SCEC Award Number 22073 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title High-resolution cross- and sub-fault comparisons of CFM, seismicity, and structural fabric imaged with receiver functions
Name Organization
Debi Kilb University of California, San Diego Vera Schulte-Pelkum University of Colorado, Boulder
Other Participants
SCEC Priorities 3b, 1b, 2c SCEC Groups CXM, SDOT, Seismology
Report Due Date 03/15/2023 Date Report Submitted 11/29/2023
Project Abstract
We combined the CFM fault model, seismicity, and receiver function anisotropic contrast imaging to improve constraints on fault geometry and deep crustal rheology.
Across the San Andreas Fault (SAF) near Parkfield, a dense network of stations shows fault-parallel contrasts in anisotropy, interpreted as contrasts in foliation in crustal rocks. Interestingly, the foliation dip is down to the northeast on both sides of the fault. Contrast depths increase nearly symmetrically away from the fault to each side but with significantly stronger conversions on the northeast side of the fault. We interpret this as northeast-dipping background foliation inherited from accretion during Farallon subduction reacting to and potentially influencing deformation along the San Andreas Fault to form a zone of interaction. One possible scenario is that the fabric on the northeastern side shows a record of progressive steepening of the SAF from the onset of strike-slip motion when it exploited the preexisting dipping subduction fabric to its current vertical geometry outlined by seismicity. Similar fault-parallel alignment trends in receiver function imaged subsurface strikes is seen in other fault zones. The strength of the anisotropic contrasts is highest on the SAF near Parkfield, weaker on fault-crossing arrays in the San Jacinto fault zone, and weakest for a network of stations crossing minor faults in the Mojave, suggesting that the strength of anisotropic fabric scales with the maturity of a fault zone.
In summary, our findings show that anisotropy imaged in conjunction with faults and seismicity illuminates the interaction between inherited fabric and present-day deformation.
Intellectual Merit Building on a previous region-wide effort by the PIs, we conducted a high-resolution and fault-zone focused cross-comparison of the CFM, SCEC-hosted seismicity catalogs, and elements that will contribute to the CRM, in particular rock fabric and shear zones as imaged by teleseismic harmonic receiver functions. During this funding cycle, we focused on three areas: (1) SAF near Parkfield, CA, which includes dense cross-fault network deployments for a high-resolution investigation of near- and sub-fault structure, seismicity, and rock fabric. (2) A band of unusually deep seismicity in the western foothills of the Sierra Nevada and a receiver function image of a strongly anisotropic layer beginning at the Moho and reaching ~10-25 km into the upper mantle in the southern Sierra Nevada region. (3) Seismicity, CFM fault orientations, and receiver function-inferred fabric in the Mojave region. We used newly available data sets to provide constraints for the refinement of the SCEC Community Fault (CFM) as well as the development of the Community Rheology (CRM) models, both in accordance with SCEC5 research goals. This suite of independent data includes the CFM 5.3 (Maechling et al., 2020; Nicholson et al., 2020; Plesch et al., 2020a-b), receiver function (RF) imaging of faults and intracrustal tectonic structural grain (Schulte-Pelkum et al., 2020b), and refined earthquake catalogs that include a template-based catalog (Ross et al., 2019) and updates to the SCSN catalog (Hauksson et al., 2012) including the GrowClust version (Trugman and Shearer, 2017). The synthesis of the CFM and catalog products with our RF deep crustal fabric imaging is an original approach that supports improvements to CFM geometries, particularly at depth, and provides constraints for the CRM’s shear zones and block rheology.
Broader Impacts A strength of this work is the cross-disciplinary nature of the study, which includes data and resources from seismicity catalogs, 3D receiver function imaging, and 3D geologic fault models. Regular interaction between researchers within these subdisciplines led to cross-fertilization between the different CXMs, increasing their value beyond the SCEC community. Improvements to the CXMs can benefit broader society through improved hazard estimates. Results were presented at the 2022 SCEC annual and 2022 AGU Fall meetings.
Exemplary Figure Fig. 1: Seismicity and receiver function results across the San Andreas Fault near Parkfield. Background shows topographic shading. Small dots without an outline are seismicity from the GrowClust double difference catalog, colored by depth. Black lines are surface fault traces from the USGS fault database (the current CFM ends in the center of the map region). Circles are receiver function results displayed at the location of each station. The circle size shows the amplitude of the largest degree-1 (360o periodicity; A1) azimuthal harmonic arrival at each station, colored by the depth of the contrast in anisotropy on the same color scale as seismicity. Red bars show the foliation strike of the stronger anisotropic fabric across the contrast. Black tickmarks show the phase of the A1 arrival; they point downdip if assuming the foliation is stronger below the contrast that generates the conversion. (Vera Schulte-Pelkum, Debi Kilb)