[S-SS14] 活断層と古地震

[SSS14-10] Geological evidence for repeated slip-to-the-trench style megathrust earthquakes at the Japan Trench

★Invited Papers

*Charlotte Olivia Pizer¹、Ken Ikehara²、Myra Keep³、Arata Kioka⁴、Shuichi Kodaira⁵、Ryo Miura⁵、Jasper Moernaut¹、Yasuyuki Nakamura⁵、Michael Strasser¹ (1.Institute of Geology, University of Innsbruck, Austria、2.National Institute of Advanced Industrial Science and Technology, Japan、3.School of Earth Sciences, University of Western Australia, Australia、4.School of Engineering, University of Tokyo, Japan、5.Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Japan)

キーワード：Sub-bottom profile data, Coseismic deformation, Megathrust earthquake, IODP Expedition 386, High resolution multi-channel seismic data, Imbricate thrust wedge formation

Deformation of trench-fill sediments at the central Japan Trench axis confirms that coseismic slip during the 2011 CE Mw 9.1 Tōhoku-oki earthquake extended to the shallowest part of the megathrust fault, contributing to the unexpectedly large tsunami that followed. Understanding the recurrence of slip-to-the-trench style earthquakes is therefore essential for diagnosing future hazard at the Japan Trench (and other subduction zones). Thermal biomarkers from the décollement indicate similar shallow slip has occurred repeatedly, but the timing has not yet been linked to specific past earthquakes. We examine the sedimentary sequence of a trench-fill basin at 38.75°N (just north of the Tōhoku-oki slip zone) to investigate archives of past deformation caused by slip to the trench. Reprocessed seismic reflection and subbottom profiler data image several stratigraphic intervals of imbricate thrust wedge formation and paleo-seafloor uplift consistent with compression induced by locally enhanced coseismic slip along the décollement. The uplifted paleo-seafloor topography is onlapped by thick seismoturbidites that have been cored and dated by International Ocean Discovery Program Expedition 386, thus providing chronostratigraphic tie points. With this, we link the youngest coseismic deformation of trench-fill sediments to the 869 CE Jogan earthquake, indicating rupture extended further north and closer to the trench than previously estimated. Documenting slip to the trench for this historical megathrust event is proof of concept for our core-to-seismic correlation approach to constrain shallow slip in past earthquakes. Hence, we infer the several deeper intervals of imbricate thrust faulting and turbidites contain the means to unlock an extensive history of slip-to-the-trench style earthquakes and quantify the recurrence of shallow, tsunamigenic slip at the Japan Trench.