The April 1992 Cape Mendocino Earthquake Sequence: Seismo-Acoustic Analysis Utilizing Fixed Hydrophone Arrays

The oceanic T-waves of earthquakes associated with the 1992 Cape Mendocino earthquake sequence were recorded and analyzed using fixed hydrophone arrays located throughout the north-east Pacific Ocean. The T-waves of these events were well recorded with high S/N ratios and strong acoustic energy present over a 0–64 Hz bandwidth. The smallest event recorded by the hydrophone arrays from the sequence had a local magnitude of 2.4. The hydrophone records of the three largest shocks in the sequence (ML 6.9, 6.2, 6.5) exhibited both T-waves and lithospheric phases from these events. Low-pass filtering (2 Hz) of the lithospheric phases yielded a clear P-wave arrival for epicentral distances of <10°, but no apparent S-wave. A seafloor cable-break was detected immediately after the second M>6 aftershock, possibly the result of a submarine slide. The direct P-wave hydrophone records from the second large aftershock showed a relatively high-amplitude, high-frequency arrival, consistent with seismic analyses which used this information to infer rupture direction. The rupture direction was toward the location of the cable break, thus rupture directivity possibly played a role in initiating the slide event. Modelling of the T-wave propagation path, using the Parabolic Equation model, produced estimates of the acoustic transmission loss from epicenter to receiver. The transmission loss to the most distant phones is typically 10-20 dB , and can be as large as 50–70 dB for acoustic propagation paths that cross the continental margin. The amount of acoustic energy each earthquake released into the ocean at the seafloor–water interface was estimated applying the transmission loss and instrument response to the recorded T-wave signals. This acoustic source power level was calculated for 41 events with magnitudes over a recorded range of 2.4ML6.9, with 17 of these events having their seismic moment estimates available through the NEIC. Ground displacement spectra were estimated from the acoustic power spectra and showed no indication of a corner frequency. Thus empirical analyses relating source level to magnitude and seismic moment were necessary to quantitatively derive an earthquake's size from hydrophone records. The results of indicator variable regression analyses suggest that T-wave source level increases linearly with the event's local magnitude and seismic moment. Furthermore, the source power level versus magnitude relationships for oceanic and continental earthquakes are significantly different, probably illustrating differences in the seismic and acoustic propagation paths from hypocenter to the hydrophone receivers. The results indicate that acoustic measurements provide a reasonable estimate of magnitude and seismic moment of an oceanic earthquake that was not detected by land-based seismic networks.