Abstract

Several earthquakes in a limited region on the coast of Northern California and off the coast of Oregon have provided a set of broadband regional seismograms at Lajitas (LTX). The significant Love and Rayleigh waves contained in these waveforms have repetitively sampled the same source-receiver path within the Basin and Range province. Previous studies for this path (using a subset of these sources) of mantle S waves suggest a smoothly varying S velocity structure above 400 km followed by a strong gradient between 400 and 700 km. These data are unable to uniquely constrain the shear velocity at shallower depths. Surface-wave inversions are performed in order to refine the upper mantle S-wave model using group- and phase-velocity dispersion for periods between 10 and 100 sec. This period range gives good resolution for sub-Moho depths extending into the low-velocity zone. Group velocities are extracted by multiple filter analysis and show significant differences for events on- and off-shore at periods around 50 sec. This effect can be shown to be an interference pattern with the mantle S body-wave arrivals. As single-station data were used, phase velocities are estimated by phase-matched filtering. The recovered surface-wave dispersion data are used in an inversion procedure to recover a shear-wave structure for the Basin and Range path to LTX that consists of a relatively thin mantle lid underlain by a low-velocity channel extending to a depth of 200 km. Structure below 200 km is poorly resolved and trades off with shallower structure. The Rayleigh-wave group-velocity data favor shear-velocity models with a ridge within the low-velocity zone with velocities 0.05 to 0.2 km/sec higher than the background.

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