Demise of ice age sparked great quakes.

In geology, as in international relations, Scandinavia has a reputation for keeping a low profile. By the rules of plate tectonics, the extremely old crust in this region should not suffer the vicissitudes that rattle younger parts of the globe. New seismic studies, however, are threatening Scandinavia's image of stability.

Evidence from northern Sweden suggests that this area may have unleashed monster earthquakes when the ice age ended, only a short geologic span ago, says Ronald Arvidsson, a seismologist at Uppsala University in Sweden. The uncharacteristic quakes within a tectonic plate occurred because the land sprang up after the heavy European ice sheet melted away, he reports in the Nov. 1 Science.

Arvidsson studied a series of northern Scandinavian faults that formed 9,000 years ago, just after the disappearance of the ice sheet there. Geologists began discovering these postglacial faults 20 years ago, but the remoteness of the faults has hindered investigations.

When Arvidsson determined the locations of minor, unmapped earthquakes that occurred between 1963 and 1993, he found signs that the faults remain active. To determine the depth of the faults, he studied seismic data taken close to one of the smaller ones. Recordings of tiny shocks revealed that the fault extends unusually deep, all the way to the base of Earth's crust, some 40 kilometers down.

Knowing the scale of the faults, Arvidsson could calculate the size of the earthquakes that ripped the crust long ago. The largest postglacial fault, the 160-km-long Parvie fault, formed during a magnitude 8.2 earthquake, he estimates. The 50-km-long Lansjarv fault came to life in a shock with a magnitude of 7.8.

"They would have been considerable earthquakes. The Parvie earthquake would have been as large as or larger than the New Madrid earthquakes," he says, referring to a trio of giant shocks that struck the U.S. Midwest in 1811-1812 (SN: 6/8/96, p. 362). The New Madrid earthquakes are the largest continental shocks ever witnessed in the interior of a tectonic plate. Most quakes strike the edges of plates, where they collide and grind against each other.

The news that large earthquakes once broke the Scandinavian crust raises a geologic paradox, says Arch C. Johnston, a seismologist at the University of Memphis. Old sections of plates tend to produce few earthquakes, but when they do, the quakes may be large because the fractures can reach great depths in this cold crust. In geologically younger regions, faults don't extend into the lower crust because the warm rock there bends rather than breaks.

The postglacial faults of Scandinavia should have cousins in North America that formed when the Canadian crust rebounded at the end of the ice age, says Johnston. Thus far, however, only one small section of fault in northern Manitoba has turned up. Johnston suspects that other, larger faults remain hidden in the wilds of Canada.