origin of high mountains in the continents the southern sierra nevada active and passive seismic experiments show that the southern sierra despite standing 1 8 to 2 8 kilometers above its surroundings is underlain by crust of similar seismic thickness about 30 to 40 kilometers thermobarometry of xenolith suites and magnetotelluric profiles indicate that the upper mantle is eclogitic to depths of 60 kilometers beneath the western and central parts of the range but little subcrustal lithosphere is present beneath the eastern high sierra and adjacent basin and range these and other data imply the crust of both the high sierra and basin and range thinned by a factor of 2 since 20 million years ago at odds with purported late cenozoic regional uplift of some 2 kilometers

structural geometry and evolution of releasing and restraining bends insights from laser scanned experimental models experimental modeling is used to study the geometry and evolution of structures and related secondary faults along releasing bends and offsets and restraining bends on strike slip faults the controls of the relative positions of adjacent strike slip faults on the geometry of the structures and the difference in geometries between bends and offsets are investigated a new method of laser scanning is used to map the geometry and evolution of the structures and related faults the models show that oblique releasing bends connecting approaching faults result in spindle shaped basins whereas transverse bends result in more s shaped or rhomboidal basins offsets result in the distribution of strain over a wider area and a larger number of faults compared with preexisting bends which result in fewer well defined basin bounding faults secondary faults include r r and y riedel shears near the main strike slip faults and oblique normal faults in the center of the basin fault patterns exhibit en echelon geometries with a progressive step down into the deepest parts of the basin symmetric asymmetric and double basins may form in any of the structural settings depending on the slip distribution among faults on the basin margins for restraining bends oblique 45 bends connecting approaching faults result in spindle shaped uplifts whereas transverse or oblique 135 bends connecting overlapping faults result in more rhomboidal or rectangular uplifts the fold trends are at increasingly higher angles with the strike faults for transverse and oblique 135 bends secondary faults include en echelon reverse faults which typically form along the steep limbs of asymmetric uplifts normal faults which are transverse or oblique to the axis of the structure and r r and y riedel shears near the main strike slip faults the aspect ratios of the basins and uplifts increase with increasing displacement on the strike slip faults the results of these models can be used to interpret the structural and fault geometries in surface and subsurface structures formed along strike slip faults

characteristic slip for five great earthquakes along the fuyun fault in china the seismic hazard associated with an individual fault can be assessed from the distributions of slip and recurrence times of earthquakes however seismic cycle models 1 that aim to predict rupture lengths and fault displacements of successive earthquakes on one fault remain poorly validated it is therefore unknown whether individual fault segments rupture independently producing earthquakes with a diverse range of magnitudes and recurrence times or slip by characteristic amounts with characteristic magnitudes here we use highresolution satellite data to document the horizontal offsets of stream channels and terraces created by strike slip motion on the fuyun fault xinjiang china during five historical earthquakes we find that the ms 7 9 11 august 1931 earthquake produced a surface rupture with a length of 160 km dispersed over three different fault segments the 290 measured stream channel and terrace offsets record an average slip of 6 3 m we use the degree of preservation of geomorphological markers to assign relative ages to individual fault offsets and identify at least four distinct older earthquakes we find that these older earthquakes also produced fault offsets with a similar distribution to the 1931 earthquake as the slip distributions during five successive earthquakes were so similar we conclude that ruptures on the fuyun fault obey a characteristic slip model very few large earthquakes are known to have ruptured the same fault segment twice in historical or instrumental time 1 the

the distribution of radiogenic heat production as a function of depth in the sierra nevada batholith california geochemical analyses and geobarometric determinations have been combined to create a depth vs radiogenic heat production database for the sierra nevada batholith california this database shows that mean heat production values first increase then decrease with increasing depth heat production is 2 aw m 3 within the 3 km thick volcanic pile at the top of the batholith below which it increases to an average value of 3 5 aw m 3 at 5 5 km depth then decreases to 0 5 1 aw m 3 at 15 km depth and remains at these values through the entire crust below 15 km below the crust from depths of 40 125 km the batholith s root and mantle wedge that coevolved beneath the batholith appears to have an average radiogenic heat production rate of 0 14 aw m 3 this is higher than the rates from most published xenolith studies but reasonable given the presence of crustal components in the arc root assemblages the pattern of radiogenic heat production interpreted from the depth vs heat production database is not consistent with the downward decreasing exponential distribution predicted from modeling of surface heat flow data the interpreted distribution predicts a reasonable range of geothermal gradients and shows that essentially all of the present day surface heat flow from the sierra nevada could be generated within the 35 km thick crust this requires a very low heat flux from the mantle which is consistent with a model of cessation of sierran magmatism during laramide flat slab subduction followed by conductive cooling of the upper mantle for 70 m y the heat production variation with depth is principally due to large variations in uranium and thorium concentration potassium is less variable in concentration within the sierran crust and produces relatively little of the heat in high heat production rocks because silica content is relatively constant through the upper 30 km of the sierran batholith while u th and k concentrations are highly variable radiogenic heat production does not vary directly with silica content d 2005 elsevier b v all rights reserved

integration of paleoseismic data from multiple sites to develop an objective earthquake chronology application to the weber segment of the wasatch fault zone utah we present a method to evaluate and integrate paleoseismic data from multiple sites into a single objective measure of earthquake timing and recurrence on discrete segments of active faults we apply this method to the weber segment ws of the wasatch fault zone using data from four fault trench studies completed between 1981 and 2009 after systematically reevaluating the stratigraphic and chro nologic data from each trench site we constructed time stratigraphic oxcal models that yield site probability density functions pdfs of the times of individual earth quakes we next qualitatively correlated the site pdfs into a segment wide earthquake chronology which is supported by overlapping site pdfs large per event displace ments and prominent segment boundaries for each segment wide earthquake we computed the product of the site pdf probabilities in common time bins which emphasizes the overlap in the site earthquake times and gives more weight to the narrowest best defined pdfs the product method yields smaller earthquake timing uncertainties compared to taking the mean of the site pdfs but is best suited to earth quakes constrained by broad overlapping site pdfs we calculated segment wide earthquake recurrence intervals and uncertainties using a monte carlo model five surface faulting earthquakes occurred on the ws at about 5 9 4 5 3 1 1 1 and 0 6 ka with the exception of the 1 1 ka event we used the product method to define the earthquake times the revised ws chronology yields a mean recurrence interval of 1 3 kyr 0 7 1 9 kyr estimated two sigma 2 range based on interevent recurrence these data help clarify the paleoearthquake history of the ws including the important question of the timing and rupture extent of the most recent earthquake and are essential to the improvement of earthquake probability assessments for the wasatch front region

evidence for seven surface ruptures in the past 1600 years on the claremont fault at mystic lake northern san jacinto fault zone california we present new data from the mystic lake paleoseismic site along the claremont segment of the northern san jacinto fault zone the site is located within a sag formed between two fault strands that pass through the eastern side of mystic lake in the san jacinto valley trenches excavated across the sag exposed faulted and folded lacustrine and alluvial strata that record at least seven ground rupturing earthquakes during the past 1600 yr evidence for past surface deformation includes upward terminating faults with associated fissure fills folding angular unconformities and pinching of strata against a paleoscarp all of the event horizons occur at the tops of paleosols and are overlain by massive lacustrine clay units we interpret this pattern to represent development of soils at the surface between earthquakes that are buried when fault rupture causes subsidence and renewed filling of the depression with lacustrine sediments the ages of the events are constrained by 50 radiocarbon dates determined from detrital charcoal the recurrence interval for the past seven events ranges from 159 to 210 yr and the most recent event occurred sometime between a d 1738 and 1850 based on radiocarbon ages trimmed by historical data some of the event ages at mystic lake overlap in time with events recorded at hog lake on the clark strand of the san jacinto fault zone to the south suggesting that these events may have jumped the san jacinto valley releasing stepover or that events on one fault triggered closely timed events on the adjacent fault

towards inferring earthquake patterns from geodetic observations of interseismic coupling ultimately seismotectonic studies seek to provide ways of assessing the timing magnitude and spatial extent of future earthquakes ample observations document the spatial variability in interseismic coupling defined as a degree of locking of a fault during the period of stress build up between seismic events fully or nearly locked fault patches are often surrounded by aseismically creeping areas however it is unclear how these observations could help assess future earthquakes here we simulate spontaneous seismic and aseismic fault slip with a fully dynamic numerical model our simulations establish the dependence of earthquake rupture patterns and interseismic coupling on spatial variations of fault friction we consider the long term evolution of slip on a model fault where two seismogenic locked segments are separated by an aseismically slipping patch where rupture is impeded we find that the probability for a large earthquake to break through the rupture impeding patch is correlated with the interseismic coupling averaged over this patch in addition the probability that an earthquake breaks through the rupture impeding patch and interseismic coupling are both related to fault friction properties through a single non dimensional parameter our study opens the possibility of predicting seismic rupture patterns that a fault system can produce on the basis of observations of its interseismic coupling and suggests that regions of low interseismic coupling may reveal permanent barriers to large earthquakes

san andreas fault zone california m 5 5 earthquake history the san andreas fault zone has been a very significant source of major california earthquakes from 1812 to 1906 it generated four major earthquakes of m 7 or larger in two pairs on two major portions of the fault a pair of major earthquakes occurred on the central to southern region where the 1857 faulting overlapped the 1812 earthquake faulting a pair of major earthquakes occurred on the northern region where the 1906 faulting overlapped the 1838 earthquake faulting also earthquakes of m 7 occurred in the san francisco bay area on the hayward fault in 1868 and the santa cruz mountains near loma prieta in 1989 and on the imperial fault near the border with mexico in 1940 the 1838 earthquake s damage effects throughout the bay area from san francisco to santa clara valley and monterey were unequalled by any historical earthquake other than the 1906 event this and numerous strong possible aftershocks during the following 3 years in the san juan bautista vicinity suggest 1838 faulting from san francisco to san juan bautista cycles of seismicity and quiescence were associated with the bay area earthquakes of 1868 1906 and 1989 the 1868 earthquake on the hayward fault was preceded by 12 earthquakes of m 5 5 from 1855 to 1866 within 60 km of the hayward fault and was followed by 13 quiet years the 1906 san andreas fault event was preceded from 1881 to 1903 by 18 earthquakes of m 5 5 and was followed by quiescence with only three earthquakes of m 5 5 until 1954 the bay area has been seismically quiet at the m 5 5 level since the 1989 loma prieta earthquake and its 1990 aftershocks which contrasts with the 10 years before 1989 when five m 5 5 6 2 events occurred the loma prieta earthquake is of similar magnitude to the 1868 hayward event and could be followed by a similarly short quiet period the 1857 earthquake had immediate foreshocks in the lonoak bitterwater region 50 km northwest of parkfield in the northern end zone of the 1857 rupture extending southeast from bitterwater 70 km to parkfield the rate of seismic moment release has decreased with time since 1857 this may reflect the decay with time of the stress loading due to the 9 m 1857 fault displacements 80 km southeast of parkfield and explain why the predicted earthquake which was based on the assumption of regular recurrence of parkfield earthquakes has not yet occurred the extent of the 1812 earthquake fault rupture is not well defined jacoby et al 1988 estimated that it extended 170 km from cajon pass to tejon pass based on this estimate we present the hypothesis that the rupture occurred in two segments in december 1812 the eastern segment generated the 8 december earthquake that damaged san juan capistrano san gabriel san fernando and san buenaventura thirteen days later the western segment ruptured generating the earthquake that damaged san fernando and san buenaventura again as well as santa barbara santa ynez and purisima concepcion

rayleigh wave phase velocities small scale convection and azimuthal anisotropy beneath southern california 1 we use rayleigh waves to invert for shear velocities in the upper mantle beneath southern california a one dimensional shear velocity model reveals a pronounced low velocity zone lvz from 90 to 210 km the pattern of velocity anomalies indicates that there is active small scale convection in the asthenosphere and that the dominant form of convection is three dimensional 3 d lithospheric drips and asthenospheric upwellings rather than 2 d sheets or slabs several of the features that we observe have been previously detected by body wave tomography these anomalies have been interpreted as delaminated lithosphere and consequent upwelling of the asthenosphere beneath the eastern edge of the southern sierra nevada and walker lane region sinking lithosphere beneath the southern central valley upwelling beneath the salton trough and downwelling beneath the transverse ranges our new observations provide better constraints on the lateral and vertical extent of these anomalies in addition we detect two previously undetected features a high velocity anomaly beneath the northern peninsular range and a low velocity anomaly beneath the northeastern mojave block we also estimate the azimuthal anisotropy from rayleigh wave data the strength is 1 7 at periods shorter than 100 s and decreases to below 1 at longer periods the fast direction is nearly e w the anisotropic layer is more than 300 km thick the e w fast directions in the lithosphere and sublithosphere mantle may be caused by distinct deformation mechanisms pure shear in the lithosphere due to n s tectonic shortening and simple shear in sublithosphere mantle due to mantle flow

platinum group element geochemistry of peridotite xenoliths from the sierra nevada and the basin and range california abstract the nature of pge re pge pt pd os ir ru behavior in subcontinental lithospheric mantle was investigated using new high precision pge re abundance measurements and previously published re os isotopic analyses of peridotite xenoliths from the sierra nevada and mojave province california ru ir ratios and ir concentrations are constant over a wide range in s content and major element fertility indices e g mg mg fe indicating that ru and ir are not only compatible during partial melting but also that their partitioning behaviors may not be controlled entirely by sulfide pt ir pd ir os ir and re ir ratios range from slightly superchondritic to distinctly subchondritic for all xenoliths except for one anomalous sample 1026v which is characterized by radiogenic 187os 188os low re os ratio and large enrichments in cu os pt pd and s relative to ir copps metasomatism assuming chondritic initial relative abundances the magnitudes of some of the depletions in pt pd os and re relative to ir and ru require incompatible behavior or substantial secondary loss in detail some samples which are otherwise characterized by fertile major element indices exhibit low s contents and subchondritic os ir and pd ir ratios indicating that depletions in pd and os relative to ir are not simple functions of the degree of melting as inferred from major elements possible mechanisms for depleting pt pd os and re relative to ir and ru include partitioning into chromian spinels and alloys partitioning between sulfide and sulfide liquids mobilization by aqueous fluids or secondary loss associated with late stage sulfide breakdown however it is not possible to explain all of the depletions in pt pd os and re by any single mechanism the preferential enrichment in os over re and ir in sample 1026v is somewhat paradoxical because this sample s radiogenic 187os 188os requires a metasomatic agent originating from a source with a high time integrated re os ratio the abundant garnet websterite xenoliths may be a suitable source because they have high re os ratios radiogenic os and abundant garnet which may sequester re over os during partial melting however their extremely low os contents require the processing of large amounts of garnet websterite to concentrate enough os into the metasomatic sulfides needed to enrich sample 1026v in os the homogeneity in 187os 188os ratio in the remaining xenoliths suggest that their os isotopic compositions were not significantly affected by pge metasomatism the singular nature of 1026v s composition emphasizes the rarity of copps metasomatism

estimating surface rupture length and magnitude of paleoearthquakes from point measurements of rupture displacement we present a method to estimate paleomagnitude and rupture extent from measurements of displacement at a single point on a fault the variability of historic ruptures is summarized in a histogram of normalized slip then scaled to give the probability of finding a given displacement within a rupture for any magnitude considered the histogram can be inverted assuming any magnitude earthquake is as likely as another yielding probability density functions of magnitude and rupture length for any given displacement measurement to improve these distributions we include a term to account for the probability that the earthquake would cause ground rupture and two alternative distributions of earthquake magnitude the gutenberg richter magnitude distribution predicts shorter rupture lengths and smaller magnitudes than does a uniform distribution where any magnitude earthquake is considered equally likely longer ruptures and larger magnitudes than the uniform model are predicted by an alternative magnitude distribution designed to return site average displacement this model is a generalization of the characteristic earthquake model and reasonably describes paleoseismic findings on the southern san andreas fault where slip is accommodated by average displacements of a few meters and earthquake recurrence times of 100 250 years our results should increase the value of paleoseismic displacement measurements for hazard assessment in particular they quantify probability estimates of earthquake magnitude and rupture length where point observations of rupture displacement are available and so can contribute to probabilistic seismic hazard analyses online material rupture profiles used to sample rupture variability

slip in the 1857 and earlier large earthquakes along the carrizo plain san andreas fault the moment magnitude mw 7 9 fort tejon earthquake of 1857 with a 350 kilometer long surface rupture was the most recent major earthquake along the south central san andreas fault california based on previous measurements of its surface slip distribution rupture along the 60 kilometer long carrizo segment was thought to control the recurrence of 1857 like earthquakes new high resolution topographic data show that the average slip along the carrizo segment during the 1857 event was 5 3 1 4 meters eliminating the core assumption for a linkage between carrizo segment rupture and recurrence of major earthquakes along the south central san andreas fault earthquake slip along the carrizo segment may recur in earthquake clusters with cumulative slip of 5 meters

fragments of the mantle and crust from beneath the sierra nevada batholith xenoliths in a volcanic pipe near big creek california a miocene volcanic pipe that cuts cretaceous granodiorite is exposed in the central sierra nevada near the town of big creek california the volcanic magma began its ascent from a mantle source as an alkalic basalt but was extensively contaminated by incorporation and assimilation of silicic plutonic rocks during ascent resulting in a pseudo andesitic composition the conduit cut through the entire thickness of the sierra nevada batholith as well as the underlying lower crust and upper mantle and the magma entrained a diverse polygenetic assemblage of igneous and metamorphic xenoliths including peridotites eclogites and granulites schists and hornfels and gabbroids and granitoids some xenoliths of the high grade metamorphic xenolith group are of particular interest in elucidating the nature of the uppermost mantle and crust beneath the sierra nevada garnet granulites the most abundant rock type of the mafic high grade metamorphic assemblage and eclogites are restricted to the central area of the pipe these xenoliths are essentially bimineralic consisting of colorless to pale green clinopyroxene and garnet eclogites contain brilliant emerald green clinopyroxene the clinopyroxenes which have a broad range of compostions mg mg fe 0 45 0 90 have a 20 jadeite component in eclogites garnets also have a range of compositions mg mg fe 0 30 0 70 with those from eclogites being the most fe rich one of the xenoliths contains ca rich garnet grs adr 90 feldspathic granulites contain plagioclase of an 45 70 and some lack garnet hornblende occurs in some granulites and a few amphibolites are present likely protoliths are ocean floor basalts for the eclogites and precambrian limestone for the grossular clinopyroxene rock the big creek xenoliths attest to the former presence of subducted oceanic lithosphere beneath the western sierra nevada

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

paleomagnetic measurement of nonbrittle coseismic deformation across the san andreas fault at pallett creek paleomagnetic data have been obtained to address a problem at the pallett creek paleoseismological site the 9 mm yr slip rate determined from three dimensional mapping of late holocene offsets across discrete faults is only a quarter of the expected value we suspected that nonbrittle deformation adjacent to the faults might account for the 26 mm yr discrepancy in our search for the missing slip we collected and analyzed 264 paleomagnetic samples from a 53 m wide transect across the fault zone half the samples came from a unit deposited immediately after a large earthquake of about a d 1480 these samples were affected by two large earthquakes that involved rupture at the site in 1812 and 1857 we collected the other half of the samples from a slightly older bed one that was deposited before the earthquake of about a d 1480 relative to control groups composed of 10 samples and collected 50 m from the fault samples closer to the fault display clockwise rotations of 30 or less if interpreted as block rotations the data from the older unit imply that it has sustained a total of 14 0 2 9 m of dextral warp during the past three major earthquakes and that the younger unit has experienced a total of 8 5 1 0 m of warp during the most recent two combining these values with the amounts of dextral slip across the mapped fault planes yields dextral offsets of 5 5 6 25 and 6 25 m for the events of a d 1480 1812 and 1857 and a slip rate of 35 6 6 7 mm yr this slip rate averaged over the past three complete seismic cycles is consistent with published rates from other sites offsets associated with the past three events are remarkably similar these amounts however appear independent of the length of interseismic cycles these observations suggest 1 that this part of the san andreas fault has a characteristic strength and 2 that conventional concepts of strain accumulation and relief for example time and slip predictable models of earthquake occurrence are unrealistic

high resolution topography derived offsets along the 1857 fort tejon earthquake rupture trace san andreas fault the great fort tejon earthquake of 1857 with an 350 km long surface rupture was the most recent major earthquake along the south central san andreas fault saf prior reconstruction of its surface slip distribution and reconstruction of preceding earthquakes along the 1857 rupture trace have contributed to formulation of the characteristic earthquake cem and uniform slip models usm for earthquake recurrence that find wide application in seismic hazard assessment and earthquake forecasting we used the high resolution b4 light detection and ranging lidar topographic data set sufficient for depiction of meter scale tectonic landforms to reevaluate the distribution of surface displacement along the 1857 rupture trace we present 450 offset measurements with displacements below 60 m increasing observation density relative to previous studies by a factor of 2 our results show that the 1857 earthquake had overall an average displacement below 3 5 m with 4 6 m released along the northwestern half of the rupture its along fault slip distribution is smooth at 10 km length scales at 28 less than prior estimates assuming a 10 km rupture depth and a 30 gpa shear modulus reconstruction of pre 1857 earthquakes is impeded by an exponential decrease in observation density with increasing displacement amount offset observation density soon reaches a level where only tentative and nonunique reconstructions are permitted the results of this study question prior surface slipreconstructions of pre 1857 rupture s that were based on a distinctly lower number of offset observations challenging the applicability of the aforementioned earthquake recurrence models to explain saf rupture behavior online material catalog of offset measurements along the 1857 surface rupture trace as well as an annotated map 1 embed inline graphic 1 gif

multicycle dynamics of nonplanar strike slip faults 1 we perform two dimensional dynamic models of strike slip faults with a change in strike a bend over multiple earthquake cycles to examine the long term effects of nonplanar fault geometry a viscoelastic model a proxy for off fault deformation and tectonic loading is introduced for the interseismic process to avoid pathological stress buildup around the bend a finite element method with an elastodynamic model is used to simulate dynamic earthquake ruptures we find that stresses near the bend differ strongly from the regional stress field and that the fault develops a relatively steady state in which the stress level and the event pattern on the fault are stable reduced normal stress on the dilatational side and increased normal stress on the compressive side of the bend during dynamic ruptures result in the bend serving as an initiation and or a termination point s for rupture typical events on such a fault consist of two classes unilateral events that rupture only the favorable segment and bilateral events that rupture the favorable segment and part of or the entire unfavorable segment in the latter class of events a time delay in rupture around the bend results from a high yield stress on the compressive side of the bend other effects of the bent fault geometry include higher displacement on the inward wall than on the outward wall higher slip on the more favorable segment than on the less favorable segment and a large slip velocity on the compressive side of the bend

thermobarometry and p t t paths the granulite to eclogite transition in lower crustal xenoliths from eastern australia lower crustal suite xenoliths in basaltic and kimberlitic magmas are dominated by mafic granulites and may also include eclogites and garnet pyroxenites pressures of up to 25 kbar obtained from such xenoliths are well in excess of an upper value of c 12 kbar for exposed granulite terranes palaeogeotherms constructed from xenoliths for the lower crust beneath the phanerozoic fold belts of eastern australia sea and beneath the eastern margin of the australian craton emac indicate two distinct thermal regimes the two geotherms have similar form with the emac curve displaced c 150 c to lower temperatures reaction microstructures show the partial re equilibration of primary igneous assemblages to granulite and eclogite assemblages and are interpreted to reflect the cooling from magmatic temperatures variations in mineral compositions and zoning are used to constrain further the history of several emac xenoliths to near isobaric trajectories detailed graphical models are constructed to predict compositional changes for isobaric p t paths at 7 14 21 kbar to transform an sea type geotherm to a cratonic geotherm the models show that for the assemblage grt cpx ky plag qtz the changes associated with falling temperature in xgr xjd increase and xan decrease will be greater at higher pressures these results indicate that discernible zoning is more likely to be preserved in the higher pressure xenoliths the zoning recorded in clinopyroxene from mafic granulite xenoliths over the pressure range c 12 22 kbar suggests isobaric cooling of a large crustal thickness 30 35 km an isobaric cooling path is consistent with magma accretion models for the transition of a crust mantle boundary from an sea type geotherm to a cratonic geotherm the coexistence of granulite and eclogite over the depth range 35 75 km beneath the emac indicates that the granulite to eclogite transition in the lower crust is controlled by p t conditions bulk chemistry and kinetic factors at shallower crustal levels typified by exposed granulite terranes isobaric cooling may not result in the transition to eclogite

can in situ measurements of mantle electrical conductivity be used to infer properties of partial melts 1 constraints on bulk conductivity from magnetotelluric measurements and petrological analyses of late quaternary peridotite xenoliths from the southern sierra nevada allow evaluation of models commonly used to relate electrical conductivity to the physical and chemical state of the upper mantle in these models two conductive melts basalt and sulfide are embedded in a resistive matrix bounds on the amount of sulfide 0 06 0 4 and the bulk conductivity 0 03 0 1 s m place constraints on the degree of interconnection between the melts because the sulfide melt is very conductive even a small fraction of well connected melt results in a bulk conductivity larger than 0 1 s m similarly completely disconnected melts result in bulk conductivities much less than 0 03 s m the only models which matched both the bulk conductivity and sulfide bounds consisted of a small fraction 1 interconnected basalt melt with a discontinuous sulfide phase such a texture is observed in laboratory experiments with much larger sulfide melt fractions but has not been reported for small melt fractions a variant of the hashin shtrikman model and a hybrid model consisting of cascaded hashin shtrikman calculations were successful in matching the magnetotelluric and petrologic constraints with a model that appropriately simulates the melt interconnectivity we suggest that electrical conductivity may be used to infer in situ melt properties in the mantle

petrogenesis of sierra nevada plutons inferred from the sr nd and o isotopic signatures of mafic igneous complexes in yosemite valley california mafic complexes in the central sierra nevada batholith record valuable geochemical information regarding the role mafic magmas play in arc magmatism and the generation of continental crust in the intrusive suite of yosemite valley major and trace element compositions of the hornblende bearing gabbroic rocks from the rockslides mafic complex and of the mafic dikes in the north america wall are compositionally similar to high alumina basalt of these rocks two samples have higher ni and cr abundances as well as higher end values than previously recognized for the intrusive suite plagioclase crystals in rocks from the north america wall and the rockslides have prominent calcic cores and sharply defined sodic rims a texture commonly associated with mixing of mafic and felsic magmas in situ analyses of 87sr 86sr in plagioclase show no significant isotopic difference from the cores to the rims of these grains we propose that the high 87sr 86sr 0 7067 and low end 3 4 of bulk rocks the homogeneity of 87sr 86sr in plagioclase and the high 18o values of bulk rocks 6 6 7 3 and zircon lackey et al in j petrol 49 1397 1426 2008 demonstrate that continental crust was assimilated into the sublithospheric mantle derived basaltic precursors of the mafic rocks in yosemite valley contamination 20 40 likely occurred in the lower crust as the magma differentiated to high alumina basalt prior to plagioclase and zircon crystallization as a consequence the isotopic signatures recorded by whole rocks plagioclase and zircon do not represent the composition of the underlying lithospheric mantle we conclude that the mafic and associated felsic members of the intrusive suite of yosemite valley represent 60 80 new additions to the crust and include significant quantities of recycled ancient crust

evolution of stress in southern california for the past 200 years from coseismic postseismic and interseismic stress changes summary the seismicity of southern california results from stresses that arise from the relative motion of the pacific and north american plates being accommodated along the san andreas fault saf system and the eastern california shear zone ecsz here we calculate how the stress field in southern california has evolved over the past two centuries due to interseismic loading as inferred from current gps observations of surface velocities from redistributions of static stress induced by large m w 6 5 earthquakes since the 1812 wrightwood quake and postseismic viscoelastic relaxation associated with these events that serves to transfer coseismic stresses from the deep warm lower crust and upper mantle to the overlying seismogenic upper crust we calculate coulomb stress changes on vertical strike slip faults striking parallel to the saf and at the hypocenters on the rupture planes of all m w 6 events over the past two centuries our results suggest that the 1857 m w 8 2 fort tejon earthquake by far the largest event to have occurred in the region over the past two centuries had a profound influence on the state of stress in southern california during the 19th century inducing significant stress increases to the north parkfield region and adjoining creeping saf and south southern saf and san jacinto fault and stress relief across the southern ecsz these stress changes were then greatly magnified by postseismic relaxation through the early part of the 20th century slow interseismic build up of stress further loads all major strike slip faults and works to reload the areas of the ecsz where stress was relieved by the 1857 quake our calculations suggest that only 56 of hypocenters were pushed closer to failure by preceding coseismic stress changes suggesting that the occurrence of large earthquakes is not strongly determined by coseismic coulomb stress changes this percentage rises to 70 when postseismic stress changes are also considered our calculations demonstrate the importance of postseismic viscoelastic relaxation in the redistribution of stress following large earthquakes we find however that postseismic processes associated with events more than about a decade old are near completion and thus do not significantly influence the regional velocity field presently observed in southern california

subduction products according to experimental prediction knowledge of the thermal structure and processes in subduction zones is so uncertain that our more extensive knowledge of source materials and phase relationships remains insufficient for prediction of magmatic products the phase relationships do provide important constraints for testing geophysical and petrogenetic models especially when considered along with geochemical constraints physical questions such as the behavior of partially molten rocks are intimately related to the compositions of magmas from source to eruption site tentative predictions and conclusions include 1 andesite is not a primary magma from oceanic crust 2 it is unlikely that liquids from subducted oceanic crust yield andesites by fractionation except by melting of amphibolite in an especially warm subducted crust 3 it is more likely that hydrous siliceous magmas from the crust leak into the overlying mantle 4 andesite magmas can be generated in peridotite only under exceptional circumstances 5 h2o undersaturated basic magmas from peridotite modified by aqueous fluids or hydrous magma may yield andesite by fractionation 6 phlogopite peridotite could escape melting and yield alkalic magmas at deeper levels 7 andesite magmas could be produced in continental crust only by extreme heating through underplating of basalts or by mixing of basalt and rhyolite derived from crust most of these conclusions need revision if convection in the asthenosphere wedge raises the temperature of the subducted slab to 1250 c at 100 km as proposed by marsh 1979a this demonstrates the dependence of experimental predictions on geophysical models

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

a more precise chronology of earthquakes produced by the san andreas fault in southern california improved methods of radiocarbon analysis have enabled us to date more precisely the earthquake ruptures of the san andreas fault that are recorded in the sediments at pallett creek previous dates of these events had 95 confidence errors of 50 100 calendar years new error limits are less than 23 calendar years for all but two of the dated events this greater precision is due to larger sample size longer counting time lower background noise levels more precise conversion of radiocarbon ages to calendric dates and better stratigraphic constraints and statistical techniques the new date ranges with one exception fall within the broader ranges estimated previously but our estimate of the average interval between the latest 10 episodes of faulting is now about 132 years variability about the mean interval is much greater than was suspected previously five of the nine intervals are shorter than a century three of the remaining four intervals are about two to three centuries long despite the wide range of these intervals a pattern in the occurrence of large earthquakes at pallett creek is apparent in the new data the past 10 earthquakes occur in four clusters each of which consists of two or three events earthquakes within the clusters are separated by periods of several decades but the clusters are separated by dormant periods of two to three centuries this pattern may reflect important mechanical aspects of the fault s behavior if this pattern continues into the future the current period of dormancy will probably be greater than two centuries this would mean that the section of the fault represented by the pallett creek site is currently in the middle of one of its longer periods of repose between clusters and sections of the fault farther to the southeast are much more likely to produce the next great earthquake in california the greater precision of dates now available for large earthquakes recorded at the pallett creek site enables speculative correlation of events between paleoseismic sites along the southern half of the san andreas fault a history of great earthquakes with overlapping rupture zones along the mojave section of the fault remains one of the more attractive possibilities

dynamic models of earthquakes on the north anatolian fault zone under the sea of marmara effect of hypocenter location 1 we perform three dimensional spontaneous dynamic rupture models of potential earthquakes on the geometrically complex north anatolian fault zone nafz under the marmara sea turkey the nafz south of the city of istanbul consists of a large scale extensional stepover with right lateral strike slip segments linked by an oblique normal segment we find that earthquakes nucleating near the stepover do not propagate across the entire fault system due to the statically unfavorable stress field on the oblique fault and lead to moderate size events only however earthquakes initiating a significant distance from the stepover cause significant dynamic unclamping of the oblique fault and thus generate large through going ruptures dynamic unclamping also produces supershear rupture propagation in the vicinity of fault discontinuities the results emphasize that estimations of earthquake size rupture propagation and slip distribution cannot be decoupled from the location of the hypocenter and the orientation of the geometrically complex fault system within the tectonic stress field

test on cyclic creep behavior of mucky clay in shanghai under step cyclic loading based on the cyclic triaxial test under step loading the variation of cyclic creep and pore water pressure with increase in cyclic stress ratio csr and load cycle of mucky clay in shanghai were analyzed the results show that there are two threshold values of stability cyclic stress ratio and failure cyclic stress ratio during the step loading which divide the development of cyclic creep strain into gradual stability stage rapid growth stage and instantaneous failure stage when the stress level is less than the failure cyclic stress ratio the recoverable elastic strain is nearly linear with csr the cyclic creep properties of mucky clay are obviously influenced by the loading frequency under the same stress level

lidar and field observations of slip distribution for the most recent surface ruptures along the central san jacinto fault we measured offsets on tectonically displaced geomorphic features along 80 km of the clark strand of the san jacinto fault sjf to estimate slip per event for the past several surface ruptures we identify 168 offset features from which we make over 490 measurements using b4 light detection and ranging lidar imagery and field observations our results suggest that lidar technology is an exemplary supplement to traditional field methods in slip per event studies displacement estimates indicate that the most recent surface rupturing event mre produced an average of 2 5 2 9 m of right lateral slip with maximum slip of nearly 4 m at anza a m w 7 2 7 5 earthquake average multiple event offsets for the same 80 kms are 5 5 m with maximum values of 3 m at anza for the penultimate event cumulative displacements of 9 10 m through anza suggest the third event was also similar in size paleoseismic work at hog lake dates the most recent surface rupture event at ca 1790 a poorly located large earthquake occurred in southern california on 22 november 1800 we relocate this event to the clark fault based on the mre at hog lake we also recognize the occurrence of a younger rupture along 15 20 km of the fault in blackburn canyon with 1 25 m of average displacement we attribute these offsets to the 21 april 1918 m w 6 9 event these data argue that much or all of the clark fault and possibly also the casa loma fault fail together in large earthquakes but that shorter sections may fail in smaller events online material topographic contour maps and hillshades generated from b4 lidar data corresponding field photographs and data tables comparing lidar based and field based slip measurements for individual geomorphic features along the clark fault

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

paleoearthquakes at frazier mountain california delimit extent and frequency of past san andreas fault ruptures along 1857 trace large earthquakes are infrequent along a single fault and therefore historic well characterized earthquakes exert a strong influence on fault behavior models this is true of the 1857 fort tejon earthquake estimated m7 7 7 9 on the southern san andreas fault ssaf but an outstanding question is whether the 330 km long rupture was typical new paleoseismic data for six to seven ground rupturing earthquakes on the big bend of the ssaf restrict the pattern of possible ruptures on the 1857 stretch of the fault in conjunction with existing sites we show that over the last 650 years at least 75 of the surface ruptures are shorter than the 1857 earthquake with estimated rupture lengths of 100 to 300 km these results suggest that the 1857 rupture was unusual perhaps leading to the long open interval and that a return to pre 1857 behavior would increase the rate of m7 3 m7 7 earthquakes

century long average time intervals between earthquake ruptures of the san andreas fault in the carrizo plain california paleoseismological data constrain the age location and associated magnitude of past surface rupturing earthquakes these are critical parameters for developing and testing fault behavior models and characterizing seismic hazard we present new earthquake evidence and radiocarbon analyses that refi ne the chronology of the six most recent earthquakes that ruptured the south central san andreas fault in the carrizo plain california united states at the bidart fan site modeled 95 percentile ranges of the earthquakes prior to the a d 1857 earthquake are a d 1631 1823 1580 1640 1510 1612 1450 1475 and 1360 1452 the average time interval between the last six earthquakes that ruptured the san andreas fault in the carrizo plain is 88 41 yr this is less than the time since the most recent a d 1857 earthquake less than all reported average intervals of prehistoric earthquakes along the entire san andreas fault and signifi cantly shorter than the 235 yr average used in recent seismic hazard evaluations the new chronological data combined with recent slip studies imply that the magnitudes of the earthquakes that ruptured the southern san andreas fault in the carrizo plain since ca a d 1360 were variable and suggest that the widely held view of rare but great surface rupturing earthquakes along this portion of the southern san andreas fault should be reevaluated 118

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

testing the stress shadow hypothesis 1 a fundamental question in earthquake physics is whether aftershocks are predominantly triggered by static stress changes permanent stress changes associated with fault displacement or dynamic stresses temporary stress changes associated with earthquake shaking both classes of models provide plausible explanations for earthquake triggering of aftershocks but only the static stress model predicts stress shadows or regions in which activity is decreased by a nearby earthquake to test for whether a main shock has produced a stress shadow we calculate time ratios defined as the ratio of the time between the main shock and the first earthquake to follow it and the time between the last earthquake to precede the main shock and the first earthquake to follow it a single value of the time ratio is calculated for each 10 10 km bin within 1 5 fault lengths of the main shock epicenter large values of the time ratio indicate a long wait for the first earthquake to follow the main shock and thus a potential stress shadow whereas small values indicate the presence of aftershocks simulations indicate that the time ratio test should have sufficient sensitivity to detect stress shadows if they are produced in accordance with the rate and state friction model we evaluate the 1989 mw 7 0 loma prieta 1992 mw 7 3 landers 1994 mw 6 7 northridge and 1999 mw 7 1 hector mine main shocks for each main shock there is a pronounced concentration of small time ratios indicating the presence of aftershocks but the number of large time ratios is less than at other times in the catalog this suggests that stress shadows are not present by comparing our results to simulations we estimate that we can be at least 98 confident that the loma prieta and landers main shocks did not produce stress shadows and 91 and 84 confident that stress shadows were not generated by the hector mine and northridge main shocks respectively we also investigate the long hypothesized existence of a stress shadow following the 1906 san francisco bay area earthquake we find that while bay area catalog seismicity rates are lower in the first half of the twentieth century than in the last half of the nineteenth this seismicity contrast is also true outside of the bay area in regions not expected to contain a stress shadow this suggests that the rate change is due to a more system wide effect such as errors in the historical catalog or the decay of aftershocks of the larger 1857 fort tejon earthquake

epistemic uncertainty in california wide synthetic seismicity simulations abstract the generation of seismicity catalogs on synthetic fault networks holds the promise of providing key inputs into probabilistic seismic hazard analysis for example the coefficient of variation mean recurrence time as a function of magnitude the probability of fault to fault ruptures and conditional probabilities for foreshock mainshock triggering i employ a seismicity simulator that includes the following ingredients static stress transfer viscoelastic relaxation of the lower crust and mantle and vertical stratification of elastic and viscoelastic material properties a cascade mechanism combined with a simple coulomb failure criterion is used to determine the initiation propagation and termination of synthetic ruptures it is employed on a 3d fault network provided by steve ward unpublished data 2009 for the southern california earthquake center scec earthquake simulators group this all california fault network initially consisting of 8000 patches each of 12 square kilometers in size has been rediscretized into patches each of 1 square kilometer in size in order to simulate the evolution of california seismicity and crustal stress at magnitude m 5 8 resulting synthetic seismicity catalogs spanning 30 000 yr and about one half million events are evaluated with magnitude frequency and magnitude area statistics for a priori choices of fault slip rates and mean stress drops i explore the sensitivity of various constructs on input parameters particularly mantle viscosity slip maps obtained for the southern san andreas fault show that the ability of segment boundaries to inhibit slip across the boundaries e g to prevent multisegment ruptures is systematically affected by mantle viscosity

upper mantle fragments from beneath the sierra nevada batholith partial fusion fractional crystallization and metasomatism in a subduction related ancient lithosphere

palaeoseismology in steep terrain the big bend of the san andreas fault transverse ranges california transpressional tectonics are typically associated with restraining bends on major active strike slip faults resulting in uplift and steep terrain this produces dynamic erosional and depositional conditions and difficulties for established lines of palaeoseismological investigation consequently in these areas data are lacking to determine tectonic behaviour and future hazard potential along these important fault segments the big bend of the san andreas fault in the transverse ranges of southern california exemplifies these problems however landslides probably seismically triggered are widespread in the rugged terrain of the big bend fluvial reworking of these deposits rapidly produces geomorphic planes and lines that are markers for subsequent fault slip the most useful are offset and abandoned stream channels for these are relatively high precision markers for identifying individual faulting events palaeoseismological studies from the central big bend involving 14c ages of charcoal fragments from trench exposures illustrate these points and indicate that the past three faulting events including the great 1857 earthquake were relatively similar in scale each producing offsets of about 7 7 5 m the mean recurrence interval is 140 220 years the pre 1857 event here may be the 1812 event documented south of the big bend or an event which took place probably between 1630 and 1690 ground breakage in both events extended south of the big bend unlike the 1857 event where rupture was skewed to the north the preceding faulting event ruptured both to the north and south of the big bend and probably occurred between 1465 and 1495 all these events centred on the big bend and may be typical for this fault segment suggesting that models of uniform long term slip rates may not be applicable to the south central san andreas a slip rate estimate of 34 51 mm a 1 for the central big bend although uncertain may also imply higher slip in the big bend and highlights difficulties in correlating slip rates between sites with different tectonic settings slip rates on the san andreas may increase within the broad compressional tectonics zone of the big bend compared to the north and south where the plate boundary is a relatively linear and sub parallel series of dominantly strike slip faults partitioning slip within the big bend is inherently uncertain and insufficient suitably comparable data are available to sustain a uniform slip model although such models are a common working assumption

a reevaluation of the pallett creek earthquake chronology based on new ams radiocarbon dates san andreas fault california 1 the pallett creek paleoseismic record occupies a keystone position in most attempts to develop rupture histories for the southern san andreas fault previous estimates of earthquake ages at pallett creek were determined by decay counting radiocarbon methods that method requires large samples which can lead to unaccounted sources of uncertainty in radiocarbon ages because of the heterogeneous composition of organic layers in contrast accelerator mass spectrometry ams radiocarbon dates may be obtained from small samples that have known carbon sources and also allow for a more complete sampling of the section we present 65 new ams radiocarbon dates that span nine ground rupturing earthquakes at pallett creek overall the ams dates are similar to and reveal no dramatic bias in the conventional dates for many layers however individual charcoal samples were younger than the conventional dates leading to earthquake ages that are overall slightly younger than previously reported new earthquake ages are determined by bayesian refinement of the layer ages based on stratigraphic ordering and sedimentological constraints the new chronology is more regular than previously published records in large part due to new samples constraining the age of event r the closed interval from event c to 1857 has a mean recurrence of 135 years 83 2 years and a quasiperiodic coefficient of variation cov of 0 61 we show that the new dates and resultant earthquake chronology have a stronger effect on cov than the specific membership of this long series and dating precision improvements from sedimentation rates

earthquake recurrence in simulated fault systems we employ a computationally efficient fault system earthquake simulator rsqsim to explore effects of earthquake nucleation and fault system geometry on earthquake occurrence the simulations incorporate rate and state dependent friction high resolution representations of fault systems and quasi dynamic rupture propagation faults are represented as continuous planar surfaces surfaces with a random fractal roughness and discontinuous fractally segmented faults simulated earthquake catalogs have up to 106 earthquakes that span a magnitude range from m4 5 to m8 the seismicity has strong temporal and spatial clustering in the form of foreshocks and aftershocks and occasional large earthquake pairs fault system geometry plays the primary role in establishing the characteristics of stress evolution that control earthquake recurrence statistics empirical density distributions of earthquake recurrence times at a specific point on a fault depend strongly on magnitude and take a variety of complex forms that change with position within the fault system because fault system geometry is an observable that greatly impacts recurrence statistics we propose using fault system earthquake simulators to define the empirical probability density distributions for use in regional assessments of earthquake probabilities

evidence for two surface ruptures in the past 500 years on the san andreas fault at frazier mountain california we conducted paleoseismic studies in a closed depression along the san andreas fault on the north flank of frazier mountain near frazier park california we recognized two earthquake ruptures in our trench exposure and interpreted the most recent rupture event 1 to represent the historical 1857 earthquake we also exposed evidence of an earlier surface rupture event 2 along an older group of faults that did not rerupture during event 1 radiocarbon dating of the stratigraphy above and below the earlier event constrains its probable age to between a d 1460 and 1600 because we documented continuous unfaulted stratigraphy between the earlier event horizon and the youngest event horizon in the portion of the fault zone exposed we infer event 2 to be the penultimate event we observed no direct evidence of an 1812 earthquake in our exposures however we cannot preclude the presence of this event at our site due to limited age control in the upper part of the section and the possibility of other fault strands beyond the limits of our exposures based on over lapping age ranges event 2 at frazier mountain may correlate with event b at the bidart fan site in the carrizo plain to the northwest and events v and w4 at pallett creek and wrightwood respectively to the southeast if the events recognized at these multiple sites resulted from the same surface rupture then it appears that the san andreas fault has repeatedly failed in large ruptures similar in extent to 1857

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

the repetition of large earthquake ruptures abstract this survey of well documented repeated fault rupture confirms that some faults have exhibited a characteristic behavior during repeated large earthquakes that is the magnitude distribution and style of slip on the fault has repeated during two or more consecutive events in two cases faults exhibit slip functions that vary little from earthquake to earthquake in one other well documented case however fault lengths contrast markedly for two consecutive ruptures but the amount of offset at individual sites was similar adjacent individual patches 10 km or more in length failed singly during one event and in tandem during the other more complex cases of repetition may also represent the failure of several distinct patches the faults of the 1992 landers earthquake provide an instructive example of such complexity together these examples suggest that large earthquakes commonly result from the failure of one or more patches each characterized by a slip function that is roughly invariant through consecutive earthquake cycles the persistence of these slip patches through two or more large earthquakes indicates that some quasi invariant physical property controls the pattern and magnitude of slip these data seem incompatible with theoretical models that produce slip distributions that are highly variable in consecutive large events

san andreas fault rupture scenarios from multiple paleoseismic records stringing pearls we present a new method to objectively combine paleoseismic event data from multiple sites into rupture scenarios and apply it to the southern san andreas fault ssaf of california first a pool is constructed of all ruptures between sites allowed by fault geometry and available event age probability distribution functions pdfs scenarios constructed by drawing from this pool are evaluated by the average quality of agreement event dating evidence by the degree of misfit of cumulative displacement over all ruptures compared to a prediction from the fault slip rate and elapsed time and by the number of events in the scenario three slip rate models from the 2008 working group on california earthquake probabilities wgcep were considered scenarios with full fault length ruptures tend to be inconsistent with low slip rates through the san bernardino and san gorgonio sections of the ssaf favorable scenarios tend to include 1857 like ruptures for three of the most recent five prehistoric ruptures in the northern half of the ssaf from carrizo to approximately wrightwood they also include one to two ruptures that involve the southern half of the ssaf but most earthquakes there appear shorter and exhibit less consistency from event to event by combining paleoseismic data into ensembles of scenarios and selecting viable scenarios using external constraints our method provides rupture histories useful for seismic hazard assessment without having to first settle which event at a site correlates with those at adjoining sites this opens a way for paleoseismic data to be used with greater power to understand the seismic hazard posed by faults like the southern san andreas

displacement and geometrical characteristics of earthquake surface ruptures issues and implications for seismic hazard analysis and the process of earthquake rupture there now exist about three dozen historical earthquakes for which in vestigators have constructed maps of earthquake rupture traces accompanied by de scriptions of the coseismic slip observed along the fault strike the maps and slip distributions are compiled here to place observational bounds on aspects of seismic hazard analysis and fault mechanics analysis leads to an initial statistical basis to predict the end points of rupture and the amount of surface slip expected at sites along the strike during earthquakes on mapped faults the observations also give support to the ideas that there exists a process zone or volume of about 3 4 km in dimension at the fronts of large laterally propagating earthquake ruptures within which stress changes may be sufficient to trigger slip on adjacent faults and that the ultimate length of earthquake ruptures is controlled primarily by the geometrical complexity of fault traces and variations in accumulated stress levels along faults that arise due to the location of past earthquakes to this may be added the observation that the form of earthquake surface slip distributions is better described by asymmetric rather than symmetric curve forms and that earthquake epicenters do not appear to correlate in any systematic manner to regions of maximum surface slip observed along strike online material maps of surface ruptures digitized values and curve fits to surface slip distributions and notes and references for tables 1 and 2