The depth dependence of earthquake T-phases at an ocean acoustic observatory

Abstract

T-phases are earthquake signals that have propagated, at least partially, in the ocean sound channel. T-phase hydrophone networks detect much smaller earthquakes over basin scales than land-based networks and they detect many more earthquakes than comparable regional scale seismic land networks. Furthermore, since T-phases travel at lower velocities than seismic phases, they result in much more precise locations of events given the same timing accuracy. T-phases are typically spread over 10's of seconds, and a common problem, however, is precisely identifying the arrival time of an event. T-phase stations usually consist of single hydrophones moored near the sound channel axis and the depth dependence of the T-phase envelope and frequency content is rarely studied. In the North Pacific Ocean, from 2004 to 2005, ambient noise and earthquakes were observed at an ocean acoustic observatory consisting of a vertical hydrophone array (from about 750 m above the seafloor to 375 m from the surface) and three co-located ocean bottom seismometers. This data set provides a unique opportunity to observe earthquake signals and their characteristics throughout the water column and to provide ground-truth to theoretical predictions on the excitation and propagation mechanisms of T-phases. In at least one case, a T-phases from a distant earthquake was readily observed even at the seafloor, well-below the conjugate depth.