While earthquakes in subduction zones seem to occur randomly in time, they tend to cluster in localized areas, which are likely controlled by fine-scale fault zone structures. Such structures can be seen from exhumed subduction zone rocks in certain regions, but direct observations of subduction zones at the depths of earthquakes and at the scale relevant to earthquake sources are still rare. As the size and spatial distribution of these structures could control the maximum size of subduction zone earthquakes, high-resolution imaging of the kilometer-size fault zone structures will significantly contribute to the assessment of earthquake hazards in subduction zones.

In this paper, we directly measure the in-situ material properties of fault zones that host frequent earthquakes at depths of 60 to 70 km along the plate interface between the Pacific plate and the Philippine plate in Japan. We find localized, anisotropic structures with a near-zero Poisson’s ratio (a median Vp/Vs ratio of 1.44). The heterogenous fault material properties, combined with stress drop estimates of deep interplate earthquakes, suggest that the fault medium is damaged, foliated, and enriched with fluid. Such small-scale material heterogeneity may cause faults stress perturbations that in turn favor the frequent occurrence of deep interplate earthquakes.

Huang, Y., Ide, S., Kato, A., Yoshida, K., Jiang, C., & Zhai, P. (2025). Fault material heterogeneity controls deep interplate earthquakes. Science Advances, 11(9), eadr9353. https://doi.org/10.1126/sciadv.adr9353