Mechanism of spontaneous and triggered continental shallow creep events

English Abstract:

Slip on tectonic faults take place over a wide range of spatial and temporal scales as earthquakes, continuous aseismic creep, or episodic creep events. Shallow creep events on continental strike-slip faults can occur spontaneously or being triggered by nearby earthquakes. Despite more than five decades of observations, the mechanism of shallow creep events and their implications for seismic hazard are still not fully understood.

To understand the mechanism of spontaneous and triggered creep events, we developed a physics-based numerical model to simulate shallow creep events on a strike-slip fault with rate- and state-dependent frictional properties. We show that a widely used synoptic model cannot explain the wide variability in observed shallow creep characteristics on strike-slip faults in California. Rather, a frictionally unstable layer embedded in the shallow stable zone is required to match the geodetic observations of the creep behavior. We then introduced static and dynamic stress perturbations to this theoretical fault model, to further investigate the observed behavior of triggered creep events. We find that many creep events were likely dynamically triggered because the static stress changes caused by nearby earthquakes are typically less than 0.1 MPa thus too small to instantaneously trigger creep events. In contrast, we can reproduce the instantaneously triggered creep with dynamic perturbations alone, where the triggering threshold depends on the peak amplitude of and the time-integrated dynamic Coulomb stress change. Based on observations and simulations, the stress change amplitude required to trigger a creep event of 1 and 0.01 mm slip is 0.8 and 0.6 MPa, respectively. This is at least one magnitude larger than the triggering threshold of non-volcanic tremor (2-60 KPa) and earthquakes in geysers (5 KPa), which may due to the difference in the effective normal stress and other friction properties in these systems or different triggering mechanisms. We conclude that shallow frictional heterogeneity on strike-slip faults can explain both the spontaneous and dynamically triggered creep events.