Some Tsunami Characteristics Deducible from Tide Records

Abstract

After an initial 40-hour ?diffusion? period, the time decay of tsunami variance was found to be uniformly exponents for 28 events among eight Pacific tide stations. Thew results essentially confirm Munk's acoustic deny hypothesis except that the decay (e-folding) time (22 h) more nearly equated the ocean's normal mode transit time (21 h) than Munk's mean collision time (14 h). Relative tsunami energy, obtained by extrapolating exponential decay back to t = 0, was found to be similarly ranked among tsunamis at all stations and roughly proportional to seismic energy. A total energy estimate for the tsunami of 9 March 1957, obtained by correcting the observed energy decay at Wake Island for instrument and island response, provided normalized total energy estimates for all tsunamis that were consistent with independent estimates by Kajiura and by Aida. Similarities between power spectra computed from these time series suggests that station response is primarily linear normal-mode forcing of the continental shelves by the isotropic tsunami spectrum in deep water, rather than resonant excitation of trapped edge waves an the shelves.