Studies See Big Quake as More Likely
A new analysis of the San Andreas fault that peers thousands of years into its turbulent past suggests that sections in the Inland Empire and Palm Springs area may be more primed for a major earthquake than previously believed.
The new work -- already the subject of dispute -- also indicates that the San Francisco Bay Area could soon emerge from the relative seismic quiet that has lasted since the magnitude 7.1 Loma Prieta quake in 1989.
The findings are included in 14 reports published today after years of study intended to help scientists forecast the next big shakers -- greater than magnitude 7 -- along the dreaded California landmark. One of the studies looked back 1,500 years at more than a dozen earthquake cycles -- a far richer information trove than the two or three cycles that earthquake archeologists typically find.
The Palm Springs study found that a stretch of the fault there, which once experienced temblors on average every 215 years, has not ruptured in 326 years -- indicating that much stress has accumulated from the movement of plates deep in the Earth. The research team believes this means that a “big one” could be more than a century overdue in that area.
A similar though even less conclusive pattern was seen on the San Bernardino strand of the San Andreas running through the now-highly populated Inland Empire, which ruptured during the 1600s, and not in 1812 as previously assumed.
“We already knew we had a high seismic risk in the Inland Empire,” said Sally McGill, a Cal State San Bernardino geologist. “This isn’t a call for new alarm. It’s just a reminder to stay prepared.”
The findings, in the Bulletin of the Seismological Society of America, are the work of 74 paleoseismologists who have spent years -- even decades -- digging deep trenches into the heart of the San Andreas fault and scrutinizing disordered layers of mud, peat and rock.
Those layers provide a record of past earthquakes. The scientists’ work is an effort to compile the most detailed record yet of the fault’s antics to see if quakes have occurred in regular, predictable patterns.
If they do, many geologists say, there would have been a large earthquake by now in tiny Parkfield in southern Monterey County, where major rumblings seemed to have occurred with striking regularity -- on average every 22 years -- since 1857. The last quake was in 1966. The U.S. Geological Survey has spent tens of millions of dollars there to “trap” the major quake they were sure would strike by 1988. They are still waiting.
“We disproved that [theory] in Parkfield, publicly and with great embarrassment,” said Lucy Jones, a seismologist who heads the survey’s Pasadena office. She was not involved in the newly reported research.
Jones is among those who think that earthquakes occur randomly and that past activity is not a significant factor.
“We know it’s not perfectly periodic,” said Ned Field, another seismologist at the survey’s Pasadena office, who is leading a project to produce more precise hazard maps. “The question is whether it’s periodic at all.”
If quakes are random, Field said, they may be triggered by small, unpredictable movements on faults that generate larger quakes, or by movement on neighboring faults.
Time elapsed since past quakes is not factored into the state’s earthquake hazard maps, Field said. The official forecast is for an 85% chance of a magnitude 7 or higher Southern California quake in the next 30 years on the San Andreas or one of the smaller faults.
The new findings may help refine estimates of the earthquake risk on segments of the San Andreas. But the results are not expected to dramatically alter the assessment of overall seismic risk in Southern California.
Still other explanations have been offered for why the strands of the San Andreas near Palm Springs and San Bernardino have not ruptured in 300 years.
The pressure that produced them may have changed, Jones said. Shifting tectonic plates under the Earth’s crust may be transferring stress to the newer, 1-million-year-old San Jacinto fault, a San Andreas offshoot.
Although the 20-million-year-old San Andreas is the best known and most studied fault on Earth, it remains a mystery. That’s because the written record of its earthquakes is just 200 years old and includes only a handful of magnitude 7 or larger quakes.
“We’ve only been here a few centuries. It’s sort of like we just met the San Andreas and we’re trying to figure out what our neighbor is going to do,” said Lisa Grant, a paleoseismologist at UC Irvine who edited the edition of the journal containing the new research.
To test the idea that earthquakes occur regularly, scientists estimate that they need data on about 30 past quakes on the same stretch of the San Andreas, a record that would probably stretch back 4,000 years. They are nearly halfway there.
The research published today includes information on a trench dug near Wrightwood in the eastern San Gabriel Mountains and studied for more than a decade. What scientists see there is 1,500 years of history, during which there were at least 14 major earthquakes.
Digging into a fault’s past becomes harder as seismologists go deeper. Sediments get smashed, thin and illegible. Ground squirrels dig holes in the layers. Storms turn the trenches into mud pits.
Work on the trench by the Geological Survey’s Tom Fumal and the University of Oregon’s Ray Weldon, both geologists, is considered a monumental achievement by many of their colleagues -- even those who don’t agree with the conclusions.
Although the Wrightwood area has had a quake about once every century through much of its history, the section of the San Andreas there has not ruptured for 215 years, Fumal said. “It’s really starting to approach the longest time we see between earthquakes. That’s true of all locations from Wrightwood south. To me, it says that whole part of the fault is ready to rupture.”
He found that many fissures on the central and southern portions of the fault were 150 miles long or more -- much longer than previously assumed -- and probably caused by big quakes. Based on this, he said, “It looks like when it does rupture, it’s going to be a large one.”
Another of the 14 studies, focusing on newspaper articles and damage reports from the early California missions, provides a wealth of new data on the fault’s relatively recent past.
In the last 200 years, the Bay Area has experienced spikes in earthquake activity in decades leading up to its major temblors. After the big quakes, the area has been still, with no magnitude 5 or larger quakes. The region has been in one of those lulls since 1989, when the 7.1 Loma Prieta quake struck.
Tousson Toppozada, the California Geological Survey seismologist who led the research, said it appears to be time for the shaking to resume.
Current forecasts say there is a 70% chance that a large quake will occur on one of the faults in the Bay Area in the next three decades.
Toppozada also found that the four largest earthquakes on the San Andreas have occurred in geographic pairs that ruptured overlapping parts of the fault.
The southern portion was hit by a 7.5 magnitude quake near Wrightwood in 1812 and an 8.0 temblor at Ft. Tejon, near the Grapevine northwest of Los Angeles, 45 years later. In the north, the great 1906 San Francisco quake, magnitude 7.8, occurred just 68 years after a 7.0 quake on the San Francisco Peninsula.
Toppozada has no explanation for the apparent pairings. But it does make him think that the Loma Prieta shaker has a twin quake yet to come -- albeit possibly decades away.
As for that nagging question of why the much-expected quake at Parkfield hasn’t happened, Toppozada now thinks he has the answer: That stretch of the fault may not be so regular after all.
By including reports of Parkfield earthquakes that had been dismissed or overlooked earlier, Toppozada found that there was no 22-year pattern. Instead, he said, the rate and force of earthquakes there have been declining since 1857.
He thinks this is because Parkfield sits at the very end of the 225-mile rupture from the Ft. Tejon quake. Such a point is typically a site of high seismic stress. Each earthquake has relieved some of that stress, he said, and that segment of the fault may no longer be vulnerable. “It’s a pretty exciting finding,” he said. “A Parkfield quake may not happen after all.”