Source depth dependence of micro-tsunamis recorded with ocean-bottom pressure gauges:: the January 28, 2000 Mw 6.8 earthquake off Nemuro Peninsula, Japan

Micro-tsunami waves with a maximum amplitude of 4-6 turn were detected with the ocean-bottom pressure gauges on a cabled deep seafloor observatory south of Hokkaido, Japan, following the January 28, 2000 earthquake (M-w 6.8) in the southern Kuril subduction zone. We model the observed micro-tsunami and estimate the focal depth and other source parameters such as fault length and amount of slip using grid searching with the least-squares method. The source depth and. stress drop for the January 2000 earthquake are estimated to be 50 km and 7 MPa, respectively, with possible ranges of 45-55 km and 4-13 MPa.. Focal depth of typical inter-plate earthquakes in this region ranges from 10 to 20 km and stress drop of inter-plate earthquakes generally is around 3 MPa. The source depth and stress drop estimates suggest that the earthquake was an intra-slab event in the subducting Pacific plate, rather than an inter-plate event. In addition, for a prescribed fault width of 30 kin, the fault length is estimated to be 15 kin, with possible ranges of 10-20 km, which is the same as the previously determined aftershock distribution. The corresponding estimate for seismic moment is 2.7 X 10(19) Nm with possible ranges of 23 X 10(19)-3.2 X 10(19) Nm. Standard tide gauges along the nearby coast did not record any tsunami signal. High-precision ocean-bottom pressure measurements offshore thus make it possible to determine fault parameters of moderate-sized earthquakes in subduction zones using open-ocean tsunami waveforms. Published by Elsevier Science B.V.