In “Earthquake Country,” California’s three major fault lines—the San Andreas, the Hayward and San Jacinto—have experienced an unusually calm century of inactivity, a new study shows.
Of course, California has plenty of earthquakes: about 10,000 annually, although most are too small to notice, occurring on more than 500 active fault lines. Earthquakes with magnitudes measuring between 2.5 and 5.5 on the Richter scale can be felt and quakes between 5.5 and 6.5 may cause minor damage to homes and roads. But major ground-rupturing earthquakes with magnitudes of 6.5 or higher can lead to serious destruction and deaths.
There hasn’t been a major quake on the state’s three major fault lines since 1918, and this strange 100-year earthquake hiatus may be setting the state up for some serious shaking in the forthcoming century, reports Amy Graff at SFGate.com.
Seismologists looked at 1,000 years of data for the San Andreas, the Hayward and San Jacinto fault lines. The team found that earth-splitting temblors magnitude 6.5 or higher typically take place along these faults three to four times a century. But in the last 100 years, the quakes have stopped. It turns out, such a pause is extremely uncommon, with just a 0.3 percent chance of occurring, according to the new study published in the journal Seismological Research Letters.
There have been major earthquakes in California in the last century on other fault lines, however, Graff reports. The 6.9 magnitude Loma Prieta shaker that took place in 1989 was a major disaster in the San Francisco Bay area which killed 67 people and caused $5 billion in damage. The massive 6.7 magnitude Northridge Earthquake in 1994 in the San Fernando Valley, which killed 57 people. The epicenter of Loma Prieta was not on a major fault, but rather a smaller parallel fault to the San Andreas. Meanwhile, Northridge was caused by a previously unknown blind thrust-fault deep underground. Other 6.5 quakes have also been recorded, but not where scientists expect to see them.
“We’re unusually quiet,” co-author Glenn Biasi, a geophysicist at the U.S. Geological Survey tells Stephanie Pappas at LiveScience. “The biggest faults and the faults carrying most of the slip have not ponied up.”
Pappas reports that the study began after David Jackson, a professor emeritus at the University of California, Berkeley, put on a talk at a conference in Alaska called “Did Someone Forget to Pay the Earthquake Bill?” in 2014. He questioned whether it was possible that data collected on earthquakes in California could be wrong, since it seemed unlikely the state could go so long without a big shake.
That’s when Biasi and Katherine Scharer, co-author of the new study and a paleoseismologist for the United States Geological Survey, began their work, Pappas reports. After analyzing 1,000 years of seismic information, however, they are confident that the data from the past century is correct, and the earthquake pause is real.
So what does the pause mean? It’s possible that the faults were simply worn out from all the slipping they did in the 19th century. Graff reports that there were 8 major ground-rupturing shakers along all of the major faults between 1800 and 1918, including the 7.8 San Francisco Earthquake of 1906 and a similar sized disaster at Fort Tejon in 1857.
“We had the flurry of very large earthquakes from 1800 to 1918,” Biasi says in a press release. “It’s possible that among them they just wrung out—in the sense of wringing out a dishrag—a tremendous amount of energy out the system.”
But pressure at the faults eventually builds back up. “We know these big faults have to carry most of the [tectonic] motion in California, and sooner or later they have to slip,” Biasi says. “The only questions are how they're going to let go and when.”
While that doesn’t mean a big shake is imminent, Pappas reports that statistically speaking a large earthquake should be forthcoming. Over the next century, Biasi says it would be reasonable to expect about six big quakes based on precedent. “If our work is correct, the next century isn’t going to be like the last one, but could be more like the century that ended in 1918,” he tells Graff.
Then again, there’s the possibility that underground features we don’t understand are syncing up the earthquake faults or causing changes below California. That's why the team is asking other seismologists to look into the century without earthquakes as well.