Anomalous pre-seismic behavior of the electromagnetic normalized functions related to the intermediate depth earthquakes occurred in Vrancea zone, Romania

Abstract. In this paper the electromagnetic normalized functions (ENF), carried out in ULF band, have been analyzed in correlation with intermediate depth seismic events occurring in Vrancea zone. To confirm the relationship between anomalous, pre-seismic behavior and an imminent earthquake, a methodology based on the temporal invariability criterion of ENF for a 2-D structure, in non-geodynamic conditions, has been used. The electromagnetic data were collected at the Geodynamic Observatory Provita de Sus (GOPS), placed on the Carpathian electrical conductivity anomaly where the epicentral distance is about 100 km, and the National Geophysical Observatory Surlari (NGOS) taken as a reference and located 140 km from the Vrancea zone. The daily mean distributions of the ENF over a span of several months in 2009, carried out at GOPS, exhibit significant enhancements from the normal trend before all the earthquakes with magnitudes higher than 4. Two correlations between the magnitudes of seismic events and Bzn have to be highlighted: (i) an earthquake of M ≥ 4 is expected to occur when Bzn ≥ 1.846; (ii) meanwhile, the anomalous behaviour of Bzn ≥ 1.851 may be use as pre- seismic value for an earthquake of M ≥ 5. The lead time is closed on 7–15 days before earthquakes occurrence.