San Andreas Speed-Up Stuns Scientists

A stretch of the San Andreas Fault running through the Santa Cruz Mountains has been sliding at a faster, steadier pace than scientists thought for decades.

Story Snapshot

San Jose State University researchers measured a 10,000-year slip record at Sanborn County Park near Saratoga.
Lead geologist Kim Blisniuk found the fault’s speed there holds steady instead of slowing down over time.
The new numbers land higher than many older estimates for this section of the fault system.
The finding feeds directly into how experts calculate earthquake hazard for the greater Bay Area.

What The New Dig Site Revealed

Kim Blisniuk, an earthquake geologist at San Jose State University, led a team that examined layers of soil and rock at Sanborn County Park. They studied alluvial fans and debris flows that had been sliced and shifted by the fault over roughly 10,000 years. That long window let them track the fault’s movement across many centuries instead of relying on a short, recent snapshot.

Blisniuk said the pattern surprised her team. Older models assumed the slip rate along this stretch tapered off over time. Her data showed something different. She said the rate “appears to not decrease along the San Andreas Fault” and instead “appears to be constant over this 10,000-year window that we were able to document”. That steadiness matters because it changes how often the ground here has been moving, and by how much.

Why The Santa Cruz Mountains Section Stands Out

Slip rate is simply how fast two sides of a fault grind past each other, usually measured in millimeters per year. Earlier work on the Santa Cruz Mountains segment put that number somewhere between one and three centimeters a year, based on federal geological survey mapping done years ago. Blisniuk’s team’s fresh dig and dating work suggests the real figure runs higher and steadier than those older estimates captured.

This is not the first time a slip-rate number on the San Andreas system has moved. Researchers working the Mission Creek strand in Southern California found a segment slipping far faster than assumed, describing it as “moving much faster than scientists previously thought”. Central California’s creeping section has separately produced right-lateral slip readings near 25 millimeters a year through GPS tracking. Fault science keeps getting refined as better dating tools and longer time windows become available.

How This Fits The Bigger Earthquake Picture

Slip-rate revisions are common across the San Andreas system, not unique to one dig site. Federal surveys already show the fault moving about 34 millimeters a year in central California, dropping to roughly 20 millimeters a year farther north, with peninsula sections running near 17 millimeters a year. Segment-to-segment variation like this is well documented, and a new number from Sanborn County Park fits into that established pattern rather than upending it.

Scientists use these rates to build earthquake recurrence models, the tools that estimate how often a fault segment might rupture and how much strain has built up since the last big quake. A federal geological study on a different California fault section found more than three feet of accumulated “slip deficit” waiting to be released in a future quake. Faster, steadier slip rates at Sanborn County Park could mean the Santa Cruz Mountains segment stores and releases strain differently than earlier models assumed.

What Comes Next For Bay Area Hazard Planning

Independent research groups have already flagged reproducibility as central to trusting slip-rate work, noting that late Holocene measurements on the San Andreas have held up “within measurement uncertainty” when checked against earlier studies. That gives Blisniuk’s team’s approach real scientific footing, even as the specific numbers get folded into broader hazard models used by state and federal agencies. Homeowners and local governments in the South Bay should expect this data to shape future building codes and emergency planning discussions.

The Santa Cruz Mountains sit close to millions of Bay Area residents, and any update to how fast that ground is actually moving carries real weight beyond academic circles. Accurate slip-rate data helps engineers, insurers, and emergency planners make decisions grounded in the best available science rather than decades-old assumptions. That is exactly the kind of rigorous, fact-driven groundwork taxpayers should want their public universities producing.