The strong motion and geodetic data were individually inverted for the source process of the 2003 Tokachi-oki, Hokkaido, earthquake with a hypocenter 25 km deep and a fault plane above the subducting Pacific slab. Both the results show a simple slip distribution with a single major asperity, but the strong motion inversion may include a trade-off between slip location and rupture time and the geodetic inversion does not have sufficient resolution for far slips. We then carred out a joint inversion of the two datasets in order to overcome these weaknesses of the single dataset inversions. The resultant slip distribution still retains the simple pattern and has a seismic moment of 2.2 × 10 21 N·m (M w 8.2). The asperity, with a peak slip of 7.1 m, is located in the center of the fault plane 50 km away from the hypocenter in the down-dip direction. The slip rate functions on subfaults around the hypocenter and asperity indicate that the rupture propagated with a supershear speed on the upper part of the fault plane and slowed down to 100-90% of the S-wave velocity on the middle and lower parts. These simple slip patterns and near-supershear rupture may imply the maturity of the Hokkaido subduction zone around the source region.

[1] High-rate GPS has the potential to recover both dynamic and static displacements accurately. We analyze 1-Hz GPS data recorded during the 2003 Tokachi-Oki earthquake. The 1-Hz GPS displacement waveforms show good agreement with integrated accelerometer records except for low frequency noise that are inherently present in integrated seismic records. The GPS waveforms were inverted to model the spatio-temporal evolution of the fault slip during the rupture. The slip is found to propagate downdip in the subduction zone with largest moment release $50 km northwest of the hypocenter. The region of largest slip agrees in general with traditional seismic studies, indicating that 1-Hz GPS can be used for finite fault studies. The 1-Hz GPS slip model shows clearer contrast with afterslip distributions than those inferred from strong motion data, possibly because 1-Hz GPS is more sensitive to cumulative slip distribution.

[1] We relocated the hypocenters of the 2004 Chuetsu earthquake sequence, Niigata, Japan, using the doubledifference method. The distribution of aftershocks reveals a complex fault system consisting of five different fault planes. Inversions of strong motion records for the five major events within the sequence indicate that the mainshock (M W 6.6) and the largest aftershock (M W 6.3) occurred on parallel WNW-dipping fault planes. The zones of large slip (asperities) of these two events are located near the hypocenters. In contrast, the three other large earthquakes (M W 5.9, 5.7 and 5.9) occurred on east-dipping fault planes oriented perpendicular to the fault plane of the mainshock. We used slip distributions deduced from waveform inversions to estimate the Coulomb failure stress changes that occurred immediately prior to each event. DCFF values were positive at the hypocenters and within the asperities of the major aftershocks. These results suggest that stress changes resulting from individual earthquakes led directly to the occurrence of subsequent earthquakes within the complex fault system. Citation: Hikima, K., and K. Koketsu (2005), Rupture processes of the 2004 Chuetsu (mid-Niigata prefecture) earthquake, Japan: A series of events in a complex fault system, Geophys.