Constraints on Friction, Dilatancy, Diffusivity, and Effective Stress From Low‐Frequency Earthquake Rates on the Deep San Andreas Fault

Abstract

Families of recurring low‐frequency earthquakes (LFEs) within nonvolcanic tremor on the San Andreas Fault in central California are sensitive to tidal stresses. LFEs occur at all levels of the tides, are strongly correlated and in phase with the ~200 Pa shear stresses, and weakly and not systematically correlated with the ~2 kPa tidal normal stresses. We assume that LFEs are small sources that repeatedly fail during shear within a much larger scale, aseismically slipping fault zone and consider two different models of the fault slip: (1) modulation of the fault slip rate by the tidal stresses or (2) episodic slip, triggered by the tides. LFEs are strongly clustered with duration much shorter than the semidiurnal tide; they cannot be significantly modulated on that time scale. The recurrence times of clusters, however, are many times longer than the semidiurnal, leading to an appearance of tidal triggering. In this context we examine the predictions of laboratory‐observed triggered frictional (dilatant) fault slip. The undrained end‐member model produces no sensitivity to the tidal normal stress, and slip onsets are in phase with the tidal shear stress. The tidal correlation constrains the diffusivity to be less than ~1 × 10−6/s and the product of the friction and dilatancy coefficients to be at most 5 × 10−7, orders of magnitude smaller than observed at room temperature. In the absence of dilatancy the effective normal stress at failure would be about ~55 kPa. For this model the observations require intrinsic weakness, low dilatancy, and lithostatic pore fluid.

The mechanism of tidal triggering of earthquakes at mid-ocean ridges

“…Low-frequency earthquakes in the tremor deep in subduction zones also show high sensitivity to tidal stresses 53,54 . This undoubtedly implies very low effective normal stresses, although if our A value is used instead of the laboratory values, effective stresses would be about an order of magnitude larger than those reported.…”

What’s down there? The structures, materials and environment of deep-seated slow slip and tremor

“…The tidal shear-stress response appears consistent with rate-dependent friction at extremely low effective normal stress, whereas purely aseismic shearing of various mineralogies and power-law or exponential viscous deformation mechanisms does not appear to allow for driving such a response [ 108 ]. Considering an undrained fault model, Beeler et al [ 109 ] suggest that the observed tremor response reflects low intrinsic friction, low dilatancy and lithostatic pore fluid pressures. Houston [ 82 ] finds that the modulation of tremors becomes stronger as slow slip accumulates during an SSE.…”

“…Considering an undrained fault model, Beeler et al [109] suggest that the observed tremor response reflects low intrinsic friction, low dilatancy and lithostatic pore fluid pressures. Houston [82] finds that the modulation of tremors becomes stronger as slow slip accumulates during an SSE.…”

Using Low‐Frequency Earthquake Families on the San Andreas Fault as Deep Creepmeters

“…Figure 10 shows that for the most extensively studied gouge from the Tejon Pass drilling project C lies in the range of 2 to 10 GPa/m for confining stresses between 50 and 200 MPa, which correspond to lithostatic pressures at shallow to midcrustal depths (Morrow et al, 1982). Pore fluid pressures in the LFE source region on the deep San Andreas Fault are thought to be near lithostatic (Beeler et al, 2018(Beeler et al, , 2013Thomas et al, 2012). The Tejon Pass gouge is the strongest and most strongly strain hardening material in Morrow et al (1982) and has a pressure dependence of 0.047 GPa/MPa-m.…”

“…The Tejon Pass gouge is the strongest and most strongly strain hardening material in Morrow et al (1982) and has a pressure dependence of 0.047 GPa/MPa-m. Pore fluid pressures in the LFE source region on the deep San Andreas Fault are thought to be near lithostatic (Beeler et al, 2018(Beeler et al, , 2013Thomas et al, 2012).…”