The results of magnetovariational (MV) soundings are usually presented in the form of induction arrows. However, many examples show that the horizontal magnetic tensor (HMT) is more informative. The distribution of some HMT invariants directly traces the location of well-conducting rocks in the crust and upper mantle. The HMT determination requires simultaneous observations in an entire region, which is a substantial disadvantage. Yet, it is possible to apply techniques capable of restoring all the magnetic field components necessary for HMT estimation from tipper data arrays alone. These techniques exploit the spatial relationships between electromagnetic field components in a non-conducting atmosphere. For Central Europe, a large data set of induction arrows has been collected by the effort of many groups during the last 50 years. Based on these data, HMT values were calculated, and the results are very significant. The spatial behavior of certain HMT invariants demonstrates the presence of deeply seated, well-conducting rocks in the crust. Anomaly maximums display an arc-shaped trend that may be genetically linked with the Caledonian and/or Variscan margin thrust belts, which developed following the collision between Baltica and Avalonia and/or Gondwana-derived terranes, respectively. This is an important finding because the position of these deformation fronts in relation to the edge of the East European Platform is still controversial.

To emphasize the relationship between the pre-seismic geomagnetic signals and Vrancea seismicity, in this work it is hypothesized that before an earthquake initiation, the high stress reached into seismogenic volume generates dehydration of the rocks and fracturing processes followed by release of electric charges along the faulting systems, which lead to resistivity changes. These changes were explored on September 2016 by the normalized function Bzn obtained from the geomagnetic data recorded in ULF range (0.001–0.0083 Hz). A statistical analysis was also performed to discriminate on the new Bzn* time series a pre-seismic signature related to the Mw5.7 earthquake. Significant anomalous behavior of Bzn* was identified on September 21, with 3 days prior to the onset of the seismic event. Similar information is provided by registrations of the magnetic and electron concentration variations in the ionosphere over the Vrancea zone, by Swarm satellites, 4 days and 1 day before the earthquake.