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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