Electric and Magnetic Field Changes Observed during a Seismic Swarm in Pollino Area (Southern Italy)

Abstract Over the last few years, seismic activity in the Pollino area (a sector of the Calabro–Lucanian Apennines in southern Italy known as a seismic gap) has been very weak. However, in 2011 the seismicity gradually intensified, culminating in an earthquake of M w  5.0 occurred on 25 October 2012. The depth of the 2011–2012 earthquake hypocenters ranges between 2 and 10 km; the seismicity results in two separate clusters and traces a north‐northwest–south‐southeast fracture more evident in the western sector. In this area, an MT station was installed on 26 September 2012 by the Institute of Methodologies for Environmental Analysis, National Research Council of Italy (IMAA‐CNR), Italy, at about 50 km from another MT station operating since 2003 in the Agri Valley (Tramutola, southern Italy). Such a seismic swarm occurred in the Pollino area (more than 3600 events in last two years with local magnitude M L ≥0.1). It has provided a rare opportunity to study the earthquake‐related temporal patterns of electromagnetic (EM) signals, and is potentially informative about ongoing seismogenic processes. In this study, we present several cases of EM field variations associated with the passage of seismic waves. The maximum amplitude of the electrical signals registered at the two MT sites and the earthquake magnitude are related by an attenuation factor that depends on the distance between the hypocenter and the MT station. Furthermore, at the two MT sites the maximum electrical anomalies seem to be more appreciable predominantly in different directions, indicating a certain directivity in the propagation of the electric field. A deep analysis of EM time series recorded during the mainshock M w  5.0 was performed. In particular, by applying time–frequency misfit criteria based on the continuous wavelet transform, we compared the electric field with seismic recordings, and we found a good waveform similarity between signals. Moreover, we also found an EM signal that significantly anticipates the theoretical first P ‐wave arrival at the Tramutola MT station.