244-3 Extreme plate boundary localization promotes shallow earthquake slip at the Japan Trench

Session: Subduction Zone Processes: Insights from Geology, Geochemistry, and Petrochronology

Presenting Author:

James Kirkpatrick

Authors:

Kirkpatrick, James¹, Savage, Heather², Regalla, Christine³, Shreedharan, Srisharan⁴, Ross, Catherine⁵, Okuda, Hanaya⁶, Nicholson, Uisdean⁷, Ujiie, Kohtaro⁸, Hackney, Ron⁹, Conin, Marianne¹⁰, Satolli, Sara¹¹, Zhang, Junli¹², Fulton, Patrick¹³, Ikari, Matt¹⁴, Kodaira, Shuichi¹⁵

(1) Department of Geological Sciences and Engineering, University of Nevada Reno, Reno, NV, USA, (2) Earth and Planetary Sciences, University of California, Santa Cruz, Santa Cruz, CA, USA, (3) School of Earth and Sustainability, Northern Arizona University, Flagstaff, AZ, USA, (4) Geosciences, Utah State University, Logan, UT, USA, (5) Geosciences, Baylor University, Waco, TX, USA, (6) Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Kochi, Japan, (7) Institute of GeoEnergy Engineering, Heriot-Watt University, Edinburgh, United Kingdom, (8) Graduate School of Science and Technology, University of Tsukuba, Tsukuba, Japan, (9) Research School of Earth Sciences, The Australian National University, Canberra, Australia, (10) GeoRessources laboratory, University of Lorraine, Nancy, France, (11) Department of Engineering and Geology, University of Chieti-Pescara, Chieti, Italy, (12) MARUM—Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany, (13) Department of Earth & Atmospheric Sciences, Cornell University, Ithaca, NY, USA, (14) MARUM—Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany, (15) Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan, (16) MarE3, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan, (17) MarE3, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan, (18) MarE3, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan, (19) MarE3, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Japan, (20) International Ocean Discovery Program, Lamont-Doherty Earth Observatory, Palisades, NY, USA,

Abstract:

International Ocean Discovery Program (IODP) Expedition 405 aims to investigate the conditions and processes that facilitated the extremely large shallow slip on the subduction interface in the 2011 Mw9.1 Tohoku-oki earthquake. In late 2024, Expedition 405 drilled multiple boreholes at two sites: Site C0026 ~8 km seaward of the Japan Trench to characterize the input sediments to the subduction zone, and Site C0019 ~6 km landward of the trench where the plate boundary fault zone is present at a depth of ~825 mbsf. The input section consists of Quaternary to Early Miocene siliceous, vitric muds that overly a ~30 m-thick Early Miocene to Cretaceous pelagic clay, which conformably overlies Mesozoic color-banded clay and chert. Notable changes to the rock physical properties occur at the stratigraphic contacts at the top and base of the pelagic clay, where measurements of sediment density and seismic velocity show pronounced stiffness contrasts. Cores and logging-while-drilling (LWD) data from Expedition 405, combined with those from the previous IODP Expedition 343 (JFAST), show that the megathrust is defined by a ~1 – 15 m-thick fault zone at the base of the frontal prism. Within the fault, a ~1 m-thick layer of clay-rich fault rock intercalated with short sections of mudstone is present and textural evidence in the cores also shows that slip was further localized onto several ~mm-thick, clay-rich slip zones within this clay-rich fault rock. Comparison with the input section shows that the megathrust is localized at either the transition from siliceous mud to pelagic clay or at the lower stratigraphic contact at the transition from pelagic clay to color banded clay and chert, suggesting the stratigraphy of the input section at the Japan Trench exerts a first-order control on the megathrust structure. Localization imposed by the input section results in a narrow, weak fault located at a major mechanical contact between frontal prism mud and subducted clay. This architecture enhances the tendency for shallow seismic slip, suggesting the Japan Trench may be more susceptible to ruptures with large shallow slip than margins without weak clays.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7832

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