When the ice age glaciers melted away from northern lands some 10,000 years ago, the Earth rebounded, much as a boat bobs upward when its cargo is unloaded. But in northern Scandinavia, at least, the process was far from gentle. Nearly 9,000 years ago, "postglacial rebound" appears to have unleashed one of the strongest earthquakes that region has ever known, estimated at 8.0 on the Richter scale, says a report in the 1 November issue of Science. Even today, the faults created as the land rose are still trembling.
Ronald Arvidsson, a seismologist at Harvard University, measured the angle and depth of faults in northern Sweden, Finland, and Norway. These calculations pointed to the existence of a massive earthquake just after the last ice age. Moreover, the stress of rebound is still being released in smaller temblors. About half of recent small quakes, he found, took place along postglacial faults--a much higher number than expected. And Arvidsson identified a cluster of even smaller microearthquakes, of a magnitude less than 2.4, near the small Lansjärv fault. "In a sense, earthquakes that occur in the region today can be thought of as very long-term aftershocks" of the original event, says Arch D. Johnston, director of earthquake research at the University of Memphis, Tennessee.
The fact that a quake of such magnitude occurred in a region that "is normally thought of as the least seismic in the world" raises the stakes of seismic hazard planning for facilities such as nuclear waste repositories, says Johnston. On the other hand, says Jim Zollweg, a seismologist at Boise State University, Idaho, Arvidsson's study should make the likely earthquake locations easy to spot. "It's comforting that the earthquakes that occur today are related to the faults," Zollweg says.