Four highly stressed seismic faults in the Bay Area’s densely populated San Andreas system are moving on the surface and could rupture in a major earthquake at any time, according to scientists tracking their movements.
The faults include the Calaveras, which runs roughly from Hollister (San Benito County) to Danville; the Hayward, between Suisun Bay and San Jose; the Rodgers Creek fault in southern Sonoma County, and the Green Valley fault near Richmond and Fremont.
”The extent of fault creep controls the size and timing of large earthquakes, and measuring that creep rate helps tell us how much strain is building up on the faults underground – although it can’t tell us when a fault will rupture in a quake," said geophysicist James J. Lienkaemper of the U.S. Geological Survey in Menlo Park who led the study.
Lienkaemper and his colleagues have been analyzing the rates of those tiny continuous movements along Northern California faults for years. They move at only a few millimeters a year, but their effects over the years can be visible in roadside cracks and offset curbs.
The surface movements of a fault can relieve a certain amount of stress, but deep underground, the faults can remain locked for centuries until they finally rupture in a full-scale, damaging earthquake, he said.
The scientists’ report was published Monday in the Bulletin of the Seismological Society of America.
They estimate that the next major quake on any of the four faults could be comparable to the 6.9- magnitude Loma Prieta quake 25 years ago, which killed 63 people and caused $6 billion in damage. Lienkaemper and his colleagues said the Hayward fault, long considered the most likely to rupture in the near future, could eventually trigger a magnitude 6.8 quake. The Calaveras fault could also rupture with a magnitude of 6.8; the Rodgers Creek and the Green Valley fault could each yield a magnitude of 7.1.
The Hayward fault last ruptured in 1868, with the magnitude now estimated at 6.8, while the Rodgers Creek fault last broke in 1745, according to evidence uncovered by trenches scientists have dug in the area. The last known major earthquake on the Calaveras fault struck about 1740, and on the Green Valley fault, the last rupture occurred about 1609.
”Given how long ago they had their last earthquakes, they are more than ready to produce a major earthquake again now," said Roland Burgmann, a geophysicist and expert on crustal deformation at UC Berkeley who was not involved in Lienkaemper’s report.
For the past 35 years San Francisco State University has maintained a "Fault Creep Monitoring Program" that enlists students, technicians and faculty to measure the barely perceptible signs of fault creep using surveyors’ tools called theodolites and – more recently – GPS instruments.
Lienkaemper now leads that project and his team’s report is based on the project’s measurements of creep rates over the years on many faults, from quake-prone Parkfield on the San Andreas fault in southern Monterey County to Humboldt County where the fault veers out to sea and joins undersea faults in what scientists call the Mendocino triple junction.
Lienkaemper’s colleagues include Robert W. Simpson of the USGS and Forrest S. McFarland and John Caskey of San Francisco State University.