Rupture processes of the 2004 Chuetsu (mid‐Niigata prefecture) earthquake, Japan: A series of events in a complex fault system

We relocated the hypocenters of the 2004 Chuetsu earthquake sequence, Niigata, Japan, using the double‐difference 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. ΔCFF 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.