By Dave Boyce
Almanac Staff Writer
This is an update to an earlier story.
"Everyone's nervous," said Sheldon Breiner, a geophysicist at Stanford University and a resident of Portola Valley who said he's gotten lots of questions from neighbors since the devastating March 11 earthquake and subsequent tsunami off the coast of northeastern Japan.
A common question: Can the Peninsula experience a 9.0 earthquake? "No," Mr. Breiner said. "We might have an 8."
The length and depth of the San Andreas fault are key factors, Mr. Breiner said, as is the fact that the San Andreas ruptures by tectonic plates sliding past one another, a so-called strike-slip fault.
In an interview, U.S. Geological Survey geologist David Schwartz agreed that the San Andreas is limited to magnitude 8, or 7.9 -- if it slips along its entire length of 292 miles. Because it is a relatively shallow 9 miles deep, a larger quake is not possible. Multiplying length times depth, "you don't have enough 'surface area' to slip to produce a magnitude 9," he said.
The dimensions of the crust material involved tends to determine the magnitude of an earthquake. If the San Andreas slips only along the section between the Golden Gate and Los Gatos, for example, the quake likely won't exceed 7.2, Mr. Schwartz said.
Tsunamis occur where a fault is both under water and along a subduction zone, Mr. Breiner said. Earthquakes in a subduction zone like the one off Japan are the result of thrust faults in which one tectonic plate moves on top of another, causing the sea floor to rise.
A rise of 18 or 24 or 30 feet over hundreds of miles will lift an immense quantity of water and create a huge wave, Mr. Schwartz said. By contrast, a strike-slip fault such as the San Andreas "has a very small if any vertical component," he said.
The difference is easily demonstrated with your hand submerged in a bathtub or sink, Mr. Breiner said. Move your hand up or down and the result is a wave. Slide it backward and forward and the surface effect is minimal.
A subduction zone with the potential to generate a tsunami does exist along the West Coast, but it begins at Mendocino County and extends north along the coasts of Oregon and Washington, Mr. Breiner noted.
But, Mr. Schwartz added, a local destructive wave is not out of the question in the case of an underwater landslide. "Strong shaking does have the potential near the coast to set something off," he said.
==N Who's next?==
Another common question concerns the Ring of Fire linking seismic activity to the nations that border the Pacific Ocean. Over the past year, major earthquakes have occurred in Chile, New Zealand and now Japan. Are we next?
"There's no evidence of any kind that I've seen" that shows such a causal relationship, Mr. Breiner said.
"We could have something soon, but there is no connection (with previous quakes)," Mr. Schwartz said. "It's really kind of coincidental. There's no A causing B causing C, which will cause D."