Eötvös balance and earthquake detection L. Völgyesi 1 , Gy. Tóth 1 , Gy. Szondy 2 , B. Kiss 3 , E. Fenyvesi 4 , G.G. Barnaföldi 4 , Cs. Égető 1 , P. Lévai 4 , E. Imre 5 , M. Pszota 4,6 , P. Ván 4,7 1 Department of Geodesy and Surveying, Faculty of Civil Engineering, Budapest University of Technology and Economics, Műegyetem rkp. 3., H-1111 Budapest, Hungary. E-mail: volgyesi.lajos@emk.bme.hu 2 Independent researcher, Society for the Unity of Science and Technology, Hungary 3 Department of Control Engineering and Information Technology, Faculty of Electrical Engineering and Informatics, Budapest University of Technology and Economics, Műegyetem rkp. 3., H-1111 Budapest, Hungary 4 Wigner Research Centre for Physics, H-1525 Budapest, P.O. Box 49, Hungary 5 Óbuda University, Faculty of Electrical Engineering, Budapest, Hungary 6 Eötvös Loránd University, Faculty of Science, Budapest, Hungary 7 Department of Energy Engineering, Faculty of Mechanical Engineering, Budapest University of Technology and Economics, Műegyetem rkp. 3. H-1111 Budapest, Hungary. Abstract Torsion-balance measurements became well known in the early 1900s, with Eötvös' famous high-precision verification of the equivalence of gravitational and inertial mass. This equiva- lence later became the basis of general relativity. A new golden age of these measurements began in 1985, when Fischbach and his colleagues discovered regularities in the small devia- tions detected by Eötvös, which had previously been considered random. This led to the short- range 5th force hypothesis, whose existence and source still have not yet been proven. Our research team started to work on the modernization of the Eötvös torsion-balance, the repeti- tion of the EPF equivalence principle measurement and the investigation of the source of the 5th force in connection with the Eötvös Year 2019. During the experiment, particular empha- sis was put on the analysis of environmental (e.g. microseismic) disturbances and the detec- tion and filtering of the resulting, sometimes systematic, errors. These perturbations included earthquakes, which have been detected by the modernized Eötvös balance since the beginning of the experiment. Another application of the torsion balance was discovered by Russian researchers around 1990-1992, who noticed that in some cases the equilibrium position of the balance shifts no- ticeably up to 2 days before earthquakes. On this basis, the researchers proposed the possibil- ity of using torsion balances to predict earthquakes. The earthquake in Greece on 9 January 2022 was recorded by both a Güralp 3T broadband seismometer and a Eötvös torsion balance, which were placed side by side in our laboratory. However, more than 30 minutes before the eruption itself, the Eötvös torsion balance registered such strange signals that are not visible on the seismogram taken by the Güralp 3T seismometer. In this article we describe the details of this observation. Keywords: Eötvös torsion balance, microseismic noise, equivalence experiment, earthquake prediction 1. Torsion balance measurements Between 1906 and 1908, Loránd Eötvös and his colleagues carried out their famous equivalence experiment (known as Eötvös-Pekár-Fekete (EPF) measurement) for validating the equivalence of gravitational and inertial mass (Eötvös, 1922). The original measurements