SCEC Award Number 25205 View PDF
Proposal Category Collaborative Research Project (Multiple Investigators / Institutions)
Proposal Title 3-D structure of the transition from convergent to transform margin in northern California as a proxy for San Andreas Fault System evolution
Investigator(s)
Name Organization
Debi Kilb University of California, San Diego Vera Schulte-Pelkum University of Colorado, Boulder
SCEC Milestones A1-1, A1-2, C1-1, A3-1, A3-2, B2-1, B3-1 SCEC Groups Seismology, CEM, PBS
Report Due Date 03/15/2026 Date Report Submitted 03/13/2026
Project Abstract
 We investigate the onshore transition from the transform plate boundary of the San Andreas Fault System (SAFS) to the convergent plate boundary of the Cascadia subduction zone in Northern California. Details of this transition zone, such as the geometry of the southern edge of the Cascadia slab and the presence and extent of a Pioneer detachment south of the slab edge, are still unknown and are of importance to seismic hazard in the region. Recently published new findings on low frequency earthquakes (LFEs) and tremor clusters south of the previously mapped Cascadia slab tremor band suggest modifications to the slab geometry and Pioneer detachment extent may be needed.
We use shear fabric contrasts as imaged by receiver functions in conjunction with local relocated seismicity catalogs, the SCEC Community Fault Model (CFM), and recent catalogs of low-frequency earthquakes and tremor to investigate these features. We find a peak in the receiver function shear fabric signal at several stations atop the proposed Pioneer detachment that matches the depth of the proposed detachment. Strikes of these arrivals are roughly N-S and similar to that of the motion of the proposed Pioneer fragment relative to the overriding North American Plate. These N-S strikes extend to the East into the region of newly located tremor. Near the transition to the subduction zone, strikes rotate to roughly E-W, similar to the strike of the band of LFEs and a previously imaged low velocity zone. Our findings support the presence of a Pioneer detachment.
SCEC Community Models Used Community Fault Model (CFM), Community Velocity Model (CVM)
Usage Description The CFM was used extensively to put imaged fabric in context with fault surfaces. The CVM was explored for receiver function depth migration, but ultimately the velocity model used for LFE and seismicity relocations was use to internal consistency.
Intellectual Merit This research addresses a gap in knowledge in the transition zone between the two largest fault plate boundary fault systems in the U.S., the Cascadia megathrust and the San Andreas fault system. Improved knowledge of fault geometries and kinematics of shear in the transition zone helps inform impactful earthquake scenarios, a SCEC priority, and has implications for seismic hazard, such as the presence of a large detachment fault under a Pioneer fragment and the influence of fault and possible detachment geometries on coordinated ruptures of the two large fault systems as recently proposed.
Broader Impacts This research contributes to the understanding of seismic hazard in the plate boundary region. The initial project included a component on focal mechanisms visualization with programming tasks for an undergraduate, but this aspect was cut in the reduced funding made available (the undergraduate worked on tasks for a related project instead).
Project Participants PIs were Debi Kilb and Vera Schulte-Pelkum. David Shelly (USGS) and Matthew Herman (Cal State Bakersfield) collaborated on this project. We exchanged results with Bo Rong (UC Berkeley).
Exemplary Figure Fig. 3: Depth slices in the larger inset box in Figure 1, showing the range of the proposed Pioneer detachment. SCEC CFM fault surfaces in semitransparent colors. Seismicity in each depth interval as black dots, and LVZ outlined in black dotted lines, LFEs as magenta squares, all from Shelly et al. (2025). Tremor from Lu and Ide (2026) in pink. Station locations with receiver function results shown as circles; empty where no A1 arrival, blue fill where arrival is present. Circle size scales with amplitude of the A1 arrival. Red bars show strike of fabric, orange tick marks are A1 phase.
Credits: Schulte-Pelkum/Kilb
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