Paleoseismic records show that the current 100-year hiatus since the last major event on the San Andreas, San Jacinto, and Hayward Faults is unprecedented in recent geologic history.
By Mary Caperton Morton
In 2018, California passed a portentous milestone: It had been 100 years since the last major earthquake struck one of the state’s three most notorious faults. Now a new study analyzing paleoseismic records along the San Andreas, San Jacinto, and Hayward Faults has shown that the 100-year earthquake-free hiatus is unprecedented in the past 10 centuries.
In 2014, University of California, Los Angeles geophysicist David Jackson presented a talk at a Seismological Society of America conference cleverly titled “Did Somebody Forget to Pay the Earthquake Bill?,” which called attention to the lack of major seismic activity at paleoseismic sites around California. At the time, Jackson suggested that the gap could be a normal statistical occurrence if paleoseismologists had systematically overestimated the number of past earthquakes.
Paleoseismic records are gleaned by digging shallow trenches that expose scars from past earthquakes that have ruptured from the depths of a fault to the surface. To determine the likelihood of a 100-year hiatus, Glenn Biasi and Katherine Scharer, both at the U.S. Geological Survey (USGS) in Pasadena, Calif., analyzed previously published paleoseismic records from 12 locations along the San Andreas, San Jacinto, and Hayward Faults.
Biasi and Scharer found that the hiatus is highly unlikely, with a 0.3% chance of being a statistical fluke.
“Statistically speaking, this outcome is highly improbable,” Biasi says. “It suggests that there must be some Earth system properties at work that we don’t fully understand yet.”
The new study, published in Seismological Research Letters, does not attempt to explain the hiatus.
Biasi says one possible explanation is that the eight major earthquakes that occurred between 1800 and 1918 may have relieved all the accumulated strain and set the faults up for a quiet century.
Another possible explanation is that faults throughout the state may be more interconnected than we realize. “Last century, all these faults ruptured together, and now they’re all being quiet together,” Biasi says.
Earthquake forecasts
It’s unlikely that the new study will affect current earthquake forecasts for the next century, says Ned Field, a geophysicist at the USGS Geologic Hazards Science Center in Golden, Colo., who was not involved in the study.
“I don’t think anybody would say this overturns the practical implications of our latest model, but it does point out there might be something missing in our understanding of this system as a whole,” Field says.
One possible impact could influence the Uniform California Earthquake Rupture Forecast, Version 3, developed by Field and used by the USGS to model hazard estimates for the state. The forecast does not take into account the relationships between parallel, adjacent faults, like the San Andreas and Hayward.
“If you have a big earthquake on the San Andreas, another fault that parallels it could be shut down in a way that the model doesn’t presently acknowledge,” Field says.
“Given what we know of the last 1,000 years of activity along these faults, it’s likely that the next century is going to busier than this last century,” Biasi says.
The long-term averages suggest that around three to four major ground-rupturing events should occur throughout the state each century. “These averages mean that the system has some catching up to do, but we don’t know where or when that will happen,” Biasi says.
“We’ve already lived through a 100-year hiatus,” Field says. “Our models include the possibility that California could come out of it with a vengeance.”