Finding fault in San Mateo County

A 100-year-old picture of a separated fence along the road between Colma and San Pedro Valley tells a story scientists have been unraveling for a century. The photo is one of many culled from archives across the state to help scientists see evidence of the fault and allow them to match environmental clues in the photos with a modern image of San Mateo County.

"With all the housing today, it’s hard to pinpoint exactly where it is,” Prentice said. "When you walk there there’s no way of knowing the fault is there.”

The photos allow scientists to see what the land looked like before it was paved over for thousands of homes that lie in its path — some of them likely built directly on the fault. In the coming weeks, the U.S. Geographical Survey will release a new database that maps the San Andreas fault on both an aerial 1906 map of the Bay Area and current satellite map. With a click of a mouse a viewer can see the undeveloped hills of South San Francisco transformed into homes packed around Skyline Boulevard.

What’s most important is the small dots that decorate the fault line on the map. Hovering the mouse cursor over those dots produces a 1906 picture and historical statement about that location. It’s one of most important tools to confirming the exact location of the fault.

The map is highlighted with a large line. The line, of course, represents the notorious San Andreas fault. It’s responsible for the devastating earthquake that shook San Francisco to its feet and sent residents running to San Mateo County. By the 1950s, homes were being built at the corner on Fleetwood Drive near Skyline Drive in San Bruno in close proximity to the fault.

In San Mateo County, the fault actually runs Mussel Rock in Pacifica through the communities in San Bruno, South San Francisco and Daly City near Skyline Boulevard and to the west of Interstate 280. The fault run directly underneath San Andreas Lake and the entire Crystal Springs Reservoir. San Andreas Lake is a natural sag pond that was enlarged by man by the creation of the dam. Two additional lakes were added. First signs of the fault were noted in San Mateo County near the San Andreas Lake — that’s how the fault got its name, Prentice said.

The fault continues toward Black Mountain Road and the areas of La Honda and Woodside.

Residents are forced by a 1970 state federal law to disclose in real estate forms whether they are in a fault’s zone. There are still plenty of homes that lie directly on the fault. With those homes aging, Prentice is sure owners will want to redevelop. The question is, Prentice said, is if officials will allow them to rebuild their homes if it sits on a fault.

Even if a house sits close to a fault, it can withstand a great shake with the help of modern engineering. However, there is no way to engineer a house to withstand the jolt delivered by a 7.8 earthquake, Prentice said.

When the 1906 earthquake hit, anything on its fault was literally ripped apart. It ruptured the earth’s surface from Shelter Cove in Humboldt County to San Juan Bauptista in San Benito County. The total length was 296 miles. For comparison, the 1989 Loma Prieta earthquake ruptured a length of about 25 miles.

Like many faults in the west, the San Andreas is a strike-slip fault with a right lateral. When the tectonic plates move, one shoves upward against the other and toward the right. Looking across a fault line, one can see objects on the fault shifted to the right. While looking the length of the fault, one end will be higher.

Before the earthquake only one minor fault was noted in an area close to the San Andreas Lake west of the San Mateo. The 1906 earthquake not only helped map the largest fault line in the United States, it led to the several well-regarded theories, Prentice said.

The most common theory is the "elastic rebound theory” which explains how areas near the fault are always in constant opposite motion, but the fault itself remains in place. Eventually the fault snaps apart to follow the movement of the land — causing an earthquake.

There’s another startling fact scientists point out. In the century leading up to the Great Quake there were many large earthquakes — with magnitudes of 6.0 — on the San Andreas Fault. It was a sign of something bigger to come, Prentice said.

"In the next 100 years, if we know anything at all, there will be more earthquakes,” Prentice said.

Dana Yates can be reached by e-mail: dana@smdailyjournal.com or by phone: (650) 344-5200 ext. 106. What do you think of this story? Send a letter to the editor: letters@smdailyjournal.com.<