Researchers looked at the history of earthquakes in the San Francisco bay area to predict what the next "Big One" will look like.
A "cluster of closely timed earthquakes" that took place over the 17th and 18th centuries put as much stress on the region's faults as the 1906 San Francisco earthquake. A research team believes that means the next big event could go one of two ways, a Seismological Society of America news release reported.
"The plates are moving," David Schwartz, a geologist with the U.S. Geological Survey and co-author of the study, said in the news release. "The stress is re-accumulating, and all of these faults have to catch up. How are they going to catch up?"
In the bay area energy released during earthquakes moves along the "San Andreas, San Gregorio, Calaveras, Hayward-Rodgers Creek, Greenville, and Concord-Green Valley faults," the news release reported.
"The 1906 quake happened when there were fewer people, and the area was much less developed," Schwartz said. "The earthquake had the beneficial effect of releasing the plate boundary stress and relaxing the crust, ushering in a period of low level earthquake activity."
The Bay Area hasn't experienced a full earthquake cycle in recorded history; people, started writing earthquake records in 1776 when the Mission Dolores took down information on damage caused by one of these events.
"We are looking back at the past to get a more reasonable view of what's going to happen decades down the road," said Schwartz. "The only way to get a long history is to do these paleoseismic studies, which can help construct the rupture histories of the faults and the region. We are trying to see what went on and understand the uncertainties for the Bay Area."
The researchers looked at trenches that occurred over faults; they also performed radiocarbon dating on detrital charcoal and looked at non-native pollen. These studies helped researchers determine large seismological events dating back to the year 1600.
The research suggests that between 1600 and 1776 there was a cluster of earthquakes ranging in magnitude between 6.6 and 7.8
"What the cluster of earthquakes did in our calculations was to release an amount of energy somewhat comparable to the amount released in the crust by the 1906 quake," Schwartz said.
The research suggests the Bay Area will either experience either another cluster of earthquakes or a "Big One." A cluster of earthquakes is believed to be more likely.
"Everybody is still thinking about a repeat of the 1906 quake," Schwartz said. "It's one thing to have a 1906-like earthquake where seismic activity is shut off, and we slide through the next 110 years in relative quiet. But what happens if every five years we get a magnitude 6.8 or 7.2? That's not outside the realm of possibility," Schwartz said.
