Geological and frictional aspects of very‐low‐frequency earthquakes in an accretionary prism

Abstract

AbstractVery‐low‐frequency earthquakes (VLFE) occur within accretionary prisms and near subduction plate boundaries at slip rates of 0.05–2 mm/s. However, the geological and frictional aspects of VLFE remain poorly understood. The thrusts in the Shimanto accretionary complex exhumed from source depths of VLFE are composed of quartz‐rich fault rocks with or without clay foliations. We examined the frictional velocity dependence of thrust materials. At slip rates of 0.0028–0.28 mm/s, the powder sample from non‐foliated fault rock shows velocity‐weakening behavior, while that from foliated fault rock exhibits velocity‐strengthening behavior. Both samples show velocity‐strengthening behavior at slip rates of 0.28–2.8 mm/s. Microstructural analysis reveals that the velocity‐weakening sample shows a shear localization, while the velocity‐strengthening sample is marked by clay foliations oblique and parallel to shear zone boundaries. Our results imply that frictional instability generates along quartz‐rich fault rock but slip becomes stable at higher slip rates, regardless of mineral composition. This is favorable for the occurrence of VLFE in subduction zones.