SCEC Award Number 17165 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Testing models of plate boundary deformation within the mantle lithosphere beneath central and southern California
Investigator(s)
Name Organization
Heather Ford University of California, Riverside
Other Participants
SCEC Priorities 3b, 1b, 3a SCEC Groups CXM, Seismology, SDOT
Report Due Date 04/30/2020 Date Report Submitted 11/19/2020
Project Abstract
 The distribution of deformation within the lithosphere in the vicinity of the plate boundary is not presently well constrained. Determining the geometry of deformation of the ductile portion of the fault system has important implications for our understanding of mantle rheology and can provide constraints on whether block models can be considered as a reasonable approximation throughout the width of the entire lithosphere. This study provides preliminary observational constraints on seismic anisotropy beneath Walker Lane in eastern California. Ps receiver function analysis was used to image lateral variations in anisotropy within the lithosphere at six stations in Walker Lane. Receiver functions were also calculated for the Mojave and central (coastal) California but are not discussed. At stations FUR, GRA and SLA anisotropic structure is present and generally consistent between stations. At CWC, MLAC and TIN anisotropic and/or dipping structure appears to be present but is significantly more complex and is not in agreement with the other stations. Small scale regional complexities, including the presence of structural basins and melt in the mid-crust may obscure similarities between the stations. The results at stations FUR, GRA and SLA bear striking resemblance to previous receiver function results at three stations (WVOR, MNV and TPNV) in the westernmost Basin and Range. Preliminary modeling indicates the presence of one or more layers of anisotropy in the lowermost crust and upper mantle. Future work will be completed to more completely characterize the orientation of anisotropy and to relate such constraints to tectonic processes.
Intellectual Merit The purpose of the proposed work was to provide preliminary observational constraints on the geometry of the ductile portion of the San Andreas fault system in southern California, and to compare these results to those currently being calculated for central California. Understanding how the base of the tectonic plates deforms will provide additional constraints on whether the block model can be considered a reasonable approximation throughout the entire lithosphere. Finally, being able to provide constraints from greater lithospheric depths will enable the community to better constrain the geometry of the ductile portion of the fault system and to better understand linkages between faults at mantle depths.
Broader Impacts This study supported an early career investigator, Heather Ford, as she began a faculty position at the University of California, Riverside. During the project the PI mentored three undergraduates in structural seismology. For each student, this project was their first exposure to research. Two of the students were non-traditional, first generation students, one of whom went on to complete an IRIS internship and was accepted to the graduate program at New Mexico Tech. The third student was a female undergraduate with learning disabilities.
Exemplary Figure Figure 2. Radial (top panels) and Transverse (bottom panels) component Ps receiver functions calculated in this study (left column) and previously (right column). Receiver functions are binned as a function of back azimuth (shown on the x-axis) in 10degree increments and filtered to 1 Hz. Positive (blue) pulses correspond to a velocity increase with increasing depth and negative (red) pulses correspond to a velocity decrease with increasing depth. On the radial component RFs, the Moho arrival is highlighted with a semi-transparent blue line, and the LAB arrivals are highlighted with a semi-transparent magenta box. The transverse component receiver functions show significant complexity. A pair of phases with polarity reversals at ~4-6 seconds (highlighted with a black box) show similarities to receiver functions calculated during an earlier analysis of Basin and Range structure. Note that the back azimuth at which polarity reversals occur (vertical black line) is similar among all stations. A model of the anisotropic structure is shown in Figure 3 and discussed in the text.