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On October 6, 2000, the 2000 Western Tottori Earthquake (Mjma 7.3) occurred in the western Tottori prefecture area, southwestern Japan. It initiated at a depth of 12 km at the bottom of the seismogenic zone, which was derived from aftershock distribution. The aftershocks extend over a 35 km length in a north-northwest direction. Spatial and temporal distribution of the aftershocks exhibits local characteristics in the fault region. The northern part consists of earthquake clusters while the southern part consists of a rather simple lineament of aftershocks, and the spreading and decaying rate of the aftershocks is slower in the northern part. This contrast is possibly due to the heterogeneity of the fault system and probably affected the rupture process of the mainshock. Two swarm sequences occurred in the surrounding region after the mainshock. One initiated 48 hours after the mainshock 25 km southwest of the main aftershock distribution. The other started 20 hours after the mainshock northeast of the mainshock on the southeast flank of Daisen volcano. These activities are probably induced seismicity due to stress changes in the focal region. Pre-seismic swarm activities occurred in the focal region from 1989 and deep low-frequency earthquakes were observed since 1999. It is important to understand the relationship between these possible precursory phenomena and the occurrence of the mainshock.

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.