Normal-mode composition of earthquake T phases recorded in the deep ocean

Abstract

For upslope propagation in an ocean environment, the place(s) where underwater acoustic field energy couples into the land seismic field is determined by the local water depth and the normal-mode composition of the acoustic energy, to first approximation. Therefore, the use of earthquake-generated T phases as natural probes of water-to-land coupling characteristics is aided by knowledge of their modal makeup. During the 1989 VAST experiment, conducted in 5000-m-deep water in the midlatitude Northeast Pacific Ocean (34.0 deg N, 140.0 deg W), the Marine Physical Lab deployed a 200-element, 3000-m-aperture vertical hydrophone array from R/P FLIP. Several earthquakes generated detectable T phases during the 11-day recording period and the modal composition of five such events, including one on land, have been determined. Because of the extended nature of the T-phase source region, the individual modes are uncorrelated, permitting an eigenanalysis of the data cross spectral matrix at a given frequency to provide approximate mode amplitude and eigenfunction information. Results indicate that for the predominant T-phase energy around 5 Hz, the first four or five modes are important, with the mode having largest amplitude often not being the lowest one. [Work supported by ONR and DSWA.]