San Andreas Fault May Be Slipping Faster Than Thought as Scientists Map Where California Could Shake Hardest

Aug 23, 2026

Earthquakes • San Andreas Fault • California Seismic Hazard

New research is sharpening California’s earthquake threat from two directions. San José State scientists say the Santa Cruz Mountains section of the northern San Andreas Fault may have a higher long-term slip rate than previously estimated, while a newly published USGS-led study shows that increasingly detailed 3D models of California’s crust can better reproduce where earthquake shaking becomes strongest.

Featured graphic showing the San Andreas Fault across California, a cracked roadway and earthquake shaking research — New research suggests part of the northern San Andreas Fault may have a higher long-term slip rate than previously estimated, while 3D seismic models are improving forecasts of where California could shake hardest.

California’s most famous fault may be carrying more tectonic movement than scientists previously assumed just south of the San Francisco Bay Area.

At the same time, researchers are getting significantly better at modeling what happens after that stored strain is released — tracing how earthquake waves travel through California’s complicated underground structure and identifying where shaking may be amplified.

The two developments are separate pieces of research, but together they attack two of the biggest questions surrounding the San Andreas Fault: how quickly is strain accumulating along the fault, and where could the strongest shaking occur when it eventually ruptures?

A Section of the Northern San Andreas May Be Moving Faster Than Thought

The first finding comes from geologist Kim Blisniuk of San José State University, whose team has been studying the Santa Cruz Mountains section of the northern San Andreas Fault near Saratoga, south of the Bay Area.

Earlier interpretations suggested that the long-term slip rate of the northern San Andreas decreased as the fault extended southward, perhaps because some plate motion was being transferred onto neighboring faults such as the Hayward and Calaveras systems.

Blisniuk’s new reconstruction suggests something different.

The fault’s long-term movement through the Santa Cruz Mountains appears to remain broadly comparable with the San Andreas farther north rather than slowing significantly.

That matters because slip rate is one of the major ingredients scientists use when estimating long-term earthquake recurrence and seismic hazard.

In general, a fault accommodating more plate motion can accumulate the displacement associated with future earthquakes more rapidly than an equivalent slower-moving fault.

But there is an important caveat.

The new San José State results have not yet been formally published or peer-reviewed. Blisniuk has said she expects to present the findings at a scientific conference, and the U.S. Geological Survey has declined to assess the result until the research is published.

So this is not yet a rewritten official earthquake forecast for California.

It is an intriguing new geological measurement that could eventually influence how scientists understand strain distribution across the northern San Andreas system.

Scientists Looked Back About 10,000 Years

One reason the research is attracting attention is the length of the geological record being reconstructed.

Previous slip-rate estimates in parts of the area often relied on records spanning roughly the past 1,000 years.

Blisniuk’s team is attempting to reconstruct approximately 10,000 years of movement.

At Sanborn County Park, where the San Andreas runs directly through the landscape, the researchers examine landforms that have been progressively displaced by repeated earthquakes.

Seasonal stream channels, for example, can preserve offsets created as opposite sides of the fault move past one another.

The team uses high-resolution drone mapping and other surveying techniques to measure those displacements.

Researchers then date exposed rocks using beryllium-10, a cosmogenic isotope produced when cosmic rays interact with minerals near Earth’s surface.

Combining the age of a displaced landform with the distance it has moved allows scientists to estimate an average geological slip rate.

Why a Higher Slip Rate Matters

The San Andreas is a transform plate boundary separating the Pacific Plate from the North American Plate.

The plates are constantly moving, but large sections of the fault can remain locked for decades or centuries while elastic strain accumulates.

Eventually, part of that locked boundary ruptures.

The result is an earthquake.

A higher long-term slip rate does not mean the fault suddenly accelerated this year.

And it certainly does not mean scientists have detected a countdown to the next “Big One.”

Instead, the new work suggests that previous models may have underestimated how much long-term tectonic displacement is being accommodated by the Santa Cruz Mountains section.

The important question is not whether the San Andreas moved faster this week. It is whether scientists have been underestimating its average movement over thousands of years.

Watch: New Research on the San Andreas Fault

But Knowing the Fault Is Dangerous Is Only Half the Problem

Even if scientists knew exactly how large the next San Andreas earthquake would be, another difficult question would remain:

Where would the shaking be strongest?

Earthquake shaking does not spread outward in a perfectly uniform circle.

Seismic waves travel through a complicated three-dimensional crust filled with sedimentary basins, dense rock bodies, fault zones, buried geological boundaries and dramatic changes in seismic-wave velocity.

Two communities located similar distances from the same earthquake can therefore experience very different levels and durations of shaking.

This is one reason scientists increasingly use 3D seismic velocity models rather than treating Earth’s crust as a simplified uniform layer.

New USGS-Led Study Simulated Five Real California Earthquakes

A separate study published in the Bulletin of the Seismological Society of America tested how well several detailed 3D models reproduce earthquake shaking that has already been recorded in California.

Researchers Oliver S. Boyd, Robert W. Graves and Evan Hirakawa simulated seismic waves from five California earthquakes using multiple underground velocity models.

Those included the USGS National Crustal Model, two established Southern California community velocity models and the USGS 3D San Francisco Bay Area model.

The scientists then compared their simulated ground motions with what seismic instruments actually recorded.

Their conclusion was encouraging:

the 3D simulations generally reproduced earthquake ground motion better than conventional broadly averaged ground-motion models.

Even more interesting, the best prediction was often not produced by any single underground model.

In most of the cases studied, averaging the results from several independent 3D velocity models performed better than relying on one model alone.

In other words, scientists may be able to improve earthquake shaking estimates by running the same hypothetical rupture through several different versions of underground California and combining the results.

California’s Hidden Geology Can Amplify Earthquake Shaking

The reason these models matter becomes obvious beneath California’s major urban areas.

Sedimentary basins beneath places such as the San Francisco Bay Area and Los Angeles contain softer material than surrounding bedrock.

Seismic waves can slow down, become trapped, reflect through basin walls and increase in amplitude.

The USGS has long documented how the softness and thickness of near-surface sediments, combined with deeper 3D crustal structure, can strongly influence local earthquake shaking.

That means the simple question:

“How far am I from the fault?”

is not enough to tell you how violently the ground might move.

Fault geometry, rupture direction, earthquake magnitude, depth, soil conditions and the underground path followed by seismic waves can all change the outcome.

Long-Period Shaking Matters for Tall Buildings and Bridges

The new California simulations examined ground motion at periods of approximately 2, 5 and 10 seconds.

These relatively long-period seismic motions are particularly important for large structures.

Tall buildings, long bridges and other flexible structures can respond strongly when the natural period of the structure overlaps with the period of incoming seismic waves.

That makes improved 3D ground-motion modeling potentially important for urban engineering and seismic hazard planning.

A future major San Andreas rupture would not simply produce one uniform “earthquake intensity” across the Bay Area.

Different parts of San Jose, Silicon Valley, the Peninsula, San Francisco and surrounding communities could experience substantially different shaking depending on their position relative to the fault and the geology beneath them.

The Underground Picture Is Improving — But It Is Still Incomplete

The new models are not perfect.

The researchers note that current simulations still simplify or omit features that can affect seismic-wave propagation, including some fault-damage zones, topography and finer-scale underground structures.

But the models are becoming increasingly detailed.

The USGS maintains a three-dimensional seismic velocity model of the San Francisco Bay region extending to roughly 45 kilometers (28 miles) beneath the surface.

The model represents faults, sedimentary basins and different geological units whose physical properties determine how quickly seismic waves travel through them.

An updated version released in August 2026 further refined the elastic properties of parts of the Bay Area model using new borehole and seismic data.

What Would a Major San Andreas Earthquake Actually Look Like?

That remains impossible to know in advance.

The San Andreas Fault system stretches roughly 1,200 kilometers through California and contains multiple segments capable of behaving differently during future ruptures.

A major earthquake could begin on one segment and stop relatively quickly.

Or rupture could propagate much farther.

Modern earthquake models increasingly allow ruptures to jump between or involve multiple faults rather than treating every fault as an isolated crack.

That is especially important in California, where the San Andreas exists within a network that includes the Hayward, Calaveras, San Jacinto and many other active faults.

Explore how those systems fit together in our Earthquakes hub.

This Does Not Mean the “Big One” Is About to Happen

Whenever new San Andreas research appears, the obvious question follows:

Does this mean a giant earthquake is imminent?

No.

Scientists still cannot predict the exact date, time or location of a future large earthquake.

The slip-rate work concerns movement averaged across thousands of years.

The 3D modeling work concerns the physics of how earthquake waves travel through the crust.

Neither provides a short-term earthquake warning.

What they do provide is a clearer picture of long-term hazard.

One important section of the northern San Andreas may be accommodating more tectonic movement than some previous estimates assumed, while scientists are simultaneously getting better at determining how future seismic energy could be distributed across California’s cities.

California Is Not the Only Major Earthquake Threat on the West Coast

The San Andreas is only one part of the tectonic machinery along western North America.

Farther north, the Cascadia Subduction Zone stretches from northern California toward British Columbia and is capable of producing much larger megathrust earthquakes and tsunamis.

Cascadia is fundamentally different from the San Andreas.

The San Andreas is predominantly a strike-slip fault, where plates slide horizontally past one another.

Cascadia is a subduction zone, where the Juan de Fuca Plate sinks beneath North America.

That subduction process also feeds the volcanic arc running through the Pacific Northwest.

Explore that tectonic connection in our guide to the Cascade volcanoes.

The Threat Is Old. The Picture Is Getting Sharper.

California has known for generations that the San Andreas Fault will rupture again.

What remains uncertain is when, how much of the fault will break, how rapidly that rupture will propagate and which communities will experience the most damaging shaking.

The latest research is beginning to sharpen both sides of that problem.

Geological fieldwork suggests the Santa Cruz Mountains portion of the northern San Andreas may have been slipping faster over thousands of years than earlier estimates indicated.

At the same time, sophisticated 3D simulations are becoming increasingly capable of reproducing how seismic energy actually travels through California’s complicated crust.

It is not evidence that disaster is imminent.

It is something arguably more useful:

a clearer view of the machinery that will eventually produce California’s next major earthquake.

Frequently Asked Questions About the San Andreas Fault Research

Is the San Andreas Fault slipping faster than scientists thought?

New research from San José State University suggests that the Santa Cruz Mountains section of the northern San Andreas Fault may have a higher long-term slip rate than some previous estimates indicated. However, these findings have not yet been formally published or peer-reviewed.

Does a higher San Andreas slip rate mean a major earthquake is imminent?

No. Slip rate describes average fault movement over long geological periods and cannot be used to predict when a particular earthquake will occur. Scientists still cannot predict the exact date or time of a major San Andreas earthquake.

Why does the San Andreas Fault slip rate matter?

Slip rate helps scientists estimate how much tectonic movement a fault accommodates and contributes to long-term assessments of earthquake recurrence and seismic hazard. A higher long-term slip rate can affect how scientists model strain accumulation along the fault system.

How do scientists measure ancient movement on the San Andreas Fault?

Researchers study displaced landforms such as stream channels and terraces, map how far they have moved across the fault and determine their ages. Combining displacement with age allows scientists to estimate the fault’s average long-term slip rate.

Why can earthquake shaking vary so much across California?

Seismic waves travel through a complex three-dimensional crust containing sedimentary basins, faults and different rock types. These geological structures can amplify, weaken or redirect earthquake waves, so communities at similar distances from an earthquake may experience very different shaking.

Can 3D earthquake models predict exactly where the strongest shaking will occur?

Not exactly, but they can improve estimates. Recent USGS-led research found that detailed 3D seismic velocity models generally reproduced observed California ground motions better than simplified average models, particularly when results from several different 3D models were combined.

What is the difference between the San Andreas Fault and the Cascadia Subduction Zone?

The San Andreas is primarily a strike-slip fault where the Pacific and North American plates move horizontally past one another. Cascadia is a subduction zone where the Juan de Fuca Plate descends beneath North America, allowing it to generate both very large megathrust earthquakes and tsunamis.