San Jose State geologist: Santa Cruz Mountain section of San Andreas may be slipping ~25 mm/yr, raising peninsula hazard questions

Professor Kimberly Blizzuk, a full professor in San Jose State University’s geology department, told a Portola Valley audience that geomorphic mapping and Beryllium‑10 exposure dating at Sanborn County Park and other sites produce higher Holocene slip rates on the Santa Cruz Mountain section of the San Andreas Fault than earlier peninsula estimates.

Blizzuk said researchers measured multiple offset landforms — including older stream offsets of roughly 320 meters, intermediate offsets near 170 ±20 meters and younger debris‑flow offsets around 50 meters — and dated associated deposits to approximately 20,000, 12,000, 8,000 and 2,500 years. Combining those offsets and ages, she reported a Holocene slip rate for the Santa Cruz Mountain section of “about 25 millimeters a year, plus or minus 3 millimeters.” (Kimberly Blizzuk, presentation.)

Why it matters: fault geometry and slip rates are first‑order inputs to earthquake probability models. Blizzuk said the higher rate is roughly 9 mm/yr faster than some earlier peninsula estimates and is consistent with long‑term rates on the north‑coast section of the San Andreas, suggesting peninsula slip may be higher than previously modeled. That shifts how the regional strain budget must be partitioned among adjacent faults and fault strands.

Methods and sites: Blizzuk described mapping of surface faults and displaced alluvial‑fan markers at Sanborn County Park (Saratoga) and explained the laboratory method used to date surface boulders and fan deposits: cosmogenic Beryllium‑10 exposure dating. She emphasized that geomorphic and paleoseismic datasets (which record multiple earthquake cycles) sample much longer time spans than short‑term geodetic measurements.

Unmapped strands and coastal hazards: Blizzuk also described work at Pillar Point Bluff on the San Gregorio Fault, where marine terraces, fault gouge and uplifted bedrock indicate Holocene activity. She said her team has identified previously unmapped fault strands that may link to the Seal Cove fault, and that the San Gregorio strand at Pillar Point shows fractures and surface deflections consistent with possible creep and enhanced landslide risk along the bluff.

Audience questions and local implications: At the Portola Valley event, residents asked about local preparedness, the Alquist‑Priolo Earthquake Fault Zoning Act, and solar‑battery fire risk after earthquakes. Blizzuk praised Portola Valley’s geologic safety commission and recommended community preparedness (emergency kits, evacuation planning and retrofits where appropriate). She summarized the Alquist‑Priolo guidance discussed in the talk: when the active fault trace is well located (Holocene rupture), a development setback of roughly 50 feet from a known fault trace is enforced; when the fault trace is not precisely located, regulatory practice typically applies wider setbacks (commonly 100 feet from each side) pending site evaluation.

On forecasting: Blizzuk reiterated that earthquake scientists forecast probabilities rather than predict exact times. During the Q&A the moderator cited the regional forecast, saying the California earthquake rupture forecast gives roughly a 23% chance of a magnitude 6.7 or greater somewhere in the Bay Area during the next 30 years.

On next steps: Blizzuk and audience members discussed the practical limits of further peninsula work: mapping in developed areas is constrained by private‑property access; she said LiDAR and targeted trenching remain essential and welcomed local committee help to secure access for additional field sites. She urged neighborhoods to coordinate with fire and emergency services about battery‑array signage and turnoff procedures.

The talk closed with an offer by Blizzuk to make virtual field materials available (Google Earth tours) and a committee offer to assist in identifying accessible trenching targets. No formal votes or regulatory actions were taken at the event.