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    Solid earth sciences


    Incoming plate structure at the Japan Trench subduction zone revealed in densely spaced reflection seismic profiles

    Yasuyuki Nakamura, Shuichi Kodaira, Gou Fujie, Mikiya Yamashita, Koichiro Obana, Seiichi Miura

    Japan Trench, Incoming structure, Sediment thickness, Normal faults, Subduction zone, earthquake

    Fig.1 (a) Map of the study area and seismic lines. The red star denotes the epicenter of the 2011 Tohoku earthquake. Green crosses and open circles indicate tremors and very low-frequency earthquakes, respectively (from Nishikawa et al. 2019). The green shaded rectangle denotes the slow slip event in 2011 (Ito et al. 2013). Gray thick dashed lines show the possible pseudofaults (Nakanishi 2011). Black contours show the bathymetry (interval, 1000 m). Abbreviation: smt, seamount.

    (b) Input sediment thickness. The vertical thickness of sediment above the chert and oceanic basement was estimated based on the interpretation of each seismic profile (color scale). The background map is a gray-shaded bathymetric map with contours every 200 m. The thick black contour line denotes a water depth of 7000 m.

    Thin yellow lines (a) and thin black lines (b) indicate seismic lines. Seismic profiles along red lines (a) and thick black lines (b) are exhibited in the paper. White (a) and magenta (b) contours show the slip distribution during the Tohoku earthquake (from Iinuma et al. 2012): Dashed, thin solid, and bold solid lines denote 10 m, 30 m, and 50 m slips, respectively. The magenta shaded rectangles (a) and yellow thick dased rectangles (b) indicate the area where chert layers are apparently missing (Fujie et al. 2020).

    The structure of the incoming plate is an important element that is often considered to be related to the occurrence of great earthquakes in subduction zones. In the Japan Trench, where the 2011 Tohoku earthquake occurred, we collected seismic profiles along survey lines separated by 2–8 km to examine the structural characteristics of the incoming Pacific plate in detail. The average thickness of the incoming sediments was < 500 m along most of the Japan Trench, and it was < 300 m at ~ 38° N, where the large shallow megathrust slip occurred during the 2011 Tohoku earthquake. We mapped bending-related normal faults, including their dip direction and amount of throw. The numbers of eastward (oceanward) and westward (trenchward) dipping normal faults were generally comparable in the Japan Trench. Eastward dipping normal faults were dominant in the northern and southern parts of the Japan Trench, whereas westward dipping normal faults were more numerous in the central part. Graben-fill sediments deposited at the landward edge of the graben were bounded by eastward dipping normal faults. Trench-fill sediments were also observed along the trench axis. The sediment fills locally increased the thickness of the input sediments where they were deposited. The along-axis variation in input sediment and sediment fill distribution, and the variations in normal fault dip direction between the central and other parts of the Japan Trench may correspond to different slip styles along the plate boundary.