OCEAN-BOTTOM PRESSURE VARIATIONS DURING THE 2003 TOKACHI-OKI EARTHQUAKE

During the 2003 Tokachi-Oki earthquake. pressure variations were registered by the ocean-bottom observatory located near the epicenter. The acoustic (pressure) waves bouncing up and down between the hard bottom and the sea surface were generated by the co-seismic seafloor displacement. The numerical simulations using the method of characteristics show that the dominant frequency of the simulated acoustic wave is in good agreement with that of the measurement if the soft sediment layers are taken into account. The quantitative analysis suggests that the co-seismic seafloor uplift took place in the duration of 6.8 similar to 7.9 seconds. The amplitude modulation of acoustic waves was found to be regular, which indicates that the neighboring geologic factors control its characteristics. Acoustic-wave attenuations for the main shock and the following aftershocks are consistent, except for one of the aftershocks. Hence, that particular aftershock must have occurred away front the location of the pressure gage.