thin crust and active upper mantle beneath the southern sierra nevada in the western united states abstract analysis of seismic refraction wide angle reflection travel time and seismic amplitude data collected during the 1993 southern sierra nevada continental dynamics sscd project resolves a small crustal root 40 42 km thick centered 80 km west of the sierran topographic crest the sscd experiment consisted of a west east profile across the sierra nevada at approximately 36 5 n and a north south profile extending the length of owens valley located eastward of the sierran topographic crest two dimensional finite difference travel time inversion of p g and p n arrivals resolves upper crustal velocities of 6 0 to 6 4 km s 1 within the sierran batholith and basin and range an upper crustal west dipping wedge of higher velocities 6 8 7 2 km s 1 consistent with ophiolitic material underlying the great valley sedimentary sequence and higher velocities in the lower crust beneath the basin and range 6 8 7 0 km s 1 than those beneath the sierran batholith 6 6 km s 1 low average p n velocities 7 6 7 9 km s 1 and a laminated transitional moho imaged beneath the sierran batholith also differ from the higher p n velocities 7 9 8 0 km s 1 and sharp first order moho observed beneath the basin and range the crust decreases in thickness both westward of the root to 28 34 km beneath the great valley and eastward to 35 km beneath the highest sierran topography and decreases further to 27 30 km beneath the basin and range crustal thickness also appears to increase southeast to northwest from 29 30 km beneath the garlock fault in the south to 38 40 km beneath the north end of chalfant valley juxtaposition of the crustal model with previous p wave tomography models of the south ernmost sierra nevada upper mantle reveal that the thickest sierran crust on the west east profile overlies a pronounced upper mantle high velocity anomaly 5 whereas the region of laminated moho overlies a flanking upper mantle low velocity region 3 the upper mantle velocity anomalies relatively low p n and relatively flat moho observed beneath the sierran crest suggest that the recent uplift of the sierra nevada is due to asthenospheric flow and or lithospheric thinning beneath the southeastern sierra nevada and basin and range

teleseismic surface wave tomography in the western u s using the transportable array component of usarray 1 fundamental mode rayleigh waves recorded by the transportable array component of earthscope usarray from january 2006 through april 2007 are used to generate phase velocity maps at periods from 25 to 100 sec across the western u s including washington oregon california nevada and western idaho at short periods 25 33 s low velocity anomalies are observed in western washington western and central oregon northern california the southern sierra nevada and the snake river plain at intermediate and long periods 50 100 s high velocities are seen in the cascades the southern central valley of california california s transverse range and the columbia river flood basalt province the phase velocity maps are consistent with those obtained from ambient noise tomography at comparable periods short period phase velocities from ambient noise tomography and the longer period phase velocities from teleseismic tomography therefore present natural data sets to invert jointly 3 d structure across the western u s

subduction induced strain in the upper mantle east of the mendocino triple junction california we observe splitting of teleseismic shear waves at five stations of the berkeley digital seismic network located east of the mendocino triple junction in northeastern california that is dependent on the arrival direction of the seismic phases the observed variations with back azimuth cannot be explained with laterally varying anisotropy with a horizontal symmetry axis and are attributed to the presence of fabric with an inclined symmetry axis we assume that the anisotropy is caused by the preferred alignment of olivine crystals a grid search over possible orientations of the olivine a axes reveals that south of the mendocino triple junction they dip to the east whereas north of the triple junction they dip to the northeast on the basis of a comparison of the ray paths of our data to the spatial distribution of fast and slow p wave velocity anomalies in the upper mantle we conclude that the anisotropy is located within seismically slow regions and that the directions are controlled by the geometry of a steeply dipping fast p wave velocity anomaly assuming that the fast p velocity anomaly represents subducted slab material we conclude that the fabric beneath the stations north of the triple junction is most likely caused by the differential motion between this rigid strong down going plate and the surrounding mantle south of the triple junction the fabric may have developed while subduction of the farallon plate was still ongoing in this region prior to 6 ma however we prefer to attribute the observations to more recent asthenospheric flow associated with the opening of a slabless window beneath the north american lithosphere the flow is modulated by the presence of rigid lithosphere to the north and east

imaging of ocean waves by synthetic aperture radar a comparison of results from seasat and sir b experiments in the north atlantic new analyses of wave spectra extracted from seasat sar images of the jasin site in the northeast atlantic are presented as tests of wave imaging theory spectra extracted from orbit 757 during a high sea state are dominated by the effects of smearing but the rms image smear is less than half that expected from in situ measurements of surface motions in a low sea state on orbit 547 fair agreement can be achieved between sar and buoy spectra but this requires that the smearing length be greater than was observed on orbit 757 these findings are compared with results obtained from the shuttle imaging radar sir b mission in order to assess the quantitative understanding of wave imaging by sar

depth dependent mantle anisotropy below the san andreas fault system apparent splitting parameters and waveforms we measure apparent teleseismic shear wave splitting parameters at several stations of the berkeley digital seismic network bdsn and three temporary broadband stations of the university of california at santa cruz located near the san andreas fault system in northern and central california previously proposed anisotropic models for the region include a two layered structure and a structure with a constant degree of anisotropy and a horizontal fast axis that gradually rotates as a function of depth we significantly increase the number of observations and confirm the existence of depth dependent anisotropy beneath the san andreas fault system we also investigate the extent to which the enhanced data set can constrain the details of the depth dependence using theoretical expressions we determine a suite of two layer models that fit our observations and result in practically indistinguishable apparent splitting parameters the wide range of acceptable models indicates that apparent splitting parameters alone cannot constrain all four parameters that specify the two anisotropic layers upper and lower fast polarization directions and delay times synthetic seismograms for three very different two layer models chosen from our suite of acceptable models and a model with a gradually rotating fast axis show subtle waveform variations and apparent splitting parameters measured from these records do not overlap hence we conclude that waveforms indeed contain additional information about the anisotropic structure we incorporate waveform data by searching for the four splitting parameters of two layer models that minimize the splitting of several records at each station simultaneously the resulting models largely overlap with the models obtained using the theoretical expressions which indicates that the addition of waveform data is not sufficient to uniquely determine all four parameters of a two layer model constraining the upper fast polarization direction to be parallel to the relative motion vector of the pacific and north american plates n35 w allows the values of the other three model parameters to be uniquely determined the lower layer fast polarization direction beneath all stations is approximately e w and the delay times vary from 0 85 to 1 70 s the delay times in the upper layer are smaller and they vary between 0 50 and 1 25 s shear within the uppermost mantle due to the relative plate motion is a satisfactory explanation for the shallow anisotropic signature the sublithospheric e w fast directions may be caused by small scale convection within a slabless window or by motions of the lithosphere relative to the deeper mantle

mantle structure beneath the western united states and its implications for convection processes 1 we present tomographic images of the mantle structure beneath the western united states our dynamic north america models of p and s velocity structure dna07 p and dna07 s use teleseismic body waves recorded at 600 seismic stations provided by the earthscope transportable array and regional networks dna07 p and s benefit from the unprecedented aperture of the network while maintaining a dense station distribution providing high resolution body wave imaging of features through the transition zone and into the lower mantle the main features imaged include 1 the juan de fuca subduction system that bottoms at 400 km beneath oregon implying interaction with the yellowstone anomaly 2 a low velocity conduit beneath yellowstone national park that bottoms at 500 km and dips toward the northwest 3 shallow low velocity anomalies upper 200 km beneath the eastern snake river plain esrp and the high lava plains and a deep low velocity anomaly 600 km beneath the esrp but not newberry 4 a low velocity slab gap to 400 km depth immediately south of the mendocino triple junction and south of the gorda slab and 5 high velocity drips beneath the transverse ranges the southern central valley sierra nevada and central nevada these observations reveal extremely heterogeneous mantle structure for the western united states and suggest that we are only just beginning to image the complex interactions between geologic objects the transportable array allows for analysis of the relationships between these anomalies in an internally consistent single tomographic model the dna velocity models are available for download and slicing at http dna berkeley edu

moho orientation beneath central california from regional earthquake travel times table al is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 011 2 50 payment must accompany order analysis of relative pn travel times recorded by 238 stations of the u s geological survey central california seismic network calnet from 77 regional earthquakes and explosions shows that arrivals are progressively delayed to the northeast in both the coast ranges and sierra nevada foothills by dividing the calnet into 10 subarrays and assuming laterally uniform velocities for a crust over an upper mantle half space the orientation of a planar moho beneath each subarray is calculated by inversion of relative pn arrival times the results show that the moho dips to the northeast between 3 and 6 beneath the coast ranges and 8 17 beneath the great valley and is horizontal beneath the foothills pn residuals west of the san andreas fault are relatively early by 0 35 s but are interpreted as resulting from a lateral velocity contrast across the fault rather than variations in crustal thickness unexplained pn travel time variance improves by approximately 60 after correction for the travel time due to dip and mean pn residual at each station the pn data corrected for the travel time due to a dipping moho indicate an isotropic upper mantle velocity of 7 97 0 05 km s beneath the coast ranges great valley and foothills evidence of upper mantle velocity anisotropy in the azimuthal variation in pn apparent velocity corrected for dip is only marginally significant at the 5 level however large azimuthal variations in corrected apparent velocity do exist and may be attributed to velocity heterogeneities in the crust unmodeled higher order mantle structure and gradients in upper mantle velocity arrival times of cape mendocino earthquakes recorded in the coast ranges south of san francisco bay indicate lateral variations in upper mantle velocity the 7 4 km s apparent velocity estimate from travel times of the oroville earthquake recorded in the coast ranges out to distances greater than 400 km is anomalous in that pn is normally observed at distances greater than 200 km these arrivals may originate from updip propagation along a 6 9 km s layer in the lower crust of the coast ranges subparallel to the moho

triggered seismicity and deformation between the landers california and little skull mountain nevada earthquakes this article presents evidence for the channeling of strain energy released by the ms 7 4 landers california earthquake within the eastern california shear zone ecsz we document an increase in seismicity levels during the 22 hr period starting with the landers earthquake and culminating 22 hr later with the ms 5 4 little skull mountain lsm nevada earthquake we evaluate the completeness of regional seismicity catalogs during this period and find that the continuity of post landers strain release within the ecsz is even more pronounced than is evident from the catalog data we hypothesize that regional scale connectivity of faults within the ecsz and lsm region is a critical ingredient in the unprecedented scale and distribution of remotely triggered earthquakes and geodetically manifest strain changes that followed the landers earthquake the viability of static strain changes as triggering agents is tested using numerical models modeling results illustrate that regional scale fault connectivity can increase the static strain changes by approximately an order of magnitude at distances of at least 280 km the distance between the landers and lsm epicenters this is possible for models that include both a network of connected faults that slip sympathetically and realistic levels of tectonic prestrain alternatively if dynamic strains are a more significant triggering agent than static strains ecsz structure may still be important in determining the distribution of triggered seismic and aseismic deformation

improvement in the estimation of dominate wavenumber and direction from spaceborne sar image spectra when corrected for ocean surface movement because a synthetic aperture radar sar is a doppler device ocean surface movement can degrade a sar ocean image ocean surface movement causes the redistribution of apparent radar cross section in a sar image to neighboring image elements it is demonstrated in this paper that this aspect of sar imaging can be treated as a simple resolution loss in a linear system such an approach explains the falloff in response at high azimuth wavenumbers experienced in sar image spectra more importantly this approach allows for a straightforward method to alleviate the problem of ocean surface motion correction of sar image spectra using a linear systems approach results in sar image spectra in which the locations in wavenumber space of surface wave spectral peaks are more consistent with the estimated location of storm sources than are the locations of peaks from uncorrected spectra

convective thermal instabilities in the wake of the migrating mendocino triple junction california whether the high seismic velocity anomalies in the upper mantle under california represent cold downwelling mantle flow or fragments of the subducted farallon plate has important implications for mantle dynamics related to the tectonic history of western north america we investigate the former possibility by simulating the time dependent thermal evolution in the asthenosphere beneath california in the wake of the migrating mendocino triple junction mtj using a two dimensional model that represents a transect parallel to the direction of the mtj migration for viscosity within the typical range for the asthenosphere beneath active tectonic regions 1 1018 2 1019 pa s our model predicts vigorous small scale convection characterized by periodic formation of cold downwelling flows near the southern edge of the migrating gorda plate within the relevant time frame the predicted convective thermal structures are comparable to the major mantle features shown by seismic imaging

a comparison of sir b directional ocean wave spectra with aircraft scanning radar spectra directional ocean wave spectra derived from shuttle imaging radar b sir b l band imagery collected off the coast of southern chile on 11 and 12 october 1984 were compared with independent spectral estimates from two airborne scanning radars in sea states with significant wave heights ranging from 3 to 5 meters the sir b derived spectra at 18 and 25 off nadir yielded reasonable estimates of wavelengths directions and spectral shapes for all wave systems encountered including a purely azimuth traveling system a sir b image intensity variance spectrum containing predominantly range traveling waves closely resembles an independent aircraft estimate of the slope variance spectrum the prediction of a u s navy global spectral ocean wave model on 11 october 1984 exhibited no significant bias in dominant wave number but contained a directional bias of about 30 espect to the mean of the aircraft and spacecraft estimates

pn travel times in southern california the pn arrivals recorded from the southern california array have been used to demonstrate regional variations in crustal thickness and pn velocity and to investigate the presence of anisotropy in the mantle over 2800 travel times were used in the analysis the classical time term method was extended to allow the moho velocity to vary in a block type structure the statistical f test shows these effects to be significant crustal delays determined depend on both crustal velocities and moho depth the ventura and los angeles basins have sediment delays which indicate sediment thicknesses near 10 kin the moho depth itself varies by 10 km over southern california with a mean of about 29 kin slightly thicker crust is prevalent in the northwestern corner of the array near the southern coast ranges five kilometers of rapid thinning between the coast and the channel islands is shown by a sharp gradient in the delay times normal crustal thicknesses occur over the rest of the array except for the area east of the salton trough where an anomalously thin crust of 22 km exists this area extends from near the plate boundary into arizona while the peninsular ranges and the san bernadino mountains have small roots the san gabriel mountains have none pn velocities range from 7 6 to 8 2 km s within the array high velocities are found in the mojave and offshore regions low velocities are found in the peninsular ranges and the transverse ranges the analysis supports a mean anisotropy of 0 15 km s with the fast direction at n 75 o w this value represents an average of a parameter that varies considerably in the region

finite frequency traveltime tomography of san francisco bay region crustal velocity structure summary seismic velocity structure of the san francisco bay region crust is derived using measurements of finite frequency traveltimes a total of 57 801 relative traveltimes are measured by cross correlation over the frequency range 0 5 1 5 hz from these are derived 4862 summary traveltimes which are used to derive 3 d p wave velocity structure over a 341 140 km2 area from the surface to 25 km depth the seismic tomography is based on sensitivity kernels calculated on a spherically symmetric reference model robust elements of the derived p wave velocity structure are a pronounced velocity contrast across the san andreas fault in the south bay region west side faster a moderate velocity contrast across the hayward fault west side faster moderately low velocity crust around the quien sabe volcanic field and the sacramento river delta very low velocity crust around lake berryessa these features are generally explicable with surface rock types being extrapolated to depth 10 km in the upper crust generally high mid lower crust velocity and high inferred poisson s ratio suggest a mafic lower crust

intraplate earthquakes and the state of stress in oceanic lithosphere abstract the sources of stress responsible for oceanic intraplate earthquakes and their usefulness for inferring the tectonic stress field are investigated through a synthesis of the source mechanisms of 58 of the largest such events determined by inversion of long period p and sh waveforms further constraints are provided by the global distribution of oceanic intraplate seismicity and a calculation of the seismic moment release per unit volume of seismogenic lithosphere as a function of lithosphere age although frequently associated with relict tectonic features large oceanic intraplate earthquakes are generally reliable indicators of the state of stress in the source region most oceanic intraplate seismicity occurs in young lithosphere

ocean wave imaging by synthetic aperture radar results from the sir b experiment in the n e atlantic waveheight spectra were extracted from synthetic aperture radar sar images close to a number of wave measuring buoys and the results of radar and in situ measurements were compared as a test of wave imaging theory sar and buoy spectra of waves traveling close to the radar range direction were in good quantitative agreement with any discrepancies probably attributable to the processing technique however sar estimates of the amplitude of azimuth traveling waves were very much lower than buoy derived values this cannot be accounted for by appealing to nonlinear imaging by the velocity bunching mechanism or by velocity smearing calculating the scene coherence time from the available buoy data rather the scene coherent time must be reduced by a factor of about two to within the range 0 022 0 027 s this result highlights a lack of knowledge of scatterer motions and lifetimes on the sea surface

spatial variations of ocean wave directional spectra from the seasat synthetic aperture radar seasat synthetic aperture radar ocean wave spectra for a 900 km pass are analyzed and interpreted in the context of both their probable generation sources and their surface current and bathymetric modifiers systematic vector wavenumber variations of several times the standard error of determination 1 5 in magnitude and 0 9 in direction occur along the entire 900 km pass the large scale spatial variation of a 200 m swell system can be accurately accounted for as a result of dispersion from a distant storm the more local variations are qualitatively well correlated in position with known currents and bathymetry but show systematic biases that appear partly due to an environmentally dependent instrument transfer function in the regions of high current and highest sea state there is also substantial evidence that a large angular deviation in the center of the pass is the result of a mesoscale eddy just to the east

early cenozoic crust at the norwegian continental margin and the conjugate jan mayen ridge multichannel seismic profiles at the voring plateau margin off norway and the northern jan mayen ridge have provided a framework for the early tertiary evolutionary history of these areas which were adjacent prior to the opening of the norwegian sea we propose that their paleogene evolution is quite similar and that the two acoustic basement reflectors ee and jo were formed in the early eocene by extrusion of flow basalts from the time of opening to about anomaly 23 time oceanic seafloor was generated by icelandic type spreading during that time the central jan mayen fracture was the main transform the seaward dipping reflector sequences are flow basalts generated by the icelandic spreading the existence of less well developed dipping sequences seaward of the primary ones probably reflects that these regions submerged later because of higher initial elevation during the early eocene subaerial spreading episode which is related to the north atlantic volcanic province basalt flows also covered the adjacent thinned and heavily intruded continental crust it is proposed that the continent ocean boundary is located just landward of anomaly 24b a short distance seaward of the voring plateau escarpment reflector k is only recognized landward of this boundary in a region of transitional crust the escarpment experienced syndepositional faulting and the most elevated parts of the voring marginal high did not subside below shallow water depths before oligocene miocene time we suggest that the voring margin and the jan mayen ridge represent volcanic passive margins the evolution of this margin type may relate to initial uplift due to rifting in previously thinned crust

comment on subduction of aseismic ridges beneath the caribbean plate implications for the tectonics and seismic potential of the northeastern caribbean by w r mccann and l r sykes

lateral variations and azimuthal isotropy of pn velocities beneath basin and range province travel time tomography was used to obtain the lateral variations in pn velocities and pn velocity azimuthal anisotropy beneath the basin and range province we used 1226 pn arrivals from 184 earthquakes recorded by 113 stations the tomographic image shows the lateral variation in pn velocity from 7 9 km s 1 in the northern province to 7 6 km s 1 in the southern province azimuthal variations in pn velocities are less than one percent as determined from the travel time residuals after taking out the contribution of the lateral velocity variations of the pn tomography although the standard errors of the azimuthal variations are 2 there is no reason to believe the existence of the pn azimuthal anisotropy in the uppermost mantle beneath the basin and range province beghoul and barazangi s 1990 suggestion of more than 3 azimuthal anisotropy is the result of the lateral heterogeneity and mantle velocity gradient before tomographic inversion we refined the pn travel times by considering the lateral variations in the crustal thickness and velocities in the region based on the numerical experiment and theoretical development given in this paper these refinements are crucial in obtaining an accurate velocity image the average velocity is 7 72 0 16 km s 1 and an average vertical mantle velocity gradient is 8 0 10 4 s 1 the resolution for pn velocity image is generally within 2 to 5

lithospheric evolution of the pacific north american plate boundary considered in three dimensions abstract tomographic images developed using the combined seismic networks of california and nevada provide a three dimensional view into neogene pacific north american plate boundary evolution images reveal structures similar in size and spatial distribution to the large scale structures observed at the surface a prominent linear anomaly in the mantle is imaged beneath the western foothills to a depth of 70 90 km called the sierran foothill anomaly sfa this 1 2 high velocity structure extends the length of the physiographic sierra nevada mountains much of the velocity increase is thermal in origin with a first episode of cooling during the same late cretaceous to paleogene low angle subduction that affected most of western north america post laramide resumption of subduction continued chilling the western sfa small thrust earthquakes at the contact between the juan de fuca slab and the sfa indicate that the contact is still relatively cold to depths approaching 90 km we propose that this chilled lithospheric section as responsible for the block like integrity of the core of the sierra nevada the peninsular ranges in southern california have also maintained a high degree of integrity at least until recent times because they shared a similar batholithic generation and laramide chilling as the sierra nevada the tomographic image includes other large scale features west of the sfa is a low velocity region interpreted as upwelling mantle filling the window behind the nw translating juan de fuca slab this low velocity feature appears to be offset by the san andreas fault which would indicate transform relations extend into the mantle compression and shortening between the peninsular ranges and the sierra nevada blocks began when the transform boundary shifted east of the peninsular ranges and baja california around 5 5 ma high velocity structures in the upper mantle developed beneath the transverse ranges and the southern great valley sgv to accommodate crustal shortening alternative explanations for the sgv anomaly are explored by comparing the volume of proximal sources to the volume of the sinking sgv anomaly no local source such as convective overturning of sierran block eclogitic roots seems adequate to provide the volume needed to explain the anomaly as a purely local feature surface and subsurface views of the plate margin are complimentary but consistent and each contributes to resolving problems not accessible to the other

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

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

seismicity and seismotectonics of norway and nearby continental shelf areas the earthquake activity of norway and nearby offshore areas is low to intermediate with few events above magnitude 5 recent significant improvements in instrumental coverage in parallel with a better utilization of older including historical data have shown that the seismicity in the south is predominantly confined to the coastal areas and to the viking graben while from the northern north sea to svalbard the earthquakes in a broad sense follow the continental margin fifty one focal mechanism solutions from these areas about half of them new reveal stress directions that clearly indicate a connection to the plate tectonic ridge push force at least for the areas at a minimum distance from the continental margin along the margin stress directions also indicate a possible connection to postglacial uplift as well as to lithospheric loading effects a dominance of normal faulting on the landward side and reverse faulting on the oceanic side agrees with this interpretation on a regional level the seismicity in these areas correlate quite well with geologic features such as grabens fault zones fault complexes fracture zones and the margin itself indicating that these structures act in a general sense as weakness zones in the presence of a regionally more stable stress field in the northern north sea however an area with quite anomalous stress orientations with strike slip faulting is found in a region transitional between normal and reverse faulting most of the earthquake foci are confined to the presumably brittle parts of the crust but many events are also located quite close to and on both sides of the moho discontinuity

sir b observations of ocean waves in the ne atlantic synthetic aperture radar sar imagery from sir b was obtained over a deep water site in the ne atlantic ocean selected images were fourier analyzed in order to compare sar measurements of ocean waves with simultaneous data obtained from wave buoys deployed in the site the wave heights were not large enough to be detected in the first pass over the site but larger wave amplitudes were present in two subsequent passes that were almost orthogonal and separated by 6 h the first of these passes showed a system of waves traveling close to the range direction and secondary wave systems were present in some spectra azimuth traveling waves were seen clearly on only one of the spectra from the second of these passes all the spectra had the appearance expected from a linear imaging theory and no significant discrepancies were found between the sar and buoy measurements of the wavelengths and directions of the dominant wave systems however the difficulty in detecting azimuth traveling waves here may be hard to explain in terms of existing theories but this is not a firm conclusion because large variations factors of about 4 in spectral intensity were observed across distances as small as 20 km this variability means that it is very difficult to determine whether sir b was observing the true sea state procedures to obtain more accurate tests of sea imaging theories are discussed

lithospheric structure of northern california from teleseismic images of the upper mantle teleseismic p wave travel time residuals from 120 earthquakes recorded across the u s geological survey california seismic network were used to determine the lithosphere p wave velocity structure beneath northern california a region characterized by complex interactions between the pacific north american and gorda plates lateral p wave velocity variations beneath the array were determined by inversion of 9383 travel time residuals inversion results for the crust show strong correlations to volcanic features the active volcanic fields shasta medicine lake lassen and clear lake are characterized by crustal low velocity anomalies that average approximately 6 possibly identifying partially molten magma bodies cooled solidified magma bodies beneath the extinct volcanic fields sonoma southern clear lake and sutler buttes are denoted by relative velocity highs averaging 3 the largest upper mantle velocity variations occur in the depth range 30 110 km where velocities vary from 5 5 to 9 5 these velocity variations reflect changes in the thickness and geometry of the pacific north american and gorda plates where they interact at the mendocino triple junction north of the mendocino triple junction the steep 70 east dipping portion of the gorda plate is imaged as a 5 velocity high to depths near 270 km a presumed segment of the gorda plate observed beneath the northern great valley and south of the inferred edge of the plate is characterized by a 9 velocity high in the depth range 30 70 km beneath the northern coast ranges shallow asthenosphere is imaged in the depth range 30 100 km as a pronounced southward tapering 4 low velocity zone which we interpret as the slab window results from this study provide improved constraints on gorda plate subduction evolution of the san andreas fault system and development of the lithosphere beneath western north america

velocity variations in the uppermost mantle beneath the southern sierra nevada and walker lane we model pn waveforms from two earthquakes in the southwestern united states mammoth lakes california and western nevada to determine a velocity model of the crustal and mantle structure beneath the southern sierra nevada and walker lane we derive a one dimensional velocity model that includes a smooth crust mantle transition east of death valley and extending south into the eastern mojave desert west of death valley and toward the sierra nevada a low velocity mantle v p 7 6 km s directly below the crust indicates the lithosphere is absent at the base of this low velocity structure at 75 100 km depth the p wave velocity jumps discontinuously to v p 8 0 km s the area of low velocity is bounded by the garlock fault to the south and the sierra nevada to the west but we cannot resolve its northern extent however on the basis of teleseismic travel times we postulate that the anomaly terminates at about 38 n the presence of a low velocity upper mantle anomaly in this area agrees with geochemical research on xenoliths from the southern sierras and recent studies of receiver functions refraction profiles tomography and gravity however the velocity discontinuity at 75 100 km is a new discovery and may represent the top of the once present now unaccounted for and possibly sunken sierra nevada lithosphere

comparison of simulated and measured synthetic aperture radar image spectra with buoy derived ocean wave spectra during the shuttle imaging radar b mission during the sir b mission over the north sea two successful synthetic aperture radar sar data takes with simultaneous buoy measurements of ocean wave spectra have been obtained on october 6 and 8 1984 on october 6 the sar imaging of ocean waves was predicted as strongly nonlinear and on october 8 as almost linear the sir b experiment confirmed the theoretical predictions by applying the sar imaging model based on velocity bunching theory the sar image spectra are calculated from the measured ocean wave spectra these calculated sar image spectra are compared with the sir b derived sar image spectra and it is shown that both agree quite well this is considered as a further experimental confirmation for the velocity bunching model that has been proposed for describing sar imaging of ocean surface waves

regional velocity structure in northern california from inversion of scattered seismic surface waves seismic surface waves recorded by the berkeley digital seismic network have been analyzed in order to constrain three dimensional lateral heterogeneity of the upper mantle under northern california a total of 2164 seismograms from 173 teleseismic events were windowed for the fundamental mode rayleigh wave followed by estimation of complex amplitude spectra over the period range 16 to 100 s using a multiple taper method since rayleigh waves at shorter periods particularly below 35 s suffer from serious multipathing or non plane wave arrivals these amplitude spectra have been interpreted as the product of wavefront distortion along the teleseismic propagation path and seismic structure beneath the network the amplitude spectra are first modeled in terms of non plane incoming wavefields and structural phase velocity perturbations period by period after corrections for moho and surface topography the phase velocity maps are inverted for three dimensional shear velocity perturbations s down to a depth of 200 km the s maps are in good agreement with the results of body studies over a broad spatial scale the dominant signals are associated with the thermal effects of the active gorda and fossil farallon subducted slab stretching from mount shasta through the western sierran foothills to the southern great valley and asthenospheric upwelling beneath the northern coast ranges the southern sierra nevada range is characterized by fast s down to 50 km and slow velocities between 60 and 120 km depth in agreement with independent inferences of a cold crust and warm upper mantle which may provide the buoyancy forces necessary to support the elevation of the range

the observation of ocean surface phenomena using imagery from the seasat synthetic aperture radar an assessment over the period july 4 to october 10 1978 the seasat synthetic aperture radar sar gathered 23 cm wavelength radar images of some 108 km2 of the earth s surface mainly of ocean areas at 25 40 m resolution our assessment is in terms of oceanographic and ocean monitoring objectives and is directed toward discovering the proper role of sar imagery in these areas of interest in general sar appears to have two major and somewhat overlapping roles first quantitative measurement of ocean phenomena like long gravity waves and wind fields as well as measurement of ships second exploratory observations of large scale ocean phenomena such as the gulf stream and its eddies internal waves and ocean fronts these roles are greatly enhanced by the ability of 23 cm sar to operate day or night and through clouds to begin we review some basics of synthetic aperture radar and its implementation on the seasat spacecraft seasat sar imagery of the ocean is fundamentally a map of the radar scattering characteristics of 30 cm wavelength ocean waves distorted in some cases by ocean surface motion we discuss how wind stress surface currents long gravity waves and surface films modulate the scattering properties of these resonant waves with particular emphasis on the mechanisms that could produce images of long gravity waves doppler effects by ocean motion are also briefly described measurements of long wavelength 100 m gravity waves using seasat sar imagery are compared with surface measurements during several experiments combining these results we find that dominant wavelength and direction are measured by seasat sar within 12 and 15 respectively however we note that ocean waves are not always visible in sar images and discuss detection criteria in terms of wave height length and direction sar estimates of omnidirectional wave height spectra made by assuming that sar image intensity is proportional to surface height fluctuations are more similar to corresponding surface measurements of wave height spectra than to wave slope spectra because seasat sar images show the radar cross section of 30 cm waves neglecting doppler effects and because these waves are raised by wind stress on the ocean surface wind measurements are possible comparison between wind speeds estimated from seasat sar imagery and from the seasat satellite scatterometer sass agreed to within 0 7 m s over a 350 km comparison track and for wind speeds from 2 to 15 m s the great potential of sar wind measurements lies in studying the spatial structure of the wind field over a range of spatial scales of from 1 km to 100 km at present the spatial and temporal structure of ocean wind fields is largely unknown because sar responds to short waves whose energy density is a function of wind stress at the surface rather than wind speed at some distance above the surface variations in image intensity may also reflect changes in air sea temperature difference thus complicating wind measurements by sar because sar images show the effects of surface current shear air sea temperature difference and surface films through their modulation of the 30 cm waves seasat images can be used to locate and study the gulf stream and related warm water rings tidal flows at inlets internal waves and slicks resulting from surface films in many of these applications sar provides a remote sensing capability that is complementary to infrared imagery because the two techniques sense largely different properties namely surface roughness and temperature both stationary ships and moving ships with their attendant wakes are often seen in sar images ship images can be used to estimate ship size heading and speed however ships known to be in areas imaged by sar are not always detectable clearly a variety of factors such as image resolution ship size sea state and winds could affect ship detection overall the role of sar imagery in oceanography is definitely evolving at this time but its ultimate role is unclear we have assessed the ability of seasat sar to measure a variety of ocean phenomena and have commented briefly on applications in the end oceanographers and others will have to judge from these capabilities the proper place for sar in oceanography and remote sensing of the ocean

fossil slabs attached to unsubducted fragments of the farallon plate as the pacific farallon spreading center approached north america the farallon plate fragmented into a number of small plates some of the microplate fragments ceased subducting before the spreading center reached the trench most tectonic models have assumed that the subducting oceanic slab detached from these microplates close to the trench but recent seismic tomography studies have revealed a high velocity anomaly beneath baja california that appears to be a fossil slab still attached to the guadalupe and magdalena microplates here using surface wave tomography we establish the lateral extent of this fossil slab and show that it is correlated with the distribution of high mg andesites thought to derive from partial melting of the subducted oceanic crust we also reinterpret the high seismic velocity anomaly beneath the southern central valley of california as another fossil slab extending to a depth of 200 km or more that is attached to the former monterey microplate the existence of these fossil slabs may force a reexamination of models of the tectonic evolution of western north america over the last 30 my

simultaneous inversion of local and teleseismic data for the crust and mantle structure of southern california we determined a detailed three dimensional seismic velocity structure of the crust and mantle to a depth of 800 km beneath southern california using local and teleseismic data simultaneously we used 131 372 p wave arrival times from 6437 local earthquakes and 12 134 travel time residuals from 193 teleseismic events recorded by 251 seismic stations of the caltech us geological survey southern california seismic network compared with previous local and teleseismic studies the station coverage is considerably improved and the number of data used is greatly increased in the present study this together with the local and teleseismic joint inversion approach results in a unified three dimensional 3 d p wave velocity model of the crust and mantle in southern california with a higher resolution than the previous models the result obtained shows very heterogeneous structures in the crust and upper mantle in southern california shallow crustal structures correlate well with surface geological features sedimentary basins such as the los angeles basin ventura basin and santa maria basin are imaged well as low velocities and batholiths such as the peninsular ranges and san gabriel mountains are imaged as high velocities the velocity in the crust is generally low in the mojave desert the crustal velocities change abruptly across major faults such as the san andreas san jacinto elsinore and garlock faults in the upper mantle three major anomalies are found beneath the transverse ranges salton trough and southern sierra nevada the transverse ranges feature appears as a curtain like east trending and north dipping high velocity anomaly with a thickness of approximately 50 km and extends most deeply at its eastern end to about 250 km the salton trough feature is composed of low velocities which trend southeast and extend to approximately 200 km depth the southern sierra nevada feature is a slab like high velocity anomaly which extends to about 250 km the mantle velocity is generally low in the upper 100 200 km beneath the volcanic areas in southern california the seismic velocity anomalies in the mantle beneath southern california are considered to result from small scale mantle convection

current sierra nevada north america motion from very long baseline interferometry implications for the kinematics of the western united states we use geodetic measurements from very long baseline interferometry to estimate the motion of the sierra nevadan microplate which is composed of the sierra nevada and the great valley the motion of the sierra nevadan microplate relative to the north american plate is described by a right handed rotation of 0 61 m y about lat 32 n long 128 w this euler pole lies only 10 southwest of the sierra nevadan microplate and predicts a significant counterclockwise rotation about a local vertical axis it further predicts a velocity of the eastern edge of the sierra nevada at 38 0 n 119 3 w relative to stable north america of 11 1 mm yr toward n36 3 w quoted uncertainties are plus or minus one standard error which accounts for about one fourth of the velocity between the pacific and north american plates and is 25 clockwise of many prior estimates the velocity nearly parallels the boundary between the sierra nevada and the great basin which implies that current motion within the great basin results in a rotational noncoaxial deformation we use this velocity to estimate how motion is distributed across the broad deforming zone taking up pacific north america plate motion we find that the vector sum of strike slip along the san andreas fault and motion of the sierra nevada relative to stable north america taken up by deformation within the great basin differs little from the pacific north america plate velocity the difference can be described at 36 n along the san andreas fault by a vector of 6 mm yr directed toward n20 w this vector resolves into components of 5 mm yr parallel to the fault and 2 mm yr perpendicular to the fault with 95 confidence intervals of 0 to 10 mm yr and 1 to 5 mm yr respectively the component perpendicular to the fault is several times smaller than found in prior studies and places a small upper bound on fault perpendicular shortening we conclude that motion previously inferred to be taken up by deformation other than strike slip along the san andreas fault or deformation within the great basin is much smaller than previously thought

density structure of the lithosphere in the southwestern united states and its tectonic significance we calculate a density model of the lithosphere of the southwestern united states through an integrated analysis of gravity seismic refraction drill hole and geological data deviations from the average upper mantle density are as much as 3 a comparison with tomographic images of seismic velocities indicates that a substantial part 50 of these density variations is due to changes in composition rather than temperature pronounced mass deficits are found in the upper mantle under the basin and range province and the northern part of the california coast ranges and adjacent ocean the density structure of the northern and central southern sierra nevada is remarkably different the central southern part is anomalous and is characterized by a relatively light crust underlain by a higher density upper mantle that may be associated with a cold stalled subducted plate high densities are also determined within the uppermost mantle beneath the central transverse ranges and adjoining continental slope the average density of the crystalline crust under the great valley and western sierra nevada is estimated to be up to 200 kg m 3 higher than the regional average consistent with tectonic models for the obduction of oceanic crust and uppermost mantle in this region

upper mantle velocity structure of the great basin teleseismic p wave travel times measured in the eastern great basin show 1 sec variations in travel time residuals inversion of these data images an alternating sequence of high and low velocity upper mantle bodies trending ne across the eastern margin of the great basin low velocity mantle lies beneath the volcanic trends of the eastern snake river plain and the st george volcanic field these two low velocity trends are separated by high velocity mantle extending from northern utah to sw wyoming combining these results with seismic data from the western great basin reveals a continuation of these upper mantle trends across the great basin

western u s upper mantle structure regional three dimensional inversions of upper mande p wave velocity structure are created from teleseismic p wave travel time residuals recorded by many of the high density high frequency regional arrays operated within the western united states these inversions are adjusted to a global international seismic centre reference and merged to obtain an image of the upper mantle beneath western united states the p wave velocities in the upper mantle are slow on average and the structure is very heterogeneous where resolution is good coherent upper mantle structures are imaged that extend as deep as 200 km the juan de fuca and gorda slabs which penetrate to greater depths are exceptions and deviate from the average velocity at a given depth by as much as 4 lateral resolution of these structures usually is very good although the magnitude of the actual seismic variations is probably greater than that imaged the long wavelength part of the imaged mantle structure defines coherent elongate features with wavelengths of 200 500 km within 250 km of the pacific coast these structures have a wavelength of 250 km and trend parallel to the surface physiography and young tectonic structures beneath the continental interior where use is made of additional seismic studies to infer average structure in regions of poor teleseismic data coverage structures have a wavelength of 500 km and trend northeasterly this northeast orientation is discordant with young tectonic structures but aligns with young volcanic activity

thermal evolution of the sierra nevada tectonic implications of new heat flow data eight new heat flow measurements in the southern sierra nevada constrain models of thermal evolution and lithospheric structure low reduced heat flows 18 to 21 mw m in the southwest sierra nevada are consistent with previous results from the northwestern and central sierra nevada and extend the known region of linear heat flow heat production correlation an additional 150 km to the south isostatic residual gravity and measured rock densities are also linearly correlated in the central sierra nevada suggesting a general association between the upper crustal distribution of density and heat production the linear residual gravity density relation implies that isostatic residual gravity anomalies have upper crustal sources and therefore is evidence against a flexural model of cenozoic sierra nevada uplift new reduced heat flows measured in the southeast sierra nevada are relatively high 32 to 57 mw m and correlate spatially with a region of high seismicity that includes extensional earthquake swarms this correlation supports the view that basin and range extensional tectonics and associated magmatic processes are encroaching on the eastern sierra nevada in the stable southwest sierra nevada as in the central and northwestern sierra nevada the persistence of low reduced heat flow at the surface is consistent with a thermal origin for the cenozoic uplift of the sierra nevada provided the conductive lithosphere is at least 60 to 90 km thick simple order of magnitude calculations show that thermal uplift models combining mechanisms such as advective warming and thinning of the mantle lithosphere advection of basaltic magma and heat into the crust simple thermal expansion and eclogite to basalt phase conversion can account for the timing and amount of cenozoic sierra nevada uplift thermal models do not need the assumptions of unusual lithospheric strength and crustal buoyancy required by mechanical models for cenozoic uplift driven by a mesozoic crustal root

rayleigh and s wave tomography constraints on subduction termination and lithospheric foundering in central california the crust and upper mantle structure of central california have been modified by subduction termination growth of the san andreas plate boundary fault system and small scale upper mantle convection since the early miocene here we investigate the contributions of these processes to the creation of the isabella anomaly which is a high seismic velocity volume in the upper mantle there are two types of hypotheses for its origin one is that it is the foundered mafic lower crust and mantle lithosphere of the southern sierra nevada batholith the alternative suggests that it is a fossil slab connected to the monterey microplate a dense broadband seismic transect was deployed from the coast to the western sierra nevada to fill in the least sampled areas above the isabella anomaly and regional scale rayleigh and s wave tomography are used to evaluate the two hypotheses new shear velocity vs tomography images a high velocity anomaly beneath coastal california that is sub horizontal at depths of 40 80 km east of the san andreas fault a continuous extension of the high velocity anomaly dips east and is located beneath the sierra nevada at 150 200 km depth the western position of the isabella anomaly in the uppermost mantle is inconsistent with earlier interpretations that the isabella anomaly is connected to actively foundering foothills lower crust based on the new vs images we interpret that the isabella anomaly is not the dense destabilized root of the sierra nevada but rather a remnant of miocene subduction termination that is translating north beneath the central san andreas fault our results support the occurrence of localized lithospheric foundering beneath the high elevation eastern sierra nevada where we find a lower crustal low vs layer consistent with a small amount of partial melt the high elevations relative to crust thickness and lower crustal low vs zone are consistent with geological inferences that lithospheric foundering drove uplift and a 3 4 ma pulse of basaltic magmatism

on the estimation of wave slope and height varnance spectra from sar imagery a procedure is described for using synthetic aperture radar sar imagery to estimate two dimensional ocean wave slope and height variance spectra the logic underpinning the procedure is based both on the results of the numerical simulation of sar wave imagery and analytic descriptions of the sar imaging process the procedure when applied to sar imagery of waves acquired during the recent shuttle imaging radar mission sir b is shown to produce spectra that agree with independent measures of both the two dimensional slope and height variance spectra the implications of these results for future sar missions aimed at measuring ocean waves are considered

theory of synthetic aperture radar ocean imaging a marsen view this paper reviews basic synthetic aperture radar sar theory of ocean wave imaging mechanisms using both known work and recent experimental and theoretical results from the marine remote sensing marsen experiment several viewpoints that have contributed to the field are drawn together in a general analysis of the backscatter statistics of a moving sea surface a common focus for different scattering models is provided by the mean image impulse response function which is shown to be identical to the spatially varying frequency variance spectrum of the local complex reflectivity coefficient from the analysis has emerged a more complete view of the sar imaging phenomenon than has been previously available a new generalized imaging model is proposed

small scale convective instability and upper mantle viscosity under california thermal calculations and convection analysis constrained by seismic tomography results suggest that a small scale convective instability developed in the upper 200 kilometers of the mantle under california after the upwelling and cooling of asthenosphere into the slab window associated with the formation of the san andreas transform boundary the upper bound for the upper mantle viscosity in the slab window 5 x 10 19 pascal seconds is similar to independent estimates for the asthenosphere beneath young oceanic and tectonically active continental regions these model calculations suggest that many tectonically active continental regions characterized by low upper mantle seismic velocities may be affected by time dependent small scale convection that can generate localized areas of uplift and subsidence

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

numerical simulation of synthetic aperture radar image spectra for ocean waves a numerical model for predicting the synthetic aperture radar sar image of a moving ocean surface is described and results are presented for two sir b data sets collected off the coast of chile wave height spectra measured by the nasa radar ocean wave spectrometer rows were used as inputs to this model and results are compared with actual sir b image spectra from orbits 91 and 106 additional parametric variations are presented to illustrate the effects of nonlinearities in the imaging process

linking sierra nevada california uplift to subsidence of the tulare basin using a seismically derived density model seismic tomography has previously imaged the high velocity isabella anomaly southwest of the sierra nevada beneath the tulare basin a region of 1 km of anomalous pliocene subsidence additionally it has been proposed that the eastern sierra has risen 1 2 km since the miocene in response to removal of dense lithospheric material the isabella anomaly has been variably interpreted as either this lithospheric material or a neutrally buoyant stalled fragment of the farallon slab to discriminate between these two we estimate upper mantle density variations from seismic velocities and show that the estimated mass anomaly accords with 60 km of cold lithospheric material removed from beneath the southern sierra sufficient for 1 3 km of range uplift a flexural model of the surface response to mantle loads predicts 1 3 1 7 km of anomalous subsidence of the tulare basin several hundred meters more than is observed nevertheless beam formed receiver functions show up to 10 km of crustal thickening beneath the basin which we attribute to viscous response of the crust to mantle loading this anomalous crustal thickness the post miocene subsidence of the tulare basin and the uplift of the sierra can all be explained by redistribution of cold continental mantle lithosphere therefore the isabella anomaly is more plausibly such continental material than a stalled farallon slab fragment

deformation across the western united states a local estimate of pacific north america transform deformation we obtain a locally based estimate of pacific north america relative motion and an uncertainty in this estimate by integrating deformation rate along three different paths leading west across southwestern north america from east of the rio grande rift to near the continental escarpment data are primarily quaternary geologic slip rate estimates and resulting deformation determinations therefore are instantaneous in a geologic sense but long term with respect to earthquake cycles we deduce a rate of motion of the pacific plate relative to north america that is 48 2 mm yr a rate indistinguishable from that predicted by the global kinematic models rm2 and nuvel 1 however we obtain an orientation that is 5 9 counterclockwise of these models a more westerly motion of the pacific plate with respect to north america is calculated from all three paths the relatively westerly motion of the pacific plate is accommodated by deformation in the north american continent that includes slip on relatively counterclockwise oriented strike slip faults including the san andreas fault which is especially relevant in and south of the transverse ranges and a margin normal component of net extension across the continent which is especially relevant north of the transverse ranges deformation of the sw united states occurs in regionally coherent domains within which the style of deformation is approximately uniform in the vicinity of the transverse ranges two important shear systems splay from the san andreas fault the eastern california shear zone trending nnw from the eastern transverse ranges and the trans peninsular faults trending sse from the western and central transverse ranges within the transverse ranges the right lateral san andreas fault steps left seemingly requiring large amounts of convergence there however most of this convergence is avoided through a funneling flow of the crust toward the western transverse ranges and into the relatively narrow central california coast ranges and the northern motion of the mojave the former process involves an alternation of rotation direction from counterclockwise in and south of the central transverse ranges to clockwise in the western transverse ranges

seismic heterogeneity and small scale convection in the southern california upper mantle we invert traveltime residuals of teleseismic p and s phases for 3 d perturbations in vp vs and vp vs structures in the southern california upper mantle the tomographic inversion uses frequency dependent 3 d sensitivity kernels to interpret traveltime residuals measured in multiple frequency bands and recent advances in regional crustal thickness and velocity models to better isolate the mantle component of traveltime residuals in addition to separate vp and vs tomography we jointly invert the p and s data sets for vp vs perturbations by imposing a smoothness constraint on the lnvs lnvp field the regional upper mantle is very heterogeneous with the greatest amplitude of velocity and vp vs perturbations in the upper 200 km and mantle structures that correlate spatially with the major physiographic provinces of southern california our imaging improves the resolution of the geometry and amplitude of these features in addition to the major structures imaged and discussed in previous papers we find a large high velocity anomaly at depths between 340 and 500 km beneath the borderland transverse ranges and northern peninsular ranges this anomaly is separate from the well known uppermost mantle transverse ranges anomaly which extends no deeper than 175 200 km the strongest low velocity high vp vs anomaly is found beneath and west of the salton trough and is attributed to relatively high partial melt in the asthenosphere as a result of lithospheric thinning the magnitudes of vp vs and vp vs perturbations and knowledge of regional 1 d velocity and attenuation lead us to conclude that the asthenosphere contains up to 1 partial melt extending to depths as great as 150 200 km

the bermuda earthquake of march 24 1978 a significant oceanic intraplate event the bermuda earthquake m 6 occurred near the westerly extension of the kane fracture zone roughly 370 km southwest of the island of bermuda it is one of the largest oceanic intraplate earthquakes to occur off the eastern coast of north america because of its size and location it has provided an excellent set of wwssn body waves they can be used to infer its depth and faulting parameters by waveform modeling techniques the results indicate a north northwest striking thrust mechanism strike n20 w dip 42 ne rake 90 with the hypocenter located at a depth of 11 km which for an oceanic crust places it predominantly in the mantle the event had a seismic moment of 3 4 1025 dyne cm and its time history was modeled with a symmetric trapezoidal time function 3 s in duration the north northwest strike of the event is in good agreement with the bathymetry of the area the epicenter being close to the southwestern edge of the bermuda rise the strike of the event is also close to that of the inferred extensions of the present ridge fracture zones in the region the presence of such fracture zones is indicative of local weak zones in the lithosphere the bermuda earthquake most likely is associated with one of these zones of weakness and is the result of the application of present day stress imposed on the region by the north american plate in the direction of its absolute motion this is an important event in terms of understanding and estimating seismic hazard on the eastern seaboard of north america

seismotectonics of portugal and its adjacent area in the atlantic abstract new elements on the seismicity of portugal and new focal mechanism solutions of earthquakes with epicentres situated off the coast of the portuguese mainland and in the azores region are presented historical seismicity data show that in the territory of the portuguese mainland there are active faults that are responsible for earthquakes that have caused important damage and many casualties however most of the intraplate earthquakes with epicentres situated in the portuguese mainland or near the shore are normally of small magnitude and this renders difficult their interpretation in the light of focal mechanisms a solution for one earthquake with magnitude 5 and epicentre at the nazare submarine canyon is presented southwestwards of cape st vincent there is an important seismic zone responsible for high magnitude earthquakes such as that of 1 november 1755 this zone is situated in the region where the extension of the messejana fault into the ocean joins with the azores gibraltar fault the seismicity of the area situated between the western coast of the portuguese mainland and the azores increases approximately along the 15 w meridian from the latitude of the azores gibraltar fault up to 44 n focal mechanisms of earthquakes with epicentres situated along this line show very similar solutions the interpretation of the focal mechanism solutions of the earthquakes with epicentres situated in the studied area shows that the stress field trends approximately nw se it is assumed that this stress field results from the interaction of the eurasian and african plates however this direction is not maintained in the azores region

source mechanisms of earthquakes near mid ocean ridges from body waveform inversion implications for the early evolution of oceanic lithosphere to investigate the early tectonic evolution of oceanic lithosphere we present a synthesis of the characteristics of near ridge earthquakes i e earthquakes which occur off the axis of major mid ocean spreading centers but in lithosphere less than 35 m y old near ridge seismicity for the past two decades has been rather evenly distributed along the major ridge systems with the exception of the central indian ocean and the eastern part of the cocos plate which have experienced a high rate of earthquake activity during that time for 32 near ridge earthquakes we determined the double couple orientation seismic moment centroid depth and source time function using a formal inversion technique based on matching synthetic and observed p and sh waveforms all types of faulting styles are observed seismogenic deformation in young oceanic lithosphere is concentrated in the first 15 m y and occurs almost entirely in the uppermost mantle from just below the moho to depths at which the temperature estimated from standard thermal models exceeds 800 c thrust faulting is not observed at depths greater than 10 km below the seafloor the deepest near ridge earthquakes are generally characterized by normal faulting focal mechanisms with the t axis oblique or perpendicular to the local spreading direction most of these earthquakes occur in the central indian ocean the transition from mechanisms characteristic of ridge axes to those characteristic of the near ridge environment occurs within the first few million years for lithosphere less than 35 m y old horizontal compressive stress associated with the cooling and subsidence of the lithosphere ridge push does not appear to be the dominant source of stress released in earthquakes this result may be used to place a lower bound of several hundred bars on the typical stress difference in young oceanic lithosphere this bound together with the observed depth of faulting implies that the strain rate associated with seismic deformation is locally at least 10 15 s 1 we propose that thermoelastic stresses associated with plate cooling play an important role in near ridge earthquakes this source of stress can account for the mechanisms and depths of many near ridge earthquakes and for the concentration of activity in very young lithosphere the comparative aseismicity of the oceanic crust may reflect the concentration of thermoelastic stress below the moho due to different material properties for the crust and upper mantle or the early release of thermoelastic stresses through hydrothermal circulation and cracking throughout the crust at or near the ridge axis the higher level of near ridge seismicity in the indian ocean may be the result of modification or enhancement of the more typical near ridge stress state by such local processes as the continental collision between india and asia or active secondary convection beneath the young lithosphere of the indian plate

intraplate seismicity in the atlantic ocean basin a teleseismic catalog a comprehensive compilation and reassessment of intraplate seismicity in the atlantic ocean basin is presented for the years 1918 through 1990 of the 846 earthquakes whose original bulletin epicenters were located in atlantic ocean crust and were at least 2 from active plate boundaries 289 were found to be reliably intraplate a significant number 237 of the original epicenters were actually mislocated interplate earthquakes that located to active plate boundaries upon relocation or whose 95 confidence locations determined through monte carlo simulations intersected seismically active boundaries many of these erroneous locations particularly in regions surrounding the mid atlantic ridge frequently appear in global seismicity maps the resulting reliable catalog of intraplate seismicity is not randomly distributed but shows significant clustering in space and time much of the seismicity is associated with the locations of hot spots such as the bermuda canary and cape verde islands seismicity is also clustered around other tectonic features such as the diffuse plate boundary between north america and south america and the fossil spreading ridge between greenland and north america while we did not examine intraplate earthquakes within 2 of the mid atlantic ridge and some of our unreliable near ridge earthquakes may be truly intraplate we do not find an indication within our reliable data set of an increase in intraplate seismicity with increased proximity to the mid atlantic ridge beyond our 2 limit

sierra nevada uplift a ductile link to mantle upwelling under the basin and range province recent geophysical studies in the southern sierra nevada found no significant airy type crustal root the mountain range seems largely supported by the buoyant asthenosphere beneath an abnormally thin mantle lithosphere we suggest that the late cenozoic uplift of the sierra nevada may have resulted from mantle upwelling under the basin and range province which tends to push ductile material within the surrounding lithosphere causing it to flow away and downward numerical modeling indicates that such ductile flow could lead to pronounced lithospheric thinning under the high sierra and lithospheric thickening under the western sierra nevada comparable to the observed structure

joint estimation of afterslip rate and postseismic relaxation following the 1989 loma prieta earthquake global positioning system gps data from campaigns carried out over the 5 years following the 1989 loma prieta earthquake and leveling data measured in 1990 and 1992 define the postseismic velocity field around the loma prieta rupture zone subtraction of a background velocity field yields a residual velocity pattern which we interpret as the product of two physical processes 1 slow afterslip along distinct planes in the upper cru st and 2 viscoelastic relaxation of the lower crust and upper mantle biirgmann et al 1997 previously derived an afterslip model involving uniform afterslip on two optimally determined planes in cluding oblique reverse slip on the coseismic rupture and reverse slip on a shallow thrust fault to the northeast of the san andreas fault we further consider models of distributed slip on these two fault planes plus a viscoelastic relaxation pattern which depends on a suitable cose ismic rupture model and crust and mantle viscosities several fault models from the literature were considered for the 1989 coseismic rupture with nearly identical impact on the results simultaneous maximum likelihood inversion of the gps and leveling data for afterslip distri bution and viscosity yields the following results 1 a good fit to the data is obtained by smooth afterslip distributions without any viscoelastic relaxation being required 2 tangible broad scale viscoelastic relaxation of the lower crust and upper mantle are present in this data set at 97 confidence and a lower crustal viscosity of 1029 pa s is obtained however the viscosity of both the lower crust and upper mantle are poorly constrained 3 for a given misfit 20 less integrated afterslip and smoother afterslip distributions result when viscoelas tic relaxation is included 4 maximum slip rates on the slip distributed models are 3 5 crn yr the dominant patches estimated on the two planes fill in the entire depth range 4 13 km without significant overlap and deeper afterslip is not required the afterslip distribution on the coseismic rupture plane is strongly dominated by reverse slip immediately southeast of the main center of coseismic reverse slip

p wave image of the crust and uppermost mantle in southern california we have determined a p wave tomographic image of the crust and uppermost mantle in southern california by using 131 372 p wave arrival times from 6 437 local earthquakes recorded by the caltech usgs southern california seismic network in the past twelve years the obtained image has a spatial resolution of 25 km in the horizontal direction and 8 11 km in depth the tomographic image is found to correlate well with major surface geological features in the shallow crust sedimentary basins such as the los angeles basin ventura basin and santa maria basin are well imaged as low velocities while batholiths such as the peninsular ranges and san gabriel mountains are imaged as high velocities in the deeper crust the velocity is low beneath the mojave desert coso volcanic area and salton trough while it is high beneath the great valley continental borderland and the major basins a high velocity layer exists in the mid crust beneath the salton trough in good agreement with a previous study using seismic explosions and gravity data in the uppermost mantle the velocity is low beneath southeastern sierra nevada and the volcanic areas while it is high beneath the mojave desert and along the pacific coast