The enormous earthquakelast November along Alaska’s Denali Fault buckled highways and shook the trans-Alaska oil pipeline. But the magnitude 7.9 shock also set off surprising swarms of small tremors thousands of kilometers to the south. This discovery is convincing geologists that far-reaching effects—only recently documented—are very likely a common result of most major shocks.

The Denali temblor is the third major earthquake in the West in the past 10 years known to have caused smaller quakes. The other two were in southern California: the Landers earthquake in 1992 and the Hector Mine quake in 1999. All three quakes affected the same geothermal volcanic fields in Wyoming’s Yellowstone National Park, Mount Rainier in Washington State, and several sites in Cali fornia. These fields, which are hot springs fu eled by magma roiling deep underground, normally rumble at low levels. But the sec ondary quakes that were triggered far ex ceeded the background seismicity, and re searchers aren’t quite sure why.

Alaska’s quake, which was centered about 283 kilometers (176 miles) northeast of An chorage, sent out a train of seismic waves. It could have caused a subtle expansion and contraction of the earth’s crust, which in turn could have tripped faults that were on the edge of failure. That’s a tidy explanation for the earthquake swarms that immediately followed the Denali shock. But some of the secondary tremors occurred a day or more later, indi cating a more complex scenario at play.

Many researchers have cited gas bubbles in the magma chamber to explain the delay. Geophysicist Alan T. Linde of the Carnegie Institution of Washington suggests that the passing waves can dislodge the bubbles, which typically stick to the walls of the cham ber like champagne bubbles to the sides of a glass. In addition, the seismic waves might stir the magma enough to create new bubbles, notes geophysicist Emily E. Brodsky of the University of California at Los Angeles. In ei ther case, as the bubbles rise, they expand, thereby increasing pressure in the fluid. They may also expand and contract as seismic waves pass through them, further changing the pres sure, according to Brodsky. The pressure changes deform the overlying rocks, possibly jarring certain faults into action.

Magma bubbles may not be the only pos sible earthquake triggers. In Greece, Brodsky has found that hot springs are fueled not by a magma chamber but by changes in the pres sure of fluids coursing through underlying crystalline rocks. Crustal deposits from the mineral-laden fluids frequently clog channels that the fluids once followed. Seismic surges from a large earthquake might crack those seals, Brodsky says. The change in pressure from renewed fluid flow is enough to start earthquakes on tiny nearby faults, a process that would apply to the hot springs in Cali fornia and Yellowstone.

Magma bubbles and cracked geothermal seals can’t account for all the secondary quakes, however. North-central Utah shook as well, but that area is a nonvolcanic, non geothermal zone. Moreover, the region saw a weeklong increase in seismicity, a phenomenon that bubbles and cracked seals cannot explain.

Research geophysicist Michael Blanpied of the U.S. Geological Survey coordinated the analyses of the Denali earthquake. He says that the Utah rumbling makes him more in clined to rely on stress changes solely from seismic waves. He points out, though, that multiple mechanisms may be responsible for the variety of events. Denali provided an enor mous amount of data over a broad area, but ultimately, Blanpied says, they “didn’t answer any questions.” It may take more tremors for the theories to shake out.

This article was originally published with the title Triggered Swarms.

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