Effect of a Shallow Weak Zone on Fault Rupture: Numerical Simulation of Scale-Model Experiments

Steven M. Day, & Geoff Ely

Published December 2002, SCEC Contribution #604

Scale-model earthquake experiments provide detailed, subsurface recordings of rupture propagation and fault motion that are unavailable for natural earthquakes and thereby offer an opportunity to test numerical earthquake simulation methods. Among the advantages realized from the use of experimental data are optimal sensor locations, precise knowledge of bulk and surface properties of the medium, detailed knowledge of the initial stress state, and experimental repeatability. We perform numerical simulations that closely reproduce the shape and duration of the acceleration pulses recorded adjacent to the fault surface in the foam rubber earthquake experiments of Brune and Anooshehpoor (1998). Outside of the event nucleation zone, experimental and simulated rupture velocities are nearly indistinguishable. With adjustment of the static friction coefficient within the constraints imposed by experimental measurements, the absolute amplitudes of the accelerations can be brought into close agreement, typically within a few tens of percent. Close agreement between simulation and experiment is also maintained when the frictional strength of the upper part of the fault plane is varied. The agreement of the numerical and experimental results verifies that the discrete numerical model accurately represents the continuum dynamics of the spontaneous rupture problem. The numerical simulations also facilitate further interpretation of the scale-model experiments. They support an interpretation in which fault displacement in the foam experiments occurs predominantly through a conventional frictional sliding mechanism rather than during fault opening episodes, resulting in a cracklike rather than pulselike mode of slip. The simulations further suggest a reinterpretation of the apparent rupture velocity measurements in the fault weak zone, with rupture slowing rather than accelerating. They also predict that the weak zone diminishes surface accelerations and velocities (relative to a uniform fault model) out to a distance that scales with weak-zone depth and may enhance amplitudes slightly at intermediate distances.

Citation
Day, S. M., & Ely, G. (2002). Effect of a Shallow Weak Zone on Fault Rupture: Numerical Simulation of Scale-Model Experiments. Bulletin of the Seismological Society of America, 92(8), 3022-3041. doi: 10.1785/0120010273.