Multi-scale imaging of a slow active fault zone: contribution for improved seismic hazard assessment in the Swiss Alpine foreland

Seismic hazard assessment of slow active fault zones is challenging as usually only a few decades of sparse instrumental seismic monitoring is available to characterize seismic activity. Tectonic features linked to the observed seismicity can be mapped by seismic imaging techniques and/or geomorphological and structural evidences. In this study, we investigate a seismic lineament located in the Swiss Alpine foreland, which was discussed in previous work as being related to crustal structures carrying in size the potential of a magnitude M 6 earthquake. New, low-magnitude (−2.0 ≤ ML ≤ 2.5) earthquake data are used to image the spatial and temporal distribution of seismogenic features in the target area. Quantitative and qualitative analyses are applied to the waveform dataset to better constrain earthquakes distribution and source processes. Potential tectonic features responsible for the observed seismicity are modelled based on new reinterpretations of oil industry seismic profiles and recent field data in the study area. The earthquake and tectonic datasets are then integrated in a 3D model. Spatially, the seismicity correlates over 10–15 km with a N–S oriented sub-vertical fault zone imaged in seismic profiles in the Mesozoic cover units above a major decollement on top of the mechanically more rigid basement and seen in outcrops of Tertiary series east of the city of Fribourg. Observed earthquakes cluster at shallow depth (<4 km) in the sedimentary cover. Given the spatial extend of the observed seismicity, we infer the potential of a moderate size earthquake to be generated on the lineament. However, since the existence of along strike structures in the basement cannot be excluded, a maximum M 6 earthquake cannot be ruled out. Thus, the Fribourg Lineament constitutes a non-negligible source of seismic hazard in the Swiss Alpine foreland

Optimizing event detection and location in low‐seismicity zones: case study from western Switzerland

Obtaining robust event catalogs in regions of low seismicity can be time-consuming, because quality events are less frequent and sensor coverage is generally sparse. Optimizing event detection and location in such regions is all the more crucial because these areas tend to host a higher density of sensitive infrastructures. The meth- odology proposed consists of reprocessing existing data recorded by a permanent net-work and boosting the final catalog resolution by temporarily deploying portable sparse mini-arrays in the target area. Sonogram analysis is applied on both existing and new datasets to detect waveforms barely emerging from the background noise. A visual interactive event analysis module is used to test for phase picking, event asso- ciation, waveform cross correlation, and location ambiguities. It also estimates back azimuth and slowness when sparse array data are available. The method is applied to a low-seismicity region in the western Swiss Molasse basin where two sparse mini- arrays were temporarily deployed. The detection of earthquakes is improved by a fac- tor of 9 when reprocessing four yrs (2009–2013) of available data recorded by two accelerometers and one broadband station in a 2500 km2 target area. Magnitude estimations are empirically calibrated over four magnitude units, down to −1:7 ML, lowering the existing catalog completeness by close to one magnitude unit. After validating picking and location accuracies with a standard residual-based scheme, 174 newly detected events are relocated, illuminating zones of previously undetected microseismic activity

3D cartographic modeling of the Alpine Arc

We built a 3D cartography of the alpine arc, a highly non-cylindrical mountain belt, using the 3D GeoModeller of the BRGM (French geological survey). The model allows to handle the large-scale 3D structure of seventeen major crustal units of the belt (from the lower crust to the sedimentary cover nappes), and two main discontinuities (the Insubric line and the Crustal Penninic Front). It provides a unique document to better understand their structural relationships and to produce new sections. The study area comprises the western alpine arc, from the Jura to the Northwest, up to the Bergell granite intrusion and the Lepontine Dome to the East, and is limited to the South by the Ligurian basin. The model is limited vertically 10 km above sea level at the top, and the moho interface at the bottom. We discarded the structural relationships between the Alps sensus stricto and the surrounding geodynamic systems such as the Rhine graben or the connection with the Apennines. The 3D-model is based on the global integration of various data such as the DEM of the Alps, the moho isobaths, the simplified geological and tectonic maps of the belt, the crustal cross-sections ECORS-CROP and NFP-20, and complementary cross-sections specifically built to precise local complexities. The database has first been integrated in a GIS-project to prepare their implementation in the GeoModeller, by homogenizing the different spatial referencing systems. The global model is finally interpolated from all these data, using the potential field method. The final document is a new tri-dimentional cartography that would be used as input for further alpine studies

3D cartographic modeling of the Alpine arc

International audienc

Karst flow processes explored through analysis of long-term unsaturated-zone discharge hydrochemistry: a 10-year study in Rustrel, France

The unsaturated zone of karst aquifers influences the dynamics and the chemistry of water. Because of a lack of direct access, other than via caves, flows in the aquifer matrix and the smallest conduits remain poorly characterized. The few artificial underground structures in the unsaturated karst provide a rare opportunity to study the variety of flow processes. At the low noise underground research laboratory (Laboratoire Souterrain à Bas Bruit, LSBB) in Rustrel (France), 12 variables (temperature, pH, electrical conductivity, alkalinity, major anions and cations, total organic carbon) have been monitored on 12 perennial or temporary flows and leakages over a 10-year period covering contrasting climatic periods. This unique dataset of 1,135 samples has been used to discriminate, identify, and rank the processes associated with the hydrochemical variability of these different types of flows. A principal component analysis and a hierarchical cluster analysis, using mean values and standard deviation of the flow along the principal components, were performed. The results indicate that seasonal variability, mean water residence time, and the depth of acquisition of the chemical characteristics are the main factors of the variability of chemistry at the monitored flow points. Distinguished clusters highlight the great diversity of flows and processes occurring in the fine pathways that may be neighboring the large and structured fractures and conduits. Long-term monitoring with various climatic conditions appears to be a useful tool for assessing this diversity