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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. Highlights • Shallow geophysics traced the Mariánské Lázně fault (MLF) on surface and at depth. • Geomorphic mapping and stream-length index revealed morphological expression of MLF. • Two Holocene earthquakes of Mw = 6.3–6.5 were identified in geological record. • The latest surface-breaking event on MLF was dated by 14C method to interval from 792 to 1020 CE. • We discovered the largest historical events reported to date for the Bohemian Massif. [ABSTRACT FROM AUTHOR]

We give an interpretive review of the geological evolution of the Nysa-Morava Zone (NMZ)-a Late Cenozoic tectonically active region of the NE Bohemian Massif located at its contact with the Western Carpathians' orogenic front. This crustal domain, delimited by generally NW-SE-striking fault system, is characterised by Oligo-Miocene and Plio-Pleistocene volcanic activity, regionally anomalous, weak historical and present-day seismicity and increased CO flux. The NMZ hosts several elongated, mostly NW-SE-trending, graben-like sedimentary basins (Upper Morava Basin System), which are filled by more than 300-m-thick succession of clastic fluvial/lacustrine sediments of Pliocene-Quaternary age. Based on geometric relations, basin architecture, coincidence of seismicity with CO escape and sparse focal mechanism data, a model is proposed, which explains this active domain as a transfer zone developed between major WNW-ESE and NW-SE faults in a right-lateral transpressional setting. It is suggested that slow horizontal slip at these faults resulted in local permutations of the largest and medium stress directions and formation of transtensional crustal domains in the NMZ. Moreover, relation of the NMZ to the Alpine-Carpathian system and sedimentary grabens in its foreland is discussed. The absence of Paleogene and Lower Miocene deposits suggests that subsidence in the NMZ was commenced later than in the European Cenozoic Rift System (ECRIS), which is in agreement with later thrusting in Western Carpathians at ~17 Ma. The quantitative contrasts to the ECRIS in terms of faulting and subsidence rates are explained by the absence of lithospheric/crustal thinning in the NMZ. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the results of two separate geophysical investigations undertaken across the Sudetic Marginal Fault zone in the Bohemian Massif. This fault zone represents one of the most important tectonic features in central Europe. The first, preliminary, investigation used electrical resistivity tomography (ERT) to define the exact position of the main fault at two localities (Kamenička & Bílá Voda). Both profiles revealed a resistivity gradient that divided a complex of crystalline rocks with high resistivities from sedimentary deposits with low resistivities. From these data, sites were selected for palaeoseismic trenching. This trenching provided detailed information regarding the surveyed structures and lithologies. The second, more extensive, investigation used electrical resistivity tomography and gravimetric survey in order to extend the previously recognised features both laterally and to depth. The fault is displayed by the presence of an expressive resistivity gradient, even on profiles undertaken in more homogeneous lithologies. Finally, the most intensive investigations were conducted at Bílá Voda, where highly detailed ERT was undertaken on the floor of the trench. From this, it was possible to trace approximately logged geological structures to far greater depths and assess sedimentary thicknesses. A gravimetric survey was also undertaken adjacent to the trench. From modelling the gravitational effects of the fault zone, it is shown that the fault dips about 75° NE and that the vertical offset of Miocene vs. bedrock on the fault is approximately 200m. The presented study has demonstrated the value of undertaking two phases of geophysical exploration separated by a trenching investigation. It is considered that this methodology is particularly suitable for the study of active tectonics in analogous intraplate settings where fault slip-rate is low. [Copyright &y& Elsevier]

Abstract: The NW–SE striking Sudetic Marginal Fault (SMF) is one of the most conspicuous tectonic structures in central Europe. It controls the pronounced morphotectonic escarpment of the Sudetic Mountains for a length of 130km. This paper presents the results of trenching on the SMF, undertaken in order to assess activity of the fault. The trenching technique has not hitherto been applied on either this particular fault system or elsewhere in the Bohemian Massif. However, it is the most effective tool for near-surface fault investigation in areas that are well vegetated and therefore devoid of fault outcrops. Moreover, the study area is situated in an intraplate region with a low displacement rate, which is exceeded by an erosion rate and does not favor the preservation of fault scarps. The trench sites were selected from prior DEM analyses, geomorphological fieldwork, and geophysical sounding. The trenches exposed a range of lithologies of Variscan crystalline rocks and Caenozoic sediments. At least four phases of faulting have been distinguished based on structural data, succession and age of the deformed sediments. Reverse faulting (N160°E), which displaced the Miocene sediments over the crystalline rocks, post-dates their deposition (15Ma) and pre-dates Late Glacial gelifluction. Horizontal movements that have affected the Miocene deposits along a structure striking N35°E have the same wide time constraint. Younger reverse faulting, which caused a co-seismic relief step, post-dates the Late Glacial gelifluction but preceded the early Holocene colluvium deposition (10,940±140calyrsBP). Normal faults striking N145°E, which cut the Miocene unit, might have been reactivated after the early Holocene colluvial sedimentation but before the buried soil (430±120calyrsBP). Moreover, based on the identified prehistoric earthquake, respective minimum moment magnitude M 6.3 and slip rate 0.03mm/year were estimated. [Copyright &y& Elsevier]

Most catastrophic earthquakes occur along fast-moving faults, although some of them are triggered by slow-moving ones. Long paleoseismic histories are infrequent in the latter faults. Here, an exceptionally long paleoseismic record (more than 300 k.y.) of a slow-moving structure is presented for the southern tip of the Alhama de Murcia fault (Eastern Betic shear zone), which is characterized by morphological expression of current tectonic activity and by a lack of historical seismicity. At its tip, the fault divides into a splay with two main faults bounding the Góñar fault system. At this area, the condensed sedimentation and the distribution of the deformation in several structures provided us with more opportunities to obtain a complete paleoseismic record than at other segments of the fault. The tectonic deformation of the system was studied by an integrated structural, geomorphological, and paleoseismological approach. Stratigraphic and tectonic features at six paleoseismic trenches indicate that old alluvial units have been repeatedly folded and thrusted over younger ones along the different traces of the structure. The correlation of the event timing inferred for each of these trenches and the application of an improved protocol for the infrared stimulated luminescence (IRSL) dating of K-feldspar allowed us to constrain a paleoseismic record as old as 325 ka. We identified a minimum of six possible paleo-earthquakes of Mw = 6-7 and a maximum mean recurrence interval of 29 k.y. This provides compelling evidence for the underestimation of the seismic hazard in the region. [ABSTRACT FROM AUTHOR]