Detection of T-phases at Island Seismic Stations: Dependence on Seafloor Slope, Seismic Velocity and Roughness

The hydroacoustic segment of the International Monitoring System (IMS) currently being installed for use in verifying compliance with the Comprehensive Nuclear Test Ban Treaty (CTBT), will consist of six hydrophone stations and five supplemental T-phase stations. The ability to detect acoustic signals at T-phase stations relies on an understanding of the acoustic to seismic coupling mechanisms. In this paper, we model upslope propagation of acoustic energy at a sloping wedge using an elastic parabolic equation (PE) modeling method. We synthesize both vertical and horizontal displacement waveforms for broadband sources, and show that the signal amplitudes are strongly dependent on the properties of the offshore slope. For all slope types, the signal amplitudes at onshore seismic stations decrease rapidly with increasing frequency. This decrease with frequency is most pronounced for sources with high mode number content at a shallow sloping wedge. Finally, we show that a significant amount of energy can be lost to surface shear waves at the sloping wedge. We investigate the dependence of surface shear wave excitation on seafloor roughness.