WASHINGTON, Sept. 13 — The powerful earthquake threw down as many as 40 castles surrounding Basel, Switzerland. Church towers toppled for miles around. The 1356 tremor was one more disaster in a Europe suffering through the Great Plague, and one that remained largely unexplained. Now researchers report they have found an active fault line south of Basel that is likely to have caused that medieval quake, the most powerful on record in central Europe.
THE FAULT runs through quiet suburbs and forests and is marked by a steep slope about 6 feet high where the ground on one side has sunk, the research team led by Mustapha Meghraoui of the University of Strasbourg, France, reports in Friday’s issue of the journal Science.
Analysis of the gravel and clay deposited along the fault indicates three different powerful quakes in the region over the last 8,500 years, the team said.
They estimated the magnitude of the 1356 quake at between 6.0 and 6.5, a strength that can do severe damage, though not as powerful as some quakes that have occurred in California, Alaska and Japan.
Even so, an earthquake similar to the 1356 event would cause an estimated $30 billion to $50 billion worth of damage if it struck today, said co-author Domenico Giardini of the Swiss Federal Institute of Technology.
The study indicates a similar quake would occur in the region about every 1,500 to 2,500 years, said Meghraoui.
While Basel may not suffer a massive earthquake in this century, the presence of nuclear and chemical industry in the area means that any seismic activity could threaten public safety, added another co-author, Peter Huggenberger of the University of Basel.
‘BREAKTHROUGH’ IN QUAKE SCIENCE
The fault discovered by the team begins near the Swiss Jura mountains south of Basel and extends nearly five miles northeasterly through the Rhine graben, a valley near the city’s southern edge. They said it may extend even farther north across the city.
“This study represents a breakthrough in evaluating earthquake recurrence along the Rhine graben,” said David P. Schwartz, head of the U.S. Geological Survey’s San Francisco Bay Area Earthquake Hazards Project.
Schwartz said he has some questions about the radiocarbon dating used, but praised the work for providing information on the long-term rate of average fault movement and direct evidence of past earthquake dates.
In addition to analyzing the ground deformation, the researchers studied water runoff patterns, used ground-penetrating radar, seismic reflection patterns and dug a series of exploratory trenches.
They used chemical and carbon-14 dating to estimate when the earlier quakes had occurred along the fault.
“These successive ruptures on the normal fault indicate the potential for strong ground movements in the Basel region,” the Science paper concludes, “and should be taken into account to refine the seismic hazard estimates along the Rhine graben.”
CATASTROPHIC LOSSES
Basel, located near the meeting point of Switzerland, France and Germany, suffered catastrophic losses during the 1356 earthquake.
According to historical accounts, a first quake struck “at the dinner time,” around 7 p.m. on Oct. 18, 1356, setting the stage for a second, stronger event “at the bed time,” probably about 10 p.m. Thirty to 40 medieval castles collapsed in the hardest-hit area. Many more churches and towers toppled within a 120-mile radius of the city.
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