Simple Faults With Complex Slip Patterns: Theoretical Arguments for Non-Characteristic Ruptures on Homogeneous Planar Faults

Date: 4/25/2019

Time: 09:15 AM

Room: Pike

While the concepts of earthquake cycle and characteristic earthquake are often used in seismic hazard, they are poorly supported by observations and have been heavily criticized in recent years. The statistical properties of regional seismicity - power law distributions in seismic moment and temporal clustering (aftershocks) - are antithetical to characteristic and periodic behavior. In contrast, some small quakes have well defined cycles: locked patches surrounded by creep can rupture periodically in repeating events. Regional statistics could simply reflect the distribution of lengthscales in a fault system; alternatively, large faults may be inherently less characteristic than small asperities.

We explore this question using fracture mechanics ideas and numerical simulations of rate-state faults with velocity-weakening (VW) regions loaded by adjacent creep, such as vertical antiplane faults with downdip creep. We suggest that slip patterns are determined by two timescales: T_f, the time required to accumulate elastic energy for full rupture; and the nucleation time T_n, controlled by creep penetrating in the VW region. The energy needed for a full rupture, and T_f, increase with fault dimension L; while the energy required for nucleation, and T_n, increase with L_n, a characteristic length determined by frictional and elastic properties. If T_n<T_f partial ruptures occur; for large faults, T_n<<T_f and we expect multiple partial ruptures. Simple crack models predict the ratio between the number of nucleations and the number of full ruptures on a 1D fault to increase as ~(L/L_n)^1/2. Numerical simulations confirm these arguments, and exhibit a transition from characteristic, periodic cycles at L/L_n <~ 10 to a power law moment distribution and temporal clustering for larger L/L_n. For L/L_n >~100, interevent times are power law distributed, consistent with Omori decay modulated by afterslip in the creeping region. We suggest that L/L_n alone can lead to variability in seismic behavior even without heterogeneous frictional properties or prestress.

Presenting Author: Camilla Cattania

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