SCEC Award Number 13024 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Investigating tectonic tremor beneath the San Andreas Fault near Parkfield with the PASO array
Investigator(s)
Name Organization
Clifford Thurber University of Wisconsin, Madison
Other Participants Dana Peterson (UW undergraduate)
Ninfa Bennington
New UW undergraduate to be recruited
SCEC Priorities 1, 4, 5 SCEC Groups Seismology, FARM
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
 This was the final year of a 3-year project. The first year was expended exploring multiple methods for stacking the LFE's observed at PASO stations. Two standard methods were applied first but were found to be ineffective. The third attempted technique used individual window start times for all the events for family 37102 near SAFOD from Shelly and Hardebeck (2010) to stack the PASO data. This approach was successful. In the second year, we applied the successful stacking approach to 14 families and 12 stations of the PASO array. The LFE stacks were picked for P and/or S arrivals. The resulting picks were used for double-difference relocation of the LFE families and nearby earthquakes. We verified the general pattern of LFE locations reported by Shelly and Hardebeck (2010). In the third year, we explored the use of arrival time data from stacked LFE's to image the 3D seismic velocity structure beneath the SAFOD area. The preliminary results indicate that the LFE's lie within or on the edge of a zone of slightly reduced Vp and more sharply reduced Vs, and thus a somewhat high value of Vp/Vs (1.85 to 1.9). From this, we infer that the zone containing the LFE's is one of elevated fluid content and fluid pressure. We also made a major advance in the quality of the LFE stacks we can obtain via the use of phase-weighted stacking, although this discovery was made too late to impact our results in a meaningful way.
Intellectual Merit For the first time, we have been able to produce a relatively high-resolution model of the lower crust beneath the San Andreas fault at the depths of low-frequency earthquake occurrence. Previous workers have speculated that fluids are involved in the generation of low-frequency earthquakes. Our discovery of a zone of reduced shear wave velocity and high Vp/Vs ratio provides direct evidence that fluids are likely present in the zone where low-frequency earthquakes are generated. We have also discovered that phase-weighted stacking is an extremely effective approach for improving the quality of low-frequency earthquake stacks.
Broader Impacts Our work provides an integrated view of the geometry of the San Andreas fault in the Parkfield region extending from the surface to the lower crust. For earthquake rupture modeling, knowledge of the fault geometry is important for determining the likelihood of a large earthquake rupture extending to depths below the normally defined seismogenic zone. In particular, our results bare on the degree of localization of the zone of shear in which low-frequency earthquakes occur, which has implications for the potential for seismic ruptures to penetrate into the deeper parts of the SAF.
Exemplary Figure Figure 5b.
Cross-sections through a three-dimensional model of the shear wave velocity structure of the crust around the SAFOD site (contours in km/s) showing the locations of normal earthquakes (black dots shallower than 15 km) and low-frequency earthquakes (black dots deeper than 15 km). We find that the low-frequency earthquakes occur within or on the edge of a zone of reduced shear wave velocity, suggesting that there are fluids present, which may influence or perhaps control the occurrence of the low-frequency earthquakes.
[Peterson et al., in prep.]