holocene rate of slip and tentative recurrence interval for large earthquakes on the san andreas fault cajon pass southern california abstract detailed mapping of the san andreas fault zone where it crosses cajon creek southern california has revealed a number of late quaternary deposits and geomorphological features offset by the fault radiocarbon dates from alluvial and swamp deposits provide a detailed chronology with which to characterize the activity of the san andreas fault four independent determinations of the slip rate on the san andreas fault yield an average rate of 24 5 3 5 mm yr for the past 14 400 years the similarity of the four values which span different intervals of time from 5900 to 14 400 years ago suggest that the slip rate has been constant during this period the slip rate confirms that the san andreas fault is accumulating slip faster than any other fault in the plate boundary throughout california also the sum of the slip rates on the san andreas 24 5 mm yr and the san jacinto 10 mm yr faults south of their junction is the same as the san andreas north of their junction 35 mm yr an excavation provided evidence for at least two and up to four earthquakes that caused rupture on the fault between 1290 and 1805 a d and tentative evidence for six earthquakes in about the last 1000 years both lines of evidence imply an average recurrence interval for large earthquakes of about 1 to 2 centuries combined with the historic record this investigation indicates that the last major earthquake at cajon creek was probably at the beginning or middle of the 18th century while the record at cajon creek may suggest that the next major earthquake on the san andreas fault is overdue the data can be interpreted to indicate that the recurrence interval for earthquakes on the san andreas south of the 1857 rupture is 3 to 4 centuries and that the anticipated event should not be expected until the next century

analytical study of hydraulic and mechanical effects on tide induced head fluctuation in a coastal aquifer system that extends under the sea summary head response to sea level fluctuations has been extensively used for characterizing coastal aquifers when the aquifer is semiconfined and extends for a certain distance d under the sea head response results from the superposition of two types of effects hydraulic i e ground water flow connection through aquifer and aquitard and mechanical induced by tidal loading onto the sea floor solutions are available for this problem that has been analyzed before but only for d zero or infinity these solutions do not allow analyzing aquifer systems that extends for a finite d or identifying them which is critical for coastal aquifer management we derive an exact analytical solution that describes separately the mechanical and hydraulic effects the proposed analytical solution is a generalization of most of existing analytical solutions a simpler approximate analytical solution is also obtained for soft aquitards with low permeabilities we find that the impact of the hydraulic component of the aquitard and the mechanical effects in the total head fluctuation at the shoreline is significant but not very sensitive to the properties of the aquitard the amplitude of these fluctuations relative to that of the sea tide ranges approximately between 1 small d and 0 5 large d this implies that aquifer penetration under the sea can indeed be identified if it is below a certain threshold beyond which the system responds as if d was infinity surprisingly the time lag is close to zero regardless of hydraulic parameters of the aquifer system

sea beam survey of an active strike slip fault the san clemente fault in the california continental borderland the san clemente fault located in the california continental borderland is an active northwest trending right lateral wrench fault sea beam data are used to map the major tectonic landforms associated with active submarine faulting in detail unavailable using conventional echo sounding or seismic reflection data in the area between north san clemente basin and fortymile bank the major late cenozoic faults are delineated by alignments of numerous tectonic landforms including scarps linear trenches benches and sags character and spatial patterns of these landforms are consistent with dextral wrench faulting although vertical offsets may be substantial locally the main trace of the san clemente fault cuts a straight path directly across the rugged topography of the region evidence of a steeply dipping fault surface basins or sags located at each right step in the en echelon pattern of faults are manifestations of pull apart basin development in a right slip fault zone seismic reflection profiles show offset reflectors and a graben in late quaternary turbidites of the navy fan where the fault zone follows a more northerly trend modern tectonic activity along the san clemente fault zone is demonstrated by numerous earthquakes with epicenters located along the fault s trend the average strike of the san clemente fault is parallel to the predicted pacific north american relative plate motion vector at this location therefore we conclude that the san clemente fault zone is a part of the broad pacific north american transform plate boundary and that the southern california region may be considered as a broad shear zone

late cenozoic tectonic evolution of the los angeles basin and inner california borderland a model for core complex like crustal extension we propose that the los angeles basin inner california borderland lab ib and the southern borderland off baja california formed by large magnitude crustal extension beginning in latest oligocene earliest miocene time and that these regions represent major late cenozoic rifts the southern borderland is analogous to the gulf of california rift but the onset of rifting in this region may have preceded the opening of the gulf by as much as 15 to 20 m y the lab ib rift is more complex than the southern borderland because it formed in the wake of the rotating western transverse ranges wtr a large crustal block that has undergone more than 90 of neogene clockwise rotation total extension in both rifted regions is estimated to be in excess of 200 km or comparable to the magnitude of late cenozoic crustal extension now estimated for the basin and range province also like the basin and range the crustal floors of the basins in these highly extended domains are characterized by metamorphic core complexes and detachment faults voluminous miocene volcanic rocks and hanging wall fragments of pre rift strata and basement rocks stranded atop detachments during extension in support of this model we synthesize a wide variety of existing geologic data and present interpretations of new geophysical data seismic reflection profiles these profiles image features that we infer are detachment faults and overlying remnants of fragmented and rifted hanging wall rocks we also suggest that a similar but slightly earlier onset of crustal extension affected the santa maria basin and adjacent regions in central california and that the southern margin of this santa maria basin rift was overridden during neogene clockwise rotation of the wtr

luminescence dates for the palaeolithic site of piekary iia poland comparison between tl of burnt flints and osl of a loess like deposit we report the thermoluminescence age estimates for burnt flints excavated at the palaeolithic site of piekary iia and discuss attempts to date quartz grains extracted from the embedding deposit using the osl technique with the sar protocol the results indicate that this loess like deposit contains a mixture of grains well bleached at different periods in the past and that the study of small aliquots seems to be the only way to discriminate between the different fractions

analogue modelling of orthogonal and oblique rifting abstract the three dimensional progressive development of orthogonal and oblique rift systems has been simulated using scaled analogue sandbox models the orientation of the zone of rifting where angle between the rift axis and the extension direction was varied systematically from 90 orthogonal rifting to 75 60 and 45 oblique rifting completed models were impregnated and sectioned both vertically and horizontally orthogonal rift models are characterized by long straight rift border faults and intra rift faults perpendicular to the extension direction oblique rift models are characterized by shorter segmented rift border faults and intrarift faults in the oblique models the rift border faults formed en echelon arrays parallel to the inderlying zone of basement stretching whereas the intra rift faults formed at a high angle to the extension vector increased extension in the moderately oblique and oblique rift systems 60 and 45 caused the intra rift faults to rotate towards parallelism with the rift border faults no strike slip or oblique slip transfer faults were found in any experiments rift border faults and intra rift faults in the models are segmented along strike with relay ramp structures formed between overlapping fault tips along the strike of the rift variations in half graben polarities and offsets of depocentres are accomplished by accommodation zones formed by interlocking conjugate fault arrays the experimental fault patterns compare well with the fault architectures found in those modern or relatively young rift systems where well defined basement controlled strike slip or oblique slip transfer faults are absent an accommodation zone model consisting of interlocking conjugate extension faults is proposed for the along strike switching of half graben asymmetries and offsetting of depocentres in rift systems the fault geometries in the analogue models can be used as templates for recognition of fault styles in both orthogonal and in oblique rift environments

upper crustal structure and neogene tectonic development of the california continental borderland multichannel seismic reflection data sonobuoy seismic refraction data and regional geology are used to define the upper crustal structure of the southern california continental borderland and to delineate the characteristics of the main lithotectonic belts of the region the catalina schist belt is separated on its west side from the gently deformed nicolas forearc belt by faults that have steep west dips and pronounced normal separations on its east side the schist belt is bounded by a large detachment fault that dips gently to the east beneath the west edge of the peninsular ranges belt at the coastline near oceanside the catalina schist was uplifted from middle crustal depths and exposed during a major event of extensional tectonism that started in early miocene time in conjunction with about 10 of clockwise rotation of the western transverse ranges belt part of the uplift of the catalina schist could have occurred on the detachment fault but it is thought to have mostly occurred on the steep faults that bound the west edge of the schist belt a large amount of uplift is required and it probably involved strong footwall flexural deformation in the wake of the translating and rotating western transverse ranges and nicolas forearc belts extension accompanied by probable large amounts of right slip continued in the borderland region during and after middle miocene time the later stage of extension was accompanied by rapid clockwise rotation of the western transverse ranges of at least 90 most of the borderland including the belt of schist that was uplifted in early miocene time was further deformed into numerous basins and ridges during this stage of oblique extension the primary driving force for the deformation is thought to have been derived from the rapid northwest motion of the pacific plate after it had become coupled to the farallon plate system which had previously been subducted beneath the borderland

tsunami sources in the southern california bight 1 locally generated tsunamis due to faulting or slope failures offshore southern california threaten nearby coastal cities disruption of operations at port facilities due to tsunami attack could severely impact the regional and national economies this study examines three faults and two landslides scenarios as potential tsunami sources computed runup ranges from 0 5 to 6 m depending on the source and location bathymetric features such as the broad san pedro shelf are shown to contribute to tsunami wave focusing while retarding tsunami arrival times suggesting the usefulness of an early warning system

complex fault interactions in a restraining bend on the san andreas fault southern santa cruz mountains california the unusual oblique thrust mechanism of the october 18 1989 loma prieta earthquake focused attention on the complex tectonic setting of this segment of the san andreas fault near the mainshock epicenter the san andreas fault curves to the left defining a restraining bend the large thrust component of the mainshock focal mechanism is consistent with the horizontal compression expected across restraining bends however repeated loma prieta type earthquakes cannot exclusively produce the observed topography of the southern santa cruz mountains the highest point of which experienced subsidence during the 1989 earthquake in this paper the authors integrate seismic geomorphic and tectonic data to investigate the possibility that motions on faults adjacent to the san andreas fault play an important role in producing the observed topography the three dimensional geometry of active faults in this region is imaged using the loma prieta preshock and aftershock sequences the most conspicuous features of the fault geometries at depth are 1 the presence of two distinct zones of seismicity corresponding to the san andreas and the sargent berrocal fault zones 2 the concave upward shape of the loma prieta rupture surface 3 the reduction in dip of the deepest portions of the rupture plane asmore the mainshock hypocenter is approached 4 the apparent transfer of shallow slip in some areas from faults in the san andreas fault zone to those in the sargent berrocal fault zone and 5 the presence of a deep northeasterly dipping plane associated with the sargent berrocal fault zone the authors find that a model of fault interactions which involves displacement on faults in both the san andreas and the sargent berrocal fault zones is consistent with loma prieta coseismic displacements preshock and aftershock seismicity and observed topography less

a transform continental margin rich in hydrocarbons gulf of california conventional and near bottom geophysical surveys mapped shallow deformational structures and sediment accumulation patterns along part of the actively shearing continental margin of sonora including the intersection with a guaymas basin spreading center the principal strike slip faults occupy a zone 1 2 km 0 6 1 2 mi wide along the lower slope with a ridge of tightly folded sediments raised by uplift of sheared basement at the abrupt boundary between continental and oceanic crust this transform ridge and a continental rise on its seaward slope grows as the fault zone ages away from the spreading center intersection shoaling at a net rate of 500 m m y 1 600 ft m y despite erosion of its crest a clathrate horizon in the upper 80 m 260 ft of sediment is inferre from bottom simulating reflectors in the rise and lower slope hydrocarbon seeps occur at 1 600 m 1 mi below sea level along the crest of the transform ridge and patchy gas accumulation and seepage are recognized on a young marginal plateau

surface ruptures of the 17 august and 12 november 1999 izmit and duzce earthquakes in northwestern anatolia turkey their tectonic and kinematic significance and the associated damage the 17 august izmit kocaeli and 12 november duzce earthquakes m w 7 4 and 7 1 respectively represent the most recent two of a series of large earthquakes along the north anatolian fault a highly active right lateral strike slip fault that crosses the anatolian peninsula in turkey from one end to the other the first earthquake rupture apparently initiated at a small step over along a relatively inconspicuous fault strand following the southern coast of the izmit bay and propagated to the west and to the east for a distance of 125 150 km the western segment enters the bay in both directions after attaining the largest surface slip of about 5 m at the town of golcuk the eastern extension of the surface rupture associated with the izmit earthquake is composed of five major segments with the maximum slip magnitudes from 3 to 4 4 5 0 5 1 4 and 0 1 m from west to east respectively the segment with 0 5 m slip trends n70 w the segment with 1 4 m slip trends n70 e and the easternmost segment with minor slip trends east west again the surface manifestation of the duzce earthquake is an extensively segmented rupture trace with dominant transtensional deformation it has a normal slip component of more than 3 7 m and a right lateral slip close to 1 4 m along the western part that showed minor rupture during the first earthquake the central part has a transpressional character with about 1 m reverse slip and about 3 m right lateral slip the highest right lateral slip nearly 5 m occurs at the central part the eastern part of the rupture displays a typical strike slip deformation with less than about 3 m right lateral slip and minor local vertical slip that exhibits changing polarity the varying orientations of the rupture segments reflect the fault mosaic of the broader region and the interaction between neighboring faults the temporal and spatial characteristics of the rupture segments as well as the significance of the nearly 3 month time period between the first and second earthquakes pose highly challenging mechanical problems regarding the rupture initiation propagation and termination processes the main surface rupture zone although unusually narrow 2 5 6 m left a path of death and destruction manuscript received 1 june 2000

structure and kinematic genesis of the quealy wrench duplex transpressional reactivation of the precambrian cheyenne belt in the laramie basin wyoming seismic and borehole data in the southern laramie basin of southeastern wyoming outline a fault bordered pop up structure that is central to the quealy wrench duplex and the site of the quealy dome oil field this contractional duplex formed at a restraining fault offset between two parallel laterally extensive northeast trending fault zones the south and north quealy faults spaced about 2 mi 3 2 km apart these parallel fault zones are interpreted as wrench straights equivalent to the floor and roof thrusts of a duplex in a fold and thrust belt but described in map view rather than in cross section the quealy wrench duplex is comprised of three east vergent basement involved thrust imbricates and a west vergent antithetic thrust detached in permian shales duplex thrusts strike at high angles to the bordering south and north quealy fault zones measurements based on interpretive piercing point evidence and fault parallel differential shortening indicate that net slip on the dominant south quealy fault zone is dextral oblique a component of dextral slip is also indicated on the north quealy fault zone the south quealy fault and north quealy overland fault trends can be traced southwesterly into the footwall of the north trending arlington basin boundary thrust and projected into the precambrian cheyenne belt of the medicine bow mountains in the hanging wall of the arlington thrust it is proposed that the quealy wrench duplex is a product of precambrian shear zone reactivation along the cheyenne belt under permian and laramide tertiary transpression

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

evolution of the northern santa cruz mountains by advection of crust past a san andreas fault bend the late quaternary marine terraces near santa cruz california reflect uplift associated with the nearby restraining bend on the san andreas fault excellent correspondence of the coseismic vertical displacement field caused by the 17 october 1989 magnitude 7 1 loma prieta earthquake and the present elevations of these terraces allows calculation of maximum long term uplift rates 1 to 2 kilometers west of the san andreas fault of 0 8 millimeters per year over several million years this uplift in concert with the right lateral translation of the resulting topography and with continual attack by geomorphic processes can account for the general topography of the northern santa cruz mountains

application of the global positioning system to crustal deformation measurements 3 result from the southern california borderlands five years of measurements from the global positioning system gps satellites collected between 1986 and 1991 are used to investigate deformation in the offshore regions of southern california gps provides the first practical technique to make precise geodetic measurements in the region the geodetic network is situated along the california coastline from vandenberg 120 6 w 34 6 n to san diego with additional sites on santa cruz san nicolas santa catalina santa rosa and san clemente islands the precision of horizontal interstation vectors is subcentimeter and the interstation vector rate between ovro and vandenberg agrees with the very long baseline interferometry derived rate to within one standard deviation no significant motion is observed in the western santa barbara channel between vandenberg and santa rosa island 0 5 1 6 mm yr where the quoted uncertainties are one standard deviation motions in the eastern santa barbara channel are consistent with compressional deformation of 6 1 mm yr at n16 3 e this motion is in agreement with seismicity and an independent geodetic analysis for the period 1971 1987 larsen 1991 san clemente island is moving relative to san diego at the rate of 5 9 1 8 mm yr at a direction of n38 20 w the motion between san nicolas island and san clemente island 0 8 1 5 mm yr is insignificant

seismic characteristics and identification of negative flower structures positive flower structures and positive structural inversion negative and positive flower structures and positive inverted structures imply specific modes of formation and their distinctive characteristics make them important criteria for the identification of certain structural styles a negative flower structure from the andaman sea consists of a shallow synform bounded by upward spreading strands of a wrench fault that have mostly normal separations paralleling monoclines and oblique en echelon normal faults flank the divergent wrench fault a positive flower structure from the ardmore basin oklahoma consists of a shallow antiform displaced by the upward diverging strands of a wrench fault that have mostly reverse separations en echelon folds are present on either side of this convergent wrench fault flower structures may resemble extensional or contractional fault blocks on individual seismic profiles flower structures are identified by their unique internal fault and fold architecture by their association with a straight and throughgoing zone of deformation and by their lack of a consistent sense of separation or regional upthrown side positive structural inversion at the rambutan oil field south sumatra basin has formed a shallow anticlinorium and has partly uplifted the underlying graben deeper fault segments bounding the graben have retained their normal fault profiles but at shallow levels some of these faults have reverse separations similar changes of separation with depth may be produced by strike slip but sets of faults with listric profiles and nearby blocklike trend patterns identify the inversion as a dip slip event where correlation of reflections is uncertain or deep control is unobtainable positive inverted structures may also resemble contrational fault blocks detached normal fault assemblages and certain negative inverted structures

large enigmatic crater structures offshore southern california summary digital mosaics of swath and conventional bathymetry data reveal large distinct near circular crater structures in the inner continental borderland offshore southern california two have maximum crater diameters that exceed 30 km and a third has a crater diameter of approximately 12 km all three features exhibit the morphology of large complex craters raised outer rim ring moat and central uplift yet their exact origin remains a mystery preliminary analyses of available seismic gravity and magnetic data over these structures reveal both similarities and distinct differences in geometry structure and geophysical signature to known impact sites all three crater structures however occur within the catalina terrane a highly extended volcanic and metamorphic province floored by catalina schist basement a likely alternative origin may thus involve explosive volcanism caldera collapse and resurgent magmatism and or possibly plutonism and schist remobilization associated with the catalina terrane no single model for crater formation whether impact caldera or pluton fully accounts for all of the present observations regarding the morphology internal structure and known geology of these near circular features timing of crater formation post dates the initial rifting and rotation of the western transverse ranges and appears to predate major right slip along the san clemente and san diego trough fault systems or approximately 18 to 16 ma regardless of their origin these complex craters represent some of the largest structures of their kind in western north america and provide a unique opportunity to better understand the development of unusual crater structures in a submarine environment

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

tectonic geomorphology of the san andreas fault zone in the southern indio hills coachella valley california geomorphic investigation of the san andreas fault zone in the indio hills indicates many tectonically produced landforms including beheaded streams right lateral deflected and offset streams sags shutter ridges pressure ridges and fault scarps near biskra palms an alluvial fan pediment complex has an apparent cumulative offset of about 0 7 km along the mission creek fault zone north branch san andreas fault many of the tectonic landforms as well as the fracture pattern that has developed during the pleistocene are explainable by simple shear or uplift associated with a small left bend in the main trace of the mission creek fault the ratio of vertical to horizontal displacement in the vicinity of the bend is about 0 04 exposed in pushawalla canyon 5 km northwest of the alluvial fan pediment complex are 1 a sequence of stream terraces 2 folded plio pleistocene fanglomerates and 3 an example of stream capture following a right lateral deflection or offset of several hundred metres a left step of the mission creek fault in pushawalla canyon is a probably cause of folds and sporadic uplift that produced the stream terraces possible cause of recent stream capture are 1 juxtaposition of pushawalla canyon with a relict canyon moving northwestward along the mission creek fault or 2 right lateral deflection or offset of pushawalla canyon along the fault with simultaneous headward erosion of a shorter steep stream flowing toward the coachella valley estimation of a slip rate and identification of paleoseismicity for the san andreas fault in the indio hills is difficult however degree of topographic dissection formation and preservation of desert pavement and relative soil profile development suggest that the age of the offset fan may be as old as 70 000 yr but that most likely it is on the order of 20 000 to 30 000 yr these age estimates for the offset fan indicate a minimum slip rate for the san andreas fault of 10 to 35 mm yr with about 23 to 35 mm yr the most likely the pattern of observed offset drainages is complex but suggests that during the past few thousand years creep events or moderately large earthquakes have periodically produced several metres of right lateral displacement

holocene and late pleistocene slip rates on the san andreas fault in yucaipa california using displaced alluvial fan deposits and soil chronology we derived slip rates on the san andreas fault from displacement of alluvial fans and from approximate dating of the fans by the degree of soil development distinctive red purple sedimentary rocks of the miocene potato sandstone exposed on yucaipa ridge in the san bernardino mountains were transported down wilson creek and shed across the modern trace of the san andreas fault soils developed in alluvial fan deposits contain red purple clasts of the potato sandstone and are progressively more developed toward the north west as a result of fault displacement we used soil development to estimate the ages of these alluvial fans and underlying deposits using dated soils from cajon pass and from the central valley of california we estimated ages of wilson creek deposits using a statistical method of maximum likelihood these methods also utilize monte carlo simulations of larger data populations and account for soil variability uncertainty of calibration dates and the limited number of soils described and sampled resulting age estimates have considerable as much as 80 uncertainty resulting largely from quality of calibration dates slip rates for three time intervals were obtained by restoring the medial axis of each displaced fan to wilson creek maximum slip rates were also obtained by restoring the westernmost extent of each fan to wilson creek best estimate rates averaged to the present for 0 to 14 000 yr are 14 to 25 mm yr for 0 to 30 000 yr 22 to 34 mm yr and for 0 to 65 000 or 90 000 yr 12 to 16 mm yr maximum rates averaged to present for 0 to 14 000 yr are as high as 65 mm yr older rates are as high as 80 mm yr when holocene rates are accounted for actual or incremental slip rates for the interval 30 000 to 65 000 or 90 000 yr b p are probably 6 to 13 mm yr these data tentatively suggest that slip rates on the modern trace have increased during the late quaternary uncertainties in slip and age data however allow for rates to have been nearly constant or even decreasing toward the present

geophysical evidence for wedging in the san gorgonio pass structural knot southern san andreas fault zone southern california geophysical data and surface geology define intertonguing thrust wedges that form the upper crust in the san gorgonio pass region this picture serves as the basis for inferring past fault movements within the san andreas system which are fundamental to understanding the tectonic evolution of the san gorgonio pass region interpretation of gravity data indicates that sedimentary rocks have been thrust at least 5 km in the central part of san gorgonio pass beneath basement rocks of the southeast san bernardino mountains subtle long wavelength magnetic anomalies indicate that a magnetic body extends in the subsurface north of san gorgonio pass and south under peninsular ranges basement and has a southern edge that is roughly parallel to but 5 6 km south of the surface trace of the banning fault this deep magnetic body is composed either of upper plate rocks of san gabriel mountains basement or rocks of san bernardino mountains basement or both we suggest that transpression across the san gorgonio pass region drove a wedge of peninsular ranges basement and its overlying sedimentary cover northward into the san bernardino mountains during the neogene offsetting the banning fault at shallow depth average rates of convergence implied by this offset are broadly consistent with estimates of convergence from other geologic and geodetic data seismicity suggests a deeper detachment surface beneath the deep magnetic body this interpretation suggests that the fault mapped at the surface evolved not only in map but also in cross sectional view given the multilayered nature of deformation it is unlikely that the san andreas fault will rupture cleanly through the complex structures in san gorgonio pass

global positioning system constraints on fault slip rates in southern california and northern baja mexico we use global positioning system gps estimates of horizontal site velocity to constrain slip rates on faults comprising the pacific north america plate boundary in southern california and northern mexico we enlist a simple elastic block model to parameterize the distribution and sum of deformation within and across the plate boundary we estimate a pacific north america relative plate motion rate of 49 3 mm yr one standard deviation consistent with nuvel 1a estimates we are able to resolve robust slip rate estimates for the southernmost san andreas san jacinto and elsinore faults 26 2 9 2 and 6 2 mm yr respectively and for the imperial and cerro prieto faults 35 2 and 42 1 mm yr respectively accounting for about 86 of the total plate motion the remaining 14 appears to be accommodated to the west of these fault systems probably via slip along the san clemente fault and or the san miguel vallecitos rose canyon and newport inglewood fault systems these results are highly consistent with paleoseismic estimates for slip rates implying that off fault strain accumulation within the deforming zone of the plate boundary is largely elastic we estimate that the seismically quiescent southernmost san andreas fault has incurred about 8 2 m of slip deficit over the last few hundred years presumably to be recovered during a future large earthquake

geophysical evidence for the evolution of the california inner continental borderland as a metamorphic core complex we use new seismic and gravity data collected during the 1994 los angeles region seismic experiment larse to discuss the origin of the california inner continental borderland icb as an extended terrain possibly in a metamorphic core complex mode the data provide detailed crustal structure of the borderland and its transition to mainland southern california using tomographic inversion as well as traditional forward ray tracing to model the wide angle seismic data we find little or no sediments low 6 6 km s p wave velocity extending down to the crust mantle boundary and a thin crust 19 to 23 km thick coincident multichannel seismic reflection data show a reflective lower crust under catalina ridge contrary to other parts of coastal california we do not find evidence for an underplated fossil oceanic layer at the base of the crust coincident gravity data suggest an abrupt increase in crustal thickness under the shelf edge which represents the transition to the western transverse ranges on the shelf the palos verdes fault merges downward into a landward dipping surface which separates basement from low velocity sediments but interpretation of this surface as a detachment fault is inconclusive the seismic velocity structure is interpreted to represent catalina schist rocks extending from top to bottom of the crust this interpretation is compatible with a model for the origin of the icb as an autochthonous formerly hot highly extended region that was filled with the exhumed metamorphic rocks the basin and ridge topography and the protracted volcanism probably represent continued extension as a wide rift until 13 m y ago subduction of the young and hot monterey and arguello microplates under the continental borderland followed by rotation and translation of the western transverse ranges may have provided the necessary thermomechanical conditions for this extension and crustal inflow

the palos verdes terraces california bathtub rings from a buried reverse fault uplift of the palos verdes peninsula has long been associated with a northwest trending southwest dipping reverse fault unfortunately the palos verdes hills fault has no obvious surface displacement and little background seismicity to substantiate its dimension orientation or earthquake potential in this paper we investigate the tectonic style and slip rate of the palos verdes hills fault and the uplift history of the palos verdes hills by analyzing the geometry of 13 marine terraces that encircle the peninsula in a bathtub ring configuration elevations of 211 terrace remnants constrain a fault model with 3 0 to 3 7 mm yr 1 of oblique dextral reverse slip on a fault dipping 67 at 6 to 12 km depth beneath the peninsula if the rate was constant through time fault inception would have occurred 2 4 3 0 ma we propose that the largest credible earthquakes on the fault have magnitude 6 and could revisit every 2000 years