the resolving power of cross borehole tomography the data collected in a cross borehole tomography problem is inherently incomplete since the boreholes do not completely surround the media under study the ability of the experiment to resolve structure is therefore limited by poor experimental geometry we use the method of backus and gilbert to investigate the ability of this technique to resolve variations in acoustic velocity and attenuation between the two boreholes both integrals that appear in the backus gilbert formulation can be performed analytically thus expediting the calculations the resolving kernels have significant horizontal width demonstrating that the horizontal position of structures can be only poorly resolved

crust and upper mantle structure in the northwest basin and range province abstract seismic waves from nuclear explosions and from well located local earthquakes have been recorded along several profiles through western nevada southeast oregon and eastern california p n delay times from these data and from additional travel times of nts explosions recorded across the western nevada seismic network have been interpreted in terms of varying crustal thickness of the western great basin our interpretation of these data implies that the crust thins from greater than 32 km thick in the vicinity of mono lake to 20 to 22 km thick over a broad region in northwest nevada the p n headwave propagates to a distance of about 550 km with a velocity of 7 8 km sec at distances greater than 600 km a low velocity 7 44 km sec larger amplitude phase is observed this phase may result from long period energy diffracting into a shadow zone a prominent high velocity second arrival following p n by 1 to 2 sec in the distance range 340 to 475 km is interpreted as a reflection from the base of the lithosphere at 60 to 70 km depth

relative array analysis of upper mantle lateral velocity variations in southern california spatial averaging of short period teleseismic p wave arrivals across scarlet constrains upper mantle lateral velocity variations beneath southern california we compare dt d back azimuth and averaged arrival time estimates determined from the entire network for 154 events to the same parameters derived from small subsets of scarlet the resulting relative array diagrams and plots of net subarray delays computed for 171 overlapping subarrays exhibit large anomalies that vary smoothly as a function of subarray position implying substantial lateral heterogeneity under the network patterns of mislocation vectors for more than 100 subarrays permit identification of an east west striking high velocity antiform beneath the transverse ranges further constraints on both the lateral configuration and depth of the anomaly are obtained through thin lens analysis of the net delay data where all the time perturbations are assumed to originate at a single depth projection of 2798 net delays requires a lens deeper than 100 km a representative lens at 150 km depth features low velocities beneath the salton trough as well as a concentrated high velocity zone between 116 3 118 3 w and 33 7 34 8 n consistent with the relative array diagrams and the pkp delay contours of hadley and kanamori 1977 therefore the bulk of the high velocity material observed by hadley and kanamori may in fact lie at or deeper than 150 km rather than 40 100 km this possibility prompts renewed speculation regarding the relationship of this body to the plate tectonic history of the region

a calculation of ionospheric currents due to semidiurnal antisymmetric tides the fully developed equivalent circuit method is used to calculate the ionospheric electric current flow due to the 2 3 and 2 5 tidal modes the 2 5 mode currents are of negligibly small amplitude the 2 3 current pattern has meridional currents in the same direction at a given local time in both north and south hemispheres there is also sometimes a minimum in the zonal current around 10 degree dip latitude and the equatorial electrojet pattern may be different in different longitude zones these features may be possible explanations of 1 the meridional current pattern observed at saint santin 2 current flows on abnormal quiet days aqds 3 the reverse equatorial electrojet 4 the seasonal phase anomaly in the lunar geomagnetic tide and 5 the invasion of one hemisphere s current pattern by that from the opposite hemisphere the recent detection of antisymmetric semidiurnal winds just below the dynamo region further demonstrates the importance of these modes

statistical characteristics of the day to day variability in the geomagnetic sq field 1 day to day variability in the geomagnetic sq field is studied by using the magnetic data from a meridian chain of magnetometers along 120 e longitude the method of natural orthogonal components noc or eigenmode analysis is applied to separate the sq variation from complicated disturbances the first eigenmode with the largest eigenvalue represents fairly well the sq variation with a conspicuous day to day variability in the daily range for the stations on the same north or south side of the sq current system focus the day to day variations show a positive correlation in contrast for the stations on the different sides of the sq focus they show a negative correlation suggesting an important role of latitudinal shift of the sq current system focus to the day to day variability in the sq daily range the sq daily range is correlated with the magnetic indices ap and dst in a peculiar way on some severe disturbed days noticeable enhancements of the sq are observed implying increases in the ionospheric conductivities and or tidal wind velocities on other severe disturbed days however dramatically reduced sq variations occur suggesting dominant effects of the disturbance dynamo process

coherence of seismic body waves from local events as measured by a small aperture array eight local earthquakes were recorded during the operation of a small aperture seismic array at pinyon flat california the site was chosen for its homogeneous granitic geology and its planar topography amplitude spectral ratios for the same signal measured at different stations had average values of less than 2 and maximum values of 7 magnitude squared coherences were estimated for all station pairs these estimates were highest for the p wave arrivals on the vertical component and lowest for the p wave recorded on the transverse component at 500 m station separation the p and s waves were incoherent above 15 hz and 10 hz respectively coherence for both the p and s waves decrease as frequency increases and as distance increases the coherence of signals from borehole sensors located at 300 and 150 m depth displays higher average coherence than equally spaced sites located on the surface the results here suggest that even for sites that appear to be very similar that is those which are located on a planar surface within a few meters of granite bedrock the measured seismic wavefield can be distorted substantially over scale lengths of 500 m coherence properties were calculated from synthetic seismograms which were computed for velocity models with exponential and self similar distribution perturbations standard deviations of 10 are not sufficient for the random velocity distributions to approximate the results from the small aperture array

source effects on regional seismic discriminant measurements source effects on high frequency regional seismic discriminant measurements involving p s type ratios pn lg pg lg are isolated for earthquakes in southern california and southern nevada the scatter in earthquake population discriminant measurements for fixed propagation paths is statistically analyzed to assess the spatial and frequency dependent contributions from source radiation pattern and source depth variations along with variable near source scattering this is useful because procedures for reducing the variance caused by propagation effects must be constrained by knowledge of the intrinsic scatter resulting from independent source effects for which corrections are not available without a priori knowledge of the source type and depth broadband waveforms for earthquakes near the nevada test site nts and in southern california recorded at stations of the livermore nts network lnn the caltech terrascope network and the berkeley digital seismic network bdsn are used to obtain pg lg and pn lg measurements and to assess source depth and mechanism contributions to measurement variance pn lg and pg lg spectral amplitude ratios for several frequency bands between 0 5 and 10 hz from vertical velocity waveforms are computed for a total of 18 tightly located clusters of earthquakes for each cluster we examine the correlations between observed pn lg and pg lg ratios and estimated source depths or predictions based on previously determined source mechanisms focal mechanism predictions do not match the observations well likely due to strong near source scattering and directivity effects that overwhelm the point source radiation pattern however the data do show significant scatter even for sources only a few kilometers apart source depth produces some coherent trends which can be emphasized by averaging results from multiple stations these trends vary from region to region presumably due to specific local crustal layering but most of the variance is not deterministically predictable and appears to arise from strong variations in near source scattering variance estimates for logarithmic amplitude ratios normalized by mean amplitude ratios vary from 0 01 to 0 05 for different phase pairs frequency dependent variations of the mean variance minimum and maximum amplitude ratio parameters show strong distance dependence out to about 300 km and then appear to become independent of event station distance these variance parameters appear to be suitable for use in spatial interpolations of observed p s type ratios for a specific unidentified event path as needed for nuclear test monitoring

lateral velocity variations in southern california i results for the upper crust from pg waves the plate boundary and major crustal blocks in southern california are imaged by a tomographic backprojection of the pg first arrivals recorded by the southern california array the method formulated specifically for local earthquake arrival times is a fast iterative alternative to direct least squares techniques with it we solve the combined problem of determining refractor velocity perturbations and source and station delays resolution and variance are found empirically by using synthetic examples a map showing lateral velocity variations at a depth of approximately 10 km is presented the results show a strong correlation with surface tectonic features clear velocity contrasts exist across the san andreas the san jacinto and the garlock faults the mojave region has the slowest velocities while the peninsula ranges have the highest the san jacinto block has velocities intermediate between mojave and peninsula range velocities and also has early station delays this may indicate that the san jacinto block has overridden mojave material on a shallow detachment surface no velocity variations are found associated with the transverse ranges which we interpret to mean that the surface batholithic rocks in this area do not extend to pg depths

the tectonic crustal stress field and style of faulting along the pacific north america plate boundary in southern california we invert for the state of stress in the southern california crust using a catalogue of high quality earthquake focal mechanisms 1981 2010 the stress field is best resolved where seismicity rates are high and sufficient data are available to constrain the stress field across most of the region from the stress field we determine the maximum horizontal compressive stress s hmax orientations and the style of faulting across southern california the trend of s hmax exhibits significant regional and local spatial heterogeneities the regional trend of s hmax varies from north along the san andreas system to nne to the east in the eastern california shear zone as well as to the west within the continental borderland and the western transverse ranges the transition zones from one state of stress to the other occur over a distance of only a few kilometres following a trend from yucca valley to imperial valley to the east and the western edge of the peninsular ranges to the west the local scale heterogeneities in the s hmax trend include nnw trends along the san andreas fault near cajon pass tejon pass and the cucapah range as well as nne trends near the northern san jacinto fault and the wheeler ridge area the style of faulting exhibits similar complexity ranging from predominantly normal faulting in the high sierra nevada to strike slip faulting along the san andreas system to three consecutive bands of thrust faulting in the wheeler ridge area and the western transverse ranges the local variations in the style of faulting include normal faulting at the north end of the san jacinto fault and scattered areas of thrust faulting the regional variations in the shmax trends are very similar to the pattern of the gps measured maximum shortening axes of the surface strain rate tensor field although the strain field tends to be smoother and appears to capture some of the upper mantle deformation field the mean trend of s hmax departs about approximately 14 to the east from the trend of the maximum shortening directions derived from anisotropy in the upper mantle

polarization tomography for p wave velocity structure in southern california we develop a tomographic inversion method that uses teleseismic p wave polarization data to obtain velocity structure polarization inversion has some intrinsic advantages over travel time inversion it is not influenced by source location and origin time errors it is not sensitive to deep mantle velocity structure and can be used iteratively to improve the tomographic result polarization inversion is more sensitive to near station velocity structure and to velocity gradient and is complementary to travel time inversion in this sense the method is applied to california institute of technology u s geological servey southern california array data the result is generally consistent with previous work and also reveals that the high velocity feature beneath the transverse ranges is bounded between 40 and 200 km depths and possibly has a second small piece at about 300 km depth the slow velocity anomaly under the salt on trough is limited to shallow depths less than about 60 km

the deep crustal structure of the mojave desert california from cocorp seismic reflection data cocorp seismic reflection profiling in the western and northern mojave desert of southern california has revealed the presence of numerous major low angle reflecting horizons within the crust these complex though laterally continuous horizons are interpreted to represent major southwesterly dipping crustal fault zones and as such they place important constraints on the tectonic evolution of the region the upper most horizon is interpreted to be the rand thrust which where exposed places precambrian and late cretaceous crystalline rocks over possibly younger pelona rand orocopia schist this reflecting horizon extends for 25 km southwest of the rand mountains where it appears to be truncated at a depth of about 7 4 km by another horizon which may be a later low angle normal fault the other reflecting horizons are not traceable to the surface and so greater ambiguity remains in their interpretation the most prominent of these horizons occurs at midcrustal depths 15 6 km exhibits a ramp and flat geometry and extends over the northern area of the mojave survey into the basin and range province a lower horizon at depths of 20 30 km in the northern part of the survey area is antiformal and appears to terminate above a flat and relatively continuous moho depth horizon the crustmantle transition appears to be represented by a continuous series of reflections which occur at about 10 s 33 km in the north of the survey and at about 8 9 s 26 29 km in the south these reflections are offset in the vicinity of the town of mojave the deep intracrustal fault zones inferred from the cocorp survey may represent 1 the deep crustal continuation of the system of mesozoic thrusts which crops out in southern nevada and southeastern california 2 late cretaceous to early cenozoic northeast vergent thrusts related to the uplift of the pelona orocopia rand schist or 3 low angle normal faults related to early miocene northeast southwest directed crustal extension the cocorp survey also traversed the major strike slip faults that bound the mojave block the san andreas fault zone appears to truncate reflectors at depths of 6 8 and 20 km within the mojave basement suggesting that it is a major vertical feature which extends to at least 20 km depth conversely the garlock fault does not offset an underlying reflecting horizon which occurs at 9 km depth and therefore appears to be a relatively shallow crustal feature

seismotectonics of the easternmost transverse ranges california relevance for seismic potential of the southern san andreas fault earthquake locations depths and focal mechanisms from the southern california regional network 1977 1985 are used to identify the orientation and sense of slip of active subsurface faults in the easternmost transverse ranges etr the etr are separated from the salton trough province by the southernmost strands of the san andreas fault saf much of the seismicity in the etr is concentrated well northeast of the saf at relatively shallow depths under the little san bernardino mountains many of these earthquakes reflect slip on steeply dipping left lateral faults striking northeast to east at relatively high angles to the adjacent saf focal mechanisms in the etr show predominantly strike slip normal or oblique normal faulting and share common near horizontal t axes striking wnw p axes range from near vertical to near horizontal and strike mostly nne in contrast reverse and strike slip focal mechanisms that exhibit persistent north trending near horizontal p axes characterize the san gorgonio pass area immediately to the west these different patterns of strain geometries are inferred to represent changes in local stress regime and clearly establish a boundary between contrasting tectonic styles of contemporary secondary deformation along the saf this boundary which in the coachella valley may be the mission creek fault is also distinguished by abrupt changes in 1 rate and depth of seismic activity 2 topography 3 quaternary vertical deformation 4 strikes and dips of major branches of the saf and 5 seismic velocities in the crust and upper mantle the preponderance of secondary normal faulting in the etr versus secondary reverse faulting in the san gorgonio pass region suggests that fault normal stress is much less across the saf adjacent to the etr if a friction law where strength is proportional to normal stress applies to the saf then a smaller tectonic shear stress would be required tor slip in large earthquakes along the salton trough segment in this case the southernmost saf may have shorter repeat times smaller average displacements and lower moment release rates than earthquakes that rupture through the major restraining bend in san gorgonio pass

the relation of crustal scattering to seismicity in southern california dense regional array recordings of 377 teleseismic events are used to map geographic variability in upper crustal p to s scattering in southern california scattered wave energy is sensitive to short wavelength heterogeneity and maps of scattering potential offer a powerful complement to travel time tomography in characterizing seismic crustal heterogeneity the scale length of resolved scattering variability is commensurate with the scale lengths of surface fault trace length and microseismicity variations and both mapped faults and seismicity are concentrated in regions of strong scattering fault proximal scattering averaged across strike of predominately strike slip fault zones is highly correlated with microseismicity levels and is predictive of the pattern of microseismicity variation but not of absolute scale north south profiles through the transverse ranges reveal coincident strong gradients in microseismicity and scattering whereas mapped segment bounds along the san andreas fault zone from san francisco to the salton sea coincide with strong along strike gradients in the level of scattering the overall correlation of microseismicity and scattering potential is consistent with structural control of aftershock and background microseismicity production strain energy control of scattering or both prior evidence for fault offset in the pattern of scattering intensity revenaugh and reasoner 1997 favors the former

pn velocity anisotropy in southern california we analyze p n propagation as a function of azimuth across a 28 station 150 km aperture subarray of the scarlet network centered near the central transverse ranges california we selected signals from 81 earthquakes and explosions with epicentral distances ranging from 150 to 400 km covering all azimuths except a 40 gap from the southwest and a lesser gap from the northeast direction for each source the apparent velocity of p n was determined using a one norm measure of misfit the apparent p n velocity does not show any systematic variation with epicentral distance but exhibits a strong azimuthal dependence our preferred interpretation calls for a slightly dipping 2 to n40w planar moho with 3 to 4 per cent anisotropy of subcrustal material transverse isotropy with a nearly horizontal symmetry axis is sufficient to explain the data the direction of sagittal symmetry is n50w the isotropic velocity of p n is 7 8 km sec in contrast a higher 8 1 km sec p n velocity is found in the mojave block with no indication of anisotropy these observations are consistent with a subcrustal model of the pacific north america plate boundary where ductile flow is characterized by simple shear in a vertical plane with strike parallel to the direction of relative plate motion

seismic tomography constrained by bouguer gravity anomalies applications in western washington tomographic inversions for velocity variations in western washington indicate a high correlation with surface geology and geophysical measurements including gravity observations by assuming a simple linear relationship between density and velocity birch s law it is possible to calculate the gravity field predicted from the velocity perturbations obtained by local tomographic inversion while the predicted gravity matches observations in parts of the model the overall correlation is not satisfactory in this paper we suggest a method of constraining the tomographic inversion to fit the gravity observations simultaneously with the seismic travel time data the method is shown to work well with synthetic data in 3 dimensions where the assumption of birch s law holds strictly if the sources of the gravity anomalies are assumed to be spatially localized integration can be carried out over a relatively small volume below the observation points and sparse matrix techniques can be applied we have applied the constrained inversion method to western washington using 4 387 shallow earthquakes to depths of 40 0 km 36 865 raypaths convering a 150 250 km region and found that the gravitational constraints may be satisfied with minor effect on the degree of misfit to the seismic data

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

slip line model for southern california crustal kinematics we propose a simple slip line model to explain the faulting and seismicity patterns of southern california we assume the sierra nevada great valley sngv block to be rigid in view of its low seismicity and other regions to undergo plastic deformation from this point of view the complex deformation patterns observed in southern california including the borderland region result from the plowing of the eastern margin of the pacific plate by the nondeforming sngv block a simple analogy can then be made between regional tectonics in southern california and two dimensional flow solutions obtained in metallurgy for the extrusion of rigid plastic metals we use a simplified geometry based on the geology and seismicity of southern california to calculate a two dimensional slip line field which predicts the shape of the san andreas fault near the big bend surprisingly well the largely aseismic high topography western mojave can then be interpreted as an analogue to the nondeforming dead metal zone which arises for certain metal extrusion geometries the slip field agrees quite well with the strikes and slip orientation of major dextral faults in southern california including the san jacinto elsinore and newport inglewood faults it also predicts correctly the strikes and slip orientation of sinistral cross faults found in some areas of the region in the transverse ranges where thrust faulting dominates the slip lines are no longer potential fault planes but a simple argument predicts that they should lie at 45 on the strikes of thrust faults in fair agreement with the observations

spatial variation of crustal seismic velocities in southern california abstract arrival time data for local earthquakes were used to study lateral velocity variations and error statistics for southern california crustal p velocities averaged over the major geomorphic provinces vary by only a few per cent regions of relatively high velocity include the peninsular ranges the imperial valley the pomona basin and the colorado desert the santa monica southern sierra san bernardino and san jacinto mountains are relatively slow as is the offshore area near surface station delays which are unknown to within several tenths of a second are a major source of uncertainty in the regional velocities arrival time errors include two types 1 small errors caused by imperfect timing and unmodeled velocity variations and 2 larger errors apparently caused by improper identification of arrivals and by blunders we found no reliable way to identify the latter errors unless they result in large arrival time residuals for a strongly edited catalog the variance of arrival time residuals appears to increase linearly with epicentral distance as predicted for wave propagation in a randomly heterogeneous medium the variance of residuals is strongly station dependent residuals are nearly gaussian within the 70 per cent confidence interval but not outside this interval the 95 per cent confidence limit occurs at about three standard deviations

p wave image of the peninsular ranges batholith southern california we invert earthquake p wave arrival times to image the 3d distribution of p wave velocities in the mesozoic peninsular ranges batholith and nearby areas in southern california there is a 3 velocity contrast between the eastern and western peninsular ranges at 4 and 20 km depth west side faster and a 1 to 1 5 velocity contrast across the san andreas fault zone south side faster in the san bernardino region at 4 to 14 km depth the san andreas velocity contrast is due to the juxtaposition of different rock types by slip along the fault zone the peninsular ranges batholith velocity contrast is due to a difference in rock composition across the batholith the maximum gradient in the crustal velocities is coincident with a compositional boundary within the batholith that reflects emplacement of the batholith across juxtaposed oceanic and continental crust quaternary fault development has been in part concentrated at this boundary

a tomographic image of mantle structure beneath southern california we determined the variations in seismic structure beneath southern california by using a tomographic method of inversion on teleseismic p delays recorded with the southern california array the algorithm employed was a modified form of an algebraic reconstruction technique art used in medical x ray imaging deconvolution with an empirically estimated point spread function was also used to help in focusing the image the inversion reveals two prominent features beneath the region the first is a thin vertical wedge directly beneath the transverse ranges that is 2 3 faster than the surrounding region this feature deepens to the east attaining a maximum depth of about 250 km beneath the san bernardino mountains the second feature is a major zone of low velocity material that is 2 4 slow under the salton trough rift valley extending to a depth of about 125 km two possible explanations for the spatial association of the transverse ranges with the velocity anomaly below are lithospheric subduction or small scale sublithospheric convection in the region of the big bend of the san andreas fault the low velocity anomaly beneath the salton trough is consistent with convective upwelling there

the study on the source of the oil seeps and bitumens in the tianjingshan structure of the northern longmen mountain structure of sichuan basin china abstract the tianjingshan structure of the northern longmen mountain is located in the north of the sichuan province china two primary source rocks are present in this region the overmature infracambrian lower cambrian source rocks and the mature upper permian source rocks oil seeps and solid bitumens occur extensively in the mesozoic and paleozoic marine strata indicating good exploration prospects in this area however oil to source correlation in the tianjingshan structure is difficult due to both high maturity of the lower cambrian source rocks and severe biodegradation of oil seeps and bitumens in this work one oil seep from the permian qixia formation and two severely degraded bitumens from the cambrian strata in the tianjingshan structure were correlated with their potential source rocks hydropyrolysis was used to release biomarkers bound to kerogen of the overmature lower cambrian source rock the released biomarkers were used for the oil to source correlation together with those indexes resistant to thermal degradation and biodegradation based on the geochemical data and the petroleum geological settings of the area it can be inferred that the cambrian ordovician and silurian bitumens with light bulk isotopic values commonly 13 c

neutral winds in the lower thermosphere from dynamics explorer 2 doppler line profile measurements of the oi 557 7 nm green line emission made by the fabry perot interferometer on dynamics explorer 2 have provided altitude profiles of the meridional component of the lower thermospheric neutral wind the wind inversion technique of nardi 1991 has been used to extract the neutral wind profiles from the line of sight measurements individual 557 7 nm doppler line profiles and inverted volume emission rate and neutral wind profiles are presented neutral wind measurements from the 120 km altitude level obtained on multiple orbital passes over the summer hemisphere polar region have been merged to produce a synthesized averaged vector wind field in geomagnetic coordinates the wind pattern exhibits a region of anticyclonic vorticity in the daytime sector of the magnetic polar cap averaged winds of 300 m sec in the equatorial direction are observed in the early morning sector the satellite winds are in reasonable agreement with the predictions of the ncar and ucl thermosphere general circulation models with significant regional discrepancies evident in both magnitude and direction

late cenozoic tectonic evolution of southwestern california we synthesize available geologic and geophysical data from southern california and adjoining areas in order to develop a reconstruction of the tectonic evolution of the southern coast ranges western transverse ranges and borderland regions of california since 30 ma premises include specified relative plate motions for times prior to 10 5 ma and after 3 ma a mid tertiary bight in the continental margin of southern california midcrustal detachment faults under much of california and adjoining regions rotation of the western transverse ranges about an eastern pivot and specified fault displacement histories prior to 18 ma about 90 of the tangential component of pacific north america relative motion was accommodated on an offshore dextral fault system near the toe of the continental slope from 18 ma to 5 5 ma dextral slip was accommodated predominantly on the offshore system but also on a second inboard system that included the san andreas fault fault bounded crustal blocks between the offshore and inboard fault systems underwent translation rotation and extension from 18 ma to 11 5 ma but not from 11 5 ma to 5 5 ma since 5 5 ma dextral slip has been accommodated predominantly on the inboard fault system dextral slip ceased on the offshore fault system north of the transverse ranges but continues on faults west of or within baja california the san gregorio san simeon hosgri fault system has accommodated less than 5 km of the late cenozoic dextral slip the distribution of dextral displacement in the plate boundary zone has been controlled by abrupt jumps of the position of a southern triple junction and to a lesser degree changes in magnitude of pacific north america relative motion based on our reconstuction the tangential component of pacific north america relative motion was 90mm yr for the period 10 5 ma to 5 5 ma and 60mm yr for the period 5 5 ma to 3 ma

quiet ionospheric currents of the southern hemisphere derived from geomagnetic records a special selection of extremely quiet 1965 geomagnetic daily variation records from north america europe central asia and east asia was used to determine monthly spherical harmonic analysis coefficients representing the sq fields from these regions the coefficients permitted a separation of the external and internal contributions to the daily variations month by month behavior of the equivalent external current system and contours of the daily range of this current showed regional differences in amplitude and location of the equivalent ionospheric currents with seasonal change current foci locations reached 35 to 42 geomagnetic latitude in summer and 18 to 35 in winter with a regional dependence the largest daily range of external current was 14 6 104 a in north america 15 3 104 a in europe 13 7 104 a in central asia and 12 3 104 a in east asia continental differences in the behavior of the current seemed to be explained by the relative position of the geographic and geomagnetic poles with respect to these regions

three dimensional vp and vp vs velocity models of the los angeles basin and central transverse ranges california we use p and s arrival times from 5225 earthquakes and 53 explosions recorded by the southern california seismographic network to invert for the three dimensional p velocity v p and the p and s velocity ratio v p v s in the central transverse ranges and the los angeles basin to model long wavelength features of the velocity structure we invert for the models by interpolating models determined successively from a sparse medium to a dense grid with 40 20 and 10 km spacing of horizontal grid nodes layers of grid nodes are placed at depths of 1 4 6 8 12 16 and 20 km the data variance decreased about 80 in the gradational inversion ample data from the 1994 northridge and other earthquake sequences the rich background seismicity and the dense station distribution along with controlled sources made the model well resolved except along the edges to the southwest in the offshore region and at depths greater than 20 km the v p model images the shape of the los angeles and east ventura basins down to depths of 8 and 12 km three low velocity areas at 1 km depth in the los angeles basin that coincide with recent sediment depositional areas are also imaged the north edge of the peninsular ranges the santa monica and the san gabriel mountains form discontinuous high velocity ridges extending to depths of 20 km the high v p v s ratios in the near surface are consistent with high pore fluid pressures in the basin sediments at depth beneath the east ventura basin the high v p v s and high v p suggest the presence of ophiolitic assemblages or mid miocene volcanics in contrast a body with normal v p and low v p v s that is identified in the upper crust beneath santa monica bay may be a fragment of quartz rich continental crust the v p model of both the ventura and the los angeles basins shows features that suggest deformation of the hanging wall or basin closure at seismogenic depths of 16 km the hypocenters of moderate sized and large earthquakes are located within or adjacent to high v p velocity bodies in most cases these high velocity bodies form the upper block consistent with shortening of the lower crust as described in thick skinned tectonic interpretations

external sq currents in the india siberia region the sq current systems for quiet days of 1976 and 1977 were studied for observatories established near 76 degrees e longitude in the northern hemisphere a special sperical harmonic analysis technique was used to separate the internal and external contributions of the quiet field variations and to determine the equivalent external current contours responsible for the sq field variations on the sixth and twenty first of each month the average ratio of the external to internal spectral power was 6 7 in 1976 and 7 4 in 1977 focus positions for the india siberia region external sq current vortex near 22 degrees to 29 degrees were found to be at lower geomagnetic latitudes than for europe and north america the usual mid latitude vortex current reaching 11 0 to 13 2 times 10 sup 4 a in summertime disappeared completely during winter months in both the years the sq current position was affected clearly by the off spin axis position of the north geomagnetic pole throughout the 2 years the authors also found small current vorticies near 70 degrees to 80 degrees geomagnetic latitude which have been ascribed to auroral region activity on quiet days 19 refs 11 figs 1 tab

cainozoic evolution of the state of stress and style of tectonism of the basin and range province of the western united states cainozoic evolution of the modern plate boundary along the western united states from subduction to a predominantly transform boundary coincided with a change from compressional to extensional deformation in the western united states extensions tectonism responsible for the modern basin and range province appears to represent a unique late stage episode of a much longer period of extension initiated in an in tra arc setting contemporaneously with calc alkaline magmatism basin range extension is distinguished from early extension on the basis of angular unconformities differences in fault trends and spacing and associated magmatism basaltic prebasin range extension i e extension preceding the break up of the region into ranges resembling the modern ones was under way locally by at least 30 ma and is now recognized by faulted and highly tilted strata exposed in uplifted range blocks by large regions of the crust underlain by passively emplaced subvolcanic batholiths and by the thickness and distribution of stratigraphic units locally high strain rates that accompanied early extensions of as much as 50 100 are implied data on preferentially orientated dyke swarms and fault slip vectors indicate a strikingly uniform wsw ene least principal stress orientation in the period ca 20 10 ma during this early extension the change from early extension to basin range style faulting of the upper 15 km of crust which resulted in broadly spaced ranges 25 35 km crest crest spacing was time transgressive and probably not abrupt locally both types occurred concurrently southern basin and range block faulting occurred largely in the period 13 10 ma in response to a stress field orientated similarly to that responsible for the early extension in contrast northern basin and range block faulting developed after 10 ma and continues to the present in response to a stress field orientated approximately 45 clockwise to the earlier stress field this modern stress field with a wnw ese to e w directed least principal stress characterizes the entire modern basin and range province and rio grande rift region the 45 change in least principal stress orientation is consistent with superposition of dextral shear associated with the development of the san andreas transform fault inclusion of pre basin range extension may help resolve the discrepancy between estimates of 15 30 for basin range block faulting and total extension estimates of 100 300 for the basin and range province

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

strange currents over saint santin analysis of 29 days of ionospheric current data obtained by saint santin radar including both magnetically quiet and disturbed periods has confirmed the existence of northward currents at all daytime hours the postulate of field aligned van sabben currents driven by interhemispherical asymmetries seems to be able to account for the anomaly the desirability of quantitative modeling of the van sabben currents on a seasonal or day to day basis is highlighted by the saint santin measurements

velocity models for the highly extended crust of death valley california we employ nonlinear optimization to obtain shallow velocity structure from cocorp seismic reflection profiles in the highly extended death valley region california we also use group velocity analysis of regional earthquake surface waves that have traversed in and near the extended domain when compared with regional models for surrounding provinces our results show no evidence for significant uplift of the middle crust or removal of the upper crust we do not find fundamental upper crustal variations between regions proposed to have widely varying extensional histories these observations constrain previously proposed extensional mechanisms toward less than extreme amounts of extension

a description of the external and internal quiet daily variation currents at north american locations for a quiet sun year summary an order 4 degree 12 spherical harmonic analysis of the smoothed quiet geomagnetic daily variations was used to separate the external and internal geomagnetic sq field at north american locations for the quiet sun year 1965 these fields were represented by a month by month display of equivalent current vortex systems with dominant pre noon foci the focus reached 40 latitude near the june solstice and about 30 latitude near the december solstice the daily range of sq current amplitudes was largest in late july to early august and smallest in mid december semi annual variations of sq currents dominated only the equatorial region daily maxima in mid latitudes occurred mostly near local noon in december to february and about 1 hr before noon in june to mid october the ratio of external to internal current amplitude vxied from about 1 5 to 1 9 in the middle latitudes with both annual and semiannual changes an error treatment indicated that the analysis reproduced the form of the surface field with a correlation coefficient of about 0 9 and the amplitudes of the surface field to about 10 per cent of the sq daily range

latitudinal variation of quasi biennial oscillation modulation of the semidiurnal tide in the lower thermosphere perturbations in the geomagnetic field at the earth s surface can result from variations in the neutral wind field in the lower thermosphere through dynamo action in the interfused ionospheric e region thus spectral analysis of the sq variations in ground based geomagnetic data extending continuously over many years and from several sites worldwide has been carried out to investigate the variation with latitude of quasi biennial oscillation qbo effects in the semidiurnal tide in the lower thermosphere this qbo modulation of the tide appears to be greatest around 60 latitude the phase of the oscillation is largely latitude independent and implies qbo modulation of the upward propagating semidiurnal tide close to its source region the semidiurnal tide appears to propagate upward more easily during the easterly phase of the tropical stratospheric qbo at 30 mbar than during the westerly phase

seismic imaging of the southern california plate boundary around the south central transverse ranges using double difference tomography we derive high resolution p and s seismic velocities vp and vs within the south central transverse ranges section of the san andreas fault saf using a new double difference tomography algorithm incorporating both event pair and station pair differential times the addition of station pair data allows for better absolute event locations and higher model resolution at shallow depths velocities within a 222 km 164 km region are inverted using 1 000 000 p and s arrival picked with an automatic detection algorithm and differential times from 10 000 local events recorded by 250 stations similarly large p and s datasets lead to high quality vp vs estimates of the region the resulting models include low velocities along major fault segments and across fault velocity contrasts they also show very high vp vs anomalies near shallow damaged rock whereas fault zones exhibit either low 1 73 vp vs characteristics at greater depth the variations in amplitude of these anomalies along the saf through san gorgonio pass sgp suggest abrupt west to east changes in elastic properties moreover their geometries imply near vertical saf segments northwest of sgp and northeast dipping faults southeast of that area the saf near coachella valley is estimated to dip by 57 regional scale low and high vp vs values are related to relative abundances of crystalline or metamorphic rocks near fault vp vs anomalies at depth are likely associated with changes in wet crack geometries the obtained results can improve future calculations of seismic motion from large earthquakes in the area and related seismic hazard estimates

mapping the crustal thickness in southwest japan using moho reflected waves in this work we have tried to detect and collect later phases associated with the moho discontinuity and use them to study the lateral variations of the crustal thickness in southwest sw japan we first compute synthetic seismograms for local earthquakes taking into account the focal mechanism solutions estimated from first motion polarity data then we compare the synthetics with the observed waveforms to detect the major later phases in seismograms of shallow crustal earthquakes in sw japan taking the advantage of the high quality and great quantity of hi net waveform data now available throughout the entire japan islands we have detected 1659 moho reflected phases pmp and sms from shallow events in sw japan we estimated the crustal thickness in the study area using travel time differences between these later phases and the first p and s arrivals our results show that the moho is deep in chubu district and becomes shallower along the coastlines of the japan sea and the pacific ocean a thinner crust appears in osaka bay and awajii island where the 1995 kobe earthquake m7 2 occurred

teleseismic tomography of the upper mantle beneath the southern lachlan orogen australia with the goal of better understanding the deep structure and tectonic setting of the lachlan orogen 50 short period seismic stations were deployed across the southern end of the great dividing range in victoria southeast australia between 2005 and 2006 to record distant earthquakes a total of 7452 relative pwave arrival time residuals from 169 teleseismic events have been extracted from the continuous records using an adaptive stacking technique which exploits the coherency of global phases across the array these residuals are mapped as three dimensional perturbations in p wavespeed in the upper mantle beneath the array using a recently developed iterative non linear tomographic procedure which combines a grid based eikonal solver and a subspace inversion technique the capability of the new scheme to include interface geometry is utilised in order to investigate the effects of a priori moho topography on the resolution of upper mantle structure the resultant images show a pattern of p wavespeed anomalies that lacks a predominant orientation and therefore does not favour a purely w e subduction accretion model for the formation of the lachlan orogen one of the main features of the three dimensional model is a zone of elevated wavespeed beneath the northern end of the array which extends to a depth of approximately 150 km and contrasts with significantly lower wavespeeds to the south this anomaly which does not appear to be an artifact of relative arrival time residual contributions from the adjoining mountainous terrane may reflect the presence of a substantial piece of proterozoic lithosphere incorporated within the phanerozoic subduction accretion setting of the lachlan orogen another key feature of the solution model is a zone of relatively low velocity beneath the newer volcanic province northwest of melbourne which extends from the crust to a depth of approximately 200 km this is likely to represent the signature of elevated temperatures associated with a diffuse mantle source for the quaternary volcanism in victoria 2008 elsevier b v all rights reserved

tomography of the western united states from regional arrival times first arrival times from regional distances 200 1200 km in the western united states extracted from the international seismological centre isc data set were inverted in a tomographic study to map the laterally varying pn velocity structure of the uppermost mantle i e the mantle lid and to estimate the crustal static delay at each seismograph station synthetic data were used to evaluate resolution results correlate well with major tectonic features we find low uppermost mantle velocities v 7 9 km s these low apparent pn velocities are primarily due to the effects of isostasy hotspot volcanism and crustal extension we interpret the low apparent pn velocities beneath the sierra nevada 7 6 7 7 km s to be the result of time delays introduced by ray paths tunneling through the deep crustal root the yellowstone hotspot migrating along the snake river plain has left low velocity 7 6 7 8 km s hot upper mantle in its wake lower pn velocities 7 8 km s beneath the extensional basin and range are also presumably due to hotter uppermost mantle based on station static delays for the basin and range we infer that moho depths there do not vary significantly from 30 km normal continental pn velocities 8 0 km s as well as thicker crust 40 50 km underlying the rocky mountains wyoming basin province colorado plateau and the northern columbia plateau indicate the presence of relatively cool uppermost mantle beneath these regions

does the great glen fault really disrupt moho and upper mantle structure many previous interpretations of deep seismic reflection profiles across the great glen strike slip fault system have postulated that the fault penetrates the crust and upper mantle as a vertical plane that sharply offsets the moho discontinuity such an interpretation has become a general feature of more recent regional syntheses of the deep geology of the british isles reprocessing of portions of four profiles across the fault improves the resolution of lower crustal and upper mantle structure and calls into question the moho step interpretation diffraction analysis and seismic migration applied to reflection data north of shetland across the northward continuation of the great glen fault walls boundary fault indicate a narrow synform or keel developed on the moho directly beneath the fault this keel is itself underlain by a possible mantle reflection that is cut off by the downward projection of the fault north and west of ireland analyses of amplitudes frequencies and the geometrical behavior of reflections upon migration show that structures previously interpreted as moho steps may be better explained as distinct packages of reflectivity that are uppermost mantle or possibly out of the plane in origin in one location north of ireland where some three dimensional control exists the vertically downward projection of the great glen fault intersects without disrupting dipping structure in the upper mantle this observation leads to a model for displacement on the fault system in which motion is laterally transferred along a dipping ramp or blind thrust in the uppermost mantle somewhat analogous to models developed for the san andreas fault that indicate displacement along the fault to be laterally offset within the middle crust one of the principal conclusions of this study that major vertical steps on the moho beneath the great glen fault are difficult to support from the available seismic data is consistent with theologically based studies which predict that moho topography such as vertical steps is unlikely to be preserved over long periods of geologic time

the south fossa magna japan revealed by high resolution p and s wave travel time tomography abstract detailed tomographic images of the collision zone between the northern edge of the philippine sea and the eurasian plates reveal a high correlation between tectonic features inferred from seismicity and p and s wave velocity structures the linear tomographic inversion covered a 150 150 60 km region in the south fossa magna centered on the northern izu peninsula thirty three three component stations were located in the target region and 3823 high quality earthquakes were selected from the catalogues of the nriesdp giving rise to 53 593 p and 50 059 s wave phase arrivals used in the inversion the model was parameterized by 60 60 10 rectilinear blocks measuring 2 5 km per side horizontally and 5 10 km varying thicknesses in depth three dimensional perturbations from the one dimensional nriesdp layered model were derived by minimizing the squared misfit of the travel time residuals regularization was employed to reduce the effects of noisy data by constraining the two dimensional laplacian of the model within horizontal layers to be small the large sparse matrix was solved using the conjugate gradient algorithm lsqr reduction of misfit was 50 for the p wave inversion and 57 for the s wave inversion there is a high correlation between the p and s wave inversions between 5 15 km depth both 3d images indicate low velocity anomalies trending northward in the west along the fujikawa from suruga bay to northwest of mt fuji north of izu peninsula there is a broad high velocity zone these results correlate with bouguer gravity anomalies previously observed starting at 5 km depth a high velocity northeast dipping anomaly beneath sagami bay is inferred to be the shallow subducting phs plate to the north a high velocity zone in the vicinity of the east yamanashi seismic swarm 15 32 km depth is clearly observed on the p wave inversion and to a lesser extent on the s wave inversion because of the high seismicity in this region we speculate this is the zone where the phs plate is grazing the eur plate to the northwest and the high velocity anomaly represents a large scale asperity to the west below thmt fuji area a broad low velocity is observed between 15 and 32 km depth we interpret the deep low velocity structure which exhibits little seismicity to be either the subduction of the volcanic arc or a region of elevated temperature associated with hot rising magma near the swarm area on the eastern edge of izu peninsula low velocity anomalies are observed near the surface on both p and s wave inversions along the eastern part of izu peninsula a prominent low velocity anomaly is observed between 10 and 20 km on the s wave image but is less pronounced on the p wave inversion

a geology based 3d velocity model of the los angeles basin sediments abstract seismic hazard studies of the los angeles basin area require a realistic seismic velocity model we use geologic information about depth to crystalline basement depths to sedimentary horizons uplift of sediments and surface geology in a velocity depth age function to construct a three dimensional velocity model in earthquake location tests the model predicts travel times satisfactorily and in earthquake ground motion simulations the model correctly determines the timing and amplitude of late arriving waves

modeling of neogene crustal block rotation case study of southeastern california numerical modeling using a two dimensional distinct element method dem was performed to give an additional understanding of the mechanism of crustal block rotation in a highly active portion of the pacific north america transform plate boundary paleomagnetic and structural investigations have shown that large 30 50 horizontal rotations of crustal blocks occurred in the eastern transverse zone and the northeast mojave block southeastern california since the end of early miocene the main aspect of the modeling was to investigate whether an anisotropic horizontal stress state can be responsible for the rotation of crustal blocks with a length of 100 km or not the simulations were performed using different stress boundary conditions and different parameters of the fault strength applying an assumed ratio of 2 for the horizontal stress components as boundary condition block rotations in the order of 40 only occur if 1 the major horizontal stress sh is oriented n20 e with a maximal deviation of 15 to the east or west and 2 the fault strength described by the friction coefficient is smaller than 0 12 or 14 only of the value 0 85 for intact rocks byerlee s law for a weakening model of the fault strength an initial friction coefficient of 0 85 allows also rotations of about 40 if the friction coefficient decreases rapidly due to an increasing shear displacement to a basic friction coefficient of 0 12 or smaller for the same amount of rotation 40 a decreasing stress ratio requires a lowering of the friction coefficient used whereas an increase of the ratio allows a higher friction coefficient the simulation also show that the modeling with elastic blocks can adequately describe the observed main features of the crustal deformation especially the cumulative slip values discrepancies in the observations and modeling results for the western parts of the mojave desert are the result of a neglect of the three dimensional deformation in our two dimensional modeling

solar and lunar geomagnetic variations in the northwestern part of turkey summary hourly mean values of x y and z components of the geomagnetic field from two observatories and the total intensity f of the geomagnetic field from eight monitoring stations were analysed to study the behaviour of solar and lunar geomagnetic variations in the northwestern part of turkey the chapman miller method was applied to the data first as a whole and then by grouping it to lloyd s seasons to calculate seasonal dependence of the harmonics amplitudes of solar and lunar harmonics increase from winter months to summer months the first two harmonics of both solar and lunar origins were calculated annually to compare with sunspot relative numbers to see annual and semi annual variation the solar harmonics show very good sunspot dependence but lunar harmonics do not it means that both ionospheric and ring current systems have an important role in the formation of solar harmonics but lunar harmonics are affected by ionospheric current system only in the northwestern part of turkey we would not expect and we do not find much ocean effect at these observatories since they are all remote from oceans with significant tides

p wave backazimuth anomalies observed by a small aperture seismic array at pinyon flat southern california implications for structure and source location abstract seismograms of earthquakes and explosions recorded at local regional and teleseismic distances by a small aperture dense seismic array located on pinyon flat in southern california reveal large 15 backazimuth anomalies we investigate the causes and implications of these anomalies by first comparing the effectiveness of estimating backazimuth with an array using three different techniques the broadband frequency wavenumber bbfk technique the polarization technique and the beamforming technique while each technique provided nearly the same direction as a most likely estimate the beamforming estimate was associated with the smallest uncertainties backazimuth anomalies were then calculated for the entire data set by comparing the results from beamforming with backazimuths derived from earthquake locations reported by the anza and caltech seismic networks and the preliminary determination of epicenters pde bulletin these backazimuth anomalies have a simple sinelike dependence on azimuth with the largest anomalies observed from the southeast and northwest directions such a trend may be explained as the effect of one or more interfaces dipping to the northeast beneath the array a best fit model of a single interface has a dip and strike of 20 and 315 respectively and a velocity contrast of 0 82 km sec application of corrections computed from this simple model to ray directions significantly improves locations at all distances and directions suggesting that this is an upper crustal feature we confirm that knowledge of local structure can be very important for earthquake location by an array but also show that corrections computed from simple models may not only be adequate but superior to those determined by raytracing through smoothed laterally varying models

p wave velocity variations in the coso region california derived from local earthquake travel times inversion of 4036 p wave travel time residuals from 429 local earthquakes using a tomographic scheme provides information about three dimensional upper crustal velocity variations in the indian wells valley coso region of southeastern california the residuals are calculated relative to a coso specific velocity model corrected for station elevation weighted and back projected along their ray paths through models defined with layers of blocks slowness variations in the surface layer reflect local geology including slow velocities for the sedimentary basins of indian wells and rose valleys and relatively fast velocities for the sierra nevada and argus mountains in the depth range of 3 5 km the inversion images an area of reduced compressional velocity in western and northern indian wells valley but finds no major velocity variations beneath the coso volcanic field to the north these results are consistent with a recent study of anomalous shear wave attenuation in the coso region between 5 and 10 km depth low velocity areas 7 slow appear at the southern end of the coso volcanics reaching east to the coso basin numerical tests of the inversion s resolution and sensitivity to noise indicate that these major anomalies are significant and well resolved while other apparent velocity variations in poorly sampled areas are probably artifacts the seismic data alone are not sufficient to uniquely characterize the physical state of these low velocity regions because of the coso region s history of pleistocene bimodal volcanism high heat flow geothermal activity geodetic deformation and seismic activity one possibility is to link the zones of decreased p velocity to contemporary magmatic activity

a comparative analysis of deformable layer tomography and cell tomography along the larse lines in southern california summary we conducted a comparative analysis of the new deformable layer tomography dlt with the traditional fixed in space cell tomography along three 2 d crustal profiles of p wave velocities in southern california using first arrivals from local earthquakes and two los angeles regional seismic experiment larse seismic lines the dlt method inverts for the depth varying geometry of velocity interfaces which include major velocity discontinuities near the top and bottom of the crust and boundaries between velocity layers in the middle crust in regions characterized by large lateral velocity changes the dlt method may improve the depth resolution of major velocity interfaces compared to traditional cell tomography using fixed in space cells or grids we also tested the use of crustal thickness estimates from a receiver function study for constraining the depth range of the moho geometry tests indicate that in areas of sufficient ray coverage the depth resolution of the dlt for major velocity interfaces is usually less than 1 km in the upper crust and less than 3 km in the lower crust our 2 d dlt models fit the first arrival data significantly better than three published 3 d cell tomography models along the profiles the configurations of basin edges and undulating velocity discontinuities are well delineated in the dlt models and indicative of the crustal architecture of the region

the complete ordered ray expansion ii multiphase body wave tomography summary in a companion paper clarke 1993 we described a method for the calculation of body wave synthetic seismograms referred to as the complete ordered ray expansion here we present a new method for the inversion of body wave seismograms based on the core algorithm this multiphase waveform tomography allows us to incorporate many body wave arrivals from each seismogram in an inversion vastly increasing the amount of structural information that may be extracted from a set of observed traces we present an example of a synthetic inversion for a low velocity anomaly representing a mantle plume and show that our method allows us to retrieve the major features of the perturbation very successfully our computational scheme does not demand excessive storage but the amount required does grow in proportion to the number of events and stations we thus investigate the effect of partitioning the procedure by initially performing trace by trace inversions and combining the information in a final step the fact that small amplitude phases are less well constrained in the preliminary inversions leads to a somewhat degraded final image nevertheless this technique is useful in situations where a single full inversion is impractical or where data become available over an extended period of time

a crustal seismic refraction study in west central arizona a reversed seismic refraction profile was recorded in the southern basin and range province between parker and globe arizona interpretation of both refracted and reflected phases indicates a two layered crust 23 25 km thick and a low pn velocity 7 67 km s pmp pn amplitude analysis on individual traces and synthetic seismogram modeling indicate that a positive velocity gradient may exist in the upper mantle the low pn velocity is indicative of partial melting in the upper mantle and implies that the crust is identical with the lithosphere the heat flow data suggest that the lithosphere was about 40 km thick approximately 5 m y ago on the basis of this and a negative free air gravity anomaly we propose that the lithosphere has been thinned from about 40 to 24 km over the past 5 m y and will probably thin and uplift further in the southern basin and range in arizona

daily geomagnetic variations on the iberian peninsula summary mean hourly values of the horizontal and vertical components of the geomagnetic field at six observatories on the iberian peninsula were analysed to study seasonal and spatial behaviour of the solar and lunar ionospheric and oceanic daily geomagnetic variations the results are tabulated and their characteristics are discussed the influence of local oceanic effects on the lunar oceanic contribution at the peninsular observatories is noted in particular the spatial behaviour of the lunar oceanic contribution is considered whereas the mean value for the peninsula is consistent with that expected according to an existing model for electric currents in the eastern atlantic ocean individual values are not the influence of tide in the mediterranean sea of water currents in the strait of gibraltar and of local effects in the atlantic ocean is proposed as the reason for this behaviour

receiver structure beneath the southern mojave block california an inversion of a receiver function recorded at lawrence livermore national laboratory nevada test site network station lac reveals a range of 1 d velocity structures that are consistent the observed waveform the shear velocity structure southeast of lac is relatively simple consisting of an approximately 30 to 34 km thick crust all models contain relatively sharp crust mantle boundaries with a change in velocity from about 3 5 km sec to approximately 4 6 km sec over a depth range of 4 to 6 kilometers the range of models can be roughly divided into two groups that differ mostly in the mid to lower crust the first model of the earth structure contains a large low velocity zone in the middle and lower crust shear wave velocities as low as 2 8 km sec the second model contains nearly constant velocities approximately 3 5 km sec through the middle to lower crust in contrast with the simplicity of the southeast receiver function the receiver functions corresponding to the northwest and southwest show the influence of lateral velocity heterogeneity and can at best be discussed qualitatively

regional gravity studies in southeastern california western arizona and southern nevada bouguer gravity anomaly low pass and band pass maps of southeastern california western arizona and southern nevada are analyzed to determine regional crustal structure based on 100 250 km band pass wavelength filtering of the bouguer anomaly map a low amplitude gravity maximum is associated with the highly extended terrane along the colorado river region of california arizona and nevada two crustal models constructed using bouguer gravity anomaly data and constrained by geological mapping aeromagnetic data and seismic refraction data are consistent with the dome shaped midcrustal lens of probable dioritic composition centered under the whipple mountains proposed by mccarthy et al 1989 although other models are not precluded this midcrustal lens extends from the western edge of the colorado river extensional corridor to east of the buckskin and hualapai mountains in arizona dioritic material is exposed in various mountain ranges in the colorado river extensional corridor and may be part of the source of the gravity maximum that is associated with each range

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

crustal structure across the san andreas fault southern california from teleseismic converted waves crustal structure along a 140 km long profile across the san andreas fault saf in southern california was imaged by stacking teleseismic p s converted phases recorded by a dense short period seismic array the crust mantle discontinuity moho is visible as a continuous feature at a depth around 30 km but is offset 6 to 8 km beneath the saf a small moho disruption can also be seen under the eastern california shear zone ecsz these results suggest that the saf and ecsz extend through the entire crust the moho upwarp under the san gabriel mountains indicates that the mountain ranges were lifted en masse as a result of crustal buckling under horizontal compression

constraints on crustal structure and complex moho topography beneath pinon flat california from teleseismic receiver functions abstract we use teleseismic p waves recorded at pinon flat observatory pfo to constrain the three dimensional crustal and upper mantle velocity structure beneath the station by forward modeling radial receiver function waveforms we construct a one dimensional crustal model that includes a significant shear velocity inversion at 9 km in depth arrivals on the tangential components indicate dip of at least the uppermost discontinuity complicated moho topography deepening to the northwest of pfo is suggested by azimuthal dependence of travel times and amplitudes of the receiver functions and travel times of crustal p wave reverberations although fine details cannot be resolved each of those sets of observations plus mislocation vectors provide strong indications of abrupt moho topography possibly including step offsets of several kilometers this is not only consistent with gravity data in implying airy isostasy with compensation at moho depth but extends that model to a much finer length scale than had been resolved

crustal structure of the borderland continent transition zone of southern california adjacent to los angeles we use data from the onshore offshore component of los angeles region seismic experiment larse to model the broad scale features of the midcrust to upper mantle beneath a north south transect that spans the continental borderland in the los angeles california region we have developed an analysis method for wide angle seismic data that consists primarily of refractions lacks near offset recordings and contains wide gaps in coverage although the data restrict the analysis to the modeling of broad scale structure the technique allows one to explore the limits of the data and determine the resolving power of the data set the resulting composite velocity model constrains the crustal thickness and location and width of the continent borderland transition zone we find that the mid to lower crust layer velocities of the inner borderland are slightly lower than the corresponding layers in the average southern california crust model while the upper mantle velocity is significantly higher the data require the moho to deepen significantly to the north we constrain the transition zone to initiate between the offshore slope and the southwest los angeles basin if the inner borderland crust is 22 km thick then the transition zone is constrained to initiate within a 2 km wide region beneath the southwest los angeles basin and have a width of 20 25 km the strong coherent and continuous pn phase suggests the moho is coherent and laterally continuous beneath the inner borderland and transition zone the inner california borderland seems to be modified and thickened oceanic crust with the oceanic upper mantle intact beneath it

ionospheric current system associated with the equatorial counterelectrojet the phenomenon of the equatorial counterelectrojet current cej is studied utilizing the d h and z components of the magnetic field at a chain of geomagnetic observatories in the indo russian zone extending from 0 to 75 n magnetic dip very significant changes in both h and d components are shown to occur on cej days at all stations even north of sq focus the current system associated with cej consists of two vortices one counterclockwise in the morning and another clockwise in the evening hours the counterelectrojet events during december solstices seem to be associated with the disappearance of the sq current loop in the indo russian sector and on a number of consecutive days the cause of the counterelectrojet is suggested to be the changes in various modes of the tidal movements in the upper atmosphere

applications of attenuation tomography to imperial valley and coso indian wells region southern california spatial variations in seismic wave attenuation were tomographically imaged from observed s to p wave amplitude ratios in the imperial valley and coso indian wells regions of southern california in the coso indian wells region a highly attenuating body s wave quality factor q 30 coincides with a slow p wave anomaly mapped by walck and clayton 1987 this coincidence suggests the presence of a 3 to 5 km depth magmatic or hydrothermal body in the indian wells region in the imperial alley slow p wave travel time anomalies and highly attenuating s wave anomalies were found in the brawicy seismic zone at a depth of 8 12 kin the effective s wave quality factor is very low q 20 and the p wave velocity is 10 slower than the surrounding area these results suggest either magmatic or hydrothermal intrusions or fractures at depth possibly related to active shear in the brawley seismic zone this attenuation tomographic technique is shown to be useful in delineating the spatial variations in seismic wave attenuation and in estimating the effective seismic quality factor of attenuation anomalies

seismological evidence for a sub volcanic arc mantle wedge beneath the denali volcanic gap alaska 1 arc volcanism in alaska is strongly correlated with the 100 km depth contour of the western aluetian wadati benioff zone above the eastern portion of the wadati benioff zone however there is a distinct lack of volcanism the denali volcanic gap we observe high poisson s ratio values 0 29 0 33 over the entire length of the alaskan subduction zone mantle wedge based on regional variations of pn and sn velocities high poisson s ratios at this depth 40 70 km adjacent to the subducting slab are attributed to melting of mantle wedge peridotites caused by fluids liberated from the subducting oceanic crust and sediments observations of high values of poisson s ratio beneath the denali volcanic gap suggest that the mantle wedge contains melted material that is unable to reach the surface we suggest that its inability to migrate through the overlying crust is due to increased compression in the crust at the northern apex of the curved denali fault

deep structure of southern california abstract we used 214 210 p wave arrival times from 7536 local earthquakes and 16 470 travel time residuals from 332 teleseismic events recorded by the southern california seismic network to determine a detailed three dimensional 3d p wave velocity structure of the crust and mantle down to 600 km depth beneath southern california in this study we have taken into account the moho topography under this region determined by a previous receiver function study we found that the undulations of the moho discontinuity affect considerably the tomographic images of the lower crust and uppermost mantle when the moho topography is taken into account ray paths and travel times can be computed more accurately the tomographic images show a very heterogeneous structure in the crust and upper mantle under southern california the velocity structure in the shallow depth correlates well with the surface geological features three major anomalies in the upper mantle are revealed clearly beneath the southern sierra nevada salton trough and transverse ranges compared with the previous result both the shape and size of the three anomalies show some differences our result revealed two prominent low velocity low v anomalies above the high velocity high v anomalies in the upper mantle under southern sierra nevada and the transverse ranges we consider that the upper mantle anomaly under the transverse ranges was formed through convergence and sinking of the entire subcrustal lithosphere vertical motions and removal of the dense crust root caused the high v anomaly under the southern sierra nevada the prominent low v anomalies above the two high v anomalies were either caused by the crustal materials pulled down to the uppermost mantle or by upwelling of partial melt in asthenosphere when the sinking of lithosphere occurs the salton trough low is the response to the lithospheric extension when the pacific plate was rifted away from the north american plate

modeling of short period surface wave propagation in southern california abstract for the 1992 landers mainshock the lateral heterogeneity of the lithosphere combined with an extended earthquake source may account for the complexity of the strong motion observations that cannot be easily interpreted by a simplified point source model in a laterally homogeneous medium both the effect of an extended source and that of lateral heterogeneity are examined using a forward modeling technique that combines distributed nonuniform slip on a fault plane in a 3d medium surface wave synthetics are constructed using surface wave gaussian beam method which allows for a mild degree of lateral heterogeneity we analyze the amplitude and waveform data of the landers strong motion recordings in southern california in order to identify the path effects and the effect of an extended source the overall source parameters of the landers earthquake are obtained by comparing synthetics with recordings at near source stations which lie within relatively homogeneous path structures this procedure yields a unilateral rupture propagating northwesterly over two distinct rupture segments with somewhat different focal mechanisms the surface wave phase velocity map is modified to fit the data recorded at larger hypocentral distances the final improved model generates a very good agreement between observations and synthetics in the presence of lateral heterogeneity the focusing defocusing and multipath interference are found to contribute significantly to the strong motion shaking in a 3d basin

3 d velocity structure of the san jacinto fault zone near anza california i p waves summary seismic arrival times from microearthquakes ml 4 on the san jacinto fault near anza california are used to find spatial variations in the seismic velocity that are related to the crustal structure of the fault zone preliminary modelling of the 1 d p wave velocity structure of the upper 25 km of crust reveals that most of the variation in velocity is lateral rather than depth dependent the traveltime anomalies due to lateral structure can be partially compensated for by applying station corrections however the variance of the traveltime residuals is still 2 25 times larger than the variance of the picking error the spatially correlated residuals show that this variance is due to localized velocity anomalies and that the data require further modelling using a 3 d velocity structure because the 3 d inverse problem is non unique smoothness constraints are applied to find the model that has the minimum structure required to fit the data to the picking error where a smooth model is defined such that the gradient of the velocity perturbation from the original 1 d model is small with small non zero station corrections a 3 d velocity model can be found that fits the data well the structure is well resolved from 3 to 9 km depth where lateral perturbations of up to 7 per cent are determined independently of the trade off between station corrections and poorly resolved near surface structure the model shows a horizontal gradient with overall faster velocities in the north east side of the fault zone at 3 6 km depth the signature of the fault zone is evident in the lower velocities beneath the surface trace of the fault however at 9 km depth higher seismic velocities are found extending into the fault zone from the north east block this higher velocity region occurs where there is a distinct lack of seismicity on the fault there is also a localized feature in the south west of the modelled region that is more than 10 km from the main trace of the fault with velocities 3 per cent slower than average

behavior of the quiet day ionospheric current system in the european region the ionospheric current system for magnetic quiet days deduced from geomagnetic field daily variations taken throughout europe at sunspot minimum has been studied special attention was paid to the selection of the representative quiet days showing the inadequacy of relying on the mean of the five international quiet days per month we used the technique of spherical cap harmonic analysis to separate the external and internal parts of the field variation from which the derivation of the equivalent current functions is straightforward this method provides a motion picture representation of the sq system through the screen formed by the spherical cap allowing the study of continuous variations with universal time detailed features of the current contours can be represented with such a regional analysis we found a strong day to day variability of the focus position along with some complexities that differ from the standard sq current behavior the average monthly or seasonal patterns are therefore averages of heterogeneous behaviors in many occasions possible explanations to these complex structures are considered

crustal deformation across and beyond the los angeles basin from geodetic measurements we combine 6 years of global positioning system gps data with 20 years of trilateration data and a century of triangulation taped distance and astronomic azimuth measurements to derive 66 interseismic station velocities in the greater los angeles region we interpolate the velocities to construct a regional strain rate map beyond the los angeles basin our results generally agree with the model proposed by the working group on california earthquake probabilities in 1995 important regional findings of this study are as follows 1 there is a significant n s convergence and e w extension about 0 22 and 0 17 0 05 strain yr respectively for the two components along the southern frontal fault system of the san gabriel mountains 2 the crustal deformation around the big bend of the san andreas fault saf cannot be explained solely by wrench style motion across the saf remaining motion could be part of a nw se extension which is the response to nfsw compression in the central transverse ranges region alternatively it could be caused by left lateral faulting on an oblique blind thrust beneath the san gabriel mountains 3 low strain rates axe found along the elsinore fault and newport inglewood 4 north south compression decreases from the raymond hill fault westward to the santa monica fault there is little east west extension along this fault system

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