The ML4 Réclère Sequence (Switzerland): Evidence for Interaction of Reverse and Strike-Slip Faults in the Basement of the Jura Fold-and-Thrust Belt

In 2021, a remarkable natural earthquake sequence initiated in the Haute-Ajoie region (Canton Jura, Switzerland), which provides new insights on how present-day deformation is accommodated in the Jura fold-and-thrust belt (JFTB) in the northern foreland of the Alps. The M_L 4.1 mainshock of December 24. 2021, located south of the village of Réclère, triggered an unusual earthquake sequence that has been lasting for at least 3 years. Initially, few aftershocks occurred in the days after the M_L 4.1 mainshock, and aftershock activity rapidly ceased within the first week. On March 22. 2023, the sequence was reactivated by an M_L 4.3 earthquake, which was followed by an intense sequence of aftershocks. Additional seismic stations were installed in the epicentral area to improve detection thresholds and hypocenter location qualities of aftershocks. Earthquake activity remained high between March 2023 and October 2024, with 14 earthquakes of M_L ≥ 2.5. By December 2024, the Swiss Seismological Service (SED) detected and located more than 430 earthquakes with standard methods and derived high-quality focal mechanisms for 17 earthquakes of this sequence.

Relative relocations in combination with the focal mechanisms image a complex fault-zone structure within the pre-Mesozoic basement at depths of about 5-6 km. It consists of a system of reverse-to-transpressive faults hosting the M_L 4.1 and M_L 4.3 earthquakes, which are limited to the west by an adjacent, roughly N-S oriented strike-slip fault zone. The spatio-temporal analysis of the sequences suggests that the reverse-to-transpressive ruptures activated the adjacent strike-slip fault zone and seismicity migrates predominantly southward since March 2023. We complement our study by an enhanced earthquake catalog derived from a cross-correlation based template-matching procedure and the analysis of the stress field at various scales.

Our preliminary results indicate significant differences in the b-value parameter over distances of few hundred meters and between the reverse-to-transpressive and strike-slip fault zones. Similarly, a Mohr-circle analysis points to differences in the distance to criticality between the two fault zones. Our results therefore provide new insights into the variability of stress and fracture conditions across upper-crustal fault zones at scales of few hundred meters and raise the question on the possible role of fluids to explain these variations. Besides adding new constraints to present-day seismotectonic processes within the JFTB, our observations are also of general relevance for geothermal exploration targeting complex fault zones.