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

ground motion prediction of realistic earthquake sources using the ambient seismic field 1 predicting accurate ground motion is critical for earthquake hazard analysis particularly in situations where sedimentary basins trap and amplify seismic waves we exploit the information carried by the ambient seismic field to extract surface wave green s functions between seismic stations and to predict long period ground motion from earthquakes to do so we modify the surface impulse response to correct for the source depth and for the double couple focal mechanism these corrections are derived under the assumption that material properties in the immediate vicinity of the source depend only on depth using this local 1 d assumption we solve the surface wave eigenproblem and compute the fundamental mode displacement eigenfunctions to express the surface wave excitation at the source we validate this technique which we call the virtual earthquake approach by comparing computed seismograms with earthquake waveforms from four moderate earthquakes that occur near broadband stations in southern california the depth and mechanism corrections show clear improvements of the predicted ground motion relative to the surface impulse response

characteristic magnetic behavior of subaerial and submarine lava units from the hawaiian scientific drilling project hsdp 2 1 this study presents rock magnetic properties and the magnetic mineralogy of subaerial and submarine lava flows of mauna loa and mauna kea volcanoes collected from the 3109 m deep hsdp 2 drill hole in hawaii three different groups of magnetic behavior are recognized in the subaerial lava flows related to the degree of high temperature oxidation during extrusion group 1 shows homogenous titanomagnetite with low xmt low curie temperatures tc 100 200 c and weak median demagnetizing fields 20 mt further subdivision into 1a and 1b subgroups is based on the low temperature behavior of magnetic susceptibility ms and hysteresis loops which indicate a contribution from ferrimagnetic cr al spinel below ca 160 c in the 1b type samples group 2 samples with exsolution lamellae of ilmenite in the titanomagnetites have higher tc 480 580 c and higher coercive forces 20 40 mt group 3 the highest oxidation stage is characterized by titanohematite bearing assemblages with enhanced median demagnetizing fields 35 85 mt and a significantly different low temperature ms behavior ms core logging shows a systematic variation occurs in the subaerial lava flows directly related to the degree of high temperature oxidation and their flow morphology aa lava flows have higher mean ms than other lava flow types besides these factors ms appears to be also affected by the magma composition of the various shield building stages mauna loa subaerial lava flows generally show lower mean susceptibilities 4 6 3 10 3 si than subaerial mauna kea lava flows 9 8 5 10 3 si as submarine lava flows show no group 3 assemblages no high temperature oxidation influenced these rocks some hyaloclastites and pillow breccias show low ms 1 10 3 si small amounts of nearly pure magnetite tc 580 c and high coercive forces up to 110 mt suggesting single domain and or superparamagnetic behavior the controlling mechanism of the magnetic properties in the submarine lava units is the cooling and quenching rate of lava flows which creates large grain size variations in titanomagnetites of varying compositions hydrothermal alteration as described from ocean floor or icelandic basalts is not an important process that influences the magnetic properties in the ocean island basalts from the hsdp 2 drill hole

on the relevance of born theory in global seismic tomography 1 does the application of seismic born theory as opposed to simpler ray theory lead to an improvement in tomographic images of the earth in recent publications montelli et al 2004a 2004b and van der hilst and de hoop 2005 among others have expressed opposite opinions we propose a quantitative approach to the comparison of tomographic images which we apply to the case of surface wave phase velocity maps derived with born vs ray theory

regional variation and frequency dependence of qβ in the crust of the united states approximate lg attenuation coefficient values are determined for paths in eastern north america at periods between 2 and 4 sec where no data were previously available these new data together with reported values at a period of 1 sec are consistent with values predicted by a frequency dependentent q of the crust in which q varies as 0 2 the new data are inconsistent with values which would be predicted by models having prominent maxima or minima in q 1 within the period range 1 to 5 sec lg attenuation coefficients were computed for the frequency independent crustal q models of cheng and mitchell 1981 for the basin and range province and colorado plateau of the western united states the predicted values at 1 sec for the colorado plateau and basin and range province are about twice as large and three times as large respectively as those predicted for the eastern united states recently reported values of q for 1 sec lg in those regions are consistent with the attenuation coefficients and q values of lg predicted by the colorado plateau and basin and range province models a model with a small degree of frequency dependence of q would be consistent but is not required by the data

ambient noise tomography of north tibet limits geological terrane signature to upper middle crust 1 we use ambient noise tomography to map regional differences in crustal rayleigh wave group velocities with periods of 8 40 s across north tibet using the international deep profiling of tibet and the himalaya phase iv arrays 132 stations deployed for 10 24 months for periods of 8 24 s sensitive to midcrustal depths of 5 30 km we observe striking velocity changes across the bangong nujiang and jinsha suture zones as well as the kunlun qaidam boundary from south to north we see higher velocities beneath the lhasa terrane lower velocities beneath the qiangtang higher velocities in the songpan ganzi and kunlun terranes and the lowest velocities beneath the qaidam basin maps at periods of 34 and 40 s sensitive to the middle and lower crust at depths of 30 60 km do not show evidence of changes across those boundaries any differences between the tibetan terrane lower crusts that were present at accretion have been erased or displaced by cenozoic processes and replaced almost ubiquitously by uniformly low velocities

attenuation tomography of the western united states from ambient seismic noise 1 we show that the spatial coherency of the ambient seismic field can be used for attenuation tomography in the western united states we evaluate the real portion of the spatial coherency with an elastic geometric spreading term a bessel function and a distance dependent decay an attenuation coefficient in order to invert the spatial coherency a weight stack inversion technique is applied we recover phase velocity and attenuation coefficient maps at periods of 8 32s which correspond to the elastic and anelastic structure at crustal and upper mantle depths the phase velocity maps obtained by this method are of similar resolution to more standard two station methods the attenuation results provide an important complement to the information extracted from earthquake based tomography several geological features are easily identifiable in the attenuation coefficient maps such as the highly attenuating sedimentary basins along the west coast of the united states and the highly attenuating yellowstone region and the boundaries of the snake river plains

imaging the deep earth insights into earth s thermal and chemical structure from the inner core to the crust have historically been gained from analyzing seismic waves excited by earthquakes 1 however the resolution of these methods is limited because nearly all earthquakes occur along tectonic plate boundaries leaving large areas that are seismically quiet recent developments in seismic noise tomography have shown great potential for high resolution imaging of earth s interior structure 2 this approach yields data wherever a seismic instrument can be deployed but the seismic noise signals 3 used to image the crustal and upper mantle structures often lack resolution for most of the remaining mantle on page 1063 of this issue poli et al 4 overcome some of these limitations enabling the imaging of deep earth structures

p wave attenuation of the yellowstone caldera from three dimensional inversion of spectral decay using explosion source seismic data using explosion source seismic refraction data recorded in the 1978 and 1980 yellowstone snake river plain seismic experiments a three dimensional inversion of differential p wave attenuation was used to assess the relative variations in q 1 in and around the volcanically active 45 km by 70 km yellowstone caldera northwestern wyoming differential attenuation was derived from spectral decay of upper crustal pg phases observed from six explosions and recorded at 90 temporary stations because of the relatively short time windows used to determine the spectral content a maximum entropy technique was employed to estimate the spectra that yielded an optimally small variance differential p wave attenuation was calculated from least squares determinations of the spectral ratios corrected for source and path effects the observed differential attenuation parameters were then inverted using a weighted least squares technique for a discretized 70 105 km three dimensional surface and upper crustal q 1 model of the yellowstone caldera and surrounding region results showed that the surface layer to depths of 2 km within the yellowstone caldera is characterized by relatively high attenuation with low q values less than 30 compared to values of 38 to 50 outside the caldera the higher attenuation in the caldera s surface layer is thought to be associated with quaternary lake sediments highly altered rhyolites and the possible influence of steam in areas of hydrothermal activity in the crystalline upper crust at depths of 2 km to 12 km q values of 40 to 70 were observed in areas of thick sedimentary fill northwest of the caldera and in areas of hydrothermal activity within the caldera upper crustal attenuation generally corresponded to q of 200 in areas that are interpreted to be associated with hot but now solidified granitic material in comparison relatively high attenuation q 40 was observed in the upper crust of the northeastern yellowstone caldera in an area that also corresponds to a 20 reduction in pg velocity a prominent negative bouguer gravity low of 20 mgal near yellowstone s largest area of hydrothermal activity and near a quaternary volcanic resurgent dome on the basis of constitutive models of velocity and density this zone of high attenuation is likely a product of unusual hydrothermal activity highly altered upper crustal rocks a shallow magmatic source or a highly porous steam saturated body outside the caldera the upper crust is relatively unaffected by high temperatures and thermal alteration and the corresponding attenuation is low with q values greater than 200

global models of surface wave attenuation 1 a large data set of fundamental mode rayleigh wave amplitudes is analyzed to derive global models of surface wave attenuation 1 q the data set consists of measurements of rayleigh wave amplitude anomalies in the period range 50 250 s for 347 earthquakes observed at 179 seismic stations the amplitude anomalies are considered to depend on four factors intrinsic attenuation along the ray path elastic focusing effects along the ray path a source factor accounting for uncertainties in the strength of excitation and a receiver factor accounting for uncertainties in the response at the station the amplitude data are inverted simultaneously for global maps of attenuation expanded in spherical harmonics up to degree 12 global maps of phase velocity expanded to degree 20 and source and receiver correction factors all four variable types are shown to be important in explaining the amplitude anomalies a data set of phase delay measurements provides additional constraints on velocity structure the maps of attenuation obtained by simultaneous inversion for elastic and anelastic models contain important features that are not robustly imaged when the effect of focusing on wave amplitude is ignored these include high attenuation along western north america and along the east pacific rise and other ridge systems and low attenuation associated with stable continental interiors the global attenuation maps exhibit a strong correlation with phase velocity maps corrected for the effect of the crust particularly for periods 200 s the correlation suggests that the variability in both q and velocity in the shallow upper mantle is primarily thermal in origin

earthquake ground motion prediction using the ambient seismic field 1 the waves generated by faulting represent the primary threat posed by most large earthquakes the effect of complex geological structures such as sedimentary basins on earthquake ground motion is a source of particular concern we show that it is possible to extract reliable phase and amplitude response that includes the effects of complex structure for the elastodynamic green s function from the ambient seismic field using deconvolution we demonstrate the accuracy of the approach by predicting complex ground motion for a moderate mw 4 6 earthquake in southern california as recorded in the los angeles basin this suggests a novel approach to seismic hazard analysis in which ground motion from hypothetical future earthquakes is simulated directly without the need for modeling the detailed heterogeneity of the earth s crust as an intermediate step

simulations of ground motion in the los angeles basin based upon the spectral element method we use the spectral element method to simulate ground motion generated by two recent and well recorded small earthquakes in the los angeles basin simulations are performed using a new sedimentary basin model that is constrained by hundreds of petroleum industry well logs and more than 20 000 km of seismic reflection profiles the numerical simulations account for 3d variations of seismic wave speeds and density topography and bathymetry and attenuation simulations for the 9 september 2001 m w 4 2 hollywood earthquake and the 3 september 2002 m w 4 2 yorba linda earthquake demonstrate that the combination of a detailed sedimentary basin model and an accurate numerical technique facilitates the simulation of ground motion at periods of 2 sec and longer inside the basin model and 6 sec and longer in the regional model peak ground displacement velocity and acceleration maps illustrate that significant amplification occurs in the basin

multiple frequency sh wave tomography of the western us upper mantle summary we estimate the sh wave velocity and attenuation structures of the western us upper mantle using the dense network of the usarray and new techniques we observe a multiple frequency data set of both traveltime and amplitude anomalies and interpret these with full 3 d finitefrequency sensitivity kernels amplitudes show stronger frequency dependence than traveltimes we perform a joint inversion on the measured traveltime and amplitude anomalies interpreting them in terms of velocity and attenuation heterogeneities aside from the expected clear division between the slow tectonically active region in the west and the fast craton in the east several interesting smaller velocity anomalies are observed the subduction along the cascades at 100 300 km depths shows lateral discontinuity with a slab hole absence of fast anomalies observed around 45 n the delaminated sierra nevada mountains root is observed to have sunk to 200 km depth the yellowstone plume seems to have an origin weak slow velocity anomalies near 1000 km depth but the plume conduit seems to be interrupted by a fast anomaly which is identified as a fragment of the farallon slab the s velocity model shows a trench perpendicular slab gap absence of fast anomalies at almost the same location as in the p model recently published by sigloch et al 2008 the methodological improvements described in the first paragraph have several benefits amplitude data help to sharpen the edges of narrow velocity heterogeneities in the shallow upper mantle the focusing effect from velocity heterogeneity dominates over that of attenuation and must be considered when interpreting amplitude anomalies in general velocity and attenuation heterogeneities correlate positively suggesting that temperature plays a major role in forming the anomalies

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

attenuation of lg in the western us using the usarray 1 the unprecedented breadth and density of the usarray allows us to resolve lateral variations of 1 hz lg attenuation to 0 5 degrees over much of the western united states we collected over 31 000 lg amplitudes from 547 events and 408 stations of the initial usarray footprint two dimensional tomographic inversion yielded q ranging from 60 to 550 with a variance reduction of 47 relative to the best fit constant q model q 155 q is low over most of the region with lowest values associated with recent volcanic activity and active shear zones high q is found in older stable crust notably the colorado and columbia plateaus while smaller islands of high q are associated with batholiths including the sierra nevada peninsular and bitterroot ranges the western mojave and okanogan highlands as well as other areas of competent crust such as the harcuvar range and two topographically bounded blocks in the basin and range

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

gps measurements of near field deformation along the southern dead sea fault system 1 analysis of short term deformation along the southern part of dead sea fault dsf provides a systematic view of kinematics this part of the continental transform the southern dsf consists of two principal segments the wadi araba and jordan valley faults in addition to other regional continuous gps data this study uses new data from 25 survey sites and 4 continuous gps stations in jordan for improved near field observations resulting velocities are reported with 1 uncertainties ranging from 0 4 1 0 mm yr application of elastic dislocation models yields estimates of slip rates for wadi araba and jordan valley faults are 4 9 0 4 mm yr and 4 7 0 4 mm yr respectively modeling also suggests different depths of effective fault locking with 15 5 km and 8 5 km for the wadi araba and jordan valley faults respectively these slip rates are generally consistent with the upper end of the range of slip rates estimated from late quaternary geology spatial variations in effective fault locking generally correspond with a heterogeneous mantle lithosphere a similar observation can be observed along the southern san andreas fault and this may reflect the influence of heterogeneity in the uppermost mantle on crustal faulting processes

seismic attenuation from recordings of ambient noise abstractwe applied seismic interferometry to data from an ocean bottom survey offshore norway and found that ambient seismic noise can be used to constrain subsurface attenuation on a reservoir scale by crosscorrelating only a few days of recordings by broadband ocean bottom seismometers we were able to retrieve empirical green s functions associated with surface waves in the frequency range between 0 2 and 0 6 hz and acoustic waves traveling through the sea water between 1 0 and 2 5 hz we discovered that the decay of these surface waves cannot be explained by geometrical spreading alone and required an additional loss of energy with distance we quantified this observed attenuation in the frequency domain using a modified bessel function to describe the cross spectrum in a stationary field we averaged cross spectra of equally spaced station couples and sorted these azimuthally averaged cross spectra with distance we then obtained frequency dependent estimates of attenuation by minimizing the misfit

seismic attenuation near the east pacific rise and the origin of the low velocity zone abstract low shear wave velocities beneath mid ocean ridges and in the low velocity zone beneath oceanic plates commonly have been attributed to the presence of melt or dissolved water but several recent studies have challenged that interpretation the alternative is that the anelastic effects of increasing temperature may cause the observed drop in velocity along with a predicted increase in attenuation we report the first measurements of surface wave attenuation within regional arrays of seismometers on the seafloor near the east pacific rise there is much less attenuation than is predicted by models in which the velocity is controlled solely by the direct elastic and anelastic effects of changing temperature suggesting that melt and water concentration do play an important role there also is somewhat less attenuation than is found in global studies we speculate that scattering from unresolved velocity heterogeneities contribute to the apparent attenuation in global studies

site response and attenuation in the puget lowland washington state simple spectral ratio ssr and horizontal to vertical h v site response estimates at 47 sites in the puget lowland of washington state document significant attenuation of 1 5 to 20 hz shear waves within sedimentary basins there amplitudes of the horizontal components of shear wave arrivals from three local earthquakes were used to compute ssrs with respect to the average of two bedrock sites and h v spectral ratios with respect to the vertical component of the shear wave arrivals at each site ssr site response curves at thick basin sites show peak amplifications of 2 to 6 at frequencies of 3 to 6 hz and decreasing spectral amplification with increasing frequency above 6 hz ssrs at nonbasin sites show a variety of shapes and larger resonance peaks we attribute the spectral decay at frequencies above the amplification peak at basin sites to attenuation within the basin strata computing the frequency independent depth dependent attenuation factor q s int from the ssr spectral decay between 2 and 20 hz gives values of 5 to 40 for shallow sedimentary deposits and about 250 for the deepest sedimentary strata 7 km depth h v site responses show less spectral decay than the ssr responses but contain many of the same resonance peaks we hypothesize that the h v method yields a flatter response across the frequency spectrum than ssrs because the h v reference signal vertical component of the shear wave arrivals has undergone a degree of attenuation similar to the horizontal component recordings correcting the ssr site responses for attenuation within the basins by removing the spectral decay improves agreement between ssr and h v estimates

strong lg attenuation in the tibetan plateau the widespread existence of strong lg attenuation in the tibetan plateau is further demonstrated by analysis of lg spectra on many paths within the plateau and quantitative estimates of the lg attenuation from low frequency lg signals for new localized path geometries strong path length dependent shifts of the lg spectra to lower corner frequencies with increasing distance are observed across the plateau consistent with a low regional average 1 hz lg attenuation value q 0 of about 125 there are clearly lateral variations within the plateau found in this and other recent studies with localized areas having q 0 values of 60 90 low enough to eliminate high frequency lg energy over path lengths of just several hundred kilometers while some localized areas may have higher q 0 values of up to 147 or higher a q 0 value of 103 is found in south central tibet compatible with recent work by others for higher frequencies on very localized scales and values from 83 to 147 are found in eastern tibet the lowest q 0 estimates found in tibet tend to be in areas for which there is evidence of volcanism and or partial melting within the crust however the strong regional attenuation may have a contribution from scattering by small scale crustal heterogeneity the strong lg attenuation in tibet gives a new constraint for understanding the tectonic development of the plateau and presents challenges to seismic monitoring of the region for possible clandestine nuclear tests

extracting surface wave attenuation from seismic noise using correlation of the coda of correlation 1 extracting surface wave travel time information from the cross correlation cc of seismic ambient noise has been a great success and remains fast growing however it is still challenging to exploit the amplitude content of the noise cc although spatial average is able to constrain somewhat meaningful attenuation using noise cc amplitudes clear bias is observed when spatially varying attenuation is estimated with the traditional noise cc calculation methods perhaps the key lies in the development of novel techniques that can mitigate the effect of the uneven distribution of natural noise sources in this paper we propose a new method to use the correlation of the coda of correlation of noise c3 for amplitude measurement we examine the ability of the method to retrieve surface wave attenuation using data from selected line array stations of the usarray by comparing c3 derived attenuation coefficients with those estimated from earthquake data we demonstrate that c3 effectively reduces bias and allows for more reliable attenuation estimates from noise this is probably because of the fact that the coda of noise correlation contains more diffused noise energy and thus the c3 processing effectively makes the noise source distribution more homogeneous when selecting auxiliary stations for c3 calculation we find that stations closer to noise sources near the coast tend to yield better signal to noise ratios we suggest to preprocess noise data using a transient removal and temporal flattening method to mitigate the effect of temporal fluctuation of the noise source intensity and to retain relative amplitudes in this study we focus our analysis on 18 s measurements

retrieving surface waves from ambient seismic noise using seismic interferometry by multidimensional deconvolution retrieving virtual source surface waves from ambient seismic noise by cross correlation assumes among others that the noise field is equipartitioned and the medium is lossless violation of these assumptions reduces the accuracy of the retrieved waves a point spread function computed from the same ambient noise quantifies the associated virtual source s spatial and temporal smearing multidimensional deconvolution mdd of the retrieved surface waves by this function has been shown to improve the virtual source s focusing and the accuracy of the retrieved waves using synthetic data we tested mdd on data recorded during the batholiths experiment a passive deployment of broadband seismic sensors in british columbia canada the array consisted of two approximately linear station lines using 4 months of recordings we retrieved fundamental mode rayleigh waves 0 05 0 27 hz we only used noise time windows dominated by waves that traverse the northern line before reaching the southern 2 5 of all data compared to the conventional cross correlation result based on this subset the mdd waveforms are better localized and have significantly higher signal to noise ratio furthermore mdd corrects the phase and the spatial deconvolution fills in a spectral f k domain gap between the single frequency and double frequency microseism bands frequency whitening of the noise also fills the gap in the cross correlation result but the signal to noise ratio of the mdd result remains higher comparison of the extracted phase velocities shows some differences between the methods also when all data are included in the conventional cross correlation

3d ground motion estimation in rome italy paleoseismic evidence and seismic hazard analysis suggest that the city of rome italy has experienced considerable earthquake ground motion since its establishment more than 2000 years ago seismic hazards in rome are mainly associated with two active seismogenic areas the alban hills and the central apennines regions located about 20 km southeast and 80 100 km east of central rome within the twentieth century m 6 8 and m 5 3 earthquakes in the apennines and the alban hills respectively have generated intensities up to mercalli cancani sieberg scale mcs vii in the city with a lack of strong motion records we have generated a 3d velocity model for rome embedded in a 1d regional model and estimated long period 1 hz ground motions for such scenarios from finite difference simulations of viscoelastic wave propagation we find 1 hz peak ground velocities pgvs and peak ground accelerations pgas of up to 14 cm sec and 44 cm sec2 respectively for a m 5 3 alban hills scenario largest near the northwestern edge of the tiber river our six simulations of a m 7 0 central apennine scenario generate 0 5 hz pgvs in rome of up to 9 cm sec as well as extended duration up to 60 sec the peak motions are similar to but the durations much longer than those from previous studies that omitted important wave guide effects between the source and the city the results from the two scenarios show that the strongest ground motion amplification in rome occurs in the holocene alluvial areas with strong basin edge effects in the tiber river valley our results are in agreement with earlier 2d sh wave results showing amplification of peak velocities by up to a factor of 2 in the alluvial sediments largest near the contact to the surrounding plio pleistocene formations our results suggest that both earthquakes from the alban hills and the central apennines regions contribute to the seismic hazards in rome although earthquakes from the former area may generate the larger peak motions seismic waves from the latter region may generate ground motions with extended durations capable of causing significant damage on the built environment

effects of the durations of crosscorrelated microtremor records on broadband dispersion measurements using seismic interferometry abstractfor ambient noise a long time series is typically used for measuring surface wave dispersion in seismic interferometry it is preferable to measure dispersions with a broad period range the reliability of such measurements is often studied using the signal to noise ratio s n of the crosscorrelation function ccf while many studies have revealed that the s n evolves as the length of a time series increases the required conditions for such measurements remain unclear we maximized the period range suitable for dispersion measurements by examining variations in the amplitudes of the signals and noise of ccfs for these purposes and to preserve the broadband amplitude information we do not apply filtering in the frequency domain or signal normalization in the time domain the preserved signals and the trailing noise levels of the ccfs exhibit different time varying features that agree with the predictions of theoretical work on amplitudes specifically as the duration of the crosscorrelated

magnetic state of 10 40 ma old ocean basalts and its implications for natural remanent magnetization abstract the natural remanent magnetization nrm of ocean basalts giving rise to the pattern of marine magnetic anomalies is known to be of comparatively low intensity for about 20 ma old oceanic crust the aim of this study is to detect possible peculiarities in the rock magnetic properties of ocean basalts of this age and to establish a link between magnetomineralogy rock magnetic parameters and the low nrm intensity ocean basalts covering ages from 0 7 to 135 ma were selected for rock magnetic experiments and their room temperature hysteresis parameters curie temperature and temperature dependence of saturation magnetization ms t was determined and complemented by reflected light microscopy the majority of samples is magnetically dominated by titanomagnetite and titanomaghemite with increasing oxidation state with age for these a strong dependence of hysteresis parameters on the age of the samples is found the samples have a minimum in saturation magnetization and a maximum in magnetic stability in the age interval ranging from approximately 10 to 40 ma coinciding with the age interval of low nrm intensity the observed change in saturation magnetization is in the same order as that for the nrm intensity a further peculiarity of the titanomaghemites from this age interval is the shape of their ms t curves which display a maximum above room temperature neel p type and sometimes a self reversal of magnetization below room temperature neel n type these special rock magnetic properties can be explained by titanomagnetite low temperature oxidation and highly oxidized titanomaghemites in the age interval 10 40 ma a corresponding measurement of the nrm at elevated temperature allows to identify a maximum in nrm intensity above room temperature for the samples in that age interval this provides evidence that the nrm is equally carried by titanomaghemites and that the low nrm intensities for about 20 ma old ocean basalts are caused consequently by the low saturation magnetization of these titanomaghemites

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

the trans atlantic geotraverse hydrothermal field a hydrothermal system on an active detachment fault abstract over the last ten years geophysical studies have revealed that the trans atlantic geotraverse tag hydrothermal field 26 08 n on the mid atlantic ridge is located on the hanging wall of an active detachment fault this is particularly important in light of the recognition that detachment faulting accounts for crustal accretion extension along a significant portion of the mid atlantic ridge and that the majority of confirmed vent sites on this slow spreading ridge are hosted on detachment faults the tag hydrothermal field is one of the largest sites of high temperature hydrothermal activity and mineralization found to date on the seafloor and is comprised of active and relict deposits in different stages of evolution the episodic nature of hydrothermal activity over the last 140 ka provides strong evidence that the complex shape and geological structure of the active detachment fault system exerts first order but poorly understood influences on the hydrothermal circulation patterns fluid chemistry and mineral deposition while hydrothermal circulation extracts heat from a deep source region the location of the source region at tag is unknown hydrothermal upflow is likely focused along the relatively permeable detachment fault interface at depth and then the high temperature fluids leave the low angle portion of the detachment fault and rise vertically through the highly fissured hanging wall to the seafloor the presence of abundant anhydrite in the cone on the summit of the tag active mound and in veins in the crust beneath provides evidence for a fluid circulation system that entrains significant amounts of seawater into the shallow parts of the mound and stockwork given the importance of detachment faulting for crustal extension at slow spreading ridges the fundamental question that still needs to be addressed is how do detachment fault systems and the structure at depth associated with these systems e g presence of plutons and or high permeability zones influence the pattern of hydrothermal circulation mineral deposition and fluid chemistry both in space and time within slowly accreted ocean crust

investigating the capability to extract impulse response functions from ambient seismic noise using a mine collapse event using recordings of a mine collapse event mw 4 2 in south korea in january 2015 we demonstrated that the phase and amplitude information of impulse response functions irfs can be effectively retrieved using seismic interferometry this event is equivalent to a single downward force at shallow depth using quantitative metrics we compared three different seismic interferometry techniques deconvolution coherency and cross correlation to extract the irfs between two distant stations with ambient seismic noise data the azimuthal dependency of the source distribution of the ambient noise was also evaluated we found that deconvolution is the best method for extracting irfs from ambient seismic noise within the period band of 2 10 s the coherency method is also effective if appropriate spectral normalization or whitening schemes are applied during the data processing

rayleigh wave tomography of the british isles from ambient seismic noise we present the first rayleigh wave group speed maps of the british isles constructed from ambient seismic noise the maps also constitute the first surface wave tomography study of the crust under the british isles at a relatively high resolution we computed interferometric interstation rayleigh waves from vertical component records of ambient seismic noise recorded on 63 broad band and short period stations across the uk and ireland group velocity measurements were made from the resulting surface wave dispersion curves between 5 and 25 s using a multiple phase matched filter method uncertainties in the group velocities were computed by calculating the standard deviation of four dispersion curves constructed by stacking a random selection of daily cross correlations where an uncertainty could not be obtained for a ray path using this method we estimated it as a function of the interreceiver distance group velocity maps were computed for 5 25 s period using the fast marching forward solution of the eikonal equation and iterative linearized inversion at short and intermediate periods the maps show remarkable agreement with the major geological features of the british isles including terrane boundaries in scotland regions of late palaeozoic basement uplift areas of exposed late proterozoic early palaeozoic rocks in southwest scotland northern england and northwest wales and sedimentary basins formed during the mesozoic such as the irish sea basin the chester basin the worcester graben and the wessex basin the maps also show a consistent low velocity anomaly in the region of the midlands platform a proterozoic crustal block in the english midlands at longer periods which are sensitive velocities in the lower crustal upper mantle the maps suggest that the depth of moho beneath the british isles decreases towards the north and west areas of fast velocity in the lower crust also coincide with areas thought to be associated with underplating of the lower crust such as northern ireland the eastern irish sea and northwest wales

anelastic earth structure from the coherency of the ambient seismic field 1 cross correlation of the ambient seismic field is now routinely used to measure seismic wave travel times however relatively little attention has been paid to other information that could be extracted from these signals in this paper we demonstrate the relationship between the spatial coherency of the ambient field and the elastodynamic green s function in both time and frequency domains through measurement of the frequency domain coherency as a function of distance we sequentially recover phase velocities and attenuation coefficients from these measurements we generate 1 d shear wave velocity and attenuation models for southern california the ambient field measurements of attenuation and the exceptional path coverage that results from the many possible interstation measurements allow us to extend q estimates over a range of frequencies that has previously been difficult to analyze using earthquake data measurements from paths that cross major sedimentary basins show both lower wave speeds and lower quality factors than other paths as expected our results indicate that there is a wealth of information available in the spatial coherency of the ambient seismic field

ambient field green s functions from asynchronous seismic observations 1 we demonstrate that the coda of station to station green s functions extracted from the ambient seismic field in southern california reach stability in the microseism band 5 10 s after correlating six months of noise data the coda stability makes it possible to retrieve green s functions between stations that operate asynchronously through scattered waves as recorded by a network of fiducial stations the green s functions extracted from asynchronous and synchronous data have comparable quality as long as stable virtual coda are used and both show good convergence to the green s functions extracted from 1 year of seismic noise with 50 fiducial stations this approach suggests that green s functions can be extracted across seismic stations regardless of whether or not they are occupied simultaneously which raises the prospect of a new mode for seismic experiments that seek to constrain earth structure

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

san andreas fault dip peninsular ranges mafic lower crust and partial melt in the salton trough southern california from ambient noise tomography we use ambient noise tomography to improve cvm h11 9 a community velocity model of southern california our new 3d shear velocity model with 0 05 x0 05 lateral and 1 km vertical blocks reveals new structure beneath the san andreas fault saf peninsular ranges batholith prb southern sierra nevada batholith snb and the salton trough st we use four years of data recorded on 849 broadband stations vastly more than previous studies and including our own broadband salton seismic imaging project a 40 station transect across the st as well as other campaign stations in both mexico and the u s a mean lower crust and upper mantle wavespeeds 3 6 km s 20km 4 2 km s 40km are low by global standards across the saf southeast of san gorgonio pass we observe vertical to steeply dipping lateral velocity contrasts that extend beneath the moho beneath the western prb and westernmost southern snb we observe relatively high shear velocities 3 8 km s in the lower crust that we interpret as the mafic roots of the overlying arc relatively high velocity upper mantle up to 4 5 km s may be part of the intact arc or possibly a remnant of the farallon plate beneath the st we observe zones of low shear velocity in the lower crust and upper mantle which permit up to 4 5 melt in the lower crust and up to 6 melt in the upper mantle depending on the assumed composition and pore geometry our results preclude the existence of older continental crust beneath the st and support the creation of new crust beneath the st this article is protected by copyright all rights reserved

extracting earth s elastic wave response from noise measurements recent research has shown that noise can be turned from a nuisance into a useful seismic source in seismology and other fields in science and engineering the estimation of the system response from noise measurements has proven to be a powerful technique to convey the essence of the method we first treat the simplest case of a homogeneous medium to show how noise measurements can be used to estimate waves that propagate between sensors we provide an overview of physics research dating back more than 100 years showing that random field fluctuations contain information about the system response this principle has found extensive use in surface wave seismology but can also be applied to the estimation of body waves because noise provides continuous illumination of the subsurface the extracted response is ideally suited for time lapse monitoring we present examples of time lapse monitoring as applied to the softening of soil after the 2011 tohoku oki earthquake the detection of a precursor to a landslide and temporal changes in the lunar soil

seismic wave attenuation in volcanic rocks from vsp experiments seismic wave attenuation in the columbia plateau basalts and snake river plain volcanics was analyzed using vertical seismic profiling vsp data the computation of attenuation coefficients is based on fitting a straight line to the logarithm of amplitude ratios computed for fixed values of frequency and variable depth this approach does not require any assumptions on the dependence of q on frequency for the columbia plateau basalts the attenuation coefficients obtained from the field data are smaller than those computed from the synthetic vsp generated using the sonic and density logs indicating that the observed attenuation is related to scattering effects and is substantially larger than the intrinsic attenuation of basalt therefore it is concluded that only a lower bound for q can be established in agreement with recent findings by other authors the effective attenuation of seismic energy in basalts about 1 1 10 4dbs m for the peak frequency is comparable to the effective attenuation observ

morin transition in hematite size dependence and thermal hysteresis 1 hematite is a frequently used mineral in paleomagnetic and environmental magnetic studies just below room temperature it undergoes a magnetic phase transition the morin transition whose nature is an important part of our basic understanding of hematite s magnetism and magnetic memory we have determined the temperature tm of the morin transition from saturation remanence warming curves to be 250 261 k for 0 5 6 mm hematite natural single crystals 257 260 k for 45 600 m sieved crystal fractions and 241 256 k for submicron synthetic hematites with grain sizes between 120 and 520 nm the variation must be due to differences in crystal morphology lattice strain and crystal defects common in both synthetic and natural crystals our results are compatible with published data for 100 nm to 10 mm hematites and show that tm is nearly size independent decreasing very gradually as particle size decreases over this broad range which includes both multidomain md and single domain sd structures however tm decreases sharply between 90 and 30 nm below 20 nm the transition disappears entirely as near surface spins deviate strongly from the antiferromagnetic easy axis our sd and md hematites exhibit a thermal hysteresis in the morin transition the values of tm in cooling and in heating are different for the same cooling warming rate tm is much greater for submicron hematites than for larger crystals we attribute the lag in the transition in both cooling and heating to crystal imperfections and resulting internal stresses and speculate that defects may serve to pin and stabilize surface spins preventing spin rotation in a region large enough to trigger the phase transition would inhibit destabilization of the weakly ferromagnetic phase in cooling and the antiferromagnetic phase in heating the wide distribution of particle sizes in our submicron samples may also play a role

shear velocities qβ and the frequency dependence of qβ in stable and tectonically active regions from surface wave observations summary velocities and attenuation coefficient values of rayleigh waves within the south american continent and indian subcontinent have been determined in order to study velocity structure and qp structure as well as their lateral variation in those regions single station and two station data are combined to yield dispersion and attenuation data from periods of a few seconds to periods between 60 s and 100 s for each region both types of data are inverted using differential inversion schemes this inversion process is found to sometimes allow the location of velocity discontinuities in the models directly from the dispersion data qp models obtained from the inversion process are not constrained to have high values at large depths a problem with earlier methods used to invert surface wave attenuation data the shear velocity models for the indian shield and eastern south america are similar to one another and have somewhat slower velocities than those reported for shield regions of north america the shear velocity models for the himalaya and andes are similar to one another in crustal thickness but the velocities in the andes are lower than those in the himalayas in the uppermost mantle a low velocity zone can marginally be resolved in the upper mantle beneath the himalaya but not beneath the other regions in this study qp models obtained for the stable regions of eastern north america and eastern south america exhibit similar qp values at mantle depths the indian shield however has much lower qp values in both the crust and upper mantle a result which may be related to the unusually high heat flow observed for that stable region for the tectonically active regions of western north america western south america and the himalaya is considerably lower than qp for the stable regions throughout most of the crust and upper mantle minimum mantle values appear to be about 120 for stable regions and 40 or less for tectonically active regions thus qp values in the upper mantle appear to vary regionally by a factor of 3 or more whereas shear velocities in the same regions vary by no more than 13 per cent

three dimensional shear wave velocity imaging by ambient seismic noise tomography summary 3 d shear wave velocity images are of particular interest for engineering seismology to obtain information about the local subsoil structure we present a one step inversion procedure based on the computation of high frequency correlation functions between stations of a small scale array deployed for recording ambient seismic noise the calculation of rayleigh wave phase velocities is based on the frequency domain spatial autocorrelation technique constitutively a tomographic inversion of the traveltimes estimated for each frequency is performed allowing the laterally varying 3 d surface wave velocity structure below the array to be retrieved we test our technique by using simulations of seismic noise for a simple realistic site and by using real world recordings from a small scale array performed at the nauen test site germany the results imply that the cross sections from passive seismic interferometry provide a clear image of the local structural heterogeneities and the shear wave velocities are satisfactorily reproduced the velocity structure is also found to be in good agreement with the results of geoelectrical measurements indicating the potential of the method to be easily applied for deriving the shallow 3 d velocity structure in urban areas and for monitoring purposes

full 3 d tomography for crustal structure in southern california based on the scattering integral and the adjoint wavefield methods we have successfully applied full 3 d tomography f3dt based on a combination of the scattering integral method si f3dt and the adjoint wavefield method aw f3dt to iteratively improve a 3 d starting model the southern california earthquake center scec community velocity model version 4 0 cvm s4 in f3dt the sensitivity frechet kernels are computed using numerical solutions of the 3 d elastodynamic equation and the nonlinearity of the structural inversion problem is accounted for through an iterative tomographic navigation process more than half a million misfit measurements made on about 38 000 earthquake seismograms and 12 000 ambient noise correlagrams have been assimilated into our inversion after 26 f3dt iterations synthetic seismograms computed using our latest model cvm s4 26 show substantially better fit to observed seismograms at frequencies below 0 2 hz than those computed using our 3 d starting model cvm s4 and the other scec cvm cvm h11 9 which was improved through 16 iterations of aw f3dt cvm s4 26 has revealed strong crustal heterogeneities throughout southern california some of which are completely missing in cvm s4 and cvm h11 9 but exist in models obtained from previous crustal scale 2 d active source refraction tomography models at shallow depths our model shows strong correlation with sedimentary basins and reveals velocity contrasts across major mapped strike slip and dip slip faults at middle to lower crustal depths structural features in our model may provide new insights into regional tectonics when combined with physics based seismic hazard analysis tools we expect our model to provide more accurate estimates of seismic hazards in southern california

assessing the impact of vertical land motion on twentieth century global mean sea level estimates near global and continuous measurements from satellite altimetry have provided accurate estimates of global mean sea level in the past two decades extending these estimates further into the past is a challenge using the historical tide gauge records not only is sampling nonuniform in both space and time but tide gauges are also affected by vertical land motion vlm that creates a relative sea level change not representative of ocean variability to allow for comparisons to the satellite altimetry estimated global mean sea level gmsl typically the tide gauges are corrected using glacial isostatic adjustment gia models this approach however does not correct other sources of vlm that remain in the tide gauge record here we compare global positioning system gps vlm estimates at the tide gauge locations to vlm estimates from gia models and assess the influence of non gia related vlm on gmsl estimates we find that the tide gauges on average are experiencing positive vlm i e uplift after removing the known effect of gia resulting in an increase of 0 24 0 08 mm yr 1 in gmsl trend estimates from 1900 to present when using gps based corrections while this result is likely dependent on the subset of tide gauges used and the actual corrections used it does suggest that non gia vlm plays a significant role in twentieth century estimates of gmsl given the relatively short gps records used to obtain these vlm estimates we also estimate the uncertainty in the gmsl trend that results from limited knowledge of non gia related vlm