EGU General Assembly 2022
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

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

Federica Lanza1, Tobias Diehl1, Nicholas Deichmann1, Toni Kraft1, Christophe Nussbaum2, Senecio Schefer2, and Stefan Wiemer1
Federica Lanza et al.
  • 1Swiss Seismological Service, ETH Zürich, Zürich, Switzerland
  • 2Swiss Geological Survey, Federal Office of Topography, swisstopo, Wabern, Switzerland

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. This trenspressive regime is confirmed by the ML4.1 Réclère earthquake of December 24. 2021, which occurred ~20 kilometres west of St. Ursanne in the uppermost crust.

How to cite: Lanza, F., Diehl, T., Deichmann, N., Kraft, T., Nussbaum, C., Schefer, S., and Wiemer, S.: The Saint-Ursanne earthquakes of 2000 revisited: Evidence for active shallow thrust-faulting in the Jura fold-and-thrust belt, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9314,, 2022.