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Digging Up the Truth: 1906 Earthquake Mystery Solved

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In 1906, a giant San Andreas Fault earthquake carved a trench through Portola Valley, now a small California village of million-dollar homes tucked in the hills above Stanford University.

More than a century later, geologists believe they’ve proved the San Andreas Fault plowed alongside the valley’s western ridges, ending decades of debate caused by a poorly reproduced historic map.

“We really didn’t understand why there could be this level of confusion as to what happened in the great earthquake when right near here there was Stanford University,” said Ted Sayre, the Portola Valley town geologist who is a consultant with the firm Cotton, Shires & Associates Inc. “There should have been a good record of what really happened.”

In the aftermath of the devastating magnitude-7.7 earthquake that destroyed San Francisco, Stanford geologists set out on horseback to survey the local damage. The scientists photographed the damage and produced an exquisitely hand-drawn map, all of which still exist in historic archives. [Album: The Great San Francisco Earthquake]

Simplified map of the San Andreas Fault zone crossing Portola Valley in California. The newly mapped fault strand is in red.

Credit: Ted Sayre

But 1908 reproductions of the map were less than clear. Through the decades, errors grew with each new geologic map of Portola Valley, Sayre said. Adding to the confusion are the San Andreas Fault’s several parallel traces in the valley.

Like many big strike-slip faults, the San Andreas can be a braid or a single thread. Sometimes the fault is a zone of several smaller faults, one or more of which may break during an earthquake. (Strike-slip faults move each side of the fault mostly parallel to each other, along the nearly vertical fault.)

In Portola Valley, two fault strands weave under the west and east sides of the valley. For decades, local geologists have debated whether the 1906 earthquake skipped across the valley, jumping from west to east and back west again as it raced northward, or if it just stayed on one strand, Sayre said. (Before Portola Valley’s housing prices went sky-high, the village was a popular home for U.S.

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