Van Noten, Koen ; Lecocq, Thomas ; Vanneste, Kris ; Lefevre, Marthe ; Camelbeeck, Thierry ; Knapmeyer-Endrun, Brigitte ; Carrasco, Sebastian ; Kadmiel, Shahar

Talk presented at 37th General Assembly of the European Seismological Commission, Corfu, Greece on 2021-09-22

Abstract: In 2021, three tectonic seismic swarms occurred in Lower Rhine Graben (LRG; Germany, The Netherlands). The Rott low-magnitude seismic swarm (North Eifel, DE) started on 1 January 2021 close to the German-Belgian border near the western rim of the LRG. The two largest events occurred in January 2021 with local magnitudes of ML = 2.6 which were felt up to Aachen (~25 km radius). Similar to other low-magnitude tectonic seismic swarms in stable continental regions, the sequence lasted for months and more than 100 seismic events were measured. During the Rott sequence, also in Voerendaal (NL) and Eschweiler (DE), in the middle of the LRG, swarm activity occurred. In this contribution we discuss swarm catalogue and statistics, waveform cross-correlation and relocation, focal mechanism and crustal stress pattern, intensity distribution of the largest events, and potential swarm causality and its link with LRG faults. The magnitude-frequency distribution of the Rott sequence indicates a completeness magnitude between ML=0.2 and ML=0.7, with corresponding estimated Gutenberg-Richter b-values of ~0.7 and ~1.0, respectively. These low b-values are typical of swarms with a tectonic origin. Focal depths are mostly between 8 and 11 km (1-sigma location uncertainty). Epicenters are located 2 - 4 km east of the Laurensberg fault, a secondary LRG fault parallel to the main graben faults with documented throw of Cretaceous strata and present-day morphology to the west. However, the dip direction and dip angle are not well known, and it is possible that the observed throw reflects Late Cretaceous inversion. In that case, the Laurensberg fault, which should currently have extensional kinematics similar to the other faults in the LRG, may well dip eastward and could potentially be related to the Rott sequence. If not, the causative structure remains unknown. This kind of seismotectonic analysis of low-magnitude tectonic swarms helps in locating and understanding seismogenic faults in stable continental regions in NW Europe.

Keyword(s): Seismic swarm ; Rott sequence ; Cross-correlation ; Collaboration