Active tectonics of the Lower Rhine Graben (NW Central Europe)

Identification of active seismogenic faults in low-strain intraplate regions is a major challenge. The understanding of intraplate earthquakes is hampered by the spatiotemporal scattering of large earthquakes and by barely detectable strain accumulation. In populated humid regions, both hillslope and anthropogenic processes are important challenges to the recognition of potentially active faults. The Lower Rhine Graben is the NW segment of the European Cenozoic Rift System. It is a prime example of a seismically active low-strain rift situated in a humid and densely populated region. The approximate location of potentially active fault segments in this region is well known, but knowledge of the recurrence of large earthquakes and of the dominant fault slip mode is still rudimentary. The current debate ranges from slip dominated by repeated large earthquakes to slip dominated by aseismic creep. The purpose of this thesis is to determine whether the Lower Rhine Graben is an exception to the usually observed deformational behaviour of the upper crust, whereby active faults fail by brittle behaviour. The thesis addresses the Holocene, historical and present-day tectonic activity of the Lower Rhine Graben. It examines the signs of coseismic deformation in the geological record, and the surface expression of active fault segments. I analyzed high-resolution LiDAR terrain models of segments of the Erft and Wissersheimer faults, in order to understand the preservation potential of active fault scarps in populated, humid settings. Results of the LiDAR analysis illustrate that the central part of the Lower Rhine Graben is characterized by severe degradation and modification of suspected seismogenic structures. Degradation is due to fluvial erosion, hillslope processes and anthropogenic overprint. This analysis shows, also for the first time, the severity of surface modification of the region resulting from aerial bombing during World War II. A large trench excavation at the Schafberg fault in Holocene sediments yielded a broadly distributed fault zone with a peculiar abundance of fractured clasts. A particular question at this site is whether or not the fault ruptured in the 1756 AD Düren event. The excavation reveals the first evidence of historical seismogenic faulting in the Lower Rhine Graben. Coseismic deformation at this site is expressed by a net vertical displacement of 1 ± 0.2 m and complex gravel fracturing. Analysis of the faulted strata and radiocarbon ages of event horizons reveal evidence of at least one, possibly two coseismic events since the Holocene. The youngest of them overlaps with the 1756 AD Düren earthquake. The complex deformation pattern in the trench included a range of features such as liquefaction, rotated, and fractured clasts in the fault zone. I developed a new analysis technique based on “fractured-clasts”, which allows insight into coseismic rupture and fracture processes in unconsolidated gravel deposits. Results of this paleoseismic study show that faults in the Lower Rhine Graben do not move dominantly by aseismic creep. They further support the observation that faults in low-strain intraplate rifts can produce large surface-breaking earthquakes. The results of this thesis further imply that specific patterns of fractured clasts in fault zones may be a detector of coseismic rupture, and could in principle be used to calculate the energy involved in the rupture process.In der Erdbebengeologie stellt die Identifizierung und Charakterisierung von seismogenen Strukturen in Intraplattenregionen mit niedrigen Deformationsraten eine große Herausforderung dar. Die stark variable, räumliche und zeitliche Verteilung von großen Erdbeben sowie kaum mit geodätischen Messmethoden erfassbare Verformungsraten erschweren das Verständnis von Intraplattenbeben. In dicht besiedelten, humiden Gebieten sind potenziell aktive Störungen noch schwerer zu erkennen, da diese, durch Hangprozesse und anthropogene Überprägung, intensiven Veränderungen unterworfen sind. Die Niederrheinische Bucht, das nordwestliche Segment des Europäischen Känozoischen Graben Systems, ist ein erstklassiges Beispiel für ein seismisch aktives Intraplattenrift in einem humiden und dicht besiedelten Gebiet. Obwohl die Lokationen potenziell aktiver Störungen bekannt sind, ist die Kenntnis bezüglich der Rekurrenzintervalle von großen Erdbeben sowie der Art der Störungsbewegungen nach wie vor rudimentär. Die derzeitige Debatte reicht von Störungsbewegungen durch starke Erdbeben, bis hin zu aseismischem Kriechen. Ziel dieser Arbeit ist es zu klären, ob der Niederrheingraben eine Ausnahme in Bezug auf das grundsätzlich beobachtete Deformationsverhalten der oberen Kruste darstellt, wo aktive Störungen durch sprödes Bruchverhalten charakterisiert sind. Hierbei befasst sich die Studie mit der Holozänen, historischen und heutigen tektonischen Aktivität des Niederrheingrabens und legt den Fokus auf die Erforschung koseismischer Prägung der Geologie sowie der Oberflächenexpression potenziell aktiver Störungen. Es wurden hochauflösende LiDAR-Geländemodelle an Segmenten der Erft- und Wissersheimer Störung in der zentralen Niederrheinischen Bucht untersucht, um das Erhaltungspotenzial aktiver Störungen in besiedelten, humiden Gebieten zu charakerisieren. Die Ergebnisse der LiDAR-Analyse offenbaren, dass seismogene Strukturen im zentralen Teil des Niederrheingrabens starker Degradation und Modifikation unterworfen sind. Hauptursachen hierfür sind fluviatile Erosion, Hangprozesse, sowie anthropogene Überprägung. In dieser Analyse konnte zudem erstmals die Schwere der Oberflächenveränderung in der Region, ausgelöst durch Fliegerbomben aus dem Zweiten Weltkrieg, dargestellt werden. In einer groß angelegten Schürfgrabenanalyse in Holozänen Sedimenten an der Schafbergstörung, wurde eine breite Deformationszone, die eine auffallend große Anzahl zerbrochener Klasten enthielt, freigelegt. Insbesondere ist es wichtig zu klären, ob diese Störung während des Dürener Bebens von 1756 AD gebrochen ist. Die Ergebnisse der Schürfgrabenanalysen an der Schafbergstörung liefern den ersten Beweis dafür, dass Erdbeben im Niederrheingraben in historischer Zeit die Oberfläche gebrochen haben. An diesem Störungssegment äußert sich die koseismische Deformation durch einen Gesamtversatz von 1 ± 0.2 m und komplexem verbreitetem Brechen von Klasten. Die Analyse mit-verworfener Schichten sowie Radiokarbondatierungen von Ereignishorizonten beweisen mindestens ein, möglicherweise zwei Holozäne koseismische Ereignisse. Das Jüngste der Beiden deckt sich hierbei mit dem Dürener Beben von 1756 AD. In der im Schürfgraben freigelegten Störungszone wurden komplexe Deformationsmuster wie Liquefaktion sowie rotierte und gebrochene Klasten dokumentiert. Basierend auf den gebrochenen Klasten habe ich in meiner Arbeit eine neue Analysetechnik entwickelt, die es ermöglicht, detaillierte Einblicke in koseismische Bruchprozesse in unverfestigten Geröllen zu erhalten. Die Ergebnisse dieser paläoseismologischen Studie weisen darauf hin, dass Störungen im Niederrheingraben nicht aseismisch kriechen. Darüber hinaus festigen sie die Beobachtung, dass Störungen in Intraplattenrifts mit niedrigen Deformationsraten, in der Lage sind, große Erdbeben mit Oberflächenruptur zu erzeugen. Die Daten dieser Studie zeigen des Weiteren, dass spezifische Muster gebrochener Klasten in Störungszonen ein möglicher Detektor koseismischer Rupturen sein können und demzufolge zur Energieberechnung des Rupturprozesses genutzt werden können

Development of a seismic source model for probabilistic seismic hazard assessment of nuclear power plant sites in Switzerland: the view from PEGASOS Expert Group 4 (EG1d)

Abstract.: We present a seismogenic source model for site-specific probabilistic seismic hazard assessment at the sites of Swiss nuclear power plants. Our model is one of four developed in the framework of the PEGASOS project; it contains a logic tree with nine levels of decision-making. The two primary sources of input used in the areal zonation developed by us are the historical and instrumental seismicity record and large-scale geological/rheological units. From this, we develop a zonation of six macrozones, refined in a series of seven decision steps up to a maximum of 13 zones. Within zones, activity rates are either assumed homogeneous or smoothed using a Gaussian kernel with width of 5 or 15km. To estimate recurrence rate, we assume a double truncated Gutenberg-Richter law, and consider five models of recurrence parameters with different degrees of freedom. Models are weighted in the logic tree using a weighted Akaike score. The maximum magnitude is estimated following the EPRI apporach. We perform extensive sensitivity analyses in rate and hazard space in order to assess the role of declustering, the completeness model, quarry contamination, border properties, stationarity, regional b-value and magnitude-dependent hypocentral dept

Seismic source characterization of the Alpine foreland in the context of a probabilistic seismic hazard analysis by PEGASOS Expert Group 1 (EG1a)

Seismic source characterization is performed as part of the PEGASOS project for the assessment of the seismic hazard at the 4 sites of the Swiss Nuclear Power Plants. The analysis is performed according to the Level 4 procedures for expert elicitation defined in the guidelines of the US Nuclear Regulatory Committee whereby the quantification of uncertainties plays a crucial role. According to our analysis, which is one amongst four that were performed in the frame of PEGASOS, the most important epistemic uncertainty is related to the question as to weather basement-rooted faults at the margins of pre-existing Permo-Carboniferous troughs are prone for compressive or transpressive reactivation under the present-day stress field or not. The question after the present-day style of deformation in the Alpine foreland (thick-skinned versus thin-skinned) is closely related to this key question. Together with the consideration of uncertainties regarding the mapping of seismogenic zones and/or line sources alternative zonations are presented in form of a logic tree with 21 branches. Area sources play a predominant role in the working area located at the margin of a diffuse plate boundary. Earthquake recurrence relationships are discussed by taking into account a series of uncertainties. These concern the evaluation of b-values and the evaluation of a-values once the b-values were fixed. Both parameters in the Gutenberg-Richter law are based on non-perfect and incomplete catalogue data that were carefully analysed beforehand. Since PEGASOS demanded an analysis of annual probabilities down to one event in 107years, the question after the value of the maximum possible earthquake magnitude Mmax and related error in Mmax estimates plays a crucial role. We estimate Mmax by using geological as well as statistical methods. Mmax = 6.9 cannot be excluded in most areas, in the Basel area Mmax = 7.3 is possible. Uncertainties in a, b and Mmax are again discussed in form of a logic tree, this time with 18 branches. Hence the final logic tree has 378 branches and represents the seismic source characterization input into PSHA that takes account of all uncertainties we are aware o

Geological Criteria for Evaluating seismicity revisited: Forty Years of Paleoseismic Investigations and the Natural Record of Past Earthquakes

The identifi cation of individual past earthquakes and their characterization in time and space, as well as in magnitude, can be approached in many different ways with a large variety of methods and techniques, using a wide spectrum of objects and features. We revise the stratigraphic and geomorphic evidence currently used in the study of paleoseismicity, after more than three decades since the work by Allen (1975), which was arguably the fi rst critical overview in the fi eld of earthquake geology. Natural objects or geomarkers suitable for paleoseismic analyses are essentially preserved in the sediments, and in a broader sense, in the geologic record. Therefore, the study of these features requires the involvement of geoscientists, but very frequently it is a multidisciplinary effort. The constructed environment and heritage, which typically are the focus of archaeoseismology and macroseismology, here are left aside. The geomarkers suitable to paleoseismic assessment can be grouped based on their physical relation to the earthquake\u2019s causative fault. If directly associated with the fault surface rupture, these objects are known as direct or on-fault features (primary effects in the Environmental Seismic Intensity [ESI] 2007 scale). Conversely, those indicators not in direct contact with the fault plane are known as indirect or off-fault evidence (secondary effects in the ESI 2007 scale). This second class of evidence can be subdivided into three types or subclasses: type A, which encompasses seismically induced effects, including soft-sediment deformation (soil liquefaction, mud diapirism), mass movements (including slumps), broken (disturbed) speleothems, fallen precarious rocks, shattered basement rocks, and marks of degassing (pockmarks, mud volcanoes); type B, which consists of remobilized and redeposited sediments (turbidites, homogenites, and tsunamites) and transported rock fragments (erratic blocks); and type C, entailing regional markers of uplift or subsidence (such as reef tracts, microatolls, terrace risers, river channels, and in some cases progressive unconformities). The fi rst subclass of objects (type A) is generated by seismic shaking. The second subclass (type B) relates either to water bodies set in motion by the earthquake (for the sediments and erratic blocks) or to earthquake shaking; in a general way, they all relate to wave propagation through different materials. The third subclass (type C) is mostly related to the tectonic deformation itself and can range from local (next to the causative fault) to regional scale. The natural exposure of the paleoseismic objects\u2014which necessarily conditions the paleoseismic approach employed\u2014is largely controlled by the geodynamic setting. For instance, oceanic subduction zones are mostly submarine, while collisional settings tend to occur in continental environments. Divergent and wrenching margins may occur anywhere, in any marine, transitional, or continental environment. Despite the fact that most past subduction earthquakes have to be assessed through indirect evidence, paleoseismic analyses of this category of events have made dramatic progress recently, owing to the increasingly catastrophic impact that they have on human society

Geological Criteria for Evaluating Seismicity Revisited: Forty Years of Paleoseismic Investigations and the Natural Record of Past Earthquakes

The identification of individual past earthquakes and their characterization in time and space, as well as in magnitude, can be approached in many different ways with a large variety of methods and techniques, using a wide spectrum of objects and features. We revise the stratigraphic and geomorphic evidence currently used in the study of paleoseismicity, after more than three decades since the work by Allen (1975), which was arguably the first critical overview in the field of earthquake geology. Natural objects or geomarkers suitable for paleoseismic analyses are essentially preserved in the sediments, and in a broader sense, in the geologic record. Therefore, the study of these features requires the involvement of geoscientists, but very frequently it is a multidisciplinary effort. The constructed environment and heritage, which typically are the focus of archaeoseismology and macroseismology, here are left aside. The geomarkers suitable to paleoseismic assessment can be grouped based on their physical relation to the earthquake\u2019s causative fault. If directly associated with the fault surface rupture, these objects are known as direct or on-fault features (primary effects in the Environmental Seismic Intensity [ESI] 2007 scale). Conversely, those indicators not in direct contact with the fault plane are known as indirect or off-fault evidence (secondary effects in the ESI 2007 scale). This second class of evidence can be subdivided into three types or subclasses: type A, which encompasses seismically induced effects, including soft-sediment deformation (soil liquefaction, mud diapirism), mass movements (including slumps), broken (disturbed) speleothems, fallen precarious rocks, shattered basement rocks, and marks of degassing (pockmarks, mud volcanoes); type B, which consists of remobilized and redeposited sediments (turbidites, homogenites, and tsunamites) and transported rock fragments (erratic blocks); and type C, entailing regional markers of uplift or subsidence (such as reef tracts, microatolls, terrace risers, river channels, and in some cases progressive unconformities). The first subclass of objects (type A) is generated by seismic shaking. The second subclass (type B) relates either to water bodies set in motion by the earthquake (for the sediments and erratic blocks) or to earthquake shaking; in a general way, they all relate to wave propagation through different materials. The third subclass (type C) is mostly related to the tectonic deformation itself and can range from local (next to the causative fault) to regional scale. The natural exposure of the paleoseismic objects\u2014which necessarily conditions the paleoseismic approach employed\u2014is largely controlled by the geodynamic setting. For instance, oceanic subduction zones are mostly submarine, while collisional settings tend to occur in continental environments. Divergent and wrenching margins may occur anywhere, in any marine, transitional, or continental environment. Despite the fact that most past subduction earthquakes have to be assessed through indirect evidence, paleoseismic analyses of this category of events have made dramatic progress recently, owing to the increasingly catastrophic impact that they have on human society

Geophysical characterization of buried active faults: the Concud Fault (Iberian Chain, NE Spain)

The Concud Fault is a ~14-km-long active fault that extends close to Teruel, a city with about 35,000 inhabitants in the Iberian Range (NE Spain). It shows evidence of recurrent activity during Late Pleistocene time, posing a significant seismic hazard in an area of moderate-to-low tectonic rates. A geophysical survey was carried out along the mapped trace of the southern branch of the Concud Fault to evaluate the geophysical signature from the fault and the location of paleoseismic trenches. The survey identified a lineation of inverse magnetic dipoles at residual and vertical magnetic gradient, a local increase in apparent conductivity, and interruptions of the underground sediment structure along GPR profiles. The origin of these anomalies is due to lateral contrast between both fault blocks and the geophysical signature of Quaternary materials located above and directly south of the fault. The spatial distribution of anomalies was successfully used to locate suitable trench sites and to map non-exposed segments of the fault. The geophysical anomalies are related to the sedimentological characteristics and permeability differences of the deposits and to deformation related to fault activity. The results illustrate the usefulness of geophysics to detect and map non-exposed faults in areas of moderate-to-low tectonic activity where faults are often covered by recent pediments that obscure geological evidence of the most recent earthquakes. The results also highlight the importance of applying multiple geophysical techniques in defining the location of buried faults

Active Faulting in Lake Constance (Austria, Germany, Switzerland) Unraveled by Multi-Vintage Reflection Seismic Data

Probabilistic seismic hazard assessments are primarily based on instrumentally recorded and historically documented earthquakes. For the northern part of the European Alpine Arc, slow crustal deformation results in low earthquake recurrence rates and brings up the necessity to extend our perspective beyond the existing earthquake catalog. The overdeepened basin of Lake Constance (Austria, Germany, and Switzerland), located within the North-Alpine Molasse Basin, is investigated as an ideal (neo-) tectonic archive. The lake is surrounded by major tectonic structures and constrained via the North Alpine Front in the South, the Jura fold-and-thrust belt in the West, and the Hegau-Lake Constance Graben System in the North. Several fault zones reach Lake Constance such as the St. Gallen Fault Zone, a reactivated basement-rooted normal fault, active during several phases from the Permo-Carboniferous to the Mesozoic. To extend the catalog of potentially active fault zones, we compiled an extensive 445 km of multi-channel reflection seismic data in 2017, complementing a moderate-size GI-airgun survey from 2016. The two datasets reveal the complete overdeepened Quaternary trough and its sedimentary infill and the upper part of the Miocene Molasse bedrock. They additionally complement existing seismic vintages that investigated the mass-transport deposit chronology and Mesozoic fault structures. The compilation of 2D seismic data allowed investigating the seismic stratigraphy of the Quaternary infill and its underlying bedrock of Lake Constance, shaped by multiple glaciations. The 2D seismic sections revealed 154 fault indications in the Obersee Basin and 39 fault indications in the Untersee Basin. Their interpretative linkage results in 23 and five major fault planes, respectively. One of the major fault planes, traceable to Cenozoic bedrock, is associated with a prominent offset of the lake bottom on the multibeam bathymetric map. Across this area, high-resolution single channel data was acquired and a transect of five short cores was retrieved displaying significant sediment thickness changes across the seismically mapped fault trace with a surface-rupture related turbidite, all indicating repeated activity of a likely seismogenic strike-slip fault with a normal faulting component. We interpret this fault as northward continuation of the St. Gallen Fault Zone, previously described onshore on 3D seismic data

Studies on "precarious rocks" in the epicentral area of the AD 1356 Basle earthquake, Switzerland

For the first time precarious rocks have been analysed in the epicentral area of the AD 1356 Basle earthquake in northern Switzerland. Several cliff sites in flat-lying, thickly bedded Upper Jurassic coral limestones in the Jura Mountains were investigated. Seven blocks are regarded as precarious with respect to earthquake strong ground motions. The age of these precarious rocks could not be determined directly as for instance by radiometric dating methods; however, based on slope degradation processes it can be concluded that the formation of these blocks predates the AD 1356 Basle earthquake. The acceleration required to topple a precarious rock from its pedestal is estimated using geometrical data for individual block sections and earthquake strong-motion records from stations on rock sites in the European Strong-Motion Database as input data for the computer program ROCKING V1.0 from the Seismological Laboratory, University of Nevada, Reno. The calculations indicate that toppling of a precarious rock largely depends on earthquake strength but also on the frequency spectrum of the signal. Although most investigated precarious rocks are surprisingly stable for ground motions similar to those expected to have occurred during the AD 1356 Basle earthquake, at least two blocks are clearly precariously balanced, with peak toppling accelerations lower than 0.3 g. Possible reasons why these blocks did not topple during the AD 1356 Basle earthquake include incomplete separation from their base, sliding of precarious rocks, their size, lower than assumed ground accelerations and/or duration of shaking

Holocene paleo-earthquakes recorded at the transfer zone of two majorfaults: the Pastores and Venta de Bravo fault (Trans-Mexican Volcanic Belt).

We present evidence of fi ve late Holocene earthquake ruptures observed at two paleoseismological trenches in the Laguna Bañí sag pond (Trans-Mexican Volcanic Belt, central Mexico). The trenches exposed two fault branches of the western termination of the Pastores fault, one of the major fault systems within the central Trans-Mexican Volcanic Belt. The site was studied by combining geomorphological and structural approaches, volcanic mapping, ground-penetrating radar, and paleoseismological analysis. The study revealed that coseismic surface rupture was noncharacteristic, and that the exposed fault branches had not always moved simultaneously. The fault tip has ruptured at least 5 times within the past 4 k.y., and the rupture events followed and preceded the deposition of an ignimbrite. The close temporal relationship of the seismic rupture with the volcanic activity of the area could be the result of volcanism triggered by faulting and its associated seismicity. The relatively high recurrence of seismic events (1.1 2.6 k.y.) and the noncharacteristic fault behavior observed at this tip of the Pastores fault suggest that the fault might have been active as a primary fault rupturing along segments of variable length or depth, and/or that the fault ruptured eventually as a secondary fault. The secondary ruptures would likely be related to earthquakes produced at major neighboring faults such as the Acambay fault, which moved during the 1912 Acambay earthquake, or the Venta de Bravo fault. A relatively large slip rate estimated for this fault branch (0.23 0.37 mm/yr) leads us to contemplate the possible connection at depth between the Pastores and the Venta de Bravo faults, increasing the maximum expected magnitude for central Mexico

A Probabilistic Approach to Seismic Hazard in Metropolitan France

In this study, we applied a probabilistic methodology to seismic hazard assessment in metropolitan France. For that purpose we determined an attenuation law adapted to the French context. This law holds for peak ground acceleration on stiff bedrock for earthquakes with local magnitudes between 2.5 and 5.6 recorded in near field (at distances between 3 and 50 km). Geological conditions are taken into account by means of a three-categories classification of lithologies based on a 1/1,000,000 geological map. The seismotectonic zonation consists of areas of diffuse seismicity characterized by a frequency-magnitude distribution. In southeastern France, active faults are considered in a test case and are assumed to follow the characteristic earthquake model. We performed hazard curves for six French cities and maps of peak horizontal ground accelerations expected for return periods of 475, 975, and 1975 years in the country. Sensitivity tests have been performed. The uncertainty introduced by ground-motion variability seems minor compared with that due to the choice of the attenuation law. This study points to the importance of testing internal consistency of the various data and laws used in any seismic hazard analysis (in particular, here the type of magnitude used to predict ground motion). If not, some systematic bias is introduced that may result in systematic errors on peak ground acceleration determination. We also show that the introduction of possibly very large and infrequent events, known only from paleoseismic investigations, may have a dramatic impact on the hazard, especially when long periods of time are considered