Geodetic Signature of Slow-Slip and Tremor in Parkfield, CA

Abstract:

It has been hypothesized that quasi-periodic slip on the deep extent of the San Andreas Fault (SAF) accompany deep tremor and low frequency earthquakes (LFEs) near Parfiield, CA. Geodetic data document that slow slip accompanies tremor in the Cascadia and other subduction zones,however there has not been an observation of deformation associated with the observed tremor in a transform fault setting such as the SAF, despite the ubiquitous presence of tremor and LFEs. The signal is likely too small to be detectable by GPS, and previous studies attempting to use borehole strainmeters in Parkfield have failed to detect any observable strain associated with tremor swarms triggered by passing surface waves and place an upper limit on the surface strain of 5 nanometers [Smith 2009]. We explore/examine strain records associated with newly identified large tremor swarms that are in inferred to be slow slip events with moment magnitudes exceeding 5. These slow slip events have durations of approximately 10 days, and the deformation is far below the detection level of GPS. In this study we utilize data from long-baseline laser strainmeters (LSM) which are particularly well suited to observe these signals, in order to place lower and upper bounds on the deformation associated with the slow-slip events. Preliminary results suggest that average surface strains associated with these events are on the order of ~1 nanometer.