Fault geometry of the 2004 off the Kii peninsula earthquake inferred from offshore pressure waveforms

Abstract

We could successfully obtain water pressure fluctuations after a series of two off the Kii peninsula earthquakes that took place on September 5 in 2004, using two pressure gauges deployed at deep ocean-bottom off Muroto. The earthquakes caused two tsunamis in the southern coast of the southwestern Japan with maximum wave heights of 0.9 m, and we could identify the tsunami signals from the acquired dataset ca. 20 min before the arrivals of the tsunamis to the coastline nearest to the sensors. Two earthquakes are assumed as one for a fore-shock and the other for a main-shock. Although seismological studies, without aftershock distributions, showed a lack of constraints to distinguish if the main-shock was caused by either southwest dipping fault or by the other, i.e., northeast dipping fault, we found that the difference in the pressure fluctuations for the tsunamis could attribute to the determination of the fault plane in the fault models. In this paper, we evaluate the fault geometry of the main shock by using tsunami waveforms obtained by those unique instruments in following procedure: (1) We first simulate pressure waveforms caused by earthquakes of the two fault geometries, one for southwest dipping place and the other for the northeast, at the location of the pressure gauges, and (2) we then compare the simulated results with observed waveforms. Our results demonstrate that the main-shock should be caused by a fault whose strike lies in northwest-to-southeast with a dip towards southwest direction.