Portola Valley: Town Center complex demolition may yield valuable geologic data Portola Valley: Town Center complex demolition may yield valuable geologic data (November 09, 2005)

By David Boyce

Almanac Staff Writer

If and when the former school buildings at Town Center in Portola Valley are torn down or salvaged for materials to be used in a new complex of buildings, scientists from the U.S. Geological Survey in Menlo Park will be waiting in the wings with trenching equipment.

Geologists from USGS and from William Lettis & Associates, a Walnut Creek-based geo-technical consulting firm, recently dug and refilled a trench at Town Center, the first of possibly several trenches 10 to 12 feet deep that may be dug and refilled when the land under the complex of buildings becomes accessible.

"Our interest out there is to try to figure out, on that stretch of the San Andreas fault, how often earthquakes occur," said Carol Prentice, a USGS geologist and a principal investigator on the team.

To estimate the location of the fault lines, Ms. Prentice said her team is relying on data prepared for the town by Lettis in 2002, when geologists dug trenches near the softball field and the tennis courts. The resulting map showed most of the 11.2-acre site, particularly the area between Portola Road and the eastern edges of the soccer field and tennis courts, to be subject to surface rupture and deformation.

Bore holes dug at the same time showed no evidence of fault activity under the tennis courts, soccer field, volleyball field and playground area.

The perfect trench at Town Center would show indications of regular seismic activity going back 1,000 to 1,500 years, said Ms. Prentice. "That's a really nice time frame to work in," she said. "You can actually read in the layers of sediment when an earthquake occurs."

But to find that perfect trench, you have to pick the right spot, she added. Several trenches may be necessary. The recently dug trench shows promise, but a thorough analysis could take several more months, she said.

If they do find a usable cross-section of the area's seismic history, the scientists would use carbon dating to narrow the estimate of when the quakes occurred.

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