Active tectonics in the Cheb Basin: youngest documented Holocene surface faulting in Central Europe?

Abstract

The NNW-SSE trending Mariánské Lázně Fault (MLF) zone is situated in the western part of the Bohemian Massif (Czech Republic, central Europe) and controls the eastern limit of Cenozoic Cheb Basin. The Cheb Basin is well-known for present-day earthquake swarms, with maximum magnitude not exceeding Mw 4.0, and abundant occurrences of mantle-derived carbon-dioxide emanations. The MLF borders the Cheb Basin towards the Krušné hory Mts. controlling their morphologically pronounced mountain front. However, no large earthquakes required for forming the tectonic morphology (Mw > 6) have been recorded historically. To examine if potential pre-historic earthquakes contributed to mountain front formation along the MLF and to Late Quaternary seismic hazard in the region, we excavated a trench at the Kopanina site where the fault was identified by geomorphological and geophysical surveys. The trench revealed repeated movements along several fault strands within the MLF of various ages, which displaced basin sediments deposited since the Oligocene. The youngest faults cut and deformed Holocene layers at least twice, with a total cumulative vertical displacement on the main fault 0.5 m. The deformations that we documented here suggest transpression within the MLF during Holocene. Based on empirical relationships between magnitude and displacement/rupture length, the MLF has produced earthquakes with a minimum magnitude of Mw = 6.5. The youngest event is dated approximately to the period 792–1020 CE. To match the youngest event with a historical earthquake, several candidates could be considered and further investigated, but the most probable one seems to be the earthquake reported in catalogs for 998 CE. Hence our study revealed the youngest proven surface faulting in central Europe and the largest Holocene event reported so far for the Bohemian Massif, which is of great importance for seismic hazard re-assessment for the broader, slowly moving region.