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  Geotimes - March 2008 - Quake shakes lake

Quake shakes lake

On Jan. 31, just before 1 p.m., people near Lake Mendota in Madison, Wis., felt a sudden tremor as their offices shook for a couple of seconds. They promptly called the police and local building officials to see what had happened, according to a Jan. 31 report in The Isthmus. A seismometer in the geology department at the University of Wisconsin at Madison (UW), not far from the lake, picked up a small signal. “It was an ice quake,” says Clifford Thurber, a UW seismologist. “They’re reasonably frequent around here, though not usually that large,” he says. One thing is certain, he adds, “they happen in Madison far more often than actual earthquakes.”

Ice quakes happen all the time on frozen lakes. Usually, they’re too small to measure on a seismometer or to be felt by anyone besides the local ice-fishermen who hear or see the cracking, Thurber says, but occasionally, big ones occur. Such ice fractures have been known to be more than a kilometer long and a meter wide, including the “Mendota quake” of Jan. 15, 1948, which UW geologist Charles Bradley detailed in a 1948 issue of the American Journal of Science. The current fracture is somewhere between half a kilometer and a kilometer long, Thurber says.

Thurber says he doesn’t really know why this ice quake was so big. Perhaps it was that the temperature in Madison shifted dramatically (dropping tens of degrees) in a single day. Regardless of the reason, despite registering only negative 0.2 on the Richter scale, the ice quake was felt because it was at the surface, unlike earthquakes, which occur deep belowground.

Megan Sever

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