crustal structure of the diablo and gabilan ranges central california a reinterpretation of existing data the compressional wave velocity structure of the crust of the coast ranges of central california was modeled from seismic refraction data reported previously for both the diablo range east of the san andreas fault and the gabilan range west of the fault two alternative velocity models were derived for each range by iterative two dimensional ray tracing each pair of models shows a sedimentary layer and an underlying crust composed of three or four layers for the diablo range the velocities in the sediments range from 2 9 to 4 8 km sec and the depth to basement varies from 0 2 km within the central diablo range to a maximum of 3 4 km north of the livermore valley in the gabilan range the velocities in the sediments overlying the granitic basement are higher north of the gabilan range 3 6 to 4 6 km sec than to the south 2 2 to 3 8 km sec and the depth to basement both north and south of the range is less than 2 km below the sediments and fractured near surface material a resolvable difference in the crustal velocity structure on opposite sides of the san andreas fault indicates compositional differences at depth the upper crust has an average velocity of 5 7 km sec at depths between 3 and 16 3 km in the diablo range and an average velocity of 6 1 km sec at depths between 2 and 9 1 km in the gabilan range the lower crust has an average velocity of 6 9 km sec at depths between 16 3 and 30 km in the diablo range and an average velocity of 6 5 km sec at depths between 10 and 24 km in the gabilan range the models also show that the depth to moho differs by several kilometers between the two ranges in the diablo range models the crust mantle boundary becomes shallower from south to north rising from a depth of 30 to 26 km in the gabilan range models the moho is at a depth of 24 to 26 km depending on the velocity assumed at the base of the crust laboratory measurements of rock velocities and the mapped surface geology allow us to interpret the velocity models in terms of crustal composition we conclude that the diablo range probably consists largely of metagraywacke to a depth of 16 3 km and gabbroic material below this depth the crust of the gabilan range probably consists of granitic material to a depth of 9 1 km and gneissic material below this depth franciscan rocks are not regionally present in the crust of the gabilan range if the gneissic lower crust consists of the same rocks as are found at the surface in the gabilan range then the presumed large scale lateral motion of the salinian block has taken place at or below the crust mantle boundary rather than along a mid crustal slip plane

crustal structure of the eastern borborema province ne brazil from the joint inversion of receiver functions and surface wave dispersion implications for plateau uplift we investigate the crustal structure of the borborema province of ne brazil by developing 44 s wave velocity depth profiles from the joint inversion of receiver functions and fundamental mode rayleigh wave group velocities the borborema province is located in the northeasternmost corner of the south american continent and represents a portion of a larger neoproterozoic mobile belt that formed during the brasiliano pan african orogeny extensional processes in the mesozoic eventually leading to the separation of africa and south america left a number of aborted rift basins in the continental interiors and episodes of diffuse intraplate volcanism and uplift marked the evolution of the province after continental breakup our velocity depth profiles reveal the existence of two crustal types in the province i the thin crustal type which consists of 30 32 5 km thick crust with an upper layer of 3 4 3 6 km s overlying a lower layer of 3 7 3 8 km s and ii the thick crustal type which consists of a 35 37 5 km thick crust with velocities between 3 5 and 3 9 km s down to 30 km depth and a gradational increase in velocity vs 4 0 km s down to upper mantle depths the crustal types correlate well with topography with the thick crustal type being mainly found in the high standing southern borborema plateau and the thin crustal type being mostly found in the low lying sertaneja depression and coastal cuestas interestingly the thin crustal type is also observed under the elevated topography of the northern plateau we argue that the thick crustal type is rheologically strong and not necessarily related to postbreakup mantle processes as it is commonly believed we propose that extensional processes in the mesozoic stretched portions of the brasiliano crust and formed the thin crustal type that is now observed in the regions of low lying topography leaving the rheologically strong thick crust of the southern plateau at higher elevations the crust making the northern plateau would have thinned and subsided during mesozoic extension as part of a greater sertaneja depression to then experience uplift in the cenozoic and achieve its present elevation

delineation of lithosphere structure and characterization of the moho geometry under the himalaya karakoram tibet collision zone using surface wave tomography

structure of the crust beneath the southeastern tibetan plateau from teleseismic receiver functions abstract southeastern tibet marks the site of presumed clockwise rotation of the crust due to the india eurasian collision and abutment against the stable sichuan basin and south china block knowing the structure of the crust is a key to better understanding of crustal deformation and seismicity in this region here we analyze recordings of teleseismic earthquakes from 25 temporary broadband seismic stations and one permanent station using the receiver function method we find that the crustal thickness decreases gradually from the tibetan plateau proper to the sichuan basin and yangtze platform but that significant intra crustal heterogeneity exists on shorter lateral scales 0 30 are detected beneath the southeastern margin of tibet but in the sichuan basin and southeastern yangtze platform the values are close to the global average combined with high regional heat flow and independent evidence for mid crustal layers of high electric conductivity the large intra crustal s wave velocity reduction 12 19 and the intermediate to high average crustal poisson s ratios are consistent with partial melt in the crust beneath parts of southeastern tibet these results could be used in support of deformation models involving intra crustal flow with the caveat that significant lateral variation in location and strength of this flow may occur

crustal velocities near coalinga california modeled from a combined earthquake explosion refraction profile crustal velocity structure for the region near coalinga california has been derived from both earthquake and explosion seismic phase data recorded along a nw se seismic refraction profile on the western flank of the great valley east of the diablo range comparison of the two data sets reveals p wave phases in common which can be correlated with changes in the velocity structure below the earthquake hypocenters in addition the earthquake records reveal secondary phases at station ranges of less than 20 km that could be the result of s to p wave conversions at velocity interfaces above the earthquake hypocenters two dimensional ray trace modeling of the p wave travel times resulted in a p wave velocity model for the western flank of the great valley comprised of a a 7 to 9 km thick section of sedimentary strata with velocities similar to those found elsewhere in the great valley 1 6 to 5 2 km sec b a middle crust extending to about 14 km depth with velocities comparable to those reported for the franciscan assemblage in the diablo range 5 6 to 5 9 km sec and c a 13 to 14 km thick lower crust with velocities similar to those reported beneath the diablo range and the great valley 6 5 to 7 30 km sec this lower crust may have been derived from subducted oceanic crust that was thickened by accretionary underplating or crustal shortening

seismic tomography and deformation modeling of the junction of the san andreas and calaveras faults local earthquake p traveltime data is inverted to obtain a three dimensional tomographic image of the region centered on the junction of the san andreas and calaveras faults the resulting velocity model is then used to relocate more than 17 000 earthquakes and to produce a model of fault structure in the region these faults serve as the basis for modeling the topography using elastic dislocation methods the region is of interest because active faults join it marks the transition zone from creeping to locked fault behavior on the san andreas fault it exhibits young topography and it has a good spatial distribution of seismicity the tomographic data set is extensive consisting of 1445 events 96 stations and nearly 95 000 travel time readings tomographic images are resolvable to depths of 12 km and show significant velocity contrasts across the san andreas and calaveras faulls a low velocity zone associated with the creeping section of the san andreas fault and shallow low velocity sediments in the southern santa clara valley and nonhem salinas valley relocated earthquakes only occur where vp s kmls and indicate that portions of the san andreas and calaveras faults are non vertical although we cannot complctcly cxclude the possibility hat all or parr of this rcsults from ray uacing problems the new dips are more consistent with geological observations that dipping faults intcrscct thc surfacc where surface traces have been mapped the topographic modeling predicts extensive subsidence in regions characterized by shallow low velocity material presumably the result of recent sedimentation some details of the topography at the junction of the san andreas and calaveras faults are not consisknl with lhe modeling results suggesting that the current position or this triple junction has changed with time the model also predicts those parts of the fault subject to contraction or extension perpendicular to the fault strike and hence the sense of any dip slip component in each locality the relative vertical motion accross the fault is consistent with the fault dips found with the new hypocentral locations

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

the crustal structure of the axis of the great valley california from seismic refraction measurements abstract in 1982 the u s geological survey collected six seismic refraction profiles in the great valley of california three axial profiles with a maximum shot to receiver offset of 160 km and three shorter profiles perpendicular to the valley axis this paper presents the results of two dimensional raytracing and synthetic seismogram modeling of the central axial profile the crust of the central great valley is laterally heterogeneous along its axis but generally consists of a sedimentary section overlying distinct upper middle and lower crustal units the sedimentary rocks are 3 5 km thick along the profile with velocities increasing with depth from 1 6 to 4 0 km s the basement upper crust consists of four units 1 1 a 1 0 1 5 km thick layer of velocity 5 4 5 8 km s 2 2 a 3 4 km thick layer of velocity 6 0 6 3 km s 3 3 a 1 5 3 0 km thick layer of velocity 6 5 6 6 km s and 4 4 a laterally discontinuous 1 5 km thick layer of velocity 6 8 7 0 km s the mid crust lies at 11 14 km depth is 5 8 km thick and has a velocity of 6 6 6 7 km s on the northwest side of our profile the mid crust is a low velocity zone beneath the 6 8 7 0 km s lid the lower crust lies at 16 19 km depth is 7 13 km thick and has a velocity of 6 9 7 2 km s crustal thickness increases from 26 to 29 km from nw to se in the model although an unequivocal determination of crustal composition is not possible from p wave velocities alone our model has several geological and tectonic implications we interpret the upper 7 km of basement on the northwest side of the profile as an ophiolitic fragment since its thickness and velocity structure are consistent with that of oceanic crust this fragment which is not present 10 15 km to the west of the refraction profile is probably at least partially responsible for the great valley gravity and magnetic anomalies whose peaks lie about 10 km east of our profile the middle and lower crust are probably gabbroic and the product of magmatic or tectonic underplating or both the crustal structure of the great valley is dissimilar to that of the adjacent diablo range suggesting the existence of a fault or suture zone throughout the crust between these provinces

ophiolitic basement to a forearc basin and implications for continental growth the coast range great valley ophiolite california we present a compilation of 18 published models from the length of the great valley forearc basin california based on seismic reflection borehole seismic refraction gravity and aeromagnetic data to address long standing questions about the nature of the basement to the great valley its origin its tectonic history and it s mechanism of incorporation into the north american continental margin the geophysical models permit a 700 km long 70 km wide complete ophiolite sequence beneath the entire great valley in the northern great valley the ophiolite is overlain by ophiolitic breccia the ophiolite crust is 7 8 km thick and the mantle section is mostly unserpentinized beneath the southern great valley there is no ophiolitic breccia the crust may be up to 10 12 km thick and the mantle section if present at all is serpentinized to such a degree that it cannot be distinguished from sierran basement or mafic ophiolite members on the basis of velocity or density data geochemical petrological and paleomagnetic data support suprasubduction zone ophiolite formation at north american paleolatitudes and geological data and geophysical models are consistent with ophiolite formation by back arc spreading behind an east facing arc in the north this was apparently followed by obduction of back arc crust onto older continental basement during the late jurassic nevadan orogeny in the south the newly formed intraoceanic arc and back arc apparently collided with the continental margin during the nevadan orogeny but were not obducted onto it instead the arc and back arc docked with the continental margin leaving the arc itself to become the basement to the great valley basin cretaceous sierran magmatism then intruded plutons beneath the docked ophiolite and mafic arc irrespective of the detailed accretionary history our cross sections show a rapid pulse of continental growth by ophiolite accretion of more than 500 km km 1 in less than 10 myr

shear wave velocity structure of the lower crust in southern africa evidence for compositional heterogeneity within archaean and proterozoic terrains 1 the nature of the lower crust across the southern african shield has been investigated by jointly inverting receiver functions and rayleigh wave group velocities for 89 broadband seismic stations located in botswana south africa and zimbabwe for large parts of both archaean and proterozoic terrains the velocity models obtained from the inversions show shear wave velocities 4 0 km s below 20 30 km depth indicating a predominantly mafic lower crust however for much of the kimberley terrain and adjacent parts of the kheis province and witwatersrand terrain in south africa as well as for the western part of the tokwe terrain in zimbabwe shear wave velocities of 3 9 km s are found below 20 30 km depth indicating an intermediate to felsic lower crust the areas of intermediate to felsic lower crust in south africa coincide with regions where ventersdorp rocks have been preserved suggesting that the more evolved composition of the lower crust may have resulted from crustal reworking and extension during the ventersdorp tectonomagmatic event at c 2 7 ga citation kgaswane e m a a nyblade j julia p h g m dirks r j durrheim and m e pasyanos 2009 shear wave velocity structure of the lower crust in southern africa evidence for compositional heterogeneity within archaean and proterozoic terrains j geophys res 114 b12304 doi 10 1029 2008jb006217

reappraisal of hypothetical franciscan thrust wedging at coalinga implications for tectonic relations along the great valley flank of the california coast ranges 1 correlation of the character and orientation of seismic reflectors imaged on profiles across the coalinga anticline with known surface geology indicates that local franciscan thrust wedging favored for past geodynamic interpretations of the 1983 coalinga earthquake is an untenable hypothesis the inference that a subhorizontal detachment surface of simple shear is present beneath the anticline is consequently unnecessary and unlikely in view of the coherent areal pattern of wrench folding within the southern diablo range elsewhere along the structural front of the california coast ranges at the western flank of the great valley interpretations of similar thrust wedging based in part on analogy with supposed structural relations at coalinga are equally questionable in a wider context the seismogenic behavior of blind thrust and reverse faults in california cannot be interpreted with the invalid concept of thrust wedging at coalinga as a guiding template for geotectonic interpretations

three dimensional modeling of intraplate stresses in the epicentral zone of the 21 may 1997 jabalpur earthquake of mw 5 8 central india abstract three dimensional intraplate stresses induced by topography and crustal density heterogeneities have been modeled for a region in central india latitude 21 26 n longitude 77 82 e that was affected by the 1927 son valley m w 6 4 and the 1997 jabalpur m w 5 8 earthquakes the modeled stresses show that these two events occurred in the zones of significant increase in gradients of the principal stress difference 1 3 contours the highest value of these differential stresses 90 mpa is obtained at 5 km depth beneath the epicentral zone of 1927 son valley earthquake whereas a value of 35 mpa is obtained at the focal depth 35 km of the 1997 jabalpur mainshock further depth sections of estimated differential stresses delineate a south dipping high stress body penetrating the whole crust probably a mafic intrusive intruded during the deccan volcanism episode beneath a zone bounded by narmada north fault in the north and narmada south fault in the south within which all aftershocks including mainshocks of the 1997 jabalpur and the 1927 son valley have occurred the modeled direction of principal stresses as well as inferred mode of faulting at 5 20 35 and 40 km depths correlates strongly with the long period 20 50 s centroid moment tensor solutions for the broadband recordings of the m w 5 8 jabalpur mainshock results suggest that estimated large intraplate stresses associated with an inferred mafic intrusive in the presence of high pore pressure resulted from the dehydration of the serpentinite minerals and deep brittle ductile transition may contribute to the failure of rock along the pre existing weak narmada south fault leading to the earthquakes in the deep crust it is proposed that the causal association of large intraplate stresses and the pre existing deep crustal faults would control the future locales of earthquakes in the narmada son lineament

a seismic refraction profile across the san andreas sargent and calaveras faults west central california in 1981 the united states geological survey recorded a seismic refraction profile across the southern santa cruz mountains in west central california to examine the shallow velocity structure of this seismogenic region this 40 km long profile which consisted of three shotpoints extended northeastward from near watsonville california to coyote lake crossing the san andreas sargent and calaveras faults this entire region is characterized by a highly heterogeneous upper crust west of watsonville 1 km of alluvium with a velocity of 2 12 km sec overlies a basement with a velocity 5 45 km sec the abrupt deepening of basement by 1 5 km just east of watsonville at a subsurface fault suggests that the zayante fault to the north and the vergeles fault to the south are connected the tertiary sediment at the san andreas fault is 2 5 km thick and has a velocity of 3 34 km sec the san andreas fault is not marked by any distinctive seismic velocity features whereas a 1 km wide low velocity zone is found at both the sargent and calaveras faults east of the sargent fault the basement surface forms a broad anticlinal structure with velocities ranging from 5 4 to 6 0 km sec from the anticlinal crest basement dips to the east beneath the santa clara valley and reaches a maximum depth of 1 km on the east side the overlying alluvium has a velocity of 2 7 km sec at the crest of the basement anticlinal structure a vertical low velocity zone coincident with exposed serpentine provides strong evidence of faulting

a seismic refraction study of the santa clara valley and southern santa cruz mountains west central california abstract two seismic refraction profiles were recorded in the santa clara valley region of central california to determine the upper crustal seismic velocity structure a reversed 8 km long profile was recorded across the valley near gilroy and an unreversed 38 km long profile was recorded in the southern santa cruz mountains the data indicate that the valley is wedge shaped in cross section with the basement dipping 10 to the east the alluvial fill of the valley has a maximum thickness of 1 5 km and has an average compressional wave velocity of 2 3 km sec the top of the basement has a velocity of 4 3 km sec the velocity reaches 6 0 km sec at a depth of 2 km the calaveras fault zone located due east of the valley was crossed by a short unreversed profile it appears to be characterized by a low velocity v p 2 7 km sec to a depth of 2 5 km the southern santa cruz mountain profile indicates a 4 6 km sec near surface refractor and a layer with a velocity of 6 1 km sec at 2 km depth thus considered together the similarity in velocity structure determined from the two profiles indicates that the franciscan rocks of the santa cruz mountains continue beneath the valley the presence of high compressional wave velocities 6 0 km sec at 2 km depth in this franciscan terrain can best be explained by an abundance of metabasalts greenstones at that depth these seismic observations indicate that an area of at least 120 km 2 is underlain by this high velocity basaltic material therefore the upper crustal structure in the study area differs dramatically from the franciscan terrain of the diablo range where greywackies and metagreywackies v p 5 4 km sec dominate in the uppermost crust

two dimensional velocity structure along the synclinal axis of the great valley california abstract in 1981 and 1982 the u s geological survey conducted a seismic refraction study of the great valley central california in order to obtian a better understanding of the deep crustal structure and tectonic evolution of the region we present the interpretation of a 147 km long seismic refraction profile recorded along the synclinal axis of the valley the northern 98 km of the profile are reversed whereas the remaining southern portion is unreversed observed travel times and amplitudes of primary and secondary arrivals from three shot points have been modeled using two dimensional ray tracing techniques the resultant p wave velocity cross section consists of a 27 km thick crust whose isovelocity contours generally rise gently to the southeast the crust is described in six layers with average velocities average layer thickness in parentheses of 2 8 km sec 3 km 4 25 km sec 3 km 5 5 km sec 2 km 6 1 km sec 6 km 6 75 km sec 6 km and 7 2 km sec 7 km the second through fifth layers are separated by 1 km thick high gradient transition zone the lower crustal boundary 6 75 7 2 km sec and the moho 7 2 8 1 km sec are modeled with first order discontinuities the first layer cenozoic sedimentary deposits is characterized by a velocity gradient of 0 7 km sec km starting with a velocity of 1 7 km sec at the surface the second layer 4 25 km sec consists of the cretaceous great valley sequence and shows a very modest velocity gradient of 0 2 km sec km the third layer 5 5 km sec basement may consist of metasedimentary rocks of the franciscan assemblage fractured crystalline rocks or possibly serpentinized ophiolite the fourth layer 6 1 km sec apparently has no velocity equivalent 30 km to the west in the diablo range it and the layer beneath it 6 75 km sec may be correlated with the metamorphic and igneous rocks of the eastern foothills of the sierra nevada the basal crustal layer 7 2 km sec is evidently mafic igneous rock an independent interpretation of this profile using the reflectivity method produced a similar model an interpretation of a parallel profile 10 to 15 km east of our line shows a structure similar to our model below 15 km but a thinner sedimentary section and velocities as much as 0 9 km sec faster 6 to 15 km deep the crustal velocity structure of the central great valley is atypical of either continental or oceanic crust but is consistent with the geologic hypotheses that the valley is a thickened marginal basin formed in either a forearc or backarc setting located west of the mesozoic sierra nevadan arc

crustal structure beneath the indochina peninsula from teleseismic receiver functions 1 we analyze teleseismic receiver functions to determine the crustal structure beneath the indochina peninsula which is located immediately south east of the eastern himalayan syntaxis basin we found that the indochina peninsula is characterized by a thin 31 km crust with a low vp vs ratio 1 68 the intra lower crustal low velocity zone lvz is observed beneath the northwestern part of our study region we hypothesize that it is an extension of the lower crustal lvz observed beneath the southeastern tibet and the south china block and that it terminates at the dien bien phu fault dbpf a lvz observed in the upper crust beneath southeast of the dbpf indicates that the crust is ductile and earthquakes are rare citation bai l x tian and j ritsema 2010 crustal structure beneath the indochina peninsula from teleseismic receiver functions geophys res lett 37 l24308 doi 10 1029 2010gl044874

temporal changes in the q of broadband p waves transmitting through a fracturing westerly granite sample under triaxial compressive conditions abstract we analyzed temporal changes in the quality factor q of p waves transmitting through a fracturing westerly granite sample under triaxial compressive conditions using the rise time of the initial wave broadband recording was performed throughout even after the differential stress reached the peak strength we calibrated a transducer assembly and corrected the recorded waveforms using theoretical waveforms we derived a new equation relating q p the rise time for the velocity waveform and the travel time q p remained at 23 before the start of dilatancy and decreased to 17 after the peak stress was reached a large nonlinear decrease in q p was not clearly observed while the first wave amplitude clearly decreased rapidly after the occurrence of the peak stress

estimation of earthquake source parameters in the kachchh seismic zone gujarat india from strong motion network data using a generalized inversion technique the generalized inversion of s wave spectral amplitude data from 18 strong motion sites in the kachchh seismic zone gujarat india has been carried out to estimate source parameters of 38 aftershocks m w 2 93 5 32 from the 2001 bhuj earthquake m w 7 7 the result shows that the seismic moment m and source radius r of aftershocks are between 3 1 1013 and graphic 1 and 226 889 m respectively while stress drops vary from 0 11 to 7 44 mpa the regression analysis between m and r shows a break in linear scaling at graphic 2 the estimated stress drop values scatter significantly with m graphic 3 for larger aftershocks graphic 4 but vary in proportion with m graphic 5 in case of smaller magnitude events this can be due to the complex rupture process associated with relatively larger aftershocks in comparison to circular rupture for smaller events the high stress drop values are observed for events located on the north wagad fault at 15 30 km depths and gedi fault at 0 5 km depths this could be attributed to the large stress develop at the hypocentral depth as a result of high pore fluid pressure beneath these two fault zones the regression between m and corner frequency f c shows m is proportional to graphic 6 which is consistent for earthquakes with short rupture lengths and high stress drop values high stress drop values commonly associated with the events from kachchh and adjacent narmada son lineament rift zones indicate strong potential for generating high peak ground accelerations and hence high seismic hazard 1 embed inline graphic 1 gif 2 embed inline graphic 2 gif 3 embed inline graphic 3 gif 4 embed inline graphic 4 gif 5 embed inline graphic 5 gif 6 embed inline graphic 6 gif

lithospheric structure across the central tien shan constrained by gravity anomalies and joint inversions of receiver function and rayleigh wave dispersion abstract shear wave velocity structure across the central tien shan orogeny was generated by jointly inverting rayleigh wave phase and group velocity with teleseismic p wave receiver functions at 40 broadband seismic stations the inferred seismic structure was validated by forward modeling of the complete bouguer anomaly data the joint inversion result reveals larger crust thicknesses beneath the kokshaal 68 72 km and kyrgyz ranges 62 64 km while other units have crustal thicknesses between 48 and 58 km a fast velocity layer vs 3 6 3 9 km s in the upper crust is found in some seismic stations within the kazakh shield our models show the presence of high velocity and density layers in the lowermost crust throughout the region consistent with the presence of mafic ultramafic lithologies the large crustal thickness is associated with a thickened mafic layer in the lower crust indicating that the thickened crust may be partly caused by magmatic underplating the low velocity and density anomaly in the middle crust and low upper mantle velocity observed in our model beneath the middle tien shan reflect the presence of partial melt in the crust due to the intrusion of hot mantle material the lack of correlation between moho depth and topography together with the gravity results suggests that the topographic compensation in the central tien shan is not confined to the crust this requires significant support from the mantle to account for the relative high elevation of the middle tien shan

crustal structure of the khartoum basin sudan abstract the crustal structure of the northern part of the khartoum basin has been investigated using data from 3 permanent seismic stations within 40 km of khartoum and two modeling methods h k stacking of receiver functions and a joint inversion of receiver functions and rayleigh wave group velocities the khartoum basin is one of several mesozoic rift basins in sudan associated with the central african rift system results from the h k stacking indicate that crustal thickness beneath the khartoum basin ranges between 33 and 37 km with an average of 35 km and that the crustal vp vs ratio ranges from 1 74 to 1 81 with an average of 1 78 from the joint inversion of receiver functions and rayleigh wave group velocities we obtained similar results for moho depth as well as an average shear wave velocity of 3 7 km s for the crust these results provide the first seismic estimates of moho depth for a basin in sudan when compared to average crustal thickness for unrifted proterozoic crust in eastern africa our results indicate that at most only a few km of crustal thinning may have occurred beneath the khartoum basin this finding is consistent with estimates of effective elastic plate thickness which indicate little modification of the proterozoic lithosphere beneath the basin and suggests that there may be insufficient topography on the lithosphere asthenosphere boundary beneath the sudanese basins to channel plume material westward from ethiopia

about the lithospheric structure of central tibet based on seismic data from the indepth iii profile signals from 11 shots and 8 earthquakes and numerous teleseismic events were recorded along the 400 km seismic line indepth iii in central tibet and interpreted together with previous seismic and tectonic data the abnormal behavior of various mantle phases reveals a complex moho transition zone especially in the northern part of the line in the changtang block where the lower crust and the mantle show unusually low velocities a shingled appearance of pn and no low velocity layer in the upper crust the strong east west anisotropy in the changtang block is related to an easterly escape movement of the whole lithosphere facilitated by the warm and weak layers in the lower crust and the upper mantle bounded apparently by two prominent west east running fault zones

crustal and uppermost mantle velocity structure along a profile across the pamir and southern tien shan as derived from project tipage wide angle seismic data summary utilizing seismic refraction wide angle reflection data from 11 approximately in line earthquakes 2 d p and s velocity models and a poisson s ratio model of the crust and uppermost mantle beneath the southern tien shan and the pamir have been derived along the 400 km long main profile of the tipage tien shan pamir geodynamic program project these models show that the crustal thickness varies from about 65 5 km close to the southern end of the profile beneath the south pamir through about 73 6 km under lake karakul in the north pamir to about 57 7 km 50 km south of the northern end of the profile in the southern tien shan average crustal p velocities are low with respect to the global average varying from 6 26 to 6 30 km s 1 the average crustal s velocity varies from 3 54 to 3 70 km s 1 along the profile and thus average crustal poisson s ratio varies from 0 23 beneath the central pamir in the south central part of the profile to 0 265 towards the northern end of the profile beneath the southern tien shan the main layer of the upper crust extending from about 2 km below the earth s surface to 27 km depth below sea level b s l has average p velocities of about 6 05 6 1 km s 1 except beneath the south central part of the profile where they decrease to around 5 95 km s 1 this is in contrast to the s velocities which range from 3 4 to 3 6 km s 1 and exhibit the highest values of 3 55 3 6 km s 1 where the p velocity is lowest thus for the main layer of the upper crust is 0 26 beneath the profile except beneath the south central part of the profile where it decreases to 0 22 the low value of 0 22 for under the central pamir the along strike equivalent of the qiangtang terrane in tibet is similar to that within the corresponding layer beneath the northern lhasa and southern qiangtang terranes in central tibet and is indicative of felsic rocks rich in quartz in the state the lower crust below 27 km b s l has p velocities ranging from 6 1 km s 1 at the top to 7 1 km s 1 at the base further for this layer is 0 27 0 28 towards the northern end of the profile but is low at about 0 24 beneath the central and southern parts of the profile which is similar to the situation found in the northeast tibetan plateau the low values can be explained by felsic schists and gneisses in the upper part of the lower crust transitioning to granulite facies and possibly also eclogite facies metapelites in the lower part within the uppermost mantle the average p velocity is about 8 10 8 15 km s 1 and is about 0 26 assuming an isotropic situation then a relatively cool 700 800 c uppermost mantle beneath the profile is indicated this would in turn indicate an intact mantle lid beneath the profile an upper mantle reflector dipping from 104 km b s l 120 km from the southern end of the profile to 86 km b s l 155 km from the northern end of the profile has also been identified the preferred model presented here for the crustal and lithospheric mantle structure beneath the pamir calls for nearly horizontal underthrusting of relatively cool indian mantle lithosphere the leading edge of which is outlined by the pamir seismic zone this cool indian mantle lithosphere is overlain by significantly shortening warm asian crust the moho trough that is a feature seen beneath some other orogenic belts for example the alps and the urals beneath the northern pamir may mark the southern tip of the actively underthrusting tien shan crust along the main pamir thrust

cenozoic volcanism in the california coast ranges numerical solutions we present the results of numerical simulations of cenozoic volcanism in the coast ranges of california associated with the northward migration of the mendocino triple junction mtj three aspects of the thermal evolution and magmatism in the wake of the mtj are investigated 1 mantle flow and thermal perturbation in the slabless window 2 pressure release partial melting in the upwelling asthenosphere and 3 thermally induced crustal anatexis numerical results show that upwelling of asthenosphere in the slabless window causes a drastic change in the thermal structure of the lithosphere and results in significant pressure release partial melting in the upwelling asthenosphere up to 4 5 km of basaltic magma may be produced most of this mafic magma is likely stored near the base of the crust thermally induced crustal anatexis mainly occurs in the deep crust below 20 km where a high degree 50 of partial melting may be predicted most of the crustal melt is probably generated within 1 m y after the passage of the mtj this work also suggests that the spatial and temporal distribution of cenozoic volcanism in the california coastal ranges is directly related to the variation of the relative velocity between the north america and pacific plates in the last 30 m y the voluminous volcanic rocks in the northern california coast ranges correlate to the relatively high velocity between these two plates in the last 10 m y the reduced volcanism in the central coast ranges may be related to the slow migration of the mtj between 10 and 20 ma

s wave velocity structure below central mexico using high resolution surface wave tomography shear wave velocity of the crust below central mexico is estimated using surface wave dispersion measurements from regional earthquakes recorded on a dense 500 km long linear seismic network vertical components of regional records from 90 well located earthquakes were used to compute rayleigh wave group velocity dispersion curves a tomographic inversion with high resolution in a zone close to the array obtained for periods between 5 and 50 s reveals significant differences relative to a reference model especially at larger periods 30 s a 2 d s wave velocity model is obtained from the inversion of local dispersion curves that were reconstructed from the tomographic solutions the results show large differences especially in the lower crust among back arc volcanic arc and fore arc regions they also show a well resolved low velocity zone just below the active part of the trans mexican volcanic belt tmvb suggesting the presence of a mantle wedge low densities in the back arc inferred from the low shear wave velocities can provide isostatic support for the tmvb

comprehensive crustal structure and seismological evidence for lower crustal flow in the southeastern margin of tibet revealed by receiver functions abstract the lower crustal flow model has become an interesting proposal to describe the growth and expansion of eastern tibet to date however the geophysical results are not definitive due to the non uniqueness of the solutions found through data inversion and the resolution limitations in this study the polarity of p to s converted phases at intra crustal interfaces are analyzed in order to reveal evidence for low seismic velocity and lower crustal flow in southeastern tibet we are interested in a joint study that is both a mapping of the crustal structure moho topography and poisson s ratio and an attempt to constrain an iclvz we have determined the crustal thickness and poisson s ratio from p prfs and s srfs receiver functions acquired in 108 permanent seismic stations deployed in the sichuan and yunnan areas also we have obtained 98 measurements of pms phase splitting parameters at the array stations the crustal thickness provided by prfs is consistent with that given by srfs it varies from 60 km in the songpan ganzi sg fold system and the northern part of the sichuan yunnan diamond shaped block sydsb near the eastern himalayan syntaxis to 33 km in southern yunnan the poisson s ratio varies mostly from 0 24 to 0 32 the highest values 0 28 0 32 are found along the axis formed by the longmenshan fault the lijiang jinhe fault and in the tengchong volcano area the value of 0 28 observed in sydsb and the sg fold system is attributed to the presence of a low velocity zone in the lower crust and partly to the great sediment thickness in sichuan basin the crust is clearly anisotropic in the southeastern margin of tibet the fast wave polarization direction exhibits a nw se regional bias while the time delay varies between 0 07 s and 1 27 s being the average 0 54 s 0 12 s implying the existence a strong shear zone in the lower crust based on the negative polarity observed after the direct p wave in teleseismic receiver functions we have detected an intra crustal low velocity zone iclvz in a wide region including east and southeast of tibet and southern yunnan so both the shear zone and iclvz suggest the existence of lower crustal flow the reverberation ppps and psps ppss phases at the moho are clearly weaker in sydsb and the sg fold system than in the indochina block and eastern yunnan implying that sydsb has a structure and tectonic history different from indochina on its west side and eastern yunnan on its east side the above results provide seismological evidence for a lower crustal flow spreading from the eastern tibet and that in its advance southward has invaded sydsb widely crossed the lijiang jinhe fault and reached southern yunnan

w and m type tetrad effects in ree patterns for water rock systems in the tono uranium deposit central japan abstract abundances of rare earth elements plus y ree were determined for granitic rocks tertiary sedimentary rocks and related groundwater in the tono area central japan tetrad effects concave curves in ree patterns were found for these rocks and groundwater samples conjugate m and w types of tetrad effects were simultaneously observed in these specimens which constitute the water rock system the granitic rocks show the m type tetrad effects while the groundwater and the sedimentary rocks exhibit the w type tetrad effects non chondritic y ho ratios were found especially in the groundwater y ho ratios showed larger values than the chondritic value for the samples showing w type tetrad effects and vice versa these results suggest that 1 preference of the groundwater for a w type tetrad effect and larger y ho ratio than chondrite produces an m type tetrad effects and lower y ho ratios in the granitic rocks during weathering processes 2 the w type tetrad effect and larger y ho ratio for the sedimentary rocks is brought about by the fixation of groundwater ree onto the sedimentary rocks the variation of the tetrad effect and y ho ratio was confirmed by laboratory experiments on the leaching of ree from the granitic rocks the w type tetrad effect and large y ho ratio in the u mineralization zone in the sedimentary rocks were also found by sims analysis which is consistent with the w type tetrad effect and larger y ho ratio in the whole rock abundances of the sedimentary rocks disequilibrium of the u series 234 u 238 u analyzed by sims in the u mineralization zone suggests that the u bearing minerals were formed through precipitation from groundwater as it became reducing these results suggest that the ree in the sedimentary rocks were transported by the groundwater from the granitic rocks in the tono area based on this study it is implied that the degree of the tetrad effects and the variation of y ho ratio can be valuable tools to investigate the migration of ree during water rock interactions in natural systems

crustal structure beneath the eastern part of the coast ranges diablo range of central california from explosion seismic and near earthquake data abstract seismic refraction and near earthquake data of the u s geological survey for central california have been compiled into record sections along profiles and interpreted in terms of crustal structure the profiles are located northeast of the san andreas fault of central california and run parallel to the general structures for the explosion seismic line through the centre of the diablo range an uppermost layer franciscan formation with p velocities of 3 6 5 0 km s 1 decreases in thickness towards the northwest the lower boundaries of layers with constant velocities of 5 75 and 6 8 km s 1 are found at almost constant depths of 12 and 21 km respectively between 21 and 26 km depth a well defined low velocity zone appears whose velocity is estimated as 5 3 km s 1 with the aid of a hedgehog inversion and the calculation of amplitudes this zone is underlain by a layer 3 5 km thick with a velocity of 7 6 km s 1 the upper mantle velocity beneath the moho at 29 30 km depth is 8 2 km s 1 the near earthquake profiles located 20 km southwest and parallel to the explosion seismic line follow more or less the hayward and calaveras fault systems the velocity depth distribution derived for the earthquake data is very similar to that found beneath the diablo range however the low velocity zone at 21 26 km depth does not seem to exist everywhere along the line the moho is not disturbed beneath the calaveras hayward and silver creek faults it rises slightly from the diablo range towards the southwest

the northern limit of the subducted juan de fuca plate system analysis of data recorded at an array of three component broadband seismograph stations deployed on northern vancouver island and the adjacent british columbia mainland at the northern end of the cascadia subduction zone provides the first constraints on the s wave velocity structure of this region and permits us to define the northern limit of the subducted juan de fuca plate system during a 2 year period more than 80 teleseisms were recorded at our five stations the method of receiver function analysis was used to constrain the s velocity structure to upper mantle depths beneath the northern three stations a relatively simple continental crust is interpreted with a well defined moho near 37 39 km depth an upper crustal s velocity discontinuity at these stations is interpreted as the top of the high velocity rocks of the wrangellia terrane in contrast more complicated structure dominated by pronounced low velocity zones dipping to the ne are interpreted beneath our southern two stations the shallower low velocity zone is 6 8 km thick has an s velocity contrast of 0 6 1 1 km s and lies within the continental crust this feature is similar to a pronounced low velocity layer the e zone imaged beneath southern vancouver island the deeper low velocity zone is interpreted as the subducted oceanic crust we interpret the pronounced change in s velocity structure that we observe as the northern limit of the subducted oceanic plate beneath vancouver island this change coincides with significant changes in topography heat flow gravity and geochemistry

crustal structure of the southern calaveras fault zone central california from seismic refraction investigations a magnitude 5 7 earthquake on 6 august 1979 within the calaveras fault zone near coyote lake of west central california motivated a seismic refraction investigation in this area a northwest southeast profile along the fault as well as two fan profiles across the fault were recorded to examine the velocity structure of this region the analysis of the data reveals a complicated upper crustal velocity structure with strong lateral variations in all directions the near surface layers consist of recent sediments with seismic p wave velocities of 2 6 to 3 2 km sec these are underlain by rocks of the great valley sequence which have an average velocity of 4 5 km sec the great valley sequence is present along the whole profile depths range from 4 3 to 4 8 km in the northwest near anderson lake and in the southeast of the profile line near hollister in the middle of the profile near coyote lake however this layer thins and we find a laterally limited higher velocity layer 5 1 km sec between depths of 2 8 to 4 8 km the high velocity zone which coincides with a gravity high also correlates spatially with the hypocentral area of the 6 august earthquake and its aftershocks and may therefore represent an asperity on the fault velocities within the fault zone were determined from the fan profiles near anderson lake a pronounced delay of first arrivals on the fan records indicates a vertical 1 to 2 km wide near surface low velocity zone along the fault near coyote lake the delays observed in the fan records correlate with two subsurface en echelon fault planes which have been previously identified from lineations in the seismicity pattern the structure of the lower crust is similar to the neighboring diablo range a 8 to 9 km thick upper crustal layer with a seismic velocity of 5 7 to 6 3 km sec is underlain by a 3 km thick layer with velocity 6 8 km sec in accordance with previous studies of the diablo range there are indications of a pronounced lower crustal low velocity zone between a depth of 17 and 23 km the presence of this low velocity zone suggests that higher velocity igneous rocks of the middle crust have been thrust over sedimentary rocks

a transect across the mesozoic accretionary margin of central california subductive accretion formed along the pacific margin of north america in the mesozoic we are exploring deep structure across this accreted margin using coordinated reflection and detailed refraction profiling gravity and magnetic interpretation together with surface and well geology although none of these techniques alone is sufficient to define the critical elements of structure in complement they place important constraints on geologic interpretation for both the foothill rocks and particularly the franciscan assemblage we find evidence of obductive emplacement overthrusting from the west in contrast to the eastward subduction of most previous views these surprises emphasize the importance of such geophysical studies to geologic analysis

lithospheric structure of the arabian shield from the joint inversion of receiver functions and surface wave group velocities abstract we estimate lithospheric velocity structure for the arabian shield by jointly modeling receiver functions and fundamental mode group velocities from events recorded by the 1995 1997 saudi arabian portable broadband deployment receiver functions are primarily sensitive to shear wave velocity contrasts and vertical travel times and surface wave dispersion measurements are sensitive to vertical shear wave velocity averages so that their combination bridge resolution gaps associated with each individual data set our resulting models correlate well with the observed surface geology the asir terrane to the west consists of a 10 km thick upper crust of 3 3 km s overlying a lower crust of 3 7 3 8 km s in the afif terrane to the east the upper crust is 20 km thick and has an average velocity of 3 6 km s and the lower crust is about 3 8 km s separating the terranes the nabitah mobile belt is made of a gradational upper crust up to 3 6 km s at 15 km overlying an also gradational lower crust up to 4 0 km s the crust mantle transition is found to be sharp in terranes of continental affinity east and gradual in terranes of oceanic affinity west the upper mantle shear velocities range from 4 3 to 4 6 km s temperatures around 1000 c are obtained from our velocity models for a thin upper mantle lid observed beneath station taif and suggest that the lithosphere could be as thin as 50 60 km under this station