The September 25, 2003 Tokachi-Oki Mw 8.3 Earthquake: Rupture Process From Joint Inversion of Tsunami Waveform, GPS, and Pressure Gages Data

We infer the slip distribution along the rupture zone of the September 25, 2003 Hokkaido Region (Japan) from tide-gages records of the tsunami, pressure gages, and GPS measured static coseismic displacements. According to USGS, this one has been the largest earthquake in 2003. We select waveforms from 16 stations, distributed along the east coast of the Hokkaido Region and the north-east coast of the Tohoku Region. Furthermore we select more than 100 GPS stations positioned on these regions and 2 high-precision pressure gages positioned in open sea near the epicenter; indeed the seafloor measurement of the water pressure is an innovative geodetic observation because the displacement of the seafloor is directly proportional to water pressure increase. We assume the fault plane to be consistent with the geometry of the subducting plate and the slip direction with the focal mechanism solutions and previous inversions of teleseismic body waves. We subdivide the fault plane into several subfaults (both along strike and down dip) and we compute the corresponding Green's function for the coseismic displacement considering a 3D Earth's model implemented in a Finite-Element code. As for the tsunami Green's function we use the shallow water equations and a bathymetric dataset with 10 arcsec of spatial resolution. The slip distribution is determined by means of a simulated annealing technique. Synthetic checkerboard tests, using the station coverage of the available data, indicate that the main features of the rupture process may be robustly inverted with a minimum subfault area of 30x30 km. We compare our results with those obtained by previous inversions of teleseismic, GPS and tsunami data.