The Great SF Quake: Disaster Preparation II

The World Will Shake... Here's a simple question: Why will a major earthquake certainly strike the San Francisco Bay area in the future? Answer: Because the Pacific and North American tectonic plates are moving in opposite directions. It's all about (mobile!) real estate.

Want proof? On a clear day, look west from the Golden Gate Bridge to the Farallon Islands, 27 miles offshore. Every year the Farallones move one inch northward. Just in my lifetime, they have moved five feet farther away from San Francisco. Amazing.

The resultant stress will eventually cause a violent movement along the San Andreas fault as the two plates slip past each other. "In the instant of an earthquake, the fault moves to catch up with the distant points that have been moving." (Mary Lou Zoback, U.S. Geological Survey, New York Times, Apr 11, 2006)

In the 1906 San Francisco Quake, earth movement along 300 miles of the San Andreas fault exceeded 20 feet in places. The San Andreas fault does NOT run through San Francisco. So the danger to the city is not so much from displacement of the earth, but rather from the violent shaking caused by nearby earth movement.

Remember Game One of the 1989 World Series between the San Francisco Giants and the Oakland A's? The Loma Prieta Earthquake struck at 5:04 p.m. on October 17 just as the stadium was filling. I was on the 21st floor of 345 California in downtown San Francisco (my partner was at the game!). The building vibrated sharply for 15 seconds, like it was being shaken by a giant hand. Not a fun experience.

But it could have been a lot worse. According to the New York Times (Apr 11), the 1906 San Francisco quake "released 30 times as much energy as the 1989 Loma Prieta quake." Uh-oh.

-- Peter Pike