The Big OneLikelihood of Calif.’s Big One within next 30 years higher than previously thought: USGS

In its Third Uniform California Earthquake Rupture Forecast (UCERF3) — created by dozens of experts in seismology, geology, paleoseismology, earthquake physics, and earthquake engineering — the U.S. Geological Survey predicts a 7 percent chance of a magnitude 8.0 or greater earthquake hitting California within the next thirty years. This is up from 4.7 percent from the last forecast. The reason for the increased estimate is due to better understanding of how different faults are connected.

“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 USGS seismologist Ned Field, the lead author of the report. “This is a significant advancement in terms of representing a broader range of earthquakes throughout California’s complex fault system.”

TheLos Angeles Times reports that for previous forecasts, models generally assumed that earthquakes were confined to separate faults or that long faults ruptured in separate segments. Recent large California earthquakes, however, showed how earthquakes can rupture across faults simultaneously. On 1 October 1987, the magnitude 5.9 Whittier Narrows earthquake struck the Puente Hills fault; three days later, a magnitude 5.6 aftershock hit on a different fault, killing one person and causing property damage in Whittier, Pico Rivera, Los Angeles, and Alhambra.

The magnitude 7.2 earthquake that hit the California-Mexico border on 4 April 2010, directed tectonic stress toward Southern California, putting the region at a higher risk for a future quake. Data showed that the same quake triggered movement on at least six faults, including the Elsinore and San Jacinto faults, both of which run close to heavily populated areas in eastern Los Angeles County and the Inland Empire. Imagery of fault lines and quake patterns at the time showed scientists for the first time that earthquakes can reverse directions and that earthquakes moving along a fault can jump over gaps beyond three miles. “As the inventory of California faults has grown over the years, it has become increasingly apparent that we are not dealing with a few well-separate faults, but with a vast interconnected fault system,” the report read. “In fact, it has become difficult to identify where some faults end and others begin, implying many more opportunities for multifault ruptures.”

The report, released on Tuesday, paid special attention to the southern San Andreas Fault, which is ripe for a large earthquake. This section of the fault has a 19 percent chance of having a 6.7 or larger earthquake in the next thirty years centered in California’s Mojave Desert. The northern section of the San Andreas fault has just a 6.4 percent chance partly due to the recent 1906 earthquake.

While experts are unable to predict the day and time of the next big earthquake, improvements have been made in modeling the possibilities. This recent rupture forecast (UCERF3) evaluated more than 250,000 fault-based earthquakes, while the last forecast considered about 10,000. The latest report reflects data from 300 earthquake faults, the 2007 forecast relied on 200 faults, and the original 1988 report was based on only sixteen earthquake faults.

The new forecasts are not meant to startle the average citizen, but property developers and homeowners should be informed. City planners will consider these predictions when forming new building codes and theCalifornia Earthquake Authority will use the predictions to evaluate insurance premiums. “The message to the average citizen hasn’t changed. You live in earthquake country, and you should live every day like it’s the day a Big One could hit,” Field said. “But what it really does help us do is refine our estimates for those designing critical facilities: hospitals, schools, bridges.”