Nonlinear interaction of high-frequency seismic waves with sliding fault planes

Norman H. Sleep

Accepted October 26, 2019, SCEC Contribution #9960

Slip during large crustal earthquakes continues for extended periods of time on spatially extensive fault planes. High-frequency body waves from other parts of the fault system thus impinge on patches of the fault that are still actively sliding. Intuitively, the material within sliding fault planes does not rheologically distinguish between the low-frequency stresses driving the gross earthquake and high-frequency dynamic stresses from impinging body waves. Nonlinear interaction occurs, interrogating the rheology of the fault plane. High-frequency S waves nonlinearly produce additional inelastic slip DELTA S on the sliding fault. The ratio of this slip to the elastic displacement (DELTA S / DELTA S_E ), where DELTA S_E is the elastic displacement, depends on the slip-reflection number, (partial V/partial tau) rho beta, where partial V /partial tau is the derivative of sliding velocity with respect to shear traction, rho is density, beta is S-wave velocity. The sliding fault transmits and weakly reflects S waves for small values <<2 of the parameter; it weakly transmits and strongly reflects for high values >> 2. It is relevant to evaluate partial V / partial tau at the long-period slip velocity V and shear traction tau of the gross earthquake. The ratio is then DELTA S/DELTA S_E = rho beta V / tau, where (partial V / partial tau) (tau/V) is measure of the PHI = nonlinearity the fault rheology. For example, the parameter PHI at high-frequency impinging S waves is the ratio mu/a of the coefficient of friction to the rate parameter a=~0.01 for rate and state friction. Weak sliding faults (low tau and high V ) strongly reflect impinging S waves. During waning slip, dynamic stresses from S waves antithetical to the gross slip direction and compressional P waves may heterogeneously lock the fault.

Key Words
nonlinear seismology, fualt rheology

Citation
Sleep, N. H. (2019). Nonlinear interaction of high-frequency seismic waves with sliding fault planes. Geophysical Research - Solid Earth, (accepted).


Related Projects & Working Groups
FARM, Fault and Rupture Mechanics (FARM)