FY 2001

Seismic-wave contributions to bottom pressure fluctuations in the North Pacific—Implications for the DART Tsunami Array

Mofjeld, H.O., P.M. Whitmore, M.C. Eble, F.I. González, and J.C. Newman

In Proceedings of the International Tsunami Symposium 2001 (ITS 2001), Session 5-10, Seattle, WA, 7–10 August 2001, 633–641, (on CD-ROM) (2001)

Over the period October 1999–January 2001, there were four separate occasions in which real-time reporting tsunami DART systems, deployed by NOAA in the North Pacific, were set into tsunami event reporting mode by regional earthquakes. Fortunately, none of these generated a dangerous tsunami. To go into event mode, the high-frequency fluctuations in the bottom pressure (BP) had to exceed a pre-programmed threshold of 3 cm HO. An explanation for the events was found by examining the seismic surface waves generated by earthquakes. By Newton's third law, they produced BP fluctuations in response to the vertical bottom acceleration induced as the waves propagated along the water-bottom interface. This hypothesis was verified for the 1999 Hector Mine, California, earthquake by analyzing seismic data from the nearby SAO seismic station and the travel time for the seismic surface waves to reach a DART system deployed off Monterey Bay, California. These results are consistent with previous studies of BP fluctuations due to seismic waves. They further suggest that the 3 cm HO threshold, applied to 15-second averaged BP data, works well to activate the DART tsunami event mode for earthquake magnitudes 7.0 and epicenter distances 610~km, while ignoring much smaller magnitude earthquakes ($mathrm{M}leq 6$) in the region.