Southern California stands a much greater chance of a huge temblor in the next 30 years than Northern California, according to a statewide earthquake forecast released Monday.
The report, which brought together experts from the U.S. Geological Survey, USC's Southern California Earthquake Center and the State Geological Survey, also found that California is virtually certain to experience at least one major temblor by 2028.
According to the research, the chance of a 6.7 magnitude temblor -- the size of the 1994 Northridge quake -- during this period is 97% in Southern California and 93% in Northern California. The chance of a 7.5 quake -- which shakes at 16 times the intensity of a 6.7 quake -- is 37% in Southern California and 15% in Northern California.
Researchers computed the likelihood of fault ruptures using new information about prehistoric earthquakes, locations of hard-to-spot faults and their slip rates, and an increasing trove of satellite-based GPS data of the Earth's crust movement.
One of the newer techniques the study used was paleoseismology, which involves digging deep into the earth to attempt to learn about ancient seismic behavior. Scientists try to use that data to help forecast quakes.
"The data has been improving, and the way we collect data has been improving," said Thomas Jordan, director of the Southern California Earthquake Center.
The report does not predict where the expected 6.7 quake will hit -- or when. This information is crucial because a temblor in a remote part of California would do far less damage than one in a populated area.
The California Earthquake Authority asked researchers to create a statewide earthquake forecast model that could be used to set insurance rates. The research will be used to update "seismic hazard maps" warning residents and local governments about areas that have the greatest danger of property damage and loss of life in the event of a catastrophic temblor.
In the past, earthquake forecasting of this kind was done on a regional basis, focusing on local fault zones. But this is the first time officials have attempted a statewide assessment that estimates the likelihood of quakes in specific areas.
"What we're presenting is the most comprehensive earthquake forecast ever developed of the entire state of California," said Ned Field, a geophysicist at the U.S. Geological Survey and the lead author of the report. "There's a much higher likelihood of [a 7.5 quake] happening in Southern California than Northern California."
The reason that Southern California is in greater danger is because the region has experienced fewer massive quakes over the last century or so than Northern California.
The 8.0 "Great Quake" of 1906 devastated San Francisco 102 years ago. In contrast, the southernmost end of the San Andreas fault, from Los Angeles to the Coachella Valley, has not had such a stress-relieving quake since 1680.
These mega-quakes happen about every 150 years along the San Andreas fault, so Southern California is overdue, experts said. Southern California also has more known faults than Northern California, according to the report.
The state has been generally less seismically active since 1930, with only about 80% the normal number of quakes. San Francisco and Los Angeles were even less active, experiencing only about half the normal number of quakes during this period, one researcher said.
"We really don't know the reason for that. Probably just because seismicity clusters," said Karen Felzer, a geophysicist and researcher for the U.S. Geological Survey in Pasadena. "At some point we're going to come out of that. We know we've been in a lull, and we don't know why that is. But we know at some point we're probably going to come out of that lull."
The report found that of all the faults in the state, the southern San Andreas, which runs from Parkfield in Central California southeast to the Salton Sea, appears most primed to break.
There is a 59% chance in the next three decades that a temblor the size of the 1994 Northridge quake will occur on the southern end of the fault, compared with 21% for the northern section, according to the research. The report also pointed with concern to two other major faults, the San Jacinto in Riverside County and the Hayward-Rodgers Creek in the East Bay.
There was some mixed news on the San Jacinto fault, which scientists fear could trigger a devastating quake. Research now shows that the chance of multiple "moderate" earthquakes -- those in the 6.7 range -- happening along segments of the fault is smaller than previously believed. But the chance of a 7.5 or greater quake, rupturing along the span of the San Jacinto fault, is now thought to be more likely than before.
The researchers were quick to point out that their forecast does not amount to an earthquake prediction. Moreover, the research focused on the probability of ruptures along the faults, not the potential destruction that can be caused by seismic waves, which do the most damage. Scientists noted that even parts of California that were not marked as the most seismically active could be vulnerable to far-reaching waves.
Still, the researchers expressed hope that their findings would be used to improve seismic codes and boost emergency response plans. There has not been a devastating temblor in urban California since the 1994 Northridge quake, which killed 57 and caused about $40 billion in damage.
In recent years, several pieces of legislation aimed at boosting earthquake safety have failed, and experts worry that households have lapsed in their quake preparedness.
"We're generally frustrated by the pace in which scientific results are incorporated into policy," said David Jackson, a UCLA seismologist who was part of a review panel for the research.
The scientists acknowledged that talk of a "Big One" has been going on for decades, with references to how the San Andreas fault was in the 10th month of its pregnancy.
"You have to realize this is a long pregnancy," Jordan said. "It's been 300 years since the southern San Andreas has ruptured. We expect ruptures to occur on the order of every 150 years. So that makes it a high probability location for a future rupture."Copyright © 2015, Los Angeles Times