11 October 2000
Headquarters for Earthquake Research Promotion
Earthquake Research Committee

Seismic Activity in Western Tottori Prefecture

*On October 6th at about 1:30 p.m., there was an M7.3 earthquake (tentative data) at a depth of about 10 km in Western Tottori Prefecture, with a maximum seismic intensity of 6 upper. Furthermore, aftershocks of M4.2, at about 4:21 p.m. on October 6th, and M5.0, at 8:51 p.m. on October 8th, were recorded, both of which registered maximum seismic intensities of 5 lower.

There have been a number of earthquakes subsequent to this event, but the incidence is decreasing over time. The epicenters of the events, as of October 7th, are distributed in a 30 km area running NNW-SSE. For this reason, it is believed that the events to present are aftershocks to the initial event. The focal mechanism was a strike slip type with an east-west pressure axis, and, from the distribution of aftershocks, it is believed there was a left lateral strike slip in the NNW-SSE strike of the source fault.

*The aftershock activity shows a trend toward a slower than average abatement. Also, the frequency distribution by magnitude shows a trend toward fewer than the average number of large events. As of 7:00 a.m. on October 10th, the largest aftershock was an M5.0 event at 8:51 p.m. on October 8th. This event occurred in the northern tip of the aftershock region.

However, on October 8th at 1:17 p.m., there was an M5.5 event 25 km WSW of the aftershock zone. It is believed that that event was triggered by the October 6th event.

*The results of recent GPS observation show low levels of strain in the area, but the area does show a trend to compression along an east-west axis, which matches with the focal mechanism of the current event. Additionally, the results of GPS observation show changes associated with the main shock of the current activity.

*There are no known events of M6.0 or greater in western Tottori Prefecture in recent years, but in 1990 in the vicinity of its zone of activity, there was activity including multiple M5 class events distributed in the same direction as the current event.

*It is estimated that there is a 10% probability of an M5.0 or greater aftershock to occur within three days of 12:00 p.m., October 11th. It is also projected that there will be around five M3.0 or greater aftershocks per day until the end of October.

*From the source fault model calculated based on an analysis of seismic and GPS observations, it is conjectured that there was a large displacement in the southern end of the aftershock zone. To date, field investigations of the area near the epicenter show the possibility of a surface earthquake fault, and reveal a number of cracks and deformations. It was conjectured that there is a short, active NW-SE fault near the aftershock zone, but it is not believed to be related to the current earthquakes.