"We are fortunate that seismic activity in California has been relatively low over the past century. But we know that tectonic forces are continually tightening the springs of the San Andreas fault system, making big quakes inevitable,” Tom Jordan, director of the Southern California Earthquake Center and a co-author of the study, said in a statement.
This latest assessment, the Third Uniform California Earthquake Rupture Forecast, is the most advanced of its kind. Researchers used two models to make their predictions: one that predicted the likelihood that the ground would slip along California's fault and another that predicted how much the ground would shake in a quake.
It also reflects a better understanding of how seismic activity in one place might influence the likelihood of earthquakes in another place.
“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 lead author and USGS scientist Ned Field in a statement. “This is a significant advancement in terms of representing a broader range of earthquakes throughout California’s complex fault system.”
Speaking to the Associated Press, Field added: "California is earthquake country,
In 1994, the most costly earthquake in U.S. history struck just 20 miles north of Los Angeles. The Northridge quake left 57 dead and more than 5,000 injured. It was just a 6.7 magnitude quake on the Richter scale.
But the damage it left in its wake is a cautionary tale: A significantly more powerful earthquake -- a magnitude of 8.0 or more for example -- near California's more densely populated regions could be catastrophic.
The good news is that while the likelihood of those massive quakes is now 7 percent instead of the 4.7 percent predicted in the 2008 estimate, smaller earthquakes are predicted to occur less frequently.
California is almost certain to experience another 6.7 magnitude earthquake at some point in the next 30 years -- the duration of a typical mortgage. But the frequency of a quake as powerful as the 1994 Northridge episode is now one every 6.3 years instead of one every 4.8 years as the previous model predicted.
In some areas, the predictions of seismic activity are greater than others -- for example, the Southern San Andreas fault near Mojave, Calif. The study predicted that the chances of an earthquake larger than 6.7 magnitude there is 19 percent in the next 30 years.
More up-to-date information -- which can be viewed down to your neighborhood thanks to a handy Google Earth tool -- can help urban planners better prepare and protect communities from the risk of earthquakes.
“This will help all those people working to make society resilient,” Field told the Desert News. “It will help them do a better job of that.”