Hiroyuki Takayama

3-D Simulation of the Occurrence of Slow Slip Events in the Tokai Region with a Rate- and State-Dependent Friction Law

A slow slip event has been progressing on the plate interface in the area near Lake Hamana, western boundary of the estimated focal region of the Tokai earthquake, since October 2000 (Ozawa et al. , 2002). Occurrence of the slow slip was revealed by the gps data of the Geographical Survey Institute. The aim of this article is to reproduce the slow slip event in a simulation of plate subduction with a rate- and state-dependent friction law in which 3D configuration of plate interface is taken into consideration. By assigning a zone of large L value within the seismic region where a − b is negative, we succeeded in producing slow slip events. Here, the a − b value expresses the rate dependence of steady-state frictional strength, and the L value represents the characteristic slip distance over which the state evolves. They were found to happen five times during one cycle of interplate earthquakes. Fifth slow slip event finally brings about the catastrophic rupture. The moment release rate of the preslip is 10 to 100 times as large as that of the preceding slow slip events. On average a seismic moment equivalent to M w 6.7 to 7.0 is released by slow slip events each year, while the moment magnitude of the earthquake is M w 8.3. Duration of slow slip events is 13 to 15 years except for the last one that starts several years before the earthquake. The temporal change of volumetric strain produced by the preslip is significantly different from that of the preceding slow slip events. That is, the moment release rate accelerates in the preslip, while it is almost constant in the preceding slow slip events. In the present model, slow slips occur in rather large areas. This may be caused by an assignment of a large L value in the seismic region. Both spatial inhomogeneity in L and curvature of the plate interface are considered indispensable to create appropriate inhomogeneous stress fields that generate slow slip events and, then, eventually a preslip and a great earthquake.

Spatiotemporal patterns of clustered and non-clustered seismic activity in the Shin-etsu and Tokai regions, central Japan

We investigated spatiotemporal patterns of seismicity in the Shin-etsu region and Tokai region, central Japan, by separating the seismicity into clustered and non-clustered activity. We found that clustered activity occurred at various sites during short periods in each investigated region and that an alternation of active and quiet periods was clear for the clustered activity. In contrast, non-clustered earthquakes occur at an almost constant rate, and active or quiet periods can not be distinguished in the non-clustered activity. Some evidence suggests that an increase of clustered activity indicates a buildup of the regional stress field.

Gravity Observation by Means of a Superconducting Gravimeter at Matsushiro, Japan

Gravity observation by means of the superconducting gravimeter #11 has been started at Matsushiro Seismological Observatory, Japan Meteorological Agency. This gravimeter is the one that used to be located at Kakioka, Japan. The new gravimeter site is inside a tunnel where the environmental conditions are very stable. The gravimeter is bottom mounted to eliminate the disturbances from local ground motions. Absolute gravity measurements were carried out to calibrate the scale factor of the superconducting gravimeter. Preliminary analysis of the gravity data shows that the noise level of the gravimeter at seismic bands has been improved with its move to Matsushiro.