Photos and sketches of samples from the study.
(a) Photo and sketch of a fault zone in core sample 316-C0007D-29R. The yellow boxes show where the close-up thin-section images in (b) and (c) were taken. The main shear zone (MSZ) contains very fine, crushed rock (“fault gouge”), and a damaged area lies just above it. The MSZ shows evidence of having slipped during past earthquakes. Small features called Riedel shears and P-foliations match structures seen in laboratory experiments. Credit: Faulkner et al. [2025], Figure 12
Editors’ Highlights are summaries of recent papers by AGU’s journal editors.
Source: Journal of Geophysical Research: Solid Earth

The Nankai subduction zone in southwest Japan has produced multiple M8+ earthquakes over the past 300 years, including the 1707 M8.7 Hōei earthquake, the 1944 M8.1 Tōnankai earthquake, and the 1946 M8.1 Nankaidō earthquake. As one of the most extensively studied subduction zones in the world, it has been the focus of numerous Integrated Ocean Drilling Program (IODP) expeditions aimed at improving our understanding of its seismogenic and tsunamigenic behavior.

Faulkner et al. [2025] compile all available laboratory friction data from Nankai Trough scientific drilling samples and integrate them with routine IODP mineralogical analyses. The dataset spans three transects—Kumano, Muroto, and Ashizuri—and includes material from 26 drilling sites. The experiments cover a wide range of slip velocities, from micrometers per second to meters per second, allowing systematic inversion of key frictional parameters.

This compilation shows that the frictional strength of these materials is generally lower than typical Byerlee friction and decreases with increasing clay content. However, the tendency for materials to weaken at higher slip rates—a key condition for earthquake nucleation—does not clearly correlate with clay abundance. Frictional stability analyses indicate a broad spectrum of possible fault-slip behaviors, from slow slip to earthquake-like failure, consistent with observations in nature. Overall, the findings highlight significant natural heterogeneity in frictional properties within a subduction environment and provide new constraints on the frictional characteristics of the shallow Nankai margin.

Citation: Faulkner, D. R., Zhang, J., Okuda, H., Bedford, J. D., Ikari, M. J., Schleicher, A. M., & Hirose, T. (2025). Synthesis of the laboratory frictional properties of a major shallow subduction zone: The Nankai Trough, offshore SW Japan. Journal of Geophysical Research: Solid Earth, 130, e2025JB031613. https://doi.org/10.1029/2025JB031613

—Alexandre Schubnel, Editor-in-Chief, JGR: Solid Earth

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