Anomalies have been observed in the radon content of thermal spring water at the Italian-Slovenian border. To distinguish the anomalies caused by environmental parameters (air and water temperature, barometric and hydrostatic pressure, rainfall) from those ascribed solely to earthquakes with M(L) from 1.2 to 2.5 and epicentres, R(E), within 2R(D) (R(D)--Dobrovolsky's radius), two approaches have been used: (i) correlation between time gradients of radon concentration and hydrostatic pressure, and (ii) regression trees within machine learning programs. The regression trees approach has been improved by introducing additional environmental parameters and prolonging the measuring period.