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

We invite contributions that explore the geologic processes of subduction zones from new field observations, analytical methodologies, and numerical modeling of metamorphic rocks along the subduction interface or try to integrate or reconcile the rocks record with geophysical observations.

Wednesday, 22 October 2025
8:00 AM - 12:00 PM
HBGCC, 217C

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

Paper number: 244-3
Program: Technical Sessions
Time: 8:45 AM – 9:00 AM

Authors

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 Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-412890

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