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. They would have been considerable earthquakes. 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.