At dinnertime on 18 October 1356, residents of Basel, Switzerland, felt the jolt of an earthquake that toppled churches and castles 200 kilometers away and triggered weeklong fires. Now researchers report that they have finally identified an active fault that may explain the earthquake and earlier quakes. The finding provides the first indication of how frequently this rift valley is shaken.
Unlike the San Andreas fault, which has prominently scarred the landscape of coastal California, European faults are hard to identify. Small earthquakes don't disturb the surface, and major earthquakes are too rare to have left much of a historical record. But with a magnitude estimated at between 6 and 6.5, the 1356 quake should have left a visible mark.
To find a trace of the quake, Mustapha Meghraoui, a geologist at the University of Strasbourg, France, and colleagues pored over aerial and satellite photographs and topographic maps. Zeroing in on an 8-kilometer-long escarpment south of Basel, they dug trenches at the base of the 50-meter-high scarp. Painstaking examination of the walls revealed blocks of sand and clay clearly separated on a steep diagonal--the trace of so-called normal faulting, in which pull-apart forces cause blocks of ruptured crust to slide up and down relative to each other. Carbon-14 dating confirmed that three earthquakes had nudged the ground upward a total of 1.8 meters over the past 8500 years. The findings suggest that the fault unleashes a 1356-type earthquake on average every 1500 to 2500 years, the team reports in the 14 September issue of Science.
Donat Fäh, a geophysicist with the Swiss Seismological Service, says that data from this and future studies will go into regional earthquake catalogs to help develop building codes, especially for critical facilities such as chemical and nuclear power plants and long-lived features such as artificial lakes.