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

Abstract

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/dΔ are attenuated more severely than those of low dT/dΔ. This agrees with an earlier study of PcP/P ratios at LASA, which had a similar though less severe depth effect.