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Earthquake faulting in subduction zones

Plate boundary subduction thrusts and out-of-sequence thrusts exhumed from seismogenic depths are expected to preserve information on subduction earthquakes. The Shimanto and Mino accretionary complexes have been considered on-land analogs of the Nankai Trough and Japan Trench, respectively. We examine earthquake faulting and the underlying physicochemical processes recorded in fault rocks (e.g., pseudotachylytes and ultracataclasites) in the on-land accretionary complexes. We also estimate coseismic shear strength, slip-weakening distance, and fracture energy through fault rock analyses and laboratory experiments for better understanding of fault behavior during subduction earthquakes.

Accretionary processes

Accretionary prisms develop through off-scraping, underplating, and out-of-sequence thrusting. We are investigating these processes in the Shimanto accretionary complex and the Nankai Trough in terms of structural geology and soil mechanics. In particular, we focus on the role of fluid on deformation in accretionary prisms. We also examine the strain histories of accreted and subducted sediments by measuring the anisotropy of magnetic susceptibility.

Geology of slow earthquakes

Crustal movements and seismic observation networks have revealed that slow earthquakes occur in subduction zones. However, materials responsible for slow earthquakes and the mechanism of low-speed slip have not been identified. We are attempting to clarify the geological perspectives of slow earthquakes based on geological surveys of accretionary prisms and metamorphic rocks exhumed from the source depths of shallow and deep slow earthquakes, respectively. This study is being conducted as part of a project called “Science of Slow Earthquakes,” funded by the Japan Society for the Promotion of Science.

Drilling into seismogenic zones

We are participating in seismogenic zone drilling projects: the Japan Trench Fast Drilling Project (JFAST) and Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). The deep-sea drilling vessel Chikyu successfully drilled into the source area of the 2011 Tohoku-Oki earthquake and plans are being made to drill through the upper portion of the seismogenic zone in the Nankai Trough. We are investigating the mechanics and mechanisms of great earthquakes in subduction zones based on observations and analysis of recovered cores and high-velocity friction experiments on fault-zone materials.