mantle flow under the western united states from shear wave splitting we show that sks splitting in the westernmost united states polarization of the fastest shear waves and splitting times including their back azimuthal dependence can be explained by a geodynamic model that includes a continuum mechanics description of plate motions and underlying asthenospheric circulation models that include a counterflow at depths of 300 km are preferred which may indicate a far field effect of the farallon slab anomaly sinking underneath the central continental united states this finding is broadly consistent with earlier suggestions and we demonstrate that a mechanically coupled system though with a strong viscosity contrast with depth is consistent with the data we explore the depth dependence of predicted anisotropy by means of computing seismogram synthetics and comparing synthetic splits with observations some patterns in the data including null observations are matched well linked models of geodynamic flow and mineral alignment in the mantle provide a means to test the relationship between strain and the saturation of texturing lower fabric saturation strains than for global models are preferred by the data which may reflect the relatively active tectonic setting and thin asthenosphere of the study region in general our results show that seismic anisotropy when interpreted jointly with mineral physics theories may be used to quantitatively constrain the spatial character of flow and the degree of force coupling at depth

surface wave tomography of the western united states from ambient seismic noise rayleigh and love wave phase velocity maps summary we present the results of rayleigh wave and love wave phase velocity tomography in the western united states using ambient seismic noise observed at over 250 broad band stations from the earthscope usarray transportable array and regional networks all available threecomponent time series for the 12 month span between 2005 november 1 and 2006 october 31 have been cross correlated to yield estimated empirical rayleigh and love wave green s functions the love wave signals were observed with higher average signal to noise ratio snr than rayleigh wave signals and hence cannot be fully explained by the scattering of rayleigh waves phase velocity dispersion curves for both rayleigh and love waves between 5 and 40 speriod were measured for each interstation path by applying frequency time analysis the average uncertainty and systematic bias of the measurements are estimated using a method based on analysing thousands of nearly linearly aligned station triplets we find that empirical green s functions can be estimated accurately from the negative time derivative of the symmetric component ambient noise cross correlation without explicit knowledge of the source distribution the average traveltime uncertainty is less than 1 s at periods shorter than 24 s we present rayleigh and love wave phase speed maps at periods of 8 12 16 and 20 s the maps show clear correlations with major geological structures and qualitative agreement with previous results based on rayleigh wave group speeds

complex upper mantle anisotropy in the pacific northwest evidence from sks splitting abstract we use a dense network of observations and an automated method of analysis to investigate complex patterns of seismic anisotropy in the pacific northwest we present sks splitting results for approximately 220 broadband seismic stations including 33 stations from the new wallowa2 array deployed between 2016 and 2018 in northeast oregon our data set contains approximately 3300 splitting measurements our anisotropy measurements indicate that anisotropy in the pacific northwest can be divided into two domains near the subduction zone anisotropy is perpendicular to the slab and inboard of the subduction zone anisotropy tends to parallel the velocity structure and is roughly oriented with absolute plate motion however splitting analysis performs poorly in northeast oregon and results are not consistent with uniform or layered anisotropy we argue that the complex splitting behavior in northern oregon is a result of spatially variable upper mantle anisotropy related to the complex upper mantle seismic structure interpreting the mantle anisotropy in this region requires methods not limited to standard ray theoretical shear wave splitting analysis

seismic anisotropy in the continental crust of northwestern canada s u m m a r y most studies of the seismic structure of continental crust assume that the wave speeds are isotropic at seismic wavelengths the ability to measure surface wave propagation speed from the cross correlation of ambient seismic noise provides new opportunities to image the crust and uppermost mantle we investigate radial anisotropy in the continental crust of northwestern canada from group velocity curves of love and rayleigh waves obtained from ambient noise cross correlation we test the null hypothesis that the love and rayleigh group speed curves can be simultaneously fit by an earth model containing isotropic seismic velocities throughout the crust group velocity is predicted for 200 000 one dimensional earth models which are generated by randomly varying the crustal shear velocity and radial anisotropy within a prescribed range the goodness of fit of the predictions is assessed by comparison with two sets of observed dispersion curves that correspond to two tectonically distinct terranes the archean early proterozoic craton and the transition from craton to cordillera the majority of best fitting models contain vsh vsv 4 5 per cent in the middle crust the finding that the middle lower crust is seismically anisotropic across a large swath of northwestern canada combined with recent observations of anisotropic crust in much of the western united states suggests that anisotropy may be ubiquitous in the continental crust

toroidal mantle flow through the western u s slab window the circular pattern of anisotropic fast axis orientations of split sks arrivals observed in the western u s cannot be attributed reasonably to either preexisting lithospheric fabric or to asthenospheric strain related to global scale plate motion a plume origin for this pattern accounts more successfully for the anisotropy field but little evidence exists for an active plume beneath central nevada we suggest that mantle flow around the edge of the sinking gorda juan de fuca slab is responsible for creating the observed anisotropy seismic images and kinematic reconstructions of gorda juan de fuca plate subduction have the southern edge of this plate extending from the mendocino triple junction to beneath central nevada and flow models of narrow subducted slabs produce a strong toroidal flow field around the edge of the slab consistent with the observed pattern of anisotropy this flow may enhance uplift extension and magmatism of the northern basin and range while inhibiting extension of the southern basin and range

seismic evidence for lithospheric modification beneath the mojave neovolcanic province southern california 1 the mojave neovolcanic province mnvp located in the mojave block of southern california comprises late miocene to quaternary small volume basaltic centers geochemistry indicates an asthenospheric source for the mnvp beginning in the late miocene but no physical evidence of missing mantle lithosphere has been presented we utilize receiver functions and ambient noise tomography to image the lithosphere beneath the mojave block regionally we find thin crust that thickens distal to sites of mnvp volcanism shear wave velocities between 40 and 75 km depth are consistent with the presence of mantle lithosphere in the southern mojave block and very thin or missing mantle lithosphere to the north with one exception mnvp volcanoes lie along this sharp boundary our observations together with the established geologic history and geochemistry of the mnvp can be explained by small scale edge driven convection producing ongoing lithospheric removal within the mojave block our results provide another example of lithospheric instability that occurs in response to rapid changes in mantle dynamics induced by major changes in tectonic plate geometry

crust and upper mantle structure beneath the pacific northwest from joint inversions of ambient noise and earthquake data 1 we perform a joint inversion of phase velocities from both earthquake and ambient noise induced rayleigh waves to determine shear wave velocity structure in the crust and upper mantle beneath the pacific northwest we focus particularly on the areas affected by mid miocene to present volcanic activity the joint inversion combined with the high density seismic network of the high lava plains seismic experiment and data from the earthscope transportable array provides outstanding resolution for this area in oregon we find that the pattern of low velocities in the crust and uppermost mantle varies between the high lava plains physiographic province and the adjacent northwestern basin and range these patterns may be due to the presence of the brothers fault zone which separates the clockwise rotating northwest basin and range from the relatively undeformed areas further north further to the east the owyhee plateau snake river plain srp and northeastern basin and range are characterized by high crustal velocities though the depth extent of these fast wave speeds varies by province of particular interest is the mid crustal high velocity sill previously only identified within the srp we show this anomaly extends significantly further south into utah and nevada we suggest that one possible explanation is lateral crustal extrusion due to the emplacement of the high density mafic mid crustal sill structures within the srp

three dimensional shear velocity anisotropic model of piton de la fournaise volcano la reunion island from ambient seismic noise we cross correlate 4 years of seismic noise from the seismic network of piton de la fournaise volcano la reunion island to measure the group velocity dispersion curves of rayleigh and love waves we average measurements from vertical and radial components to obtain 577 rayleigh wave dispersion curves the transverse components provided 395 love wave dispersion curves we regionalize the group velocities measurements into 2 d velocity maps between 0 4 and 8 s finally we locally inverted these maps for a pseudo 3 d anisotropic shear velocity model down to 3 km below the sea level using a neighborhood algorithm the 3 d isotropic shear wave model shows three distinct high velocity anomalies surrounded by a low velocity ring the anomaly located below the present plaine des sables could be related to an old intrusive body at the location of the former volcanic center before it migrated toward its present location the second high velocity body located below the summit of the volcano likely corresponds to the actual preferential dyke intrusion zone as highlighted by the seismicity the third high velocity anomaly located below the grandes pentes and the grand brule areas and is an imprint of the solidified magma chamber of the dismantled les alizes volcano radial anisotropy shows two main anomalies positive anisotropy above sea level highlighting the recent edifice of piton de la fournaise with an accumulation of horizontal lava flows and the second one below the sea level with a negative anisotropy corresponding to the ancient edifice of piton de la fournaise dominated by intrusions of vertical dykes

localized shear in the deep lithosphere beneath the san andreas fault system seismic images of the base of the lithosphere across the san andreas fault system california usa yield new constraints on the distribution of deformation in the deep lithosphere beneath this strikeslip plate boundary we show that conversions of shear to compressional waves sp across the base of the lithosphere are systematically weaker on the western side of the plate boundary indicating that the drop in seismic shear wave velocity from lithosphere to asthenosphere is either smaller or occurs over a larger depth range in central and northern california the lithosphere asthenosphere boundary changes character across a distance of 50 km and does so directly beneath the san andreas fault along its simple central segment and beneath the calaveras green valley bartlett springs faults to the north given the absolute velocities of the north america and pacific plates and low viscosities inferred for the asthenosphere these results indicate the juxtaposition of mantle lithospheres with different properties across these faults the spatial correlation between the central san andreas fault and the laterally abrupt change in the velocity structure of the deepest mantle lithosphere points to the accommodation of relative plate motion on a narrow shear zone 50 km in width and a rheology that enables strain localization throughout the thickness of the lithosphere

crust and uppermost mantle structure of greenland and the northwest atlantic from rayleigh wave group velocity tomography the greenland landmass preserves 4 billion years of tectonic history but much of the continent is inaccessible to geological study due to the extensive inland ice cap we map out for the first time the 3 d crustal structure of greenland and the nw atlantic ocean using rayleigh wave anisotropic group velocity tomography in the period range 10 80 s from regional earthquakes and the ongoing glatis glisn seismograph networks 1 d inversion gives a pseudo 3 d model of shear wave velocity structure to depths of 100 km with a horizontal resolution of 200 km crustal thickness across mainland greenland ranges from 25 km to over 50 km and the velocity structure shows considerable heterogeneity the large sedimentary basins on the continental shelf are clearly visible as low velocities in the upper 5 15 km within the upper continental basement velocities are systematically lower in northern greenland than in the south and exhibit a broadly nw se trend the thinning of the crust at the continental margins is also clearly imaged upper mantle velocities show a clear distinction between typical fast cratonic lithosphere vs 4 6 km s 1 beneath greenland and its ne margin and anomalously slow oceanic mantle vs 4 3 4 4 km s 1 beneath the nw atlantic we do not observe any sign of pervasive lithospheric modification across greenland in the regions associated with the presumed iceland hotspot track though the average crustal velocity in this region is higher than that of areas to the north and south crustal anisotropy beneath greenland is strong and complex likely reflecting numerous episodes of tectonic deformation beneath the north atlantic and baffin bay the dominant anisotropy directions are perpendicular to the active and extinct spreading centres anisotropy in the subcontinental lithosphere is weaker than that of the crust but still significant consistent with cratonic lithosphere worldwide

a 3 d model of the crust and uppermost mantle beneath the central and western us by joint inversion of receiver functions and surface wave dispersion 1 rayleigh wave phase velocity maps from ambient noise and earthquake data are inverted jointly with receiver functions observed at 828 stations from the usarray transportable array west of 100 w longitude for data recorded in the years 2005 through 2010 to produce a 3 d model of shear wave speeds beneath the central and western us to a depth of 150 km eikonal tomography is applied to ambient noise data to produce about 300 000 rayleigh wave phase speed curves and helmholtz tomography is applied to data following 1550 ms 5 0 earthquakes so that rayleigh wave dispersion maps are constructed from 8 to 80 s period with associated uncertainties across the region harmonic stripping generates back azimuth independent receiver functions with uncertainty estimates for each of the stations a nonlinear bayesian monte carlo method is used to estimate a distribution of shear wave speed vs models beneath each station by jointly interpreting surface wave dispersion and receiver functions and their uncertainties the assimilation of receiver functions improves the vertical resolution of the model by reducing the range of estimated moho depths improving the determination of the shear velocity jump across moho and improving the resolution of the depth of anomalies in the uppermost mantle a great variety of geological and tectonic features are revealed in the 3 d model that form the basis for future detailed local to regional scale analysis and interpretation

vs and density structure beneath the colorado plateau constrained by gravity anomalies and joint inversions of receiver function and phase velocity data 1 the colorado plateau is a physiographic province in the western us with an average elevation of 1 9 km where in contrast to neighboring provinces there is little evidence of large scale tectonic deformation or magmatism recent availability of earthscope usarray seismic data allow us to better examine the crust and upper mantle structure beneath the region and test proposed explanations for the plateau s uplift and relative stability using phase velocities for fundamental mode rayleigh waves and p receiver functions we perform over 800 joint inversions for 1 d shear wave velocity vs profiles sampling the plateau and surrounding regions down to 150 km depth we image a sharp change in crustal thickness at the western edge of the colorado plateau with a more gradual increase eastward moving into the rocky mountains a relatively thick 100 km lithosphere beneath the plateau extends into the rocky mountains to the north we use empirical scaling relations to estimate densities from our vs results and predict the associated gravity anomalies which are inconsistent with the observed distribution of the bouguer gravity anomalies we somewhat reconcile the prediction and observations by assuming that lateral density variations below 50 km can be ignored and the lithospheric root is therefore neutrally buoyant while there is some evidence for small scale convection and lithospheric removal at its edges the shape of the lithospheric mantle anomaly is consistent with a large scale uplift of the plateau by heating since removal of the farallon slab we conclude that the lithospheric root is key to the long term stability of the colorado plateau leading to a colder stronger crust

structure and anisotropy of the mexico subduction zone based on rayleigh wave analysis and implications for the geometry of the trans mexican volcanic belt 1 we develop a three dimensional model of shear wave velocity and anisotropy for the mexico subduction zone using rayleigh wave phase velocity dispersion measurements this region is characterized by both steep and flat subduction and a volcanic arc that appears to be oblique to the trench we give a new interpretation of the volcanic arc obliqueness and the location of the tzitzio gap in volcanism based on the subduction morphology we employ the two station method to measure rayleigh phase velocity dispersion curves between periods of 16 s to 171 s the results are then inverted to obtain azimuthally anisotropic phase velocity maps and to model 3 d variations in upper mantle velocity and anisotropy our maps reveal lateral variations in phase velocity at all periods consistent with the presence of flat and steep subduction we also find that the data are consistent with two layers of anisotropy beneath mexico a crustal layer with the fast directions parallel to the north american absolute plate motion and a deeper layer that includes the mantle lithosphere and the asthenosphere with the fast direction interpreted in terms of toroidal mantle flow around the slab edges our combined azimuthal anisotropy and velocity model enables us to analyze the transition from flat to steep subduction and to determine whether the transition involves a tear resulting in a gap between segments or is a continuous deformation caused by a lithospheric flexure our anisotropy results favor a tear which is also consistent with the geometry of the volcanic belt

vertical mantle flow associated with a lithospheric drip beneath the great basin rapid surface uplift or subsidence and voluminous magmatic activity have often been ascribed to regional scale downwelling of lithospheric mantle however because lithospheric drips sinking plumes of cold and dense lithosphere are relatively small and transient features direct evidence of their existence has been difficult to obtain moreover the significant vertical mantle flow that should be associated with such structures has not been detected here we integrate seismic anisotropy data with tomographic images to identify and describe a lithospheric drip beneath the great basin region of the western united states the feature is characterized by a localized cylindrical core of cooler material with fast seismic velocities and mantle flow that rapidly shifts from horizontal to vertical our numerical experiments suggest that the drip can be generated by gravitational instability resulting from a density anomaly of as little as 1 and a localized temperature increase of 10 the drip tilts to the northeast opposite to the motion of the north american plate in the hotspot reference frame and thereby indicates northeast directed regional mantle flow seismic anisotropy data for the great basin region of the western united states coupled with tomographic images help delineate a northeast dipping lithospheric drip numerical experiments suggest that the drip could have formed owing to gravitational instability triggered by a density increase of about 1 and a temperature increase of about 10

focal spot imaging based on zero lag cross correlation amplitude fields application to dense array data at the san jacinto fault zone we image the subsurface below a dense seismic array straddling the clark branch of the san jacinto fault zone in southern california the analysis is based on focal spots of surface waves associated with the zero lag amplitudes of noise cross correlations computed between all stations of the dense array local medium properties are inferred from the spatially variable focal spot size and shape based on the first zero crossing of amplitude versus distance distributions the method provides simultaneous estimates of wave speed apparent attenuation and anisotropy without solving a tomographic inverse problem the obtained images of the frequency dependent seismic velocity distributions are consistent with independent estimates from a far field rayleigh wave tomography we observe an anticorrelation between our apparent attenuation coefficient and seismic velocity and a fault parallel alignment of fast propagation directions with greater structural complexity to the southwest of the fault the results imply a complex fault zone structure including a waveguide to the northeast of the fault that is continuous across the observed depth range and a low velocity structure to the southwest associated with a shallow sedimentary basin

seismic anisotropy and shear wave splitting in lower crustal and upper mantle rocks from the ivrea zone experimental and calculated data to quantify the relationships between anisotropy s wave splitting and tectonics we determined the seismic properties of lower crustal and upper mantle rocks outcropping in the ivrea zone northern italy we obtained p and s wave seismic velocities by laboratory direct velocity measurements and or by calculations based on the modal compositions of the rocks the lattice preferred orientations lpos and the single crystal stiffness coefficients measured p and s wave velocities 6 0 7 5 km s 1 and 3 6 4 2 km s 1 are typical of the lower crust the p wave anisotropy is in the range 0 10 shear wave birefringence is in the range 0 0 0 6 km s 1 with typical values between 0 0 and 0 2 km s 1 in many cases the birefringence is clearly related to fabric elements foliation lineation mafic rocks such as anorthosite or pyroxene bearing gabbros exhibit low p wave anisotropies 5 and low shear wave birefringences less than 0 1 km s 1 in contrast the seismic properties of felsic rocks such as biotite bearing gneisses and mafic rocks such as amphibolites display high vp anisotropy 10 and strong birefringence 0 3 km s 1 biotite and amphibole preferred orientations clearly control seismic anisotropy and particularly shear wave splitting in these rocks maximum splitting is observed in directions parallel to the foliation with the fast split shear wave polarized parallel to the foliation plane to have an overview of the seismic properties of this lower crustal section at a broader scale we calculated from our data the anisotropic seismic properties of several hypothetical samples that are perhaps more representative of the regional anisotropy than each sample individually for instance the average lower crustal sample displays an anisotropy of 5 5 for p waves and a birefringence around 0 14 km s 1 for s waves propagating parallel to the foliation we observe little splitting for waves propagating at high angle to the foliation

anisotropy and vp vs in the uppermost mantle beneath the western united states from joint analysis of pn and sn phases pn and sn phases are valuable for resolving velocity structure in the mantle lid as they propagate horizontally right below the moho relatively few sn tomography attempts have been made compared to pn because sn is often highly attenuated or buried in p wave coda usarray has greatly increased data coverage for regional phases and both pn and sn are routinely picked by network analysts here we jointly invert pn and sn arrival time residuals with a modified time term analysis and a regularized tomography method and present new maps of crustal thickness uppermost mantle p velocity perturbations vp vs ratios and azimuthal anisotropy strength and orientation beneath the western united states the results indicate partially molten mantle below the snake river plain and the colorado plateau the seismic structure of the top 40 km of the mantle below the colorado plateau differs from that seen at greater depths in other studies such as surface wave or teleseismic body wave tomography whereas the snake river plain anomaly just below the moho is comparable to structures seen at about 200 km depth pn fast axes provide complementary information to sks shear wave splitting observations and our analysis indicates that in several regions in the western united states the orientation of azimuthal anisotropy changes with depth in the upper mantle however we have so far been unable to resolve shear wave splitting directly in sn waveforms which seem to be dominated by sn sv energy

correlated crust and mantle strain fields in tibet vertical to subvertical planes of shear within the active crustal deformation field in tibet align with fast directions of shear wave polarization this observation suggests that the present day velocity gradient tensor field within the crust of tibet correlates with the lithospheric mantle velocity gradient tensor field beneath tibet this inference requires the following 1 the 100 axes of olivine are aligned within established zones of lithospheric mantle fabric or anisotropy built up through finite strain 2 present day shear in the mantle lithosphere is occurring within these established zones of fabric parallel to the direction of olivine 100 axis alignment 3 the zones of mantle lithospheric fabric or zones of mantle shear are aligned with zones of crustal shear faults the correlation of crustal and mantle strain fields most simply results from the fact that both crust and mantle lithosphere are under the influence of similar velocity boundary conditions furthermore the observations confirm the distributed nature of lithospheric mantle deformation beneath east central tibet and suggest that left lateral shear has been the dominant component of finite shear there

anisotropic rayleigh wave tomography of northeast china using ambient seismic noise abstract the ambient noise data recorded by 249 seismic stations in the permanent and temporary networks in northeast china are used to invert for the isotropic phase velocity and azimuthal anisotropy of rayleigh waves in the period band 5 50 s the inversion results reflect the structure from the shallow crust to upper mantle up to approximately 120 km depth beneath the songliao basin both the fast direction in shallow crust and strike of a low velocity anomaly in the middle crust are nne ssw which is coincident with the main tectonic trend of the paleo pacific tectonic domain this indicates that the rifting of the songliao basin is influenced by the subduction of paleo pacific plate the upper mantle of songliao block except the central area of songliao basin to the west of mudanjiang fault and the east of the north south gravity lineament is characterized by high velocity and weak anisotropy up to approximately 120 km depth we infer that there is delamination of lithospheric mantle beneath the songliao block obvious n s ne sw and e w trending fast directions are found in the lithospheric mantles of the east west and south sides of songliao block respectively which coincide with the strikes of the paleozoic tectonic in these areas this suggests that the frozen in anisotropic fabric in the lithospheric mantle can be used to indicate the historical deformation of the lithosphere in the northern margin of the north china craton the spatial variations of phase velocity and azimuthal anisotropy are more dramatic than those in northeast china blocks which indicates that the lithosphere of the north china craton has experienced more complicated tectonic evolution than that of the northeast china blocks

seismic interferometry with antipodal station pairs in this study we analyze continuous data from all global seismographic network stations between year 2000 and 2009 and demonstrate that several body wave phases e g pp pcppkp sksp and pps propagating between nearly antipodal station pairs can be clearly observed without array stacking using the noise coda cross correlation method based on temporal correlations with global seismicity we show that the observed body waves are clearly earthquake related moreover based on single earthquake analysis we show that the earthquake coda energy observed between 10 000 and 30 000 s after a large earthquake contributes the majority of the signal we refine our method based on these observations and show that the signal can be significantly improved by selecting only earthquake coda times with our improved processing the pkikp phase which does not benefit from the focusing effect near the antipode can now also clearly be observed for long distance station pairs

crust mantle coupling mechanism in cameroon west africa revealed by 3d s wave velocity and azimuthal anisotropy abstract to understand the depth variation of deformation beneath cameroon west africa we developed a new 3d model of s wave isotropic velocity and azimuthal anisotropy from joint analysis of ambient seismic noise and earthquake surface wave dispersion we found that the cameroon volcanic line cvl is well delineated by slow phase velocities in contrast with the neighboring congo craton in agreement with previous studies apart from the congo craton and the oubanguides belt the uppermost mantle revealed a relatively slow velocity indicating a thinned or thermally altered lithosphere the direction of fast axis in the upper crust is mostly ne sw but trending approximately n s around mt oku and the southern cvl the observed crustal azimuthal anisotropy is attributed to alignment of cracks and crustal deformation related to magmatic activities a widespread zone of weak to zero azimuthal anisotropy in the mid lower crust shows evidence for vertical mantle flow or isotropic mid lower crust in the uppermost mantle the fast axis direction changed from ne sw to nw se around mt oku and northern cameroon this suggests a layered mechanism of deformation and revealed that the mantle lithosphere has been deformed ne sw fast azimuths are observed beneath the congo craton and are consistent with the absolute motion of the african plate suggesting a mantle origin for the observed azimuthal anisotropy our tomographically derived fast directions are consistent with the local sks splitting results in some locations and depths enabling us to constrain the origin of the observed splitting the different feature of azimuthal anisotropy in the upper crust and the uppermost mantle implies decoupling between deformation of crust and mantle in cameroon

complex and variable crustal and uppermost mantle seismic anisotropy in the western united states deformation in the crust and mantle measured using seismic anisotropy is poorly constrained in the western united states because of inconsistencies in the existing data a three dimensional model that reconciles these discrepancies reveals that seismic anisotropy in the crust correlates with different geological provinces but is unrelated to anisotropy in the underlying mantle

high resolution 3d shallow crustal structure in long beach california application of ambient noise tomography on a dense seismic array ambient noise tomography has proven to be effective in resolving shallow earth structure we applied ambient noise tomography on a dense seismic array in long beach california the array was composed of more than 5200 stations with an average spacing close to 100 m three weeks of passive ambient noise were crosscorrelated between each station pair which resulted in more than 13 5 million crosscorrelations within the area clear fundamental mode rayleigh waves were observed between 0 5 and 4 hz which were most sensitive to structure above 1 km depth for each station pair we applied frequency time analysis to determine the phase traveltime dispersion and for each frequency we applied eikonal tomography to determine the rayleigh wave phase velocity map the eikonal tomography accounted for ray bending by tracking the wavefront and allowed uncertainties to be estimated through statistical analysis the compilation of phase velocity maps was then used to invert for 3d shear velocity structure the inverted model showed clear correlation with the known geologic features such as the shallow south north velocity dichotomy and a deeper fast anomaly associated with the newport inglewood fault zone our results can potentially be used to complement traditional active source studies

the crustal structure of the arizona transition zone and southern colorado plateau from multiobservable probabilistic inversion the arizona transition zone is a narrow band that separates two of the main and most contrasting tectonic provinces in western us namely the southern colorado plateau and the southern basin and range provinces as such the internal crustal structure and physical state of this transitional zone hold clues for understanding i the amalgamation of these provinces ii the partitioning of deformation due to both past and present day stress fields and iii the role of thermal versus compositional effects in controlling surface observables here we employ and expand a novel multiobservable probabilistic inversion method and jointly invert fundamental mode rayleigh phase velocities receiver functions surface heat flow geoid height and absolute elevation to obtain an internally consistent 3 d model of the temperature density vs and vp of the arizona transition zone and the southern portions of the colorado plateau and basin and range our results confirm a significant crustal thickening from 28 km in the sw of the arizona transition zone and southern basin and range to 48 km beneath the southern colorado plateau inverted temperatures agree well with the location of recent volcanism and indicate that the lithosphere asthenosphere boundary is not deeper than 70 km in most of the region we find that major pre cambrian surface structures and or shear zones separate crustal domains with distinct bulk properties suggesting that the juxtaposed crustal blocks still retain at least in part their original characteristics however widespread intrusions of significant volumes of mafic magmas have affected these blocks at different depths locally overprinting their original compositions and creating highly heterogeneous crustal sections a dominant and large scale internal crustal pattern of sw dipping planes structures is evident in our models coinciding with the orientation of deep faults previously inferred from earthquake focal mechanisms while we cannot categorically corroborate the presence of melt or aqueous fluids within the crust our results are compatible with these scenarios beneath some parts of the basin and range the mogollon datil and springerville volcanic fields

full wave multiscale anisotropy tomography in southern california the southern california earthquake data center scedc operates a densely distributed seismic network in southern california the high quality seismic data it provides have enabled seismologists to obtain high resolution 3d isotropic velocity models for this region owing to the limitations of ray theory assumption used in anisotropy analysis the 3d anisotropic structure in southern california is still poorly understood in this chapter we apply our full wave tomography method to the seismically densely covered southern california to obtain a 3d anisotropic model of this region reliable 3d anisotropic model can help us understand the processes of the upper mantle flow and the crustal deformation

continental transforms a view from the alpine fault abstract continental transform faults are dominantly highly localized strike slip shear zones hundreds of kilometers long that accumulate tens to hundreds of kilometers of displacement from work on the alpine fault we pose the questions what is the deep structure of a continental transform and how does the displacement become localized we review research on the alpine fault and propose a model in which the fault partitions at depth into a steep zone extending into the mantle with largely fault parallel motion and a flat ductile decollement in the lower crust the fault localizes around two thirds of the plate motion within a 100 km wide zone of distributed deformation a review of other active continental fault systems suggests that variation between them may reflect their tectonic origins the nature of the crust in which they develop the presence of a significant oblique component of motion and the displacement rate all however have evidence for the development of a single principal fault zone that carries 50 of the total displacement and extends as a localized zone of shear into the upper mantle we review mechanisms of strain weakening and suggest that localization of a principal fault may be initiated in the seismogenic crust and through a series of positive feedbacks eventually extend through the lower crust into the upper mantle

pn tomography of the western united states using usarray 1 usarray has now provided several years of high quality seismic data and improved ray coverage for much of the western united states which will enable increased resolution for studies of the lithospheric and deeper structure of the north american continent here we analyze pn arrival times from the transportable stations of usarray to resolve crustal thickness and uppermost mantle structure we use 123 008 pn picks from april 2004 to october 2009 as measured by the array network facility at epicentral distances from 180 to 1450 km these picks are derived from 778 stations at 70 km spacing and 7903 earthquakes and quarry blasts applying the classic time term method we use a regularized least squares inversion to estimate crustal thickness variations and image velocity perturbations in the uppermost mantle just below the moho we also consider upper mantle anisotropy and describe the velocity perturbations with a cos 2 azimuthal variation our crustal thickness map generally agrees with receiver function results from other researchers but differs in some details we obtain an average upper mantle velocity of 7 93 km s with higher velocities beneath eastern washington and northern idaho and lower velocities near the california mexico border the sierra nevada the northern coastal california region and the greater yellowstone area we observe large anisotropic anomalies in southern california as well as in the snake river plain area these results should complement other seismic studies e g body and surface wave tomography and shear wave splitting to provide information about composition temperature and tectonic processes in the western united states

3d shear wave velocity structure beneath the southeastern tibetan plateau from ambient noise we perform high resolution 3d shear wave structures of the crust and upper mantle using continuous wave data recorded at 170 stations from national and regional networks in the southeastern tibetan plateau first empirical green s functions are obtained between all the possible station pairs then rayleigh wave group velocity and phase velocity dispersion curves are retrieved using the frequency time analysis method the rayleigh wave group velocity images at periods from 8 to 50 s and phase velocity images at periods from 8 to 40 s are inverted on a 1 1 grid finally the 1d shear wave structure beneath each grid and the 3d shear wave velocity structure are acquired by linear interpolation our results indicate that low velocity layers lvls are ubiquitous in the middle lower crust beneath the southeastern tibetan plateau but vary in depth 20 35 km thickness 5 20 km and strength 5 4 10 8 velocity reduction in different blocks the 3d geometry of the lvls is complex and cannot be interpreted as a model with a uniform flow channel the shear velocity in the chuandian block is unevenly distributed indicating that the block is inhomogeneous the 12 may 2008 wenchuan and 20 april 2013 lushan earthquake sequences m s 3 0 all occurred in the crustal layers in which lvls are inhibited by the strong sichuan basin and in which the topography and crustal thickness vary sharply this formed the earthquake preparation environment of the two earthquakes the ningnan qiaojia pull apart region is characterized by low shear wave velocity in the shallow crust which agrees well with the sedimentary features of the upper crust a relatively high velocity is observed in the central part of the low velocity sichuan basin in the shallow layers indicating the basement beneath the sichuan basin may be uplifted around the central part of the basin

western u s seismic anisotropy revealing complex mantle dynamics abstract the origin of the complex pattern of sks splitting over the western united states u s remains a long lasting debate where a model that simultaneously matches the various sks features is still lacking here we present a series of quantitative geodynamic models with data assimilation that systematically evaluate the influence of different lithospheric and mantle structures on mantle flow and seismic anisotropy these tests reveal a configuration of mantle deformation more complex than ever envisioned before in particular we find that both lithospheric thickness variations and toroidal flows around the juan de fuca slab modulate flow locally but their co existence enhances large scale mantle deformation below the western u s the ancient farallon slab below the east coast pulls the western u s upper mantle eastward spanning the regionally extensive circular pattern of sks splitting the prominent e w oriented anisotropy pattern within the pacific northwest reflects the existence of sustaining eastward intrusion of the hot pacific oceanic mantle to beneath the continental interior from within slab tears below oregon to under the snake river plain and the yellowstone caldera this work provides an independent support to the formation of intra plate volcanism due to intruding shallow hot mantle instead of a rising mantle plume

3 d crustal structure of the western united states application of rayleigh wave ellipticity extracted from noise cross correlations we present a new 3 d seismic model of the western united states crust derived from a joint inversion of rayleigh wave phase velocity and ellipticity measurements using periods from 8 to 100 s improved constraints on upper crustal structure result from use of short period rayleigh wave ellipticity or rayleigh wave h v horizontal to vertical amplitude ratios measurements determined using multicomponent ambient noise cross correlations to retain the amplitude ratio information between vertical and horizontal components for each station we perform daily noise pre processing temporal normalization and spectrum whitening simultaneously for all three components for each station pair amplitude measurements between cross correlations of different components radial radial radial vertical vertical radial and vertical vertical are then used to determine the rayleigh wave h v ratios at the two station locations we use all earthscope usarray tranportable array data available between 2007 january and 2011 june to determine the rayleigh wave h v ratios and their uncertainties at all station locations and construct new rayleigh wave h v ratio maps in the western united states between periods of 8 and 24 s combined with previous longer period earthquake rayleigh wave h v ratio measurements and rayleigh wave phase velocity measurements from both ambient noise and earthquakes we invert for a new 3 d crustal and upper mantle model in the western united states correlation between the inverted model and known geological features at all depths suggests good resolution in five crustal layers use of short period rayleigh wave h v ratio measurements based on noise cross correlation enables resolution of distinct near surface features such as the columbia river basalt flows which overlie a thick sedimentary basin

on the interaction of the north andes plate with the caribbean and south american plates in northwestern south america from gps geodesy and seismic data

seismic tomography of the southern california plate boundary region from noise based rayleigh and love waves we use cross correlations of ambient seismic noise between pairs of 158 broadband and short period sensors to investigate velocity structure over the top 5 10 km of the crust in the southern california plate boundary region around the san jacinto fault zone sjfz from the 9 component correlation tensors associated with all station pairs we derive dispersion curves of rayleigh and love wave group velocities the dispersion results are inverted first for rayleigh and love waves group velocity maps on a 1 5 1 5 km2 grid that includes portions of the sjfz the san andreas fault saf and the elsinore fault we then invert these maps to 3d shear wave velocities in the top 7 km of the crust the distributions of the rayleigh and love group velocities exhibit 2 azimuthal anisotropy with fast directions parallel to the main faults and rotations in complex areas the reconstructed 3d shear velocity model reveals complex shallow structures correlated with the main geological units and strong velocity contrasts across various fault sections along with low velocity damage zones and basins the sjfz is marked by a clear velocity contrast with higher v s values on the ne block for the section se of the san jacinto basin and a reversed contrast across the section between the san jacinto basin and the saf velocity contrasts are also observed along the southern parts on the saf and the elsinore fault with a faster southwest block in both cases the region around the salton trough is associated with a significant low velocity zone strong velocity reductions following flower shape with depth are observed extensively around both the sjfz and the saf and are especially prominent in areas of geometrical complexity in particular the area between the sjfz and the saf is associated with an extensive low velocity zone correlated with diffuse seismicity at depth and a similar pattern including correlation with deep diffuse seismicity is observed on a smaller scale in the trifurcation area of the sjfz these results augment local earthquake tomography images that have low resolution in the top few km of the crust and provide important constraints for studies concerned with behavior of earthquake ruptures generation of rock damage and seismic shaking hazard in the region

seismic evidence for widespread western us deep crustal deformation caused by extension morgan moschetti and colleagues use ambient noise tomography data from the western united states to show that strong radial seismic anisotropy exists in the deep crust confined mainly to the geological provinces that have undergone significant extension during the cenozoic era since about 65 million years ago the coincidence of crustal radial anisotropy with these extensional provinces supports the hypothesis that the deep crust within these regions has undergone widespread and relatively uniform strain in response to crustal thinning and extension here a method of seismic wave imaging known as ambient noise tomography is used to generate high resolution images of seismic wave speeds in the crust and uppermost mantle the observations reveal strong and uniform anisotropy where waves travel through rock at different speeds depending on direction in the deep crust in areas of the western united states that have undergone significant extension during the past 65 million years laboratory experiments have established that many of the materials comprising the earth are strongly anisotropic in terms of seismic wave speeds1 observations of azimuthal2 3 and radial4 5 anisotropy in the upper mantle are attributed to the lattice preferred orientation of olivine caused by the shear strains associated with deformation and provide some of the most direct evidence for deformation and flow within the earth s interior although observations of crustal radial anisotropy would improve our understanding of crustal deformation and flow patterns resulting from tectonic processes large scale observations have been limited to regions of particularly thick crust6 here we show that observations from ambient noise tomography in the western united states reveal strong deep middle to lower crustal radial anisotropy that is confined mainly to the geological provinces that have undergone significant extension during the cenozoic era since 65 myr ago 7 8 the coincidence of crustal radial anisotropy with the extensional provinces of the western united states suggests that the radial anisotropy results from the lattice preferred orientation of anisotropic crustal minerals caused by extensional deformation these observations also provide support for the hypothesis that the deep crust within these regions has undergone widespread and relatively uniform strain in response to crustal thinning and extension9 10 11

seismic anisotropy and mantle flow from the great basin to the great plains western united states shear wave splitting and p sks and s travel time residuals are calculated for teleseismic arrivals recorded on the colorado plateau great basin program for array seismic studies of the continental lithosphere passcal portable broadband seismic deployment and for permanent stations in the western united states little shear wave splitting is observed for broadband recordings in the northern colorado plateau the rocky mountains or the central great basin the transition between the colorado plateau and the great basin is marked by moderate shear wave splitting 1 0 s and unusually late teleseismic phase arrivals this suggests material with a higher content of mantle melt or volatiles than regions to either side splitting in the transition between the colorado plateau and great basin is part of a pattern of fast polarizations that align in a semicircle surrounding a central great basin region of null no splitting measurements away from the california plate boundary splitting to the north and south of our study region aligns roughly parallel to the absolute plate motion of the north american plate no simple spatial relation of splitting with geological and geophysical features such as mountain ranges velocity anomalies gravity magnetics or heat flow is evident in most of the western united states however splitting in the great basin is compatible with asthenospheric flow the smallest shear wave splitting delay times coincide with the eureka low in heat flow also having low s velocity at 300 km depth we suggest that the circumferential pattern of fast polarization directions ringing a central region of nulls in the great basin is caused by mantle flow by the interaction of mantle up welling and the absolute motion of the north american plate

the matuyama brunhes boundary and the nature of magnetic remanence acquisition in the loess palaeosol sequence from the western part of the east european loess province abstract loess palaeosol sequences from the western part of the east european loess province were palaeomagnetically studied the position of the matuyama brunhes boundary was found in the sequence at zahvizdja east carpathian foreland overlying the surface that corresponds to the vi dniestre terrace this boundary is located within the accumulation horizon of an interglacial soil developed during the time equivalent of oxygen isotope stage 19 the lowermost palaeosol of this sequence located just below the soil containing the matuyama brunhes boundary record can thus be correlated with oxygen isotope stage 21 and the vi dniestre terrace was developed during oxygen isotope stage 22 samples from the palaeosols especially from their accumulation horizons revealed the presence of secondary postdepositional components of magnetisation some resistant to alternating field demagnetisation even as high as 100 mt fine grained maghaemite is most probably the carrier of these components subsequent thermal demagnetisation can remove them to establish the primary palaeodirections haematite and magnetite are the main carriers of remanent magnetisation of the loess layers the degree of possible delay of remanence acquisition in loess sequences was assessed on sediments from three sections sampled in the volhynia and lublin uplands good correlation of directional changes recorded in coeval segments of loess sections shows that the characteristic magnetisation of aeolian deposits from the western part of the east european loess province is approximately syndepositional

surface wave tomography from microseisms in southern california received 5 april 2005 revised 23 may 2005 accepted 9 june 2005 published 26 july 2005 1 since it has already been demonstrated that point topoint seismic propagation green functions can be extracted from seismic noise it should be possible to image earth structure using the ambient noise field seismic noise data from 148 broadband seismic stations in southern california were used to extract the surface wave arrival times between all station pairs in the network the seismic data were then used in a simple but densely sampled tomographic procedure to estimate the surface wave velocity structure within the frequency range of 0 1 0 2 hz for a region in southern california the result compares favorably with previous estimates obtained using more conventional and elaborate inversion procedures this demonstrates that coherent noise field between station pairs can be used for seismic imaging purposes citation sabra k g p gerstoft p roux w a kuperman and m c fehler 2005 surface wave tomography from microseisms in southern california geophys res lett 32 l14311 doi 10 1029 2005gl023155

3 d upper mantle shear velocity model beneath the contiguous united states based on broadband surface wave from ambient seismic noise ambient noise seismic tomography has been widely used to study crustal and upper mantle shear velocity structures most studies however concentrate on short period 50 s surface wave from ambient noise while studies using long period surface wave from ambient noise are limited in this paper we demonstrate the feasibility of using long period surface wave from ambient noise to study the lithospheric structure on a continental scale we use broadband rayleigh wave phase velocities to obtain a 3 d vs structures beneath the contiguous united states at period band of 10 150 s during the inversion 1 d shear wave velocity profile is parameterized using b spline at each grid point and is inverted with nonlinear markov chain monte carlo method then a 3 d shear velocity model is constructed by assembling all the 1 d shear velocity profiles our model is overall consistent with existing models which are based on multiple datasets or data from earthquakes our model along with the other post usarray models reveal lithosphere structures in the upper mantle which are consistent with the geological tectonic background e g the craton root and regional upwelling provinces the model has comparable resolution on lithosphere structures compared with many published results and can be used for future detailed regional or continental studies and analysis

the geological signature of a slab tear below the aegean in this paper we explore the geology of the aegean region in the miocene to identify the geological signature of a first order slab tear observed in all tomographic models from 15 to 8 ma the tear is associated spatially and timely with a fast clockwise rotation of the external hellenides alkaline volcanism high temperature metamorphic domes with a predominance of north dipping detachments and south westward migration of granitoid intrusions these features suggest a warmer geodynamic environment during the rotation and the impact of a hot mantle flow associated with the tear the 8 ma duration between the first high temperature metamorphic domes in the centre of the archipelago and the beginning of the fast rotation may correspond to the time needed for the slab to bend stretch and finally tear

seismic and electrical signatures of the lithosphere asthenosphere system of the normal oceanic mantle although plate tectonics started as a theory of the ocean basins nearly 50 years ago the mechanical details of how it works are still poorly known our understanding of these details has been hampered partly by our inability to characterize the physical nature of the lithosphere asthenosphere system las beneath the ocean we review the existing observational constraints on the seismic and electrical properties of the las particularly for normal oceanic regions away from mid oceanic ridges hot spots and subduction zones where plate tectonics is expected to present its simplest form whereas a growing volume of seismic data on land has provided remarkable advances in large scale pictures seafloor observations have been shedding new light on essential details by combing through these observational constraints researchers are unveiling the nature of the enigmatic las future directions for large scale seafloor observations are also discussed

on the relationship between azimuthal anisotropy from shear wave splitting and surface wave tomography 1 seismic anisotropy provides essential constraints on mantle dynamics and continental evolution one particular question concerns the depth distribution and coherence of azimuthal anisotropy which is key for understanding force transmission between the lithosphere and asthenosphere here we reevaluate the degree of coherence between the predicted shear wave splitting derived from tomographic models of azimuthal anisotropy and that from actual observations of splitting significant differences between the two types of models have been reported and such discrepancies may be due to differences in averaging properties or due to approximations used in previous comparisons we find that elaborate full waveform methods to estimate splitting from tomography yield generally similar results to the more common simplified approaches this validates previous comparisons and structural inversions however full waveform methods may be required for regional studies and they allow exploiting the back azimuthal variations in splitting that are expected for depth variable anisotropy applying our analysis to a global set ofskssplitting measurements and two recent surface wave models of upper mantle azimuthal anisotropy we show that the measures of anisotropy inferred from the two types of data are in substantial agreement provided that the splitting data is spatially averaged so as to bring it to the scale of long wavelength tomographic models and reduce spatial aliasing observed and tomography predicted delay times are significantly correlated and global angular misfits between predicted and actual splits are relatively low regional anisotropy complexity notwithstanding our findings imply that splitting and tomography yield a consistent signal that can be used for geodynamic interpretation

shear wave splitting beneath western united states in relation to plate tectonics we have examined shear wave splitting in teleseismic shear waves from 26 broadband stations in the western united states fast polarization directions and delay times t show spatial variations that are coherent within geologic provinces stations located near the san andreas fault show clear evidence for fault parallel anisotropy in the crust and upper mantle 115 125 km thickness this can be explained by the finite strain associated with the relative plate motion between the north american and pacific plates the lateral extent of this strain field is probably narrow to the west because stations 55 km west of the san andreas fault do not show fault parallel anisotropy in southern california station lac located 80 km east of the san andreas fault shows large fault parallel anisotropy this suggests that the pacific north american plate boundary in the mantle might be displaced to the east in southern california a deeper e w oriented fast direction of anisotropy underlies the fault parallel anisotropic layer in the vicinity of the san andreas fault an e w fast feature is also present beneath the western basin and range and the foothills of the sierra nevada although local variations are present the magnitude of delay times suggests that this feature resides in the asthenosphere we interpret this feature as the asthenospheric flow in the slabless window left behind the farallon plate the flow induced anisotropy may partially be frozen in at shallow depths station orv is located near the southern edge of the gorda slab where no anisotropy is detected the absence of anisotropy at this location could therefore mark a boundary between farallon associated flow and regions where e w oriented asthenospheric flow did not occur the lack of evidence for ne sw fast orientation within the walker lane shear belt of western nevada suggests that this crustal feature does not extend into the mantle or that is not as well developed as that beneath the san andreas fault stations located over the young subducting gorda plate mark a change in the fast direction to nearly ne sw this direction aligns well with the maximum compressive stress direction in the overlying north american plate and the ne sw directed internal shearing of the gorda plate the anisotropic thicknesses calculated from delay times suggest roughly double that expected for purely lithospheric contributions this implies that the anisotropic thickness may include some of the asthenosphere alternatively using a higher anisotropy of 8 can bring thicknesses in line with other measures of lithospheric thicknesses the correspondence between the fast directions and the present plate tectonic deformations suggest that mapping upper mantle deformation through seismic anisotropy is a viable method and that asthenospheric flow may be a significant contributor to seismic anisotropy

shear wave anisotropy from cross correlation of seismic noise in the parkfield pilot hole summary we use cross correlation of seismic noise recorded at stations in the san andreas fault observatory at depth safod pilot hole to extract p and s waves and measure s wave anisotropy on the horizontal components the data are recorded at seven three component stations at depths from 1857 to 2097 m in the pilot hole in late september and early 2004 october drilling noise underneath the stations generated propagating waves which were absent in the rest of october estimates of the p and s wave velocities from the cross correlations on the vertical and horizontal components respectively are consistent with velocity measurements taken directly in the borehole we observe polarization of the s wave with a fast polarization direction of 120 130 that is 4 per cent faster than the slow direction cross correlation of the seismic noise can accurately determine s wave anisotropy

how to relate body wave and surface wave anisotropy seismic anisotropy is one of the most efficient geological and geodynamical tools for understanding the dynamics of the earth upper mantle anisotropy is evident in seismic data sets for the last 30 years primarily from surface wave dispersion curves and body wave sks data we demonstrate in this paper that surface wave and body wave derived anisotropy can be explained by the same anisotropic parameters l gc gs in the simplest case of a horizontal fast symmetry axis one application of this method is to display the sks delay time and the corresponding azimuth which can be derived from surface wave global with a lateral resolution of around 2000 km and regional lateral resolution of 350 km anisotropy tomography a global scale comparison is disappointing since there are only a few areas where both data types are correctly retrieved at the same spatial scale same lateral resolution the anisotropy is well resolved in oceanic basins from surface waves whereas most measurements of sks splitting are available below continents however there is a good agreement between surface wave and body wave anisotropy in regions where large scale processes primarily tectonic are taking place such as the western united states and in central asia in particular we show that at high frequency 1hz the observed sks delay time and azimuth depend on the order in which s waves propagate through the layers a comparison between synthetic and observed sks phases in central asia highlights the varying sensitivity of sks waves at depth we can obtain a good correlation between synthetic and observed sks by considering anisotropy only between 80 and 200 km depth in the tomographic model rather than in the entire upper mantle however it is more difficult to explain the discrepancy between synthetic and observed sks under the eastern united states where a large scale coherent sks anisotropy is present such results call for more sophisticated sks waveform modeling and for enhanced lateral resolution in anisotropic tomographic models

the mantle flow field beneath western north america although motions at the surface of tectonic plates are well determined the accompanying horizontal mantle flow is not we have combined observations of surface deformation and upper mantle seismic anisotropy to estimate this flow field for western north america we find that the mantle velocity is 5 5 1 5 centimeters per year due east in a hot spot reference frame nearly opposite to the direction of north american plate motion west southwest the flow is only weakly coupled to the motion of the surface plate producing a small drag force this flow field is probably due to heterogeneity in mantle density associated with the former farallon oceanic plate beneath north america

crustal structure determined from ambient noise tomography near the magmatic centers of the coso region southeastern california we apply seismic ambient noise tomography to image and investigate the shallow shear velocity structure beneath the coso geothermal field and surrounding areas data from a passcal experiment operated within the coso geothermal field between 1998 and 2000 and surrounding broadband stations from the southern california seismic network are acquired and processed daily cross correlations of ambient noise between all pairs of stations that overlapped in time of deployment were calculated and then stacked over the duration of deployment phase velocities of rayleigh waves between 3 and 10 s periods are measured from the resulting cross correlations depending on the period between about 300 and 600 reliable phase velocity measurements are inverted for phase velocity maps from 3 to 10 s periods which in turn are inverted for a 3 d shear velocity model beneath the region the resulting 3 d model reveals features throughout the region that correlate with surface geology beneath the coso geothermal area shear velocities are generally depressed a prominent low velocity anomaly is resolved clearly within the top 2 km no significant anomaly is seen below about 14 km depth and a weakly resolved anomaly is observed between 6 and 12 km depth the anomaly in the top 2 km probably results from geothermal alteration in the shallow subsurface no magmatic body is imaged beneath 14 km depth but the shear velocity anomaly between 6 and 12 km may be attributable to partial melt the thickness and amplitude of the magma body trade off in the inversion and are ill determined low velocities in the regions surrounding coso at depths near 7 km underlie areas with miocene to recent volcanism suggesting that some magmatic processing of the crust could be focused near this depth

crustal anisotropy beneath hudson bay from ambient noise tomography evidence for post orogenic lower crustal flow the crust underlying hudson bay canada records a long and complex tectonic history in this study we investigate this region using tomographic inversion based on continuous ambient noise recordings from 37 broadband seismograph stations that encircle hudson bay the ambient noise data were processed to obtain group velocity dispersion measurements from 10 35 s period which were inverted using an algorithm that incorporates the effects of anisotropy this work is among the first in which ambient noise data have been used to investigate azimuthal anisotropy the inversion method uses smoothing and damping to regularize the solution due to the significantly increased number of model parameters relative to the isotropic case we performed a careful analysis for parameter selection to determine whether leakage occurs between isotropic and anisotropic model parameters we observe a robust pattern of anisotropic fast directions in the mid crust that are consistent with large scale tectonic trends based on magnetic anomaly patterns in particular a distinctive double indentor shape for the superior craton is clearly expressed in both data sets this pattern breaks down deeper in the crust suggesting that some degree of lithospheric decoupling in the lower crust such as channel flow occurred during orogenesis given regional evidence for vertically coherent deformation in the crust and underlying mantle we interpret this pattern in the lower crust as a tectonic overprint that post dates the main phase of trans hudson deformation at most levels in the crust we observe a profound change in direction of anisotropic fast direction across an inferred suture beneath hudson bay