Geotimes
Web Extra Tuesday, August 19, 2003

Fast earthquakes break speed limit

Some earthquakes may move faster than seismologists once thought possible. A new study published in the Aug. 8 Science shows the most convincing data yet that a large earthquake can travel down a fault at velocities that surpass theoretical limits. The new speeds could have an impact on earthquake models — and related prediction and hazards for very large earthquakes.

Limits on the speed at which a rupture can propagate down a fault stem from past observations and assumptions about crack mechanics and the material strength of rocks. Historically, seismologists assumed that a fault wouldn't break faster than the two types of waves that radiate from an earthquake's epicenter. Known as P and S waves, they pack a one-two punch to the rocks in the region of a fault. Though P waves move faster than faults can break, some recent observations of earthquakes in Turkey and California (such as the Imperial Valley) have given an inkling that a fault could break faster than S waves, with rock motions outrunning the S wave along a fault. The findings complicate a relatively simple picture of how earthquakes unfold.

In the most recent study to challenge past assumptions, Michel Bouchon and Martin Vallée of the Université Joseph Fourier and Centre National de la Recherche Scientifique in Grenoble, France, looked closely at the Kunlunshan earthquake in Tibet. The magnitude-8.1 earthquake struck Nov. 14, 2001, rupturing a distance of about 400 kilometers, the longest earthquake yet observed on land. Several regional broadband stations in the Incorporated Research Institutions for Seismology (IRIS) and China Digital Seismic networks recorded the event. Bouchon and Vallée chose several stations to trace movement at positions along the fault, as well as off the fault for a different viewpoint. They then modeled different velocities for ground motions moving down four equal consecutive segments of the 400-kilometer-long fault, in order to find the best match for the shifts clocked by the stations.

Movement along the first segment ripped at about 2.4 kilometers per second, which is well below the expected limit of S wave speeds — around 3.0 to 3.2 kilometers per second in the brittle crust. But the team found that the next three segments of the fault ruptured at closer to 5 kilometers per second, at a rate referred to as "supershear" because it is well above the shear-wave velocity of the rocks in the region. The average rupture velocity along the entire fault was 3.7 to 3.8 kilometers per second, also definitively faster than previously accepted as possible, according to all of the models that fit the seismic stations' data.

Although "people have seen this before," says John Vidale, a seismologist at the University of California at Los Angeles, "this is the best observation" because of the density of data and the length of the fault rupture — making it more convincing than observations in Turkey and elsewhere. "Previous cases had holes in them," he says, that allowed the older theoretical speed limits to stand. Vidale compares an earthquake moving at supershear speeds to "an airplane going supersonic": Just as a listener would hear the airplane's sound all at once, an observer might feel all the energy of such an earthquake rupture at once. Depending on how fast the crack rupture propagates, it's a different pattern of damage, he explains.

The new findings may change how seismologists model ground movements of very large earthquakes in the future, says Ruth Harris, a seismologist at the U.S. Geological Survey in Menlo Park, Calif. "There are still big puzzles out there about how large earthquakes work and if their physics is different from moderate and small earthquakes."

Naomi Lubick

Links:

Martin Vallée's home page

Images of the Kunlunshan 2001 rupture from the China Seismological Bureau (Seismological Research Letters)

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