Excitation of T-phases by seafloor scattering.

Abstract

T-phases excited by suboceanic earthquakes are classified into two types: abyssal phases which are excited near the earthquake epicenter at seafloor depths far below the SOFAR velocity channel, and slope T-phases which are excited at continental, or ocean island slopes and ridges at distances up to several hundreds of kilometers from the epicenter. In this article, it is demonstrated that approximate time-frequency characteristics of both classes of T-phase can be synthesized under the assumption that T-phases are excited by scattering from a rough seafloor. Seafloor scattering at shallow depths preferentially excites low order acoustic modes that propagate efficiently within the ocean sound channel minimum. At greater depths, scattering excites higher order modes which interact weakly with the seafloor along much of the propagation path. Using known variations in near-source bathymetry, T-phase envelopes are synthesized at several frequencies for several events south of the Fox Islands that excited both types of T-phase. The synthesized T-phases reproduce the main time vs frequency features of each type of arrival; a higher frequency, nearly symmetric arrival excited near the epicenter and a longer duration, lower frequency arrival excited near the continental shelf, with a peak amplitude at about 5 Hz.