Long Range Hydroacoustic Propagation from Small Explosives to Large Tectonic Events (Invited)

A wide variety of hydroacoustic signals have been detected near Diego Garcia, in the Indian Ocean, and at other hydro-acoustic stations operated by the Comprehensive Test Ban Treaty Organization. The durations range from several seconds for signals from explosive shots in temperate latitudes, tens of seconds for explosions within the Antarctic Circumpolar Current (ACC), several minutes for small earthquakes near the continental shelf, to over half an hour for propagation from a large tsunamigenic event, for source-receiver distances of several thousands of kilometers. The primary mechanisms used to explain the variability in duration are the existence of a distributed source region, source size, and dispersion. Travel time, and hence dispersion is computed using normal modes; bottom interacting higher order modes are neglected. For the explosive shots, the observed and predicted travel times and signal dispersion agree to within 2 s, under the assumptions that propagation is adiabatic and follows a geodesic path. Dispersion is predicted and observed to increase with propagation distance through the ACC. The duration of the underwater earthquakes along the continental shelf are independent of source size, and significantly exceeds the dispersion times computed for the given location, indicating a distributed source region; the azimuths derived from the hydro-acoustic signals further corroborate this. Finally, the long duration of the acoustic signal from the tsunamigenic event is used to derive the velocity and direction of rupture.