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We have identified an active normal fault in the epicentral area of the Basel (Switzerland) earthquake of 18 October 1356, the largest historical seismic event in central Europe. The event of 1356 and two prehistoric events have been characterized on the fault with geomorphological analysis, geophysical prospecting, and trenching. Carbon-14 dating indicates that the youngest event occurred in the interval 610 to 1475 A.D. and may correspond to the 1356 Basel earthquake. The occurrence of the three earthquakes induced a total of 1.8 meters of vertical displacement in the past 8500 years for a mean uplift rate of 0.21 millimeters per year. These successive ruptures on the normal fault indicate the potential for strong ground movements in the Basel region and should be taken into account to refine the seismic hazard estimates along the Rhine graben.
To understand the damage produced by past earthquakes and to forecast future earthquake scenarios, an analysis of potentially active faults and related seismogenic behavior is required. An earthquake on 18 October 1356 destroyed the city of Basel, Switzerland, and damaged large parts of the upper Rhine graben (Fig. 1). This large earthquake had an epicentral intensity of IX to X on the MSK scale (1) and an estimated magnitude of 6.0 to 6.5 (2, 3). Thirty to 40 castles were destroyed within a 10-km radius around Basel, and castles, towers, and churches within a 200Ian radius were damaged (2, 3) (Fig. 2). The Basel region belongs to an area with a low seismicity level and a long seismic cycle. Basic knowledge on seismogenic faults in this area is lacking, and the hazards evaluation depends on the sporadic occurrence of large earthquakes. To determine the activity and recurrence rate of earthquakes near Basel, we need, first, to identify the fault along which the 1356 earthquake occurred and, second, to reconstruct its seismogenic history.
The Basel region belongs to the upper Rhine graben and its related Cenozoic rift system, which is limited to the south by the fold-and-thrust belt of the Jura Mountains. The background seismicity of the graben is relatively modest in comparison with that of other continental rifts. Regional studies with local seismic networks show a more pronounced seismicity to the east and south of the graben; the...