6th Colloquium on historical earthquakes & paleoseismology studies

Historical surface-breaking earthquake in central Europe?

24 Oct 2018, 13:45

35m

Han-sur-Lesse

Poster Preference Session 2 Posters Poster Session 1

Speaker

Dr Petra Štěpančíková (Czech Academy of Sciences)

Description

We analysed morphologically pronounced, NNW-SSE trending Mariánské Lázně Fault (MLF) situated in the western part of the Bohemian Massif (Czech Republic, central Europe). The MLF controls the eastern limit of Cheb-Domažlice Graben. In the northern part it borders the Cenozoic Cheb basin towards mountain front of Krušné hory Mts and intersects with NE-trending Cenozoic Eger rift. The Cheb basin is known for Mid-Pleistocene volcanism, abundant occurrences of mantle-derived carbon-dioxide emanations, and present-day earthquake swarms with maximum magnitude not exceeding Mw 4.0. These present-day swarms are mostly aligned along a NNW-trending fault known only from the foci at the depth with no geological or morphological expression on the surface and which intersects mountain front of the Krušné hory Mts controlled by the MLF, trending NW-SE in the Cheb basin segment. Since no significant recent seismicity or large earthquakes (Mw>6) have been reported for the MLF, we used 3D paleoseismic trenching to look for a large surface-rupturing prehistoric earthquake, which could be responsible for the pronounced mountain range front. We excavated seven trenches and six hand-dug trenches at the site Kopanina with most suspicious morphology related to the MLF zone. The trenches revealed a complex geology and deformation probably as a result of right-lateral transpression during Late Quaternary. Even Holocene colluvia appeared to be faulted, indicating at least two Holocene surface-breaking earthquakes on the MLF with possible minimum magnitude Mw=6.3-6.5. OxCal modelling based on radiocarbon dating of charcoals sampled from the faulted Holocene deposits shows a prehistoric earthquake in the period 1134 BC – 192 BC and the latter one during 792 – 1020 AD. Several candidates for a historical earthquake of the latter period were analysed from the historic earthquakes catalogues. So far it seems that the surface-breaking earthquake that we revealed from the trenching could have matched with the reported historical earthquake that occurred 998 AD. Further investigation on this match is under process. Nevertheless, our paleoseismic study revealed the youngest surface-breaking earthquake in central Europe and the largest one in the Bohemian Massif proven so far. This might have a great implication for seismic hazard for areas with slow-moving faults.