Constraining the velocity structure near the tremorogenic portion of the San Andreas Fault near Parkfield, CA using Ambient Noise Tomography

The central section of the San Andreas Fault (SAF) displays a range of seismic phenomena including tectonic tremor, low frequency earthquakes (LFEs), repeating micro-earthquakes (REQ), and aseismic creep. We investigate the seismic velocity structure associated with this section of the SAF using surface wave tomography from ambient seismic noise. A high-resolution model of shear velocities in the crust and upper mantle is derived from data recorded by the USArray, the Parkfield Hi-Resolution Seismic Network (HRSN), and other regional networks between 2003 and 2015. Fault perpendicular sections show that the SAF is highlighted by lateral contrasts in relative velocities in the deeper lithosphere, where it appears to be localized to within about 50 km at depths between 15 and 30 km. This localization of the fault at depth is consistent with the hypothesis that LFEs are shear-slip events on a deep extension of the SAF. Fault parallel sections show that LFE and REQ locations occur within low velocity structures, suggesting that the presence of fluids, weaker minerals, or hydrous phase minerals may play an important role in the generation of slow slip phenomena. Such correlations between seismic waves speeds and seismicity can provide insight and constraints on the physical mechanics that produce LFEs and REQs.