Analysis of pressure variations observed at the ocean bottom during the 2003 Tokachi-Oki earthquake

Graduation date: 2007Most tsunami studies have been conducted based on the incompressible fluid theory. However water compressibility must be considered for the process of tsunami generation by bottom displacements during submarine earthquakes. During the 2003 Tokachi-Oki earthquake, the ocean-bottom pressure variations were registered by the pressure gauges PG1 and PG2 located right in the tsunami source area. The co-seismic ocean-bottom displacements were estimated by the observed ocean-bottom pressure variations. The pressure waves (acoustic waves) traveling back and forth between the hard bottom and the water surface were generated due to the ocean bottom displacements during the\ud earthquake. In this study, the e-folding time for the attenuation of pressure wave was estimated for the mainshock and the following three aftershocks. The amplitude modulation of the pressure fluctuations at PG1 caused by the mainshock was investigated by applying the variable-frequency complex demodulation method (VFCDM). The temporal evolution of amplitude spectra indicates that the dominant frequency of the pressure wave decreases gradually. The numerical simulation of the pressure wave is conducted by using a one-dimensional model with the method of characteristics and the finite-difference scheme. The numerical model is base on the linear momentum equation and the conservation of mass. Results show that the dominant frequency of the simulated pressure wave is in good coincidence with that of the measurement if sediment layers are taken into account. This strongly suggests that the sediment layers must play a role in the formation of the pressure wave caused by the earthquake