No. 917

Title:

Synchronous seismicity changes in and around the northern Japan preceding the 2003 Tokachi-oki earthquake of M8.0

Author(s):

Ogata, Yosihiko (The Institute of Statistical Mathematics)

Key words:

Coulomb failure stress change; ETAS model; Precursory slip; Relative quiescence and activation.

Abstract:

The Epidemic-Type Aftershock Sequence (ETAS) model predicts the normal seismic rate that superposes the empirical decreasing laws of aftershock activity. Deviations (lowering and enhancement) of the seismic activities from the predicted rate by the ETAS model are revealed to have concurrently occurred during 1996, in three interplate aftershock activities and with an inland seismicity, in and around northern Japan. The respective deviation is explained by the change in Coulomb failure stress, caused by the same aseismic slip that is assumed to have occurred along a plate interface of the 2003 Tokachi rupture of M8.0. The slip is also supported by a significant change in the distribution of fault mechanisms of moderate earthquakes, before and after 1996, in northern Japan. According to the occurrence data of strong earthquakes of M5.0 or larger for the long period 1926-2003, the similar relative quiescence lasting about 10 years is indicated not only preceding the recent 2003 earthquake but also preceding the 1952 and 1968 great Tokachi earthquakes.