Constitutive parameters for earthquake rupture dynamics based on high-velocity friction tests with variable sliprate

Abstract

To reproduce the dynamic rupture process of earthquakes, the fault geometry, initial stress distribution and a frictional constitutive law on the fault are important parameters as initial and boundary conditions of the system. Here, we focus on the frictional constitutive relation on the fault. During a high-speed rupture, fault strength decreases as slip develops which can be described by a slip weakening equation. To understand the physical process of stress breakdown during the dynamic rupture of earthquakes, we investigated the friction behavior of rocks in the laboratory by direct measurements of traction evolution with slip in response to a given slip history. We employed a high-speed rotary shear apparatus introduced at National Research Institute for Earth Science and Disaster Prevention (NIED). This apparatus has a capability of sliding with predefined variable velocities using a servo-controlled system. We used a pair of granite cylindrical specimens with a diameter of 25 mm. As an input signal, we used a regularized Yoffe function to investigate the scale dependence of fracture energy and slip weakening distance (D c). We observed a positive correlation between D c and total slip, keeping the maximum slip velocity constant. These conditions correspond to those for earthquakes with the same stress drop and varying magnitudes. Finally, we used a real fault motion; a fault parallel velocity seismogram observed at PS10, 3 km away from the surface fault trace and above the high slip region during the 2002 Denali, Alaska, earthquake. We compared the seismological fracture energy with the corresponding D c. The relation is linear with an inflection point at D c = 0.2 m, where the gradient changes. Another interesting feature is that the maximum value of D c is about 4m even if the total slip exceeds 12 m.