JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. B 1l, PAGES 22,331-22,347, NOVEMBER 10, 1994 Crustal structure and seismicity beneath the forearc off northeastern Japan Kiyoshi Suyehiro Ocean Research Institute, University of Tokyo, Tokyo, lapan Azusa Nishizawa Hydrographic Department, MaritimeSafety Agency of Japan, Tokyo, lapan Abstract. Marine seismological and otherdatain theJapan Trencharea (northof 38.5øN) were used to infer therelation of subducting plate structure, seismicity, and focal mechanisms especially at theaseismic to seismogenic zone along theinterface between thetwo interacting plates.Previous crustal models fromocean bottom seismographic refraction (OBS) surveys were improved by taking intoaccount therelative seismic amplitude characteristics and constraining the shallow structure using multichannel seismic data. A wave speed discontinuity in these models is interpreted to be the contact zoneof thecrust of the overriding plateandthe subducting Pacificplatecrust. Its dip angle increases to about 7ø, 110 km landward of the trench axis.A large increase beneath thedeep sea terrace is required to reach thewell-defined angle of 25 øbeneath theTohoku east coast. A largewavespeed gradient within layer2, commonly observed under normal oceans, seems to vanish beneath the innertrench slope at about 10-kmdepth. Within theoverriding plate,apparently brittle material with a P wave speed of -6 km/s canbe foundas nearas 35 km from the trench axis. The upperlimit of the seismogenic zone of interplate low-angle thrust events is about 15-kmdepth fromOBS seismicity andlarge-event analyses. Boththestrength of thecrust of theoverriding plate andthe characteristics of subducting sediments must be investigated to define the seismic coupling of interacting plates. Introduction Large interplateearthquakes occur east off the northeastern Japan arc. These events have been studied to delineate the mechanical interaction between the Pacific and the overriding Eurasian plate [Kanamori, 1977; Yoshii, 1979; Chen et al., 1982; Kawakatsu and Seno, 1983; Seno and Kroeger, 1983; Jarrard, 1986; Byrne et al., 1988; Tichelaar and Ruff, 1993; Pacheko et al., 1993]. Historically, great thrustearthquakes have recurred north of 40øN every •-80-100 years at depths less than 40 km. But southward between 38ø-40øN, great events seem to occur more irregularly and less frequently. However, M7 class thrust eventsat relatively larger (30-60 km) depthsdo recur every •-30 years between 38 ø and 40øN [e.g., Kawakatsu and Seno, 1983] . Both changes in seismicity and focal mechanisms take place within the shallow (<50 km depth) part of the subducting Pacific plate [e.g., Yoshii, 1979; Chen et al., 1982; Seno and Kroeger, 1983] (Figure 1). Earthquakes with normal fault focal mechanisms occuroceanward of the Japan Trench axis within the Pacific plate, and interplate thrust types occur on the landward side [Yoshii, 1979; Seno and Gonzalez, 1987]. Large thrust-type eventsare known to occur to about 200 km landward of the trench axis, where the intra- Copyright 1994 bytheAmerican Geophysical Union. Paper number 94JB01337. 0148-0227/94/94JB-01337505.00 Pacific plate double seismic zone is observed[Kawakatsu and Seno, 1983; Seno and Kroeger, 1983]. Defining an interplate seismogenic zone helps to determine how much plate motion is accommodated by seismicslips and understand how that zone is formed [Byrne et al., 1988; Pacheko et al., 1993; Ruff, 1989; Tichelaar and Ruff, 1993; Wang, 1980]. Byrne et al. [1988] suggested that the "seismic front" (Figure 1) defined as the shallowestlimit of the interplatethrustearthquakes, coincides with a boundary within the overriding plate acrosswhich seismicity increases landward and material changes from weak accretedsediments to hard rocks.Ruff [1989] suggested that sediment subduction provides a larger contact area for interplate thrust faulting. Wang [1980] used lab data to infer that both stable and unstable slip can occur along a plate boundary, depending on the mechanical properties of different sediments. In the northeast Japan subduction zone the accretionary prism is barely developed: most sediments subduct [ Murauchi and Ludwt•g, 1980]. The upper limit of the seismogenic zone has not previously been defined so that its control by the overriding material and subductingsedimentshas not been determined. Detecting such details beneath the trench and forearc is not possiblewithout high-resolution seismicityand seismic structure mapping. Marine seismic surveys provide accurate and direct methods of defining the geometry and structure of the subducting plate near the trench. In the vicinity of the northern Japan Trench, seismic reflection, two-ship refraction, and ocean bottom seismograph (OBS) refraction 22,331