SCEC Award Number 24141 View PDF
Proposal Category Collaborative Research Project (Multiple Investigators / Institutions)
Proposal Title Greater Bay Area fault junction geometry, stress state, and sub- and near-fault rock fabric from machine learning-based focal mechanisms combined with anisotropic receiver functions and seismicity
Investigator(s)
Name Organization
Debi Kilb University of California, San Diego Vera Schulte-Pelkum University of Colorado, Boulder
SCEC Milestones A1-1 SCEC Groups Seismology, CEM, SDOT
Report Due Date 03/15/2025 Date Report Submitted 03/16/2025
Project Abstract
 This project targeted the geometry of a major fault junction in the San Andreas plate boundary system, the Hayward-Calaveras junction in the East Bay fault system. We combined constraints on stress and geometry from seismicity, rock fabric contrasts imaged with anisotropy-aware receiver functions (RF), and machine-learning (ML) improved focal mechanisms to investigate the Hayward-Calaveras fault junction. The SCEC Community Fault Model, based on the 3D Bay Area fault model in this area, shows the Hayward fault merging with the Calaveras fault at depth. Our results show fabric contrasts that appear to track the projection of the merged steeply dipping fault plane to below the brittle-ductile transition. They suggest that a shear zone forming the downward extension of the merged fault plane exists below the seismogenic depth range to depths of close to 30 km. The implication is that the Hayward-Calaveras in the East Bay fault system may be a crustal scale feature. Foliation strikes at the imaged fabric contrasts are parallel to near-parallel to the strikes of the fault surfaces. A change in imaged strikes to near fault-perpendicular at the latitude of a pronounced shallowing of the fault junction line in depth suggests complexity in the deformation signature at depth due to the 3-D geometry of the fault junction line.
SCEC Community Models Used Community Fault Model (CFM)
Usage Description The SCEC CFM was used in 3-D representation of the subsurface structure in combination with seismicity, focal mechanisms, and receiver function strikes, depths, and amplitudes. The results are relevant to shear zone geometries in the SCEC CRM.
Intellectual Merit This work combines observations from seismology and geology to constrain features in the SCEC Community Fault Model and contributes to the SCEC Community Rheological Model. It uses an original method (anisotropic contrast imaging with receiver functions) to extend fault observations to below seismogenic depths. Imaging below the brittle-ductile transition identifies the deep roots of faults and shear zones and contributes to understanding of the interactions between faults and their roles in the plate boundary system.
Broader Impacts Results from this work were used in teaching activities at UCSD. Constraints from this work can be incorporated into SCEC Community Earth Models. Results were presented at the SCEC annual meeting, at the Fall AGU meeting, and at the Northern California Earthquake Hazards workshop in an invited talk. Improved understanding of deep fault geometry and fault interactions in the deep crust enhances seismic hazard assessments.
Project Participants Deborah Kilb (UCSD - Scripps Institution of Oceanography) – Seismicity
Vera Schulte-Pelkum (University of Colorado Boulder) – Receiver functions
Thorsten Becker (UTIG & DGS, JSG, UT Austin) – Interpretation
Jeanne Hardebeck (USGS, Moffett Field, California) – Focal mechanisms
Robert Skoumal (USGS, Moffett Field, California) – Focal mechanisms
Exemplary Figure Fig. 4. 3-D views of Bay Area fault model, seismicity, and receiver function conversions. View from SSE; Hayward in green, Calaveras in brown (yellow - additional SCEC CFM faults). Seismicity shown as grey dots. Spheres are receiver function first azimuthal harmonic conversions plotted under each station location at converting depth (color, see Fig. 2), scaled by amplitude of the first azimuthal harmonic (Fig. 2); red bars show foliation strike, brown tick marks plunge of anisotropy. Conversions outline shear zones continuing below the seismogenic portions of the faults. Strikes are subparallel to fault surfaces except for the region of the constraining wedge.
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