Analysis of the Szarvas, Hungary, Earthquake Swarm of 19 August 2023: A Case Study in a Low-Seismicity Sedimentary Basin

ABSTRACT Between 19 August and 26 September 2023, an earthquake swarm of 105 events occurred beneath an otherwise low-seismicity sedimentary basin near Szarvas, southeastern Hungary. There were seven ML≥3.5 earthquakes in the first 24 hr, including three ML≥4.0 events. As much as 90% of all the recorded events occurred in the first 48 hr. Then, on the fourth day, another ML 3.7 earthquake was observed. After 25 August, only two events happened. Taking advantage of the dense temporary seismic network operating in the country, in this article, we thoroughly analyze the source parameters of this unusual earthquake swarm. Utilizing the BayesLoc multiple-event location algorithm, we have refined the hypocentral locations of 83 earthquakes with ML between 0.9 and 4.1. The events took place within a small area with a radius of a few kilometers near Csabacsűd, just on top of the Dévaványa–Gyomaendrőd fault zone. The 2023 Szarvas earthquake swarm provides the first clear evidence that this fault zone is active. More than one-third of the events, all with ML<3.0, occurred at shallow depths in sedimentary layers, whereas the larger ML≥3.0 events originated between 6 and 10 km in the upper crust. By employing a probabilistic waveform inversion methodology, we have estimated the full moment tensors for eight earthquakes with ML≥3.5. All of the investigated events have strike-slip mechanisms with either right-lateral slip on a north-northeast–south-southwest-striking, or left-lateral movement on a west-northwest–east-southeast-striking nodal plane. The negligible isotropic component indicates that the events are of a tectonic nature. Both the P and T principal axes are horizontal, and the P axis exhibits an east-northeast–west-southwest orientation, in agreement with the maximum-horizontal stress direction recently published for the epicentral region.