Active intraplate deformation in central Europe: paleoseismology of the Lower Rhine Embayment
Final Report Abstract
The focus of this study was to investigate the Holocene and historical fault activity of the Lower Rhine Graben (NW Germany) by combining paleoseismical approaches, analysis of high-resolution LiDAR data, and outcrop- to micro-scale analysis of coseismically deformed gravel deposits. In the slowly-deforming Lower Rhine Graben, the tectonic imprint on young sediments is difficult to identify. The reasons for this are (1) the long recurrence intervals of large earthquakes, relative to the sedimentation rates in the adjacent basins, and, (2) the extensive overprint of the landscape due to hillslope processes, soil erosion, and anthropogenic influence. Former paleoseismological studies carried out in the Lower Rhine Graben had successfully identified earthquake-related deformation in Pleistocene sediments. Due to a lack of fault exposures in younger sediments, however, the present-day seismogenic potential of most of the faults in the Lower Rhine Graben is unknown. Seismic activity of the Schafberg fault, which is considered a subsidiary fault can be related to Holocene and historical surface rupture, as the youngest identified event in the trench most likely matches the 1756 AD Düren earthquake. This implies that at least one, perhaps neighboring faults, too, can produce coseismic surface ruptures. The results of this study imply that future paleoseismic and tectonogeomorphic research in the Lower Rhine Graben must focus on faults concealed by Holocene sediments, rather than on analysis of the main border faults. Recurrence interval and slip-rate estimates on the Schafberg fault, as well as historical and instrumental earthquake records further imply that the tectonic activity of the Lower Rhine Graben is currently concentrated in the SW sector of the rift system. In order to better understand the spatio-temporal evolution of the Lower Rhine Graben, however, additional paleoseismological studies, for example, in the eastern and central sectors of the region are needed. Understanding coseismic rupture and aseismic creep processes, as well as their distinction in the geological record, are important tasks for earthquake geologists in both, intraplate and plate boundary settings. In particular, unconsolidated sediments cut by active faults often lack distinct features for differentiating aseismic and coseismic slip. In low-strain regions, however, most seismogenic ruptures barely reach the surface. Therefore, coseismic features are intrinsically small, such as those near the ends of la rge-magnitude rupture segments or those in the center of ruptures just large enough to break the surface. We have developed a new approach, using 'fractured clasts' to analyze tectonically deformed gravel deposits. Systematic analysis of the spatial distribution of fractured clasts, as well as fracture-orientation measurements are valuable observables to detect seismogenic rupture processes in unconsolidated sediments. Under specific condifions, this approach may be useful for the identification of earthquake-related deformation in intraplate settings. Elsewhere, the fractured-clast analysis may also be useful for potential supershear faults such as the Kunlun, Denali, or Dead Sea faults. Here, the distribution of fractured clasts may be used to pin point the relative timing between the propagating rupture on the one hand, and shaking-related deformation on the other hand. The methods developed in this thesis expand the tools and strategies available for future paleoseismic research. Such strategies include trench site selection specifically in gravel deposits, as well as the combination of field observations of deformed gravels with laboratory tests and modeling approaches. This study showed that at least one fault in the Lower Rhine Graben has been seismically active in historic time. I postulate that aseismic creep, if any, may result from near-surface motion of faults due to mining-induced subsidence and does not represent the dominant fault-slip mode in this region. However, a distinction between seismically and aseismically generated features in the geological record remains difficult for small-offset faults, and needs further investigation. The fractured-clast approach could be tested some of the larger faults in the central Lower Rhine Graben, e.g., at the Erft-, or Rurrand faults, where mining-induced near-surface creep overlaps with larger offset seismogenic ruptures. Promising future paleoseismic approaches combine traditional trenching and high-resolution remote sensing analysis with micro-deformation analysis and modeling approaches.
Publications
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2005, Moment release in the Lower Rhine Embayment, Germany: seismological perspective of the deformation process. Geophys. J. Int. 160, p. 901-909
Schmedes, J., Hainzl, S., Reamer, S. K., Scherbaum, F., Hinzen, K. G.
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2010 "Paleoseismic evidence for seismogenic fauhing in the epicentrical area of the 1755/56 Düren earthquake series. Lower Rhine Embayment, NW Germany"; Abstract, EGU, Annual Meeting 2010, Vienna, Austria
Kübler S., Friedrich A., Strecker M.
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2011, Seismogenic surface faulting in the area of Germany's strongest historical earthquake, evidence from fractured clast analysis. In: Grützner, C et al. (eds.): Proceedings 2nd INQUA-IGCP7International Workshop on Active Tectonics, Earthquake Geology, Archaeology and Engineering 19-24 September 2011, Corinth (Greece), ISBN 978-960-466-093-3, 111-113
Kübler, S., Friedrich, A. M., Strecker, M. R.
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2011, Seismogenic surface faulting in the area of Germany's strongest historical earthquake, Lower Rhine Embayment, NW Germany, In: Breitkreuz, C., Gursky, H. J. (eds.): Geo-risk management, a German Latin American approach, Freiberger Forschungshefte C538, ISBN 978-3-86012-419-2, 13-16
Kübler, S., Friedrich, A. M., Strecker, M. R.
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2013, Active tectonics of the Lower Rhine Graben (NW central Europe) based on new paleoseismological constraints and implications for coseismic rupture processes in unconsolidated gravels, PhD Thesis, LMU Munich, Germany
Kübler, S.
