Sep. 17, 2001 BASEL, SWITZERLAND -- Beneath the suburban neighborhoods and forests immediately south of this cultural mecca, an active fault continues to tremble, some 645 years after it caused the worst earthquake in central European history, the journal Science reports.
The study--completed by researchers with ETH Zurich, the University of Basel in Switzerland, and the University of Strasbourg in France--finally pinpoints the exact source of the devastating 1356 Basel earthquake. It also suggests a time-frame for when the next major earthquake may strike Basel.
An active fault, marked at ground-level by a ridge or escarpment called a fault scarp, has caused three successive ruptures, moving the Earth's surface upward by 1.8 meters (about 6 feet), over the past 8,500 years, researchers discovered.
Basel may not suffer a massive earthquake in this century, said Peter Huggenberger of the University of Basel, a co-author on the study. But, the presence of nuclear and chemical industry in the area means that any seismic activity could threaten public safety. And, according to governmental and insurance estimates, an earthquake similar to the 1356 event would cause an estimated 30 to 50 billion U.S. dollars worth of damage (50 to 80 billion Swiss francs), added co-author Domenico Giardini of ETH Zurich.
The Science study describes 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," explained lead author Mustapha Meghraoui of the University of Strasbourg.
Though researchers emphasized that they can't predict the next major earthquake with certainty, the seismic pattern they identified would give the region time to safeguard infrastructure and fine-tune emergency procedures. "We need to take precautionary measures now," Giardini said.
Basel--renowned as a Renaissance city of artistic and intellectual rebirth at the intersection 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 7:00 pm on 18 October 1356, setting the stage for a second, stronger event "at the bed time," probably about 10:00 pm. Some 30 to 40 medieval castles collapsed in the hardest hit area. Many more churches and towers toppled within a 200-kilometer radius of Basel, as the earthquake reached a Mercalli intensity of IX to X, comparable to the disaster in Izmit, West Turkey, two years ago.
Beginning near the Swiss Jura Mountains south of Basel, the fault scarp covers at least eight kilometers, or nearly five miles. It extends in a northeasterly direction through a rift valley south of the Rhine River (the Rhine graben), traversing the flat fields of the Birs Valley to reach the city's southern edge. Indeed, scientists said, it's possible that the fault extends even farther north across the city, and deeper south, into the Jura Mountains. The Basel-Reinach fault is part of a larger "seismogenic layer."
Finding the precise source of the 1356 Basel earthquake has been difficult: The fault is partly obscured by dense Alpine forests, and seismic activity in the region is so infrequent that researchers can't easily determine the fault's location.
To more fully characterize the Basel-Reinach fault, scientists first studied the deformations it has caused at ground level--including ancient meanders from the nearby Birs River and steplike outcroppings.
Water runoff patterns along the scarp offered early clues to the type of fault in the Rhine graben: Tributary creeks of the Birs River cut short, deep incisions into the scarp's eastern face, whereas long, meandering streams run down the more gently sloping western side. Such drainage patterns suggest an "active normal fault," where the intersection of two tectonic domains (the Rhine graben and the related Cenozoïc rift system) form a cliff-like structure, with a western ridge that drops 100 meters to a shelf or "footwall" on the east.
Using ground-penetrating radar, seismic reflection profiling and measurements of electrical resistance, researchers established two key sites for trenching, beginning in September 1999. At the first site, just north of the Jura Mountains, they dug three exploratory trenches. Confined by trees, these ran no longer than 10 meters each. At the second site, located farther north in the Rhine graben, four parallel trenches were created, crossing 75 meters of the scarp.
From the base of the scarp's steep slope, researchers examined three thick wedges of mixed gravels and silty clay. Each wedge was deposited when debris washed over the fault, following a medium-to-large magnitude event. Biochemical analysis, based on a count of long-living radioactive carbon-14 in organic matter washed into each of the wedges, gave the dates that bracketed the timing of three seismic events from the late Quaternary period: the Basel earthquake, and two earlier episodes.
Based on the carbon-14 ages of the wedges, researchers determined that the most recent earthquake had to occur between 1475 and 610 AD, corresponding with the time-frame for the 1356 Basel earthquake. Two other events happened between 890 AD and 850 BC; and before the age range of the oldest wedge, dated as being deposited between 850 BC and 6480 BC.
"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."
With Meghraoui, Giardini and Huggenberger, authors on the Science paper were Bertrand Delouis and Matthieu Ferry of ETH Zurich; Ina Spottke of the University of Basel; and Michael Granet of the University of Strasbourg.
This research received support from the INSU-PNRN (Institut National des Sciences de l'Univers-Programme National sur les Risques Naturels); the BRGM (Bureau de Recherches Geologiques et Minieres); the European Commission SAFE project; and the Swiss PALEOSEIS program sponsored by the Swiss National Foundation, and by the Swiss Commission for the Safety of Nuclear Installations.
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