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    Solid earth sciences


    Fault geometry of M6-class normal-faulting earthquakes in the outer trench slope of Japan Trench from ocean bottom seismograph observations

    Obana K, Takahashi T, Yamamoto Y, Iinuma T, Nakamura Y, Fujie G, Miura S, Kodaira S

    Outer trench slope, Japan Trench, Outer-rise normal-faulting earthquake, Pacific Plate, Stress change, Seismicity

    Three M6-class normal-faulting earthquakes and their aftershocks occurred in the outer trench slope of the Japan Trench in 2017. (left) Map of the OBS survey area with hypocenters of the M6-class earthquakes (green stars), their aftershocks within 24 hours (colored circles), and GCMT solutions. White contours indicate coseismic slip of the 2011 Tohoku-oki earthquake. (right) Cross sections and magnitude-time diagrams of three M6-class earthquakes and their aftershocks. The GCMT solutions and focal mechanisms from the OBS first-motion polarities of the mainshocks are indicated on the cross sections. The gray rectangles on the cross sections indicate the area of the earthquakes that occurred within 1 h after the mainshock.

    Since the 2011 Mw 9.0 Tohoku-oki earthquake, intra-plate normal-faulting earthquakes, including several M7-class earthquakes, have occurred in the outer trench slope area from the trench to the outer rise along the Japan Trench. Concerns regarding large earthquakes and associated tsunamis have also arisen. Based on aftershock distributions, several outer trench slope normal-faulting earthquakes (hereinafter referred to as outer-rise earthquakes) are likely related to the rupture of multiple faults. However, few observations have clearly shown how multiple faults act during outer-rise earthquakes. During the ocean bottom seismograph (OBS) observations in the outer trench slope area of the central Japan Trench from September 2017 to July 2018, three M6-class normal-faulting earthquakes (Mw 6.2 on September 20, Mw 6.2 on October 06, and Mw 6.0 on November 12) occurred around the OBS network. The near-field OBS observations provided detailed information on hypocenter locations and focal mechanisms of the mainshocks and aftershocks, including immediately after the mainshocks. We investigated the fault configurations of normal-faulting earthquakes based on OBS observations. During the September 2017 earthquake, the mainshock ruptured high-angle normal faults with a dip angle of 65°. Off-fault aftershock activities that were not directly related to the mainshock rupture and could be explained by the stress changes caused by the mainshock were confirmed. However, hypocenter distributions and focal mechanisms of the main and aftershocks of the October and November 2017 earthquakes suggest that the mainshock ruptured multiple faults with various dipping directions, angles, and strike orientations. The complicated fault geometry should be considered a possible fault model for large outer-rise earthquakes and related tsunamis.