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

Very-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. Citation: Saito, T., K. Ujiie, A. Tsutsumi, J. Kameda, and B. Shibazaki (2013), Geological and frictional aspects of very-low-frequency earthquakes in an accretionary prism, Geophys. Res. Lett., 40, 703-708, doi:10.1002/grl.50175.