Inferring fault rheology from low‐frequency earthquakes on the San Andreas

Abstract

[1] Families of recurring low-frequency earthquakes (LFEs) within nonvolcanic tremor (NVT) on the San Andreas fault in central California show strong sensitivity to shear stress induced by the daily tidal cycle. LFEs occur at all levels of the tidal shear stress and are in phase with the very small,~400 Pa, stress amplitude. To quantitatively explain the correlation, we use a model from the existing literature that assumes the LFE sources are small, persistent regions that repeatedly fail during shear of a much larger scale, otherwise aseismically creeping fault zone. The LFE source patches see tectonic loading, creep of the surrounding fault which may be modulated by the tidal stress, and direct tidal loading. If the patches are small relative to the surrounding creeping fault then the stressing is dominated by fault creep, and if patch failure occurs at a threshold stress, then the resulting seismicity rate is proportional to the fault creep rate or fault zone strain rate. Using the seismicity rate as a proxy for strain rate and the tidal shear stress, we fit the data with possible fault rheologies that produce creep in laboratory experiments at temperatures of 400 to 600°C appropriate for the LFE source depth. The rheological properties of rock-forming minerals for dislocation creep and dislocation glide are not consistent with the observed fault creep because strong correlation between small stress perturbations and strain rate requires perturbation on the order of the ambient stress. The observed tidal modulation restricts ambient stress to be at most a few kilopascal, much lower than rock strength. A purely rate dependent friction is consistent with the observations only if the product of the friction rate dependence and effective normal stress is~0.5 kPa. Extrapolating the friction rate strengthening dependence of phyllosilicates (talc) to depth would require the effective normal stress to be~50 kPa, implying pore pressure is lithostatic. If the LFE source is on the order of tens of meters, as required by the model, rate-weakening friction rate dependence (e.g., olivine) at 400 to 600°C requires that the minimum effective pressure at the LFE source is~2.5 MPa.

Fortnightly modulation of San Andreas tremor and low-frequency earthquakes

“…The semidiurnal shear stress is only a few hundred Pascals-six orders of magnitude smaller than the lithostatic stress at the tremor depth of 16-30 km. These observations suggest a very weak fault with high pore pressure and poorly drained hydrologic conditions (12,13).…”

“…Individual LFE families (spatially localized patches of repeating LFEs) show varying sensitivity to tidal stresses, reflecting heterogeneities in the local stress state, pore pressure, frictional rheology, or other properties (12,13).…”

“…The short-period nature of the semidiurnal tides complicates the physical interpretation of triggered LFEs, because the shortterm response may be influenced by the time-dependent process of LFE nucleation (13,20,21) or by fault weakening as the tremor episode accelerates (16). To shed additional light on the mechanics of triggering for LFEs and creep episodes, we here analyze the effect of the fortnightly tides, which modulate the semidiurnal tidal amplitude on a 14-d cycle and are relatively far removed from the timescale of LFE nucleation.…”

“…Tidal triggering of tremor, on the other hand, has been found almost everywhere that tectonic tremor is observed (6)(7)(8)(9)(10). Tidal triggering acts as a probe of the properties of faults at depth, generating insights into the mechanics of the brittle−ductile transition (11)(12)(13)(14)(15)(16).…”

“…Previous studies have analyzed the amplitude and phase of the semidiurnal tidal modulation of tremor on the San Andreas fault near Parkfield, CA (12,13) (Fig. 1).…”