late cenozoic tectonics of the east ventura basin transverse ranges california the east ventura basin originated in the middle miocene as a rift system bounded on one side by the oak ridge simi hills structural shelf and on the other side by a granitic ridge parallel to the san gabriel fault this fault began accumulating right slip 10 12 m y ago at a rate of 4 5 9 mm yr depending on whether total slip is 45 or 60 km slowing to about 1 mm yr in the quaternary north of the santa clara river rifting ended prior to deposition of the uppermost miocene lower pliocene towsley formation south of the santa clara river the rift axis shifted southwest toward the oak ridge simi hills shelf as the towsley formation accumulated against a normal fault ancestor of the santa susana fault a change to contractile tectonics occurred in the pliocene with depos tion of the fernando formation when the newhall potrero anticline developed as a monocline above a blind reverse fault the pico anticline to the southeast and the temescal and hopper ranch modelo anticlines to the northwest may have a similar origin tectonic inversion and displacement on the southwest verging santa susana fault began about 0 5 ma based on appearance of locally derived clasts in the upper saugus formation and its equivalents and continues today along with the southwest verging san cayetano fault farther west also active are northeast verging backthrusts occurring in the east ventura basin fold belt and a segment of the san gabriel fault which now acts as a northeast dipping oblique slip reverse fault northeast trending discontinuities and structures divide the present deformation zone into four segments in the hopper canyon segment at the west end of the area in the west ventura basin the san cayetano fault places miocene modelo formation over pliocene pleistocene strata more than 5 km thick largely at maximum burial to the southeast the newhall potrero segment is characterized by north vergent backthrusts within the east ventura fold belt and by southward thrusting of the basin sequence tectonic inversion over the structural shelf on the santa susana fault farther southeast the placerita segment is marked by reverse faulting on both the santa susana fault and the san gabriel fault southeast of the basin in the san fernando valley the sylmar segment contains a thick pli cene pleistocene sequence overridden by basement rocks of the san gabriel mountains as well as the south verging mission hills granada hills and northridge hills fault zones

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 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

near surface expression of the eastern part of the san cayetano fault a potentially active thrust fault in the california transverse ranges examined in this study are the near surface geometry late cenozoic geologic history and seismic potential of the eastern part of the san cayetano fault a major north dipping thrust fault of the transverse ranges california the fault generally dips with a low angle close to the surface but the dip increases up to 40 at about 5 km below sea level the fault may become nearly horizontal at depth and root into a midcrustal detachment surface of the transverse ranges the late cenozoic reverse movement along the san cayetano fault may have begun during the deposition of the lower part of the saugus formation the fault has also moved after the deposition of the saugus formation within the last 0 2 m y the san cayetano fault should be considered at least potentially active and may be capable of producing damaging earthquakes this conclusion is based collectively on 1 the presence of several surficial geological features suggestive of recent movement along the fault 2 the presence of microearthquakes on the fault plane and 3 the structural similarity of the fault to seismically active north dipping thrust faults of the region

convergence rates across a displacement transfer zone in the western transverse ranges ventura basin california three cross sections are balanced and retrodeformed to the top of the pleistocene saugus formation and to a horizon close to the base of the jaramillo subchron to yield crustal shortening and shortening rates for the western transverse ranges of california the cross sections compare the shortening that occurs along a transfer zone in which displacement is transferred eastward from a surface reverse fault red mountain fault to a blind thrust and from that to a combination of both a surface reverse fault san cayetano fault and the blind thrust using an age of 250 50 ka for the top of the saugus formation based on amino acid racemization of fossil mollusks crustal shortening rates appear to have abruptly accelerated through time from 3 4 3 to 5 4 mm yr between 250 and 975 ka to 20 6 to 28 1 mm yr since 250 50 ka using an age of 500 ka for the top of the saugus formation based on paleomagnetic stratigraphy there is little increase in deformation rates through time ranging from 5 5 to 8 6 mm yr between 975 500 ka and from 10 3 to 14 1 mm yr since 500 ka more crustal convergence rates determined by global positioning system surveys taken over 4 6 years indicate a shortening rate of 7 10 mm yr across the basin this is consistent with the slower rates of deformation calculated using an age of 500 ka for the top of the saugus formation less

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

a model of tectonic stress state and rate using the 1994 northridge earthquake sequence a model of the spatial and temporal distribution of seismicity both before and after the northridge california earthquake has been used to estimate the background stress state loading rate and some parameters relating to fault friction a detailed source model was used to calculate the stress change field from the northridge mainshock which was fit to the spatial distribution of seismicity agreement between the best fitting background stress state from seismicity modeling and the background stress state inferred from focal mechanisms was found to be 99 significant showing that the spatial distribution of seismicity contains information about the stress state the stress rate found for the northridge area 1 2 pa day 0 004 bar a is approximately a factor of 9 less than the stress rate expected from observed strain rates which implies that aseismic deformation is reducing seismic hazard in the los angeles basin the resulting estimate of the friction parameter a d 0 008 is similar to laboratory values if an effective normal stress much smaller than the overburden and similar to background stress magnitudes 70 mpa from the seismicity modeling is assumed to apply estimates of the magnitude of background stress gradient of stress with depth and effective coefficient of friction are quite uncertain

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

three dimensional imaging of active structures using earthquake aftershocks the northridge thrust california abstract aftershocks of the 1994 northridge earthquake m 6 8 allow us to image the structure beneath the san fernando valley in northwestern los angeles in three dimensions we combine aftershocks and geological data to build an image of the three dimensional 3 d geometry of the previously unrecognized north vergent northridge blind thrust to a depth of 21 km the most striking feature of the imaged fault is mega corrugations oriented parallel to the mean aftershock slip vector with most of the 1994 slip confined to west of the largest corrugation we also image the partially overlying south vergent san fernando thrust which broke to the surface in a complex rupture in 1971 m 7 1 both thrusts produce fault related folding because of either fault propagation or fault bends however this deeper folding is masked by overlying complex deformation in the cover which is one reason why the northridge thrust was not identified until it ruptured in 1994 we use trishear fold modeling based on our 3 d fault geometry to evaluate possible folding due to slip on the northridge thrust as well as its interaction with the overlapping san fernando thrust and with shallow structures in the cover this example illustrates the importance of earthquake data to structural geology and the value of its 3 d integration with surface and near surface geological data

comment on faulting apparently related to the 1994 northridge california earthquake and possible co seismic origin of surface cracks in potrero canyon los angeles county california by r d catchings m r goldman w h k lee m j rymer and d j ponti catchings et al 1998 hypothesized that several shallow faults 100 m deep and two deeper reverse faults 400 1500 m deep cut through gently dipping bedding in the core of a broad syncline below potrero canyon based on shallow high resolution seismic reflection profiles modeled from a seismic line completed by the u s geological survey usgs in 1994 following the northridge california earthquake catchings et al 1998 then attempted to correlate the faults with ground cracking produced in potrero canyon by the earthquake and with the oak ridge fault system based on map relations and projection of the profiles into nearby published cross sections however in contrast to the flat structural geometry inferred at depth by catchings et al 1998 published maps outcrop geology and subsurface geologic data all indicate that the bedrock at the seismic line dips 40 to 60 northward the subsurface structure inferred by catchings et al 1998 cannot be reasonably reconciled with the known surface geology the structure shown in nearby published cross sections or structure contours based on oil well data this indicates that the geologic structure at depth is not accurately represented by the seismic profile and therefore the interpreted faults are not substantiated in addition field evidence and ground rupture modeling overwhelmingly indicate that fault rupture during the northridge earthquake ceased below a depth of 4 km and that surface cracking in potrero canyon was the result of strong ground motions and secondary effects rather than surface faulting a combination of errors in the p wave velocity model and reflection artifacts are a likely cause for the inaccurate seismic profile our main concerns outlined previously are discussed in detail along with apparent inconsistencies in the correlation of the seismic profile with published cross sections catchings et al 1998 referred to herein as the authors stated that potrero canyon

overlapping fault planes of the 1971 san fernando and 1994 northridge california earthquakes aftershocks of the 1971 san fernando and 1994 northridge earthquakes were relocated using a three dimensional velocity model that was derived from inverting p wave travel time data the hypocenters show clear orientations of the dipping fault planes the san fernando aftershocks form a plane extending from a depth of 15 km to the surface dipping toward the northeast at about 40 the northridge aftershocks delineate a fault extending from a depth of 18 km up to about 5 km dipping toward the southwest at about 40 in the region the aftershocks overlap in map view the san fernando plane cuts off the northridge plane at a depth of 5 to 8 km preventing it from reaching the surface the similar but oppositely dipping fault planes suggest a pair of conjugate planes reflecting a horizontal northeast southwest compression

how do horizontal frictional discontinuities affect reverse fault propagation folding abstract the development of new reverse faults and related folds is strongly controlled by the mechanical characteristics of the host rocks in this study we analyze the impact of a specific kind of anisotropy i e thin mechanical and frictional discontinuities in affecting the development of reverse faults and of the associated folds using physical scaled models we perform analog modeling introducing one or two initially horizontal thin discontinuities above an initially blind fault dipping at 30 in one case and 45 in another and then compare the results with those obtained from a fully isotropic model the experimental results show that the occurrence of thin discontinuities affects both the development and the propagation of new faults and the shape of the associated folds new faults 1 accelerate or decelerate their propagation depending on the location of the tips with respect to the discontinuities 2 cross the discontinuities at a characteristic angle 90 and 3 produce folds with different shapes resulting not only from the dip of the new faults but also from their non linear propagation history our results may have direct impact on future kinematic models especially those aimed to reconstruct the tectonic history of faults that developed in layered rocks or in regions affected by pre existing faults

how sensitive are fault slip rates in the los angeles basin to tectonic boundary conditions geodetic studies have produced different interpretations of the relative vertical thickening versus escape tectonics occurring within the los angeles basin based on calculated horizontal strain rates ranging from 56 10 9 yr contraction at n36 et o 85 10 9 yr north south contraction and 60 10 9 yr east west extension three dimensional forward mechanical models of faults in the los an geles basin are used to compare modeled slip rates and slip styles represented as the rakes of average net slip vectors under four representative horizontal strain condi tions with geologically determined late quaternary slip rates and observed slip styles the comparisons strive to identify the most mechanically viable tectonic boundary conditions for the los angeles basin the three dimensional fault surfaces used within the mechanical models have been defined by southern california earthquake center scec workers based on surface geology and subsurface data the model results suggest that northeast southwest principal contraction is inappropriate for the los angeles basin because this tectonic boundary condition produces left lateral slip along the whittier and san gabriel faults in contrast with abundant geologic evidence of right lateral slip of the four tectonic boundary conditions evaluated north south contraction accompanied by either zero or slow east west contraction gives the low est mismatch of modeled and observed fault slip style these results suggest that vertical thickening may be more predominant than escape tectonics in this region

map restoration of folded and faulted late cenozoic strata across the oak ridge fault onshore and offshore ventura basin california the ventura basin lies within the east west trending active fold and thrust belt of the western transverse ranges california this basin has been the site of significant earthquakes on structures within it and bordering it the purpose of our study is to identify the main structures in the basin and its borders and to quantify their rate of deformation our study includes the onshore and offshore ventura basin the arcuate basin bounding oak ridge reverse fault and the oxnard shelf to the south shortening fault slip and crustal block motions were studied using a three dimensional map restoration technique structure contour maps on the 6 ma surface and other horizons were digitized and restored to the initial horizontal state by unfolding them using the computer program unfold and then fitting the unfolded surfaces across faults comparing the restored and present configuration allows us to estimate total net finite displacements relative to a fixed horizontal reference line average post 5 ma shortening rates estimated from our restoration are slower than both post 1 ma rates and present rates determined by global positioning systems most shortening due to folding in the onshore basin is post 1 ma although slip on the oak ridge fault has occurred both before and after 1 ma displacement due to faulting and folding includes left lateral strike slip motion on the northeast southwest coastal segment of the oak ridge fault and associated clockwise rotation of the adjacent ventura basin the oxnard shelf is bordered to the south by mountains and islands that have been previously interpreted as folds above thrust fault ramps this onshore offshore block moves as one continuous thrust sheet similarly beyond the well studied onshore fault a kinematically continuous offshore oak ridge mid channel left oblique fault system is interpreted to continue at least an additional 100 km westward beneath the santa barbara channel

paleomagnetic constraints on the initiation of uplift on the santa susana fault western transverse ranges california the plio pleistocene nonmarine saugus formation is widely exposed in the east ventura basin and the northern san fernando valley north and south of the santa susana fault in the east ventura basin the saugus is overlain by the pacoima formation magnetostratigraphy of the fine grained saugus and pacoima strata and the presence of the 0 76 ma bishop ash are used to calculate their average sedimentation rates 0 9 km m y in the east ventura basin and 1 1 km m y in the northern san fernando valley extrapolation of the saugus and pacoima sedimentation rates shows that they were deposited from about 2 3 to 0 5 ma both north and south of the santa susana fault the shift from remote to locally derived clasts marks the initiation of uplift of the santa susana mountains the extrapolated age of this boundary is about 0 7 0 6 ma

high resolution 3d p wave velocity model for the east ventura san fernando basin california and relocation of events in the northridge and san fernando aftershock sequences we determined a high resolution 3d p wave velocity model for an 80 km 80 km area around the san fernando valley extending 22 km below sea level we used events recorded from 1981 to 2000 as well as 799 aftershocks of the 1971 san fernando earthquake recorded by a portable network the total number of events and stations used were 13 455 and 101 respectively most of the events are aftershocks of the 1994 northridge earthquake the inversion software includes a raytracing subroutine that can handle the sharp lateral velocity variations that exist in the area the resulting model shows an overall good agreement with the southern california earthquake center scec 3d velocity model but images the deeper structure of the san fernando valley in more detail in addition our results are supported by the inversion of synthetic data and by the remarkable agreement with a density model derived from gravity data along a 55 km long profile an approximately 10 km wide and sharply defined zone of northridge aftershocks occurred in basement rocks between about 8 and 20 km depth and identifies the fault that slipped during the mainshock horizontal slices at deep 8 18 km levels show that the northridge earthquake occurred within a high velocity basement block that may have controlled the size of the earthquake the fault plane inferred from the seismicity is in good agreement with the geodetic fault plane determined by hudnut et al 1996 assuming uniform slip most of the aftershocks of the northridge earthquake occurred within the sedimentary rocks of the san fernando basin with a large concentration of events located to the east of the eastern boundary of the mainshock fault plane the latter events form a zone that dips about 45 to the southwest at depths less than about 14 km these may represent a lateral ramp this zone terminates at the steeply inclined eastern edge of the san fernando basin these events do not lie on the mainshock fault plane and because the san fernando aftershocks also lie east of the northridge fault plane the san fernando and the northridge mainshocks must have occurred on conjugate en echelon faults i e they do not abut each other in addition an 8 km wide cross section along the density model referred to previously suggests the northridge aftershocks east of the mainshock and most of the aftershocks of the san fernando earthquake illuminate two thrust faults that have a common high velocity basement block in their footwalls the velocity model produced by our analysis suggests that thrust faults formed by modern contraction in this region are strongly influenced by prior basin and basement block geometry online material 3d p wave velocity models without seismicity

tectonic setting of the 1971 sylmar and 1994 northridge earthquakes in the san fernando valley california the san fernando valley lies above the north dipping 1971 sylmar and south dipping 1994 northridge earthquake faults to understand the tectonic setting of these two earthquakes we mapped subsurface geology of the san fernando valley down to a depth of 3 km using industry oil well and seismic data the 1994 northridge earthquake did not rupture the surface and the south dipping aftershock zone terminated against the north dipping 1971 aftershock zone at a depth of 5 8 km however the blind northridge fault has a near surface geologic expression fault propagation folding related to the northridge fault has preserved a thick forelimb sequence of plio pleistocene saugus formation in the sylmar basin and merrick syncline which are located on the hanging wall side of north dipping reverse faults the north dipping mission hills verdugo and northridge hills reverse faults are interpreted to be potential seismic sources because fault propagation folds above these faults have tectonic geomorphic expression these north dipping reverse faults were initiated during the deposition of the saugus formation between 2 3 and 0 5 ma and have minimum dip slip rates of 0 35 to 1 1 mm yr based on the oldest possible age of the initiation of faulting the northridge hills and mission hills faults are interpreted to merge at depth and are located at the updip extension of the 1971 aftershock zone even though these faults did not rupture during the 1971 earthquake surface breaks appeared north of these faults mostly along north dipping bedding planes and are interpreted as secondary features related to flexural slip folding rather than a direct extension of the 1971 seismogenic fault surface and subsurface geology together with seismological data of the 1971 and 1994 earthquakes suggests that the north and south dipping deformation zones in the san fernando valley are divided into multiple segments separated by northeast trending structural discontinuities

compression directions north of the san fernando valley determined from borehole breakouts borehole breakouts in 4 nearly vertical oil wells and several other deviated holes in the san fernando valley and santa susana mountains suggest a maximum horizontal compressive stress direction s h of n49 w these wells provide information about the stress field from 1974 to 1983 in the uppermost crust 3 km depth near the aftershock zones of the january 1994 northridge earthquake and the february 1971 sylmar earthquake this direction of s h is anomalous with respect to the n to nne directions of s h seen in other regional data but is consistent with the structural complexity of this zone including local changes in strike of major thrust fault zones and the presence of lateral ramps in both the santa susana and san fernando faults

multidisciplinary approaches to australian island pasts late pleistocene to historical perspectives on australian island use in this review volume we bring together researchers and institutions involved in australian island research to provide a contemporary synthesis and benchmark on archaeological and palaeoenvironmental research of australian island environments the last major overview of australian islands archaeological research appeared 16 years ago in the volume australian coastal archaeology hall and mcniven 1999 at that time the main geographical areas of dynamic islands research in australia were northwest western australia and central and southeast queensland since 1999 torres strait has developed as the most dynamic region of islands research in australia and this new volume showcases the range of recent research from torres strait and the adjacent region of southern papua new guinea all five inqua commissions coastal and marine processes humans and the biosphere palaeoclimates stratigraphy and chronology terrestrial processes deposits and history are represented in the papers in the volume key environmental themes explored through the volume are geoarchaeology taphonomy impacts of sea level rise and climate change chronology of island formation and colonisation anthropogenic environmental modifications island terrestrial and marine resource management and intra and inter island occupation mobility and exchange relationships key socio cultural themes explored include ritual and ceremonial sites mortuary practices canoe voyaging and responses to colonial impact site types discussed include shell deposits cultural and natural villages long houses gardens rock art ritual shrines artificial reefs and ceremonial stone arrangements the contributions represent diverse multidisciplinary approaches adopting coordinated methodologies to the task of understanding the diversity of human use and modification of islands in the australian context culturally and chronologically the volume surveys the depth and breadth of australian island societies including studies of late pleistocene island use in northwest australia late holocene deltaic island societies of papuan lowland rainforests early to mid holocene tropical island use of torres strait and the great barrier reef and holocene selective occupation and abandonment of bass strait and other southern ocean islands the volume s geographic coverage is thus broadscale spread from the hot and humid tropical north to the cold temperate waters of the southern ocean uniquely the papers in this volume reveal the diversity and complexity of cultural practices e g subsistence mobility technology socio religious institutions cosmology resource manipulation and management and historical contingencies that structure and mediate long term human use engagement and modification of islands

discrepancy between geological and geodetic deformation rates in the ventura basin space geodesy provides a new tool for measuring neotectonic deformation which when combined with geological and geophysical observations and models can be used to infer rates and styles of deformation in the ventnra basin region of southern california the geodetically measured convergence rates of 7 10 mm yr 1 ref 1 are significantly lower than the 20 25 mm yr 1 estimated from geological data2 3 here we investigate the possible implications of this discrepancy it might be for example that temporal variations cause the short and long term convergence rates to differ but if this were the case we would expect to observe the long term rate by extending the geodetic measurements over a broader region yet only 11 3 mm yr 1 of regional convergence is observed4 our preferred explanation of the discrepancy is that the geological models require revision and to this end we present two deformation models that satisfy both the geological and geodetic constraints our modelling suggests that the faults bounding the basin are locked at the surface but are slipping at depths below 2 5 km

mechanical modeling of compressional basins origin and interaction of faults erosion and subsidence in the ventura basin california using geophysical observations and some basic geological ideas on the present structure of the ventura basin we model its possible mechanical evolution during the last 4 million years we use a two dimensional finite element model with frictional and elastoviscoplastic rheologies that allow strain localization this allows us to study the mechanical origin of the san cayetano thrust fault scf and its interaction with the oak ridge fault orf the results of our modeling indicate that the geometry and activity of a shear zone which mimics the san cayetano fault are conditioned both by the preexistence of the oak ridge fault and by the shape of the brittle ductile transition furthermore the presence of a decollement level is shown to favor the development of the san cayetano fault shear zone as a low angle structure the value of the effective friction on the oak ridge fault is evaluated from this model possible values of the viscosity of the sediments are also estimated it is shown that low sediment viscosity may completely hinder the activity of such a fault at the surface while slip at depth continues the stratigraphic profile of the basin is also shown to depend on the relative strengths of the orf and scf which also provide an explanation for the shortening and rapid subsidence of the basin

genesis of hydrothermal silver antimony sulfide veins of the braunsdorf sector as part of the classic freiberg silver mining district germany the peripheral regions of the freiberg vein type silver mining district comprise several sub districts of which braunsdorf was among the richest in terms of ag grade historically 114 t about 3 9 million ounces of ag were produced from the neue hoffnung gottes mine near braunsdorf the neuer segen gottes stehender is a sigmoidally shaped nne ssw trending vein which varies significantly in thickness 0 05 to 3 m and extends over about 2 6 km strike length at the surface the vein infill is marked by a pb zn cu fe sulfide quartz stage 1 and an abundant ag sb sulfide sulfosalt quartz carbonate assemblage stage 2 to develop a sound genetic understanding of the polymetallic mineralisation in the braunsdorf sub district we conducted detailed textural analyses of ore and gangue minerals fluid inclusion analyses electron microprobe analyses and thermodynamic computations in order to characterise the ore fluids and ore forming processes the early stage pb zn cu fe sulfide mineralisation stage 1 is related to fluids with low salinities 0 5 4 eq w nacl and formed at temperatures 300 c microthermometric data related to the ag sb sulfide sulfosalt assemblage stage 2 show similar salinites compared to ore stage 1 but have significantly lower homogenisation temperatures in the range of 180 280 c based on fluid inclusion data cooling can be regarded as the major ore forming process reaction path model calculations for cooling of fluids with different initial ph values 4 5 5 and 7 reproduce the observed mineral assemblages very well and predict spatial zonation of the ag sb and sb sulfide minerals that are in excellent agreement with field observations we conclude that ag rich zones may well occur below sb rich zones in hydrothermal vein type systems similar to those of the freiberg district this relationship may be of potential use for exploration targeting

interseismic deformation associated with three dimensional faults in the greater los angeles region california 1 existing interseismic models are not well suited to simulate deformation within the network of finite intersecting nonplanar faults observed in the greater los angeles region instead of applying fault slip rates to a model a priori we allow three dimensional fault surfaces to interact and accumulate mechanically viable slip distributions and then use the deep nonseismogenic portion of slip to calculate interseismic deformation we apply this approach to the los angeles region and find that the geologic timescale model results match well geologic slip rate data and the interseismic timescale model results match well the heterogeneous gps velocity pattern in the los angeles region model results suggest that localized geodetic convergence in the san gabriel basin can be achieved with slip on multiple active fault surfaces in the los angeles region including relatively fast slip on the sierra madre fault and slow slip on the puente hills thrusts in agreement with geologic data the ability of the three dimensional model to reproduce well both geologic slip rates and interseismic geodetic velocity patterns suggests that current day contraction rates in the greater los angeles region are compatible with long term geologic deformation rates and disputes suggestions of significant temporal variations in fault slip rates inferred from existing investigations

a balanced cross section of the 1994 northridge earthquake southern california the northridge earthquake of 17 january 19941 was the latest in a series of very damaging thrust fault generated earthquakes to strike california following the san fernando2 1971 coalinga3 1983 and whittier narrows4 5 1987 events like the last two of these the northridge event occurred along a fault that did not reach the surface and which had not been detected by traditional seismic hazard methods6 7 balanced cross sections8 9 which flatten and remove the crustal deformation can be used to identify and quantify the seismic hazard posed by thrust faults here we present a balanced cross section through the northridge portion of the transverse ranges fold and thrust belt10 which shows that the earthquake occurred on what we call the pico thrust a cross section of this sort constructed before the earthquake would have revealed the fault although it would not have predicted the earthquake cross sectional modelling of the pico thrust yields an average slip rate of 1 4 1 7 mm yr 1 and a recurrence interval of northridge sized mw 6 7 earthquakes every 1 500 1 800 years we show that the pico thrust is the back thrust to the 170 km elysian park thrust3 4 which underlies some of the most densely urbanized portions of the los angeles basin

three dimensional vp and vp vs velocity models of the los angeles basin and central transverse ranges california we use p and s arrival times from 5225 earthquakes and 53 explosions recorded by the southern california seismographic network to invert for the three dimensional p velocity v p and the p and s velocity ratio v p v s in the central transverse ranges and the los angeles basin to model long wavelength features of the velocity structure we invert for the models by interpolating models determined successively from a sparse medium to a dense grid with 40 20 and 10 km spacing of horizontal grid nodes layers of grid nodes are placed at depths of 1 4 6 8 12 16 and 20 km the data variance decreased about 80 in the gradational inversion ample data from the 1994 northridge and other earthquake sequences the rich background seismicity and the dense station distribution along with controlled sources made the model well resolved except along the edges to the southwest in the offshore region and at depths greater than 20 km the v p model images the shape of the los angeles and east ventura basins down to depths of 8 and 12 km three low velocity areas at 1 km depth in the los angeles basin that coincide with recent sediment depositional areas are also imaged the north edge of the peninsular ranges the santa monica and the san gabriel mountains form discontinuous high velocity ridges extending to depths of 20 km the high v p v s ratios in the near surface are consistent with high pore fluid pressures in the basin sediments at depth beneath the east ventura basin the high v p v s and high v p suggest the presence of ophiolitic assemblages or mid miocene volcanics in contrast a body with normal v p and low v p v s that is identified in the upper crust beneath santa monica bay may be a fragment of quartz rich continental crust the v p model of both the ventura and the los angeles basins shows features that suggest deformation of the hanging wall or basin closure at seismogenic depths of 16 km the hypocenters of moderate sized and large earthquakes are located within or adjacent to high v p velocity bodies in most cases these high velocity bodies form the upper block consistent with shortening of the lower crust as described in thick skinned tectonic interpretations

the role of bed parallel slip in the formation of blind thrust faults abstract a finite element code is used to model the mechanical behaviour of elastoplastic sedimentary layers that are deformed by the movement of a faulted deformable basement the propagation of the blind thrust fault is analysed in terms of the evolution of strain localization in the elastoplastic layers of strain softening and strain hardening materials the role of layer thickness bedding parallel slip and fault dip are also studied we found that if no interlayer slip is allowed blind thrusts tend to propagate with constant dip regardless of the rheological properties of the sediment layers however if bedding parallel slip is present a mechanical decoupling appears between layers allowing stress transfers along large distances and favouring the formation of backthrusts fault dip also has significant influence on the qualitative behaviour when interlayer slip is possible a steep fault will tend to split

mechanical models favor a ramp geometry for the ventura pitas point fault california recent investigations have provided new and significantly revised constraints on the subsurface structure of the ventura pitas point fault system in southern california however few data directly constrain fault surfaces below 6 km depth here we use geometrically complex three dimensional mechanical models driven by current geodetic strain rates to test two proposed subsurface models of the fault system we find that the model that incorporates a ramp geometry for the ventura pitas point fault better reproduces both the regional long term geologic slip rate data and interseismic gps observations of uplift in the santa ynez mountains the model calculated average reverse slip rate for the ventura pitas point fault is 3 5 0 3 mm yr although slip rates are spatially variable on the fault surface with 8 mm yr predicted on portions of the lower ramp section at depth

the oak ridge fault system and the 1994 northridge earthquake the 17 january 1994 northridge earthquake moment magnitude m w 6 7 in the san fernando valley in southern california illuminated a hitherto unrecognized blind reverse fault with a moderate southward dip 1 this fault lies beneath the active north dipping santa susana fault system and uplifted both the footwall and the hanging wall of the santa susana fault during the earthquake1 here we argue that footwall uplift on the santa susana fault before the earthquake could have been used to identify the northridge blind thrust as an active fault moreover we propose that the northridge earthquake occurred on a continuation of the oak ridge fault system which reaches the surface in the ventura basin to the west slip rates on the western part of this fault system 2 3 are nearly three times larger than on the northridge blind thrust 4 increasing the probability of a n or t bridge sized earthquake in the heavily populated ventura basin

earthquake focal mechanisms in the eastern transverse ranges and san emigdio mountains southern california and evidence for a regional decollement earthquake focal mechanisms obtained from p wave first motions are presented for the eastern transverse ranges and the san emigdio mountains in southern california the former region shows a predominance of strike slip faulting whereas quaternary faults in the region show thrust motion we suggest that the observed strike slip mode of deformation cannot continue indefinitely without the occurrence of more thrust faulting fault deformation in the san emigdio mountains inferred from focal mechanisms is in accord with displacements across quaternary faults in the area this study and a search of the literature has yielded 19 mechanisms with shallow dipping nodal planes previous workers have interpreted such mechanisms as evidence for a regional decollement if such a regional decollement exists these data give some indication of its regional extent slip directions inferred from the focal mechanisms with shallow dipping nodal planes show some regional consistency but this pattern cannot be entirely explained with current tectonic models a comparison of the stress drop of an event having a shallow dipping nodal plane with an event with steeper planes gave inconclusive results

anomalously deep crustal earthquakes in the ventura basin southern california earthquakes at depths of 20 30 km occur beneath the ventura basin in southern california the epicentral distribution of deep seismicity outlines an east trending ellipse which corresponds closely with the mapped santa clara syncline the main structural element of the ventura basin with 15 km thickness of sediments clear pn and pg phases are seen for even the deepest earthquakes which demonstrate that these earthquakes are occurring within the crust travel time curves from shallow and deep earthquakes in the western transverse ranges require that the depth to the moho under the ventura basin must be depressed by 7 10 km relative to the surrounding area indicating crustal thickening of 20 35 other researchers have shown that the ventura basin has the lowest heat flow in western california thus low heat flow exceptionally thick 15 km sediments rapid shortening of the crust the deep earthquakes and a depressed moho all coincide within a small area strongly suggesting a common cause for all five phenomena the confinement of the deep earthquakes and depressed moho to the ventura region suggests that the rapid shortening of the ventura basin is anomalous and not representative of the rate of shortening across all of the western transverse ranges

geodetic measurement of deformation in the ventura basin region southern california we have measured the deformation in the ventura basin region southern california with global positioning system gps measurements carried out over 4 6 years between 1987 and 1992 the deformation within our network is spatically variable on scales of tens of kilometers with strain rates reaching 0 6 1 micro rad yr in the east central basin blocklike rotations are observed south and northwest of the basin where the maximum shear strain rates are an order of magnitude lower 0 06 1 micro rad yr to the south we also observed clockwise rotations of 1 deg 7 deg m y shear strain rates determined by comparing angle changes from historical triangulation spanning several decades and gps measurements give consistent though less precise results the geodetic rates of shortening across the basin and western transverse ranges are lower than those estimated from geological observations but the patterns of deformation from the two methods agree qualitatively

the magnitude 6 7 northridge california earthquake of january 17 1994 the most costly american earthquake since 1906 struck los angeles on 17 january 1994 the magnitude 6 7 northridge earthquake resulted from more than 3 meters of reverse slip on a 1 5 kilometer long south dipping thrust fault that raised the santa susana mountains by as much as 70 centimeters the fault appears to be truncated by the fault that broke in the 1971 san fernando earthquake at a depth of 8 kilometers of these two events the northridge earthquake caused many times more damage primarily because its causative fault is directly under the city many types of structures were damaged but the fracture of welds in steel frame buildings was the greatest surprise the northridge earthquake emphasizes the hazard posed to los angeles by concealed thrust faults and the potential for strong ground shaking in moderate earthquakes

late quaternary fold deformation along the northridge hills fault northridge california deformation coincident with past northridge blind thrust earthquakes and other nearby structures paleoseismic investigation of the northridge hills fault in the northern san fernando valley california helps assess the timing and style of near surface late quaternary deformation in the epicentral area of the 1994 northridge earthquake the northridge hills fault a 15 km long north dipping reverse fault exhibits geomorphic evidence of late quaternary surface deformation including topographic scarps across late quaternary fluvial terraces and aligned alluvial fan apices on the footwall block we excavated one 40 m long trench and six test pits and drilled nine boreholes across a 2 m high scarp developed on a probable holocene fluvial terrace adjacent to aliso canyon wash a continuous clayey gravel identified in the trench test pits and boreholes defines a south facing monocline with 6 1 m of vertical separation across the northridge hills fault based on pedochronology the clayey gravel ranges in age from 6 to 30 ka the borehole data also suggest that an unconformity developed on the plio pleistocene saugus formation is warped into a monocline that has 13 2 m of vertical separation across the fault these preliminary data yield a dip slip rate of 1 0 0 7 mm yr for the northridge hills fault the absence of distinct scarp derived colluvium in trench exposures at the base of the scarp and secondary brittle fracturing or faulting suggests that the monocline is related to folding during small incremental uplifts rather than large uplifts that generate distinct scarp relief we postulate that such uplift could be produced via moderate magnitude earthquakes m w 6 on the northridge hills fault or secondary deformation induced by earthquakes on other faults for instance evidence of surface uplift near the trench site during or following the 1994 earthquake suggests that all or part of the observed deformation is a result of secondary slip on the northridge hills fault produced by movement on the underlying northridge blind reverse fault or other nearby large structures based on our geologic investigations the distribution of aftershocks following the 1994 earthquake and pre and post 1994 leveling and geodetic surveys we interpret that the northridge hills fault underwent triggered slip during the 1994 earthquake

restraining segments and reactivation of the santa monica dume malibu coast fault system offshore los angeles california 1 a regional west striking system of surface and blind faults transects northern metropolitan los angeles separating the santa monica mountains from two deep sedimentary basins the surface faults include the santa monica dume and malibu coast faults the three dimensional 3 d geometries of these faults and deformed dated strata were examined in order to determine how oblique shortening is accommodated how structural relief grows along a mountain front and how block translation is related to block rotation industry seismic reflection well and outcrop data were used to construct digital structure contour maps of several of these faults and three stratigraphic horizons that intersect them these maps swath bathymetry digital elevation models and seismicity were incorporated into a 3 d digital database modeling included new approaches to determining strike slip displacement by separate analyses of shortening and structural relief in a restraining double bend stratigraphic thicknesses indicate miocene extension across the santa monica and dume faults which have been reactivated as a single or linked moderately dipping arcuate fault that accommodates one coherent block motion via left lateral and left reverse displacement modeling indicates 5 km 8 1 km of left lateral displacement on part of the santa monica dume fault and 11 of clockwise rotation of the santa monica mountains during the last 4 1 m y modern displacement rates modeled from gps data are similar to our modeled post 4 ma rates of westward escape and clockwise rotation of the santa monica mountains

crustal imaging in southern california using earthquake sequences abstract an inexpensive means to further understand the geometry of active faults in southern california arises from the use of aftershock recordings to image crustal structures the advent of regional seismic networks that record digital seismograms from hundreds of stations makes this crustal reflectivity profiling possible even in the absence of conventional active source seismic data we show that it is feasible to image fault structure using three dimensional wide angle prestack kirchhoff migration we achieve this with the use of aftershock traces recorded on the short period vertical stations of the southern california seismic network this work complements seismicity and focal mechanism work by imaging reflectivity volumes and cross sections rather than having to associate events with certain faults further it can image below the seismogenic zone to resolve current geologic controversies on how proposed faults extend below focal depths we demonstrate the validity of these images as showing reflective structures and the ability to use clipped high gain seismograms as sign bit data to yield valid geometric imaging work with data from the 1991 sierra madre earthquake sequence images the prominent lower crustal reflective zone observed beneath most of the san gabriel mountains by the los angeles region seismic experiment line 1 aftershocks of the 1994 northridge earthquake allow us to image a north dipping structure that may represent the fault plane of a crustal penetrating blind thrust the images serve as a test for the existence and geometry of thrust ramps and detachments proposed from balanced section reconstructions of shallow crustal profiles and borehole data our results are more consistent with a thick skinned tectonic regime in the vicinity of the northridge earthquake rather than a thin skinned model

kinematics of postseismic relaxation from aftershock focal mechanisms of the 1994 northridge california earthquake geodetic observations of surface deformation associated with the 1994 northridge southern california earthquake generally are reproduced by simple models of a large scale elastic dislocation on a blind or buried thrust fault the smaller scale aftershocks of the northridge earthquake are distributed throughout much of the volume of crust that appears to have deformed elastically during the mainshock these aftershocks averaged over volumes that are large relative to their rupture radii reflect a distributed permanent deformation that is accommodated by local brittle fracture we use a micropolar continuum model to invert the aftershocks in such volumes for the average incremental strain and we compare that deformation both with the elastic strain from the dislocation model of the mainshock and with geodetically measured strain aftershock deformation that occurred at depths below about 6 km and which is associated with the primary rupture zone is consistent with slow continuation of the southwest side up reverse slip on the blind northridge thrust fault in contrast aftershock deformation from the upper 5 7 km of the hanging wall block directly above the thrust fault can be characterized by horizontal ne sw shortening and horizontal nw se i e fault parallel extension this pattern of deformation is similar to that associated with the mainshock as observed geodetically and as calculated from the elastic dislocation model we interpret that the aftershock activity in the hanging wall represents the quasi ductile accommodation by brittle deformation mechanisms of a permanent strain distributed through the hanging wall block the aftershocks along the mainshock rupture zone are interpreted as resulting from either 1 the time dependent release along a weakened fault zone of part of the remaining accumulated elastic strain in the upper crust or 2 the continued slip in the weakened fault zone driven by the deformation of a ductile elastic lower crustal layer that relaxes under the stress transferred by the coseismic loss of cohesion in the upper crust in either case the aftershock activity suggests that the crust undergoes quasi ductile flow as a brittle elastic material and is not a strictly elastic material

heat flow and thermotectonic problems of the central ventura basin southern california the ventura basin southern california is located near the big bend area of the san andreas fault system within the transverse ranges physiographic province continuous equilibrium temperature logs were measured in 12 idle oil wells located within the onshore ventura avenue san miguelito filmore oxnard and west montalvo fields to an average depth of about 3100 m 10 200 feet thermal conductivities were measured on all available samples heat flows were calculated with the aid of a thermostratigraphic scheme based on correlative gradient intervals and average thermal conductivity for the appropriate units negative curvature of the ventura avenue temperature profiles may be explained by an increase in thermal conductivity associated with tectonic compaction of the underlying pliocene clastic sequence temperature profiles at fillmore are enigmatic but suggest highly unusual geotectonic conditions basinwide heat flow averages about 48 mw m2 a value which is low relative to most of southern california as heat flow does not vary systematically to the maximum measured depth of about 4 km this anomaly is not easily explained in terms of hydrologic effects or recent uplift and erosion however a diminution of heat flow is an expectable consequence of the accumulation of cold sediments up to 12 km since eocene time if 70 mw m2 is accepted as the background heat flow then the sedimentation effect is probably sufficient to explain the anomaly

evidence from a cluster of small earthquakes for a fault at 18 km depth beneath oak ridge southern california abstract a swarm of about 50 small earthquakes m 1 5 occurred for a month during 1989 beneath oak ridge southern california location accuracy using conventional analysis of arrival time picks is limited for these events by the weak emergent nature of arrivals on the available seismograms however waveform cross correlation techniques are found to provide precise relative event locations due to the similarity of the waveforms recorded at individual stations the relocated events form a small cluster about 1 km across at a depth of 18 km and are aligned along a plane that dips 35 to the northwest estimated standard errors for the locations are generally less than 50 m the time evolution of the sequence shows a gradual migration of activity away from its initiation point three additional events occurred several months later these align along the same plane but are displaced about 500 m to the southeast from the main swarm reliable fault plane solutions are difficult to obtain for these events due to the small number of station records available the limited range of takeoff angles and the weak initial arrivals on many of the seismograms stacking the records at each station over the different events greatly reduces prearrival noise levels and assists in resolving the average p first motions analysis of these first motion data indicates that the slip planes of probable focal mechanisms are not in agreement with the plane defined by the seismicity the seismicity alignment may represent the extension of the simi fault in which case some shallowing of the fault dip would be required to match the observed 35 dip at 18 km

late quaternary slip rate on the oak ridge fault transverse ranges california implications for seismic risk abstract the slip rate on the oak ridge fault at south mountain in the densely populated lower santa clara valley california is estimated as 6 15 to 12 3 mm yr since the end of saugus deposition at 0 4 to 0 2 ma slightly lower slip rate estimates result from a different projection of the top of the saugus in the hanging wall block of the fault an assumed displacement of 3 m per event gives an average recurrence interval of 250 to 500 yr with the major uncertainty being the age of the top of the saugus displacement of 2460 m of the top of the saugus at south mountain includes piercing point displacement and distributed displacement from drag folds near the fault both are the near surface expression of faulting at potential mainshock depths beneath well control slip rates for the nearby san cayetano and red mountain faults are not well constrained because post miocene strata are absent in their hanging wall blocks but available evidence suggests that their slip rates are about the same as that for the oak ridge fault earthquake return times measured in hundreds of years have been shown by others for the 1971 san fernando fault and a small fault near the red mountain fault and comparable return times are suggested for a normal fault in the hanging wall block of the oak ridge fault a trench on the harmon alluvial fan near the ventura county government center revealed evidence of cracks filled with sediments from below suggesting liquefaction during an earthquake the lower santa clara valley has not had a large damaging earthquake in 200 years of record keeping but one is expected there in the near future the december 21 1812 earthquake may have occurred on the offshore continuation of the oak ridge or red mountain fault

geomorphology denudation rates and stream channel profiles reveal patterns of mountain building adjacent to the san andreas fault in northern california usa relative horizontal motion along strike slip faults can build mountains when motion is oblique to the trend of the strike slip boundary the resulting contraction and uplift pose off fault seismic hazards which are often difficult to detect because of the poor vertical resolution of satellite geodesy and difficulty of locating offset datable landforms in active mountain ranges sparse geomorphic markers topographic analyses and measurement of denudation allow us to map spatiotemporal patterns of uplift along the northern san andreas fault between jenner and mendocino california emergent marine terraces found southwest of the san andreas fault record late pleistocene uplift rates between 0 20 and 0 45 mm yr 1 along much of the coast however on the northeast side of the san andreas fault a zone of rapid uplift 0 6 1 0 mm yr 1 exists adjacent to the san andreas fault but rates decay northeastward as the coast becomes more distant from the san andreas fault a newly dated 4 5 ma shallow marine deposit located at 500 m above sea level masl adjacent to the san andreas fault is warped down to just 150 masl 15 km northeast of the san andreas fault and it is exposed at just 60 110 masl to the west of the fault landscape denudation rates calculated from abundance of cosmogenic radionuclides in fluvial sediment northeast of and adjacent to the san andreas fault are 0 16 0 29 mm yr 1 but they are only 0 03 0 07 mm yr 1 west of the fault basin average channel steepness and the denudation rates can be used to infer the erosive properties of the underlying bedrock calibrated erosion rates can then be estimated across the entire landscape using the spatial distribution of channel steepness with these erosive properties the lower elevation areas of this landscape that show high channel steepness and hence calibrated erosion rate are distinct from higher elevation areas with systematically lower channel steepness and denudation rates these two areas do not appear to be coincident with lithologic contacts assuming that changes in rock uplift rates are manifest in channel steepness values as an upstream propagating kinematic wave that separates high and low channel steepness values the distance that this transition has migrated vertically provides an estimate of the timing of rock uplift rate increase this analysis suggests that rock uplift rates along the coast changed from 0 3 to 0 75 mm yr 1 between 450 and 350 ka this zone of recent relatively rapid crustal deformation along the plate boundary may be a result of the impingement of relatively strong crust underlying the gualala block into the thinner weaker oceanic crust left at the western margin of the north american plate by the westward migration of the subduction zone prior to establishment of the current transform plate boundary the warped pliocene marine deposits and the presence of a topographic ridge support the patterns indicated by the channel steepness analyses and further indicate that the zone of rapid uplift may herald elevated off fault seismic hazard if this uplift is created by periodic stick slip motion on contractional structures

geomorphic evidence for the geometry and slip rate of a young low angle thrust fault implications for hazard assessment and fault interaction in complex tectonic environments abstract we present surface evidence and displacement rates for a young active low angle 20 reverse thrust fault in close proximity to major population centers in southern california usa the southern san cayetano fault sscf active faulting along the northern flank of the santa clara river valley displaces young landforms such as late quaternary river terraces and alluvial fans geomorphic strain markers are examined using field mapping high resolution lidar topographic data 10be surface exposure dating and subsurface well data to provide evidence for a young active sscf along the northern flank of the santa clara river valley displacement rates for the sscf are calculated over 103 104 yr timescales with maximum slip rates for the central sscf of 1 9 1 0 0 5 mm yr 1 between 19 7 ka and minimum slip rates of 1 3 0 5 0 3 mm yr 1 since 7 ka uplift rates for the central sscf have not varied significantly over the last 58 ka with a maximum value of 1 7 0 9 0 6 mm yr 1 for the interval 58 19 ka and a minimum value of 1 2 0 3 mm yr 1 since 7 ka the sscf is interpreted as a young active structure with onset of activity at some time after 58 ka the geometry for the sscf presented here with a 20 north dip in the subsurface is the first interpretation of the sscf based on geological field data our new interpretation is significantly different from the previously proposed model derived geometry which dips more steeply at 45 60 and intersects the surface in the middle of the santa clara river valley we suggest that the sscf may rupture in tandem with the main san cayetano fault additionally the sscf could potentially act as a rupture pathway between the ventura and san cayetano faults in large magnitude multi fault earthquakes in southern california however given structural complexities including significant changes in dip and varying holocene displacement rates along strike further work is required to examine the possible mechanism likelihood and frequency of potential through going ruptures between the ventura and san cayetano faults confirmation of the sscf in a previously well studied area such as southern california demonstrates that identification of young faults is critical for accurate seismic hazard assessment we suggest that many young active faults remain undetected in other structurally complex and tectonically active regions globally and that significant seismic hazards can be overlooked

southern california earthquake center geologic vertical motion database the southern california earthquake center geologic vertical motion database vmdb integrates disparate sources of geologic uplift and subsidence data at 104 to 106 year time scales into a single resource for investigations of crustal deformation in southern california over 1800 vertical deformation rate data points in southern california and northern baja california populate the database four mature data sets are now represented marine terraces incised river terraces thermochronologic ages and stratigraphic surfaces an innovative architecture and interface of the vmdb exposes distinct data sets and reference frames permitting user exploration of this complex data set and allowing user control over the assumptions applied to convert geologic and geochronologic information into absolute uplift rates online exploration and download tools are available through all common web browsers allowing the distribution of vertical motion results as html tables tab delimited gis compatible text files or via a map interface through the google maps web service the vmdb represents a mature product for research of fault activity and elastic deformation of southern california

the scec southern california reference three dimensional seismic velocity model version 2 we describe version 2 of the three dimensional 3d seismic velocity model of southern california developed by the southern california earthquake center and designed to serve as a reference model for multidisciplinary research activities in the area the model consists of detailed rule based representations of the major southern california basins los angeles basin ventura basin san gabriel valley san fernando valley chino basin san bernardino valley and the salton trough embedded in a 3d crust over a variable depth moho outside of the basins the model crust is based on regional tomographic results the model moho is represented by a surface with the depths determined by the receiver function technique shallow basin sediment velocities are constrained by geotechnical data the model is implemented in a computer code that generates any specified 3d mesh of seismic velocity and density values this parameterization is convenient to store transfer and update as new information and verification results become available

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

large scale quaternary detachments in ventura basin southern california the ventura basin is an east north east trending trough in the california transverse ranges which records major quaternary detachment faulting at three levels the earliest thrusting occurred along weak siltstone interbeds in a sequence dominated by competent basin plain turbidite sandstone because sedimentation continued during thrusting the age rate and direction of thrusting can be worked out faulting began 1 3 m y ago and ceased 0 65 m y ago with a maximum slip rate of 2 8 mm yr to the southeast the fault set moved up a 45 ramp and ended as a blind thrust the ramp had topographic expression on the seafloor diverting turbidites around the ramp and preserving ash beds along with other hemipelagic sediments on its crest following the end of deposition 0 2 m y ago the competent basin plain turbidites underwent flexural slip folding over an incompetent miocene sequence dominated by shale underlying competent paleogene strata were not folded the south flank of the ventura avenue anticline tilted at 3 4 rad yr the anticlinal crest rose at a rate of 15 16 mm yr decelerating to 4 3 5 2 mm yr and the anticline and an adjacent syncline shortened at a rate of 20 mm yr the high rate of folding in the ventura avenue oil field resulted in overpressured sandstone reservoirs and oil water interfaces which have not had time to reach gravity equilibrium the red mountain san cayetano and santa susana faults mark the surface expression of a seismically active midcrustal detachment which produced convergence across the ventura basin at rates as high as 23 mm yr total convergence across the eastern san cayetano fault near fillmore is 11 600 2000 m in the last million years

quaternary flake tectonics of the california transverse ranges p residual studies suggest that the pacific american plate boundary in the upper mantle lies in the central mojave desert northeast of its crustal position at the san andreas fault focal mechanisms of some relatively deep microearthquakes suggest a flattening with depth of north dipping thrust faults resulting in a mid crustal detachment in which the central transverse ranges move south relative to the mantle this makes sense geologically if the central transverse ranges and western mojave desert are considered as tectonic flakes of brittle crystalline rocks underlain by ductile pelona schist or catalina schist which makes up the zone of decollement such ductile schists would occur today at depths comparable to those at which they were metamorphosed the detachment breaks through to the surface at the point dume malibu coast fault zone and the red mountain san cayetano santa susana sierra madre cucamonga fault zone the latter coinciding with the southern edge of the palmdale bulge the transverse ranges flake now moves west northwest relative to the western mojave flake south southwest relative to the pacific plate and northwest relative to the great valley sierra nevada the mantle boundary may rejoin the san andreas fault north of a southwest trending zone of high seismicity in the tehachapi mountains which may be produced by convergence the age of the present tectonic system may be the same as the age of the break through thrust faults these are probably no older than 2 m y and may be even younger

paleoseismologic evidence for a very large mw 7 post a d 1660 surface rupture on the eastern san cayetano fault ventura county california was this the elusive source of the damaging 21 december 1812 earthquake the first paleoseismologic data from the san cayetano fault a major reverse fault that extends along the northern edge of the ventura basin northwest of los angeles reveal that the most recent event on the eastern part of the fault generated at least 4 3 m of surface slip age determinations from detrital charcoal recovered from the faulted section indicate that this surface rupture occurred after a d 1660 the faulted deposits are overlain by unfaulted historical alluvium containing abundant metal fragments and a leather glove comparison of the large surface slip in this event with data from other earthquakes indicates that the most recent eastern san cayetano surface rupture was larger than m w 7 and was probably of the order of magnitude 7 5 much larger than any earthquakes that have occurred on los angeles metropolitan region faults during the past 150 yr such a large event almost certainly would have been recorded had it occurred during the historical period even if it had occurred during the earliest part of the historical era between ca a d 1780 and 1850 although this surface rupture could have occurred during the century immediately preceding the historic era the location and large size of the event lead us to suggest that the trench exposed evidence for the damaging earthquake of 21 december 1812 whether the most recent surface rupture on the eastern san cayetano fault was the 21 december 1812 event these paleoseismologic data add to a growing body of evidence that shows that large earthquakes have occurred on faults within and adjacent to the los angeles metropolitan region reemphasizing the need to include such large events on urban faults in future seismic hazard analyses of the region

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

thin skinned tectonics of the upper ojai valley and sulphur mountain area ventura basin california by integrating surface mapping with subsurface well data and drawing cross sections and subsurface maps the geometry of shallow structures and their geologic history of the upper ojai valley of california can be reconstructed the geometry of shallow structures the geologic history and the location of earthquake foci then offer constraints on the deep structure of this complex area the upper ojai valley is a tectonic depression between opposing reverse faults its northern border is formed by the active north dipping san cayetano fault which has 6 0 km of stratigraphic separation in the silverthread area of the ojai oil field and 2 6 km of stratigraphic separation west of sisar creek the fault dies out farther west in ojai valley where the south vergent shortening is transferred to a blind thrust the southern border of the upper ojai valley is formed by the quaternary lion fault set sisar big canyon and lion faults which dips south and merges into the sisar decollement within the south dipping ductile lower miocene rincon formation folds with north dipping axial planes including the lion mountain anticline and reeves syncline are middle pleistocen or older and are related to movement on an unnamed fault in the footwall block of the san cayetano fault and on the san cayetano fault respectively by the middle pleistocene the sulphur mountain anticlinorium and the big canyon syncline began forming as a fault propagation fold the fault propagation fold is rooted in the sisar decollement a passive backthrust rising from a blind thrust at depth the formation of the sulphur mountain anticlinorium was followed closely by the ramping of the south dipping lion fault set to the surface over the nonmarine upper pleistocene saugus formation to the east the san cayetano fault overrides and folds the lion fault set near the surface area balancing of the deformation shows shortening of 15 5 km and suggests a 17 km depth to the brittle d ctile transition