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San Andreas Fault May Be Rare Quake "Superhighway"

Kate Ravilious
for National Geographic News
August 16, 2007
 
Some faults can send earthquakes zooming along the ground faster than the speed of sound, scientists say—and California's San Andreas Fault may be one of them.

Most earthquake faults "unzip" at around 2 miles (3 kilometers) a second. But evidence is growing that some faults can send quakes zooming much faster—up to 3.7 miles (6 kilometers) a second.

"They are moving faster than the speed of sound, like a sonic boom," said Shamita Das, a seismologist at Oxford University in the U.K.

These hasty earthquakes cause much more damage on the ground and are more likely to topple buildings, crumple bridges, and buckle highways than regular upheavals.

Now it turns out that the San Andreas Fault may be one of these earthquake "superhighways." (Related: "Southern California Long Overdue for Quake, Experts Say" [August 13, 2007].)

Quake "Freeways"

Superfast earthquakes are rare, and it has taken Das nearly 30 years to prove their existence.

But on November 14, 2001, a magnitude 7.8 earthquake struck the Kunlunshan region of Tibet. That quake unzipped over 250 miles (400 kilometers) of fault, providing the long-awaited opportunity.

By studying the seismic waves and analyzing the damage on the ground, Das and her colleagues have been able to map out the earthquake's journey.

"The rupture started slowly, but then accelerated to a super shear-wave speed, traveling for more than 100 kilometers [62 miles] at a speed of nearly 6 kilometers per second," Das said.

Das and colleagues also realized that the fast section of the Tibetan fault happened to be very long and straight, like a freeway.

"When the fault has curves and bumps in it, then the earthquake has to travel slowly," Das said. "But on a long, straight stretch it can reach very high speeds."

Only a few of the world's earthquake faults contain these freeway sections, but there is one major fault that Das is concerned about.

"The Kunlushan Fault in Tibet is very similar to the San Andreas Fault in California," Das said. Both faults move horizontally rather than vertically, and both faults contain long straight sections.

Das and her colleagues believe that a superfast rupture may have been partially responsible for the extreme devastation caused by California's 1906 earthquake.

"The long straight bit goes directly underneath San Francisco," Das said. (See a photo gallery of San Francisco after the 1906 quake and now.)

Engineering a Fix

So can this information help the people of San Francisco prepare for the next big one?

"There is quite a lot that can be done in terms of building design to ensure they can cope with the higher frequencies of a faster wave," said Ian Main, a seismologist at Edinburgh University, who wasn't involved in Das's work.

"New buildings can be built on rollers and sited on bedrock rather than soft sediments."

San Francisco isn't the only major city at risk. Further down the San Andreas Fault, another section of "freeway" exists underneath the Carrizo Plain, Das and colleagues say (see a map of California).

"As an earthquake moves along this section it is likely to send out shock waves in front, which may focus on cities like Santa Barbara and Los Angeles, some of the most highly populated parts of California," Das said.

In tomorrow's issue of the journal Science, Das outlines the need to categorize all the world's earthquake faults according to their potential earthquake speed.

By predicting which faults are the freeways and which faults are mere "country lanes," Das and her colleagues believe that communities can be better prepared.

"You wouldn't want to build a nuclear power plant above a fast section of fault," Das said.

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