Fortnightly modulation of San Andreas tremor and low-­‐frequency earthquakes

Abstract:

Earth tides modulate tremor and low-­‐frequency earthquakes (LFEs) on faults in the vicinity of the brittle-­‐ductile transition. The response to the tidal stress carries otherwise inaccessible information about fault strength and rheology. Here we analyze the LFE response to the fortnightly tide, which modulates the amplitude of both shear and normal stress components of the daily tidal stress over a 14–day cycle. Analyzing the fault-­‐tide interactions at a period that is long with respect to the duration of triggered LFEs provides a clearer picture of the triggering process than can be obtained by examining the daily tides alone. We find that LFE rate is highest during the rising fortnightly tide (-­‐90° tidal phase), that is, not when the daily stress reaches a maximum, but rather when it exceeds the previous day’s stress by the widest margin. This demonstrates that the tidal triggering of LFEs is at least partly a
threshold-­‐triggering phenomenon, and not exclusively a modulation of creep rate in the surrounding medium. Armed with this insight, we use the amplitude of the fortnightly modulation to directly quantify the tectonic loading rate at depth and map its variability. The inferred variations in loading rate are spatially coherent, with loading rate apparently smaller for LFE families located within the interiors of tremor-­‐producing regions, and higher at their margins. This suggests either stress-­‐ shielding of the interior LFEs, or variations in the rate of truly aseismic creep in the bulk fault zone.