SCEC Project Details
| SCEC Award Number | 24004 | View PDF | |||||
| Proposal Category | Individual Research Project (Single Investigator / Institution) | ||||||
| Proposal Title | Evaluating Uplift Patterns Adjacent to Restraining Bends and Steps Along the San Andreas Fault System: Insights from the Gabilan Mesa and East Bay Hills | ||||||
| Investigator(s) |
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| SCEC Milestones | A1-3, A2-2, A3-4 | SCEC Groups | Geology, Geodesy, PBS | ||||
| Report Due Date | 03/15/2025 | Date Report Submitted | 06/23/2025 | ||||
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Project Abstract |
This project investigated uplift patterns and timing within restraining structures along the San Andreas Fault system, focusing on the northern Gabilan Mesa adjacent to a major restraining bend southwest of Hollister, California. Through topographic analysis of river profiles combined with cosmogenic ¹⁰Be erosion rate measurements from six watersheds, we determined both modern uplift rates and the timing of uplift initiation. High-resolution DEM analysis revealed prominent knickpoints at ~610-660 m elevation that separate low-relief plateau surfaces from steep downstream canyons. Cosmogenic ¹⁰Be analyses show plateau erosion rates of 9-31 m Myr⁻¹ contrasted with canyon erosion rates of 64-87 m Myr⁻¹. Combining knickpoint relief with these erosion rate contrasts indicates that accelerated rock uplift began 8-14 Ma in the northern Gabilan Mesa. This timing predates the geomorphically continuous ≤2.8 Ma surface farther south, demonstrating that the Gabilan Mesa comprises diachronous fragments assembled over ~10 Myr rather than a single, synchronous geomorphic surface. These results provide important constraints on restraining bend kinematics and suggest that present-day uplift rates and associated seismic hazard in the northern Gabilan block are modest relative to other Coast Range segments. While the original proposal included analysis of the East Bay Hills, field reconnaissance and logistical constraints led us to focus efforts on the Gabilan Mesa, where we achieved more comprehensive results that address the core scientific objectives. |
| Intellectual Merit | This research advances SCEC's understanding of restraining bend evolution and seismic hazard assessment by quantifying modern erosion rates (64-87 m/Myr) in the northern Gabilan Mesa that are significantly lower than other Coast Range segments (150-600 m/Myr in Santa Cruz Mountains), suggesting relatively modest current seismic hazard. This directly supports SCEC objectives A1-3 and A2-2 through novel geomorphic approaches to regional fault characterization. |
| Broader Impacts | This project enhances SCEC's research infrastructure by developing publicly available cosmogenic datasets and analysis codes, and establishing protocols for future restraining bend studies. The work expands SCEC's geographic reach into Northern California, demonstrating methodological transferability beyond Southern California. Results inform regional seismic hazard models by quantifying relatively modest current uplift rates compared to other Coast Range segments, contributing to public safety through improved hazard understanding. |
| Project Participants | People involved: George Hilley, Aaron Steelquist, Stephan Graham |
| Exemplary Figure |
Figure 2. (A) Shaded-relief elevation map showing location of the SAF and upland, low-relief landscape (approximated here as elevations > 650 m amsl). (B) Location of collected samples, source watersheds, and measured erosion rates. Elevation and Chi profiles for (C) Gabilan Creek, (D) McCoy Creek, and (E) Tres Pinal Creek. In (C-D), dashed lines show location of base-level (lower) and mesa surface (upper) determined using elevation profiles of distance and chi. The middle dashed line in (C) shows a break in the elevation-chi relationship; however, the average slope of the chi-elevation plot is virtually identical in the section above and below this elevation. |
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Linked Publications
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