E\"otv\"os balance and earthquake detection

Torsion-balance measurements became well known in the early 1900s, with E\"otv\"os' famous high-precision verification of the equivalence of gravitational and inertial mass. This equivalence later became the basis of general relativity. A new chapter of these measurements began in 1985, when Fischbach and his colleagues discovered regularities in the small deviations detected by E\"otv\"os, which had previously been considered random. This led to the short-range 5th force hypothesis, whose existence and the source still have not yet been proven. Our research team started to work on the modernization of the E\"otv\"os torsion-balance, the repetition of the EPF equivalence principle measurement and the investigation of the source of the 5th force in connection with the E\"otv\"os Year 2019. During the experiment, particular emphasis was put on the analysis of environmental (e.g. microseismic) disturbances and the detection and filtering of the resulting, sometimes systematic, errors. These perturbations included earthquakes, which have been detected by the modernized E\"otv\"os 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 noticeably up to 2 days before earthquakes. On this basis, the researchers proposed the possibility of using torsion balances to predict earthquakes. The earthquake in Greece on 9 January 2022 was recorded by both a G\"uralp 3T broadband seismometer and an E\"otv\"os torsion balance, which were placed side by side in our laboratory. However, more than 30 minutes before the eruption itself, the E\"otv\"os torsion balance registered such strange signals that are not visible on the seismogram taken by the G\"uralp 3T seismometer. In this article, we describe the details of this observation.