The Saint-Ursanne earthquakes of 2000 revisited: evidence for active shallow thrust-faulting in the Jura fold-and-thrust belt

The interpretation of seismotectonic processes within the uppermost few kilometers of the Earth’s crust has proven challenging due to the often significant uncertainties in hypocenter locations and focal mechanisms of shallow seismicity. Here, we revisit the shallow seismic sequence of Saint-Ursanne of March and April 2000 and apply advanced seismological analyses to reduce these uncertainties. The sequence, consisting of five earthquakes of which the largest one reached a local magnitude (ML) of 3.2, occurred in the vicinity of two critical sites, the Mont Terri rock laboratory and Haute-Sorne, which is currently evaluated as a possible site for the development of a deep geothermal project. Template matching analysis for the period 2000–2021, including data from mini arrays installed in the region since 2014, suggests that the source of the 2000 sequence has not been persistently active ever since. Forward modelling of synthetic waveforms points to a very shallow source, between 0 and 1 km depth, and the focal mechanism analysis indicates a low-angle, NNW-dipping, thrust mechanism. These results combined with geological data suggest that the sequence is likely related to a backthrust fault located within the sedimentary cover and shed new light on the hosting lithology and source kinematics of the Saint-Ursanne sequence. Together with two other more recent shallow thrust faulting earthquakes near Grenchen and Neuchâtel in the north-central portion of the Jura fold-and-thrust belt (FTB), these new findings provide new insights into the present-day seismotectonic processes of the Jura FTB of northern Switzerland and suggest that the Jura FTB is still undergoing seismically active contraction at rates likely < 0.5 mm/yr. The shallow focal depths provide indications that this low-rate contraction in the NE portion of the Jura FTB is at least partly accommodated within the sedimentary cover and possibly decoupled from the basement.

The Swiss Alpine zero degree line: methods, past evolution, sensitivities

&lt;p&gt;The near-surface zero degree line (ZDL) is a key isotherm in mountain regions worldwide, but a detailed analysis of methods for the ZDL determination, their properties and applicability in a changing climate is missing. We here test different approaches to determine the near-surface ZDL on a monthly scale in the Swiss Alps. A non-linear profile yields more robust and more realistic ZDLs than a linear profile throughout the year and especially in the winter-half year when frequent inversions disqualify a linear assumption. In the period 1871-2019, the Swiss ZDL has risen significantly in every calendar month: In northern Switzerland, the monthly ZDL increases generally amount to 300-400 m with smaller values in April and September (200-250 m) and a larger value in October (almost 500 m). The largest increases of 600-700 m but also very large uncertainties (&amp;#177;400 m, 95% confidence interval) are found in December and January. The trends have accelerated in the last decades especially in spring and summer. The ZDL has increased by ~160 m per &amp;#176;C warming in the summer-half year and up to 340&amp;#177;45 m/&amp;#176;C in winter months. In southern Switzerland, ZDL trends and temperature scalings are somewhat smaller, especially in winter. Sensitivity analyses using a simple shift of the non-linear temperature profile suggest that the winter ZDL-temperature scalings are at a record high today or will reach it in the near future, and are expected to decrease with a strong future warming. Nevertheless, the cumulative ZDL increase for strong warming is considerably larger in winter than in summer. Based on a few key criteria, we also present best practises to determine the ZDL in mountain regions worldwide. The outlined methods lay a foundation for the analysis of further isotherms and to study the future ZDL evolution based on climate scenario data.&lt;/p&gt;

Exploring possible links between Quaternary aggradation in the Upper Rhine Graben and the glaciation history of northern Switzerland

The Quaternary filling of the Upper Rhine Graben is an excellent archive to reconstruct sediment dynamics in response to climate change, in particular related to past glaciations of the Swiss Alpine Foreland. Here, a sediment sequence recovered by drilling for exploration purposes near Kronau is investigated, using a combination of sedimentological logging, provenance studies (heavy minerals and clast petrography), and luminescence dating. Several phases of coarse sediment aggradation are identified that possibly correlate to Marine Isotope Stages (MIS) 12 (478–424 ka), 10 (374–337 ka), 8 (300–243 ka), 6 (191–130 ka) and/or 4 (71–57 ka), and 2 (29–14 ka). Several of these phases have previously also been reported from cores recovered in the major Quaternary depo-centre near Heidelberg. This suggests that the observed coarse aggradation in the Upper Rhine Graben can be assigned to various glaciations in northern Switzerland: Möhlin (MIS 12), Habsburg (MIS 10 or 8), Beringen (MIS 6), an unnamed glacial advance during MIS 4, and Birrfeld (MIS 2). However, due to the limited data available, this hypothesis and the suggested correlations require further confirmation by applying the approach presented here to further cores from the Upper Rhine Graben.

Quantification of Lithological Heterogeneity Within Opalinus Clay: Toward a Uniform Subfacies Classification Scheme Using a Novel Automated Core Image Recognition Tool

The Opalinus Clay is notable in Switzerland as being the selected host rock for deep geological disposal of radioactive waste. Since the early 1990’s, this argillaceous mudstone formation of Jurassic age has been intensively studied within the framework of national and international projects to characterize its geological, hydrological, mechanical, thermal, chemical, and biological properties. While there is no formal stratigraphic subdivision, the Opalinus Clay lithology is classically divided into several, dam- to m-scale sub-units (or facies), depending on location. Recent multi-proxy studies (combining petrographic, petrophysical, geochemical, and mineralogical analyses) have however demonstrated that high, intra-facies, lithological heterogeneity occurs at the dm- to cm-scale. To constrain this small-scale heterogeneity into distinct lithological units (subfacies), the present study aims at defining and presenting a convenient subfacies classification scheme covering the overall Opalinus Clay lithology across northern Switzerland. Petrographic (macro- and microfacies), mineralogical (X-ray diffraction) and textural (image analysis, machine learning and 3D X-ray computed tomography) analyses are performed on diverse drill cores from the Mont Terri rock laboratory (northwestern Switzerland), and results are extended further to the east (Riniken, Weiach, and Benken). Most of the investigated Opalinus Clay can be described by the use of five distinctive subfacies types (SF1 to SF5), which are visually and quantitatively distinguishable by texture (grain size, bedding, fabric, and color) and composition (nature and mineralogy of components). The five subfacies types can be further refined by additional attributes and sedimentary characteristics (biogenic, diagenetic, and structural). Eventually, the widespread and consistent use of standardized Opalinus Clay subfacies types provides the means to harmonize petrographic descriptions within multidisciplinary research projects, enhance reproducibility of in situ experiments, and further evidence the tight relations between lithology and various rock properties.

3-D Qp and Qs Seismic Attenuation for the Central Alps and their Foreland

&lt;p&gt;In the framework of the SeismoTeCH project, which aims at advancing our understanding of seismotectonic processes in Switzerland, we present the first 3-D attenuation model of the upper crust for the Central Alps and their northern foreland. The 3-D inversions derive the quality factor Q (1/attenuation) using path attenuation t&lt;sup&gt;&amp;#8727;&lt;/sup&gt; observations for 4,192 distributed earthquakes recorded on permanent and temporary stations, including both velocity and acceleration records for the period 2002-2019. We followed a procedure of gradational inversions, in which a series of inversions with increasingly grid complexity are performed, with the goal of obtaining a useful Q model everywhere despite the varied data distribution. The Qs and Qp results show large-scale features in the upper crust, which are consistent with a recently improved high-resolution velocity models of the same region and serve to refine the interpretations of crustal structures from Vp and Vp/Vs. For example, the foreland region of southern Germany and northern Switzerland show a low Q crustal block bounded by high Q regions in the uppermost layer between -2.5 and 2.0 km depth. This markedly correlates with the overlying surface geology, where low Q areas coincide with the Molasse Basin, and the transition between low and high Q regions outline the geological boundary between the Molasse and the Mesozoic sediments towards north and the Alpine front to the south. At depths ranging between 2.0 - 6.5 km, low Q is imaged along the Rhone valley in the Valais in southwest Switzerland. This region presents the transition between the Centrals and Western Alps and hosts the presently seismically most active fault zones. As the attenuation of fractured areas is enhanced by fluids, low Q values may relate here to distributed microfractures that produce greater fracture connectivity and permeability in a relatively higher strain-rate zone. These geophysical constraints seem to support crustal scale fluid flow along fracture networks as manifest by the prominent occurrence of hot springs in this area. On the other hand, the moderate-to-high Qs and Qp (400-800) along with low Vp/Vs ratio and high Vs observed in the external Aar Massif could be indicative of metamorphic processes leading to different Vp/Vs ratios compared to the basement in the northern foreland (Black Forest Massif), and possibly image the continuation of the massif 20-30 km further to the northeast. In combination with recently developed Vp and Vs velocity models, the developed 3-D attenuation models provide additional constraints in terms of composition and physical properties of the uppermost crust of the central Alps as well as crucial input for next generation seismic hazard models of Switzerland, allowing for a more realistic prediction of earthquake related ground motions.&lt;/p&gt;

3D overdeepened trough morphology revealed as a product of bedrock geology (northern Switzerland)

&lt;p&gt;Subglacial overdeepenings are a common element of once glaciated mountain forelands and have considerable implications for society, e.g. in construction projects, water production and radioactive waste disposal. Yet the processes of overdeepening erosion, especially the influence of bedrock lithology and structure, are poorly understood. We present a case study of the Gebenstorf-Stilli Trough in northern Switzerland, a unique overdeepening with complex underlying geology: In contrast to the Molasse-hosted majority of the Alpine foreland overdeepenings, it is to a large part incised into Upper Jurassic limestones and marls. In order to constrain its morphology in 3D, it was targeted with scientific boreholes as well as a seismic campaign based on analysis of surface waves. The results reveal unexpected trough morphology with two nested sub-basins that appear to be closely related to the bedrock geology. We suggest that this morphology is a product of low erosional efficiency in Jurassic limestones in comparison with both underlying marls and overlying Molasse deposits as well as secondary paleoglaciological effects. We further infer that the glacier&amp;#8217;s basal drainage system was the main driver of subglacial erosion of the Gebenstorf-Stilli Trough.&lt;/p&gt;

Delving deeper into cosmogenic 26Al-10Be isochron-burial dating of Swiss Deckenschotter deposits

&lt;p&gt;The oldest Quaternary deposits of the Swiss Northern Alpine Foreland are found on numerous hilltops, up to 300 m above the current valley bottoms. These Deckenschotter deposits consist mainly of glaciofluvial sediments intercalated with glacial sediments. Traditionally, the Deckenschotter are divided into two units: H&amp;#246;here Deckenschotter (HDS &amp;#8211; Higher Deckenschotter) and Tiefere Deckenschotter (TDS &amp;#8211; Lower Deckenschotter). Elevation differences between the two suggest a phase of 100-150 m of incision (Graf, 2009).&lt;/p&gt;&lt;p&gt;Knowledge of their age of deposition is necessary for understanding the long-term landscape evolution as well as for assessing the long-term safety of the planned deep geological repository for nuclear waste in northern Switzerland (NTB 14-01, 2014). In this study, the method of isochron-burial dating was implemented to address the question of the age of the Deckenschotter. We aim to reconstruct the chronology of the alternating deposition and incision of the gravel units in the Northern Alpine Foreland. Our focus is placed on similar and complementary Deckenschotter sites located in the Northern Alpine Foreland in crucial locations in order to establish sound long-term landscape evolution scenarios. One of these is a former gravel pit, Feusi, situated in the southern slope of the hill chain called &amp;#8216;Egg&amp;#8217; or &amp;#8216;Schliniker Platten&amp;#8217;, north of the village Oberweningen. The outcrop comprises several gravel units intercalated with glacigenic diamict layer in the upper part. Previous age estimates with the isochron-burial dating method indicate an age of 1.1 &amp;#177; 0.2 Ma for the diamict layer (NAB 19-025, 2020). Knudsen et al. (2020) reported an age of 0.93 &amp;#177; 0.13 Ma for the same layer based on a slightly different age calculation approach.&lt;/p&gt;&lt;p&gt;We sampled the lowermost accessible horizon, the Egg Schotter, of the Feusi outcrop at an altitude of ~580 m a.s.l. This horizon is located close to the base of the outcrop, just a few meters above the contact with the underlying Molasse and in a clear stratigraphic position, 20 m below the previously dated diamict. Study of the lowermost unit will allow us to temporally examine the earliest phases of Deckenschotter accumulation. Weathering horizons in the gravel layers overlying the Egg Schotter suggests periodic subaerial exposure. Therefore, the total time contained in the sediment package is difficult to estimate. Having two horizons dated at different depth in the same outcrop may provide insight into the timespan hidden between the deposition and weathering of different gravel layers. Indications of the timespan of HDS activity could be further gleaned by comparing to the age from the glacigenic sediment. In order to achieve this, eight clast samples of quartz-rich lithologies, of various shapes and sizes were collected in the Egg Schotter and processed for isochron-burial dating. The cosmogenic nuclides &lt;sup&gt;10&lt;/sup&gt;Be and &lt;sup&gt;26&lt;/sup&gt;Al were extracted and measured with the new MILEA accelerator at the accelerator mass spectrometry facility, ETH Zurich. The first results of this study will be presented.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Graf, H.R. 2009: Quaternary Science Journal 58, 12&amp;#8211;53&lt;/p&gt;&lt;p&gt;Nagra, NTB 14-01, 2014&lt;/p&gt;&lt;p&gt;Nagra, NAB 19-025, 2020&lt;/p&gt;&lt;p&gt;Knudsen, M.F. et al. 2020. Earth and Planetary Science Letters, 549, 116491&lt;/p&gt;

Luminescence properties and dating of glacial to periglacial sediments from northern Switzerland

Luminescence dating has become a pillar of the understanding of Pleistocene glacial advances in the northern foreland of the Swiss Alps. However, both quartz and feldspar from the region are equally challenging as dosimeters with anomalous fading and partial bleaching being some of the obstacles to overcome for the establishment of decisive chronologies. In this study, luminescence properties of coarse- and fine-grained quartz, feldspar, and polymineral fractions of eight samples from a palaeovalley, Rinikerfeld in northern Switzerland, are systematically assessed. Standard performance tests are conducted on all four fractions. Deconvolution of luminescence signals of the quartz fractions is implemented and shows the dominance of stable fast components. Reader-specific low preheat temperatures are investigated on the infrared stimulated luminescence (IRSL) signal of feldspar. Thermal stability of this signal is found for low preheats, and thermal quenching could be excluded for higher preheats. However, anomalous fading is observed in the feldspar and polymineral IRSL signals and two correction approaches are applied. For one approach, fading corrected coarse-grained feldspar ages are consistent with those derived from quartz. In general, coarse-grained quartz and feldspar, as well as the fine-grained polymineral fraction of one sample, are in chrono-stratigraphic agreement and present negligible evidence for partial bleaching. However, ages derived from fine-grained quartz are found to underestimate those of the coarse-grained quartz fractions. Hence, the impact of alpha efficiency and water content on the dose rate and thus the ages are assessed. A finite explanation for the observed discrepancies remains lacking, but this systematic investigation of different luminescence signals allows for the establishment of a chronology for the palaeovalley fill dating back to at least Marine Isotope Stage 6 (MIS 6).