Investigation for anisotropic attenuation in the Pacific

Abstract

Anisotropic attenuation arises through preferential orientation of dislocations, crystals, and scattering bodies. As seismic energy loss is an exponential function of distance, whereas traveltime is a linear function, attenuation anisotropy is potentially as important as velocity anisotropy. In an examination for anisotropic attenuation, oceanic S-waves, So, were analysed from 10 earthquakes near Hokkaido, Japan which were recorded naturally rotated into SV and SH components of motion at the HIG borehole seismometer in the northwest Pacific, OSS IV. Some evidence for velocity polarization anisotropy is seen. The observed W / S H amplitude ratios, which vary from $ to 2, limit the possible contribution of scattering between orthogonal components of motion. The effect of local scattering of SV into SH and vice versa, which contaminates the measurement, can be estimated from observed transverse energy on P-waves. In the frequency band with a good signalto-noise ratio, 3 to 20Hz, the So phases have propagated about 900-6000 wavelengths. Spectral ratios of SV over SH are formed for each event. Frequency independent source, propagation, and receiver effects are removed by normalizing the spectra. The normalized spectral ratios are then stacked. The resultant stacked data are essentially flat, varying by less than 15 per cent about their geometric mean with no frequency-dependent trend. The results are consistent with Q values of SV polarized shear waves within 5 per cent of those for SH polarized shear waves.

Structure of the Earth: Mantle and Core

“…Short‐period P wave attenuation and its frequency dependence has continued to be a very active area of research in this review period, particularly in shield areas [ Der et al , 1986; Walk , 1988, 1989; Burger et al , 1986]. The oceanic lithosphere is also a surprisingly efficient waveguide for high frequency seismic energy [ Butler et al , 1987; Butler and Cessaro , 1990] with the Q's of S n and P n being increasing functions of frequency. Sereno and Orcutt [1987] interpret the observations as refractions from the lower oceanic lithosphere with the subsequent coda being reverberations in the sediment and water column.…”