Observations of sediment shear velocity in the San Diego Trough

Abstract

We report observations of shallow seafloor shear velocity structure in the San Diego Trough, whose depth is about 1100 m. Dispersed Scholte waves were generated using a new implosive source, described elsewhere in this meeting, and observed using ocean-bottom seismographs. Preliminary estimates of the surficial shear velocities are in the 16–25 m/s range, which is very low. The Scholte waves are observed in the 1–8 Hz frequency range and dispersion is well-developed after 100 m of propagation. Higher modes appear in addition to the fundamental mode, which exhibits a smooth increase in frequency from 2 to an Airy phase at about 3 Hz. The group velocity of the Airy phase approximates the surficial shear velocity. In the work area, the topographic relief is only a few meters. Geologically, the sediments are turbidites distributed by a channel and levee system. High-resolution multibeam bathymetry is available from NOAA and is being used to aid geological interpretation.