for National Geographic News
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].)
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."
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