SCEC Award Number 11113 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Mapping the Crust-Mantle Transition Beneath Parkfield, Non-Volcanic Tremor and the Deep Extension of the San Andreas Fault
Name Organization
Susan Y. Schwartz University of California, Santa Cruz Geoffrey Abers Lamont Doherty Earth Observatory
Other Participants 1 UCSC graduate student to be determined
SCEC Priorities A11, A4, C SCEC Groups Seismology, LAD, FARM
Report Due Date 02/29/2012 Date Report Submitted N/A
Project Abstract
We used teleseismic data recorded on all broadband stations within about 100 km of Parkfield to image the shallow lithospheric velocity structure with the goal of relating variations in structure to the spatial pattern of non-volcanic tremor. P and PP wave receiver functions were computed at eight individual stations as well as 26 stations of the PASO array straddling the fault. Stations located on the fault produce complex radial receiver functions requiring better estimates of event source time functions than provided by the vertical component seismograms. However, station PKD, located just to the west side of the SAF reveals a consistent Moho conversion at a depth of 26 km for rays arriving from the west. This depth is consistent with the EARS determination (27 km) and with prior work. Array receiver function analysis of 26 PASO stations that straddle the SAF indicates a clear step in the Moho depth across the fault. The Moho deepens from 26 km on the southwest side to ~30 km on the northeast. The array processed image also reveals a velocity inversion at 12-14 km, the depth of the deepest earthquakes. This suggests that the base of the brittle deformation zone corresponds to a significant velocity decrease. Thurber’s relocation of tremor event families (SCEC Project 11016 report) indicates that most of the tremor events occur on the southwest side of the fault at depths between 15-26 km. Many of these tremor events locate directly beneath the deepest earthquakes where the velocity inversion is the strongest.
Intellectual Merit Slow slip and tremor is one of the most exciting geophysical discoveries of the last few decades. It is a new mode of fault motion that has profound scientific and hazard implications. Thus far, most slow slip and tremor appears to locate below the seismogenic zone, providing a view into processes occurring at the deep extension of fault zones. Shelly and Hardebeck [2010] report a shallowing in tremor depth from 25-30 km northwest and southeast of Parkfield to ~16 km, adjacent to the base of the seismogenic zone near Parkfield. Stacking event families at stations of the dense PASO array near Parkfield, Thurber [2011] relocated some of the tremor events identified by Shelly and Hardebeck [2010] and found that most of the tremor events occur on the southwest side of the fault at depths between 15-26 km. Our work is investigating how the shallowing of tremor and its concentration on the southwest side of the fault might be controlled by lower crust/upper mantle structure. Our preliminary results indicate that a change in Moho depth across the SAF and a strong velocity inversion below 11 km on the southwest side of the fault may influence tremor locations.
Broader Impacts This project engaged a graduate student at UC Santa Cruz and this work will be part of his dissertation research. The project also strengthened collaborations between diverse SCEC institutions (Columbia and UC Santa Cruz).
Exemplary Figure Figure 3. Back-projected scattered P coda wavefield from the PASO array, near SAFOD. Colors show amplitude of source-normalized scattered P coda (akin to receiver functions), following array-based deconvolution, back-projected along ray paths collapsed onto a two-dimensional section crossing the San Andreas Fault. Signals are back-projected both as the direct, upgoing P-S forward-scattered conversion and a surface reflected Ppxs back-scattered conversion, and stacked, weighted by their semblance. Reflections imaged by a 1977 COCORP transect nearby are overlain [McBride and Brown, 1986]. Note positive conversions associated with the Moho, showing a change in depth across the SAF, and a strong negative-amplitude conversion at 11-12 km depth on the SW side of the fault.