Geometry and Pore Pressure Shape the Pattern of the Tectonic Tremors Activity on the Deep San Andreas Fault with Periodic, Period-Multiplying Recurrence Intervals

Abstract:

A never before recorded pattern of periodic, chaotic, and doubled, earthquake recurrence intervals was detected in the sequence of deep tectonic tremors of the Parkfield segment of the San Andreas Fault (Shelly, 2010). These observations may be the most puzzling seismological observations of the last decade: The pattern was regularly oscillating with a period doubling of 3 and 6 days from mid-2003 until it was disrupted by the 2004 Mw 6.0 Parkfield earthquake. But by the end of 2007, the previous pattern resumed.

Here, we assume that the complex dynamics of the tremors is caused by slip on a single asperity on the San Andreas Fault with homogeneous friction properties. We developed a three-dimensional model based on the rate-and-state friction law with a single patch and simulated fault slip during all stages of the earthquake cycle using the boundary integral method of Lapusta & Liu (2009).

We find that homogeneous penny-shaped asperities cannot induce the observed period doubling, and that the geometry itself of the velocity-weakening asperity is critical in enabling the characteristic behavior of the Parkfield tremors. We also find that the system is sensitive to perturbations in pore pressure, such that the ones induced by the 2004 Parkfield earthquake are sufficient to dramatically alter the dynamics of the tremors for two years, as observed by Shelly (2010).

An important finding is that tremor magnitude is amplified more by macroscopic slip duration on the source asperity than by slip amplitude, indicative of a time-dependent process for the breakage of micro-asperities that leads to seismic emissions. Our simulated event duration is in the range of 25 to 150 seconds, closely comparable to the event duration of a typical Parkfield tectonic tremor.

Our simulations reproduce the unique observations of the Parkfield tremor activity. This study vividly illustrates the critical role of geometry in shaping the dynamics of fault slip evolution on a seismogenic fault.