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Mapping shallow heterogeneity with teleseismic P to Rg scattered waves

Institute of Tectonics University of California, Santa Cruz, California 95064
Department of Geophysics Stanford University, Stanford, California 94305

Abstract

Over 9000 records of teleseismic P-wave coda from the Southern California Seismic Network are used to map lateral variations in P to Rg scattering strength and the group velocity of scattered Rg. Building on the results of Revenaugh (1995a), we use the correlation of scattering strength and topographic roughness as an indicator of model fit to estimate group-velocity dispersion over five frequency bands between 0 and 3 Hz. Rg group velocity displays normal dispersion, decreasing from 2.7 ± 0.3 km/sec for frequencies between 0 and 0.6 Hz to a low of 2.2 ± 0.4 km/sec for the 2.0 to 3.0 Hz band. Lateral heterogeneity is introduced by assuming Rg velocity is a function of local topographic roughness. Grid-search/migration analysis yields group velocities of 2.2 and 2.85 km/sec for "smooth" and "rough" terrain, respectively, and a 150-m threshold for the local standard deviation of topography distinguishing "smooth" from "rough". The map of scattering strength approximates topographic roughness throughout southern California, implying that teleseismic P-wave coda duration and relative amplitude level depend strongly on near-surface (<3 km) geology.