automated detection and location of microseismicity at mount st helens with a large n geophone array in the summer of 2014 a dense array of 904 geophones was deployed at mount st helens along the road and trail system within 15 km distance of the summit crater the array recorded continuous data for approximately 2 weeks and presents an unprecedented seismic observation of an active volcano a reverse time imaging method is applied to short term average over long term average time series data to automatically detect and locate microseismicity these efforts resulted in an order of magnitude increase in earthquake detections over the normal monitoring operations of the pacific northwest seismic network earthquake locations resolve a narrow 1 km wide vertical lineament of seismicity which extends from the surface to 4 km depth directly beneath the summit crater this feature is interpreted as a fracture network that acts as a conduit connecting an underlying magma chamber to the surface

extracting time domain green s function estimates from ambient seismic noise 1 it has been demonstrated experimentally and theoretically that an estimate of the green s function between two seismic stations can be obtained from the time derivative of the long time average cross correlation of ambient noise between these two stations this tdgf estimate from just the noise field includes all tensor components of the green s function and these green s function estimates can be used to infer earth structure we have computed cross correlations using 1 to 30 continuous days of ambient noise recorded by over 150 broadband seismic stations located in southern california the data processing yielded thousands of cross correlation pairs for receiver separations from 4 500 km which clearly exhibit coherent broadband propagating dispersive wavetrains across frequency band 0 1 2 hz

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

magma reflection imaging in krafla iceland using microearthquake sources the details of magma plumbing beneath active volcanoes remain a major challenge in geochemistry geophysics and volcanic hazard evaluation here we apply a relatively novel variation of seismic interferometry which we call virtual reflection seismic profiling vrsp to produce a high resolution image of a known crustal magma body the technique takes advantage of recent advances in both seismic instrumentation dense arrays and seismic analysis seismic interferometry we have applied this technique to data recently acquired at an iconic volcanic system krafla which lies on the mid atlantic ridge as exposed in northern iceland what make this particular site exceptional are encounters with rhyolitic magma in two drillholes k 39 and iceland deep drilling project 1 iddp 1 these known magma bodies represent a unique calibration opportunity for surface geophysical measurements of magma distribution at depth in this study we produced a stacked seismic reflection section by applying common depth point processing techniques to virtual shot gathers derived from interferometry of p waves from microearthquakes generated by tectonic magmatic and or geothermal activity we observe a strong coherent reflection on the seismic section at a travel time corresponding to the depth at which magma was encountered in the iddp 1 wellbore we interpret this reflection to be from magma or magma related fluids additional coherent reflections may correspond to other components of the magma plumbing beneath krafla these results represent a promising new technique for structural imaging with natural sources that can be applied to a wide array of geologic and energy problems that involve natural or induced seismic clusters

spatial coherence of the seismic wavefield continuously recorded by the usarray we use a method based on the array covariance matrix eigenvalues to study the level of spatial coherence and of isotropy of the seismic wavefield continuously recorded during 2010 by the usarray first we observe that the raw data is often dominated by local sources to remove their influence we apply spectral and temporal normalizations to the input signals we notice that this widely used pre processing in ambient noise seismology does not fully homogenize the seismic wavefield and that some strongly coherent arrivals persist among these persistent signals generated by teleseismic sources we detect 1 seismic waves emitted by strong earthquakes 2 a nearly continuous quasi monochromatic signal at 26 seconds period and 3 multi day coherent wave trends in the spectral band of oceanic microseisms 0 07 0 2 hz for the latter beamforming analysis shows that while most of the signals are composed of surface waves some are dominated by body waves likely generated in the deep ocean

reflection imaging with earthquake sources and dense arrays the mw 5 8 earthquake that occurred in louisa county virginia on 23 august 2011 provided an opportunity to record with several high density seismic arrays in addition to traditional sparse temporary seismic networks traditional aftershock networks consist of a few dozen stations spread over tens of kilometers as a result the recorded seismic waveforms suffer from spatial aliasing that is so severe that many types of waveform processing are not applicable here we report the results of recording with a large number of oil industry type instruments deployed at a spacing that is an order of magnitude closer than in traditional deployments the objective was to image subsurface structure with array methods using the aftershocks as sources the dense array recorded continuously for 12 days and consisted of 172 vertical component seismometers that were placed at 200 400 m and a 60 km long three component regional profile with stations every 2 km we demonstrate how processing techniques from vertical seismic profiling can produce high resolution 3 d reflection images of structure beneath the array these images display reflectivity that correlates with that observed on a nearby deep reflection survey collected by the u s geological survey of particular interest is a strong reflector imaged across multiple profiles our analysis demonstrates how a surface array of seismometers can provide 3 d images of structure using microearthquake sources when wavefields are sampled sufficiently densely

seismic noise based time lapse monitoring of the valhall overburden we used two vintages of ambient seismic noise recorded at the valhall life of the field seismic network in 2004 and 2005 to perform a passive time lapse imaging of the subsurface first the cross correlations between each pair of stations were computed for both vintages to extract scholte waves second the relative velocity variations between the 2004 and 2005 cross correlations were measured on the ballistic waves using the moving window cross spectral technique finally the best quality relative velocity variation measurements were regionalized using a modified eikonal tomography technique the results albeit noisy because of the short duration of the available records show a large patch of increased seismic velocity in the southern part of the network and a weaker anomaly in the northern part the southern increase of velocity can be attributed to the exploitation of the southern flank of the valhall reservoir with new wells and is in good agreement with a previous time lapse study using scholte waves

ambient noise cross correlation observations of fundamental and higher mode rayleigh wave propagation governed by basement resonance measurement of basement seismic resonance frequencies can elucidate shallow velocity structure an important factor in earthquake hazard estimation ambient noise cross correlation which is well suited to studying shallow earth structure is commonly used to analyze fundamental mode rayleigh waves and increasingly love waves here we show via multicomponent ambient noise cross correlation that the basement resonance frequency in the canterbury region of new zealand can be straightforwardly determined based on the horizontal to vertical amplitude ratio h v ratio of the first higher mode rayleigh waves at periods of 1 3 s the first higher mode is evident on the radial radial cross correlation functions but almost absent in the vertical vertical cross correlation functions implying longitudinal motion and a high h v ratio a one dimensional regional velocity model incorporating a 1 5 km thick sedimentary layer fits both the observed h v ratio and rayleigh wave group velocity similar analysis may enable resonance characteristics of other sedimentary basins to be determined

elliptical anisotropic eikonal phase velocity tomography we formulated an anisotropic eikonal tomography approach for phase velocities based on a two dimensional elliptical anisotropic wave equation we can fit the parameters of the ellipse directly from measured first order traveltime surface gradients and constrain these parameters to vary smoothly over space the method is applied to scholte waves in virtual seismic sources from stations in the life of field seismic ocean bottom cable array installed over the ekofisk field the fast directions of the azimuthally anisotropic scholte wave velocities form a large circular pattern over the ekofisk field this pattern dominates the scholte wave phase velocities at ekofisk between 0 7 and 1 1 hz it results from the overburden stress state and from seafloor subsidence induced by decades of hydrocarbon extraction

a detailed image of the continent borderland transition beneath long beach california new crustal images beneath long beach california show the region of the inner borderland to continent transition the cross sections are obtained from stacked autocorrelations of virtual sources generated from oil industry data recorded in the city of long beach ca they show that the moho is dipping at 65 and obliquely truncates an 10 km thick flat lying lower crustal fabric the moho appears to be fault controlled and an integral part of the extrusion of the catalina schist that underlays the inner borderland the basement interface has significant offsets of up to 2 km none of which correspond to the mapped trace of the newport inglewood fault

crustal shear wave velocity structure of the western united states inferred from ambient seismic noise and earthquake data 1 surface wave dispersion measurements from ambient seismic noise and array based measurements from teleseismic earthquakes observed with the earthscope usarray transportable array ta are inverted using a monte carlo method for a 3 d vs model of the crust and uppermost mantle beneath the western united states the combination of data from these methods produces exceptionally broadband dispersion information from 6 to 100 s period which constrains shear wave velocity structures in the crust and uppermost mantle to a depth of more than 100 km the high lateral resolution produced by the ta network and the broadbandedness of the dispersion information motivate the question of the appropriate parameterization for a 3 d model particularly for the crustal part of the model we show that a relatively simple model in which vs increases monotonically with depth in the crust can fit the data well across more than 90 of the study region except in eight discrete areas where greater crustal complexity apparently exists the regions of greater crustal complexity are the olympic peninsula the mendocinotriple junction the yakima fold belt the southern cascadia back arc the great central valley of california the salton trough the snake river plain and the wasatch mountains we also show that a strong rayleigh love discrepancy exists across much of the western united states which can be resolved by introducing radial anisotropy in both the mantle and notably the crust we focus our analysis on demonstrating the existence of crustal radial anisotropy and primarily discuss the crustal part of the isotropic model that results from the radially anisotropic model by voigt averaging model uncertainties from the monte carlo inversion are used to identify robust isotropic features in the model the uppermost mantle beneath the western united states is principally composed of four large scale shear wave velocity features but lower crustal velocity structure exhibits far greater heterogeneity we argue that these lower crustal structures are predominantly caused by interactions with the uppermost mantle including the intrusion and underplating of mafic mantle materials and the thermal depression of wave speeds caused by conductive heating from the mantle upper and middle crustal wave speeds are generally correlated and notable anomalies are inferred to result from terrane accretion at the continental margin and volcanic intrusions

the 11 march 2011 tohoku tsunami wavefront mapping across offshore southern california the 11 march 2011 m w 9 0 tohoku tsunami was recorded by a temporary array of seafloor pressure gauges deployed off the coast of southern california demonstrating how dense array data can illustrate and empirically validate predictions of linear tsunami wave propagation characteristics a noise cross correlation method was used to first correct for the pressure gauge instrument phase response phase and group travel times were then measured for the first arrival in the pressure gauge tsunami waveforms filtered in narrow bands around 30 periods between 200 and 3000 s for each period phase velocities were estimated across the pressure gauge array based on the phase travel time gradient using eikonal tomography clear correlation was observed between the phase velocity and long wavelength bathymetry variations where fast and slow velocities occurred for deep and shallow water regions respectively in particular velocity gradients are pronounced at the patton escarpment and near island plateaus due to the abrupt bathymetry change in the deep open ocean area clear phase velocity dispersion is observed comparison with numerically calculated tsunami waveforms validates the approach and provides an independent measure of the finite frequency effect on phase velocities at long periods

seismic fault zone trapped noise systematic velocity contrasts across and within fault zones can lead to head and trapped waves that provide direct information on structural units that are important for many aspects of earthquake and fault mechanics here we construct trapped waves from the scattered seismic wavefield recorded by a fault zone array the frequency dependent interaction between the ambient wavefield and the fault zone environment is studied using properties of the noise correlation field a critical frequency fc 0 5 hz defines a threshold above which the in fault scattered wavefield has increased isotropy and coherency compared to the ambient noise the increased randomization of in fault propagation directions produces a wavefield that is trapped in a waveguide cavity like structure associated with the low velocity damage zone dense spatial sampling allows the resolution of a near field focal spot which emerges from the superposition of a collapsing time reversed wavefront the shape of the focal spot depends on local medium properties and a focal spot based fault normal distribution of wave speeds indicates a 50 velocity reduction consistent with estimates from a far field travel time inversion the arrival time pattern of a synthetic correlation field can be tuned to match properties of an observed pattern providing a noise based imaging tool that can complement analyses of trapped ballistic waves the results can have wide applicability for investigating the internal properties of fault damage zones because mechanisms controlling the emergence of trapped noise have less limitations compared to trapped ballistic waves

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

imaging widespread seismicity at midlower crustal depths beneath long beach ca with a dense seismic array evidence for a depth dependent earthquake size distribution we use a dense seismic array composed of 5200 vertical geophones to monitor microseismicity in long beach california poor signal to noise ratio due to anthropogenic activity is mitigated via downward continuation of the recorded wavefield the downward continued data are continuously back projected to search for coherent arrivals from sources beneath the array which reveals numerous previously undetected events the spatial distribution of seismicity is uncorrelated with the mapped fault traces or with activity in the nearby oil fields many events are located at depths larger than 20 km well below the commonly accepted seismogenic depth for that area the seismicity exhibits temporal clustering consistent with omori s law and its size distribution obeys the gutenberg richter relation above 20 km but falls off exponentially at larger depths the dense array allows detection of earthquakes two magnitude units smaller than the permanent seismic network in the area because the event size distribution above 20 km depth obeys a power law whose exponent is near one this improvement yields a hundred fold decrease in the time needed for effective characterization of seismicity in long beach

significance of stable isotope variations in crustal rocks from the kola superdeep borehole and their surface analogues abstract the kola superdeep borehole the deepest ever drilled into the earth s crust with a depth of 12 2 km is located in the kola peninsula russia and consists of high grade metasedimentary and metaigneous lithologies ranging in age from archean to proterozoic metabasic rocks and amphibolites from the borehole have 18o values in the range from 4 4 to 9 2 and d values in the range from 73 to 40 gneisses and a migmatite have 18o values in the range from 7 8 to 9 4 and d values in the range from 63 to 44 and metasedimentary rocks show large variations in 18o values from 6 3 to 15 and have d values in the range from 57 to 33 the isotopic compositions of the deep borehole samples are identical with their surface equivalents indicating that the processes of emplacement and exposure of deep crustal rocks have not affected their h and o isotopic compositions in marked contrast to what has been observed for crust in tectonically active areas fluid compositions calculated from the 18o and d values of coexisting amphibole feldspar biotite and garnet indicate that two isotopically distinct fluids have interacted with the kola superdeep borehole rocks one fluid has isotopic compositions consistent with locally rock buffered regional metamorphic fluids and the other fluid has compositions expected for paleoproterozoic seawater the 18o values of co existing amphibole feldspar and biotite garnet pairs in amphibolites and quartzite have apparent equilibration temperatures from ca 500 c to 800 c which are in good agreement with temperatures determined by cation exchange geothermometry and co existing assemblages only rocks associated with two major faults and the thrust zone that marks the proterozoic archean boundary indicate isotopic exchange with regional metamorphic fluids circulating along these structures the stable isotopic compositions of the kola superdeep borehole rocks indicate that there was interaction of the rocks with channelized fluids moving along major faults but there was no pervasive fluid flow during metamorphism

theoretical limits on detection and analysis of small earthquakes we investigate theoretical limits on detection and reliable estimates of source characteristics of small earthquakes using synthetic seismograms for shear tensile dislocations on kinematic circular ruptures and observed seismic noise and properties of several acquisition systems instrument response sampling rate simulated source time functions for shear tensile dislocation events with different magnitudes static stress drops and rupture velocities provide estimates for the amplitude and frequency content of p and s phases at various observation angles the source time functions are convolved with a green s function for a homogenous solid assuming given p s wave velocities and attenuation coefficients and a given instrument response the synthetic waveforms are superposed with average levels of the observed ambient seismic noise up to 1 khz the combined seismograms are used to calculate signal to noise ratios and expected frequency content of p and s phases at various locations the synthetic simulations of signal to noise ratio reproduce observed ratios extracted from several well recorded data sets the results provide guidelines on detection of small events in various geological environments along with information relevant to reliable analyses of earthquake source properties

the san andreas and walker lane fault systems western north america transpression transtension cumulative slip and the structural evolution of a major transform plate boundary abstract relative right lateral pacific north american transform motion across the western us is largely taken up by the san andreas and walker lane fault systems cumulative lateral displacement on active strands of the san andreas system is 3 4 or more times greater than across faults of the walker lane the san andreas system is transpressional while the walker lane system is transtensional the relatively more complex discontinuous and broader system of faults composing the walker lane system is attributed to lower cumulative slip and an attendant extensional component of motion greater slip accompanied by a small component of contraction has yielded the simpler more continuous and generally throughgoing set of faults that comprise the san andreas system with respect to the contrasting amounts of cumulative slip the san andreas is at a more mature stage of structural development than the walker lane despite differences in gross patterns of faulting the systems also share similarities in deformation style slip along each is locally accommodated by clockwise rotation of crustal blocks that has produced structural basins and oblique components of slip are commonly partitioned into subparallel strike slip and dip slip faults

retrieval of moho reflected shear wave arrivals from ambient seismic noise theoretical studies on ambient seismic noise asn predict that complete green s function between seismic stations can be retrieved from cross correlation however only fundamental mode surface waves emerge in most studies involving real data here we show that moho reflected body wave sms and its multiples can be identified with asn for station pairs near their critical distances in the short period band 1 5 s we also show that an uneven distribution of noise sources such as mining activity and wind topography interaction can cause surface wave precursors which mask weaker body wave phases

magma bright spots mapped beneath krafla iceland using rvsp imaging of reflected waves from microearthquakes abstract the geometry and distribution of magma in the crust remain controversial topics with recent studies questioning the role of large magma chambers in this investigation high resolution 3d reflection images of crustal discontinuities beneath the krafla geothermal field in northern iceland were generated by applying vertical seismic profiling vsp techniques adapted from reflection seismology to microearthquake data exceptionally large amplitude reflections bright spots at a depth of 2 1 km correlate with rhyolitic magma encountered in the iddp 1 borehole although similarly bright reflectors at about 4 km correspond in depth to the top of an inferred magma chamber from previous seismic studies the scattered reflectivity that persists beneath this deeper reflector argues for a distributed magma system rather than a large feeder chamber

amplification and attenuation across usarray using ambient noise wavefront tracking as seismic traveltime tomography continues to be refined using data from the vast usarray data set it is advantageous to also exploit the amplitude information carried by seismic waves we use ambient noise cross correlation to make observations of surface wave amplification and attenuation at shorter periods 8 32 s than can be observed with only traditional teleseismic earthquake sources we show that the wavefront tracking approach can be successfully applied to ambient noise correlations yielding results quite similar to those from earthquake observations at periods of overlap this consistency indicates that the wavefront tracking approach is viable for use with ambient noise correlations despite concerns of the inhomogeneous and unknown distribution of noise sources the resulting amplification and attenuation maps correlate well with known tectonic and crustal structure at the shortest periods our amplification and attenuation maps correlate well with surface geology and known sedimentary basins while our longest period amplitudes are controlled by crustal thickness and begin to probe upper mantle materials these amplification and attenuation observations are sensitive to crustal materials in different ways than traveltime observations and may be used to better constrain temperature or density variations we also value them as an independent means of describing the lateral variability of observed rayleigh wave amplitudes without the need for 3 d tomographic inversions

analysis of teleseismic p waves with a 5200 station array in long beach california evidence for an abrupt boundary to inner borderland rifting we analyze teleseismic p waves from four mw 6 5 earthquakes recorded by a petroleum industry survey in long beach california the survey used a 2 d array with up to 5200 seismometers 120 m mean spacing and 7 10 km aperture at frequencies near 1 hz p wave travel times and amplitudes exhibit coherent lateral variations over scales as short as 400 m including locally delayed travel times and increased amplitudes at the crest of the long beach anticline deeper heterogeneity is indicated by p wave phase velocities that deviate from reference model predictions for events from southwestern azimuths we postulate that a sharp northeastward increase in moho depth from the inner borderland ib to mainland southern california causes the anomalous phase velocities elastic forward modeling finds the travel times are fit well by a moho that dips 65 to the northeast and flattens 10 km southwest of the newport inglewood fault zone constraining the felsic thickness of mainland crust to 28 km requires an 8 km thick layer with a p velocity of 7 km s beneath it which could result from basal accretion of former farallon ocean crust or magmatic underplating during miocene volcanism forward models with a 65 moho dip predict a p to s conversion with a phase velocity of 5 km s deconvolution of the array s mean p wave signal isolates a similar later arriving phase the steep crust thickness transition supports a locally abrupt boundary to ib rifting our results highlight the utility of dense short period arrays for passive imaging at near surface to uppermost mantle depths

shallow crustal imaging using distant high magnitude earthquakes global phases viz seismic phases that travel through the earth s core can be used to locally image the crust by means of seismic interferometry this method is known as global phase seismic interferometry glopsi traditionally glopsi retrieves low frequency information up to 1 hz recent studies however suggest that there is high frequency signal present in the coda of strong distant earthquakes this research quantifies the potential of these high frequency signals by analysing recordings of a multitude of high magnitude earthquakes 6 4 mw and their coda on a selection of permanent usarray stations nearly half of the p pkp and pkikp phases are recorded with a signal to noise ratio of at least 5 db at 3 hz to assess the viability of using the high frequency signal the second half of the paper highlights two case studies first a known sedimentary structure is imaged in malargue argentina secondly the method is used to reveal the structure of the midcontinent rift below the spree array in minnesota usa both studies demonstrate that structural information of the shallow crust 5 km below the arrays can be retrieved in particular the interpreted thickness of the sedimentary layer below the malargue array is in agreement with earlier studies in the same area being able to use global phases and direct p phases with large epicentral distances 80 to recover the earth s sedimentary structure suggests that glopsi can be applied in an industrial context

tomography of southern california via bayesian joint inversion of rayleigh wave ellipticity and phase velocity from ambient noise cross correlations the data used in this work are archived and distributed by the southern california earthquake data center scedc http scedc caltech edu research tools waveform html and the iris earthquake data center https ds iris edu ds nodes dmc both the scec community velocity models used in this work cvms 4 26 and cvmh 15 1 0 are available from the southern california earthquake center https github com sceccode ucvmc this study was supported by the southern california earthquake center award 17195 the national science foundation grants cybersees 1442665 and ear 162061 and the king abdullah university of science and technology award ocrf 2014 crg3 2300 we also thank the reviewers of this paper an anonymous individual and jan dettmer for their valuable insight and constructive reviews of the manuscript the 3 d shear wave velocity model will be available from iris data services products earth model collaboration https ds iris edu ds products emc

surface wave array tomography in se tibet from ambient seismic noise and two station analysis i phase velocity maps summary empirical green s functions egfs between pairs of seismographs can be estimated from the time derivative of the long time cross correlation of ambient seismic noise these egfs reveal velocity dispersion at relatively short periods which can be used to resolve structures in the crust and uppermost mantle better than with traditional surface wave tomography we combine rayleigh wave dispersion estimates from egfs and from traditional two station ts analysis into a new approach to surface wave array tomography with data from dense receiver arrays we illustrate the methodology with continuous broad band recordings from a temporary seismographic network on the southeastern part of the tibetan plateau in sichuan and yunnan provinces sw china the egfs are robust under temporal changes in regional seismicity and the use of either ambient noise approximated by records without signal from events with magnitude mb 5 or 4 or surface wave coda produces similar results the egfs do not strongly depend on the presence of large earthquakes but they are not reciprocal for stations aligned in the n s direction this directionality reflects the paucity of seismicity to the north of the array using a far field representation of the surface wave green s function and an image transformation technique we infer from the egfs the rayleigh wave phase velocity dispersion in the period band from 10 30 s a classical ts approach is used to determine rayleigh wave phase velocity dispersion between 20 120 s together they constrain phase velocity variations for t 10 120 s which can be used to study the structure from the crust to the upper mantle beneath se tibet short and intermediate period 10 80 s phase velocities are prominently low suggesting that the crust and upper mantle beneath se tibet is characterized by slow shear wave propagation

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

potential misidentification of love wave phase velocity based on three component ambient seismic noise people have calculated rayleigh wave phase velocities from vertical component of ambient seismic noise for several years recently researchers started to extract love waves from transverse component recordings of ambient noise where transverse is defined as the direction perpendicular to a great circle path or a line in small scale through observation sensors most researches assumed rayleigh waves could be negligible but rayleigh waves can exist in the transverse component when rayleigh waves propagate in other directions besides radial direction in study of data acquired in western junggar basin near karamay city china after processing the transverse component recordings of ambient noise we obtain two energy trends which are distinguished with rayleigh wave and love wave phase velocities in the frequency velocity domain using multichannel analysis of surface waves masw rayleigh waves could be also extracted from the transverse component data because rayleigh wave and love wave phase velocities are close in high frequencies 0 1 hz two kinds of surface waves might be merged in the frequency velocity domain rayleigh wave phase velocities may be misidentified as love wave phase velocities to get accurate surface wave phase velocities from the transverse component data using seismic interferometry in investigating the shallow geology our results suggest using masw to calculate real love wave phase velocities

shear wave structure of a transect of the los angeles basin from multimode surface waves and h v spectral ratio analysis we use broad band stations of the los angeles syncline seismic interferometry experiment lassie to perform a joint inversion of the horizontal to vertical spectral ratios h v and multimode dispersion curves phase and group velocity for both rayleigh and love waves at each station of a dense line of sensors the h v of the autocorrelated signal at a seismic station is proportional to the ratio of the imaginary parts of the green s function the presence of low frequency peaks 0 2 hz in h v allows us to constrain the structure of the basin with high confidence to a depth of 6 km the velocity models we obtain are broadly consistent with the scec cvm h community model and agree well with known geological features because our approach differs substantially from previous modelling of crustal velocities in southern california this research validates both the utility of the diffuse field h v measurements for deep structural characterization and the predictive value of the cvm h community velocity model in the los angeles region we also analyse a lower frequency peak 0 03 hz in h v and suggest it could be the signature of the moho finally we show that the independent comparison of the h and v components with their corresponding theoretical counterparts gives information about the degree of diffusivity of the ambient seismic field

joint inversion of rayleigh wave phase velocity and ellipticity using usarray constraining velocity and density structure in the upper crust rayleigh wave ellipticity or h v ratio observed on the surface is particularly sensitive to shallow earth structure in this study we jointly invert measurements of rayleigh wave h v ratio and phase velocity between 24 100 and 8 100 sec period respectively for crust and upper mantle structure beneath more than 1000 usarray stations covering the western united states upper crustal structure in particular is better constrained by the joint inversion compared to inversions based on phase velocities alone in addition to imaging vs structure we show that the joint inversion can be used to constrain vp vs and density in the upper crust new images of uppermost crustal structure 3 km depth are in excellent agreement with known surface features with pronounced low vs low density and high vp vs anomalies imaged in the locations of several major sedimentary basins including the williston powder river green river denver and san juan basins these results demonstrate not only the consistency of broadband h v ratios and phase velocity measurements but also that their complementary sensitivities have the potential to resolve density and vp vs variations

strong near surface seismic anisotropy of taiwan revealed by coda interferometry abstract we report the near surface v p shear wave velocity v s v p v s ratio poisson s ratio and v s anisotropy of taiwan by applying seismic coda interferometry to 34 borehole surface station pairs we find clear characteristic cos 2 dependence of v s in all the determinations and about half of the amplitudes of anisotropy are larger than 10 with the largest amplitudes up to 34 the patterns of anisotropy fall into two categories opa orogeny parallel anisotropy and saa stress aligned anisotropy both types of anisotropy fit well the local geological fabrics and or the ambient stress and show strong correlation with the poisson s ratios at the borehole sites with these new findings and reported tomographic results we infer that the saa are likely confined to the uppermost portion of the crust in particular to the fluid saturated late quaternary deposits the strong near surface anisotropy also implies that delay times contributed by the shallow crust might have been underestimated in studies of shear wave splitting measurements using the direct arrivals of earthquake waves

body wave imaging of earth s mantle discontinuities from ambient seismic noise ambient seismic noise correlations are widely used for high resolution surface wave imaging of earth s lithosphere similar observations of the seismic body waves that propagate through the interior of earth would provide a window into the deep earth we report the observation of the mantle transition zone through noise correlations of p waves as they are reflected by the discontinuities associated with the top 410 kliometers km and the bottom 660 km of this zone our data demonstrate that high resolution mapping of the mantle transition zone is possible without using earthquake sources

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

surface wave array tomography in se tibet from ambient seismic noise and two station analysis ii crustal and upper mantle structure summary we determine the 3 d shear wave speed variations in the crust and upper mantle in the southeastern borderland of the tibetan plateau sw china with data from 25 temporary broad band stations and one permanent station interstation rayleigh wave phase velocity dispersion curves were obtained at periods from 10 to 50 s from empirical green s function egf derived from ambient noise interferometry and from 20 to 150 s from traditional two station ts analysis here we use these measurements to construct phase velocity maps from 10 to 150 s using the average interstation dispersion from the egf and ts methods between 20 and 50 s and estimate from them with the neighbourhood algorithm the 3 d wave speed variations and their uncertainty the crust structure parametrized in three layers can be well resolved with a horizontal resolution about of 100 km or less because of the possible effect of mechanically weak layers on regional deformation of particular interest is the existence and geometry of low shear velocity layers lvls in some regions prominent lvls occur in the middle crust in others they may appear in the lower crust in some cases the lateral transition of shear wave speed coincides with major fault zones the spatial variation in strength and depth of crustal lvls suggests that the 3 d geometry of weak layers is complex and that unhindered crustal flow over large regions may not occur consideration of such complexity may be the key to a better understanding of relative block motion and patterns of seismicity

high frequency rayleigh wave tomography using traffic noise from long beach california abstractusing a dense seismic array in long beach california we have investigated the effectiveness of using traffic noise for passive subsurface imaging spectral analysis revealed that traffic induced vibrations dominate the ambient seismic noise field at frequencies between 3 and 15 hz using the ambient noise crosscorrelation technique at these frequencies we have extracted fundamental and first order mode rayleigh waves generated by interstate 405 and local roads we picked group traveltimes associated with the fundamental mode and used them in a straight ray tomography procedure to produce group velocity maps at 3 0 and 3 5 hz the velocity trends in our results corresponded to shallow depths and coincided well with lithologies outlined in a geologic map of the survey area the most prominent features resolved in our velocity maps were the low velocities to the north corresponding to less consolidated materials high velocities to the south corresponding to more consolidated materials a low vel

stress induced upper crustal anisotropy in southern california 1 we use an automated method to analyze shear wave splitting from local earthquakes recorded by the southern california seismic network between 2000 and 2005 the observed fast directions of upper crustal anisotropy generally are consistent with the direction of maximum horizontal compression hmax suggesting that one major mechanism of anisotropy in the top 20 km of crust under southern california is regional stress however at other stations fast directions are aligned with the trends of regional faulting and local alignment of anisotropic bedrock splitting delay times range widely within 0 2 s these upper crustal anisotropy observations together with previous studies of sks shear wave splitting surface waves and receiver functions suggest different mechanisms of anisotropy at different depths under southern california anisotropy in the upper crust appears to be in response to the current horizontal maximum compression hmax which differs from the cause of anisotropy in the lower crust and mantle we also explore possible temporal variations in upper crustal anisotropy associated with preearthquake stress changes or stress changes excited by surface waves of great earthquakes but do not observe any clear temporal variations in fast directions or time delays

shallow crustal velocities and volcanism suggested from ambient noise studies using a dense broadband seismic network in the tatun volcano group of taiwan abstract the tatun volcano group tvg is situated adjacent to the taipei metropolis and was active predominantly around 0 8 0 2 ma pleistocene various recent lines of evidence suggest that the tvg is a potentially active volcano and that future volcanic eruptions cannot be ruled out geothermal activities are largely constrained to faults but the relationship between volcanism and detailed velocity structures is not well understood we analyzed ambient seismic noise of daily vertical components from 2014 using a dense seismic network of 40 broadband stations we selected a 0 02 grid spacing to construct 2d and 3d shallow crustal phase velocity maps in the 0 5 3 s period band two s wave velocity profiles transect chishingshan mt cs in the shallow 3 km crust are further derived the footwall of the shanchiao fault is dominated by low velocity which may relate to tertiary bedrock buried under andesitic lava flows dozens to hundreds of meters thick the hanging wall of the shanchiao fault is the location of recent major volcanic activities low velocity zones in the southeast of dayoukeng dyk may be interpreted as hydrothermal reservoirs or water saturated tertiary bedrock related to cenozoic structures in the shallow crust high velocities conspicuously dominate the east of the tvg where the earliest stages of volcanism in the tvg are located but where surface hydro geothermal activities were absent in recent times between the shanchiao fault and kanchiao fault high velocities were detected which converge below mt cs and may be related to early stages of magma conduits that gradually consolidated these two faults may play a significant role with the tvg the submarine volcanism adjacent to the keelung coastline also requires further attention

delineating shallow s wave velocity structure using multiple ambient noise surface wave methods an example from western junggar china abstract aiming for a geophysical survey to around 1 5 km depth we conduct a passive source surface wave survey along a 70 km long line on the western side of the junggar basin in northwest china we use frequency time analysis and spatial autocorrelation methods to extract surface wave dispersion curves from ambient noise data of station pairs as well as ambient noise multichannel analysis of surface waves from multistations dispersion curves corresponding to the same positions obtained by these three methods independently are comparable which shows the validity of using these methods after inverting more than 400 dispersion curves and aligning their results together we construct a 2d shear s wave velocity image around 1 5 km deep and 60 km long a less than 6 difference between the inverted s wave velocity profile and the borehole data in situ shows the relatively high accuracy of our method the 2d s wave velocity image clearly delineates the main geologic structures including transition of landform intrusion of rock mass and shape of bedrock beneath the survey line

body wave reconstruction from ambient seismic noise correlations in an underground mine abstractthe reconstruction of seismic green s functions from correlations of ambient seismic noise has recently developed as a promising approach for exploring the earth s interiors without the requirement of costly active seismic sources this approach is widely used for imaging the crust at a kilometer scale however few studies report noise based green s function reconstruction at smaller scales in industrial environments we have investigated the possibility of constructing seismic green s functions between sensors in an active underground mine garpenberg sweden by crosscorrelating seismic noise generated by mining activities we have determined with realistic numerical simulations that the mining excavations in an underground mine lead to a regime of strong scattering which is favorable for constructing seismic green s functions by crosscorrelating seismic noise one month of continuous data was recorded by 18 seismic sensors located more than 1 km below surface a variety of broadband 10 3000

shallow crustal structure of the tanlu fault zone near chao lake in eastern china by direct surface wave tomography from local dense array ambient noise analysis ambient noise tomography ant has been used successfully to image shallow earth structure here we perform ant on a local dense seismic array around the tanlu fault zone tfz to the southeast of hefei city anhui province in eastern china the array consists of 53 stations with average spacing close to 5 km cross correlations of vertical component ambient noise data of different station pairs are computed in 1 h segments and stacked over 1 month from 17 march to 26 april 2017 clear fundamental mode rayleigh waves are observed between 0 2 and 5 s period we then use the direct surface wave tomographic method with period dependent ray tracing to invert group and phase dispersion travel time data simultaneously for three dimensional 3d shear wave velocity vs structure the vs model shows clear correlation with the known geologic features the tfz is associated with a high velocity anomaly zone in the shallow crust corresponding to metamorphic rocks due to magma intrusion low velocity anomaly zones are mainly located to the west of the tfz caused by thick sedimentary layers in the hefei basin our study shows that with ambient noise data recorded on a dense array and an advanced surface wave inversion method we can image detailed structure around the fault zone

chemical weathering controls on variations in the molybdenum isotopic composition of river water evidence from large rivers in china abstract mo isotopic composition in large rivers is very important for understanding global mo cycle at present temporal variation signatures in the 98 95 mo in large rivers have not been investigated which hinder a comprehensive understanding on the mechanism for the variations of mo isotopic compositions in river water in this study we report a one year long time series march 2010 to march 2011 of the 98 95 mo of both the water and suspended particulates collected at guiping from the middle reaches of the xijiang river xjr and of the water 98 95 mo from the lower reaches of the huanghe river hhr or yellow river collected at lijin china the results indicate that the temporal variations in the concentration and 98 95 mo of dissolved mo in the xjr range from 4 32 to 10 5 nmol l mean 7 31 nmol l and 1 04 to 1 31 relative to nist 3134 mean 1 20 respectively but that the particulates have a lower 98 95 mo 0 18 to 0 58 analysis of the suspended particulates and other chemical parameters of the river water suggests that the weathering of silicates and sulfides is the main contributor to the dissolved mo content in the xjr subsequently the highly efficient preferential trapping of lighter 98 95 mo by weathering products such as clay minerals fe mn oxides and organic materials in soils and saprolites which are abundant in the tropical catchment is the key to the heavy mo isotope signatures in the xjr furthermore it seems that neither mo scavenging by suspended particulates during riverine transportation nor the weathering of sulfates significantly influences the 98 95 mo of the xjr water cross comparison among the 98 95 mo in the waters of the three largest rivers in china the changjiang cjr hhr and xjr indicates that the hhr has the highest mo concentrations but the lightest 98 95 mo the xjr has the lowest mo concentrations but the heaviest 98 95 mo and the values for the cjr are intermediate between those for the other two rivers this supports the conclusion that the efficiency of the selective trapping of mo by soils and saprolites is the main factor controlling 98 95 mo in these large rivers the tropical subtropical xjr catchment experiences intense chemical weathering the semi arid temperate hhr catchment experiences very little and the cjr catchment falls somewhere between the two such a relationship between water mo concentrations and 98 95 mo agrees with previous observations from many large rivers worldwide it is therefore suggested that chemical weathering on continents is the key to variations in the isotopic composition of mo in the waters of large rivers

fault zone imaging from correlations of aftershock waveforms we image an active fault zone environment using cross correlations of 154 15 s long 1992 landers earthquake aftershock seismograms recorded along a line array a group velocity and phase velocity dispersion analysis of the reconstructed rayleigh waves and love waves yields shear wave velocity images of the top 100 m along the 800 m long array that consists of 22 three component stations estimates of the position width and seismic velocity of a low velocity zone are in good agreement with the findings of previous fault zone trapped waves studies our preferred solution indicates the zone is offset from the surface break to the east 100 200 m wide and characterized by a 30 velocity reduction imaging in the 2 6 hz range resolves further a high velocity body of similar width to the west of the fault break symmetry and shape of zero lag correlation fields or focal spots indicate a frequency and position dependent wavefield composition at frequencies greater than 4 hz surface wave propagation dominates whereas at lower frequencies the correlation field also exhibits signatures of body waves that likely interact with the high velocity zone the polarization and late arrival times of coherent wavefronts observed above the low velocity zone indicate reflections associated with velocity contrasts in the fault zone environment our study highlights the utility of the high frequency correlation wavefield obtained from records of local and regional seismicity the approach does not depend on knowledge of earthquake source parameters which suggests the method can return images quickly during aftershock campaigns to guide network updates for optimal coverage of interesting geological features

reflection images from ambient seismic noise one application of seismic interferometry is to retrieve the impulse response green s function from crosscorrelation of ambient seismic noise various researchers show results for retrieving the surface wave part of the green s function however reflection retrieval has proven more challenging we crosscorrelate ambient seismic noise recorded along eight parallel lines in the sirte basin east of ajdabeya libya to obtain shot gathers that contain reflections we take advantage of geophone groups to suppress part of the undesired surface wave noise and apply frequency wavenumber filtering before crosscorrelation to suppress surface waves further after comparing the retrieved results with data from an active seismic exploration survey along the same lines we use the retrieved reflection data to obtain a migrated reflection image of the subsurface

seasonal variations of seismic velocities in the san jacinto fault area observed with ambient seismic noise s u m m a r y we observe seasonal seismic wave speed changes dv v in the san jacinto fault area and investigate several likely source mechanisms velocity variations are obtained from analysis of 6 yr data of vertical component seismic noise recorded by 10 surface and six borehole stations we study the interrelation between dv v records frequency dependent seismic noise properties and nearby environmental data of wind speed rain ground water level barometric pressure and atmospheric temperature the results indicate peak to peak seasonal velocity variations of 0 2 per cent in the 0 5 2 hz frequency range likely associated with genuine changes of rock properties rather than changes in the noise field phase measurements between dv v and the various environmental data imply that the dominant source mechanism in the arid study area is thermoelastic strain induced by atmospheric temperature variations the other considered environmental effects produce secondary variations that are superimposed on the thermal based changes more detailed work with longer data on the response of rocks to various known external loadings can help tracking the evolving stress and effective rheology at depth

observation of shallow anisotropy on local earthquake records at the garner valley southern california downhole array abstract we study very shallow s wave anisotropy effects on local earthquake seismograms recorded by a vertical array of accelerometers at garner valley southern california using seismic events recorded by sensors buried at depth in the crystalline bedrock 220 m in weathered crystalline rocks 22 m and in sediments 15 6 and 0 m we analyze the s wave vertical travel times and polarizations at different depths hodograms plotted for horizontal components at s onset exhibit a clear linear and constant polarization independent of both earthquake location and mechanism this alignment varies with depth and is n 325 13 at 220 m while n 25 20 at 22 15 6 and 0 m a for the deep propagation 220 m the s wave direction of polarization at 220 m is identical to the observation obtained on a crystalline outcrop at the knw usgs stations 7 km away studies conducted on this site concluded to a rock fabric anisotropy due to the alignment of minerals and or microcracks along the lineation direction b for the shallow s fast s slow travel time delays roughly correspond to half the dominant period of the signal and the subsurface linear polarization is interpreted as a degenerated ellipse the north 20 anisotropy fast axis coincides with the maximum horizontal compressive stress associated with the san andreas fault system the shallow anisotropy extension corresponds to slow velocities associated with altered granite it suggests either a stress induced anisotropy due to pore crack deformation in an altered e g hydrothermally medium or the superposition of two crystalline units having different lineation directions these data show that in presence of slow 0 to 200 m subsurface layers shallow anisotropy strongly affects the earthquake surface records in the usual frequency range below 30 hz used in short period seismology

seismic anisotropy of the shallow crust in the umbria marche italy region abstract small magnitude earthquake data recorded during the 1997 umbria marche italy seismic crisis have been used to study the shear wave splitting in the central apennine using the method of diagonalization of the covariance matrix a set of 1234 s waves impinging the earth at an angle less than the critical angle has been selected the cross correlation method was used to estimate the splitting time between the two horizontal s components and the direction of the symmetry axis of the anisotropic structure a further analysis based on visual inspection of corrected traces allowed us to estimate the anisotropic parameters for 884 s waves an average splitting time 32 3 4 ms has been estimated splitting times do not generally show a clear pattern with distance which is an indication of possible spatial variations of seismic anisotropy a minimum 2 km depth of the anisotropic layer is estimated the trend of normalized splitting time vs depth roughly indicates a maximum 4 5 km depth of the anisotropic layer the distribution of the fast symmetry axis indicates a main symmetry axis oriented along the direction of the maximum compressive stress in the area and a secondary symmetry axis almost orthogonal to the first one the stereographic plots of fast axis direction seem exclude a depth dependence of anisotropic parameters a clear spatial correlation between the directions of the fast symmetry axes at the considered stations and the orientations of the nearest fault systems and thrust fronts is found which confirms the spatial variability in the anisotropic properties of the area as inferred by the analysis of splitting times based also on previous studies results are interpreted in terms of the extensive dilatancy anisotropy eda model the estimated crack density is always smaller than the critical coalescence value enforcing the hypothesis that fluid filled separated crack systems control the polarization of s waves

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

the seismic traffic footprint tracking trains aircraft and cars seismically although naturally occurring vibrations have proven useful to probe the subsurface the vibrations caused by traffic have not been explored much such data however are less sensitive to weather and low visibility compared to some common out of road traffic sensing systems we study traffic generated seismic noise measured by an array of 5200 geophones that covered a 7 10 km area in long beach california usa with a receiver spacing of 100 m this allows us to look into urban vibrations below the resolution of a typical city block the spatiotemporal structure of the anthropogenic seismic noise intensity reveals the blue line metro train activity departing and landing aircraft in long beach airport and their acceleration and gives clues about traffic movement along the i 405 highway at night as low cost stand alone seismic sensors are becoming more common these findings indicate that seismic data may be useful for traffic monitoring

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others