Swarm-like seismic activity in 1989, 1990 and 1997 preceding the 2000 Western Tottori Earthquake

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.