California's San Francisco Bay Area grew into a metropolis during the eerily quiet earthquake gap following its devastating 1906 temblor. Scientists predict a 63 percent chance of another big quake before 2032, but when the shaking starts, it may not be a single "Big One" as in 1906, according to a new study.
Instead, the Bay Area could face a cluster of deadly earthquakes that deliver a series of rapid punches, researchers report today (May 19) in the Bulletin of the Seismological Society of America.
The report is another nail in the coffin for the idea that earthquake faults repeat their behaviors in the same way every time. "The historical record is not sacrosanct," said lead study author David Schwartz, a geologist with the U.S. Geological Survey (USGS). "It's really clear that the frequency of different earthquake sizes varies over time."
Schwartz and his co-authors have pieced together a detailed record of Bay earthquakes back to 1600. The bygone earthquakes come from several sources, such as paleoseismology (digging up evidence of shaking), and historic records from Spanish missions.
Between 1690 and 1776, the Bay Area's most dangerous faults unleashed a series of earthquakes between magnitude 6.6 and 7.8, Schwartz and his co-authors report. (The 1906 San Francisco quake was probably a magnitude 7.9, using modern estimates.) These include the Hayward fault, the San Andreas Fault, the northern Calaveras fault, the Rodgers Creek fault and the San Gregorio fault.
