Slowly Propagating Slip Events in a Composite Sample of Granite and Marble

Abstract

Slip events with anomalously low rupture velocities are examined through experiments by the use of a rock sample with a pre-existing fault. The rock sample consisted of a block of Akiyoshi marble sandwiched between two blocks of Westerly granite. Sliding along the composite fault is low stress-drop stick-slip motion over the entire fault, though sliding is stable on a pure marble fault and unstable on a pure granite fault under the same experimental conditions. The observed rupture velocity, VR, ranges from 0.1 to 500 m/s, and the observed breakdown stress drop from 0.04 to 0.2 MPa. Combining the data for events in the composite fault with those obtained for stick-slip on faults in pure granite, the breakdown stress drop Δτb can be related to both VR and strain rate by Δτb ∝ VR0.25 for 0.1 m/s ≲ VR≲ 3 km/s, Δτb ∝ ̇εmax0.20 for 6 × 10−7/s ≲ ̇εmax ≲ 2 × 10−1/s, where ̇εmax stands for the maximum value of observed strain-rate. This clearly indicates that the shear strength between fault surfaces is dependent on strain-rate. We emphasize the possibility of slow earthquakes or episodic creep events in the lower crust and the upper mantle. © 1991, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.