ELECtRICAL oSCILLAtIonS RECoRdEd by CIEn StAtIonS At tIME of tWo SEISMIC SWARMS In thE UMbRIA REGIon , CEntRAL ItALy , In 2013-2014

Introduction. Characteristic ELF signals were monitored in relation to seismic activity in Fermo, Marche Region, Italy, from January 2006 to July 2014 (Fidani, 2009). They were detected also during low seismic activity at the Perugia CIEN station from October 2008 to July 2014 (Fidani, 2010). A CIEN update is shown in Fig. 1, with a total number of 13 stations, where the Gubbio, Pozzuolo del Friuli and Camerino stations were added in January, April and May 2014, respectively. From the summer 2012, four stations were not operative because of technical problems: Torre Pellice, Fagnano Amiterno, Siena and Capitignano stations. The Torre Pellice station has been recently closed because of unresolved technical problems. At the time of the April 2009 L’Aquila earthquakes two stations were in operation, the Fermo and the Perugia stations. Both of these detected several ELF electric perturbations (Fidani, 2011). Whereas, at the time of the May 2012 Emilia earthquakes, nine stations were in operation. Of these, the Zocca station detected several horizontal ELF electrical oscillations from April 2012 to the end of June 2012 (Fidani et al., 2012). Each of the 9 stations had a couple of wide band amplifiers operative in a VLF range: 1 to 25 kHz. Three of these stations (Chieti, Fermo and Torre Pellice) were in LF band, 1 to 50–100 kHz. The VLF and LF ranges allowed to monitor several sub-ionospheric signals from different VLF and LF transmitters (Fidani, 2011). Recently, the Città di Castello station was rendered operative in LF band, 1 to 50 kHz. Some significant perturbations have been revealed in the VLF and LF band data related to the 2013 Ancona earthquake (Siciliani et al., 2013), while no significant perturbations were recorded during the Emilia seismic activity (Fidani et al., 2012). A moderate earthquake swarm in the Pietralunga area, 35 km north of Perugia, started on March 24, 2013, and it migrated to the Gubbio area, 30 km NE of Perugia, in December 2013. Another moderate seismic swarm started at the half of March 2014, in the Massa Martana area, 45 km south of Perugia, migrating to the Colfiorito area, 35 km SE of Perugia, in April 2014. The swarms occurred at shallow crustal levels from 4 to 12 km in depth. They covered the area over the Alto Tiberina Fault, a low angle normal fault that acts as a basal seismic detachment and accommodates extensional deformation together with a complex normal fault system that is located in its hanging wall (Chiaraluce et al., 2007). The structure bounds the western side of the upper Tiber Quaternary basin, and it is about 70 km long, dipping from 15° to 20° ENE (Mirabella et al., 2011). It is clearly evident in the CROP03-NVR seismic reflection profiles (Barchi et al., 1998a, 1998b; Pialli et al., 1998). The micro-seismicity associated with this system of faults is probably triggered by frictional instabilities that are created by fluid overpressure, while the most of the extension along the fault is accommodated by aseismic slip and creep (Collettini and Barchi, 2002). The possible occurrence of moderate-to-large earthquakes generated by low angle normal fault is an open question (Wernicke, 1995). Thus, it is relevant in terms of seismic hazard. In the tectonic setting of the Apennines, a seismicity migration episode along the strike of a fault system or along a preferred direction was observed during seismic sequences of the Colfiorito earthquake in 1997 (Catalli et al., 2008). Over the past 5 years, characteristic ELF signals were detected at the Perugia station as well as in the recently installed Avigliano Umbro, Città di Castello and Gubbio stations. While, no evident perturbations were revealed over the last year from the three latter stations in VLF and LF bands. Electrical oscillations at the time of the Pietralunga-Gubbio seismic swarm in 2013. An increase in the seismicity rate occurred in the Pietralunga area after the seismic swarm starting on April 15, 2010, with a shock of M = 3.8 (Marzorati et al., 2014). The swarm epicentre was about 35 km north of the Perugia station, which started to record data from October 2008.