HR: 1340h
AN: S53A-1032    [Abstracts]
TI: Use for Hydroacoustic (T-phases) for Tsunami Warning
AU: * Salzberg, D
EM: david.h.salzberg@saic.com
AF: SAIC, 1710 SAIC Dr M/S 1-11-15, MCLEAN, VA 22102, United States
AU: Newton, J
EM: John.l.newton@saic.com
AF: SAIC, 3049 UALENA STREET SUITE 1100, Honolulu, HI 968199, United States
AB: We have identified a method to exploit T-phases recorded at the Comprehensive Test Ban Treaty's International Monitoring System in-water hydroacoustic arrays for tsunami warning by examining the T-phase spectral slope and high-frequency duration. Over a dataset consisting of several hundred M>=5.5 events recorded at the hydroacoustic stations, long-duration and shallow spectral slope T-phases originating from subduction-zones region are associated with tsunamigenic earthquakes. These observations can be explained based on the following concepts: the observed T-phase results from propagation within the earth convolved with propagation within the water column nd a earth-water conversion factor. Propagation within the water column at 5-100 Hz is simple as there is minimal spectral deformation. Within the earth, the propagation is more complicated, with scattering, multiple phases, and anelastic attenuation. As the primary factor impacting the spectral shape is the anelastic attenuation, and assuming Q is constant with frequency, that means that the observed T-phase spectrum is dependent only on the value of Q and the number of wave cycles. Thus, the presence of a shallow slope means that either Q is high or the distance is small. In subduction zones, the energy travels through the low-Q accretionary wedge; thus implying that a shallow slope in subduction zone environment means the rupture is very close to the water interface. Similarly, the T-phase duration can be used to estimate the source duration. At higher frequencies, the less direct seismic energy will be attenuated prior to entering the water column. Therefore, the duration of the higher frequency energy gives the source duration. We had also hoped to use the direction of the T-phase to determine source finiteness. However, the analysis indicated that the broadband T-phase directionality is influenced by many factors including the source finiteness; we found that smaller, deep events show a broad range of back-azimuths, presumably the result of a longer solid-earth T-phase path. In addition, if the propagation to the hydroacoustic station aligns with the fault orientation, then source finiteness estimates cannot be obtained. It is possible that directionality from higher frequency T-phase energy will prove to be a robust indicator.
DE: 4259 Ocean acoustics
DE: 4564 Tsunamis and storm surges
DE: 7215 Earthquake source observations (1240)
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Seismology [S]
MN: 2007 Fall Meeting