a referenced empirical ground motion model for arias intensity and cumulative absolute velocity based on the nga east databasea referenced empirical ground motion model for arias intensity and cumulative absolute velocity based on the nga east database

stratigraphic record of pliocene pleistocene basin evolution and deformation within the southern san andreas fault zone mecca hills california abstract a thick section of pliocene pleistocene nonmarine sedimentary rocks exposed in the mecca hills california provides a record of fault zone evolution along the coachella valley segment of the san andreas fault saf geologic mapping measured sections detailed sedimentology and paleomagnetic data document a 3 5 myr history of deformation and sedimentation in this area sw side down offset on the painted canyon fault pcf starting 3 7 ma resulted in deposition of the mecca conglomerate southwest of the fault the lower member of the palm spring formation accumulated across the pcf from 3 0 to 2 6 ma during regional subsidence sw side up slip on the pcf and related transpressive deformation from 2 6 to 2 3 ma created a time transgressive angular unconformity between the lower and upper members of the palm spring formation the upper member accumulated in discrete fault bounded depocenters until initiation of modern deformation uplift and basin inversion starting at 0 7 ma some spatially restricted deposits can be attributed to the evolution of fault zone geometric complexities however the deformation events at ca 2 6 ma and 0 7 ma are recorded regionally along 80 km of the saf through coachella valley covering an area much larger than mapped fault zone irregularities and thus require regional explanations we therefore conclude that late cenozoic deformation and sedimentation along the saf in coachella valley has been controlled by a combination of regional tectonic drivers and local deformation due to dextral slip through fault zone complexities we further propose a kinematic link between the 2 6 2 3 ma angular unconformity and a previously documented but poorly dated reorganization of plate boundary faults in the northern gulf of california at 3 3 2 0 ma this analysis highlights the potential for high precision chronologies in deformed terrestrial deposits to provide improved understanding of local to regional scale structural controls on basin formation and deformation along an active transform margin

interpretation of three dimensional seismic refraction data from western hecate strait british columbia structure of the queen charlotte basin the queen charlotte basin consists of up to 6 km of tertiary clastic sediments in a complex sequence of fault bounded subbasins the tectonic evolution of the basin in still being debated with recent interpretations including distributed strike slip extension oblique or en echelon rifting simple extension orthogonal to the plate margin and block faulting and vertical tectonics a combined seismic reflection and refraction survey was carried out in 1988 to investigate the structure and tectonic evolution of the basin and underlying crust while the marine multichannel reflection data were being collected refracted and wide angle reflected energy from the large air gun array was recorded at surrounding land sites in both two dimensional in line and three dimensional broadside geometries the broadside refraction data recorded on the queen charlotte islands provide good three dimensional coverage of western hecate strait these data are modelled to determine the three dimensional structure of the qu

finite difference calculation of traveltimes in three dimensions the traveltimes of first arriving seismic rays through most velocity structures can be computed rapidly on a three dimensional numerical grid by finite difference extrapolation head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 11 percent are found between the results of this technique and ray tracing for a complex but smooth model this scheme has proven useful for earthquake location and shows promise as an inexpensive well behaved substitute for ray tracing in forward modeling and kirchhoff inversion applications

estimation of moment magnitude and stress parameter from shakemap ground motion parameters we describe an efficient method to determine moment magnitude and stress parameter in near real time in the immediate aftermath of a small to moderate earthquake m 3 6 from shakemap ground motion parameters 5 damped pseudospectral acceleration at 1 hz 0 33 hz 10 hz and or peak ground acceleration the methodology is based on relating shakemap parameters to source and attenuation parameters within the context of a generic stochastic point source model to provide an event specific ground motion prediction equation gmpe that will reliably predict amplitudes it is an approach that has been optimized for applications in regions with sparse networks an example is provided for southern ontario in brief we initially develop a simulation guided generic gmpe model based on the available database in the region we derive the applicable regional anelastic attenuation stress parameter and site amplification models then using the estimated regional models we show how to invert shakemaps parameters to estimate moment magnitude and stress parameter in near real time to define an event specific gmpe for an event that just happened the event specific gmpe can be used to provide robust calibrated shakemaps that are fully consistent with ground motion observations

a numerical study of strike slip bend formation with application to the salton sea pull apart basin how stepovers of strike slip faults connect to form bends is a question important for understanding the formation of push up ranges restraining bends and pull apart basins releasing bends we investigated the basic mechanics of this process in a simple three dimensional viscoelastoplastic finite element model our model predicts localized plastic strain within stepovers that may eventually lead to the formation of strike slip bends major parameters controlling strain localization include the relative fault strength geometry of the fault system and the plasticity model assumed using the drucker prager plasticity model in which the plastic yield strength of the crust depends on both shear and normal stresses our results show that a releasing bend is easier to develop than a restraining bend under similar conditions these results may help explain the formation of the salton sea pull apart basin in southern california 0 5 0 1 ma ago when the stepover between the imperial fault and the san andreas fault was connected by the brawley seismic zone

regionally adjustable generic ground motion prediction equation based on equivalent point source simulations application to central and eastern north america abstract we develop a generic ground motion prediction equation gmpe that can be adjusted for use in any region by modifying a few key model parameters the basis of the gmpe is an equivalent point source simulation model whose parameters have been calibrated to empirical data in california in such a way as to determine the decoupled effects of basic source and attenuation parameters on ground motion amplitudes we formulate the generic gmpe as a function of magnitude distance stress parameter geometrical spreading rate and anelastic attenuation this provides a fully adjustable predictive model allowing users to calibrate its parameters using observed motions in the target region we also include an empirical calibration factor to account for residual effects that are different from and or missing in simulations compared to observed motions in the target region as an example we show how the generic gmpe can be adjusted for use in central and eastern north america cena and calibrated with the next generation attenuation east database we provide median predictions of ground motions in cena for average horizontal component peak ground motions and 5 damped pseudospectral acceleration periods up to t 10 s for magnitudes m 3 8 and distances up to 600 km

crustal structure and seismicity distribution adjacent to the pacific and north america plate boundary in southern california new three dimensional 3 d v p and v p v s models are determined for southern california using p and s p travel times from local earthquakes and controlled sources these models confirm existing tectonic interpretations and provide new insights into the configuration of geological structures at the pacific north america plate boundary the models extend from the u s mexico border in the south to the southernmost coast ranges and sierra nevada in the north and have a 15 km horizontal grid spacing and an average vertical grid spacing of 4 km down to 22 km depth the heterogeneity of the crustal structure as imaged by v p and v p v s models is larger within the pacific plate than the north american plate similarly the relocated seismicity deepens and shows more complex 3 d distribution in areas of the pacific plate exhibiting compressional tectonics the models reflect mapped changes in the lithology across major geological terranes such as the mojave desert the peninsular ranges and the transverse ranges the interface between the shallow mono of the continental borderland and the deep moho of onshore california forms a broad zone to the north beneath the western transverse ranges ventura basin and the los angeles basin and a narrow zone to the south along the peninsular ranges the near surface increase in velocity from the surface to up to 8 km depth is rapid and has a logarithmic shape for stable blocks and mountain ranges but is slow with a linear shape for sedimentary basins at midcrustal depths a rapid increase in v p is imaged beneath the sediments of the large sedimentary basins while beneath the adjacent mountain ranges the increase is small or absent

crustal and upper mantle velocity structure of the salton trough southeast california this paper presents data and modelling results from a crustal and upper mantle wide angle seismic transect across the salton trough region in southeast california the salton trough is a unique part of the basin and range province where mid ocean ridge transform spreading in the gulf of california has evolved northward into the continent in 1992 the u s geological survey usgs conducted the final leg of the pacific to arizona crustal experiment pace two perpendicular models of the crust and upper mantle were fit to wide angle reflection and refraction travel times seismic amplitudes and bouguer gravity anomalies the first profile crossed the salton trough from the southwest to the northeast and the second was a strike line that paralleled the salton sea along its western edge we found thin crust 21 22 km thick beneath the axis of the salton trough imperial valley and locally thicker crust 27 km beneath the chocolate mountains to the northeast we modelled a slight thinning of the crust further to the northeast beneath the colorado river 24 km and subsequent thickening beneath the metamorphic core complex belt northeast of the colorado river there is a deep apparently young basin 5 6 km unmetamorphosed sediments beneath the imperial valley and a shallower 2 3 km basin beneath the colorado river a regional 6 9 km s layer between 15 km depth and the moho underlies the salton trough as well as the chocolate mountains where it pinches out at the moho this lower crustal layer is spatially associated with a low velocity 7 6 7 7 km s upper mantle we found that our crustal model is locally compatible with the previously suggested notion that the crust of the salton trough has formed almost entirely from magmatism in the lower crust and sedimentation in the upper crust however we observe an apparently magmatically emplaced lower crust to the northeast outside of the salton trough and propose that this layer in part predates salton trough rifting it may also in part result from migration of magmatic spreading centers associated with the southern san andreas fault system these spreading centers may have existed east of their current loca tions in the past and may have influenced the lower crust and upper mantle to the east of the current salton trough

subsurface geometry of the san andreas fault in southern california results from the salton seismic imaging project ssip and strong ground motion expectations the san andreas fault saf is one of the most studied strike slip faults in the world yet its subsurface geometry is still uncertain in most locations the salton seismic imaging project ssip was undertaken to image the structure surrounding the saf and also its subsurface geometry we present ssip studies at two locations in the coachella valley of the northern salton trough on our line 4 a fault crossing profile just north of the salton sea sedimentary basin depth reaches 4 km southwest of the saf on our line 6 a fault crossing profile at the north end of the coachella valley sedimentary basin depth is 2 3 km and centered on the central most active trace of the saf subsurface geometry of the saf and nearby faults along these two lines is determined using a new method of seismic reflection imaging combined with potential field studies and earthquakes below a 6 9 km depth range the saf dips 50 60 ne and above this depth range it dips more steeply nearby faults are also imaged in the upper 10 km many of which dip steeply and project to mapped surface fault traces these secondary faults may join the saf at depths below about 10 km to form a flower like structure in appendix d we show that rupture on a northeast dipping saf using a single plane that approximates the two dips seen in our study produces shaking that differs from shaking calculated for the great california shakeout for which the southern saf was modeled as vertical in most places shorter period t 1 s shaking is increased locally by up to a factor of 2 on the hanging wall and is decreased locally by up to a factor of 2 on the footwall compared to shaking calculated for a vertical fault

evidence for a minimum 52 1 km of total offset along the northern branch of the north anatolian fault in northwest turkey abstract the north anatolian fault naf splits into two major branches in northwestern turkey with most of the present strain accumulation and holocene displacement being along the northern branch naf n estimates of total offset along the naf n range between 4 km and 70 km in the marmara sea region these different estimates lead to different interpretations on the formation of marmara sea basins in this study we use cretaceous faults sub perpendicular to the naf n as precise offset markers based on these faults as well as the offset of the middle eocene volcanic belt we report a minimum 52 1 km cumulative dextral displacement along the naf n east of marmara sea near 31 e longitude the displacement of the middle eocene volcanic belt shows that the offset is post middle eocene if we assume an additional 15 km dextral displacement on the second strand of the naf n duzce fault the total offset along the naf n can be estimated as 67 km in the eastern marmara region adding the published offsets that range from 16 to 26 km on the southern branch of the naf give a total offset estimate of whole naf zone as 88 5 km in the eastern marmara region the gps velocity estimate indicates 23 mm yr 1 of total plate motion across and near eastern marmara sea that would take 3 9 million years to accumulate 88 km of displacement on the naf additionally the anatolian plate would not have instantaneously accelerated to its modern rate of motion thus initiation of transform displacement must somewhat pre date 3 9 ma

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 seismic refraction survey of the imperial valley region california the u s geological survey conducted an extensive seismic refraction survey in the imperial valley region of california in 1979 the imperial valley is located in the salton trough an active rift between the pacific and north american plates forty shots fired at seven shot points were recorded by 100 portable seismic instruments at typical spacing of 0 5 1 km more than 1300 recording locations were occupied and more than 3000 usable seismograms were obtained we analyzed five profiles using a standard ray tracing program constructed a contour map of reduced travel times from our most widely recorded shot point and modeled an existing gravity profile across the salton trough results are itemized 1 all models have in common a sedimentary layer vp 1 8 5 0 km s a transition zone vp 5 0 5 65 km s a basement vp 5 65 km s in the imperial valley 5 9 km s on the bordering mesas and subbasement vp 7 2 km s 2 the sedimentary layer ranges in thickness along the axis of the salton trough from 3 7 km salton sea to 4 8 km u s mexican border on the bordering mesas it is quite variable in thickness 3 the transition zone is about 1 km thick in most places in the imperial valley there are no marked velocity discontinuities in this zone between the sedimentary layer and basement on the bordering mesas however there is a discontinuity at the top of this zone 4 there are apparently two types of basement on the bordering mesas basement is crystalline igneous and metamorphic rocks in the imperial valley basement is mostly lower greenshist facies sedimentary rocks based primarily on the smooth transition in character from sediment to basement arrivals the low value of basement velocity and the fact that deep 4 km wells in the valley penetrate only the upper part of the known cenozoic stratigraphic column for the salton trough 5 the subbasement or intermediate crustal layer ranges in depth along the axis of the salton trough from 16 km salton sea to 10 km u s mexican border gravity modeling requires that this layer deepen and or pinch out beneath the bordering mesas and mountain ranges based on its high velocity and the presence of intrusive basaltic rocks in the sedimentary section in the imperial valley the subbasement is thought to be a mafic intrusive complex similar to oceanic middle crust 6 several structures are seen that affect basement transition zone and deeper parts of the sedimentary layer they include a scarp along the imperial fault as much as 1 km down to the northeast and a scarp passing roughly along the topographic boundary between the imperial valley and the bordering mesa to the west as much as 3 km down to the east we interpret the latter scarp to be the suture or rift boundary between the older crystalline basement on the mesa and the younger metasedimentary basement in the imperial valley 7 on a contour map of reduced travel time from our most widely recorded shot point subtle patches of early arrivals among otherwise late arrivals in the central imperial valley correlate well with known geothermal resource areas having reservoir temperatures of more than 150 c apparently the salton trough is a location where new crust is being generated as the rift opens mafic intrusive rocks fill it from below as sedimentary rocks fill it from above rifting and intrusion produce high heat flow that metamorphoses the sedimentary rocks to shallow depth metasedimentary basement in the imperial valley and thus consolidates the new crust

insights into the kinematic cenozoic evolution of the basin and range colorado plateau transition from coincident seismic refraction and reflection data estimates of surface extension in the southern basin and range province and transition into the colorado plateau range from a few percent to several hundred percent locally yet the crustal thickness varies perhaps only 10 15 km across these provinces within the southern basin and range and the metamorphic core complex belt extremely extended crust is directly juxtaposed against equally thick or thinner crust that underwent far milder extension unless preextension crustal thickness varied dramatically over a short distance the crust must have maintained its thickness during extension through mechanisms that involve crustal flow and magmatism we employ a 300 km long profile of seismic refraction and coincident vertical incidence reflection data to investigate the geophysical signature of these processes from the extended southern basin and range province to the unextended colorado plateau by integrating the seismic velocity with the pattern of reflectivity along the profile we estimate the amounts of tertiary magmatism and flow that have occurred we estimate an upper bound of 8 km of mafic material intruded beneath the metamorphic core complex belt and 4 and 5 km of intruded material beneath the transition zone and southern basin and range province respectively we emphasize that this 8 km estimate is strictly an upper bound and that the actual amount of magmatism was probably less 3 to 4 km we further speculate that several kilo meters of silicic rock was added to the metamorphic core complex belt via ductile flow as suggested by numerous numerical models of crustal extension we conclude that a mobile felsic midcrustal layer accommodated most of this crustal flow this ductile midcrustal layer appears to be thickest beneath the most extended terranes and thinnest beneath the less extended transition zone and colorado plateau in contrast the lowermost crust appears to have thinned passively in an amount that corresponds more directly to the regional surface extension

applicability of the site fundamental frequency as a vs30 proxy for central and eastern north america we introduce a new proxy measure for v s 30 time averaged shear wave velocity in the upper 30 m for central and eastern north america cena the new proxy is the site fundamental frequency f peak measured from the horizontal to vertical h v spectral ratios of recorded ground motion or ambient noise in this study h v spectral ratios are obtained from 5 damped pseudospectral acceleration psa from seismograph stations in cena using the next generation attenuation east nga east database we correlate the measured v s 30 values at recording stations with the corresponding f peak values to obtain a predictive relationship for v s 30 the uncertainty of the v s 30 estimate using the f peak based model is small 0 14log10 units in comparison to that for the proxy based methods e g topographic slope and surface geology proxies used in the nga east database 0 25log10 units however values of f peak can be obtained for only about 23 of the nga east recording stations due to data and site type limitations reducing the error in v s 30 estimates can potentially reduce the variability of ground motion prediction equations gmpes to test this hypothesis we use the f peak based v s 30 estimates and select one of the proponent nga east gmpe models for a selected database we are able to reduce the gmpe variability by 3 on average for psa at 1 10 hz just using the f peak based proxy to estimate v s 30 greater variability reductions could be achieved by replacing v s 30 with improved site characterization parameters including f peak in the gmpes

site effects model for central and eastern north america based on peak frequency abstract we develop a regional site effects model for central and eastern north america based on an analysis of the residuals of observed ground motion parameters relative to two regional ground motion prediction equations one model has a hard rock site class a reference site condition whereas the other is referenced to a b c boundary site condition site classification of national earthquake hazard reduction program in both cases the residuals are well described by a site effects model based on site fundamental frequency f peak in which f peak is as determined from the horizontal to vertical component response spectral ratios accordingly we derive an f peak based site amplification model with respect to b c and hard rock reference site conditions implementing the f peak based model we reduce random variability in amplitudes by 10 on average for a selected database from the next generation attenuation east project relative to the value obtained when using a generic site effects model parameterized by near surface shear wave velocity time averaged shear wave velocity in the upper 30 m v s 30 the reduction in comes from the site to site component of the variability reduced by 20 on average whereas the single station variability is unaffected

crust mantle structure of the central north island new zealand based on seismological observations abstract two geophysical features characterise the anomalous crust mantle structure beneath central north island of new zealand the apparent thinning of the felsic crust by a factor of at least 50 and upper mantle p wave velocities pn phase about 10 less than normal 7 3 7 4 km s pn velocities increase slowly to a maximum of 7 8 km s at a depth of about 80 100 km above a depth of 15 km seismic p wave velocities vp are 6 km s or less representing what is likely to be a mix of the original greywacke crust that has been stretched along with an unknown volume of intruded igneous rocks that are in various stages of cooling defining the moho is problematic in this dynamic region below a depth of 15 km there is a continuous increase in vp such that velocities 7 km s are reached at a depth as shallow as 20 km on a east west line and 7 4 km s on a north south line we interpret these wave speeds as a transition zone into low pn velocities rather than high lower crustal values of vp pn wave speeds as low as 7 4 km s are detected by tomographic methods to depths of at least 65 km and laterally over an area of at least 10 000 km2 a 1 4 partial melt in the mantle wedge beneath the central north island is estimated based on qp seismic anisotropy low values of pn and amplitudes of deep reflections these upper mantle features are more indicative of asthenosphere than lithosphere and coupled with the geological evidence for rapid uplift here at 5 ma are consistent with convective removal of the mantle lithosphere from western and central north island in late miocene times

finite difference calculation of travel times the travel times of the first arriving seismic waves through any velocity structure can be rapidly computed on a two or three dimensional numerical grid by finite difference extrapolation from point to point wavefronts rather than the traditional rays are tracked head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 03 per cent are found between the results of this technique and raytracing for a complex two dimensional model this scheme is useful for the windowing of finite difference calculations to increase computing speed and promises to aid in earthquake location tomographic inversion and kirchhoff migration in structures that have strong lateral velocity gradients

crustal thickness variations beneath the peninsular ranges southern california we investigate the crustal thickness under the peninsular ranges using p to s converted phases of teleseismic body waves recorded on a temporary broadband seismometer array and isolated by the receiver function method ps minus p times at sites west of a compositional boundary that separates the peninsular ranges batholith into east and west zones indicate a relatively flat deep moho ps minus p times at sites east of the compositional boundary decrease eastward moho depth estimates made from the ps delays and crustal velocities from seismic tomography indicate a relatively constant 36 to 41 km thick crust in the western zone in the eastern zone the crust thins rapidly from 35 km thick at the compositional boundary to 25 km at the edge of the salton trough a lateral distance of 30 km the lack of correlation between topography and moho depths suggests compensation via lateral density variations in the lower crust or upper mantle we propose that the compositional boundary decouples the eastern and western portions of the batholith and that the eastern portion has thinned in response to regional miocene extension or salton trough rifting or both

development of a vs30 nehrp map for the city of ottawa ontario canada four different seismic methods were used extensively to evaluate the shear wave velocity of soils and rock in the city of ottawa canada from which the travel time weighted average shear wave velocity vs from surface to 30 m in depth vs30 and the fundamental frequency f0 were computed three main geological or geotechnical units were identified with distinct shear wave velocities these consist of very loose thick post glacial fine grained sands silts and clays vs 150 m s thickness up to 110 m firm glacial sediments vs 580 m s thickness 3 m and very firm bedrock vs 1750 3550 m s the seismic methods applied were downhole interval vs measurements at 15 borehole sites seismic refraction reflection profile measurements for 686 sites high resolution shear wave reflection landstreamer profiling for 25 km in total and horizontal to vertical spectral ratio hvsr of ambient seismic noise to evaluate the fundamental frequency for 400 sites most of these methods are able to distinguis

tomographic image of the southern california mantle p wave teleseismic travel time delays recorded by the southern california array are inverted by the method of backprojection tomography to obtain images of variations in the p wave velocity structure to a depth of 750 km two major upper mantle features arc resolved one beneath the transverse ranges region and another beneath the salton trough region the transverse ranges feature appears as a curtainlike east trending high velocity anomaly this feature is 60 km thick extends most deeply on its eastern end to 250 km and attains a maximum velocity 3 greater than average southern california mantle of the same depth the salton trough feature which is not as well resolved as the transverse ranges feature is composed of low velocities in the upper 70 100 km these p wave velocities arc depressed 3 4 compared to average southern california mantle tests of the inversion indicate the major aspects of the imaged structure are authentic

algal constraints on the cenozoic history of atmospheric co2 future acidification of the oceans due to raised carbon dioxide levels will cause a drastic change in ocean chemistry that has not been experienced for at least the last 650 000 years and will likely reduce marine calcification coccolithophores calcareous haptophyte algae constitute a key biological group subjected to this global process the rate at which the natural populations can acclimatise or adapt to changes in ocean chemistry is an essential factor in how their natural feedback mechanisms will operate in future novel experiments testing the environmental tolerance of different extant coccolithophore species to various conditions of seawater carbonate chemistry reveal the need to consider species specific effects when evaluating whole ecosystem responses to elevated pco2 langer 2006 specifically pic poc ratios in coccolithus pelagicus appeared unaffected by the range in co2 tested langer 2006 which to date remains unexplained we argue that the evolutionary history of the coccolithus genus which originated in the early paleocene holds not only invaluable information on how species evolve within planktic super species de vargas 2004 whilst keeping rather conservative coccolith morphologies as will be demonstrated it potentially is also a crucial factor in constraining maximum levels of atmospheric co2 experienced in the geological past

continental rupture and the creation of new crust in the salton trough rift southern california and northern mexico results from the salton seismic imaging project a refraction and wide angle reflection seismic profile along the axis of the salton trough california and mexico was analyzed to constrain crustal and upper mantle seismic velocity structure during active continental rifting from the northern salton sea to the southern imperial valley the crust is 17 18 km thick and approximately one dimensional the transition at depth from colorado river sediment to underlying crystalline rock is gradual and is not a depositional surface the crystalline rock from 3 to 8 km depth is interpreted as sediment metamorphosed by high heat flow deeper felsic crystalline rock could be stretched preexisting crust or higher grade metamorphosed sediment the lower crust below 12 km depth is interpreted to be gabbro emplaced by rift related magmatic intrusion by underplating low upper mantle velocity indicates high temperature and partial melting under the coachella valley sediment thins to the north and the underlying crystalline rock is interpreted as granitic basement mafic rock does not exist at 12 18 km depth as it does to the south and a weak reflection suggests moho at 28 km depth structure in adjacent mexico has slower midcrustal velocity and rocks with mantle velocity must be much deeper than in the imperial valley slower velocity and thicker crust in the coachella and mexicali valleys define the rift zone between them to be 100 km wide in the direction of plate motion north american lithosphere in the central salton trough has been rifted apart and is being replaced by new crust created by magmatism sedimentation and metamorphism

age constraints for the present fault configuration in the imperial valley california evidence for northwestward propagation of the gulf of california rift system releveling and other geophysical data for the imperial valley of southern california suggest the northern section of the imperial brawley fault system which includes the mesquite basin and brawley seismic zone is much younger than the age of the valley itself a minimum age of 3000 years is calculated for the northern segment of the imperial fault from correlations between surface topography and geodetically observed seismic interseismic vertical movements calculation of a maximum age of 100 000 years is based upon displacements in the crystalline basement along the imperial fault inferred from seismic refraction surveys this young age supports recent interpretations of heat flow measurements and the evolution of geothermal systems which also suggest that the current patterns of seismicity and faulting in the imperial valley are not long lived the current fault geometry and basement morphology suggest a northwestward growth of the imperial fault and a northwestward migration of the brawley seismic zone if this localized process is representative of more regional tectonic processes along the extent of the salton trough we suggest that this migration is a manifestation of the propagation of the gulf of california rift system into the north american continent

alteration and remelting of nascent oceanic crust during continental rupture evidence from zircon geochemistry of rhyolites and xenoliths from the salton trough california abstract rhyolite lavas and xenoliths from the salton sea geothermal field southern california provide insights into crustal compositions and processes during continental rupture and incipient formation of oceanic crust salton buttes rhyolite lavas contain xenoliths that include granophyres fine grained altered rhyolites felsite and amphibole bearing basalts zircon is present in lavas and xenoliths surprisingly even in the basaltic xenoliths where it occurs in plagioclase rich regions interpreted as pockets of crystallized partial melt zircons in the xenoliths are exclusively late pleistocene holocene in age and lack evidence for inheritance u th isochron ages are 20 5 1 2 1 2 ka granophyres 18 3 3 5 3 6 ka felsite 30 1 12 4 14 1 ka and 9 2 6 6 7 0 ka basalts all errors 1 the dominant zircon population in the rhyolite lavas yielded u th ages between 18 and 10 ka with few pre quaternary xenocrysts present 18ozircon values are lower than typical crustal basement values thus ruling out rhyolite genesis by melting of continental crust moreover 18ozircon values are 0 5 1 0 lower than compositions achievable by zircon crystallization from residual melt in equilibrium with unaltered mid ocean ridge basalt suggesting that basaltic crust and silicic plutons in the subsurface of the salton sea geothermal field isotopically exchanged with meteoric waters this is evidence for deep reaching hydrothermal circulation and indicates rhyolite genesis by episodic remelting of altered basalts instead of fractional crystallization of unaltered basaltic magma

comparison between crustal density and velocity variations in southern california we predict gravity from a three dimensional v p model of the upper crust and compare it to the observed isostatic residual gravity field in general this comparison shows that the isostatic residual gravity field reflects the density variations in the upper to middle crust both data sets show similar density variations for the upper crust in areas such as the peninsular ranges and the los angeles basin both show similar variations across major faults such as the san andreas and garlock faults in the mojave desert the difference between the two data sets in regions such as the salton trough the eastern california shear zone and the eastern ventura basin where depth to moho is 30 km however suggests high density middle to lower crust beneath these regions hence the joint interpretation of these data sets improves the depth constraints of crustal density variations

dating of pliocene colorado river sediments implications for cosmogenic burial dating and the evolution of the lower colorado river we applied cosmogenic 26 al 10 be burial dating to sedimentary deposits of the ancestral colorado river we compared cosmogenic burial ages of sediments to the age of an independently well dated overlying basalt flow at one site and also applied cosmogenic burial dating to sediments with less precise independent age constraints all dated gravels yielded old ages that suggest several episodes of sediment burial over the past 5 3 m y comparison of burial ages to the overlying 4 4 ma basalt yielded good agreement and suggests that under the most favorable conditions cosmogenic burial dating can extend back 4 5 m y in contrast results from other sites with more broadly independent age constraints highlight the complexities inherent in burial dating these complexities arise from unknown and complicated burial histories insufficient shielding postburial production of cosmogenic isotopes by muons and unknown initial 26 al 10 be ratios nevertheless and in spite of the large range of burial ages and large uncertainties we identify samples that provide reasonable burial age constraints on the depositional history of sediment along the lower ancestral colorado river these samples suggest possible sediment deposition and burial at ca 5 3 4 7 and 3 6 ma our calculated basinwide erosion rate for sediment transported by the modern colorado river 187 mm k y 1 is higher than the modern erosion rates inferred from the historic sediment load 80 100 mm k y 1 in contrast basinwide paleo erosion rates calculated from pliocene sediments are all under 40 mm k y 1 the comparatively lower denudation rates calculated for the pliocene sediment samples are surprising given that the sampled time intervals include significant pliocene aggradation and may include much incision of the grand canyon and its tributaries this conflict may arise from extensive storage of sediment along the route of the colorado river slower paleobedrock erosion or the inclusion of sediments that were derived preferentially from higher elevations in the watershed

applicability of the nga west2 site effects model for central and eastern north america abstract we examine the applicability of the next generation attenuation west2 nga west2 site effects model seyhan and stewart 2014 which is a v s 30 based time averaged shear wave velocity in the upper 30 m model to sites in central and eastern north america cena using the nga east ground motion database we determine residual site terms by comparing the observed cena ground motion amplitudes adjusted to b c site condition v s 30 760 m s using the western site effects model to the corresponding predicted amplitudes of a cena ground motion prediction equation for b c site condition yenier and atkinson 2015 the cena prediction model used the same western site model to level the database before model development thus residual trends reveal inadequacies of the western site effects model when applied to cena ground motions plotting the residual site terms versus their corresponding site fundamental frequencies f peak reveals significant f peak dependent trends at all frequencies average residual site terms for cena sites after the western site amplifications have been removed can be as large as 0 45 in log10 units 2 8 in nonlog units around f f peak correlating the site terms with western site effects predictions reveals that at f v s 30 in a manner that is similar to the way they behave in the nga west2 database however at higher frequencies the correlation of site amplification with v s 30 decreases markedly in cena in contrast the horizontal to vertical h v component spectral ratio as obtained from the nga east ground motion data is a good predictor of the observed f peak dependent site terms suggesting that h v spectral ratio is a more reliable predictor of site amplification for the recording stations of the nga east database the results of this study demonstrate the importance of a well calibrated regional site effects model for cena and the importance of f peak as a site indicator

a deterministic and stochastic velocity model for the salton trough basin and range transition zone and constraints on magmatism during rifting as a high resolution addition to the 1992 pacific to arizona crustal experiment pace a 45 km long deep crustal seismic reflection profile was acquired across the chocolate mountains in southeastern california to illuminate crustal structure in the transition between the salton trough and the basin and range province the complex seismic data are analyzed for both large scale deterministic and fine scale stochastic crustal features a low fold near offset common midpoint cmp stacked section shows the northeastward lateral extent of a high velocity lower crustal body which is centered beneath the salton trough off end shots record a high amplitude diffraction from the point where the high velocity lower crust pinches out at the moho above the high velocity lower crust moderate amplitude reflections occur at midcrustal levels these reflections display the coherency and frequency characteristics of reflections backscattered from a heterogeneous velocity field which we model as horizontal intrusions with a von karman fractal distribution the effects of upper crustal scattering are included by combining the mapped surface geology and laboratory measurements of exposed rocks within the chocolate mountains to reproduce the upper crustal velocity heterogeneity in our crustal velocity model viscoelastic finite difference simulations indicate that the volume of mafic material within the reflective zone necessary to produce the observed backscatter is about 5 the presence of wavelength scale heterogeneity within the near surface upper and middle crust also produces a 0 5 s thick zone of discontinuous reflections from a crust mantle interface which is actually a first order discontinuity

an axial view of a metamorphic core complex crustal structure of the whipple and chemehuevi mountains southeastern california a 135 km long nw se trending seismic refraction wide angle reflection profile provides a unique along strike view of the crustal structure of a belt of metamorphic core complexes in southeastern california the whipple chemehuevi and sacramento mountains metamorphic core complexes interpretation of the seismic data was done by two dimensional forward modeling of travel times and amplitudes the final model consists of 1 a thin 1 5 km veneer of upper plate and fractured lower plate rocks velocities of 1 5 5 3 km s 1 overlying a fairly homogeneous basement with velocities of 6 0 km s 1 2 a localized high velocity 6 4 km s 1 body situated directly beneath the whipple mountains 3 a 6 3 6 4 km s 1 middle crust that is thickest beneath the core complexes 4 a 6 65 0 15 km s 1 lower crust 5 crustal thickness of 27 km with a deeper crustal root 3 km beneath the whipple mountains metamorphic core complex and 6 a pn velocity of 8 0 0 10 km s 1 the crustal structure that underlies the belt of metamorphic core complexes provides new insights into the processes that control extension in the deep crust upper crustal velocities are higher beneath the whipple mountains where velocities increase to 6 4 km s 1 at 5 km depth than beneath the chemehuevi and sacramento mountains in addition midcrustal discontinuities rise 2 5 km beneath the whipple complex compared to the other complexes these observations support greater uplift and a slightly deeper midcrustal origin for the rocks now exposed in the core of the whipple mountains compared to rocks in the chemehuevi and sacramento mountains despite the enhanced uplift and extension in the whipple mountains the crust is thicker here 30 km than anywhere else along the colorado river extensional corridor this may be in part a relic of compressional and magmatic thickening during the mesozoic however we suggest that inflation of the crust during tertiary extension was the dominant mechanism both mantle derived magmatism and lateral ductile inflow in the crust are proposed

through thick and thin a new model for the colorado plateau from seismic refraction data from pacific to arizona crustal experiment we have constructed a model for the crust and uppermost mantle beneath the colorado plateau by combining newly acquired seismic refraction wide angle reflection data from the 1989 pacific to arizona crustal experiment pace cross profile with data collected by roller 1965 a combination of forward modeling methods was used to model travel times and relative amplitudes of crustal and upper mantle phases the salient features of the model for the colorado plateau hereafter referred to as the plateau are 1 an upper crust having an average velocity of 6 1 km s with a low vertical velocity gradient 2 midcrustal discontinuities at depths of 30 to 38 km beneath the plateau 3 a lower crust that varies in thickness 5 to 16 km and has an average velocity of 6 8 km s 4 a transitional lower crust upper mantle boundary and 5 a crustal thickness of 45 3 km a comparison of our new model for the colorado plateau with recent models for the basin and range shows that the upper crusts of the two provinces are similar but that the lower crust of the plateau is thicker particularly in the plateau s central region these observations are consistent with the idea that the basin and range could have formed from crust like that of the colorado plateau

attenuation relations of strong ground motion in japan using site classification based on predominant period a spectral acceleration attenuation model for japan is presented in the present study the data set includes a very large number of strong ground motion records up to the 2003 off tokach main and aftershocks site class terms instead of individual site correction terms are used the site classes of recording stations are from a recent study on site classification for strong motion recording stations in japan according to a classification scheme that has been used in japanese engineering design the use of site class terms enables tectonic source type effects to be identified and accounted for in the present model the effects of a faulting mechanism for crustal earthquakes also are accounted for for crustal and interface earthquakes a simple form of an attenuation model with respect to distance is able to capture the main strong motion characteristics and achieves unbiased estimates for subduction slab events a simple distance modification factor is employed to achieve plausible and unbiased predictions the effects of source depth tectonic source type and faulting mechanism of crustal earthquakes are significant the need for magnitude squared terms is evaluated and the use of magnitude squared terms reduces the interevent error further

site effect evaluation using spectral ratios with only one station the spectral ratio technique is a common useful way to estimate empirical transfer function to evaluate site effects in regions of moderate to high seismicity the purpose of this paper is to show that it is possible to estimate empirical transfer function using spectral ratios between horizontal and vertical components of motion without a reference station the technique originally proposed by nakamura to analyze rayleigh waves in the microtremor records is presented briefly and it is discussed why it may be applicable to study the intense s wave part in earthquake records results are presented for three different cities in mexico oaxaca oax acapulco gro and mexico city these cities are very different by their geological and tectonic contexts and also by the very different epicentral distances to the main seismogenic zones affecting each city each time we compare the results of nakamura s technique with standard spectral ratios in all three cases the results are very encouraging we conclude that if site effects are caused by simple geology a first estimate of dominant period and local amplification level can be obtained using records of only one station

velocity structure of the upper crust under the san francisco volcanic field arizona abstract in addition to traditional in line wide angle seismic profiles the 1989 pacific to arizona crustal experiment pace 1989 included some off line shots and seismic recorders which along with the data from the main profiles provide tomographic coverage of the late cenozoic san francisco volcanic field this major volcanic center is interesting because of its location on the margin of the colorado plateau and because of the diversity of the composition of the volcanics we employed arrival times for the p g phase to undertake a tomographic analysis of the structure of the upper crust the tomographic image indicates high velocity zones at upper crustal depths under the san francisco mountain region the p wave velocities within these zones are 6 higher than the surrounding basement rocks although other interpretations are possible we believe that these high velocity regions most likely represent igneous intrusions associated with the volcanic centers that formed the san francisco volcanic field geologic studies indicate that these intrusions would be of intermediate to mafic composition gravity data in the san francisco volcanic field area also indicate that these high velocity regions are coincident with high density bodies at upper crustal depths

anatomy of a metamorphic core complex seismic refraction wide angle reflection profiling in southeastern california and western arizona the metamorphic core complex belt in southeastern california and western arizona is a nw se trending zone of unusually large tertiary extension and uplift midcrustal rocks exposed in this belt raise questions about the crustal thickness crustal structure and the tectonic evolution of the region three seismic refraction wide angle reflection profiles acquired and analyzed as a part of the u s geological survey s pacific to arizona crustal experiment were collected to address these issues the results presented here which focus on the whipple and buckskin rawhide mountains yield a consistent three dimensional image of this part of the metamorphic core complex belt the seismic refraction wide angle reflection data are of excellent quality and are characterized by six principal phases that can be observed on all three profiles these phases include refractions from the near surface and crystalline basement reflections from boundaries in the middle and lower crust and reflections and refractions from the upper mantle the final model consists of a thin veneer 2 km of upper plate and fractured lower plate rocks 1 5 5 5 kms 1 overlying a fairly homogeneous basement 6 0 km s 1 and a localized high velocity 6 4 km s 1 body situated beneath the western whipple mountains a prominent midcrustal reflection is identified beneath the whipple and buckskin rawhide mountains between 10 and 20 km depth this reflector has an arch like shape and is centered beneath or just west of the metamorphic core complex belt this event is underlain by a weaker approximately subhorizontal reflection at 24 km depth together these two discontinuities define a lens shaped midcrustal layer with a velocity of 6 35 6 5 km s 1 the apex of this midcrustal layer corresponds roughly to a region of major tectonic denudation and uplift 10 km defined by surface geologic mapping and petrologic barometry studies the layer thins to the northeast and is absent in the transition zone the 6 35 6 5 km s 1 velocities are compatible with a diorite composition or a mixture of mafic and silicic rocks this midcrustal layer is underlain by a higher velocity lower crustal layer that is modeled as only 3 6 km thick beneath the metamorphic core complex belt and regions to the southwest to the northeast however this layer thickens to 8 10 km as the midcrustal layer pinches out above it the velocity of the lower crust is constrained by traveltime modeling and is 6 6 0 15 kms 1 beneath the western transition zone and the metamorphic core complex belt higher velocities may be present farther to the southwest where the layer is thin the velocity of the lower crust is too low to accommodate significant amount of mafic underplating at the base of the crust instead we interpret the velocities to indicate that the lower crust is passively thinned beneath these regions without significant addition of mafic mantle derived intrusions the crust mantle boundary does not dome up beneath the core complexes but remains approximately subhorizontal at a depth of 26 28 km or in the case of the whipple mountains actually deepens a 3 km crustal root is modeled this lack of upward doming of the moho together with the vertical alignment of the metamorphic core complex belt over what are believed to be extension related structures in the middle and lower crust suggest that there is no lateral offset of upper crustal deformation from deeper zones of extension as one would expect if extension occurred along crust penetrating shear zones wernicke 1981 wernicke et al 1985 instead domed and inflated middle crust and thinned lower crust directly underlie the region of greatest thinning of the upper crust

san andreas fault dip peninsular ranges mafic lower crust and partial melt in the salton trough southern california from ambient noise tomography we use ambient noise tomography to improve cvm h11 9 a community velocity model of southern california our new 3d shear velocity model with 0 05 x0 05 lateral and 1 km vertical blocks reveals new structure beneath the san andreas fault saf peninsular ranges batholith prb southern sierra nevada batholith snb and the salton trough st we use four years of data recorded on 849 broadband stations vastly more than previous studies and including our own broadband salton seismic imaging project a 40 station transect across the st as well as other campaign stations in both mexico and the u s a mean lower crust and upper mantle wavespeeds 3 6 km s 20km 4 2 km s 40km are low by global standards across the saf southeast of san gorgonio pass we observe vertical to steeply dipping lateral velocity contrasts that extend beneath the moho beneath the western prb and westernmost southern snb we observe relatively high shear velocities 3 8 km s in the lower crust that we interpret as the mafic roots of the overlying arc relatively high velocity upper mantle up to 4 5 km s may be part of the intact arc or possibly a remnant of the farallon plate beneath the st we observe zones of low shear velocity in the lower crust and upper mantle which permit up to 4 5 melt in the lower crust and up to 6 melt in the upper mantle depending on the assumed composition and pore geometry our results preclude the existence of older continental crust beneath the st and support the creation of new crust beneath the st this article is protected by copyright all rights reserved

seismic constraints on the nature of lower crustal reflectors beneath the extending southern transition zone of the colorado plateau arizona events maintain a constant amplitude with offset which is most consistent with a mafic layering model one exception is a high amplitude 10 db above background event near the base of lower crustal reflectivity which abruptly decreases in amplitude at increasing offsets the event has a pronounced s wave response which along with its negative avo trend is a possible indication of the presence of fluids in the lower crust the arizona transition zone is an active but weakly extended province which causes us to discard models of lower crustal layering resulting from shearing because of the high degree of strain required to create such layers instead we favor horizontal basaltic intrusions as the primary origin of high impedance reflectors based on 1 the fact that most xenoliths in eruptive basalts of the transition zone are of mafic igneous composition 2 indications that a pulse of magmatic activity crossed the transition zone in the late tertiary period and 3 the high regional heat flow observed in the transition zone the apparent presence of fluids near the base of the reflective zone may indicate a partially molten intrusion we present a mechanism by which magma can be trapped and be induced to intrude horizontally at rheologic contrasts in extending crust

buried rhyolites within the active high temperature salton sea geothermal system abstract previously unrecognized pulses of rhyolite volcanism occurred in the salton trough between 420 8 ka and 479 38 ka 2 based on high spatial resolution u pb zircon geochronology presently these rhyolite lavas tuffs and shallow subvolcanic sills are buried to depths between 1 6 and 2 7 km at ambient temperatures between 200 and 300 c and are overprinted by propylitic to potassic hydrothermal alteration mineral assemblages consisting of finely intergrown quartz k feldspar chlorite epidote and minor pyrite alteration resistant geochemical indicators whole rock nd isotopes zircon oxygen isotopes reveal that these rhyolites are derived from remelting of morb type crust that was chilled and hydrothermally altered by deep circulating hydrothermal waters u pb zircon dating confirms the presence of bishop tuff in well state 2 14 at 1 7 km depth approximately 5 km ne of the geothermal wells that penetrated the buried rhyolites these results indicate accelerated subsidence towards the center of the salton trough increasing from 2 2 mm a to 3 8 mm a based on these results the present day salton sea geothermal field is identified as a focus zone of episodic rhyolitic volcanism intense heat flow and metamorphism that predates present day geothermal activity and holocene volcanism by at least 400 ka

cenozoic volcanism and two stage extension in the salton trough southern california and northern baja california pronounced differences in age and geochemistry between cenozoic volcanic rocks within the salton trough southern california and northern baja california imply their formation within two distinct tectonic settings rather than the previously proposed single phase of continental rifting the emplacement of oligocene miocene alkaline lavas overlapped with crustal extension and basin formation in the region during this time post 4 ma subalkaline magmatism related to the gulf of california east pacific rise divergent plate margin exploited crust already ruptured during miocene extension new rare earth element and sr nd isotopic data and k ar 40ar 39ar and u pb ages for basalt andesite and granophyric granite xenoliths contained within the salton buttes rhyolite domes part of the post 4 ma oceanic spreading regime indicate that they are the components of a differentiated magmatic system these data also refute the presence of crystalline basement beneath this part of the salton trough we suggest that the rupture of crystalline basement beneath the axis of the salton trough was more effective during oligocene miocene time

geothermal resources of rifts a comparison of the rio grande rift and the salton trough the rio grande rift and the salton trough are the best developed rift systems in the united states and both share many features common to rifts in general including geothermal resources these two rifts have different tectonic and magmatic histories however and these differences are reflected in the nature of their geothermal resources the salton trough is a well developed and successful rift it is the landward extension of the gulf of california spreading center which has separated baja california from the remainder of mexico quaternary silicic magmatization has occurred and several of the geothermal resources are associated with recent rhyolitic intrusions such resources tend to be high temperature 200 c greenschist facies metamorphism has been observed in several of the geothermal wells localized upper crustal melting is a distinct possibility and there is increasing speculation that very high temperature 300 c geothermal fluids may underlie a large portion of the central trough at depths in excess of 4 km low temperature geothermal resources associated with shallow hydrothermal convection are less common and tend to be located on the flanks of the trough or in the coachella valley to the north of the zone of active rifting in contrast the rio grande rift is less well developed recent volcanism consists primarily of mantle derived basalts which have not had sufficient residence time within the crust to generate significant crustal melting the geothermal resources within the rio grande rift do not correlate well with these young basalts rather the quantity of geothermal resources are low temperature 100 c and result from forced hydrothermal convection which discharges at constrictions within or at the end of the major sedimentary basins high temperature resources are less common and the only discovered example is the valles caldera of northern new mexico t 250 300 c the deep interiors of the sedimentary basins of the rio grande rift do not appear to be major geothermal exploration targets

fault zone characteristics and basin complexity in the southern salton trough california ongoing oblique slip at the pacific north america plate boundary in the salton trough produced the imperial valley california usa a seismically active area with deformation distributed across a complex network of exposed and buried faults to better understand the shallow crustal structure in this region and the connectivity of faults and seismicity lineaments we used data primarily from the salton seismic imaging project to construct a three dimensional p wave velocity model down to 8 km depth and a velocity profile to 15 km depth both at 1 km grid spacing a vp 5 65 5 85 km s layer of possibly metamorphosed sediments within and crystalline basement outside the valley is locally as thick as 5 km but is thickest and deepest in fault zones and near seismicity lineaments suggesting a causative relationship between the low velocities and faulting both seismicity lineaments and surface faults control the structural architecture of the western part of the larger wedge shaped basin where two deep subbasins are located we estimate basement depths and show that high velocities at shallow depths and possible basement highs characterize the geothermal areas

novel type of crust produced during continental rifting a normal oceanic crust cannot be generated after continental crust splitting of rifts filled by a thick pile of young sediments the magma released by the rising mantle is trapped mainly in these sediments as basalts sills and plugs the sediments are themselves metamorphosed by the heat flux from the underlying mantle a metasedimentary crust is formed in direct contact with an abnormal mantle vp 7 5 km s l produced by partial serpentinization

pre pliocene extension around the gulf of california and the transfer of baja california to the pacific plate late miocene 12 5 ma extension around the edges of the gulf of california has been alternatively attributed to basin and range extension back arc extension or development of the pacific north america plate boundary this extension was ene directed and similar in structural style to extension in the basin and range province timing constraints permit nearly synchronous onset of this deformation in a belt extending sse from northernmost baja california to the mouth of the gulf where this extensional faulting continued through pliocene time to the present synchronous with motion on the modern transform plate boundary in the gulf of california no change in direction of extension can be resolved revised constraints on pacific north america plate motion support the development of this late miocene extension as a component of pacific north america displacement that could not be accommodated by strike slip displacement along the existing plate boundary west of the baja california peninsula this scenario implies that transfer of baja california from the north america plate to the pacific plate was a gradual process beginning about 12 10 ma when motion of the pacific plate relative to north america was partitioned into separate regimes of strike slip and dip slip displacement on opposite sides of baja california