Characteristics of T phases from earthquake sources, or how earthquakes ‘‘talk’’ into the ocean

Abstract

We review the mechanisms by which earthquake sources within the solid Earth generate acoustic energy into the SOFAR channel. We examine individual cases involving both the relatively inefficient mechanism of ‘‘downslope conversion,’’ leading to T phases with low amplitudes and long durations, and efficient conversion at steep interfaces allowing direct penetration of the SOFAR, larger amplitudes, and shorter durations. We derive several ways to quantify T waves generated by an earthquake source. To avoid the saturation of their amplitude as source size grows, we use the concept of a T-phase energy flux (TPEF), which mirrors the radiated energy measured on conventional seismic body waves. TPEF is scaled to the seismic moment of the earthquake to define a parameter, gamma, characterizing the earthquake’s efficiency for T-phase generation. In another approach, we compare the amplitude of T-phase envelopes to the duration of their signal. The resulting discriminant has been proposed to identify earthquakes from explosions. We show that it varies for several categories of nonstandard earthquakes, notably those occurring in volcanic islands and events with slow rupture giving rise to large tsunamis. Finally, we discuss a few examples of T waves from underwater landslides.