Frictional characteristics change due to spatial distribution of velocity-weakening patches

Abstract:

Coupling on the fault is brought by the microscopic geometrical irregularity on the fault. True contact areas are usually much smaller than total fault area. Frictional force is determined by the breakage and recovery of true contact areas. Meanwhile, rock friction experiments provide macroscopic information of friction.

In the numerical simulation, it is often assumed that one seismic event is characterized by one uniform velocity-weakening (VW) patch. However, such seismic patch should be composed of the mixture of VW patch and velocity-strengthening (VS) background. Hence, this study investigates macroscopic frictional property of a heterogeneous fault, where seismic patch and VS background are mixed. For simplicity, we conduct 2D (linear fault) numerical simulation with rate-and-state friction law. We especially focus on how macroscopic friction changes with spatial distribution of microscopic seismic patches.

First, we consider one seismic patch in VS background. When stress loading is too fast, seismic patch slips stably, resulting in macroscopic a-b being simple spatial average of a-b on the fault. If stress loading is slower and stick-slip occurs, spatial averaged stress becomes lower than that expected from spatial averaged a-b, and macroscopic a-b becomes larger than spatial average of a-b on the fault.

Next, we consider the fault where several seismic patches are distributed. When two seismic patches are separated by VS region, rapid slip cannot overcome the barrier if the size of the barrier exceeds an order of one seismic patch. However, if the distribution of seismic patch is fractal, entire fault can slip rapidly even if the total area of seismic patch is just a half of total fault area. This result implies that seismic patches with very small area in total can rupture the huge area as an earthquake if the hierarchy of spatial distribution is well established. We will also present how macroscopic frictional parameters change with the upper limit of hierarchy, and what kind of fractal distribution enables entire rupture.