In this section, we summarize existing data regarding the seismotectonic state of the northeast Japan arc system, where the Pacific plate is subducting nearly normal to the trench axis at 9-10 cm/yr (Fig. F1). Figure F2 shows recent seismicity in the area. Recent major (M > 7.5) interplate events in this area are the 1968 Tokachi-oki earthquake (Ms = 8.0) (Kanamori, 1971) and 1994 far east off Sanriku earthquake (Ms = 7.5) (Nishimura et al., 1996; Nakayama and Takeo, 1997). The depths of large interplate earthquake rupture zones range from 12 to 55 km (Hasegawa et al., 1994; Suyehiro and Nishizawa, 1994).
Recently, the distribution of microearthquakes in spatial relation to structure in the area has been summarized by Suyehiro and Nishizawa (1994) and Hino et al. (2000). An outstanding feature is the existence of a seismic gap beneath the trench inner slope, the water depth of which is 4-6 km (Hirata et al., 1983, 1985). These microearthquakes occur in clusters, and they have not moved over decades, only changed frequency of activity.
Von Huene et al. (1994) suggested that subducted strata along the plate boundary exists to 12 km depths, thickens beneath the upper and middle slope, and acts as a buffer to control interplate friction. The location of the subducted strata appears to correspond to the low-velocity zone as detected by wide-angle reflection data (Fujie et al., 2002; Takahashi et al., 2000).
For our purpose, it is important to summarize the existing results regarding the transitional change toward the trench axis. The crustal structure of the arc shows a slow Pn velocity of 7.5 km/s and a crustal thickness of 30 km (Yoshii and Asano, 1972). Pn is the upper mantle P-wave (VP) velocity. A more recent model for the Kitakami crustal structure put the Mohorovicic discontinuity (Moho) depth at 32-34 km with Pn velocity >7.7 km/s (Iwasaki et al., 1994). The Moho becomes shallower toward the trench (Zhao et al., 1992). Forearc crustal structures were estimated by many offshore wide-angle refraction surveys (Murauchi and Ludwig, 1980; Nagumo et al., 1980; Suyehiro and Nishizawa, 1994; Takahashi et al., 2000; Ito et al., 2002; Fujie et al., 2002). Suyehiro and Nishizawa (1994) estimated that the subducting angle is 7° at a distance of 110 km from the trench axis and 25° beneath the coastline. Hino et al. (1996) also estimated the subduction angle near the updip limit of the seismogenic zone as ~5°.
Many multichannel seismic (MCS) studies obtained images of the Cretaceous unconformity and subducting strata along the plate boundary (e.g., Nasu et al., 1980; von Huene et al., 1994; Tsuru et al., 2000). Tsuru et al. (2000) imaged deep reflection images of the plate boundary down to ~15 km depth.
Figure F3 summarizes the outstanding features of these structural models. The tip of the mantle wedge has a significantly higher velocity than beneath the arc and terminates at 20 km depth (Hino et al., 2000; Takahashi et al., 2000; Ito et al., 2002). The lower crust, with VP > 6.5 km/s, slows to 6.2-6.6 km/s and terminates at a depth range of 12-20 km. We also plotted a zone of high electrical resistivity (Goto et al., 2001) as shown in Figure F3.