Almost half of California's faults — including San Andreas — are overdue for earthquakes

California is an earthquake oddball in that the state's faults are more likely to be "overdue" compared with faults elsewhere in the world, new research finds.

The study could help geoscientists make more accurate calculations of when faults are likely to break and cause earthquakes — even in misfit California, said study lead author Vasiliki Mouslopoulou, a research geoscientist at the National Observatory of Athens in Greece.

"The main message to take away is that everyone could benefit, including California," Mouslopoulou told Live Science.

Mouslopoulou and her co-authors were interested in studying the recurrence intervals between earthquakes on faults with a long geological record of their ruptures. Earthquake scientists use these records to calculate the chance of a future quake on the fault by comparing the time since the last known major earthquake to the average time between two large earthquakes on the fault throughout history. If the rocks of the fault show signs of rupturing every 150 years and it's been 200 years since the last earthquake, that quake is said to be "overdue."

Many of California's faults, including the southern portion of the San Andreas, are overdue, Mouslopoulou said. She and her colleagues wanted to compare this overdue pattern to the patterns in other regions of the world.

They gathered records from the state and four other quake-prone regions: Japan, Greece, New Zealand, and the Basin and Range province, which covers much of Nevada, Utah, Arizona and northwestern Mexico. These geological records come from trenches that were dug into fault lines so researchers can see when and where the ground broke in the past.

The study showed that while about 45% of California's faults are overdue compared with the mean duration of their seismic cycle, faults in the rest of the world are early — less than 20% were overdue in each of the other regions studied, the researchers reported April 18 in the journal JGR Solid Earth.

Related: Part of the San Andreas fault may be gearing up for an earthquake

That may be an issue, Mouslopoulou said, because California's earthquake data feed into seismic forecast models for faults around the world. This means the models may be skewed by California's current outlier patterns.

She and her colleagues suggest that to better understand faults globally, researchers may use the elapsed time between the historic rupture and their next-to-last events, rather than the time elapsed since the most recent quake. This method best predicts the earthquake patterns actually seen at faults outside California, Mouslopoulou said.

One reason California is an earthquake outlier is that the San Andreas Fault and the faults around it are fast-slipping faults, meaning that they move quickly and generate earthquakes regularly. That relatively frequent earthquake recurrence means there's a good historical and geological record of quakes on those faults, Mouslopoulou said. In comparison, a slow fault that generates an earthquake every 10,000 years offers far fewer historical comparisons. Geologists will be lucky to glean even one or two earthquake records from that fault, much less to demonstrate that it's overdue for a quake.

California is currently in a quiescent period for quakes, but that likely won't last forever; there's some evidence that San Andreas quakes come in "supercycles," or clusters, Mouslopoulou said. No one knows what drives these patterns, but they may be linked to the giant subduction zone off California's coast where the Pacific Ocean floor grinds under the North American continent. Thus, in the long term, California is not likely to keep up this overdue pattern; if it ends up in a more active phase, it will probably fall closer in line with faults around the rest of the world.

The next step, Mouslopoulou said, is to compare California's fast faults to other fast-slip systems around the world, such as in Turkey's Anatolia region or the subduction zones, such as the one off the coast of Chile.

"It will be very interesting to see how these fast-moving faults and regions compare to the Californian faults," Mouslopoulou said.