Large pre-historic earthquakes on the Sudetic Marginal Fault in the low-strain intraplate Bohemian Massif (Central Europe)

The Sudetic Marginal Fault (SMF) is a component of the WNW-striking Elbe Fault System that forms part of the north-eastern limit of the Bohemian Massif (central Europe). It controls the pronounced mountain front of the Sudetic Mountains for a length of 130 km, although it does not show present-day seismicity and the historic earthquakes recorded within the fault zone only reached epicentral intensities of I=4-7 (MSK), which corresponds to macroseismic magnitudes of M=3-4.9. Consequently, we initiated a paleoseismic search for larger prehistoric earthquakes responsible for the origin of the mountain front. Eight trenches were excavated across the SMF at Bila Voda (Czech Republic) in order to study the activity of the fault that produced the mountain front. The trenches revealed a subvertical fault zone (striking 135°-150°/75°NE) with a flower structure, suggesting strike-slip motion, juxtaposing Paleozoic crystalline rocks (phyllites, gneisses, granitic aplite) on the footwall from Late Quaternary colluvial and fluvial deposits overlying warped Miocene sediments on the downthrown hanging-wall. The colluvial deposits close to the fault show characteristics of fault-derived colluvial wedges. Along with deformed overlying banded layers dated to ca 2.6 ka to 0.8 ka by radiocarbon dating, these observations suggest the occurrence of at least four to five large morphogenic earthquakes during the Late Quaternary (Holocene). The youngest recorded faulting (probably horizontal movement), which deformed the banded layers, displays about 35 cm of vertical motion. The amount of this youngest movement is in accordance with our previous results from former trenching at Vlcice u Javornika site, and based on the empirical relationship of magnitude versus maximum vertical displacement, the minimum moment magnitude on the SMF is expected to be M 6.3. However, as the prevailing movements here are likely horizontal, additional trenching parallel to the fault is a subject of further work to specify the amount of horizontal offset and the related slip-rate.