Shear Wave Velocity Distribution Research Articles

THE INFLUENCE OF S-WAVE VELOCITY OF SUBSURFACE LAYER ON TRANSFER FUNCTION OF MODEL GROUND

In this study, we investigate how the vertical distribution of shear wave velocity has effects on the shape of transfer function of subsurface layer. The effective coefficients for transfer function and its maximum peak value are defined on the basis of partial derivatives of transfer function regarding to shear wave velocity and thickness of a layer. The effective coefficients are calculated to several subsurface layer models in Japan. It is concluded that the effective coefficients of the shear wave velocity of upper layer and base rock are generally larger than those of the medium layers. However, in the subsurface layer model which has relatively large shear wave velocity and large thickness in medium layers, it is found that the effective coefficients in the medium layers are considerably large.

Anisotropy and shear‐velocity heterogeneities in the upper mantle

Long‐period surface waves are used to map lateral heterogeneities of velocity and anisotropy in the upper mantle. The dispersion curves are expanded in spherical harmonics up to degree 6 and inverted to find the depth structure. The data are corrected for the effect of surface layers and both Love and Rayleigh waves are used. Shear wave velocity and shear polarization anisotropy can be resolved down to a depth of about 450 km. The shear wave velocity distribution to 200 km depth correlates with surface tectonics, except in a few anomalous regions. Below that depth the correlation vanishes. Cold subducted material shows up weakly at 350 km as fast S‐wave anomalies. In the transition region a large scale pattern appears with fast mantle in the South‐Atlantic. S‐anisotropy at 200 km can resolve uprising or downwelling currents under some ridges and subduction zones. The Pacific shows a NW‐SE fabric.

Ray-theoretical approach to torsional oscillations of a two-layered spherical shell and application to an inversion problem.

A ray-theoretical approach was adopted to obtain an asymptotic frequency equation for torsional oscillations of a spherically symmetrical Earth consisting of a two-layered solid shell, that is, the crust and mantle. It is shown that the equation is utilized for the inversion problem of getting the shear wave velocity distribution in the mantle from torsional eigenfrequencies. Two different methods of solving the problem are then presented. Numerical experiments for a model Earth confirm the validity of these methods and show the necessity of taking the Moho discontinuity into consideration for this kind of inversion method.

Lunar near‐surface shear wave velocities at the Apollo Landing Sites as inferred from spectral amplitude ratios

We reexamined the horizontal‐to‐vertical amplitude ratios of the long‐period seismograms to determine the shear wave velocity distributions at the Apollo 12, 14, 15, and 16 lunar landing sites. Average spectral ratios, computed from a number of impact signals, were compared with spectral ratios calculated for the fundamental mode Rayleigh waves in media consisting of homogeneous, isotropic, horizontal layers. The shear velocities of the best fitting models at the different sites resemble each other and differ from the average for all sites by not more than 20% except for the bottom layer at station 14. The shear velocities increase from 40 m/s at the surface to about 400 m/s at depths between 95 and 160 m at the various sites. Within this depth range the velocity‐depth functions are well represented by two piecewise linear segments, although the presence of first‐order discontinuities cannot be ruled out.

Shear-wave velocity structure for a spherically averaged earth

The shear-wave velocity distribution in a spherically averaged Earth is estimated statistically from previously published short-period S travel-time measurements (Uhrhammer). An algorithm is defined for integral inversion techniques which allows estimation of the variance of the velocity distribution from the uncertainties in the S slowness model. Comparisons are made between the resulting S-velocity solution and other solutions in common use. There are significant differences (at the 95 per cent confidence level) between the S-velocity model determined here and the Jeffreys—Bullen model over the depth ranges of 150–550 km and 2100–2350 km. The 95 per cent confidence level in the present velocity distribution ranges from ± 0.025 km/s at 625 km to ±0.32km/s at 2766 km and averages about ±0.063 or ±1 percent. Correlations between azimuthally dependent source and station adjustments (which were previously determined (Uhrhammer)) indicate widespread lateral inhomogeneities (up to 3.4 per cent) to depths of approximately 700 km. Up to three-quarters of the source adjustments are due to lateral velocity variations in the source regions. Station adjustments for differential S minus P times are significantly correlated with elevation and crustal age, but not with station instrumental magnification.

A geophysical, geochemical, petrological model of the sub-marine lithosphere

A schematic but quantitative geochemical, petrological, model of the sub-marine lithosphere and its genesis is given. With this model we calculate numerically, a priori, the geophysical characteristics of the lithosphere, its acoustic properties, density, oceanic heat flow and ocean bottom topography. Comparison with observational data for these characteristics shows good agreement. Particular attention is given to anomalous upper mantle in the vicinity of spreading centres. Compressional and shear wave velocity distributions are given in tabular form for the submarine lithosphere as a function of age. Comparison between observations for V p , V s and the calculated acoustic properties suggests that the lower marine lithosphere is anisotropic. Possible thickening of layer 3 with age is discussed. Melt distribution in the ridge axial region has been evaluated. All calculations were done for a plate velocity of 1 cm y −1.

Shear wave velocity measurement during a standard penetration test

AbstractAn experiment was carried out to develop a technique to measure shear wave velocity simultaneously with the standard penetration test popular in soil engineering. In the standard penetration test an impact at the bottom of a borehole is produced by weight dropping and may be expected to generate seismic waves. A three‐component geophone was set on the ground surface near the borehole and the waves generated were recorded with a magnetic recorder at successive depths of the penetration test. The predominance of the SV wave obtained with this simple method was assured by measurement of the particle orbit. Signal amplitudes decrease with depth and become less than the noise level at a certain depth. Therefore records from deeper sources must be processed to disclose the shear waves. Since waveforms of SV events generated by blows of the penetration test at a given depth are very similar, the signal to noise ratio would be expected to be improved by a stack of wave trains. A paste‐up of the radial component after stacking was compared with that before stacking and a refinement was clearly recognized.A vertical distribution of shear wave velocity was obtained by reading the onset time at each depth. Shear wave velocities thus obtained were compared with N values from the standard penetration test and specific resistivities from electrical logging in the same borehole. The data were mutually consistent.This experiment showed that a convenient, precise shear wave velocity measurement can be conducted during the routine work of a standard penetration test.

Fine structure of the crust inferred from linear inversion of Rayleigh-wave dispersion

abstract The Backus-Gilbert inversion formulation applied to surface-wave dispersion measurements can be used to infer detailed shear-velocity distribution in the Earth9s crust. In this study, Rayleigh-wave group-velocity dispersion measurements were used to derive the shear-wave velocity distribution within a crustal model consisting of 15 to 20 layers whose thicknesses were held fixed. Simultaneous inversion of fundamental mode (4- to 36-sec period) and first higher-mode (3- to 8-sec period) data was sufficient to obtain crustal models displaying high resolution. In order to avoid the ambiguity associated with large side lobes in the resolving kernels, the inversion scheme was modified to yield smooth resolving kernels. This improvement was particularly important to the problem of recognition of a crustal low-velocity layer. The inversion scheme was successfully applied to both real and idealized data. It is suggested that, by using Rayleigh-wave group-velocity measurements with standard deviations of 0.03 km/sec, crustal shear-velocity models may be derived which have estimated standard deviations of 0.15 km/sec and resolving kernels with spreads of less than 7 km.

静粘性係数, 内部摩擦および弾性定数の温度変化とそれらの間の関係についての実験式の提案

The static viscosity η, the internal friction Q-1, and the elastic wave velocities Vp and Vs in resin sample are measured from 10°C to the temperature close to its softening point, 50°C. The result shows that the change of η and Q with temperature can not be explained by a single thermally activated process, as the activation energy increasing with temperature. The shear wave velocity decreases with increasing temperature in a similar manner to the variations of log η and log Q. From the data obtained, the following empirical relations are derived:logQ=AVs+B logQ=Clogη+D logη=EVs+F where A, B, …, and F are constants. Therefore, the temperature variations of these properties are interrelated.Although the material composing the earth's interior is quite different from the resin used in the present experiment, similar empirical relation appear to be valid in the earth's mantle, too. From the available data on informations of mechanical property of the earth's mantle, following constants were obtained; A=0.42, B=0.17, C=0.26, D=-3.6, E=1.6, and F=14.5 when Vs and η are represented by km/sec and poise. The distributions of Q and η in the mantle are estimated using these empirical relations and the observed distribution of shear wave velocity.

Effect of low velocity channel on SH wave

The propagation of SH wave was investigated using the velocity distribution function compatible with the actual distribution of shear wave velocity inside the earth. The constants involving the velocity distribution function were calculated from Gutenberg-Birch model and the results discussed.

Seismic waves and earth structure in the Canadian shield

Abstract Careful measurements of phase velocities in the Canadian shield have been made in the period range 3 to 90 sec for Rayleigh waves and 12 to 60 sec for Love waves by phase correlation of wave trains. The continental Love wave phase velocity data are the first to be reported in the literature. The phase and group velocities are higher than yet found in any other continental area, indicating relatively higher shear velocities in the crust and upper mantle. For paths in the Canadian shield a prominent Lg arrival with a velocity of about 3.65 km/sec is observed and an Sn arrival is recorded clearly to distances of about 4000 km with a velocity of about 4.72 km/sec. A theoretical layered model of the crust and upper mantle consistent with the various types of data has been derived by an inversion method employing least-squares curve-fitting of phase velocity data. This model has a three-layered crust 35.2 km thick with shear velocity increasing to about 3.85 km/sec in the lower crust. The upper mantle has a high speed layer with shear velocity 4.72 km/sec down to about 115 km below which the low velocity channel has a shear velocity of about 4.5 km/sec down to a depth of about 315 km. At greater depths the shear velocity closely follows the Gutenberg model. Higher mode phase velocity dispersion curves computed from the theoretical model are used for computing theoretical seismograms for the Canadian shield. These theoretical seismograms possess many of the features of the observed Lg arrivals, showing that Lg can be explained by normal mode wave propagation in a simple, layered crust. This paper shows that new methods of measuring and interpreting surface wave dispersion, combined with travel time data, can provide detailed and reliable information on shear wave velocity distribution down to depths of a few hundred kilometers in regions of horizontally uniform structure.