How to Explain California’s Recent Earthquake Hiatus?

25 April 2019–There have been no major ground rupturing earthquakes along California’s three highest slip rate faults—the San Andreas, the San Jacinto and the Hayward—in the past 100 years. At the 2019 SSA Annual Meeting, researchers discussed why this “hiatus” might exist, and what we might expect for California earthquakes over the next century.

“There have been other big earthquakes in California in the past 100 years, but they haven’t happened in the places that we would have expected them, the really fast-moving faults,” says U.S. Geological Survey seismologist Morgan Page. “Given the rates at which we see past events on these faults, we would expect there to be something in the past 100 years, if they were just going off randomly.”

Between 1800 and 1918, there were eight large ground-rupturing earthquakes (magnitude 6.5 or larger) along the faults, including the well-known 1906 earthquake in San Francisco and the similar-sized 1857 rupture of the San Andreas in southern California.

Morgan and her colleagues have analyzed the statistical significance of the current hiatus with the idea that earthquake timing between these major faults may be correlated. They conclude that the hiatus is unlikely but may occur if earthquakes are clustering in time. Such clustering might occur if rupture of one fault triggers rupture on others, or if a “stress shadow” effect occurs, where large earthquake rupture lowers the overall stress on faults in a region for years to come.

By adding known effects such as stress shadows and clustering to the gold standard earthquake prediction models for California, Page says, “you can get occasionally centuries where there are not any earthquakes at these places, although it’s still not extremely likely.”

Researchers can’t predict when earthquake clusters may occur again, she adds, but if clustering is occurring, “it’s likely that we’re going to have a lot of earthquakes when they do start happening again.”

At the meeting, USGS seismologist Kate Scharer will also present results of a recent study published in the journal Seismological Research Letters on the unlikely nature of the California hiatus. The analysis by Scharer and her USGS colleague Glenn Biasi concludes that the current hiatus has no precedent in the last 1000 years of paleoseismic records along the three major faults.

They agree that the next 100 years of California earthquakes along these faults could be a busy one. “If our work is correct,” Scharer and Biasi note, the next century isn’t going to be like the last one, but could more like the century that ended in 1918.”