SeisTecNoS

This project is carried out under a contract with the National Cooperative for the Disposal of Radioactive Waste (Nagra). It provides an independent monitoring and analysis of the earthquake activity in the area of the proposed nuclear waste repositories in northern Switzerland. Detailed microseismic analysis will help to identify active fault zones and provide insights into the underlying seismotectonic processes in the vicinity of proposed sites, which has direct implications on the seismic hazard assessment. The Swiss Seismological Service (SED) provides results of this project in a transparent manner and all data acquired with the permanent stations (network code CH) are made available for public access (see SED declaration on transparency).

Within this project, the SED constructed ten new seismic stations in northeastern Switzerland and southern Germany between 2012 and 2013. These stations help to improve monitoring capabilities for very small earthquakes. Monitoring of weak seismic events in this region is challenging, because the study region is densely populated and sediments of the Molasse basin dominate the surface geology. To reduce the seismic background noise, three of the ten new sites were equipped with borehole-sensors, located at depths of 120-150 m below the surface. The new stations are fully operational since December 2013 and observe the local seismicity in northern Switzerland since then. The newly installed stations complement the five stations installed in 2003 under a first agreement with Nagra. With these ‘Nagra’ stations, together with stations of the Swiss National Seismic Network and stations of neighboring networks in Germany, the project aims to monitor earthquakes down to magnitudes of 1.0 and smaller in the study area. In combination with novel detection methods and an improved 3D seismic velocity model, we achieve an overall catalog completeness of Mc 1.0 throughout northeastern Switzerland and location errors less than about 0.5 km in epicenter and less than about 1-2 km in focal depth within the study region. In addition, the dense network allows for reliable estimation of focal mechanisms of micro earthquakes and for high-precision relative hypocenter relocation, imaging the geometry and kinematics of seismogenic faults in detail.

In addition to those permanent stations, Nagra also funded a pool of eight stations for temporary deployments that are currently installed in a semi-permanent manner to further improve monitoring of clustered seismicity near the town of Eglisau and along the Neuhausen fault. These stations are operative since 2016 and 2019, respectively, and allow event detection down to about ML=0 and reliable focal-depth estimation for these local seismicity clusters. Between 2018 and 2022, additional temporary stations were deployed during Nagra’s deep drilling program. A local monitoring station was installed near each of the nine drilling sites to locally monitor potential seismicity induced during the drilling operations. In 2022, one of the boreholes near Bülach (ZH) was equipped with an accelerometer, installed at the depth of the Opalinus clay formation (about 900 m below surface), the target formation for nuclear waste repositories in northern Switzerland. Together with an accelerometer installed at the surface, the borehole instrument provides important information on the expected ground motions at the depth of a future waste repository. This estimate is a key parameter in the downstream site-specific seismic hazard assessment of the future repository.  

A first comprehensive seismotectonic study based on the densified network (complemented by geodetic data) analyzed seismogenic faults associated with the Hegau-Bodensee Graben in the border region between southern Germany and northern Switzerland. In the next steps of this project, we will extend the analysis to gain a better understanding of seismogenic faults and the underlying seismotectonic processes in northern Switzerland. In combination with the analysis of data acquired at the Bülach borehole and other stations, the project will provide crucial input for the next generation hazard models of this region.