon fault zone three dimensional ray tracing through this structure shows that focusing of rays from the northridge earthquake epicenter occurs in the region where damage was greatest and where aftershock amplitudes are magnified we also find that near surface p velocities while low in the santa monica basin do not correlate with the zone of damage in a related study baher et al 2002 site effects determined from coda waves are better correlated with damage suggesting that compliant site effects were also important in localizing the damage distribution we conclude that the anomalous damage was caused by a combination of site and basin focusing effects such geometries may be quite common in earthquake zones worldwide where similar results have been observed

dependency of near field ground motions on the structural maturity of the ruptured faults little work has been undertaken to examine the role of specific long term fault properties on earthquake ground motions here we empirically examine the influence of the structural maturity of faults on the strong ground motions generated by the rupture of these faults and we compare the influence of fault maturity to that of other source properties slip mode and blind versus surface rupturing we analyze the near field ground motions recorded at rock sites for 28 large mw 5 6 7 8 crustal earthquakes of various slip modes the structural maturity of the faults broken by those earthquakes is classified into three classes mature intermediate and immature based on the combined knowledge of the age slip rate cumulative slip and length of the faults we compare the recorded ground motions to the empirical prediction equation of boore et al 1997 at all frequencies earthquakes on immature faults produce ground motions 1 5 times larger than those generated by earthquakes on mature faults the fault maturity appears to be associated with larger differences in ground motion amplitude than the style of faulting factor of 1 35 between reverse and strike slip earthquakes and the surface rupture occurrence factor of 1 2 between blind and surface rupturing earthquakes however the slip mode and the fault maturity are dependent parameters and we suggest that the effect of slip mode may only be apparent actually resulting from the maturity control we conclude that the structural maturity of faults is an important parameter that should be considered in seismic hazard assessment

shear wave splitting observations and implications on stress regimes in the los angeles basin california a systematic analysis of three component seismograms recorded at 15 stations from earthquakes occurring at depths of 5 to 18 km beneath the los angeles basin and adjacent areas during the period between 1988 and 1994 shows 20 to 160 ms shear wave splitting shallow events exhibit little splitting while deeper events show progressively greater splitting with depth the preferred polarization direction of the fast shear wave is nearly n s consistent with the direction of the regional maximum horizontal compressive stress but independent of the azimuth between the event and station we interpret that the shear wave splitting is caused by fluid filled crustal microcracks and macrofractures aligned in the n s direction the shear wave splitting observations of 2 8 to 7 8 ms km can be explained in terms of an anisotropic crust containing vertical cracks with the apparent crack density of 0 023 0 08 on a regional basis the crack density may vary from station to station but we find that the apparent crack density in the strike slip region of the newport inglewood fault and the whittier fault is higher than in the reverse thrusting santa monica mountains and palos verdes hills no systematic change of shear wave splitting in the los angeles basin is found in this study

deformation rates across the placerita northridge mw 6 7 aftershock zone and hopper canyon segments of the western transverse ranges deformation belt abstract aftershocks of the 1994 northridge m w 6 7 earthquake provide insights into the geometry of the seismic source faults in the san fernando valley and the east ventura basin and allow the calculation of deformation rates for the region the northridge thrust and santa susana faults dip in opposite directions with the northridge thrust entirely beneath the santa susana fault these opposing reverse faults interact resulting in a folded active santa susana fault and uplift in the footwall block of that reverse fault two balanced cross sections suggest thick skinned deformation of the western transverse ranges the western section across the modelo lobe segment of the north dipping san cayetano fault and the easternmost surface trace of the south dipping oak ridge fault is west of any aftershocks of the northridge earthquake and has been termed the hopper canyon segment of the deformation belt structural modeling predicts a dip of 46 s on the oak ridge fault at seismogenic depths horizontal shortening rates are calculated by adding the products of the dip slip displacements and the cosines of the dips of both faults the eastern cross section shows the northridge mainshock with a 45 south dipping nodal plane at a depth of 18 km aftershocks reach a depth of 20 km in a thin skinned paradigm a hinge should occur at the surface near the santa monica mountains due to rocks moving from a decollement at the brittle plastic transition and changing dip as they move up the ramp no hinge of that magnitude occurs there calculation of horizontal short ening rates across this part of the western transverse ranges must take into account the displacement on both the northridge thrust eastern extension of the oak ridge fault and the santa susana fault placerita segment horizontal shortening rates are 8 2 2 4 mm yr across the modelo lobe segment of the san cayetano fault and the oak ridge fault and 5 7 2 5 mm yr across the northridge thrust and the santa susana fault these rates are consistent with those based on tectonic geodesy using gps dip slip displacement rates on the faults are 1 7 mm yr for the northridge thrust since 2 3 ma 4 1 0 4 mm yr for the oak ridge fault since 500 ka 5 9 3 9 3 8 mm yr for the santa susana fault since 600 to 2300 ka and 7 4 3 0 mm yr for the modelo lobe segment of the san cayetano fault since 500 ka this indicates that the slip rates on the north dipping the eastern san cayetano and the santa susana faults are comparable but the slip rate on the south dipping faults decreases eastward the slip rate on the oak ridge fault in the ventura basin is more than double that of the northridge thrust

3d simulations of multi segment thrust fault rupture thrust faults such as those that underlie the los angeles basin are typically segmented by tear faults that offset the thrust fault segments we perform 3d finite difference simulations of earthquake rupture to evaluate the effectiveness of these offsets in retarding rupture the simulations include orthogonal intersecting faults to model the interaction of the thrust fault segments with the tear fault for reasonable assumptions for fault segment length strength and stress drop rupture can jump offsets of up to 2 km if a tear fault is present consistent with observations of well studied thrust earthquakes absent a tear fault the maximum offset that can be breached is an order of magnitude smaller

constraining the kinematics of metropolitan los angeles faults with a slip partitioning model due to the limited resolution at depth of geodetic and other geophysical data the geometry and the loading rate of the ramp decollement faults below the metropolitan los angeles are poorly understood here we complement these data by assuming conservation of motion across the big bend of the san andreas fault using a bayesian approach we constrain the geometry of the ramp decollement system from the mojave block to los angeles and propose a partitioning of the convergence with 25 5 0 5 mm yr and 3 1 0 6 mm yr of strike slip motion along the san andreas fault and the whittier fault with 2 7 0 9 mm yr and 2 5 1 0 mm yr of updip movement along the sierra madre and the puente hills thrusts incorporating conservation of motion in geodetic models of strain accumulation reduces the number of free parameters and constitutes a useful methodology to estimate the tectonic loading and seismic potential of buried fault networks

shallow seismic imaging of folds above the puente hills blind thrust fault los angeles california 1 high resolution seismic reflection profiles image discrete folds in the shallow subsurface 600 m above two segments of the puente hills blind thrust fault system los angeles basin california the profiles demonstrate late quaternary activity at the fault tip precisely locate the axial surfaces of folds within the upper 100 m and constrain the geometry and kinematics of recent folding the santa fe springs segment of the puente hills fault zone shows an upward narrowing kink band with an active anticlinal axial surface consistent with fault bend folding above an active thrust ramp the coyote hills segment shows an active synclinal axial surface that coincides with the base of a 9 m high scarp consistent with tip line folding or the presence of a backthrust the seismic profiles pinpoint targets for future geologic work to constrain slip rates and ages of past events on this important fault system

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

simulations of ground motion in the los angeles basin based upon the spectral element method we use the spectral element method to simulate ground motion generated by two recent and well recorded small earthquakes in the los angeles basin simulations are performed using a new sedimentary basin model that is constrained by hundreds of petroleum industry well logs and more than 20 000 km of seismic reflection profiles the numerical simulations account for 3d variations of seismic wave speeds and density topography and bathymetry and attenuation simulations for the 9 september 2001 m w 4 2 hollywood earthquake and the 3 september 2002 m w 4 2 yorba linda earthquake demonstrate that the combination of a detailed sedimentary basin model and an accurate numerical technique facilitates the simulation of ground motion at periods of 2 sec and longer inside the basin model and 6 sec and longer in the regional model peak ground displacement velocity and acceleration maps illustrate that significant amplification occurs in the basin

evidence for large earthquakes in metropolitan los angeles the sierra madre fault along the southern flank of the san gabriel mountains in the los angeles region has failed in magnitude 7 2 to 7 6 events at least twice in the past 15 000 years restoration of slip on the fault indicated a minimum of about 4 0 meters of slip from the most recent earthquake and suggests a total cumulative slip of about 10 5 meters for the past two prehistoric earthquakes large surface displacements and strong ground motions resulting from greater than magnitude 7 earthquakes within the los angeles region are not yet considered in most seismic hazard and risk assessments

three dimensional seismo tectonic imaging an example from the southern california transverse ranges abstract we present a new technique to image deformation 3d seismo tectonic imaging which juxtaposes instantaneous and accumulated deformation by combining earthquake and structural geology data we test the technique using three independent data sets we find concordance between the predicted finite deformation field and related instantaneous deformation surfaces inferred from aftershock and background seismicity data as well as structure imaged by seismic reflection data these results indicate that 1 the 1994 northridge event ruptured a transverse high angle reverse fault linking two ew striking listric thrust systems the pico and oak ridge the rupture terminated on the thrusts and was capped by a north dipping forward breaking imbricate of the santa susana fault 2 the los angeles seismic zone is a region of thrusting associated with the development of a new mountain front forming south of the present one represented by the santa monica anticline the thrust is rooted on a mid crustal ramp forming beneath the los angeles basin we conclude that seismo tectonic imaging by linking earthquake data to an empirically based quantitative description of the finite deformation field provides a new and fruitful method for integrating geological and seismological data

recent plate motions and crustal deformation reports by u s workers on geodetic measurements of recent plate motions or crustal deformation published in 1987 1990 are reviewed the review begins with global plate motions proceeds through plate boundaries in california alaska and the pacific northwest and finishes with volcanic phenomena monument stability and longevity and gps relative position measurements 184 refs

3d effects of strain vs velocity weakening on deformation patterns in accretionary wedges abstract shear zones are weaker than surrounding rocks whether weakening depends on accumulated strain or strain rate is debated we used a three dimensional numerical code with a visco plastic brittle rheology to investigate the influence of strain and strain rate weakening often referred to as velocity weakening on the evolution of thin skinned fold and thrust belts two model setups are used i a uniform setup to recognize the effects of each weakening mode on the structural evolution and dynamics of fold and thrust belts and ii a dual setup with two adjacent domains of decollement strength to investigate the structural response of laterally varying systems strain weakening and velocity weakening lead to remarkably different structural patterns in particular strain weakening favours out of sequence shear zones whilst some amount of finite plastic strain has to be achieved before the structural development initiates weakening in contrast velocity weakening starts with the onset of high strain rate bands and does not favour out of sequence deformation we also tested separately the influences of shortening velocity cover sequence thickness and weakening style and amount on the structural evolution of the dual systems the shortening velocity has a minor effect in strain weakened models velocity weakening and a thinner cover sequence enhance the occurrence of strike slip shear zones we conclude that the weakening mode strongly influences the fault patterns and the dynamics of thrust wedges inversely natural fault patterns are keys to discriminate weakening processes

coseismic folding earthquake recurrence and the 1987 source mechanism at whittier narrows los angeles basin california static deformation associated with the october 1 1987 whittier narrows california m 6 0 earthquake was detected by geodetic elevation changes the earthquake uplifted a 1 5 km high quaternary fold the santa monica mountains anticlinorium by 50 mm but caused no fault rupture at the ground surface this suggests that folding and faulting of the los angeles basin sediments are coincident and continuing after correction for surveying errors and nontectonic subsidence we model the 214 geodetic observations with a simple dislocation in an elastic half space a thrust fault with reverse slip of 1 1 0 3 m and dipping 30 4 n with an upper edge at a depth of 12 1 km and a lower edge at 17 1 km fits the geodetic data best and is consistent with the main shock hypocenter fault plane solution and initial aftershock distribution the upper limit on the static stress drop is 17 5 5 0 mpa 175 50 bar the lower limit on the geodetic moment is 1 0 0 2 1025 dyne cm in accord with seismic estimates indicating that most of the slip took place during seismic rupture if earthquakes of m 6 characterize the blind thrust fault on which the earthquake occurred we estimate a 200 year repeat time at whittier narrows and a 5 18 year rate of earthquake occurrence within a 150 km long band in the northern los angeles basin and santa monica bay to the west the 25 year rate of historical occurrence since 1860 is less than this prediction the deficit in moment release implies that either vigorous aseismic slip or infrequent larger earthquakes occur here

using vertical rock uplift patterns to constrain the three dimensional fault configuration in the los angeles basin comparison of geologic uplift patterns with results of three dimensional mechanical models provides constraints on the fault geometry compiled by the south ern california earthquake center community fault model in the northern los angeles basin california the modeled uplift matches well the geologic pattern of uplift as sociated with the santa fe springs and coyote hills segments of the puente hills thrust fault but does not match structures to the west of the san gabriel river to better match the geologic patterns in this area alternative fault configurations were tested the best match to geologic uplift is attained with a model incorporating 1 a steep blind thrust fault at the location of the los angeles segment of the puente hills thrust system following interpretations of the las cienegas fault geometry at this location 2 removal of an inferred linking fault between the raymond and holly wood faults and 3 lateral continuation of the lower elysian park fault a blind low angle detachment at 10 km depth along strike to the northwest these geometric revisions alter the connectivity of northern los angeles basin faults and significantly improve the match of model uplift pattern to geologic data model results suggest that fault connectivity may be more important in governing fault slip rate than are fault dip and fault area the preferred model alters slip rates by 0 2 mm yr for the upper elysian park hollywood lower elysian park raymond sierra madre west and verdugo faults additionally the preferred model alters the surface area of several faults in the northern los angeles basin such as the puente hills thrust and the lower elysian park fault which may have important implications for seismic hazard assess ment in the northern los angeles basin

structure of the cuyama valley caliente range and carrizo plain and its significance to the structural style of the southern coast ranges and western transverse ranges a concerted exploration program in the cuyama area by arco during the last 10 years employing surface mapping deep drilling high quality seismic profiles biostratigraphic analysis and balanced cross sections has revealed several significant structural characteristics 1 thrust faults such as the morales and white rock flatten under the caliente range and merge into a single system with up to 14 km of slip this system intersects the san andreas fault at a high angle and may detach the san andreas fault at depth from 7 to 8 km 2 integration of surface and subsurface mapping shows that these thrust faults have little or no component of strike slip and are younger than 3 ma 3 major surface folds such as the caliente mountain anticline are due to thrust faults ramping upsection and producing fault bend and fault propagation folds 4 the young thrust system has cut across a late oligocene to early miocene extensional fault controlled basin and both tectonically transported and overthrust portions of this older basin these structural characteristics strongly suggest that the structural style of the cuyama area and possibly adjacent areas in the southern coast ranges and western transverse ranges is that of a fold and thrust belt and is not relatedmore to wrench tectonics less

seismotectonics of the newport inglewood fault zone in the los angeles basin southern california the newport inglewood fault zone nif strikes northwest along the western margin of the los angeles basin in southern california the seismicity 1973 to 1985 of m l 2 5 that occurred within a 20 km wide rectangle centered on the nif extending from the santa monica fault in the north to newport beach in the south is analyzed a simultaneous full inversion scheme velest is used to invert for hypocentral parameters two velocity models and a set of station delays arrival time data from three quarry blasts are included to stabilize the inversion the first velocity model applies to stations located along the rim and outside the los angeles basin and is well resolved it is almost identical to the starting model which is the model routinely used by the cit usgs southern california seismic network for locating local earthquakes the second velocity model applies to stations located within the los angeles basin it shows significantly lower velocities down to depths of 12 to 16 km which is consistent with basement of catalina schist below the sediments in the western los angeles basin the distribution of relocated hypocenters shows an improved correspondence to mapped surface traces of late quaternary fault segments of the nif a diffuse trend of seismicity is observed along the inglewood fault from the dominguez hills across the baldwin hills to the santa monica fault in the north the seismicity adjacent to long beach however is offset 4 to 5 km to the east near the trace of the subsurface los alamitos fault the depth distribution of earthquakes along the nif shows clustering from 6 to 11 km depth which is similar to average seismogenic depths in southern california thirty nine single event focal mechanisms of small earthquakes 1977 to 1985 show mostly strike slip faulting with some reverse faulting along the north segment north of dominguez hills and some normal faulting along the south segment south of dominguez hills to newport beach the results of an inversion of the focal mechanism data for orientations of the principal stress axes and their relative magnitudes indicate that the minimum principal stress is vertical along the north segment while the intermediate stress is vertical along the south segment the maximum principal stress axis is oriented 10 to 25 east of north reverse faulting along the north segment indicates that a transition zone of mostly compressive deformation exists between the los angeles block and the central transverse ranges

the mission link between the hayward and calaveras faults the left stepover between the calaveras and hayward faults gives rise to compressive strain that is accommodated by vertical deformation producing the highest hills bordering the east side of san francisco bay monument peak to mission peak the projection of the active trace of the hayward fault is separated 3 5 km from the junction of the northern and southern segments of the calaveras fault at calaveras reservoir for pure strike slip on the hayward and calaveras faults the stepover region is compressed in area at a rate given by the separation times the long term slip rate of 9 mm yr on the hayward fault assuming a seismogenic depth of 10 km the horizontal compression accumulated over 150 years is equivalent to a dip slip seismic moment of 2 8 1018 n m this moment if released in a single event would generate an earthquake of magnitude 6 3 due to dip slip alone dip slip may occur on buried faults throughout the stepover region the base of the southwest face of monument peak suggests a buried thrust or reverse fault dipping northeast the prominent scarp on mission peak has the same strike and sense of slip current microearthquakes between the calaveras and hayward faults align parallel to the mission fault but show only horizontal slip on a vertical fault plane a dipping fault is needed to satisfy the geometric constraint of the convergence therefore the mission fault zone must consist of two or more fault surfaces inclined at different dip angles as is observed in the san andreas fault zone in the loma prieta area horizontal slip is building up stress on the inclined fault which is currently locked growing compressive stress will increasingly impede horizontal slip until a thrust or reverse slip event occurs allowing then more horizontal slip to be transferred from the calaveras to the hayward fault the dip slip fault might be the unstable element governing slip in the larger system

kinematics of present crust and mantle flow in southern california in the southern california region published geologic studies of prehistoric fault offsets yield at least 81 rates of slip covering various periods of time chiefly since the opening of the gulf of california these rates together with the known geometries of active faults are used to constrain a hand fit block model for present horizontal velocity of the crust the assumptions employed are that these rates apply to the present that the blocks are internally rigid down to the greatest depth of earthquakes and that the over all pacific north american plate motion is correctly given by global models the resulting model fits 67 data within their uncertainties 10 more with a small discrepancy 1 mm yr and fails to accommodate only 4 data 2 of which are inconsistent with other data nominal precision is good 3 mm yr but accuracy and uniqueness are both suspect because many block boundaries are questionable despite its many local defects this model suggests four reliable regional conclusions first the rate of shortening on the south side of the transverse ranges varies from 6 to 14 mm yr increasing eastward which implies an unrecognized seismic hazard unless the motion is taken up by folding or fault creep second the pattern of block rotation rates is quite different from the pattern of rotations shown by tertiary paleomagnetic declinations which implies some major pliocene reorganization of blocks third the great bend left step of the plate boundary fault system in the transverse ranges appears to be straightening gradually by a 3 m y clockwise rotation relative to both plates finally if it is assumed that plate shortening has been occurring at this bend since the early pliocene and that crust and upper mantle velocities are similar then this model predicts mantle lithosphere downwelling to at least 250 km depth beneath the transverse ranges this prediction is consistent with upper mantle seismic velocity structure

mechanical fault interaction within the los angeles basin a two dimensional analysis using mechanical efficiency 1 mechanical models examine deformation within eight different structural cross sections proposed by davis et al 1989 and shaw and suppe 1996 along a northeast southwest transect across the los angeles basin california horizontal contraction of the models constrained by geodetic measurements yields varying dip slip rates along frictionally sliding faults within the different cross sections mechanical efficiency analysis using effective stiffness and strain energy density assesses the overall fault system deformation as well as the partitioning of work between fault slip and host rock strain the cross section interpreted by shaw and suppe 1996 has the best fit to paleoseismically determined slip rates and the greatest mechanical efficiency greatest proportion of work toward fault slip however this model produces excessive reverse slip along the newport inglewood fault a modified fault configuration with a wedge or blind puente hills thrust fault rather than a ramp detachment configuration better matches the paleoseismic data with slightly lower mechanical efficiency slip rates in the mechanical models based on interpretations of shaw and suppe 1996 have much closer match to the geologically determined rates than those estimated from kinematic models this difference is due to 1 differing time spans of slip rate estimates and 2 deformable rather than rigid host rock in the mechanical models the mechanical efficiency analysis provides quantitative indicators of overall fault system deformation including the cumulative effect of interaction between individual faults assessment of effective stiffness and strain energy density furthers our understanding of two dimensional fault interactions in the los angeles basin and offers great potential for future applications

the wilmington blind thrust fault an active concealed earthquake source beneath los angeles californiathe wilmington blind thrust fault an active concealed earthquake source beneath los angeles california

denudation vertical crustal movements and sedimentary basin infill denudation and vertical crustal movements are linked through the isostatic equation for conservation of lithospheric mass recent empirical results from a wide range of sources are presented on the rates of these processes denudation alone can cause non tectonic isostatic surface uplift of the right magnitude predicted by theory a case is made following ahnert for a single linear relationship although with much scatter between denudation rate and mean drainage basin relief for large basins some recent determinations of denudational time constants have neglected to include tectonic and isostatic terms which oppose the reduction of relief by denudation application of diffusional transport models to determine the rate of sediment deposition are severely limited by inherent one dimensionality and neglect of suspended and washload contributions basin infill sequences may be used to determine past sediment transfer coefficients from a hinterland but problems arise in the apportionment of the various hinterland fluxes that occur in real world three dimensional basins sediment yields in smaller half graben and growth fold basins must reflect the magnitude of drainage basins and the complex local controls upon sediment yields examples are given from extensional basins in the western usa and greece the length of tectonic relief produced in tilt blocks and growth folds will control the area of drainage basin developed through the fractal form of hack s law and hence the magnitude of deposition in sedimentary basins adjoining tectonic uplands

puente hills blind thrust system los angeles california we describe the three dimensional geometry and quaternary slip history of the puente hills blind thrust system pht using seismic reflection profiles petroleum well data and precisely located seismicity the pht generated the 1987 whittier narrows moment magnitude m w 6 0 earthquake and extends for more than 40 km along strike beneath the northern los angeles basin the pht comprises three north dipping ramp segments that are overlain by contractional fault related folds based on an analysis of these folds we produce quaternary slip profiles along each ramp segment the fault geometry and slip patterns indicate that segments of the pht are related by soft linkage boundaries where the fault ramps are en echelon and displacements are gradually transferred from one segment to the next average quaternary slip rates on the ramp segments range from 0 44 to 1 7 mm yr with preferred rates between 0 62 and 1 28 mm yr using empirical relations among rupture area magnitude and coseismic displacement we estimate the magnitude and frequency of single m w 6 5 6 6 and multisegment m w 7 1 rupture scenarios for the pht manuscript received 16 november 2001

the 1987 whittier narrows earthquake in the los angeles metropolitan area california the whittier narrows earthquake sequence local magnitude m l 5 9 which caused over 358 million damage indicates that assessments of earthquake hazards in the los angeles metropolitan area may be underestimated the sequence ruptured a previously unidentified thrust fault that may be part of a large system of thrust faults that extends across the entire east west length of the northern margin of the los angeles basin peak horizontal accelerations from the main shock which were measured at ground level and in structures were as high as 0 6g where g is the acceleration of gravity at sea level within 50 kilometers of the epicenter the distribution of the modified mercalli intensity vii reflects a broad north south elongated zone of damage that is approximately centered on the main shock epicenter

the 3 december 1988 pasadena california earthquake evidence for strike slip motion on the raymond fault the pasadena earthquake m l 4 9 occurred on 3 december 1988 at a depth of 16 km the hypocenters of the earthquake and its aftershocks define a east northeast striking steeply northwest dipping surface that projects up to the active surficial trace of the raymond fault one of the nodal planes of the focal mechanism of the earthquake parallels the raymond fault with left lateral strike slip movement on that plane and is consistent with geomorphic and paleoseismic evidence that the raymond fault is dominantly a left lateral strike slip fault the existence of a component of sinistral slip along the raymond fault had been suspected prior to the earthquake but the northward dip of the fault and the prominent scarp along the western portion of its trace had led most workers to conclude that slip along the fault was dominantly reverse

estimated ground motions in los angeles due to mw 7 earthquake on the elysian thrust fault abstract the elysian thrust fault has been identified as a blind thrust fault davis et al 1989 hauksson and jones 1989 representing a potentially serious seismic hazard to the metropolitan los angeles and its neighboring areas we have simulated time histories peak ground accelerations and their uncertainties using a semi empirical method for a m w 7 0 earthquake on the elysian thrust fault using a flat layered crustal structure the accelerograms from the 4 october 1987 whittier narrows aftershock 10h 50m m l 5 3 are used to represent the source functions of each subfault on the fault surface to account for the velocity variation in the surface sediments we compared simulated ground motions using two separate shearwave velocities 0 6 km sec and 0 9 km sec respectively in the top layer of the crustal model the use of such a simple crustal model is validated by modeling the accelerograms recorded during the 1 october 1987 whittier narrows mainshock m l 6 0 the duration and the relative frequency content observed on these accelerograms are successfully modeled at some stations the simulated peak accelerations agree well with the observed values however at some sites the simulated and observed values differ by a factor of 2 to 3 the variation is attributed to a laterally varying crustal structure that is significant for small earthquakes additional site specific study is needed for an improved prediction of the observed peak accelerations for large magnitude earthquakes the near source peak ground acceleration appears to be controlled by the source receiver geometry relative to the fault as well as the location of asperities on the fault surface this is validated by simulating the peak acceleration data of the 1989 loma prieta earthquake recorded within 30 km of the source for our simulations on the elysian thrust fault an additional component is added to the uncertainty by analyzing several asperity models finally an analytical representation is given to the simulated average peak horizontal ground accelerations y r which is expressed by ln y r 5 38 0 085 2 09 0 0268 ln r 8 0 0 343 in the range of 7 5 r 35 km r is the closest distance to the seismogenic rupture zone this functional form predicts a rapid fall off rate for the simulated ground motions compared with the fall off rate predicted by the other published empirical attenuation relations we use the finite difference method to investigate the effects of irregular structure on ground motions resulting from point sources on the elysian park fault the response computed at several depths beneath the basin suggests that the response is dominated by the initial direct arrivals for sources located interior to the basin when the receivers are located at one end of the basin and the seismic sources are located at the opposite edge the characteristic features on the seismograms are the long durations caused by the trapped energy within the basin the level of trapped energy decreases as the depth of the source increases thus the fault model becomes important in determining the level of peak ground motions and shaking duration

space geodetic measurement of crustal deformation in central and southern california 1984 1992 we estimate the velocity field in central and southern california using global positioning system gps observations from 1986 to 1992 and very long baseline interferometry vlbi observations from 1984 to 1991 our core network includes 12 gps sites spaced approximately 50 km apart mostly in the western transverse ranges and the coastal borderlands the precision and accuracy of the relative horizontal velocities estimated for these core stations are adequately described by a 95 confidence ellipse with a semiminor axis of approximately 2 mm yr oriented roughly north south and a semimajor axis of approximately 3 mm yr oriented east west

preliminary analysis of long period basin response in the los angeles region from the 1994 northridge earthquake long period 1 10 sec ground motions recorded in the los angeles basin region during the northridge earthquake show complex waveforms extended durations and multiple sets of arrivals which cannot be attributed solely to source processes or wave propagation within a plane layered medium these features suggest a strong interaction between the propagating seismic wave field and the laterally varying subsurface geologic structure of this region recorded motions at a hard rock site in the santa monica mountains scrs are smaller by nearly a factor of three in peak velocity compared to recordings at more distant sites in the adjacent northwest portion of the los angeles basin in addition although the rock site recording has a relatively simple wave shape the basin site recordings are dominated by late arriving large amplitude pulses of energy we interpret these arrivals to be surface waves which are generated by body waves interacting with the thickening margin of the basin a preliminary modeling analysis of these data indicates that a combination of both large scale deep basin and small scale shallow micro basin structures are needed to explain the observed responses

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

satellite interferometric observations of displacements associated with seasonal groundwater in the los angeles basin 1 the newport inglewood fault zone nifz displays interferometric synthetic aperture radar sar phase features along most of its length having amplitudes of up to 60 mm however interpretation in terms of right lateral shallow slip along the fault fails to match the range of geologic estimates of slip recently bawden et al 2001 proposed that these phase features as well as a broader deformation pattern in the los angeles basin are due to vertical motion related to annual variations in the elevation of the water table we confirm this hypothesis through the analysis of a longer span of data consisting of 26 sar images collected by the ers 1 and ers 2 spacecraft between june 1992 and june 2000 moreover we use continuous gps measurements from 1995 to the present to establish the amplitude and phase of the vertical deformation the los angeles basin becomes most inflated one quarter of the way through the year which is consistent with water table measurements as well as with the end of the rainy season when the aquifer should be at a maximum the spatial pattern of the amplitude of the annual signal derived from continuous gps measurements is consistent with the shape of the interferometric fringes gps sites both near the nifz and in a 20 by 40 km zone within the basin also show significant n s annual variations that may be related to the differential expansion across the fault since these horizontal signals have peak to trough amplitudes of 6 mm they mask the smaller tectonic signals and need to be taken into account when interpreting gps time series of site position moreover since the groundwater signal appears to have a long term vertical trend which varies in sign depending on location it will be difficult to distinguish interseismic tectonic slip along the nifz and within the affected areas in the basin

recent crustal deformation in southern california deduced from the restoration of folded and faulted strata a rigid element method of restoration unfold is used to restore competent folded and faulted layers of the ventura and los angeles basins to their initial horizontal state comparison of initial undeformed state with present deformed state allows one to estimate both the finite crustal deformation and its associated horizontal displacement field relative to an arbitrary fixed line including data from the santa barbara channel basin the total finite displacement field for the western transverse ranges and vicinity within the pacific plate is inferred from this map restoration and is modeled as a double fan closure this model implies a 10 clockwise rotation of the northeen boundary of the western transverse ranges and a 5 counterclockwise rotation of its northeast boundary lateral variation of the deformation reveals the heterogeneity of the subsurface deformation most of the major thrusts appear to initiate as en echelon structures along the left lateral northern margin and the right lateral northeastern margin of the studied area the resulting deformation and displacement values closely match those derived by other geological methods section balancing techniques or fault slip measurements and by geophysical methods geodetic paleomagnetic and focal mechanism data map restoration thus is a method that can independently quantify both local and regional deformation including folds and faults this method also reveals the zones where problems of compatibility appear with the available geological and geophysical data and thus where the next studies might be focused

style and rate of holocene slip palos verdes fault southern california recent studies of the palos verdes fault in the los angeles outer harbor characterize the fault s holocene activity the studies included areal and local geophysical surveys as well as drilling sampling and dating of the subsurface materials the studies identify the fault s location in the shallow subsurface document evidence of holocene strike slip quantify dip slip versus sediment age and quantify the net slip where an early holocene paleochannel crosses the fault the slip rate for the palos verdes fault is constrained to between 2 7 and 3 mm yr for the past 7 8 to 8 ka because the new evidence shows that the palos verdes fault has one of the highest slip rates in the los angeles basin the studies provide significant new insight into the regional tectonics and the seismic risk of the los angeles region using this rate of slip and segmentation models for the fault the coastal los angeles region can expect an mw 7 to 7 2 event about every 400 to 900 years from this potential seismic source

the 1991 sierra madre earthquake sequence in southern california seismological and tectonic analysis the m l 5 8 sierra madre earthquake of 28 june 1991 occurred at a depth of 12 km under the san gabriel mountains of the central transverse ranges since at least 1932 this region had been quiescent for m 3 the mainshock focal mechanism derived from first motion polarities exhibited almost pure thrust faulting with a rake of 82 on a plane striking n62 e and dipping 50 to the north the event appears to have occurred on the clamshell sawpit fault a splay of the sierra madre fault zone the aftershock sequence following the mainshock occurred at a depth of 9 to 14 km and was deficient in small earthquakes having a b value of 0 6 twenty nine single event focal mechanisms were determined for aftershocks of m 1 5 the 4 km long segment of the clamshell sawpit fault that may have ruptured in the mainshock is outlined by several thrust focal mechanisms with an east northeast striking fault plane dipping to the north to the west several thrust aftershocks with east striking nodal planes suggest some complexity in the aftershock faulting such as a curved rupture surface in addition several strike slip and normal faulting events occurred along the edges of the mainshock fault plane indicating secondary tear faulting the tectonic stress field driving the coexisting left lateral strike slip and thrust faults in the northern los angeles basin is north south horizontal compression with vertical intermediate or minimum principal stress axis

a kinematic model of southern california we propose a kinematic model for southern california based on late quaternary slip rates and orientations of major faults in the region internally consistent motions are determined assuming that these faults bound rigid blocks relative to north america most of california west of the san andreas fault is moving parallel to the san andreas fault through the transverse ranges and not parallel to the motion of the pacific plate this is accomplished by counterclockwise rotation of california south of the san andreas fault and by the westward movement of central california north of the gar lock fault the velocities of the blocks are calculated along several paths in southern california that begin in the mojave desert and end off the california coast a path that crosses the western transverse ranges accumulates the accepted relative north america pacific plate velocity whereas paths to the north and south result in a significant missing component of motion this implies the existence of a zone of active deformation in southern california that is interpreted to include the western transverse ranges and northwest trending predominately strike slip faults close to the coast both north and south of the transverse ranges strain on this system accounts for about a third of the total north america pacific plate motion

an integrated 3d velocity inversion joint hypocentral determination relocation analysis of events in the northridge area abstract a subset of 3371 events recorded in the northridge area by the southern california seismic network during january to april 1994 was relocated with the joint hypocentral determination jhd technique this analysis showed two unexpected results a the jhd locations are shifted about 3 9 km on average in a northwest direction with respect to the locations determined using a single event location sel program and b the station corrections vary between 0 55 and 1 26 sec a rather large range in addition the jhd locations are less scattered than the sel locations for each station the weighted average of the arrival time residuals obtained when the events are located with the sel program which does not apply distance or error weighting are generally smaller than the corresponding jhd corrections the locations determined with sel and using the weighted average residuals as station corrections do not differ much from the sel locations but on average the rms residuals become as small as those corresponding to the jhd locations as the magnitude of the station corrections indicates the presence of large lateral velocity variations a 3d velocity model for the area was determined using the arrival times of 1012 events recorded by at least 17 stations the initial velocity model was that used routinely by the southern california earthquake center the first two layers 5 5 and 10 5 km thick were subdivided into 100 blocks each 12 12 km these layers show a pronounced low velocity anomaly 24 and 16 respectively immediately to the northwest of the epicentral area this low velocity zone coincides with the west ventura basin another pronounced low velocity zone to the southeast of the epicentral area reflects the presence of the los angeles basin the locations obtained with the 3d velocity model are consistently to the southeast of the jhd locations 2 4 km on average to establish the effect of these pronounced lateral velocity variations on the sel and jhd locations synthetic travel times were analyzed the synthetic times were generated for event locations determined by jhd shifted by various amounts and the 3d velocity model and were subsequently treated as the actual data the most important result of this analysis is that the jhd locations are affected by a quasi systematic shift in a northwest direction up to about 2 7 km on average depending on the initial shift but that the relative locations are well preserved therefore both the velocity inversion of the actual data and the analysis of the synthetic data indicate that the jhd locations determined for the actual data are quasi systematically mislocated to account for this mislocation an overall shift of 2 5 km to the southeast was applied to all the jhd locations one of the most important implications of the shifted locations is the possibility that the northeasterly dipping santa susana fault to the northwest of the epicentral area was seismically active during the aftershock sequence this feature is more diffuse in other published locations

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

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

earthquake hazards of active blind thrust faults under the central los angeles basin california we document several blind thrust faults under the los angeles basin that if active and seismogenic are capable of generating large earthquakes m 6 3 to 7 3 pliocene to quaternary growth folds imaged in seismic reflection profiles record the existence size and slip rates of these blind faults the growth structures have shapes characteristic of fault bend folds above blind thrusts as demonstrated by balanced kinematic models geologic cross sections and axial surface maps we interpret the compton los alamitos trend as a growth fold above the compton ramp which extends along strike from west los angeles to at least the santa ana river the compton thrust is part of a larger fault system including a decollement and ramps beneath the elysian park and palos verdes trends the cienegas and coyote hills growth folds overlie additional blind thrusts in the elysian park trend that are not closely linked to the compton ramp analysis of folded pliocene to quaternary strata yields slip rates of 1 4 0 4 mm yr on the compton thrust and 1 7 0 4 mm yr on a ramp beneath the elysian park trend assuming that slip is released in large earthquakes we estimate magnitudes of 6 3 to 6 8 for earthquakes on individual ramp segments based on geometric segment sizes derived from axial surface maps multiple segment ruptures could yield larger earthquakes m 6 9 to 7 3 relations among magnitude coseismic displacement and slip rate yield an average recurrence interval of 380 years for single segment earthquakes and a range of 400 to 1300 years for multiple segment events if these newly documented blind thrust faults are active they will contribute substantially to the seismic hazards in los angeles because of their locations directly beneath the metropolitan area

on a possible connection between three major earthquakes in california and oil production abstract the 1983 coalinga m 6 5 the 1985 kettleman north dome m 6 1 and the 1987 whittier narrows m 5 9 earthquake sequences each occurred beneath major producing oil fields and caused an aftershock sequence whose epicentral extent coincides approximately with that of the overlying oil field despite this coincidence a causal relationship between these sequences and oil production has been discounted because the earthquakes were located at depths of the order of 10 km whereas oil production is confined to the topmost several km moreover because all three oil fields coalinga kettleman north dome and montebello are situated on anticlines that have undergone recent uplift as a consequence of horizontal tectonic compression of the crust natural earthquakes are not unexpected a mechanical connection between oil production and the earthquake sequence is suggested however by the observation that in each case the total seismic deformation was just that required to offset the force imbalance caused by oil production specifically for each of the three sequences m 0 2 m c where m 0 is the sum of the seismic moments is the modulus of rigidity m is the mass of the fluid removed due to oil production is the fraction of the upper crust that is seismogenic and c is the average upper crustal density in essence net extraction of oil and water reduces slightly the average density of the upper crust causing an isostatic imbalance the ductile lower crust deforms in response to this imbalance thus increasing the load on the seismogenic layer which fails seismically to thicken the crust so as to restore static equilibrium locally accordingly earthquakes near the base of the upper crust may be an expected outcome of major oil production from growing anticlines irrespective of the depths of the producing formations

shallow structure of the tangshan fault zone unveiled by dense seismic array and horizontal to vertical spectral ratio method abstract the tangshan fault zone has been considered to be a potential seismogenic fault from the devastating 1976 ms 7 8 tangshan earthquake based on the dense seismic array with interstation distance of 1 km and length of 40 km hvsr horizontal to vertical spectral ratio curves at 37 stations are obtained with the micro tremor seismic waveforms and the two dimensional quaternary sedimentary structures cross the tangshan fault zone are imaged by frequency to depth conversion results suggest that two significant seismic impedance interfaces at 100 m and 300 800 m depth are clearly revealed respectively the interface at 100 m depth can be attributed to the strong contrasts between the unconsolidated sand clay and semi consolidated silt which suggests very small variations and is well consistent with the seismic reflection interface from shallow seismic reflection exploration while the deeper interface at 300 800 m depth can be attributed to the quaternary sedimentary basement which increases from 300 to 800 m from the west to the east and is also consistent with the results in previous studies rapid variations 200 m of the quaternary sedimentary basement depth just beneath the fault zone well agrees with spatial characteristics of the fault revealed by deep seismic reflection profiling our work suggests that the tangshan fault zone has been significantly ruptured and modified by strong earthquake activities since quaternary this study also suggests that the hvsr method with dense seismic array is an efficient effective non invasive and low cost method for investigating active faults in shallow sediments it is useful for probing the active faults buried by thick sediments in the densely populated urban settings where seismic investigation with explosive sources are too hazardous to carry out

active tectonics paleoseismology and seismic hazards of the hollywood fault northern los angeles basin california data from geotechnical boreholes and trenches in combination with geomorphologic mapping indicate that the hollywood fault is an oblique reverse left lateral fault that has undergone at least one surface rupturing earthquake during latest pleistocene to middle or late holocene time geomorphologic observations show that the fault extends for 14 km along the southern edge of the eastern santa monica mountains from the los angeles river westward through downtown hollywood to northwestern beverly hills where the locus of active deformation steps 1 2 km southward along the west beverly hills lineament to the santa monica fault rupture of the entire hollywood fault by itself could produce a m w 6 6 earthquake similar in size to the highly destructive 1994 northridge earthquake but even closer to more densely urbanized areas assuming a 0 35 mm yr minimum fault slip rate consistent with available geologic data we calculate an average maximum recurrence interval for such moderate events of 4000 yr although occurrence of such moderate events is consistent with the elapsed time since the poorly constrained age of the most recent surface rupture the data do not preclude a longer quiescent interval suggestive of larger earthquakes if earthquakes much larger than m w 6 6 occurred in the past we speculate that they may have been generated by the hollywood fault together with other faults in the transverse ranges southern boundary fault system

the 1987 whittier narrows california earthquake a metropolitan shock just 3 hours after the whittier narrows earthquake struck it became clear that a heretofore unseen geological structure was seismically active beneath metropolitan los angeles contrary to initial expectations of strike slip or oblique slip motion on the whittier fault whose north end abuts the aftershock zone the focal mechanism of the mainshock showed pure thrust faulting on a deep gently inclined surface hauksson et al 1988 this collection of nine research reports spans the spectrum of seismological geodetic and geological investigations carried out as a result of the whittier narrows earthquake although unseen the structure was not unforeseen namson 1987 had published a retrodeformable geologic cross section meaning that the sedimentary strata could be restored to their original depositional position 100 km to the west of the future earthquake epicenter in which blind or subsurface thrust faults were interpreted to be active beneath the folded southern transverse ranges working 25 km to the west hauksson 1987 had also found a surprising number of microearthquakes with thrust focal mechanisms south of the santa monica mountains another clue to a subsurface system of thrust faults finally davis 1987 had presented a preliminary cross section only 18 km to the west of whittier narrows that identified as fault b the thrust that would rupture later that year not only was the earthquake focus and its orientation compatible with the 10 15 km depth and north dipping orientation of davis proposed thrust but fault b appears to continue beneath the northern flank of the los angeles basin skirting within 5 km of downtown los angeles an area of dense commercial high rise building development these results are refined and extended by davis et al this issue

crustal deformation across and beyond the los angeles basin from geodetic measurements we combine 6 years of global positioning system gps data with 20 years of trilateration data and a century of triangulation taped distance and astronomic azimuth measurements to derive 66 interseismic station velocities in the greater los angeles region we interpolate the velocities to construct a regional strain rate map beyond the los angeles basin our results generally agree with the model proposed by the working group on california earthquake probabilities in 1995 important regional findings of this study are as follows 1 there is a significant n s convergence and e w extension about 0 22 and 0 17 0 05 strain yr respectively for the two components along the southern frontal fault system of the san gabriel mountains 2 the crustal deformation around the big bend of the san andreas fault saf cannot be explained solely by wrench style motion across the saf remaining motion could be part of a nw se extension which is the response to nfsw compression in the central transverse ranges region alternatively it could be caused by left lateral faulting on an oblique blind thrust beneath the san gabriel mountains 3 low strain rates axe found along the elsinore fault and newport inglewood 4 north south compression decreases from the raymond hill fault westward to the santa monica fault there is little east west extension along this fault system

oceanside and thirtymile bank blind thrusts implications for earthquake hazards in coastal southern california we define an active blind thrust system in offshore southern california that extends from los angeles south to the united states mexico international border these blind thrusts formed by tectonic inversion of miocene extensional detachments we attribute the 1986 oceanside ml 5 3 earthquake local uplift of marine terraces seafloor fold scarps and observed geodetic convergence to motion on these faults single and multisegment fault rupture scenarios suggest the potential for large m 7 1 7 6 but infrequent earthquakes that would affect the los angeles and san diego metropolitan areas

tsunamis within the eastern santa barbara channel several locally generated tsunamis have been reported in southern california during the past 200 years yet the hazard from locally generated tsunamis has received considerably little attention we consider here tsunamis generated by coseismic displacements on the channel islands thrust cit system as well as waves generated by slope failures along the walls of the santa barbara channel we find that purely tectonic sources could generate regional tsunamis with 2m runup whereas combinations of tectonic sources and submarine mass movements could generate local runup as large as 15m