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A large proportion of the tsunami warnings issued by the Pacific Tsunami Warning Center (PTWC) were never realized (Bernard 1998). For tsunami warning, the earthquake magnitude and location alone are clearly not enough, especially for so-called tsunami earthquakes (Kanamori 1972) that produce tsunamis larger than might be expected from the high-frequency content of the seismic signal of the earthquake. Additional information on the earthquake source and seismic signals may help to reveal the tsunamigenic nature of such events. The mechanism of tsunami earthquakes was explained by Kanamori in 1972. He studied 5–100-s period seismic waves of two earthquakes (Aleutian 1946, Mw 8.3 and Sanriku 1896, Mw 8.4) and reported that observed seismic amplitude spectra could be better fitted with dislocation functions having a longer source time constant for tsunami earthquakes. Brune and Engen (1969) also pointed out that the effective moment evaluated at long-period seismic signals ( i.e. , 100 s) was significantly larger compared with that determined at 20 s for the Aleutian 1946 earthquake, which caused a Pacific-wide tsunami. But broadband long-period seismograms became available only in the 1980s, so there were some uncertainties in the magnitude values used (which may have been measured from seismograms of lower quality than what is available today) in those studies. However, the 1992 Nicaraguan earthquake, which was measured with sensitive broadband instruments, offered clearer evidence that tsunami earthquakes stem from a “slow-slip” motion, which is enriched in long-period seismic signals (Kanamori and Kikuchi 1993). Based on 20-s short-period seismic signals, the 1992 Nicaraguan earthquake was evaluated to have a magnitude Ms 7.0. However, by using long-period waves (∼250 s), the magnitude was evaluated at Mw 7.6. Kanamori and Kikuchi (1993) reported that the slow-slip associated with this earthquake might have masked its true size. They thus suggested that … |
