What 'Slipping Faster' Means for the San Andreas Fault
The Santa Cruz Mountains finding is a lesson in geological averages, not a live countdown. Here is what a slip rate records, what it cannot tell us, and why a bent creek matters.
Part of the Weather, Earth and sky collection.
A seasonal creek in Saratoga's Sanborn County Park makes two sharp right-angle turns in quick succession. That odd little zigzag is the sort of landscape clue a hurried walker might never notice. For a field geologist, it can be a remarkably patient record of how ground has shifted over a very long time. Reporting from the park on August 19, 2026 described San Jose State geologist Kim Blisniuk and her students using that kind of clue in work on the Santa Cruz Mountains section of the northern San Andreas Fault.
The headline version is that this section may be slipping faster than earlier estimates suggested. The useful version is more precise: the preliminary work reportedly finds that its long-term average motion may not taper off southward in the way some earlier models assumed. The Chronicle said the work reaches back about 10,000 years, is not yet published, and may be presented at a conference in October 2026. That last detail matters. A preliminary geological result deserves curiosity, not a siren.
The plain-English translation
- Slip rate
- An average amount of motion measured across a stated span of time.
- What is newly reported
- A Santa Cruz Mountains rate that appears similar to the northern section's rate over thousands of years.
- What it is not
- A clock, a date, or a forecast for a particular event.
- Why a creek helps
- Its offset and the age of nearby landforms can turn shape into an average rate.
A geological average is not a speedometer
The word rate tempts people to picture a dashboard with a needle. That is not the job here. In field-based work, researchers find a feature that has been displaced, determine how far it moved, establish the feature's age, and divide one by the other. Blisniuk's San Jose State profile describes this mix of field samples, geochronology, geomorphic mapping, high-resolution topography, and modeling as the way her group measures how a fault moves over time. Her listed research includes a 2015 Sanborn County Park abstract about beryllium-10 dating of an offset alluvial fan.
That makes a slip rate more like the average pace inferred from a very old footpath than the current speed of a runner. It is powerful because it compresses a huge slice of Earth history into a number. It is limited for the same reason. A long record smooths over the individual chapters that made it. The SJSU description of Blisniuk's work puts the human scale back into the science: students collect and process samples, then carry field observations into the lab rather than conjuring a result from a map alone. The university's 2016 account explains that field-to-lab chain.
Why this particular result drew attention
According to the August 19 report, one earlier idea was that motion on the northern San Andreas would decline farther south as some of it was transferred to nearby faults. The preliminary finding instead indicates a similar rate from the North Coast section to the Santa Cruz Mountains section. If later work confirms that interpretation, it changes how researchers apportion long-term motion across that part of the fault system. It does not turn a landscape measurement into a same-day announcement. The U.S. Geological Survey declined to comment on the finding because it had not yet been published. That caveat is in the original report.
The creek is doing the charming part
The most delightful detail in this story is also the most useful one. At Sanborn, Blisniuk's team reportedly uses drones, laser pulses, and thousands of photographs to map how far a seasonal stream has been shifted. They then sample nearby sandstone boulders, whose quartz can be dated with beryllium-10 made as cosmic rays strike exposed rock. Distance plus age gives the long-term average. It is hard not to admire the arrangement: a creek bends, a stone catches particles from space, and a student with careful notes gets a number out of both. The field method is described in the Chronicle's account.
That approach belongs to a broader family of measurements. USGS material distinguishes deep slip rates from shallow surface creep, using GPS velocity fields to help infer the difference. A 2008 study of the central creeping section combined GPS and other data and reported a deep long-term rate of 31 to 35 millimeters a year and a shallow creep rate of 28 millimeters a year for that particular section. Those are not interchangeable measurements, and they are not a number for every stretch of the San Andreas. USGS's overview and the paper record are useful companions to the new field report.
| Term | What it is really describing |
|---|---|
| Long-term slip rate | Average movement inferred across years, centuries, or millennia. |
| Surface creep | Slow movement measured near the fault trace in some sections. |
| GPS velocity | How points on the ground change position through repeated satellite-based measurements. |
| Creepmeter | A local instrument that tracks tiny changes between markers on opposite sides of a fault. |
What happens next is refreshingly ordinary
For now, the next step is the unglamorous one: publication, review, comparison with other records, and debate about the model. That is how a provocative field result earns its place. The USGS already maintains instruments and public data for fault deformation, including creepmeters that take repeated measurements across parts of the San Andreas system. Its monitoring guide shows how many different scales of observation are in play.
The honest reward for paying attention is not a dramatic prediction. It is a sharper way to read a phrase that will keep appearing in headlines. A fault's slip rate tells a long story about motion. A pair of bent turns in a creek can hold a chapter of it. For the close-up science, read why some sections creep quietly and how geologists turn a crooked creek into a rate.
Sources
Every factual claim above traces to one of these. Links open in a new tab.
- San Andreas Fault just south of Bay Area is slipping faster than previously thought, geologist says
- Kim Blisniuk
- Dr. Kim Blisniuk introduction in the SJSU Research Foundation Annual Report
- Earthquake Processes and Effects
- Fault Slip Rates
- Aseismic slip and fault-normal strain along the central creeping section of the San Andreas fault
- Monitoring Instruments
- Can you predict earthquakes?
