A new report released by the U.S. Geological Survey (USGS) states that California’s likelihood of experiencing a magnitude 8.0 or larger earthquake in the next 30 years has increased from 4.7 percent to 7 percent. Scientists believe the last such mega-quake to hit Southern California was a magnitude 7.9 in 1857, according to the Los Angeles Times.
The Third California Earthquake Rupture Forecast (UCERF3), released Tuesday, was the follow-up to the previous report from 2008. Part of the reason the chances of a high-magnitude earthquake increased since 2008 is that scientists are learning that fault lines are interconnected. “The new likelihoods are due to the inclusion of possible multi-fault ruptures, where earthquakes are no longer confined to separate, individual faults, but can occasionally rupture multiple faults simultaneously,” said Ned Field, USGS scientist and lead study author, in a statement.
Scientists have witnessed earthquakes jumping fault lines before. The Los Angeles Times reports that the largest, most recent quakes in California, including the 7.2-magnitude temblor on Easter Sunday 2010 near the California-Mexico border, all pushed beyond fault boundaries. That earthquake and its aftershocks triggered movement on six other faults, including two that run near heavily populated areas of Los Angeles.
Good news from the report is that the estimated rate of earthquakes around magnitude 6.7 has dropped by about 30 percent, meaning that California can expect about one every 6.3 years instead of every 4.8 years.
The forecast pegs the most likely location for the next big quake as the section of the San Andreas Fault that runs through the Mojave Desert—but the study authors can’t predict when it’ll hit. “We are fortunate that seismic activity in California has been relatively low over the past century,” co-author Tom Jordan said in the statement. “But we know that tectonic forces are continually tightening the springs of the San Andreas fault system, making big quakes inevitable.”
“The message to the average citizen hasn’t changed,” Field told the Los Angeles Times. “You live in earthquake country, and you should live every day like it’s the day a Big One could hit.”