SCEC Award Number 12007 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Investigating tectonic tremor beneath the San Andreas Fault near Parkfield with the PASO array
Name Organization
Clifford Thurber University of Wisconsin, Madison
Other Participants Dana Peterson, undergraduate
Ninfa Bennington, post-doc
SCEC Priorities 5a, 4a, 1b SCEC Groups Seismology, FARM, USR
Report Due Date 03/15/2013 Date Report Submitted N/A
Project Abstract
We extended the successful stacking approach developed with 2011 SCEC support to process and pick P and S arrivals recorded by the 59-station PASO array for dozens of LFE families identified and located by Shelly and Hardebeck (2010). We then used a combination of P and S arrival times and corresponding differential times from 14 LFE families directly beneath the PASO array, along with absolute and differential times from shallower microearthquakes, to provide higher-precision locations of the LFE's and to image the three-dimensional (3-D) P- and S-wave velocity structure to ~20 km depth. Our initial focus was on the crustal volume directly below the PASO array, near SAFOD. Our results indicate that the LFE's align close to the vertically-downward extension of the SAF, but they show some variability along strike, suggesting a somewhat complex structure for the SAF at lower crustal depths. Furthermore, the LFE's near SAFOD lie within or adjacent to zones with slightly reduced P-wave velocity and more sharply reduced S-wave velocity. The estimated Vp/Vs values are approximately 1.9 in these zones. The elevated Vp/Vs values are interpreted to reflect high pore fluid pressure and low effective stress (Shelly et al., 2007). This is consistent with results from previous findings in subduction zones and with observations of triggering and tidal modulation of LFE’s and tremor on this deep extension of the SAF.
Intellectual Merit We have produced the first detailed image of the three-dimensional seismic velocity structure in the tectonic tremor zone beneath the San Andreas fault. We find slightly reduced P-wave velocity and more sharply reduced S-wave velocity, and hence a high Vp/Vs ratio. This is consistent with the presence of fluids in the tremor-generating zone.
Broader Impacts This project has supported the research experience of a female UW-Madison undergraduate student, Dana Peterson, as well as providing partial support for an early-career female post-doc, Ninfa Bennington. Peterson has learned a range of seismological analysis techniques and has gained substantial experience in computational methods and computer programming, particularly MATLAB. Peterson will be continuing her career in geophysics, initially with a research position at the Earth Observatory of Singapore.
Exemplary Figure Figure 3b