Seismic moment tensor and focal mechanism of the October 9, 2023 earthquake in Eastern Slovakia

The accuracy of the focal mechanism solution mainly depends on the number of stations used and becomes problematic especially in the case of weak earthquakes and sparse networks. In our study, we retrieve the seismic moment tensor of the M=5 earthquake on 9 October 2023 (18:23:09 UTC, 21.783°E, 49.086°N, depth 11.5 km) in Eastern Slovakia from its records at only four seismic stations. Our seismic moment tensor inversion is based on the point source approach and the use of only direct waves calculated by the matrix method. Displacements on the surface of an elastic, horizontally-layered medium are generated using the frequency-wavenumber integration technique. The advantage of using only direct P- and S- waves in our inversion method is that they are less sensitive to path effects compared to reflected and converted waves, which reduces the impact of an inaccurate velocity structure and improves the accuracy and reliability of the result. Based on the forward modeling, a numerical technique was developed for the inversion of the observed waveforms for the components of the moment tensor M(t) using the generalized inversion solution. Before applying our method to the earthquake of October 9, 2023, it was also tested on the April 23, 2020 earthquake (23:18:26.42 UTC, 21.945°E, 48.781°N, magnitude M=5, depth 9 km), also in Eastern Slovakia, using data from only three stations. The resulting versions of the mechanism compare well with a very reliable version previously determined from the polarities of the first P-waves at a much larger number of stations, which only confirms the reliability of our inversion method and the very possibility of obtaining useful results from data of only limited number of stations.