In 1356 a devastating earthquake shook Basel, Switzerland and almost completely destroyed the city, leveling nearly every fortified castle within a 30-kilometer radius. The cause of the temblor remained a mystery for 645 years. But according to a report in the latest issue of Science, Mustapha Meghraoui and his colleagues in Strasbourg, France and at the University of Basel have now pinpointed the quake's exact source. Furthermore, they have identified a pattern of seismic activity that may threaten the area again.

The researchers blame the1356 earthquake on an active fault near the city that appears at the surface as an abrupt change in ground level. During the past 8,500 years the fault has shifted some 1.8 meters (about six feet) and has produced three major earthquakes, of which the 1356 event was the most recent. The rock on either side of the fault line continues to move and "points to a recurrence time for a 1356-type earthquake in the Basel area of about 1,500 to 2,500 years," Meghraoui and team write.

Although the researches say that the pattern they identified doesn't indicate a recurrence in the near future, they are hopeful that steps will be taken to prepare for another earthquake of similar magnitude. Previously, the best estimate of the recurrence time of such an event for the Basel area was about 6,000 years. Several Swiss pharmaceutical and chemical concerns operate in the city, and three nuclear reactors lie within 50 kilometers. "We need to take precautionary measures now," co-author Domenico Giardini says.