GUEST OPINION: Fault still active at Portola Valley's Town Center GUEST OPINION: Fault still active at Portola Valley's Town Center (December 17, 2003)

By Rowland W. Tabor

As a long-time resident of Portola Valley and a geologist (U.S.G.S, retired), I have served on several committees-the permanent Geologic Safety Committee, the Ad Hoc Committee to Re-evaluate the Setback along the San Andreas Fault, and the recently established Ad Hoc Town Center Citizens Advisory Committee -addressing the problem of the Town Center's location astride the active Fault Zone.

Bob Brown's October 15 letter to the Almanac characterizes the eastern Trancos Trace of the San Andreas Fault as inactive. Although geologists have not all agreed on interpretation of features revealed in trenches dug to the south some years ago across the Trancos Trace, officially it is still considered active.

Very costly deep trenching might prove the fault inactive, but until we know, the fault rightly remains on the town's zoning map. Thus the No-Build-Zone encompasses almost all of the present buildings and adjacent lands to the east. New public buildings in this zone would not be allowed by the state.

Even if the Trancos Trace were officially deleted from the zoning map, the No-Build-Zone associated with the complex deformational zone of the western Woodside Trace would still include most of the existing buildings.

Although retrofitting the present buildings at great expense would not technically violate the Town's own zoning laws and would probably be allowed by the state, a major earthquake might well make the buildings unusable due to ground movements that would tip or warp them.

Furthermore, architects and structural engineers consulted by the Town Council and the aforementioned committees emphasize that new construction is cheaper than remodeling existing buildings, especially if they are brought up to standards able to withstand offsets. Buildings just outside of the actual zone of ground deformation, even though subject to mighty shaking during a major earthquake, can be designed to resist collapse.

The Town Council is taking steps to rebuild the Town Center with increased safety, as outlined in Mayor Ted Driscoll's excellent Almanac article October 8. We should support the council's efforts now. During many years of deliberation, the Pacific Tectonic Plate has continued moving north at about 1.6 inches a year relative to the North American Plate. From the east side of our town to the west side, the crust has been stretching across the fault. Eventually the crust will break, producing a major earthquake. We should prepare for it now as best we can.

Rowland W. Tabor is a Portola Valley resident and a retired geologist for the U.S. Geological Survey.

E-mail a friend a link to this story.