The 2004 Mid Niigata Prefecture Earthquake (Mj = 6.8) occurred on 23 October 2004 in the northeastern part of the Niigata-Kobe Tectonic Zone where large contraction rates were observed. The mainshock was followed by an anomalously intense aftershock activity that included nine Mj ≥ 5.5 aftershocks. We deployed three temporary online seismic stations in the aftershock area from 27 October, combined data from the temporary stations with those from permanent stations located around the aftershock area, and determined the hypocenters of the mainshock and aftershocks with a joint hypocenter determination (JHD) technique. The resulting aftershock distribution showed that major events such as the mainshock, the largest aftershock (Mj = 6.5), the aftershock on 27 October (Mj = 6.1), etc. occurred on different fault planes that were located nearly parallel or perpendicular to each other. This might be due to heterogeneous structure in the source region. The strain energy was considered to have been enough accumulated on the individual fault planes. These features are probably a cause of the anomalous intensity of the aftershock activity.

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Swarm-like seismic activity including six moderate events (Mj = 5.1-5.4) occurred in 1989, 1990 and 1997 in the same area as the 2000 Western Tottori Earthquake (Mj = 7.3). For each time period, we carried out temporary seismic observations in and around the source area and processed the data together with data from permanent stations, to determine the hypocenters precisely. In this study we also redetermined the earthquake locations in each seismic activity using a two-step master event technique with common master events, so that the accuracy in the relative locations of the events was improved. The purpose of this study is to clarify the relationship between the preceding seismic activity and the mainshock in 2000 by comparing the hypocenter distributions. The relocated hypocenter distributions show that the three preceding swarms occurred in different parts of the same fault plane as the 2000 Western Tottori Earthquake. The b-values of the preceding swarms were low (0.51-0.67), suggesting a high stress level in the area. The mainshock initiated in the area of the preceding swarms. The rupture propagated with relatively small slip (∼1 m) in the area for the first three seconds. Then, it developed to main rupture with large slip (2-4 m) outside the area toward the southeast.

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