Triggered Swarms; March 2003; Scientific American Magazine; by Naomi Lubick; 2 Page(s)

The enormous earthquake last 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 California. These fields, which are hot springs fueled by magma roiling deep underground, normally rumble at low levels. But the secondary quakes that were triggered far exceeded the background seismicity, and researchers aren't quite sure why.