LATE QUATERNARY FAULTING ON THE SUDETIC MARGINAL FAULT; BÍLÁ VODA SITE (BOHEMIAN MASSIF)

The Sudetic Marginal Fault (SMF) is a part of the WNW-striking Elbe Fault System and is situated in the north-eastern limit of the Bohemian Massif (central Europe). For a length of 130 km the SMF controls the pronounced mountain front of the Sudetic Mountains. The SMF does not show present-day seismicity and the historic earthquakes recorded within the fault zone had epicentral intensity estimated to reach only I₀=4-7 (MSK). These intensities would correspond to macroseismic magnitude M_M=3-4.9, so the earthquakes were not large enough to create the morphology. Thus, the study of a potential presence of large prehistoric earthquakes responsible for the origin of the mountain front was carried out.

Five trenches across the SMF were performed at the locality Bílá Voda (Czech Republic) in order to study the mountain front. The trenches revealed subvertical fault zone (striking 135°-150°/75°NE) with a flower structure suggesting strike-slip movements, which divides Paleozoic crystalline rocks (phyllites, gneisses, granitic aplite) on the footwall from Late Quaternary colluvial deposits mostly overlaying warped Miocene sediments on the hanging-wall. These colluvial deposits close to the fault show characteristics of fault-derived colluvial wedges and their ages constrained by OSL and radiocarbon dating range from 9.5 ka to 2.6 ka. Along with deformed overlaying banded layers dated as ca 2.6 ka to 0.8 ka they suggest at least four to five large morphogenic earthquakes during late Quaternary (Holocene). The youngest recorded faulting (probably horizontal movement with a vertical compression component), which deformed the banded layers displays the vertical component around 35 cm. The amount of this youngest movement is in accordance with the authors´ published results from the previous trenching at the site Vlčice u Javorníka, and based on the empirical relationship ‘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 horizontal, further trenching parallel to the fault is a subject of further work to specify the amount of horizontal offset and the related slip-rate.