Deciphering seismic sequences with high-precision hypocenter locations, focal mechanisms, and waveform modelling

To better understand seismotectonic processes and the associated seismic hazards within a region, it is imperative to explore the causality between earthquakes, active tectonics, and individual fault structures. Hypocenter locations represent a well-established method to identify active faults, their spatial geometry and temporal evolution. First motion focal mechanisms provide insights into source processes and fault kinematics, aiding in the reconstruction of seismic strain and seismogenic stress regimes. Waveform modelling is key to refine earth structure models and constrain source process parameters.

Here, we present the challenging case of the seismic sequence of Saint-Ursanne of March and April 2000 in Switzerland, where we applied advanced seismological analyses to reduce uncertainties in hypocenter locations and focal mechanisms, commonly encountered in shallow seismicity. The sequence, consisting of five earthquakes of which the largest one reached a local magnitude (M_L) of 3.2, occurred in the vicinity of two critical sites, the Mont Terri rock laboratory and Haute-Sorne, which has been approved as a site for the development of a deep geothermal project. Our 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 sequence. These new findings provide new insights into the present-day seismotectonic processes of the Jura fold-and-thrust belt (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 transpressive regime is confirmed by the ongoing Réclère earthquake sequence, ca. 20 kilometres west of St. Ursanne, which initiated with a reverse event of ML 4.1 on December 24, 2021, and was followed by few aftershocks in January 2022. Seismic activity started again in March 2023 with another ML4.3 reverse event, which activated a left-lateral strike-slip secondary fault just west of the reverse fault. This fault seems to be more active in terms of microseismicity and is responsible for the increased activity over the last years.