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Evidence for anisotropic scattering of short-period P phases in the upper mantle

DEPARTMENT OF EARTH AND SPACE SCIENCES INDIANA UNIVERSITY-PURDUE UNIVERSITY OF FORT WAYNE, FORT WAYNE, INDIANA 46805
APPLIED SEISMOLOGY GROUPLINCOLN LABORATORY MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASSACHUSETTS 02142

Abstract

In an attempt to isolate the transmission effects of the upper mantle and crust on short-period compressional phases, amplitude ratios (pPcP/pP)/(PcP/P) were studied from 18 earthquakes using steered beams of LASA data. This ratio eliminates the source radiation pattern and earth transmission effects below the earthquake focus including spherical spreading and Q in the lower mantle, the core-mantle reflection, and the upper mantle and crust structure under LASA. What remains can be attributed to geometric spreading, attenuation and scattering along the two-way ray paths of pPcP and pP from focus to surface. The measured ratios vary from ~1 for shallow events to ~4 or greater for deep events independently of epicentral distance. This strong depth effect cannot be due to geometric spreading or attenuation using a typical low Q model (60 < Q < 450 in the upper mantle). Attenuation models which could produce the depth effect require unrealistically low Q's <40 in the upper 200 km. A possible cause of the depth effect is anisotropic scattering in the upper mantle whereby waves of high dT/d are attenuated more severely than those of low dT/d. This agrees with an earlier study of PcP/P ratios at LASA, which had a similar though less severe depth effect.