Fault rocks record the dynamic slip processes during subduction earthquakes

Most of the global seismic moment releases are achieved through earthquakes on subduction thrusts (e. g., the Mw9.2 Sumatra earthquake in 2004). However, the dynamics of subduction earthquakes remain poorly understood due to lack of sufficient information on the ambient conditions. The paleo-subduction thrusts exhumed from seismogenic depths are expected to provide an invaluable opportunity to understand the dynamic slip processes during subduction earthquakes. We have examined the fault rocks of the paleo-subduction thrusts exposed in the Shimanto accretionary complex in eastern Shikoku, Japan. The Shimanto accretionary complex represents an ancient analog of the Nankai Trough accretionary margin, where great subduction earthquakes (Mw ¿ 8.0) occur repeatedly. Our results indicate that the seismic slip concentrated in a narrow slip zone of thickness ranging from ~ 1 mm to a few centimeters, which led to the generation of frictional heat. As a result, the slip zone melted at temperatures higher than 1100 °C or fluidized under elevated fluid pressures with a temperature rise of 50-720 °C. The frictional melting of the slip zone produced pseudotachylytes, while the fluidization induced the injection of comminuted material into the host rocks by hydrofracturing. The dynamic strength of faults in the presence of the frictional melt and fluidized material is estimated to be low. Our fault rock analyses suggest that both frictional melting and fluidization along the thin slip zone can induce dynamic weakness of the fault and large dynamic stress drops during subduction earthquakes.