vs30 and spectral response from collocated shallow active and passive source vs data at 27 sites in puerto rico shear wave velocity vs and time averaged shear wavevelocity to 30 m depth vs30 are the key parameters used in seismic site response modeling and earth quake engineering design where vs data are limited available data are often used to develop and refine map based proxy models of vs30 for predicting ground motion intensities in this paper we present shallow vs data from 27 sites in puerto rico these data were acquired using a multimethod acquisition approach consisting of noninvasive collocated active source body wave refraction reflection active source surface wave at nine sites and passive source surface wave refraction micro tremor remi techniques vs versus depth models are constructed and used to calculate spectral response plots for each site factors affecting method reliability are analyzed with respect to site specific differences in bedrock vs and spectral response at many but not all sites body and surface wave methods generally determine similar depths to bedrock and it is the difference in bedrock vs that influences site ampli fication the predicted resonant frequencies for the majority of the sites are observed to be within a relatively narrow bandwidth of 1 3 5 hz for a first order comparison of peak frequency position predictive spectral response plots from eight sites are plot ted along with seismograph instrument spectra derived from the time series of the 16 may 2010 puerto rico earthquake we show how a multimethod acquisition approach using collocated arrays compliments and corroborates vs results thus adding confi dence that reliable site characterization information has been obtained

geometry mechanisms and significance of extensional folds from examples in the rocky mountain basin and range province u s a abstract geologic mapping and structural analysis in paleogene half graben of idaho and montana have revealed over 70 extensional folds that form orthogonal sets with mostly ne and se trending axes on a regional scale they parallel or lie perpendicular to the strikes of the largest normal faults transverse folds at a high angle to the fault and folds oblique to the fault comprise more than half of the folds and reflect the highly three dimensional nature of the bulk strain detailed geometric analysis of 15 folds in the salmon horse prairie and medicine lodge basins shows that they typically have an upright to steeply inclined bisecting surface an interlimb angle of 141 and a plunge of 19 based on our synthesis of the eight common mechanisms of extensional folding and their distinguishing characteristics we were able to determine how some of the investigated folds formed fault bend folding above both fault parallel and fault perpendicular bends in the underlying normal fault produced most of the folds in the study area but displacement gradients along normal faults fault drag isostatic adjustments and transtension were also factors in the deformation most of the compound folds which result from more than one mechanism of folding are oblique to all adjacent normal faults recognition of such extensional folds is critical because they might be misinterpreted as contractional structures they influence sediment thickness patterns and dispersal in rift basins and may control the migration and trapping of petroleum and groundwater resources

stream driven high density gravelly traction carpets possible deposits in the trabeg conglomerate formation sw ireland and some theoretical considerations of their origin within high density flood flows a prominent mechanism of gravel transport and deposition is by stream driven high density traction carpet with a rheology similar to grain flow these gravel carpets are envisaged to form the basal portion of a bipartite high density flood flow decoupled from an overlying sand and silt laden turbulent flow several examples already documented in the literature are reviewed and an additional case from the lower old red sandstone of southwest ireland is presented two mechanisms of traction carpet initiation are discussed by rapid entrainment of gravel into suspension on rising stage followed by settling into the gravel traction carpet at peak and falling stage and by overconcentration of a normal low density bedload gravel entrainment suspension and traction carpet development are significantly easier if the flood water already carries a high concentration of sand and silt in suspension theoretical consideration further shows that gravelly traction carpets can be maintained in channels of relatively low gradient by the shear stress exerted by the high density sand bearing turbulent flood flow above this tangential shear stress is converted to dispersive pressure which aids buoyancy and quasi static grain to grain contacts in the support of the clasts within the gravel carpet the carpet is thought to have a quasi plastic rheology but behave much like a viscous fluid at high shear rates stream driven gravelly traction carpets are expected to produce sheet like units of clast to matrix supported conglomerate characterized by a parallel or an a p a i clast fabric these units may be ungraded normally or inversely graded depending on the rate of shear the viscosity of the flow and the celerity of deposition

age of the peach springs tuff southeastern california and western arizona sanidine separates from pumice of the early miocene peach springs tuff are concordantly dated at 18 5 0 2 ma by two isotopic techniques the peach springs tuff is the only known unit that can be correlated between isolated outcrops of miocene strata from the central mojave desert of southeastern california to the western colorado plateau in arizona across five structural provinces a distance of 350 km thus the age of the peach springs tuff is important to structural and paleogeographic reconstructions of a large region biotite and sanidine separates from bulk samples of the peach springs tuff from zones of welding and vapor phase alteration have not produced consistent ages by the k ar method published ages of mineral separates from 17 localities ranged from 16 2 to 20 5 ma discordant 40ar 39ar incremental release spectra were obtained for one biotite and two of the sanidine separates ages that correspond to the last gas increments are as old as 27 ma the 40ar 39ar incremental release determinations on sanidine separated from blocks of peach springs tuff pumice yield ages of 18 3 0 3 and 18 6 0 4 ma laser fusion measurements yield a mean age of 18 51 0 10 the results suggest that sanidine and biotite k ar ages older than about 18 5 ma are due to inherited ar from pre tertiary contaminants which likely were incorporated into the tuff during deposition sanidine k ar ages younger than 18 ma probably indicate incomplete extraction of radiogenic 40ar whereas laser fusion dates of biotite and hornblende younger than 18 ma likely are due to postdepositional alteration laser fusion ages as high as 19 01 ma on biotite grains from pumice suggest that minerals from pre tertiary country rocks also were incorporated in the magma chamber

tertiary extension and contraction of lower plate rocks in the central mojave metamorphic core complex southern california several authors recently have proposed the rolling hinge model for the formation of extensional detachment faults in metamorphic core complexes in this model isostatic uplift of the denuded footwall of a major normal fault causes the near surface part of the fault to rotate from a higher to a lower dip for the rolling hinge model to be acceptable the footwalls of detachment faults must have undergone the strain pattern dictated by the rolling hinge in this paper the structural development of the footwall of the central mojave metamorphic core complex is used as a field test tertiary structures in the lower plate of the core complex record ne directed ductile extensional shear followed by cataclastic ne side up shear with a significant component of contraction along the previous extension direction this strain history agrees with the rolling hinge model and particularly with a version of the model in which footwall uplift occurs by a combination of elastic flexure and flexurally driven upper crustal failure similar late stage layer parallel contraction appears to be present in some other tectonically denuded terranes of the basin and range suggesting that the flexural failure rolling hinge model for extensional terranes may be widely applicable

tertiary basin development and tectonic implications whipple detachment system colorado river extensional corridor california and arizona a stratigraphic and structural record of synchronous extension and detachment faulting is preserved in predominantly miocene deposits of the whipple detachment system colorado river extensional corridor in the mohave mountains and aubrey hills of arizona and the eastern whipple mountains of california near parker dam these deposits comprise four unconformity bounded sequences composed of locally derived epiclastic and volcanic rocks and the peach springs tuff the three older sequences represent syntectonic units that were deposited coeval with detachment faulting and the fourth is interpreted to be postextensional the sequences are correlated between four fault bounded regions which are the remnants of four different depositional basins similar sequences can be correlated over broad areas of the extensional corridor despite the general lack of widespread units the basins developed in about the same positions relative to each other and to volcanic sources as they occupy at present this is shown by gradational changes of pre tertiary rock types between regions systematic variations in the abundance of magmatic units and correlative volcanic units that occur in two adjacent regions the basins formed in the early miocene from segmentation of the upper crust into blocks bounded by high angle faults that trended both parallel and perpendicular to the direction of extension and which were terminated at middle crustal depths by a low angle detachment fault extreme rotation of one large crustal block which constitutes the central mohave mountains is recorded by a major unconformity in the lower miocene section of one basin because coeval sections that formed in adjoining basins do not record this rotation the underlying crustal blocks must have been separated by transfer faults that allowed them to rotate independently these proposed transfer faults are represented at present by major faults with trends that parallel the direction of extension on the whipple detachment system

large magnitude miocene extension in the central mojave desert implications for paleozoic to tertiary paleogeography and tectonics this is the published version copyright 1990 american geophysical union all rights reserved

topographic slope as a proxy for seismic site conditions and amplification we describe a technique to derive first order site condition maps directly from topographic data for calibration we use global 30 arc sec topographic data and v s 30 measurements here v s 30 refers to the average shear velocity down to 30 m aggregated from several studies in the u s as well as in taiwan italy and australia v s 30 values are correlated against topographic slope to develop two sets of parameters for deriving v s 30 one for active tectonic regions where topographic relief is high and one for stable shields where topography is more subdued by taking the gradient of the topography and choosing ranges of slope that maximize the correlation with shallow shear velocity observations we can recover to first order many of the spatially varying features of sitecondition maps developed for california our site condition map for the low relief mississippi embayment also predicts the bulk of the v s 30 observations in that region despite rather low slope ranges we find that maps derived from the slope of the topography is often well correlated with other independently derived regional scale site condition maps but the latter maps vary in quality and continuity and subsequently also in their ability to match observed v s 30 measurements contained therein alternatively the slope based method provides a simple approach to uniform site condition mapping

a terrain based site conditions map of california with implications for the contiguous united states abstract we present an approach based on geomorphometry to predict material properties and characterize site conditions using the v s 30 parameter time averaged shear wave velocity to a depth of 30 m our framework consists of an automated terrain classification scheme based on taxonomic criteria slope gradient local convexity and surface texture that systematically identifies 16 terrain types from 1 km spatial resolution 30 arcsec shuttle radar topography mission digital elevation models srtmdems using 853 v s 30 values from california we apply a simulation based statistical method to determine the mean v s 30 for each terrain type in california we then compare the v s 30 values with models based on individual proxies such as mapped surface geology and topographic slope and show that our systematic terrain based approach consistently performs better than semiempirical estimates based on individual proxies to further evaluate our model we apply our california based estimates to terrains of the contiguous united states comparisons of our estimates with 325 v s 30 measurements outside of california as well as estimates based on the topographic slope model indicate our method to be statistically robust and more accurate our approach thus provides an objective and robust method for extending estimates of v s 30 for regions where in situ measurements are sparse or not readily available

on the characteristics of local geology and their influence on ground motions generated by the loma prieta earthquake in the san francisco bay region california strong ground motions recorded at 34 sites in the san francisco bay region from the loma prieta earthquake show marked variations in characteristics dependent on crustal structure and local geological conditions peak horizontal acceleration and velocity inferred for sites underlain by rock generally occur on the transverse component of motion they are consistently greater with lower attenuation rates than the corresponding mean value predicted by empirical curves based on previous strong motion data theoretical amplitude distributions and synthetic seismograms calculated for 10 layer models suggest that bedrock motions were elevated due in part to the wide angle reflection of s energy from the base of a relatively thin 25 km continental crust in the region characteristics of geologic and geotechnical units as currently mapped for the san francisco bay region show that average ratios of peak horizontal acceleration velocity and displacement increase with decreasing mean shear wave velocity ratios of peak acceleration for sites on soil alluvium fill bay mud are statistically larger than those for sites on hard rock sandstone shale franciscan complex spectral ratios establish the existence of predominant site periods with peak amplifications near 15 for potentially damaging levels of ground motion at some sites underlain by alluvium and fill bay mud average spectral amplifications inferred for vertical and the mean horizontal motion are respectively 1 1 for sites on the franciscan complex kjf 1 4 1 5 for sites on mesozoic and tertiary rocks tmzs 2 1 2 0 for sites on the santa clara formation qts 2 3 2 9 for sites on alluvium qal and 2 1 4 0 for sites on fill bay mud qaf qhbm these mean values are not statistically different at the 5 significance level from those inferred from previous low strain data analyses suggest that soil amplification and reflected crustal shear energy were major contributors to levels of ground motion sufficient to cause damage to vulnerable structures at distances near 100 km in the cities of san francisco and oakland

nonlinear and linear site response and basin effects in seattle for the m 6 8 nisqually washington earthquake we used recordings of the m 6 8 nisqually earthquake and its m l 3 4 aftershock to study site response and basin effects for 35 locations in seattle washington we determined site amplification from fourier spectral ratios of the recorded horizontal ground motions referenced to a soft rock site soft soil sites generally national earthquake hazard reduction program nehrp class e on artificial fill and young alluvium have the largest 1 hz amplifications factors of 3 7 for both the mainshock and aftershock these amplifications are correlated with areas of higher damage from the mainshock to major buildings and liquefaction there are several indications of nonlinear response at the soft soil sites for the mainshock ground motions despite relatively modest peak accelerations in the s waves of 15 22 g first the mainshock spectral ratios do not show amplification at 2 8 hz as do the aftershock spectral ratios spectral peaks at frequencies below 2 hz generally occur at lower frequencies for the mainshock spectral ratios than for the aftershock ratios at one soft soil site there is a clear shift of the resonant frequency to a lower frequency for the mainshock compared with the aftershock the frequency of this resonance increases in the coda of the mainshock record indicating that the site response during the weaker motions of the coda is more linear than that of the initial s wave three of the soft soil sites display cusped one sided mainshock accelerograms after the s wave these soft soil sites also show amplification at 10 20 hz in the s wave relative to the rock site that is not observed for the aftershock the cusped waveforms and 10 20 hz amplification are symptomatic of nonlinear response at the soft soil sites these sites had nearby liquefaction the largest amplifications for 0 5 hz occur at soft soil sites on the southern portion of the seattle basin stiff soil sites nehrp classes d and c on pleistocence age glacial deposits display similar spectral amplification for the mainshock and aftershock indicating approximately linear response the stiff soil sites generally have moderate amplification factors of 1 1 2 4 at 0 5 and 1 hz amplifications at 1 and 5 hz for all sites generally increase with decreasing shear wave velocity measured in the top 30 m v s 30 however larger amplifications at 0 5 and 1 hz for sites with similar v s 30 values are observed for sites in the seattle basin illustrating the amplification from the deeper 30 m sediments and the contribution from basin surface waves record sections for the mainshock and aftershock show that basin surface waves produce the peak velocities for many of the sites in the seattle basin and often dominate the amplitude at 1 hz and lower frequencies

selective recharge and isotopic composition of shallow groundwater within temperate epigenic carbonate aquifers summary this paper considers the variation of 18o and 2h vsmow in precipitation and shallow groundwater from carbonate aquifers that lend insight into the source and timing of recharge within temperate epigenic karst the shallow groundwater collected during 2010 and 2011 at stream cave sc and natural bridge caverns nbc represent one input to and the primary output from the redmond creek karst aquifer in the cumberland plateau of southeast kentucky respectively these data are compared with the isotopic composition of concurrent samples of precipitation from the same watershed that covers some 1900 ha values of 18o and 2h at sc and nbc are statistically similar and cluster at the midpoint of the local meteoric water line these values remain surprisingly constant despite seasonal changes in temperature regimens and discharge samples in 2012 from regional springs that include redmond creek are more depleted in the heavier isotope and similarly stable despite coming from aquifers of a range of sizes and physical characteristics applying a priestly taylor model for daily values of potential evapotranspiration only 43 of the 1 10 m of precipitation in the 2010 2011 dataset remains as potential recharge primarily during cooler months with lower solar insolation weighting 18o and 2h values of precipitation by potential recharge creates a better match with the isotopic composition of shallow groundwater than by weighting by precipitation amount the isotopic composition and deuterium excess of precipitation samples are directly and inversely proportional to temperature respectively deuterium excess in this study and displays intra and inter annual variation that ranges from a minimum of 11 1 to a maximum of 29 5 that demonstrate the higher than average deuterium excess in greater appalachia and the shifting latitude of moisture sources including a significant winter component of re evaporated continental moisture

sedimentary basin effects in seattle washington ground motion observations and 3d simulations seismograms of local earthquakes recorded in seattle exhibit surface waves in the seattle basin and basin edge focusing of s waves spectral ratios of s waves and later arrivals at 1 hz for stiff soil sites in the seattle basin show a dependence on the direction to the earthquake with earthquakes to the south and southwest producing higher average amplification earthquakes to the southwest typically produce larger basin surface waves relative to s waves than earthquakes to the north and northwest probably because of the velocity contrast across the seattle fault along the southern margin of the seattle basin s to p conversions are observed for some events and are likely converted at the bottom of the seattle basin we model five earthquakes including the m 6 8 nisqually earthquake using 3d finite difference simulations accurate up to 1 hz the simulations reproduce the observed dependence of amplification on the direction to the earthquake the simulations generally match the timing and character of basin surface waves observed for many events the 3d simulation for the nisqually earthquake produces focusing of s waves along the southern margin of the seattle basin near the area in west seattle that experienced increased chimney damage from the earthquake similar to the results of the higher frequency 2d simulation reported by stephenson et al 2006 waveforms from the 3d simulations show reasonable agreement with the data at low frequencies 0 2 0 4 hz for the nisqually earthquake and an m 4 8 deep earthquake west of seattle

complex rupture during the 12 january 2010 haiti earthquake initially the devastating mw 7 0 12 january 2010 haiti earthquake seemed to involve straightforward accommodation of oblique relative motion between the caribbean and north american plates along the enriquillo plantain garden fault zone here we combine seismological observations geologic field data and space geodetic measurements to show that instead the rupture process involved slip on multiple faults primary surface deformation was driven by rupture on blind thrust faults with only minor deep lateral slip along or near the main enriquillo plantain garden fault zone thus the event only partially relieved centuries of accumulated left lateral strain on a small part of the plate boundary system together with the predominance of shallow off fault thrusting the lack of surface deformation implies that remaining shallow shear strain will be released in future surface rupturing earthquakes on the enriquillo plantain garden fault zone as occurred in inferred holocene and probable historic events we suggest that the geological signature of this earthquake broad warping and coastal deformation rather than surface rupture along the main fault zone will not be easily recognized by standard palaeoseismic studies we conclude that similarly complex earthquakes in tectonic environments that accommodate both translation and convergence such as the san andreas fault through the transverse ranges of california may be missing from the prehistoric earthquake record

sedimentation and extensional basin evolution in a miocene metamorphic core complex setting alvord mountain central mojave desert california usa the lower miocene clews formation at alvord mountain mojave desert california comprises an upward coarsening sequence of synextensional continental deposits of variable thickness this sequence pinches out to the west against the alvord mountain pre tertiary basement complex and thickens eastward to c 300 m over a distance of 7 km two stages of sedimentation are recognized in the formation during the initial stage of sedimentation the depocentre was delineated by a lacustrine system that was bounded to the west by small sheetflood dominated alluvial fans and to the north by a southerly flowing fluvial braidplain lacustrine mudstone siltstone and carbonate suggest an initially closed basin conglomerate to the west represents small sheetflood dominated fans igneous clasts and a south east palaeoflow direction suggest a proximal low relief source in the western alvord mountain area pebbly sandstone of the southward prograding braidplain has a metasedimentary provenance in the paradise range to the north the second stage of deposition was dominated by coarse conglomerate and breccia west to southwest palaeotransport indicators and a distinctive metaigneous petrofacies indicate a provenance in the cronese hills 8 km to the east these strata reflect rapid westward progradation of sheetflood and debris flow dominated alluvial fans that advanced across the axial braidplain and lacustrine system basin development is interpreted to have been controlled by regional ne directed extension on a detachment fault associated with the central mojave metamorphic core complex 30 km to the west the alvord mountain cronese hills region was initially transported as a single hangingwall block on the e dipping detachment initial lacustrine sedimentation reflects the development of a flexural or sag basin in the hangingwall subsequent westward progradation of alluvial fans out of the cronese hills is believed to record the propagation of a nw striking sw dipping normal fault antithetic to the low angle e dipping detachment fault

sediments and processes on a small stream flow dominated devonian alluvial fan shetland islands abstract the main conglomerate type of a small devonian alluvial fan in shetland northern britain is an inverse to normally graded framework supported gravel the sedimentological details of these beds and fundamental considerations of the mechanics of movement of highly concentrated flows suggests that neither debris flow nor grain flow were responsible for the deposition of these conglomerates on the contrary these inverse to normally graded conglomerates were deposited by water as thick gravel sheets with little topographic relief but broadly analogous to longitudinal bars they were deposited under high aggradation conditions first on the rising and then on the falling flood the high concentration of material in transport on the rising and peak flood was responsible for the polymodal and unstratified nature of the conglomerates as the flood waned normal grading was developed and a preferred horizontal fabric was produced under low concentrations of sediment other conglomerate types comparatively of lesser importance were deposited from highly concentrated clast dispersions and are notably richer in matrix and locally possess inverse grading the hydrological environment was one of flashy discharges of short duration but of high velocity flow was dissipated rapidly perhaps due to extreme water loss this example from the devonian of shetland provides an interesting alternative to the mass transport dominated fan models currently in vogue

integrated seismic texture segmentation and cluster analysis applied to channel delineation and chert reservoir characterization in recent years 3d volumetric attributes have gained wide acceptance by seismic interpreters the early introduction of the single trace complex trace attribute was quickly followed by seismic sequence attribute mapping workflows three dimensional geometric attributes such as coherence and curvature are also widely used most of these attributes correspond to very simple easy to understand measures of a waveform or surface morphology however not all geologic features can be so easily quantified for this reason simple statistical measures of the seismic waveform such as rms amplitude and texture analysis techniques prove to be quite valuable in delineating more chaotic stratigraphy in this paper we coupled structure oriented texture analysis based on the gray level co occurrence matrix with self organizing maps clustering technology and applied it to classify seismic textures by this way we expect that our workflow should be more sensitive to lateral changes rather than vertical changes in ref

determination of site amplification in the los angeles urban area from inversion of strong motion records abstract the amplification of strong ground motion at sites in the greater los angeles california region is determined using the generalized inverse method of andrews 1986 site amplification estimates are determined at 281 strong motion sites that provided horizontal component accelerograms from the 1971 san fernando 1987 whittier narrows 1991 sierra madre or 1994 northridge mainshocks the estimates are determined relative to the spectral level recorded at a single reference site in a second inversion a source site interaction term is added to andrews9s 1986 model to quantify the effect selected mainshock records have on site amplification estimates the source site interaction term is applied to the san fernando valley sites9 records of the northridge earthquake and to three los angeles basin sites9 records of the whittier narrows mainshock site amplification spectra are averaged within two frequency bands the intermediate frequency band ifb from 0 5 to 1 5 hz and the high frequency band hfb from 2 to 6 hz results are displayed on maps of surficial geology average spectral levels are correlated with average shear wave velocity in the uppermost 30 m a geotechnical parameter used by the national earthquake hazards reduction program nehrp to characterize site amplification statistically significant correlation is found in both frequency bands average spectral amplification levels for nehrp class b c and d sites in the la urban area are determined these averages display an expected increase in spectral amplification with category with a more pronounced variation in the ifb than in the hfb considerable overlap in the one standard deviation range of the c and d site levels is found in both ifb and hfb suggesting that site specific spectral amplification is influenced by more than just near surface shear wave velocity average site amplification levels are compared with those obtained from northridge aftershock records at 28 collocated sites and are found to be in reasonably good agreement in both frequency bands some interesting outliers that is sites for which the mainshock and aftershock determined amplifications differ significantly are identified as sites that experienced ground failure during the northridge mainshock

paleomagnetism geochronology and possible tectonic rotation of the middle miocene barstow formation mojave desert southern california the barstow formation of the central mojave desert in the mud hills consists of 965 m of epiclastic and volcanic sediments deposited in an extensional rift this widespread unit is of importance because its mammalian faunas characterize the barstovian land mammal age previous isotopic ages on interbedded tuffs have been used to calibrate middle miocene time and it was deposited in a tectonically active region volcanic ash samples were collected from the barstow formation for isotopic dating using conventional 40 k 40 ar and 40 ar 39 ar single crystal laser fusion methods multiple analyses on minerals from 5 superposed tuffs yielded the following mean 40 k 40 ar ages rak tuff at 16 3 0 3 m y oreodont tuff at 15 9 0 2 m y dated tuff at 14 8 0 2 m y hemicyon tuff at 14 0 0 1 m y and lapilli tuff at 13 4 0 2 m y similar mean ages were also obtained on the following tuffs using single crystal 40 ar 39 ar laser fusion methods oreodont tuff at 15 9 0 06 m y dated tuff at 14 8 0 06 m y and hemicyon tuff at 14 0 0 09 m y in order to develop a local magnetostratigraphy 272 paleomagnetic sites were collected from 7 important sections within the mud hills saturation isothermal remanence experiments suggested the presence of either hematite or magnetite as carriers of remanence after either stepwise alternating field or thermal demagnetization characteristic directions were usually isolated between 30 and 50 mt or 300 and 500 c positive reversal and fold tests indicate a stable remanence acquired prior to folding there are 21 magnetic polarity zones interpreted from the continuous part of the barstow formation in the study area by using the 5 isotopic ages determined from interbedded tuffs the local composite polarity pattern is correlated to the top of chron c5d through the bottom of c5ab on the magnetic polarity time scale or an age range from 17 6 to 13 4 m y the boundary between hemingfordian and barstovian land mammal ages herein occurs 30 m below the oreodont tuff 15 8 0 2 m y within c5b and is taken at about 16 0 m y ago the barstow formational mean declination is 347 1 inc 47 5 95 6 51 d 7 97 suggesting negligible or possibly slight counterclockwise rotation r 4 1 r 10 5 for this part of the mojave block during the past 18 m y a 40 ar 39 ar mean age of 19 3 0 02 m y for the red tuff which is near the local base of the barstow formation and unconformably underlying our composite section constrains a minimum age for the beginning of local basin filling with principally epiclastic sediments

a site conditions map for california based on geology and shear wave velocity one simple way of accounting for site conditions in calculating seismic hazards is to use the shear wave velocity in the shallow subsurface to classify materials the average shear wave velocity to 30 m v 30s has been used to develop site categories that can be used for modifying a calculated ground motion to account for site conditions we have prepared a site category map of california by first classifying the geologic units shown on 1 250 000 scale geologic maps our classification of geologic units is based on v 30s measured in 556 profiles and geological similarities between units for which we have v s data and the vast majority of units for which we have no data we then digitized the geologic boundaries from those maps that separated units with different site classifications v s data for california shows that several widespread geologic units have ranges of v 30s values that cross the boundaries between nehrp ubc site categories the franciscan complex has v 30s values across ubc categories b and c with a mean value near the boundary between those two categories older alluvium and late tertiary bedrock have v 30s values that range from about 300 to about 450 m sec across the boundary between categories c and d to accommodate these units we have created intermediate categories which we informally call bc and cd geologic units that have or are interpreted to have v 30s values near the boundary of the ubc categories are placed in these intermediate units in testing our map against the available v 30s measurements we have found that 74 of the measured v 30s values fall within the range assigned to the v s category where they fall on the map this ratio is quite good considering the inherent problems in plotting site specific data on a regional map and the variability of physical properties in geologic units we have also calculated the mean and distribution of v 30s for each of our map units and prepared composite profiles showing the variation of v s in the upper 100 m from the available v s data these data show that the map categories that we have defined based on geologic units have different v s properties that can be taken into account in calculating seismic hazards

the tiger formation paleogene fluvial sediments deposited adjacent to a deforming cordilleran metamorphic core complex northeastern washington abstract the eocene to oligocene tiger formation and associated upper plate rocks of northeastern washington are juxtaposed along low angle fault zones with structurally lower or lower plate rocks of the spokane dome a metamorphic core complex strata of the tiger formation that were derived solely from upper plate rock are termed intraplate strata and those that include clasts of lower plate rock are termed extraplate strata the cliff ridge and ione regions are situated on opposite sides of an arch of typical lower plate rock the tiger formation here consists of predominantly conglomeratic intraplate strata that were deposited by braided streams on both valley fills and alluvial fans these strata were derived from a tectonically active source terrane where lower plate rock now exists intraplate strata of the cusick region consist of conglomeratic sandstone and siltstone facies alluvial fans and a sandy braided alluvial plain graded downstream to a floodplain dominated environment intraplate strata exposed in the cliff ridge ione and cusick regions record deposition in response to differential uplift of the core complex in the subsurface extraplate conglomeratic strata deposited in alluvial fan and braided alluvial plain environments conformably and unconformably overlie the intraplate sandstone and siltstone facies of the cusick region these extraplate conglomerates were derived from the area now occupied solely by the core complex and record the erosion of intraplate tiger strata and other upper plate rocks exposure of the fault zone at the paleosurface and erosion of the core complex the tiger formation provides valuable insight to the spatial and temporal distribution of deformation that accompanies the development of a cordilleran metamorphic core complex

magnitude and significance of miocene crustal extension in the central mojave desert california the newly recognized waterman hills detachment fault whdf of the central mojave desert california is significant because it provides the first unambiguous evidence for large scale core complex style crustal extension in the central mojave desert and because it has significantly rearranged the pre miocene paleogeography of the mojave desert the whdf places steeply dipping to overturned miocene volcanic and sedimentary rocks upon mylonitic pre tertiary basement the mylonites which apparently formed during extension are predominantly l tectonites which manifest top to northeast shear the whdf dips to the northeast beneath dominofaulted ranges of the central mojave desert and detachment faults of the colorado river trough forming an imbricated early miocene system of detachment faults extension continued in the colorado river trough after extension had ceased in the central mojave desert tentative correlations of mesozoic intrusions suggest about 40 km of slip across the whdf which carries eugeoclinal paleozoic rocks in its hanging wall and cratonal miogeoclinal paleozoic rocks in its footwall restoration of 40 km of slip 1 removes a prominent kink in the boundary between eugeoclinal and cratonal miogeoclinal facies 2 aligns cratonal miogeoclinal strata near victorville more closely with the late paleozoic continental margin farther north 3 places cratonal miogeoclinal rocks structurally beneath eugeoclinal rocks implying that the facies were stacked by thrusting and 4 straightens the western margin of the late jurassic independence dike swarm

refraction microtremors data analysis and diagnostics of key hypotheses surface wave methods are quite popular for site characterization in geotechnical earthquake engineering among these techniques a particular role is taken by passive methods for their ability to yield information on the low frequency range and consequently on large depths one such passive method the refraction microtremors remi technique has been proposed as a simple alternative to 2d array techniques to estimate surface wave dispersion by using linear arrays of geophones the technique owes its name to the use of widely available instruments also adopted for seismic refraction the basic hypotheses underlying remi are that noise is distributed isotropically in azimuth or is aligned exactly with the array these conditions often are not met and in most cases they are not verified because such analysis requires an accurate approach to data processing that is rarely applied we have developed an algorithm that verifies remi s basic hypotheses by analyzing experimental data in addition we have propo

assessment of site effects in alpine regions through systematic site characterization of seismic stations abstract in the framework of the renewal project of the swiss strong motion network ssmnet a procedure for site characterization has been established the aim of the procedure was to systematically derive realistic 1d velocity profiles at each station it is mainly based on the analysis of surface waves particularly from passive experiments and includes cross checks of the derived amplification functions with those obtained through spectral modeling of recorded earthquakes the systematic use of three component surface wave analysis allowing the derivation of both rayleigh and love dispersion curves also contributes to the improvement of the quality of the retrieved profiles the procedure is applied to the 30 ssmnet stations installed on various site types within the project covering different aspects of seismic risk the characterization of these 30 sites gives an overview of the variety of possible effects of surface geology on ground motion in the alpine area such effects ranged from deamplification at hard rock sites to amplification up to a factor of 15 in lacustrine sediments with respect to the swiss reference rock velocity model the derived velocity profiles are shown to reproduce observed amplification functions from empirical spectral modeling although many sites are found to exhibit 1d behavior the procedure allows the detection and qualification of 2d and 3d effects the sites are therefore classified with respect to the occurrence of 2d 3d resonance and edge generated surface waves in addition to the large and deeply incised alpine valleys of the rhone the rhine and the aar smaller structures such as local alpine valleys and alluvial fans are shown to exhibit 2d 3d behavior

site characterization for urban seismic hazards in lower manhattan new york city from microtremor array analysis 1 we characterize the shear wave velocity vs of soil deposits in lower manhattan at six sites using the spac and hvsr microtremor methods the soil vs ranges from 140 m s to 300 m s over bedrock we believe that bedrock depth is constrained to 15 with these microtremor data but an accurate quantification of bedrock vs is unattainable because of low spectral coherency at frequencies below 3 hz hvsr data corroborate the spac microtremor results at sites based on good alignment of theoretical rayleigh ellipticity peaks 1d sh amplifications and peak hvsr resonant frequencies vary between 1 4 and 5 5 hz between the sites from both observed hvsr and predicted 1d sh amplification shear wave velocities to 30 m classify the sites as c d or e this study demonstrates that given relatively shallow bedrock depth the spac method can work in extremely urbanized areas

intramethod variability in remi dispersion measurements and vs estimates at shallow bedrock sites the refraction microtremor remi method uses a linear array of receivers to record passive noise stress waves thereby negating the need for a strong active source such as those typically employed in other linear array surface wave methods the simplicity of such an approach is very appealing however there is a potential for significant uncertainty in shear wave velocity vs estimates obtained in this manner because much is unknown about the seismic wavefield that is being sampled this article examines intramethod variability in remi dispersion measurements and vs estimates at shallow bedrock sites the focus is not on evaluating whether remi measurements yielded the true vs profiles for the study sites but rather on evaluating how the dispersion curves and vs profiles at the sites were affected by variations in the seismic wavefield sampled by a linear array of receivers it was found that changes in the sampled seismic wavefield obtained through employing various array orientations at a site an

the mojave extensional belt of southern california it is proposed that the regional pattern of early miocene low and high angle normal faulting and dike swarm emplacement in the western and central mojave desert is the result of regional extension developed within a roughly east trending zone named here the mojave extensional belt this zone of extension although now disrupted by late cenozoic right slip faulting can be traced from the intersection of the san andreas and garlock faults to the eastern mojave desert near bristol lake and the granite mountains fault restoration of post oligocene movement along the san andreas fault system places the now buried western part of the mojave extensional belt opposite a similar age extended area of west central california surface mapping and limited subsurface data suggest that the dominant tectonic process responsible for development of the mojave extensional belt was low angle normal sense simple shear detachment faulting strain within the mojave extensional belt is partitioned between four domains edwards waterman daggett and bullion terranes that each consist of one or more half grabens each half graben is composed of tilted normal fault bounded blocks that lie above a rooted low angle brittle ductile normal sense shear zone tilting and extension of the upper plate resulted from the superposition of several generations of originally steeply dipping 80 planar to slightly curviplanar normal faults at least one generation of high angle faults postdates the formation of the detachments differential extension between the domains was accommodated by strike slip faults transfer zones similarly the lack of early miocene extensional elements north of the western segment of the garlock fault suggests that the garlock may have been active in the early miocene and served as the northern transfer boundary of the mojave extensional belt the mojave extensional belt initially began to open about 22 m y ago with the major phase of extension occurring between 22 and 20 ma local high angle normal faults cut older detachments and were active between 20 and 17 ma kinematic indicators suggest that the western part of the mojave extensional belt opened in a ne sw direction whereas the eastern portion extended in an ene wsw direction present day reference frame rotation of the region about vertical axes as revealed by paleomagnetic studies suggests that the direction of extension within the mojave extensional belt was originally oriented n s and was rotated as much as 50 clockwise between 20 and 18 ma early miocene

faster better shear wave velocity to 100 meters depth from refraction microtremor arrays current techniques of estimating shallow shear velocities for assessment of earthquake site response are too costly for use at most construction sites they require large sources to be effective in noisy urban settings or specialized independent recorders laid out in an extensive array this work shows that microtremor noise recordings made on 200 m long lines of seismic refraction equipment can estimate shear velocity with 20 accuracy often to 100 m depths the combination of commonly available equipment simple recording with no source a wavefield transformation data processing technique and an interactive rayleigh wave dispersion modeling tool exploits the most effective aspects of the microtremor spectral analysis of surface wave sasw and multichannel analysis of surface wave masw techniques the slowness frequency wavefield transformation is particularly effective in allowing accurate picking of rayleigh wave phase velocity dispersion curves despite the presence of waves propagating across the linear array at high apparent velocities higher mode rayleigh waves body waves air waves and incoherent noise two locations illustrate the application of this technique in detail coincident with a large accelerometer microtremor array in reno nevada and atop a borehole logged for shear velocity in newhall california refraction equipment could duplicate microtremor results above 3 hz but could not estimate velocities deeper than 100 m refraction microtremor cannot duplicate the detail in the velocity profile yielded by a suspension logger but can match the average velocity of 10 to 20 m depth intervals and suggest structure below the 100 m logged depth of the hole eight additional examples from southern california and new zealand demonstrate that the refraction microtremor technique quickly produces good results from a wide range of hard and soft sites

oblique collision in the northeastern caribbean from gps measurements and geological observations 1 previous caribbean gps studies have shown that the rigid interior of the caribbean plate is moving east northeastward 070 at a rate of 18 20 3 mm yr relative to north america this direction implies maximum oblique convergence between the island of hispaniola on the caribbean plate and the 22 27 km thick crust of the bahama carbonate platform on the adjacent north america plate we present a tectonic interpretation of a 15 site gps network which spans the hispaniola bahama oblique collision zone and includes stable plate interior sites on both the north america and caribbean plates measurements span the time period of 1994 1999 in a north america reference frame gps velocities in puerto rico st croix and the lesser antilles indicate that these areas move as a single block in an east northeast direction 070 at a rate of 19 20 mm yr consistent with the movement of the rigid interior of the caribbean plate gps velocities at six sites in central and eastern hispaniola dominican republic show drastically different behavior with more eastwardly strikes 080 and much slower rates 4 17 mm yr than areas of the stable caribbean plate the boundary between the relatively slower moving hispaniola collisional zone and the relatively faster moving uncollided puerto rico virgin islands area is the mona passage where late neogene rifting occurs in a broad zone elastic modeling favors strain partitioning with oblique slip on the outer low angle submarine thrust faults north hispaniola muertos and strike slip on the inner subvertical subaerial strike slip faults septentrional enriquillo

cenozoic evolution of the mojave block of southern california the recorded cenozoic history of the mojave desert region of southern california began in the latest oligocene when intense volcanism and tectonism interrupted a long early tertiary silence volcanism commenced across the region in an east west band ca 24 22 ma northwest of barstow volcanism was accompanied by intense crustal extension and development of a metamorphic core complex outside of this relatively restricted area extension was minor or absent after extension ceased ca 18 ma volcanism shifted to small volume eruptions of basalt post extensional deformation has largely been by strike slip faulting along northwest striking dextral faults and west striking sinistral faults and total dextral slip across the mojave desert region since the early miocene is 45 60 km strike slip deformation has been overprinted locally by intense north south contraction that is the dominant style of deformation in the western mojave block paleomagnetic data indicate that parts of the mojave block were rotated clockwise although the magnitude and timing of this rotation are poorly determined the best evidence for large 45 rotation comes from the area east of barstow where large clockwise declination anomalies and mesozoic and cenozoic dikes with anomalous strikes may reflect early miocene clockwise deflection along the mojave river fault volcanism and tectonism in the mojave block resulted from interactions among the north american pacific and various oceanic plates patterns of volcanism and tectonism do not correlate with growth of slab windows beneath the continent but do correlate with the position of the subducted mendocino fracture zone plate circuit reconstructions suggest that the driving force for extension was divergence between the pacific and north american plates along the transform margin that separated the two this hypothesis accounts for the direction magnitude and rate of extension in the mojave block

early neogene continental sedimentation in the vallecito and fish creek mountains western salton trough california abstract lower neogene sedimentary rocks in the vallecito and the fish creek mountains along with locally intercalated basalts were deposited in a half graben basin and range setting three depositional environments are recognized 1 braided stream 2 alluvial fan and 3 landslide a platte type braided stream system with local accumulation of seif dunes drained a westward tilted pediment surface coarse sand was deposited as transverse and linguoid bars sand dunes and flat beds and gravel was deposited as longitudinal bars modified by tractive flow in broad shallow channels this system joined a donjek type northward flowing axial drainage characterized by channels with marked topographic relief deep channel reaches were infilled with longitudinal gravel bars and massive granular very coarse sand from concentrated dispersions giant sand waves were rarely present in deeper channel reaches in shallower channel tracts coarse sand was deposited as transverse bars sand dunes and flat beds and gravel was deposited as small longitudinal bars the alluvial fan deposits accumulated along the faulted western margin of the basin and prograded eastward over the axial drainage they consist of debris flow sheetflood and donjek type channel fill deposits subenvironments are defined by the distribution of these deposits and their bedding geometries the upper fan above the intersection point is characterized by debris flow and channel fill deposits with numerous internal erosional contacts and laterally discontinuous beds the middle fan below the intersection point is marked by crudely developed thickening upward cycles of laterally continuous repetitively interbedded very thick debris flow and thinner bedded sheetflood deposits the lower fan is represented by interbedded debris flow and sheetflood deposits that are thinner bedded and less organized than the middle fan subfacies the landslide deposits locally top the sequence they are massive laterally continuous sedimentary megabreccias containing clasts that are up to 5 m in diameter and display a distinctive jigsaw puzzle texture these breccias formed as two cataclysmically emplaced air cushioned landslides one was dislodged from the vallecito mountains and traveled eastward while the other was released from the fish creek mountains and traveled westward

coarse grained nonmarine volcaniclastic sediment terminology and depositional process modern and ancient volcaniclastic sedimentary sequences contain depositional units whose features cannot be attributed to fully turbulent dilute stream flow or viscous debris flow the characteristics of these poorly sorted sediments suggest rapid deposition from high concentration dispersions but not en masse sedimentation thus appears related to high discharge flows intermediate in sediment water ratio between stream flow and debris flow the term hyperconcentrated flood flow is proposed for describing this intermediate condition hyperconcentrated flood flow deposits are distinguished from debris flow deposits by lack of matrix support or reverse grading and instead exhibit distribution normal grading and horizontal stratification these deposits are distinguished from normal dilute stream flow deposits by lack of cross stratification in sand facies and by very poor sorting poor imbrication and numerous clasts with long axes oriented parallel to flow direction in gravel facies the horizontal bedding that dominates sandy hyperconcentrated flood flow deposits consists of sediment too coarse grained and strata too thick to have been produced in the boundary layer of the upper flow regime and should not be confused with the more familiar thin graded laminae of fine to medium grained sand often associated with parting lineation hyperconcentrated flood flow deposits are not unique to volcanic settings they also occur in arid alluvial fan sequences debris flow and hyperconcentrated flood flow deposits however are much thicker and more extensive in volcanic regions than on alluvial fans because explosive volcanism leads to rapid mobilization of large volumes of sediment and water on a scale unparalleled in nonvolcanic settings in volcanic regions therefore these deposits have greater preservation potential show greater lateral variability and are more voluminous transformation of channelized debris flow to hyperconcentrated flood flow by dilution with stream water recently observed at mount st helens is recorded in ancient volcaniclastic sequences and may serve as the primary mechanism for generating hyperconcentrated flood flow

linked fault families in basin formation abstract recent models of faulting tectonics in basins and passive margins have emphasized the linked nature of faults in a way that is analogous to that developed for thrust regimes a linked fault system requires three components ramps flats and sidewalls or transfers basins that are the result of other processes are not considered in this contribution and the examples used are drawn from the midland valley and northumberland basins of the u k where existing mapping by the british geological survey as well as field work undertaken by the author indicate that such models are appropriate this paper summarizes the geometric features and constraints of such models and emphasizes the role of each of the three critical components of a linked system for large deformations that result in the formation of major extensional and strike slip basins the principal geometries can be understood by reference to regionally and locally balanced fault models that involve detachment ramp and sidewall or transfer elements tip strains and both lateral and vertical accommodation zones are included in the general model but these probably account for less than 10 of the total upper crustal strain the basic analysis of a linked basin system requires the transfer of displacement on and between the fault and shear zone components the geometry of the stratigraphic units is used to deduce the active linkage on the fault system in oblique slip systems and where the dominant extensional ramp is connected to oblique transfer and detachment systems lateral and sidewall sub basins and intra basin uplifts occur as part of continuing slip on the fault system these control facies distribution and depocentre evolution the existence of a basin forming linked system frequently controls later deformation of basins and margins and an appreciation of basin growth faults and stratigraphic geometry is essential in the interpretation of inverted basins and orogenic belts involving supracrustal formations

petrology depositional environments and structural development of the mineta formation teran basin cochise county arizona abstract the oligocene mineta formation of the teran basin in southeastern arizona contains approximately 1000 m of interfingering alluvial fan fluvial and lacustrine strata with minor intercalated volcanics sedimentation began prior to the middle oligocene and ended with the eruption of the oligocene miocene galiuro volcanics about 28 m y ago several related depositional facies are present in a cyclic pattern within the mineta formation a complete cycle contains proximal coarse grained alluvial fan conglomerates that fine upward into sandstone and pebble conglomerate of a distal fan or braided stream environment medium to fine grained sandstones are interpreted as sandflat deposits that lie stratigraphically above the distal fan deposits and below lacustrine sandstone and algal limestone the cycle culminates with deeper water lacustrine mudstone with intercalated thin bedded turbidite sandstone discrete cycles of bedded gypsum and mudstone are found locally within the lacustrine deposits these deposits represent a temporary playa lake phase of the mineta depositional system the mineta formation filled a half graben that developed in response to extensional stresses associated with the development of the rincon catalina metamorphic core complex paleocurrents flowed southwestward from a source area of predominantly sedimentary and volcanic rocks sedimentation was contemporaneous with extensional deformation that produced low angle normal faults and tilted the section toward the northeast maximum extension was directed about n55 e s55 w roughly parallel to the direction of paleocurrent flow parallel to the mean slip direction for low angle normal faults and perpendicular to the strike of the mineta section

blind shear wave velocity comparison of remi and masw results with boreholes to 200 m in santa clara valley implications for earthquake ground motion assessment multichannel analysis of surface waves masw and refraction micro tremor remi are two of the most recently developed surface acquisition techniques for determining shallow shear wave velocity we conducted a blind comparison of masw and remi results with four boreholes logged to at least 260 m for shear vel ocity in santa clara valley california to determine how closely these surface meth ods match the downhole measurements average shear wave velocity estimates to depths of 30 50 and 100 m demonstrate that the surface methods as implemented in this study can generally match borehole results to within 15 to these depths at two of the boreholes the average to 100 m depth was within 3 spectral amplifi cations predicted from the respective borehole velocity profiles similarly compare to within 15 or better from 1 to 10 hz with both the masw and remi surface method velocity profiles overall neither surface method was consistently better at matching the borehole velocity profiles or amplifications our results suggest masw and remi surface acquisition methods can both be appropriate choices for estimating shear wave velocity and can be complementary to each other in urban settings for hazards assessment

comparison of p and s wave velocity profiles obtained from surface seismic refraction reflection and downhole data high resolution seismic reflection refraction data were acquired on the ground surface at six locations to compare with near surface seismic velocity downhole measurements measurement sites were in seattle wa the san francisco bay area ca and the san fernando valley ca we quantitatively compared the data in terms of the average shear wave velocity to 30 m depth vs30 and by the ratio of the relative site amplification produced by the velocity profiles of each data type over a specified set of quarter wavelength frequencies in terms of vs30 similar values were determined from the two methods there is 15 difference at four of the six sites the vs30 values at the other two sites differ by 21 and 48 the relative site amplification factors differ generally by less than 10 for both p and s wave velocities we also found that s wave reflections and first arrival phase delays are essential for identifying velocity inversions the results suggest that seismic reflection refraction data are a fast non invasive and less expensive alternative to downhole data for determining vs30 in addition we emphasize that some p and s wave reflection travel times can directly indicate the frequencies of potentially damaging earthquake site resonances a strong correlation between the simple s wave first arrival travel time apparent velocity on the ground surface at 100 m offset from the seismic source and the vs30 value for that site is an additional unique feature of the reflection refraction data that could greatly simplify vs30 determinations

shear wave velocity structure of the shallow sediments in the bohai sea from an ocean bottom seismometer survey abstractto investigate the seismic velocity structure of the shallow sediments in the bohai sea of china we conducted a shear wave velocity inversion of the surface wave dispersion data from a survey of 12 ocean bottom seismometers obss and 377 shots of a 9000 in3 air gun with obs station spacing of approximately 5 km and air gun shot spacing of approximately 190 m high quality scholte wave data were recorded by the obss within 0 4 5 km offset we retrieved the scholte wave phase velocity dispersion for the fundamental mode and first overtone in the frequency band of 0 9 3 0 hz with the phase shift method and inverted for the shear wave velocity structure of the shallow sediments with a damped iterative least squares algorithm pseudo 2d shear wave velocity profiles with a depth of approximately 400 m revealed coherent features of relatively weak lateral velocity variation we also estimated the uncertainty in shear wave velocity structure based on the pseudo 2d profiles from six trial inversions wi

comparisons between vs30 and spectral response for 30 sites in newcastle australia from collocated seismic cone penetrometer active and passive source vs data although the time averaged shear wave velocity down to 30 m depth v s 30 can be a proxy for estimating earthquake ground motion amplification significant controversy exists about its limitations when used as a single parameter for the prediction of amplification to examine this question in absence of relevant strong motion records we use a range of different methods to measure the shear wave velocity profiles and the resulting theoretical site amplification factors afs for 30 sites in the newcastle area australia in a series of blind comparison studies the multimethod approach used here combines past seismic cone penetrometer and spectral analysis of surface wave data with newly acquired horizontal to vertical spectral ratio passive source surface wave spatial autocorrelation spac refraction microtremor remi and multichannel analysis of surface wave data the various measurement techniques predicted a range of different afs the spac and remi techniques have the smallest overall deviation from the median af for the majority of sites we show that v s 30 can be related to spectral response above a period t of 0 5 s but not necessarily with the maximum amplification according to the modeling done based on the measured shear wave velocity profiles both v s 30 and af values are influenced by the velocity ratio between bedrock and overlying sediments and the presence of surficial thin low velocity layers 0 5 s do the amplification curves consistently show higher values for soft site classes and lower for hard classes

sheet flow dominated gravel fans of the non marine middle cenozoic simmler formation central california abstract alluvial fan conglomerates of the middle cenozoic simmler formation in the cuyama gorge and southern la panza range of west central california were deposited in two half graben basins on the basis of paleocurrents and down fan fining of sediment both to the northeast the basins are thought to have been adjacent to the then active nacimiento and la panza faults three contiguous conglomerate lithofacies from most proximal to most distal are 1 clast supported crudely stratified coarse conglomerate with imbrication proximal talus and powerful sheet flow 2 clast supported well bedded internally unstratified conglomerate with common imbrication less common grading and thin interbedded coarse sands mid to upper fan deposits of highly sediment charged density modified grain flow sheet flows 3 stratified sandy conglomerate with imbrication distal alluvial sheet flows two additional lithofacies cut across the first three 4 parallel bedded graded coarse sands turbidity currents in floodbasin lakes and 5 matrix supported very poorly sorted conglomerates cohesive debris flows the alluvial fan deposits of the more northeasterly basin are overlain directly by the marine vaqueros formation the fans were dominated by sheet flow events the lack of cross bedding and minor occurrence of channels indicate that permanent streams were not established cohesive debris flows were unimportant the relationship of fan gravels both to the lake turbidites and to the conformably overlying marine sands indicates the existence of fan deltas as a result of late cenozoic transcurrent movement on the la panza fault a strand of the early san andreas system the two half graben basins have been moved so as to create a sedimentary mismatch

the effect of quaternary alluvium on strong ground motion in the coyote lake california earthquake of 1979 abstract the effect of alluvium on strong ground motion can be seen by comparing two strong motion records of the coyote lake california earthquake of 6 august 1979 m l 5 9 one record at a site on franciscan bedrock had a peak horizontal acceleration of 0 13 g and a peak horizontal velocity of 10 cm sec the other at a site 2 km distant on 180 meters of quaternary alluvium overlying franciscan had values of 0 26 g and 32 cm sec amplifications by factors of 2 and 3 horizontal motions computed at the alluvial site for a linear plane layered model based on measured p and s velocities show reasonably good agreement in shape with the observed motions but the observed peak amplitudes are greater by a factor of about 1 25 in acceleration and 1 8 in velocity about 15 per cent of the discrepancy in acceleration and 20 per cent in velocity can be attributed to the difference in source distance the remainder may represent focusing by refraction at a bedrock surface concave upward there is no clear evidence of nonlinear soil response fourier spectral ratios between motions observed on bedrock and alluvium show good agreement with ratios predicted from the linear model in particular the observed frequency of the fundamental peak in the amplification spectrum agrees with the computed value indicating that no significant nonlinearity occurs in the secant shear modulus computations show that nonlinear models are compatible with the data if values of the coefficient of dynamic shear strength in terms of vertical effective stress are in the range of 0 5 to 1 0 or greater the data illustrate that site amplification may be less a matter of resonance involving reinforcing multiple reflections and more the simple effect of the low near surface velocity application of traditional seismological theory leads to the conclusion that the site amplification for peak horizontal velocity is approximately proportional to the reciprocal of the square root of the product of density and shear wave velocity

on the earthquake hazards of puerto rico and the virgin islands abstract puerto rico and the virgin islands are located near the northeastern corner of the caribbean seismic zone numerous large earthquakes have struck these islands some with disastrous results the 400 yr long earthquake record of puerto rico describes shocks affecting nearly all portions of the island the last destructive shock in 1918 7 5 m s did not occur along the main seismic zone but rather on an intraplate fault near mona canyon off the northwest coast a possible great earthquake in 1787 8 to 8 25 m s appears to have occurred along the main seismic zone near the puerto rico trench to the north of the island but data for the event are scarce a disastrous earthquake in the virgin islands 1867 7 5 m s also occurred on an intraplate fault this fault is one of a series that bound the anegada trough separating saint croix from the main chain of the virgin islands seafloor morphology microearthquakes and the record of historic earthquakes define a zone of deformation extending from the puerto rico trench northeast of the virgin islands trending southwest along the anegada trough and then westerly along the muertos trough the muertos trough is the locus of convergence between the floor of the caribbean sea and puerto rico maximum dimensions of future large earthquakes are inferred from sizes of blocks in the anegada and muertos troughs as well as mona canyon shocks as large as 7 5 can occur on these intraplate faults although strain rates on these faults may be an order of magnitude less than on faults in the puerto rico trench the large number of potential sources suggest that damaging earthquakes in this part of the caribbean can come from either the puerto rico trench or the intraplate faults with nearly equal probability

neotectonics of puerto rico and the virgin islands northeastern caribbean from gps geodesy the boundary between the north american and caribbean plates is characterized primarily by left lateral motion along predominantly east west striking faults seismicity and marine geophysical survey data are consistent with at least two and possibly three microplates in the diffuse boundary zone in the northeastern caribbean 1 the gonave 2 the hispaniola and 3 the puerto rico northern virgin islands prvi we discuss results from gps geodetic measurements acquired since 1994 to test the microplate hypothesis define prvi translation and rotation within the boundary zone and constrain prvi neotectonics gps derived velocities are analyzed with respect to both north american and caribbean plate reference frames integrated displacements across prvi are limited to a few millimeters per year consistent with a rigid prvi and permitting calculation of an average velocity for prvi the motions of prvi relative to north america and the caribbean are 16 9 1 1 mm yr toward n68 e 3 1 and 2 4 1 4 mm yr toward s79 w 26 1 respectively in contrast with some recent models ongoing rotation of prvi about a nearby 25 distant vertical axis is not supported by the geodetic data in addition we argue against eastward tectonic escape of prvi and favor a simple progressive increase in velocity across the plate boundary zone requiring that the summed magnitude of strike slip fault slip rates will equal the total plate motion rate between the caribbean and north america gps data are consistent with components of left lateral strike slip faulting along the muertos trough south of puerto rico and shortening across the puerto rico trench comparison of gps velocities for prvi with respect to north america with total north america caribbean relative motion suggests up to 85 of north american caribbean plate motion is accommodated by the puerto rico trench and offshore faults north of puerto rico differences in gps derived velocities from hispaniola and prvi yield east west extension across the n s trending mona rift of a few millimeters per year when estimated elastic strain accumulation effects along the north hispaniola deformed belt and the septentrional fault zone are considered the opening rate implies an age of the mona rift of 2 3 million years agreeing with marine geophysical data that support a young age for the structure

sedimentation of the neogene recent alasehir gediz continental graben system used to test alternative tectonic models for western aegean turkey abstract the alasehir gediz graben exemplifies clastic sedimentation in a long lived continental half graben in a semi arid setting developed within relatively incompetent metamorphic rocks early miocene to recent rift related sediments are exhumed on both flanks of the graben allowing detailed three dimensional study during the early miocene small fan delta lobes were shed northwards from the rugged menderes metamorphic massif into a bordering lacustrine basin during early to mid miocene time large alluvial fans prograded northwards into this basin through drainage to the aegean sea was established as the basin widened and filled discrete lobes of coarse alluvial fan sediments of latest miocene pliocene age also shedding northwards are likely to have been climatically influenced quaternary alluvium party infills the modern alasehir rift basin the sedimentary information can be used to test two alternative tectonic models for the alasehir graben in the first model an e w graben bounded by high angle faults was active during latest miocene recent time whereas earlier miocene sedimentation was controlled by n s faulting related to a n s compressional stress regime in the second hypothesis the alasehir graben was initiated much earlier in the early miocene and was then either continuously or episodically active until recent our results especially facies and palaeocurrent data from alluvial sediments indicate that clastic sedimentation was controlled by mainly e w faulting in a n s stress regime assuming the early miocene clastic sediments are correctly dated this supports the second long lived extension model however rather than steady state extension for ca 15 ma the sedimentary evidence and regional context are consistent with a pulsed extension model whereby initial early to mid miocene extension and related clastic sedimentation was followed by a second phase of extension in latest miocene pliocene time the driving force of initial early miocene extension was probably gravity spreading towards a south aegean subduction zone whereas the inferred second extension pulse is seen as being triggered by westward tectonic escape of anatolia towards the extending aegean back arc region