Study says California is overdue for a major earthquake. Does that mean ‘the big one' is coming?

San Francisco Chronicle​

07-06-2025

Unlike other earthquake-prone places around the planet, California is overdue for a major quake, according to a recent study. But that doesn't mean a catastrophic event like the 1906 San Francisco earthquake is on the verge of striking.
'A fault's 'overdue' is not a loan payment overdue,' said Lucy Jones, founder of the Dr. Lucy Jones Center for Science and Society and a research associate at the California Institute of Technology, who wasn't part of the work.
The new study reported that a large share of California faults have been running 'late,' based on the expected time span between damaging temblors.
The researchers compiled a geologic data set of nearly 900 large earthquakes on active faults in Japan, Greece, New Zealand and the western United States, including California.
Faults are cracks in the planet's crust, where giant slabs of earth, known as tectonic plates, meet. The Hayward Fault is slowly creeping in the East Bay and moves around 5 millimeters per year, according to the U.S. Geological Survey. But sometimes plates get stuck and pressure builds. Earthquakes occur when plates suddenly slip, producing a jolt of energy that causes the ground to shake.
Scientists study ruptured rock layers deep beneath the surface to estimate when large earthquakes occurred in the past. In the new study, the authors collected data stretching back tens of thousands of years. For a region spanning the Great Basin to northern Mexico, this paleoearthquake record stretched back about 80,000 years. For California, the record extended back about 5,000 years.
The scientists used these records to calculate how much time typically passes between large surface-rupturing earthquakes around the planet. The average interval was around 100 years for some sites on the San Andreas Fault; it was 2,100 years on the less famous Compton thrust fault beneath the Los Angeles area.
About 45% of the faults analyzed for California are running behind schedule for a major earthquake, meaning that more time has passed since the last large quake on a fault than the historical average. In the other regions studied, this statistic ranged from 9% to 18%.
The researchers' analysis only included large surface-rupturing earthquakes. It didn't include the magnitude 6.9 Loma Prieta earthquake in 1989, which was below the magnitude 7 threshold that the study authors used for quakes on the San Andreas Fault.
The authors associated seismic punctuality with slip rates, or how fast the two sides of a fault move past each other.
'Our analysis showed that the faster the faults are moving, the more likely it is that they will appear overdue,' said study author Vasiliki Mouslopoulou, a senior scientist at the National Observatory of Athens, in Greece.
In tectonically active California, the San Andreas Fault has a particularly high slip rate. The Pacific and North American plates slide past each other an average of more than inch per year in some spots.
'Faults in California are among the fastest-slipping faults in the world,' Mouslopoulou said, adding that other factors are also probably contributing due to the pattern of chronically late large earthquakes.
Previous studies had also shown that seismic activity has been unusually subdued in California, compared with paleorecords. A 2019 study reported that there's been a 100-year hiatus in ground-rupturing earthquakes at a number of paleoseismic sites in California, including on the San Andreas and Hayward faults.
The authors of the 2019 study treated large earthquakes at these sites as independent events, akin to flipping pennies and counting how many turn up heads. They calculated a 0.3% probability that there'd be a 100-year hiatus in ground-rupturing quakes across all the California sites.
Scientists have suggested that there could be earthquake 'supercycles,' with large quakes occurring in clusters, with less active periods in between.
'There are these longer-term, decadal, century-long ups and downs in the rate of earthquakes,' Jones said. Potentially, California is in a quiet time and large earthquakes are currently less likely.
Katherine Scharer, a U.S. Geological Survey research geologist who wasn't part of the new research, commended the authors of the study, explaining that compiling the paleoseismic records was a 'tremendous amount of work' and will enable more scientists to investigate earthquakes.
California's relatively sparse big earthquake activity could be connected to the geometry of its faults. While the analyzed faults in California were more or less in line with each other, those in other regions resembled 'a plate of spaghetti,' Scharer said.
'From the study, I think you would say that the main California faults are mechanically different somehow than the averages from these other places,' Glenn Biasi, a geophysicist with the U.S. Geological Survey, who wasn't part of the new work.
Biasi emphasized that it's impossible to say if California's faults are truly overdue for a big earthquake.
'The faults slip on their own schedule and for their own reasons,' Biasi said.
Scientists can't accurately predict large earthquakes in advance but paleoearthquake data could help. The authors of the new study found that, excluding California's recent lack of large earthquakes, faults around the entire planet have generally produced surface-rupturing quakes at intervals expected from paleoearthquake and historic records. Considering such data could improve earthquake forecasts, Mouslopoulou said.