Earthquakes of magnitude 6 to 6.9 are considered strong enough to cause widespread devastation across populated areas, according to USGS.
In the Bay Area, just four earthquakes have passed the magnitude 6 barrier in the past hundred years: two magnitude 6.1 temblors in 1926, a magnitude 6.2 in 1982, and the 6.9 Loma Prieta quake in 1989.
Why the relative calm? Geologists believe the 1906 earthquake was so large it released all the pent up stress in the San Andreas and neighboring faults.
As a result, the Bay Area has been in a so-called stress shadow, explained David Schwartz, a research geologist who heads the San Francisco Bay Area Hazards Project with the USGS in Menlo Park.
Schwartz co-chaired a team of geologists, seismologists, and geophysicists that released a 2002 report concluding the Bay Area is due for a magnitude 6.7 or greater quake in about 30 years.
The probability study assumes the Bay Area is still in the stress shadow of the 1906 earthquake. Whether that is true remains uncertain, Schwartz said.
The assessment will be updated next year. If the team decides the Bay Area is out of the shadow, the probability for another major earthquake would increase.
"The seismic activity since 1906 has been amazingly low compared to what it had been before 1906," Schwartz said.
"Looking at the next 30 years, are we still feeling the effect of 1906? Or are we out of that effect of 1906? Or are we just coming out?"
The Bay Area sits on the boundary between the Pacific and North American tectonic plates. These two major pieces of Earth's crust jostle about the surface in fits and starts, creating mountains and trenches in the process.
The Pacific plate slips northwestward relative to the North American plate at a rate of 1.6 inches (40 millimeters) per year.
Stress from this motion accumulates on seven major faultsthe San Andreas and six others that run relatively parallel to each other across the region.
"All that strain is what loads faults and what causes them to break in earthquakes," Zoback said.
About half of the stress accumulates on the San Andreas, which was the source of the 1906 magnitude 7.9 quake.
In general, Zoback said, it takes about 250 years for sufficient strain to build up on the San Andreas to produce an earthquake of equal size.
But in 1838 an earthquake of about magnitude 7 occurred on the San Andreas on the San Francisco peninsula.
"This tells us [that] we could still accumulate enough stress for a magnitude 7 on the San Francisco peninsula" before the next 250-year mark, but a quake as powerful as the 1906 event isn't likely to occur soon, Zoback said.
"It just depends on how the fault decides to behave."
The USGS probability study says the chance that the next big quake will originate in the San Andreas fault, versus other area faults, is about 21 percent.
Of even greater concern, however, are the branches of faults that run roughly parallel to the San Andreas on the eastern side of the San Francisco Bay.
The most prominent is the Hayward Fault and its northwestern extension, the Rodgers-Creek Fault.
"In the analysis we did, the highest probability of an earthquake fell on this fault system an estimate of 27 percent chance by 2032," Schwartz said.
The Hayward fault last ruptured in 1868. The event had a magnitude of about 7 and was known as the Big One until the 1906 quake occurred.
Recent studies on the fault show it has ruptured, on average, every 150 years for the past 11 cycles.
In the last three cycles, the Hayward has ruptured every 135 years, on average. The fault last ruptured 138 years ago.
"Fundamentally, however you slice it, we are in that time window of additional concern," Schwartz said.
The impact of a major earthquake on the Hayward Fault would be devastating, Zoback said.
That fault cuts through the towns of Fremont, Hayward, Oakland, and Berkeleyhome to roughly two million people.
And even if the Hayward Fault doesn't break, there is stress slowly building up on the five other faults in the Bay Area, four of them on the East Bay region.
But unlike the Hayward, none of these East Bay faults have experienced a major earthquake in recorded history, leaving researchers uncertain as to when or if they might rupture.
The uncertainty and tectonic complexity of the region leaves Zoback concerned for northern California residents.
"Because we have seven major faults traversing the core of the Bay Area, and all of them could have earthquakes, there is no safe place in the Bay Area," she said.
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