High-Velocity Frictional Properties of Basalt-Derived Fault Rocks

Abstract:

Recent geophysical surveys in subduction zones suggest that subduction earthquakes could occur along the uppermost part of oceanic crust composed of basaltic rocks. In fact, the fluidization of comminuted material and mineralogical signatures of frictional heating were reported from a few-centimeters-thick ultracataclasite in the underplated basalt that were formed in association with the downward step of the subduction thrust. Thus, high-velocity frictional properties of basaltic rocks are keys for understanding rupture propagation during subduction earthquakes. We conducted high-velocity (1.3 m/s) friction experiments on the disaggregated basaltic rocks under wet (water-saturated) conditions and at normal stresses of ~2.0 MPa. The samples were taken from pillow basalt, foliated cataclasite, and ultracataclasite developed in the underplated basalt in the Shimanto accretionary complex of eastern Shikoku, southwest Japan. All samples show slip weakening behavior and lower steady-state friction of <0.14. The samples from pillow basalt and foliated cataclasite show higher peak friction of 0.49–0.61 and 0.59–0.75, respectively, whereas those from the ultracataclasite exhibit lower peak friction of 0.26–0.29. Compared to the samples from pillow basalt and ultracataclasite, that from foliated cataclasite has longer slip weakening distance. The calculated fracture energy in the sample from ultracataclasite is considerably small owing to smaller stress drop and slip weakening distance, which could be due to the higher clay content of 50 wt.% relative to the samples from others (21–29 wt.%). Our results imply that the propagation of earthquake rupture through the ultracataclasite is energetically favorable. This is consistent with the fact that the geological evidence of earthquake faulting was found from the ultracataclasite.