Setsuro Nakao

The 2000 Western Tottori Earthquake—Seismic activity revealed by the regional seismic networks—

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.

Study of Focal Mechanisms in Eastern and Central Areas of Tottori Prefecture

The tectonic stress, which was found from focal mechanisms of earthquakes in the eastern and the central area of Tottori prefecture, were investigated. Focal mechanisms in this area were the strike slip type of WNW-ESE compression, which is nearly agreed with the mechanism of the 1943 Tottori earthquake. Tectonic stress in this area has conserved the same pattern as the Tottori earthquake, 1943. But the seismic activity near the Yoshioka-Shikano faultsystem shows lower than the other regions. Though their focal mechanisms show a large irregularity, the averaged P-axis indicates NW-SE compression compatible with the strike of the Yoshioka-Shikano faultsystem. This may be attributed to the situation that this area agrees approximately with the bright spots of the 1943 Tottori earthquake.