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Expert: San Andreas strand near Portola Valley may be slipping faster than thought
Summary
At a Portola Valley Geologic Safety Committee meeting, Professor Kimberly Blizzuk summarized field dating and geomorphic evidence indicating the Santa Cruz Mountain section of the San Andreas fault is slipping about 25 ± 3 mm/yr—higher than earlier peninsula estimates—and discussed implications for local hazard assessments and preparedness.
Kimberly Blizzuk, a professor of geology at San Jose State University, told the Portola Valley Geologic Safety Committee that new field-based dates and geomorphic mapping indicate the Santa Cruz Mountain section of the San Andreas fault has a Holocene slip rate of about 25 millimeters per year, plus or minus 3 millimeters.
Blizzuk said her team dated offset alluvial fans and debris-flow deposits at Sanborn County Park using cosmogenic beryllium‑10 exposure dating, then compared measured offsets to deposit ages to calculate slip rates. “This section of the fault is slipping at about 25 millimeters a year, plus or minus 3 millimeters,” she said. She added those higher long‑term geologic rates are consistent with data from northern sections of the San Andreas and suggest the peninsula segment’s long‑term slip might be faster than previously estimated.
Why it matters: slip rate and fault geometry are principal inputs to earthquake‑probability models. Blizzuk said higher slip rates on the Santa Cruz Mountain section would change hazard calculations for the region and should prompt review of local preparedness and planning assumptions. She cautioned that mapped faults are typically broader zones rather than single lines, so setback and land‑use decisions should factor a fault zone's width as well as trace location.
Methods and evidence: Blizzuk summarized field mapping of offset channels and fan surfaces, showing examples of offsets of roughly 50–320 meters for deposits dated at about 2,500 to 20,000 years ago. She explained how combining minimum and maximum offset reconstructions with deposit ages yields long‑term slip‑rate estimates.
Public questions and local implications: Residents and committee members asked what the findings mean for local setback rules and emergency planning. Blizzuk summarized the state Alquist‑Priolo act framework in general terms—noting that confirmed active faults that ruptured within roughly the last 11,000 years trigger special review and that recognized setbacks are used to avoid placing structures directly across known rupture traces—but she emphasized the committee should check legal specifics with town planning staff. On household preparedness she advised standard measures—emergency kits, water, food, and neighborhood coordination—so that a major earthquake does not become a catastrophe.
Committee response and next steps: Committee members said the presentation raises new questions for the town’s Safety Element comments and for local mapping priorities; several members suggested identifying priority sites on private property where additional trenching or mapping could clarify fault traces, and offered to help secure property access for target investigations.
The presentation and Q&A were part of the committee’s meeting agenda; no formal action followed the presentation itself. The committee adjourned after the speaker’s Q&A.
