Near-field Observations of Very-low-frequency Earthquakes on the San Andreas Fault

Abstract:

Fault rupture at varying time scales has been detected in multiple subduction zones, e.g., in slow-slip events (SSEs), very-low-frequency earthquakes (VLFEs), and low-frequency earthquakes (LFEs) or tectonic tremor. However, only LFEs or tremor have been identified and studied in detail along strike-slip faults, like the San Andreas Fault (SAF). Here, we present evidence for VLFEs on the SAF near Parkfield, California. Using data from permanent broadband stations and a temporary deployment of 13 broadband stations installed in 2010-2011 near Cholame, California, we detect 5 VLFEs, with 1 VLFE occurring unambiguosly when there is visible tremor activity. We check that the signals we detect are local by confirming that they appear only on stations within a 70 km radius, and removing time periods when teleseismic events occur, as identified in the global Centroid Moment Tensor (CMT) and the Northern California Seismic Network (NCSN) catalogs. VLFEs have to-date been observed to only occur simultaneously in time and space with tremor activity, but our detections suggests that VLFEs can occur independent of tremor along strike-slip faults. This may indicate that the slipping patches that produce slow earthquakes in transform faults have different mechanical properties than the patches in subducting plates, althought it does not rule out that VLFEs are only observed with tremor in subduction zones simply due to detection methods. An approximate estimation of the apparent velocity, based on a grid-search location using variance reduction, suggests that the observed phase velocity of the VLFEs is ~ 3km/s, corresponding to surface waves. We perform a focal mechanism inversion with a grid search to find a more precise location, depth and orientation of the VLFEs. These results provide new insight into the behavior of the SAF and more generally contribute to an improved understanding of transform fault systems.