Fault found that destroyed Basel in 1356

A FAULT that is still active caused one of the strongest European earthquakes on record, a 1356 quake centred on the Swiss city of Basel from which the region did not recover for a century, scientists said yesterday.

In research appearing in the journal Science, scientists led by Mustapha Meghraoui of the University of Strasbourg in France identify the long-elusive geological cause of the 1356 Basel earthquake and suggest the region could be hit by another devastating quake.

Finding the fault responsible for the earthquake has been a tough task because it is concealed by dense forests and seismic activity is so rare that it yields few clues. Scientists using ground-penetrating radar, measurements of electrical resistance and other advanced techniques identified two possible sites for the geological cause of the quake.

At the second site they tackled, Mr Meghraoui and colleagues from the University of Basel and the Swiss Federal Institute of Technology Zurich (ETH Zurich) identified an active fault, marked at ground-level by a ridge or escarpment called a fault scarp, that has caused three ruptures that thrust the Earth’s surface upward by about 6ft during the past 8,500 years.

The fault runs beneath the suburbs and forests immediately south of Basel. At this site, the researchers dug four parallel trenches. They examined three thick wedges of gravel and clay deposited when debris washed over the fault following large or moderate earthquakes. Biochemical analysis yielded the dates of three seismic events - one that might correspond to the 1356 quake and two earlier ones.

Beginning near the Jura Mountains south of Basel, the fault scarp covers at least five miles and extends north-east through a rift valley south of the Rhine River, then through the Birs Valley to the city’s southern edge, the researchers said. They did not rule out the possibility that the fault extends farther north across the city and south into the Jura Mountains.

Disturbingly, this active fault continues to tremble 645 years after it caused the worst earthquake in central European history. On 18 October, 1356, an earthquake with an estimated magnitude of well over 7 on the Richter scale caused severe destruction in the city of Basel and in an area with about a 120-mile radius.

In the earthquake’s twin jolts - one in the early evening and another hours later - churches as far away as Geneva were toppled, and 30 to 40 medieval castles collapsed. The roof and spires of Basel’s cathedral caved in.

The cities in the region were not heavily populated during this period and the loss of life has been estimated in the low thousands. But Alpine society in the region was devastated by the quake and did not recover for 100 years.

Basel went on to become famous as a Renaissance city of artistic and intellectual vigour at the crossroads of Switzerland, France and Germany.

Archaeological excavations also have provided evidence that an earthquake in AD 250 destroyed the Roman city of Augusta Raurica, about six miles east of Basel.

Mr Meghraoui said his team found a consistent pattern of seismic activity that "points to a recurrence time for a 1356-type earthquake in the Basel area of about 1,500 to 2,500 years." The researchers noted that they cannot predict the timing of a next major earthquake with any certainty.

Another researcher, Domenico Giardini of ETH Zurich, said a quake similar to the 1356 one now would inflict an estimated 20 billion to 35 billion in damage. The researchers also said the presence of the nuclear and chemical industry in the area poses an even more acute threat to public safety in the event of an earthquake.

"We need to take precautionary measures now," Mr Giardini said. - Reuters