Fundamental Mode Rayleigh Waves Research Articles

Depth constraints on azimuthal anisotropy in the Great Basin from Rayleigh-wave phase velocity maps

We present fundamental-mode Rayleigh-wave azimuthally anisotropic phase velocity maps obtained for the Great Basin region at periods between 16 s and 102 s. These maps offer the first depth constraints on the origin of the semi-circular shear-wave splitting pattern observed in central Nevada, around a weak azimuthal anisotropy zone. A variety of explanations have been proposed to explain this signal, including an upwelling, toroidal mantle flow around a slab, lithospheric drip, and a megadetachment, but no consensus has been reached. Our phase velocity study helps constrain the three-dimensional anisotropic structure of the upper mantle in this region and contributes to a better understanding of the deformation mechanisms taking place beneath the western United States. The dispersion measurements were made using data from the USArray Transportable Array. At periods of 16 s and 18 s, which mostly sample the crust, we find a region of low anisotropy in central Nevada coinciding with locally reduced phase velocities, and surrounded by a semi-circular pattern of fast seismic directions. Away from central Nevada the fast directions are ∼ N–S in the eastern Great Basin, NW–SE in the Walker Lane region, and they transition from E–W to N–S in the northwestern Great Basin. Our short-period phase velocity maps, combined with recent crustal receiver function results, are consistent with the presence of a semi-circular anisotropy signal in the lithosphere in the vicinity of a locally thick crust. At longer periods (28–102 s), which sample the uppermost mantle, isotropic phase velocities are significantly reduced across the study region, and fast directions are more uniform with an ∼ E–W fast axis. The transition in phase velocities and anisotropy can be attributed to the lithosphere–asthenosphere boundary at depths of ∼ 60 km. We interpret the fast seismic directions observed at longer periods in terms of present-day asthenospheric flow-driven deformation, possibly related to a combination of Juan de Fuca slab rollback and eastward-driven mantle flow from the Pacific asthenosphere. Our results also provide context to regional SKS splitting observations. We find that our short-period phase velocity anisotropy can only explain ∼ 30% of the SKS splitting times, despite similar patterns in fast directions. This implies that the origin of the regional shear-wave splitting signal is complex and must also have a significant sublithospheric component.

The lithosphere of North China Craton from surface wave tomography

Fundamental mode Rayleigh wave group velocity dispersion data were used in a tomographic study to reconstruct the crust and upper mantle shear velocity structure in east Asia. This article expounds the features of lithosphere structure in different sub-regions of North China, and discusses their implications for the tectonic evolution and present tectonics of North China Craton (NCC). The study revealed that strong earthquakes took place in the low velocity zones and along the boundaries between high and low velocities in the middle and lower crust; the active NW-trending Zhangjiakou–Bohai seismic belt corresponds to a prominent low velocity zone in the crust and upper mantle lid. In addition to the well-known structural difference between western and eastern NCC, we are able to resolve clearly the lithospheric features of sub-regions in eastern NCC. The southern North China Basin and north Yellow Sea are characterized by a high velocity crust and upper mantle lid down to 70–80 km depth, while the northern North China Basin has a low velocity crust and the velocity below Moho to about 130 km is rather uniform and low. There is also a general trend of velocity decrease in the upper mantle from west to east. The velocity images bear the imprints of subduction and collision that took place in the periphery of NCC since Mesozoic, and shed some light on the mechanism of Mesozoic lithospheric thinning of eastern NCC.

Analysis of Microseismic Noise at LASA, NORSAR and ALPA†

Summary An analysis has been made of the properties of both the short-period and long-period microseismic noise in the 0.2- to 40-s period range at LASA, NORSAR and ALPA. The high-resolution frequency–wavenumber spectra are presented for the noise, as well as power spectra and coherence results. These data show that the long-period noise in the 10- to 40-s period range consists of fundamental-mode Rayleigh and Love waves, as well as a non-propagating component. The results of a simultaneous analysis of long-period noise at ALPA and LASA provide for the first time an indication of the location of the noise source. It is shown that this location can be associated with low-pressure atmospheric disturbances. The short-period noise in the 2- to 10-s period range consists primarily of higher-mode Rayleigh waves. The 0.2- to 2-s microseisms consist of a significant non-propagating, as well as a propagating, component. This propagating component of the noise has a frequency-wavenumber spectrum whose structure is highly diffuse in wavenumber space. The signal-to-noise ratio gain of maximum-likelihood processing relative to that obtained with beamforming is computed using a theoretical model for the wavenumber structure of the long-period noise which is based on the measurements. These results show that this gain is not substantial in most cases of practical interest.

Interpreting Surface-wave Data for a Site with Shallow Bedrock

The inversion of dispersive Rayleigh-wave data has been shown to be successful in providing reliable estimated shear-wave velocities within unconsolidated materials in the near surface. However, in a case where the multi-channel analysis of surface waves method was applied to a site consisting of clay residuum overlying basalt bedrock, inversion for the fundamental-mode Rayleigh wave resulted in shear-wave velocities within the rock that are less than half of expected values. Forward modeling reveals that the fundamental-mode dispersion curve is hardly sensitive to bedrock velocity perturbations over a practical range of wavelengths, leading to poorly constrained solutions. Standard surface-wave methods can fail because of a shortage of phase-velocity estimates at the low frequencies that are necessary to properly constrain shear-wave velocities at depth. The commonly used guideline that maximum investigation depth is roughly half of the largest recorded wavelength can be misleading. Data at much lower frequencies (i.e., longer wavelengths) than typically acquired might be required to obtain a meaningful shear-wave velocity profile, particularly for a site with a high-velocity half-space beneath a low-velocity layer. For such cases, layer geometry appears to have a large impact on inversion results. Consequently, Rayleigh-wave methods can be effective in determining depth to bedrock in simple, layered geologies (e.g., soft sediment over hard bedrock) when independent information of shear-wave velocity is available. Analysis techniques that address higher modes of Rayleigh-wave propagation may be useful for more accurately resolving depth and velocity of a high-velocity half-space. In the studied case, higher modes can theoretically reach the asymptotic high-velocity limits within the range of recorded frequencies.

Surface wave tomography in the European and Mediterranean region

We present a new surface-wave tomographic study of the broad European and Mediterranean region. Our goal is to enhance the resolution of previously published group velocity models using new data from European permanent seismic networks and a dense broad-band array in Northern Apennines (RETREAT). We measure fundamental mode Rayleigh and Love wave group velocities from long-period seismograms recorded at regional distance (between 600 and 7000 km). Our measurement technique is based on iterative application of multiple filters and phase-matched filters; we accurately estimate dispersion curves for more than 1500 Rayleigh wave and about 850 Love wave paths in the period range 35–170 s. Consistency of measurements is evaluated by comparing ray clusters from sample earthquakes to closely spaced RETREAT stations. In the whole data set, measurement errors in group velocity decrease with increasing distance and show to be caused by inaccuracy in the estimate of group arrival time. We calculate maps of Love and Rayleigh group velocity at selected periods by linear tomographic inversion, accounting for group arrival time errors and evaluating a posteriori group slowness errors. Data coverage in this region is not uniform, and it is highly influenced by the uneven distribution of earthquakes and seismic stations. We therefore build a laterally heterogeneous reference model by inverting a global data set of group velocity derived from the phase velocity library of Ekström et al. (1997). Use of this reference as an a priori model during inversion improves preliminary data coverage at the borders of our study region and warrants consistency with global models. The implications of different regularization constraints (mathematically equivalent to norm damping or smoothing with different criteria) are analysed and compared. Group velocity maps confirm the large-scale geological lineaments known for the region: short-periods maps differentiate well among thinner oceanic and thicker continental crust; the most dominant feature in long-period maps is the difference between the fast Precambrian East European Platform and the low velocity signature of central Europe and western Mediterranean, separated by a sharp gradient in correspondence of the Tornquist—Tesseyre Zone. The seismically active Tethyan Belt is clearly marked by a continuous slow anomaly. Smaller scale, possibly thermally related, low velocity anomalies are found under Iceland and Mid-Atlantic Ridge, Rhine Graben and Tyrrhenian back-arc basin, whereas the Hellenic Arc is characterized by fast velocity.

Dipping-interface Mapping Using Mode-separated Rayleigh Waves

Multichannel analysis of surface waves (MASW) method is a non-invasive geophysical technique that uses the dispersive characteristic of Rayleigh waves to estimate a vertical shear (S)-wave velocity profile. A pseudo-2D S-wave velocity section is constructed by aligning 1D S-wave velocity profiles at the midpoint of each receiver spread that are contoured using a spatial interpolation scheme. The horizontal resolution of the section is therefore most influenced by the receiver spread length and the source interval. Based on the assumption that a dipping-layer model can be regarded as stepped flat layers, high-resolution linear Radon transform (LRT) has been proposed to image Rayleigh-wave dispersive energy and separate modes of Rayleigh waves from a multichannel record. With the mode-separation technique, therefore, a dispersion curve that possesses satisfactory accuracy can be calculated using a pair of consecutive traces within a mode-separated shot gather. In this study, using synthetic models containing a dipping layer with a slope of 5, 10, 15, 20, or 30 degrees and a real-world example, we assess the ability of using high-resolution LRT to image and separate fundamental-mode Rayleigh waves from raw surface-wave data and accuracy of dispersion curves generated by a pair of consecutive traces within a mode-separated shot gather. Results of synthetic and real-world examples demonstrate that a dipping interface with a slope smaller than 15 degrees can be successfully mapped by separated fundamental waves using high-resolution LRT.

Seismic waveform modelling in a 3-D Earth using the Born approximation: potential shortcomings and a remedy

S U M M A R Y Although the use of the first-order Born approximation for the computation of seismic observ-ables and sensitivity kernels in 3-D earth models shows promise for improving tomographic modelling, more work is necessary to systematically determine how well such methods forward model realistic seismic data compared with more standard asymptotic methods. Most work so far has been focused on the analysis of secondary data, such as phase velocity, rather than time domain waveforms. We here compare synthetic waveforms obtained for simple models using standard asymptotic approximations that collapse the sensitivity to 3-D structure on the great circle plane and those obtained using the 3-D linear Born approximation, with accurate numerical 3-D synthetics. We find, not surprisingly, that 3-D Born more accurately models the perturbation effects of velocity anomalies that are comparable in wavelength to or are smaller than the first Fresnel zone. However, larger wavelength and amplitude anomalies can easily produce large phase delays that cause the first-order (linear) Born approximation to break down, whereas asymptotic methods that incorporate the effect of heterogeneity in the phase rather than in the amplitude of the waveform are more robust. Including a path average phase delay to the Born calculated waveforms significantly improves their accuracy in the case of long-wavelength structure, while still retaining the ability to correctly model the effect of shorter-wavelength structure. Tests in random models with structural wavelengths consistent with existing global seismic models indicate that the linear Born approximation frequently breaks down in realistic earth models, with worse misfit for first and second orbit Rayleigh and higher mode surface waveforms than the great-circle based approximations at all distances tested (>20 •). For fundamental modes, the average misfit for the waveforms calculated with the linear Born formalism is quite poor, particularly for distances larger than 60 •. The modified Born formalism consistently improves the fit relative to the linear Born waveforms, but only outperforms the great-circle based approximations for the higher mode surface waveforms. We note, however, that phase delay kernels for multitaper measurements of fundamental mode Rayleigh wave phase velocities developed from the Born approximation do not demonstrate the problems associated with the linear waveform kernels. There is general agreement with measurements and moderate improvement relative to phase delays predicted by the path-average approximation.

Partial melt in the upper-middle crust of the northwest Himalaya revealed by Rayleigh wave dispersion

Seismic shear-wave velocities are sensitive to the partial melts that should be present in the Himalayan orogen if low-viscosity channel flow is active at the present day. We analyzed regional earthquakes in the western Himalaya and Tibet recorded on 16 broadband seismometers deployed across the NW Indian Himalaya, from the Indian platform to the Karakoram Range. We used a multiple filter technique to calculate the group velocity dispersion of fundamental-mode Rayleigh waves, and then inverted the dispersion records to obtain separate one-dimensional shear-wave velocity models for five geologic provinces: the Tibetan plateau, Ladakh arc complex, Indus Tsangpo suture zone, Tethyan Himalaya, and Himalayan thrust belt. Our velocity models show a low-velocity layer (LVL) with 7–17% velocity reduction centered at ~ 30 km depth and apparently continuous from the Tethyan Himalaya to the Tibetan plateau. This LVL shows good spatial correspondence with observations of low resistivity from magnetotelluric studies along the same profile. Of the possible explanations for low velocity and low resistivity in the mid-crust, only the presence of melts or aqueous fluids (or both) satisfactorily explains both sets of observations. Elevated heat flow observed in the NW Himalaya implies that if aqueous fluids are present in the mid-crust, then the mid-crust is well above its solidus. Comparison of our results with laboratory measurements and theoretical models suggests 3–7% melt is present in a channel in the upper-middle crust of the NW Himalaya at the present day, and the physical conditions to enable active channel flow may be present.

Upper mantle structure beneath the Caribbean‐South American plate boundary from surface wave tomography

We have measured shear wave velocity structure of the crust and upper mantle of the Caribbean‐South American boundary region by analysis of fundamental mode Rayleigh waves in the 20‐ to 100‐s period band recorded at the BOLIVAR/GEODINOS stations from 2003 to 2005. The model shows lateral variations that primarily correspond to tectonic provinces and boundaries. A clear linear velocity change parallels the plate bounding dextral strike‐slip fault system along the northern coast of Venezuela, illustrating the differences between the South American continental lithosphere, the Venezuelan archipelago, and the Caribbean oceanic lithosphere. At depths up to 120 km beneath the Venezuelan Andes and the Maracaibo block, there is evidence of underthrusting of the Caribbean plate, but there is no other evidence of subduction of the Caribbean plate beneath the South American plate. In eastern Venezuela, linear crustal low velocities are associated with the fold and thrust belts whereas as higher crustal velocities are imaged in the Guayana shield lithosphere. The subducting oceanic part of the South American plate is imaged beneath the Antilles arc. The surface wave images combined with seismicity data suggest shear tearing of the oceanic lithosphere away from the buoyant continental South American plate offshore of northeastern Venezuela. The continental lithosphere south of the slab tear is bent down toward the plate boundary in response to the propagating tear in the lithosphere. We interpret a nearly vertical low‐velocity “column” west of the tear centered beneath the Cariaco Basin, with three‐dimensional asthenospheric flow around the southern edge of the subducting oceanic lithosphere, with the asthenosphere escaping from beneath continental South America and rising into the plate boundary zone. The complex plate boundary structure is best examined in three dimensions. We discuss the new surface wave tomographic inversion in the context of results from other researchers including local seismicity, teleseismic shear wave splits, and interpretations from active source profiling.

Multichannel analysis of surface waves for bedrock depth estimation over granites, Hyderabad, India

The dispersive characteristics of Rayleigh type surface waves were utilized to estimate shear wave velocity (Vs) profile followed by imaging the shallow subsurface granitic layers near Hyderabad India. The reliability of Multichannel Analysis of Surface Waves (MASW) depends on the accurate determination of phase velocities for horizontally traveling fundamental mode Rayleigh waves. Multichannel recording leads to effective identification and isolation of various factors of noise. Calculating the 1-D shear wave velocity (Vs) field from surface waves ensures high degree of accuracy irrespective of cultural noise. The main advantage of mapping the bed rock surface with shear wave velocity (Vs) is the insensitivity of MASW to velocity inversion besides being free from many constraints such as contrast in physical properties etc. Modeling of surface waves data results a shear wave velocity (Vs) of 250-750 m/sec covering the top soil to weathering and up to bedrock corresponding to a depth range of 10-30 m. A pair of selected set of results over granites are presented here as a case study highlighting the salient features of MASW.

Deep structure of the Mt. Karabetov Mud volcano

The results of multidisciplinary geological‐geophysical investigations of mud volcanism in the Taman mud-volcanic province are presented. Geophysical data on the internal structure of Mt. Karabetov mud volcano were obtained for the first time, and pathways of fluid migration down to a depth of 15‐25 km were determined. Mud volcanism is a surprising and quite rare natural phenomenon, whose mechanisms had not been fully studied. At present, we can consider that the correlation of mud volcanism with the dynamics of deep fluids and hydrocarbon pools has been established [2, 9]. Intense mud-volcanic activity is observed in the territory of Russia, first of all, on the Taman Peninsula, which is an integral part of the modern evolution of fluid-magmatic systems in the Northern Caucasus [9]. During the last few years, Mt. Karabetov, which is one of the most active mud volcanoes in the Taman region, has been the object of multidisciplinary geological‐geophysical and geochemical investigations [3, 4]. This volcano is characterized by explosive type of eruptions with periodic manifestation of the whole power of this seemingly harmless natural phenomenon. During the field works of the Institute of Physics of the Earth (IPE RAS) in August‐September 2007, a detailed geological-geomorphological mapping of Mt. Karabetov mud volcano was carried out and supplemented by remote sounding data. As a result, it became possible to trace the tectonic deformations of young forms of topography and various manifestations of exogenous geological processes in the study region. Simultaneously, profile geophysical measurements were carried out using the microseismic sounding method [5, 6]. It was shown experimentally and using numerical models that inhomogeneities of the Earth’s crust specifically distort the spectrum of the low-frequency microseismic field. In particular, spectral amplitudes of specific frequency f decrease above highvelocity anomalies at the Earth’s surface and increase above low-velocity anomalies. Frequency f is related to the depth of the anomaly H location and velocity of the fundamental mode of Rayleigh wave V R ( f ) as H = . The low-frequency microseismic field is considered as a superposition of wave packets of fundamental Rayleigh modes with different frequency compositions. The method was used as a principally new technology of microseismic sounding of near-surface (0‐0.5 km) and deep (up to 50 km) structures of the Earth’s crust. The technology was successfully tested in practice on different geological objects in terms of scale and genesis [14].

Observations and interpretation of fundamental mode Rayleigh wavefields recorded by the Transportable Array (USArray)

Broadband recordings of the dense Transportable Array (TA) in the western United States provide unparalleled detailed images of long‐period seismic surface wavefields. With 400 stations spanning most of the western United States, wavefronts of fundamental mode Rayleigh waves may be visualized coherently across the array at periods ≳40 s. In order to constrain the Rayleigh wave phase velocity structure in the western United States, I assemble a data set of vertical component seismograms from 53 teleseismic events recorded by the TA from April 2006 to October 2007. Complex amplitude spectra from these recordings at periods 27–100 s are interpreted using the multiplane wave tomographic method of Friederich and Wielandt (1995) and Pollitz (1999). This analysis yields detailed surface wave phase velocity and three‐dimensional shear wave velocity patterns across the North American plate boundary zone, elucidating the active processes in the highly heterogeneous western U.S. upper mantle.

The global attenuation structure of the upper mantle

A large data set of fundamental mode Rayleigh wave amplitudes is analyzed to derive a new global three‐dimensional model of shear wave attenuation in the upper mantle. The amplitude observations span a range of periods between 50 and 250 s and are derived from earthquakes with MW > 6.0 that occurred between 1993 and 2005. Four separate factors may influence an amplitude anomaly: intrinsic attenuation along the raypath, elastic focusing effects along the raypath, uncertainties in the strength of excitation, and uncertainties in the response at the station. In an earlier paper (Dalton and Ekström, 2006a), dependence of the retrieved attenuation structure on these terms was shown to be significant and an approach was developed to invert the amplitudes simultaneously for each term. The new three‐dimensional attenuation model QRFSI12, which is the subject of this paper, is derived using this method. The model contains large lateral variations in upper‐mantle attenuation, ±60% to ±100%, and exhibits strong agreement with surface tectonic features at depths shallower than 200 km. At greater depth, QRFSI12 is dominated by high attenuation in the southeastern Pacific and eastern Africa and low attenuation along many subduction zones in the western Pacific. Resolution tests confirm that the change in pattern of attenuation above and below 200‐km depth can be determined with confidence using the fundamental mode data set. The new model is highly correlated with global models of shear wave velocity, particularly in the uppermost mantle, suggesting that the same factors may control both seismic attenuation and velocity in this depth range. However, forcing the lateral perturbations in attenuation to match those found in global velocity models decreases the data variance reduction, which suggests that subtle differences between patterns of attenuation and velocity are robust.

S-wave velocity structure beneath the High Lava Plains, Oregon, from Rayleigh-wave dispersion inversion

The High Lava Plains (HLP) “hotspot” track is a prominent volcanic lineament that extends from the southeast corner of Oregon in the northern Great Basin to Newberry volcano in the eastern Cascades. With the age of silicic volcanism decreasing along track to the northwest, the HLP and Newberry volcano are a rough mirror image to the Eastern Snake River Plain and Yellowstone but, in the case of the HLP, at an orientation strongly oblique to North American plate motion. Since this orientation is incompatible with plate motion over a fixed hotspot, other proposed origins for the HLP, such as asthenospheric inflow around a steepening slab, residual effects of a Columbia River/Steens plume, backarc spreading, and Basin and Range extension, relate it to various tectonic features of the Pacific Northwest. To begin distinguishing between these hypotheses, we image upper-mantle structure beneath the HLP and adjacent tectonic provinces with fundamental-mode Rayleigh waves recorded by stations of the USArray Transportable Array, the recently-initiated HLP seismic experiment, the United States National Seismograph Network, and the Berkeley seismic network. We estimate phase velocities along nearly 300 two-station propagation paths that lie within and adjacent to the HLP and cross the region along two azimuths, parallel to and perpendicular to the HLP track. The dispersion curves, which typically give robust results over the period range 16–171 seconds, are grouped by tectonic region, and the composite curves are inverted for S-wave velocity as a function of depth. We also fit a single plane wave to phase delays across the region to identify laterally homogeneous subregions. The resulting variations in upper-mantle structure correlate with variations in surface volcanism and tectonics. The lowest velocities (∼ 4.1 km/s) occur at ∼ 50 km depth in the SE corner of Oregon, where there has been extensive basaltic volcanism in the past 2–5 kyr, and suggest uppermost mantle temperatures sufficient to produce basaltic partial melting. While the seismic velocities of the uppermost mantle beneath the volcanic High Lava Plains are low relative to the standard Tectonic North America (TNA) model, they are only slightly lower than those found for the adjacent northern Great Basin and they appear to be significantly higher than upper-mantle velocities beneath the Eastern Snake River Plain. Our results provide no evidence for a residual plume signature beneath the HLP region, leaving open questions as to the origin of the HLP volcanic track itself.

Lithosphere structure across the Dead Sea Transform as constrained by Rayleigh waves observed during the DESERT experiment

SUMMARY The interdisciplinary Dead Sea Rift Transect (DESERT) project that was conducted in Israel, the Palestine Territories and Jordan has provided a rich palette of data sets to examine the crust and uppermost mantle beneath one of Earth’s most prominent fault systems, the Dead Sea Transform (DST). As part of the passive seismic component, thirty broad-band sensors were deployed in 2000 across the DST for roughly one year. During this deployment, we recorded 115 teleseismic earthquakes that are suitable for a fundamental mode Rayleigh wave analysis at intermediate periods (35‐150 s). Our initial analysis reveals overall shear velocities that are reduced by up to 4 per cent with respect to reference Earth model PREM. To the west of the DST, we find a seismically relatively fast but thin lid that is about 80 km thick. Towards the east, shallow seismic velocities are low while a deeper low velocity zone is not detected. This contradicts the currently favoured thermomechanical model for the DST that predicts lithospheric thinning through mechanical erosion by an intruding plume from the Red Sea. On the other hand, our current results are somewhat inconclusive regarding asthenosphere velocities east of the DST due to the band limitation of the recording equipment in Jordan.

Crust and upper mantle velocity structure of the Yellowstone hot spot and surroundings

The Yellowstone hot spot has recently been shown to be a plume that extends into the transition zone. At roughly 60–120 km depth, the plume material rising beneath Yellowstone Park is sheared SW by North America Plate motion, producing a profound low velocity layer emplaced beneath the thin lithosphere. To constrain the absolute seismic velocity of the plate‐sheared plume layer, fundamental mode Rayleigh wave observations have been inverted for phase velocity using the two plane wave technique. The resulting phase velocity models are inverted with Moho‐converted P to S arrival times to better constrain crustal thickness and absolute S wave velocity structure to ∼120 km depth. A regionalized S wave velocity model has an extremely low velocity minimum of 3.8 ± 0.1 km/s at 80 km depth beneath the hot spot track. Nonregionalized 3‐D velocity models find a velocity minimum of 3.9 km/s beneath the hot spot track. Below 120 km depth, our resolution diminishes such that the lateral spreading of the plume track is not resolved. The volume of the low velocity plume layer is small and the estimated buoyancy flux for the Yellowstone plume is <0.1 Mg/s which contrasts with the ∼9 Mg/s value for Hawaii. In addition, a notable region of thick crust and high lower crustal velocities is found around Billings, Montana, consistent with previous refraction and receiver function studies that interpret this as evidence for a massive Precambrian underplating event.

Global upper-mantle tomography with the automated multimode inversion of surface andS-wave forms

SUMMARY We apply the Automated Multimode Inversion of surface and S-wave forms to a large global data set, verify the accuracy of the method and assumptions behind it, and compute an S vvelocity model of the upper mantle (crust‐660 km). The model is constrained with ∼51 000 seismograms recorded at 368 permanent and temporary broadband seismic stations. Structure of the mantle and crust is constrained by waveform information both from the fundamentalmode Rayleigh waves (periods from 20 to 400 s) and from S and multiple S waves (higher modes). In order to enhance the validity of the path-average approximation, we implement the automated inversion of surface- and S-wave forms with a three-dimensional (3-D) reference model. Linear equations obtained from the processing of all the seismograms of the data set are inverted for seismic velocity variations also relative to a 3-D reference, in this study composed of a 3-D model of the crust and a one-dimensional (1-D), global-average depth profile in the mantle below. Waveform information is related to shear- and compressional-velocity structure within approximate waveform sensitivity areas. We use two global triangular grids of knots with approximately equal interknot spacing within each: a finely spaced grid for integration over sensitivity areas and a rougher-spaced one for the model parametrization. For the tomographic inversion we use LSQR with horizontal and vertical smoothing and norm damping. We invert for isotropic variations in S- and P-wave velocities but also allow for S-wave azimuthal anisotropy—in order to minimize errors due to possible mapping of anisotropy into isotropic heterogeneity. The lateral resolution of the resulting isotropic upper-mantle images is a few hundred kilometres, varying with data sampling. We validate the imaging technique with a ‘spectral-element’ resolution test: inverting a published global synthetic data set computed with the spectral-element method using a laterally heterogeneous mantle model we are able to reconstruct the synthetic model accurately. This test confirms both the accuracy of the implementation of the method and the validity of the JWKB and path-average approximations as applied in it. Reviewing the tomographic model, we observe that low-S v-velocity anomalies beneath mid-ocean ridges and backarc basins extend down to ∼100 km depth only, shallower than according to some previous tomographic models; this presents a close match to published estimates of primary melt production depth ranges there. In the seismic lithosphere beneath cratons, unambiguous high velocity anomalies extend to ∼200 km. Pronounced low-velocity zones beneath cratonic lithosphere are rare; where present (South America; Tanzania) they are neighboured by volcanic areas near cratonic boundaries. The images of these low-velocity zones may indicate hot material—possibly of mantle-plume origin—trapped or spreading beneath the thick cratonic lithosphere.

Effects of Love Waves on Microtremor H/V Ratio

Abstract The horizontal-to-vertical (H/V) method has the potential to significantly contribute to site effects evaluation, in particular in urban areas. Within the European project, site effects assessment using ambient excitations (SESAME), we investigated the nature of ambient seismic noise in order to assess the reliability of this method. Through 1D seismic noise modeling, we simulated ambient noise for a set of various horizontally stratified structures by computing efficiently the displacement and stress of dynamic Green’s functions for a viscoelastic-layered half-space. We performed array analysis using the conventional semblance-based frequence-wavenumber method and the three-component modified spatial autocorrelation method on both vertical and horizontal components and estimated the contribution of different seismic waves (body/surface waves, Rayleigh/Love waves) at the H/V peak frequency. We show that the very common assumption that almost all the ambient noise energy would be carried by fundamental-mode Rayleigh waves is not justified. The relative proportion of different wave types depends on site conditions, and especially on the impedance contrast. For the 1D horizontally layered structures presented here, the H/V peak frequency always provides a good estimate of the fundamental resonance frequency whatever the H/V peak origin (Rayleigh wave ellipticity, Airy phase of Love waves, S -wave resonance). We also infer that the relative proportion of Love waves in ambient noise controls the amplitude of the H/V peak.

Teleseismic surface wave tomography in the western U.S. using the Transportable Array component of USArray

Fundamental mode Rayleigh waves recorded by the Transportable Array component of EarthScope/USArray from January 2006 through April 2007 are used to generate phase velocity maps at periods from 25 to 100 sec across the western U.S., including Washington, Oregon, California, Nevada and western Idaho. At short periods (25–33 s), low velocity anomalies are observed in western Washington, western and central Oregon, northern California, the southern Sierra Nevada and the Snake River Plain. At intermediate and long periods (50–100 s), high velocities are seen in the Cascades, the southern Central Valley of California, California's Transverse Range, and the Columbia River Flood Basalt Province. The phase velocity maps are consistent with those obtained from ambient noise tomography at comparable periods. Short period phase velocities from ambient noise tomography and the longer‐period phase velocities from teleseismic tomography, therefore, present natural data sets to invert jointly 3‐D structure across the western U.S.

The domain of existence of prograde Rayleigh-wave particle motion for simple models

The existence of prograde particle motion for fundamental-mode Rayleigh waves is studied systematically in models of increasing complexity by using an exact expression of the ellipticity. This expression, together with the secular equation for the phase velocity, are useful to find the most relevant parameters for prograde particle motion, namely Poisson’s ratio in the layer and the shear-wave velocity contrast between the layer and the half-space. The density contrast between layer and half-space, and up to a certain degree Poisson’s ratio in the half-space, are usually less important. The domain of existence of prograde Rayleigh-particle motion is specified for typical combinations of parameters.