BART study sees potential quake damageBy Betsy Mason
The BART commute between Oakland and Concord traverses some treacherous territory, crossing as many as six active faults that could potentially warp or break the tracks in the event of a big earthquake.
Danger from the Hayward fault in the Berkeley Hills Tunnel is well known, but a new study commissioned by BART found a zone of at least four faults between Lafayette and Walnut Creek about which little is known.
The study found that BART could see three feet or more of movement in the Berkeley tunnel if there is a big quake on the Hayward fault. Recent work by the U.S. Geological Survey uncovered traces of 11 big earthquakes on that fault that occurred roughly every 150 years over the past 1,650 years. The last big quake there was 137 years ago, so it may be ready for another.
Geologists Keith Kelson and Stephen Thompson of William Lettis and Associates Inc. in Walnut Creek estimated how far the BART line might be displaced by an quake about a magnitude 7 on the Hayward and Concord faults, as well as on the four new faults together, which they dubbed the Contra Costa shear zone.
The Contra Costa shear zone doesn't yet appear on any published geologic maps, but Kelson and Thompson believe it could contain at least four faults that cross beneath the Pittsburg/Bay Point line and Highway 24.
"These are potentially active," Thompson said Tuesday at the 100th Anniversary Earthquake Conference in San Francisco, where he presented the research.
It's possible the shear zone faults join together into one main fault deeper in the earth. And it's likely they are a splintered northern continuation of the Calaveras fault, which runs from Hollister to Danville. The geologists are nowy hunting for clues as to whether these faults have moved in the last 10,000 years.
"The most likely event would be one that would nucleate on the Calaveras fault and rupture north through the Contra Costa shear zone," Thompson said.
If the shear zone has a major quake, all four faults could potentially move for a total of four feet or more. There is an outside chance that there would be as much as 13 feet of total slip, but the geologists' best guess is that it would be more moderate.
One of those faults, the Reliez Valley fault, crosses beneath BART and Highway 24 at the Pleasant Hill Road overpass. Thompson thinks it is probably the biggest of the four and would slip the most, perhaps a foot and a half or more. Movement on that fault alone would be unlikely to be much more than five feet.
Another of the shear zone faults, called the Saklan fault runs beneath the BART tracks where they cross Interstate 680 just before entering the Walnut Creek station. Thompson expects this fault would move less than the Reliez Valley one.
BART engineers are now assessing how existing track will handle all this movement.
In Concord, BART runs across the Concord fault precisely where it crosses Systron Road. Fortunately, this fault prefers to creep slowly rather than break in big quakes, and poses more of a maintenance issue. Evidence of its slow movement can be seen in curb cracks on Systron road.
"The continuing creep is already putting some visible strain on the bridge," Thompson said.
Copyright ©2006 Contra Costa Times. Published 04/18/2006.