Spatio-temporal Changes in Stress Field and Occurrence of the 2003 Tokachi Oki Earthquake in Hokkaido, Northern Japan

Abstract

From the historical times, many large subduction earthquakes with magnitude 8 or more have been rupturing different segments of the Kurile and Japan trenches in Hokkaido, northern Japan. The most recent recurrent earthquake in this region is the 2003 Tokachi Oki earthquake with magnitude M=8.0 that ruptured about 100km long fault segment (Yamanaka and Kikuchi, 2003) in the Kurile subduction zone. As shown in Fig. 1, in this region the Pacific plate subducts beneath the Okhotsk plate along the Kurile and Japan trenches at a rate of 8-10cm per year (DeMets, 1992). The tectonics in this region is complicated by the collision of Kurile and Japan arcs along the Hidaka Collision Zone, HCZ in Fig. 1 (Seno, 1985; Moriya, 1986; Tsumura et al., 1999) where the subducted Pacific slab exhibits bulging or distortion (Kanamori, 1971, Hashimoto, 1984, Moriya, 1986). Apparent complexity in seismotectonics in Hokkaido region can not be explained by simple models of downdip compression or downdip extension (Isacks and Molnar, 1971, Astiz et al., 1988) based on the relative positions of the Pand T-axes (pressure and tension axes from earthquake focal mechanism solution) with respect to the geometry of subducting slab. The Pand T-axes may not represent the exact direction of the principal stresses because of two reasons: (i) an earthquake might be a result of slip on a preexisting fault plane with frictional strength far below than the intact rock mass, and (ii) even if an earthquake is resulted from the shear failure of intact rock mass, the frictional criteria (Byrlee, 1978; Sibson, 1994) would lead to a deviation up to ±20o of the principal stress directions from the Pand T-axes (McKenzie, 1969). Christova and Tsapanos (2000) based on inversion of focal mechanism data determined the stress conditions at different depths in Hokkaido region. Ghimire and Kasahara (2009) identified many seismotectonic zones with different stress conditions in this region. Ghimire et al. (2005) explored the temporal variation in this region after the occurrence of the 2003 Tokachi Oki earthquake. In this paper, we first review the distribution of stress in Hokkaido region both spatially and temporally and then discuss the implications of the stress conditions on ongoing seismic activities.