Love Wave Group Research Articles

Love wave tomography in Italy from seismic ambient noise

We estimate Love wave empirical Green’s functions from cross-correlations of ambient seismic noise to study the crust and uppermost mantle structure in Italy. Transverse-component ambient noise data from October 2005 through March 2007 recorded at 114 seismic stations from the Istituto Nazionale di Geofisica e Vulcanologia (INGV) national broadband network, the Mediterranean Very Broadband Seismographic Network (MedNet) and the Austrian Central Institute for Meteorology and Geodynamics (ZAMG) yield more than 2 000 Love wave group velocity measurements using the multiple-filter analysis technique. In the short period band (5–20 s), the cross-correlations show clearly one-sided asymmetric feature due to non-uniform noise distribution and high local activities, and in the long period band (>20 s) this feature becomes weak owing to more diffusive noise distribution. Based on these measurements, Love wave group velocity dispersion maps in the 8–34 s period band are constructed, then the SH wave velocity structures from the Love wave dispersions are inverted. The final results obtained from Love wave data are overall in good agreement with those from Rayleigh waves. Both Love and Rayleigh wave inversions all reveal that the Po plain basin is resolved with low velocity at shallow depth, and the Tyrrhenian sea is characterized with higher velocity below 8 km due to its thin oceanic crust.

Ambient noise Love wave tomography in the eastern margin of the Tibetan plateau

We collected continuous waveform data from June to December 2007 recorded by 49 broadband seismic stations in western Sichuan and eastern Tibet, and applied an ambient noise technique to obtain a high-resolution tomography model of the crust beneath the eastern margin of the Tibetan plateau. Cross-correlations of ambient noise data are calculated in one-hour segments and stacked over 7 months. Then Love wave group dispersion curves along approximately 700-station pairs were determined using the multiple-filter analysis method. The study region was divided into a grid of 0.2° × 0.2°, and group velocity distribution maps between 8 and 40 s were obtained. The Love wave group velocity maps show good correlations with surface features. From the group velocity maps we inverted a 3-D model for SH wave velocity. The inferred SH velocity heterogeneities generally agree with variations in SV derived from Rayleigh wave dispersion, discrepancies between them are examined in view of non-uniqueness of the models and radial anisotropy. Both the Rayleigh and Love wave inversions show the significant difference of the crustal structure and reveal a prominent low-velocity zone within the mid-to-lower crust beneath the eastern margin of the Tibetan plateau.

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.

Broadband ambient noise surface wave tomography across the United States

This study presents surface wave dispersion maps across the contiguous United States determined using seismic ambient noise. Two years of ambient noise data are used from March 2003 through February 2005 observed at 203 broadband seismic stations in the US, southern Canada, and northern Mexico. Cross‐correlations are computed between all station‐pairs to produce empirical Green functions. At most azimuths across the US, coherent Rayleigh wave signals exist in the empirical Green functions implying that ambient noise in the frequency band of this study (5–100 s period) is sufficiently isotropically distributed in azimuth to yield largely unbiased dispersion measurements. Rayleigh and Love wave group and phase velocity curves are measured together with associated uncertainties determined from the temporal variability of the measurements. A sufficient number of measurements (>2000) is obtained between 8 and 25 s period for Love waves and 8 and 70 s period for Rayleigh waves to produce tomographic dispersion maps. Both phase and group velocity maps are presented in these period bands. Resolution is estimated to be better than 100 km across much of the US from 8–40 s period for Rayleigh waves and 8–20 s period for Love waves, which is unprecedented in a study at this spatial scale. At longer and shorter periods, resolution degrades as the number of coherent signals diminishes. The dispersion maps agree well with each other and with known geological and tectonic features and, in addition, provide new information about structures in the crust and uppermost mantle beneath much of the US.

Fully nonlinear inversion of fundamental mode surface waves for a global crustal model

We use neural networks to find 1‐dimensional marginal probability density functions (pdfs) of global crustal parameters. The information content of the full posterior and prior pdfs can quantify the extent to which a parameter is constrained by the data. We inverted fundamental mode Love and Rayleigh wave phase and group velocity maps for pdfs of crustal thickness and independently of vertically averaged crustal shear wave velocity. Using surface wave data with periods T > 35 s for phase velocities and T > 18 s for group velocities, Moho depth and vertically averaged shear wave velocity of continental crust are well constrained, but vertically averaged shear wave velocity of oceanic crust is not resolvable. The latter is a priori constrained by CRUST2.0. We show that the resulting model allows to compute global crustal corrections for surface wave tomography for periods T > 50 s for phase velocities and T > 60 s for group velocities.

Surface wave tomography of the Barents Sea and surrounding regions

The goal of this study is to refine knowledge of the structure and tectonic history of the European Arctic using the combination of all available seismological surface wave data, including historical data that were not used before for this purpose. We demonstrate how the improved data coverage leads to better depth and spatial resolution of the seismological model and discovery of intriguing features of upper-mantle structure. To improve the surface wave data set in the European Arctic, we extensively searched for broad-band data from stations in the area from the beginning of the 1970s until 2005. We were able to retrieve surface wave observations from regional data archives in Norway, Finland, Denmark and Russia in addition to data from the data centres of IRIS and GEOFON. Rayleigh and Love wave group velocity measurements between 10 and 150 s period were combined with existing data provided by the University of Colorado at Boulder. This new data set was inverted for maps showing the 2-D group-velocity distribution of Love and Rayleigh waves for specific periods. Using Monte Carlo inversion, we constructed a new 3-D shear velocity model of the crust and upper mantle beneath the European Arctic which provides higher resolution and accuracy than previous models. A new crustal model of the Barents Sea and surrounding areas, published recently by a collaboration between the University of Oslo, NORSAR and the USGS, constrains the 3-D inversion of the surface wave data in the shallow lithosphere. The new 3-D model, BARMOD, reveals substantial variations in shear wave speeds in the upper mantle across the region with a nominal resolution of 1° × 1°. Of particular note are clarified images of the mantle expression of the continent-ocean transition in the Norwegian Sea and a deep, high wave speed lithospheric root beneath the Eastern Barents Sea, which presumably is the remnant of several Palaeozoic collisions.

Path-specific Velocity Structure of Western China from Surface-wave Dispersion

We develop one-dimensional (1-D) path-specific velocity models in western China using new Rayleigh and Love wave group and phase velocity dispersion measurements for 20 events in the region. The earthquakes were grouped into three geographic clusters from which we compute the average phase and group velocity dispersion. We invert the average dispersion curves simultaneously for 1-D shear-velocity models appropriate for the three central Asian paths, using three previous shear-velocity models as initial models. The models are validated by forward modeling waveforms of recent events. The crustal thickness beneath western China in the vicinity of the Lop Nor test site is 50–60 km and our velocity models are consistent with major geologic features (e.g., basins and mountain ranges) and previous structural models for this region.

Global surface wave diffraction tomography

We determine the effect of replacing geometrical ray theory in surface wave tomography with scattering theory. We describe a tomographic method based on a simplified version of the scattering sensitivity kernels that emerge from the Born or Rytov approximations in which surface wave travel times are a weighted average of phase or group slowness over the first Fresnel zone of the wave. We apply this “diffraction tomography” to Rayleigh and Love wave group velocity measurements to produce group velocity maps from 20 to 150 s period on a 2° × 2° grid globally. Using identical data and damping parameters, we also produce maps using “Gaussian tomography” which is based on ray theory with intuitive Gaussian smoothing constraints. Significant differences in the amplitude and geometry of the imaged features appear primarily at long periods but exist even in the short‐period maps in regions where average path lengths are large. Diffraction tomography, therefore, is significant in most oceanic regions at all periods, but it is also important on continents at long periods at least. On average, diffraction tomography produces larger velocity anomalies in a period‐dependent band of spherical harmonic degrees, and diffraction and Gaussian tomography maps decorrelate past a critical spherical harmonic degree that also depends on period. The widths of resolving kernels that emerge from diffraction tomography are systematically larger than those from Gaussian tomography. Finally, mantle features inferred from diffraction tomography tend to have larger amplitudes and extend deeper than those from Gaussian tomography.

New constraints on the arctic crust and uppermost mantle: surface wave group velocities, P n, and S n

We present the results of a study of surface wave dispersion across the Arctic region (>60°N) and compare the estimating group velocity maps with new maps of the body wave phases P n and S n . Data recorded at about 250 broadband digital stations from several global and regional networks were used to obtain Rayleigh and Love wave group velocity measurements following more than 1100 events with magnitudes M s>5.0 that occurred in the northern hemisphere from 1977 to 1998. These measurements were used to construct both isotropic and 2 Ψ azimuthally anisotropic group velocity maps from 15 to 200 s period. As elsewhere in the world, the observed maps display the signatures of sedimentary and oceanic basins, crustal thickness variations, and upper mantle anomalies under both continents and oceans. We also present P n and S n maps produced from a groomed data set of travel times from the ISC and NEIC bulletins. The long period group velocity maps correlate well with P n and S n velocities. Finally, at long wavelengths, the estimated 2 Ψ azimuthal anisotropy in Rayleigh wave group velocity correlates well with the azimuthal anisotropy in phase velocity obtained in a global scale study of Trampert and Woodhouse. Because attempts to improve the resolution to regional scales change both the amplitude and the pattern of the inferred azimuthal anisotropy, caution should be exercised in interpreting the anisotropy maps.

Average shear-wave velocity structure of the Kamchatka peninsula from the dispersion of surface waves

An average shear-wave velocity structure has been estimated for the path between the Kamchatka Isthmus and Petropavlovsk-Kamchatski. It is obtained from the Monte Carlo inversion of the Rayleigh and Love wave group velocity dispersion curves measured using broad-band seismograms of events in Northern Kamchatka recorded by the IRIS station PET in Petropavlovsk-Kamchatski. The Moho interface was found at a depth of 35±5 km and the Konrad one at 18±4 km. An important feature of the found structure is a low velocity in the upper mantle. This result is coherent with the recent and present-day volcanic activity in Kamchatka. Synthetic long period seismograms computed for the obtained structure are in good agreement with observed ones.

Transitional continental-oceanic structure beneath the Norwegian Sea from inversion of surface wave group velocity data

SUMMARY We have analysed the fundamental mode of Love and Rayleigh waves generated by 12 earthquakes located in the mid-Atlantic ridge and Jan Mayen fracture zone. Using the multiple filter analysis technique, we isolated the Rayleigh and Love wave group velocities for periods between 10 and 50 s. The surface wave propagation paths were divided into five groups, and average group velocities calculated for each group. The average group velocities were inverted and produced shear wave velocity models that correspond to a quasi-continental oceanic structure in the Greenland‐Norwegian Sea region. Although resolution is poor at shallow depth, we obtained crustal thickness values of about 18 km in the Norwegian Sea area and 9 km in the region between Svalbard and Iceland. The abnormally thick crust in the Norwegian Sea area is ascribed to magmatic underplating and the thermal blanketing eVect of sedimentary layers. Maximum crustal shear velocities vary between 3.5 and 3.9 km s’1 for most paths. An average lithospheric thickness of 60 km was observed, which is lower than expected for oceanic-type structure of similar age. We also observed low shear wave velocities in the lower crust and upper mantle. We suggest that high heat flow extending to depths of about 30 km beneath the surface can account for the thin lithosphere and observed low velocities. Anisotropy coeYcients of 1‐5 per cent in the shallow layers and >7 per cent in the upper mantle point to the existence of polarization anisotropy in the region.

Lithospheric structure of the Arabian Shield and Platform from complete regional waveform modelling and surface wave group velocities

SUMMARY Regional seismic waveforms reveal significant differences in the structure of the Arabian Shield and the Arabian Platform. We estimate lithospheric velocity structure by modelling regional waveforms recorded by the 1995-1997 Saudi Arabian Temporary Broadband Deployment using a grid search scheme. We employ a new method whereby we narrow the waveform modelling grid search by first fitting the fundamental mode Love and Rayleigh wave group velocities. The group velocities constrain the average crustal thickness and velocities as well as the crustal velocity gradients. Because the group velocity fitting is computationally much faster than the synthetic seismogram calculation this method allows us to determine good average starting models quickly. Waveform fits of the Jon and Sn body wave arrivals constrain the mantle velocities. The resulting lithospheric structures indicate that the Arabian Platform has an average crustal thickness of 40 km, with relatively low crustal velocities (average crustal P- and S-wave velocities of 6.07 and 3.50 km s -1 , respectively) without a strong velocity gradient. The Moho is shallower (36 km) and crustal velocities are 6 per cent higher (with a velocity increase with depth) for the Arabian Shield. Fast crustal velocities of the Arabian Shield result from a predominantly mafic composition in the lower crust. Lower velocities in the Arabian Platform crust indicate a bulk felsic composition, consistent with orogenesis of this former active margin. P- and S-wave velocities immediately below the Moho are slower in the Arabian Shield than in the Arabian Platform (7.9 and 4.30 km s-1, and 8.10 and 4.55 km s_', respectively). This indicates that the Poisson's ratios for the uppermost mantle of the Arabian Shield and Platform are 0.29 and 0.27, respectively. The lower mantle velocities and higher Poisson's ratio beneath the Arabian Shield probably arise from a partially molten mantle associated with Red Sea spreading and continental volcanism, although we cannot constrain the lateral extent of a zone of partially molten mantle.

A joint inversion algorithm to process geoelectric and surface wave seismic data. Part II: applications

Seismic and geoelectric methods are often used in the exploration of near‐surface structures. Generally, these two methods give, independently of one other, a sufficiently exact model of the geological structure. However, sometimes the inversion of the seismic or geoelectric data fails. These failures can be avoided by combining various methods in one joint inversion which leads to much better parameter estimations of the near‐surface underground than the independent inversions. In the companion paper (Part I: basic ideas), it was demonstrated theoretically that a joint inversion, using dispersive Rayleigh and Love waves in combination with the well‐known methods of DC resistivity sounding, such as Schlumberger, radial dipole‐dipole and pole‐pole arrays, provides a better parameter estimation. Two applications are shown: a five layer structure in Borsod County, Hungary, and a three‐layer structure in Thüringen, Germany. Layer thicknesses, wave velocities and resistivities are determined. Of course, the field data sets obtained from the ‘real world’ are not as complete and as good as the synthetic data sets in the theoretical Part I. In both applications, relative model distances, in percentages, serve as quality control factors for the different inversions; the lower the relative distance, the better the inversion result. In the Borsod field case, Love wave group slowness data and Schlumberger, radial dipole‐dipole and pole‐pole (i.e two‐electrode) data sets are processed. The independent inversion performed using the Love wave data leads to a relative model distance of 155%. An independent Schlumberger inversion results in 41%, a joint geoelectric inversion of all data sets in 15%, a joint inversion of Love wave data and all geoelectric data sets in 15% and the robust joint inversion of Love wave data and the three geoelectric data sets in 10%. In the Thüringen field case, only Rayleigh wave group slowness data and Schlumberger data were available. The independent inversion using Rayleigh wave data results in a relative model distance of 19%. The independent inversion performed using Schlumberger data leads to 34%, the joint and robust joint inversion of Rayleigh wave and Schlumberger data gave results of 18% and 20%, respectively.

Shear-wave velocity and attenuation structure beneath Antarctica determined from surface waves

SUMMARY Fundamental and higher mode surface waves generated by nine earthquakes, which occurred in Antarctica and its nearby regions and were recorded at Scott Base (SBA), South Pole (SPA), Novolazarevskaya (NVL) and Mirnyy (MIR) seismic stations are used to determine the shear-wave velocity and attenuation structure beneath these regions. The Frequency-Time Analysis method is used to determine group velocities for periods ranging from 6 to 80 seconds for fundamental and higher mode Rayleigh and Love waves. Crustal thickness is found to be 41 km beneath the eastern part and 30 km beneath the western part of Antarctica. Rayleigh and Love wave group velocities show lower values in the eastern part as compared to the western part of Antarctica. A shear-wave velocity of 3.52 to 3.7 km s−1 is estimated in the lower part of the crust (15–41 km from the surface) beneath eastern Antarctica. Similarly a shear-wave velocity of 3.48–3.6 km s−1 is estimated in the lower part of the crust (15–30 km from the surface) beneath western Antarctica. Rayleigh and Love wave group velocities are found to be lower for eastern Antarctica compared to Australia, Canada, India and other shield models of the world. Love and Rayleigh wave attenuation coefficients are estimated at periods of 10–110 s using the spectral amplitude of these waves across the eastern and western parts of Antarctica. Backus & Gilbert inversion theory is applied to the surface-wave attenuation data to obtain average Q−1β models for the crust and upper mantle beneath Antarctica. Inversion of Love and Rayleigh wave attenuation data shows a high-attenuation zone (Q= 125–200) at a depth of 10 to 40 km beneath eastern Antarctica. Similarly, a high-attenuation zone (Q= 65) occurs at a depth of 20 to 90 km beneath western Antarctica. The Q−1β models show a lithospheric thickness of 80–100 km beneath western Antarctica. The base of the lithosphere is identified as the depth at which there is a significant change in the Q−1β value. The Q−1β models for Antarctica show that there is a decrease in Q−1β value by an average of three factors in the asthenosphere as compared to lithosphere. In general, Q−1β in the asthenosphere show a higher value (an average of 2–3 times) compared to the lithosphere. This implies that the asthenosphere beneath Antarctica is cooler compared to other shield structures of the world.

Analysis of Rayleigh wave refraction from three-component seismic spectra

SUMMARY Lateral refraction of intermediate period surface waves is caused by horizontal gradients in local phase velocity in the vicinity of the source-receiver minor arc. For smooth structural perturbations, this is manifested by an off-azimuth arrival (azimuth deviation) of the Airy phase. We present a method for estimating azimuth deviation as a function of frequency for Rayleigh waves observed on a three-component seismogram. By taking advantage of the different dispersion characteristics of the Rayleigh and Love wave group arrivals, the method essentially constructs an optimal narrow-band taper for each frequency component, in order to minimize the effect of Love wave contamination in the Rayleigh wave azimuth estimation. We have measured the frequency-dependent azimuth deviation of fundamental mode Rayleigh waves of period 20-100s under a variety of conditions. The most coherent results are obtained for (1) seismic records with a relatively high Rayleigh wave excitation, (2) ray paths which are confined to purely oceanic or purely continental lithosphere, and (3) ray paths of intermediate offset (30°-50°).

Crustal structure and possible anisotropy in Turkey from seismic surface wave dispersion

SUMMARY The velocity structure of two regions of Turkey are determined using single-station measurements of Rayleigh and Love wave group velocities in the period range 8-50s. A differential inversion scheme yields models for Turkey in which crustal and upper mantle shear-wave velocities are slower than those of most of Europe. Comparisons of upper mantle shear-wave velocities we have obtained with reported P,, velocities leads to Poisson's ratio values in the upper mantle between 0.29 and 0.30 for eastern Turkey and between 0.27 and 0.31 for western Turkey. Crustal velocities are slightly slower and upper mantle velocities are slightly faster in western Turkey than in eastern Turkey. The crust-mantle boundary obtained in our studies is gradational, but if a shear velocity of 4.2 km s-l is taken to define the upper mantle then the crust appears to be about 40 km thick throughout all of Turkey. A sharp crust-mantle boundary may occur, but cannot be resolved. The data of this study require neither a low-velocity zone in the upper mantle nor polarization anisotropy in the crust or upper mantle. Azimuthal variations of Rayleigh and Love wave group velocities in western Turkey are consistent with velocities predicted by an azimuthally anisotropic upper crust in which vertical cracks are orientated in an approximate E-W direction. This interpretation is consistent with geological information, fault-plane solutions, lineations mapped from satellite observations, and reported heat flow values, but the possibility that these variations are caused by lateral changes of velocity in the crust of western Turkey cannot be completely ruled out at the present time.

Crust and upper mantle velocity structure beneath north and central India from the phase and group velocity of Rayleigh and Love waves

Crust and upper mantle velocity structure is determined using the fundamental mode Rayleigh and Love waves for 23 different paths across north and central India. The earthquakes occurring in Kashmir, Nepal Himalaya, Burma and nearby regions and recorded at Hyderabad, New Delhi and Kodaikanal seismic stations are used for dispersion studies. Frequency Time Analysis (FTAN) using the multiple filter technique is done for the fundamental mode surface waves to estimate group velocities from the time periods of 13–71 s. The single station technique is used to estimate Rayleigh and Love wave phase velocities at time periods of 15–75 s. Velocity structure down to a depth of 120 km is determined using both Rayleigh and Love wave phase and group velocities. The crustal thickness is found to be 50 km, with 4 km sedimentary thickness, for the northern India across Burma to New Delhi path, and 44 km for the central India across Kashmir, Nepal Himalaya and northeast India to Hyderabad and Kodaikanal path. A low-velocity zone ( V p = 8.4 km/s, V s = 4.15 km/s ) is found from 100 km below the surface. Velocity structure shows that the shield structure of the peninsula extends up to northernmost India below sub-Himalayan regions with an overlying sedimentary thickness of 4 km. The lower value of observed group velocity and higher phase velocity across northern and central India may be caused by the presence of varying sedimentary thickness in the northernmost part of India (the Indo-Gangetic Plain) below the sub-Himalayan region. The observed lower value of Rayleigh wave group velocities below 40 s time periods can be explained by assuming a thin layer of a high contrast material with a P-wave velocity of 4.6 km/s, an S-wave velocity of 2.5 km/s, a density of 3.24 g/cm 3 and a thickness of 3 km, centred above the Moho.

Regional variations in the crustal structure of Northern Canada from surface wave dispersion

Rayleigh and Love wave group velocities in the Arctic islands and adjacent mainland of northern Canada exhibit large regional variations which can be attributed to variations in crustal structure. The shield and platform regions, the eastern Sverdrup basin, and the western Sverdrup basin are characterized by progressively slower velocities. Inversions of the regionalized group velocity data lead to three distinct shear-velocity models. The shield and platform model is similar to earlier models obtained for the Canadian shield. The Sverdrup basin models have low near-surface velocities which may correspond to thick accumulations of sediment. A sediment thickness of about 12 km obtained for the eastern basin corresponds to the maximum thickness inferred from earlier geological studies. An even thicker zone of low velocities occurs in the upper crust of the western basin. This zone may represent the true thickness of a very deep basin or slow velocities in basement rock may underlie the shallower sediments. A third alternative is that low apparent velocities for the western Sverdrup basin result from systematic errors in group velocity determinations produced by epicentral mislocations or origin time errors of the earthquakes used, but such mislocation or errors would have to be quite large to explain the observations. It is not possible to explain the group velocities across the basin areas by simply adding sediments to the shield and platform model. The resulting discrepancies with the data suggest an altogether different crustal structure beneath the basin than that beneath the shield and platform areas.

Surface wave dispersion, crustal structure, and sediment thickness variations across the Barents shelf

Summary. Rayleigh and Love wave group velocities were determined for 21 paths across the Barents shelf. Those group velocities exhibit regional variations of 1.0 km-' or more at short periods, depending upon the location of the path within the shelf. Only two different crustal shear-velocity models beneath sedimentary layers are required, however, to explain all of the group velocity data. One model pertains to most of the shelf from a longitude near the eastern coast of Svalbard to Novaya Zemlya. The other pertains to a 200 or 300km wide region at the western edge of the shelf. Shear velocities in the upper crust of the western region are significantly higher and the crust is much thinner than they are for the rest of the shelf. That region is known to have moved to its present prosition from a point several hundred kilometres to the north during the Caledonian orogeny. Surface wave group velocities within each of the two regions are strongly influenced by sediments which have accumulated in basins within the Barents shelf. Some of these basins, in the southern portion of the shelf, may be 10 km or more in thickness.

Azimuthal anisotropy of surface waves and the possible type of the seismic anisotropy due to preferred orientation of olivine in the uppermost mantle beneath the Pacific Ocean.

The group velocities of surface waves in the period range 22-89 s were measured to retrieve the anisotropy pattern of the surface wave velocities in the Eastern Pacific Ocean. WWSSN long period records at KIP, Hawaii, from ridge events in the Gulf of California, the East Pacific rise, and the Easter Island Cordillera were used to cover propagation directions N60°E-N140°E. These station and epicenters are in very favorable positions to minimize the effects of the lateral heterogeneity owing to ocean floor ages. The fast direction of the Rayleigh wave group velocity is normal to ridge, showing 3-5% azimuthal anisotropy. The Love wave group velocity shows little azimuthal anisotropy. A model of a possible type of preferred orientation of constituent olivine beneath the Pacific Ocean is proposed, based on the elastic constants and the petrofabrics of dunite from Hidaka, Hokkaido, Japan, and that of olivine nodules in the ophiolite complex from the Bay of Islands and Oman. In this model, a-axis is concentrated along a particular direction and b- and c-axes form a girdle. Elastic constants of the model are C11=2.44, C22=C33=2.05, C12=C13=0.72, C23=0.67, C44=0.69, and C55= C86=0.75 (1012 dyn·cm), where X1-axis, or the direction of the a-axis concentration, is normal to ridge and X3-axis is vertical. If we assume that the type of the preferred orientation is an age-independent property of suboceanic lithosphere, this model can explain the overall features of the large-scale anisotropy in the Pacific Ocean: the azimuthal anisotropy of surface wave velocities as described above; the SH-SV anisotropy of surface waves of about 0.15 km/s; the Pn, anisotropy of 7.9 km/s to 8.6 km/s; the high Sm, velocity of about 4.8 km/s; and the ScS polarization anisotropy