Tremor-tide correlations and near-lithostatic pore pressure on the deep San Andreas fault

Parkfield tremors reach out Amanda Thomas and colleagues identify a correlation between non-volcanic tremor activity near Parkfield, California, and extremely small, tidally induced shear stress parallel to the San Andreas fault. Non-volcanic tremor is a weak seismic signal observed periodically on some major faults. Thomas et al . suggest that the Parkfield tremors may represent shear failure on a critically stressed fault in the presence of near-lithostatic pore pressure. Similarities with tremor in subduction-zone environments, such as Cascadia and Japan, indicate that these findings may also be relevant to other tectonic settings. Non-volcanic tremor was discovered nearly a decade ago; however, a thorough explanation of the geologic process responsible for tremor generation has yet to be determined. A robust correlation is now identified between extremely small, tidally induced shear stress parallel to the San Andreas fault and non-volcanic tremor activity near Parkfield, California. Such tremor may represent shear failure on a critically stressed fault in the presence of near-lithostatic pore pressure. Since its initial discovery nearly a decade ago 1 , non-volcanic tremor has provided information about a region of the Earth that was previously thought incapable of generating seismic radiation. A thorough explanation of the geologic process responsible for tremor generation has, however, yet to be determined. Owing to their location at the plate interface, temporal correlation with geodetically measured slow-slip events and dominant shear wave energy, tremor observations in southwest Japan have been interpreted as a superposition of many low-frequency earthquakes that represent slip on a fault surface 2 , 3 . Fluids may also be fundamental to the failure process in subduction zone environments, as teleseismic and tidal modulation of tremor in Cascadia and Japan and high Poisson ratios in both source regions are indicative of pressurized pore fluids 3 , 4 , 5 , 6 , 7 . Here we identify a robust correlation between extremely small, tidally induced shear stress parallel to the San Andreas fault and non-volcanic tremor activity near Parkfield, California. We suggest that this tremor represents shear failure on a critically stressed fault in the presence of near-lithostatic pore pressure. There are a number of similarities between tremor in subduction zone environments, such as Cascadia and Japan, and tremor on the deep San Andreas transform 3 , 4 , 5 , 6