HR: 0800h
AN: S51D-1041    [Abstracts]
TI: Hydroacoustic identification of tsunamogenic events based on the shape of the T-phase spectrum
AU: * Salzberg, D H
EM: david.h.salzberg@saic.com
AF: SAIC/OSD, 1710 SAIC Dr. M/S 1-11-15, Mclean, va 22102 United States
AU: Mikhalevsky, P N
EM: peter.n.mikhalevsky@saic.com
AF: SAIC/OSD, 1710 SAIC Dr. M/S 1-11-15, Mclean, va 22102 United States
AB: We have identified a discriminate between seismic events which excite tsunamis and those that do not generate Tsunamis. We have found that tsunamogenic earthquakes have significantly more high-frequency (>50 Hz) relative to low-frequency (< 10 Hz) T-phase energy with longer durations (>30 seconds) than non-tsunamogenic events. Our approach is to model the spectral slope of the T-phase; the T-phase spectrum is the product of the source spectrum, the propagation loss in the solid earth, the propagation loss in the ocean, the conversion from seismic to T, and the instrument response. Since the oceanic T-phase propagation can be accurately modeled, and assuming that the seismic-to-acoustic is constant in frequency, the remaining unknown terms are solid-earth propagation and the source excitation. Assuming the solid earth transfer spectral shape is primarily anelastic attenuation, and the source spectrum is Aki's corner frequency model. We are able to obtain the distance traveled prior to conversion to T based the anelastic attenuation, and possibly the corner frequency of the source. If the approach is valid, then any significant high-frequency energy means that the source was close to (or in) the water column, and did not travel within the earth for significant distances. We analyzed three tsunomogenic events (Dec 26, 2004, Mar 28, 2005 and July 17, 1998) and several non-tsunamogenic events. All of the tsunomogenic events had significant high frequency energy with long durations. This is probably the result of either very shallow rupture (hence sea floor deformation) or secondary sources within the water column. The examined non-tsunomogenic thrust events did not have significant, long duration high frequency energy. However, it is likely that that a shallow large strike-slip event would generate significant high-frequency T-phase energy without exciting a Tsunami. However, since all events that triggered tsunamis had long-duration high-frequency T-phases. Thus, the absence of the long-duration high-frequency T-phase energy strongly suggests that an event did not excite a tsunami. In addition, an unexpected long-duration high frequency source may indicate an unexpected tsunami, perhaps triggered by a submarine land slide.
DE: 4564 Tsunamis and storm surges
DE: 7215 Earthquake source observations (1240)
SC: Seismology [S]
MN: Fall Meeting 2005