Electrical oscillations recorded by CIEN stations at time of two seismic swarms in the Umbria Region, Central Italy, in 2013-2014 P.M. Siciliani 1, C. Fidani 1,2, D. Marcelli 2, M. Arcaleni 1, S. Tardioli 1 1 Osservatorio Sismico Andrea Bina, Perugia, Italy 2 Central Italy Electromagnetic Network (CIEN), Fermo, Italy 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. 87
Fig. 1 The geographical positions of the 13 CIEN stations are indicated by red circles. Several shocks of M > 3 occurred near the same epicentre over the following years and, at the same time, many ELF oscillations were recorded by the Perugia instruments. Being that the earthquake distances and the magnitudes were compatible with the Dobrovolsky condition (Dobrovolsky, 1979), an analysis was carried out looking for a possible correlation between ELF electrical oscillations and moderate seismic activity. The recently installed CIEN stations in Città di Castello and Gubbio contributed to this analysis, though, a thorough verification needs to be performed on the signals recorded by the two new stations. Electrical oscillations, such as those reported in connection with the 2009 L Aquila earthquake (Fidani, 2011) were also detected at the Perugia station during moderate seismic activity. Specifically, electrical oscillations were recorded on many occasions where M = 2.8 3.7 in the Pietralunga territory. The pattern recorded from the S-N electrode lasted from a of few minutes up to two hours, and concerned electrical oscillations between 40 Hz and 350 Hz. The electrical oscillation intensities observed during moderate seismic activity near the Perugia station ranged between -70 db and -55 db. To verify if observed electrical oscillations could have been influenced by the meteorological precipitation, data from the meteorological stations in Perugia were retrieved. Precipitation events were also recorded in ELF data at the Perugia 88
Fig. 2 ELF oscillation intensities recorded from the beginning of April 2013 to the end of April 2014, are shown on the top panel; yellow areas indicate lost data; green bars under ELF oscillations indicate rainfall detected by the Perugia station; the black bars indicate the electrical oscillations occurred on E-W and S-N directions; the red bars indicate the seismic events of M > 2 occurred inside a 30 km radius from the station and finally the seismic events of M > 3 occurred inside a 70 km radius. station, as water drops are charged and produced intense signals on spectrograms. These data were used to label ELF oscillations occurring during rainfalls. Finally, ELF data were analysed from April 2013 to April 2014 for a total of 9,480 hours of records, producing about 146 Gb of data. Spectrograms were prepared to better localise ELF anomalies, compared to electromagnetic noise. A scale of colours, corresponding to 5 db variations in power spectral amplitudes, was sufficient enough to distinguish the oscillations. Being so, all anomalies > 5 db above noise were collected. In order to verify when meteorological phenomena could have produced the electric perturbations, rainfall data were superimposed on ELF data. Moreover, electrical oscillations occurring less then 30 minutes from rainfall events were excluded from the analysis. Strong electrical oscillations were identified from records taken a few hours or days before the moderate shocks. A summary of recordings from the Perugia station of both electrodes, N-S and E-W, over the last year is shown in Fig. 2. To verify an association between earthquakes electric perturbations, seismic activity in a radius of 30 km and 70 km around Perugia were plotted on the same graph and shown in red. Electrical oscillations at the time of Massa Martana-Colfiorito seismic swarm in 2014. An increase in the seismicity rate occurred in the Massa Martana area since de middle of March 2014 and on March 26, 2014, with a shock of M = 3 (Bina Observatory, 2014). The swarm epicentre was about 45 km south of the Perugia station that started to record data in October 2008. Likewise, the epicentre was about 10 km east of the Avigliano Umbro station, which started to record data in June 2013. Two shocks with a magnitude M = 3.1 struck the same area seven days later, on April 2. The seismic swarm migrated to the Colfiorito area on April 15, 2014, with a shock of M = 3.3 and other smaller seismic events (Bina Observatory, 2014). 89
GNGTS 2014 Sessione 2.1 Fig. 3 The ELF electric perturbations recorded at the Avigliano Umbro station on March 30, 2014, are shown; the oscillations lasted 150 minutes and followed an irregular pattern between 10 Hz and 100 Hz with a relative magnitude of up to -50 db. After July 21, 2013, when an earthquake of M = 5 struck Ancona, 100 km east of Perugia, the Perugia station recorded a particular electrical oscillation on a recently installed instrument which measures the vertical electric component. Similar detectors were installed in another six CIEN stations: Fermo, Città di Castello, Gubbio, Pozzuolo del Friuli, Camerino and Avigliano Umbro. Clear signals were detected at the Avigliano Umbro Station a few hours before the two strongest shocks occurring in Massa Martana on March 2014. The unusually clear electric disturbances recorded before the second moderate shock are shown in Fig. 3. This figure shows a 3 hour spectrogram of vertical component electric field variations recorded at the Avigliano Umbro station. The spectrogram frequency range is 1-450 Hz. The electrical oscillations ranges between 10 Hz and 100 Hz, following a continuous and irregular path. The total time of the phenomena was about 150 minutes, with maximum intensities 25 db above the noise power. A similar phenomena was also observed before the first moderate shock. No similar oscillations were recorded on the vertical detector by other CIEN stations during the same period. The day before the moderate shock of M = 3.3 in the Colfiorito area, a signal of -55 db was recorded at the Perugia station along the E-W electrode. This electrical oscillation lasted about one hour and had the characteristic shape of an arc, as described in previous publications (Fidani, 2011). This signal was very well defined in frequency but had a very irregular pattern, similar to past cases (Fidani, 2011). Conclusions. Data recorded from the Perugia CIEN station suggests that electric perturbations become slightly more intense and frequent during meteorological and seismically active periods, as recorded before and after several Pietralunga and Gubbio earthquakes. In fact, in these periods, the power spectra of the electrical oscillations became greater than other natural electromagnetic phenomena, such as the Schumann Resonances. Candidate pre-seismic oscillations in electric intensity were quantified and resulted being of the same order of the 2009 L Aquila earthquakes (Fidani, 2011). Moreover, the moderate seismic activity in the Umbria Region has been associated to degassing activity, which are often related to the presence of over pressurized fluids and to diffusion processes (Noir et al., 1997; Miller et al., 2004; Antonioli et al., 2005). CIEN observations in the atmosphere, near the Earth s surface, have discovered that sources of electrical oscillations induced in the electrodes are localised near the stations. The presence of air ionisation has been suggested when seismic activity increases (Freund et al., 2009). Rainfalls have also been known to be preceded by electric charges in the atmosphere (Takahashi, 1972). All of these observations have evidenced that electrical oscillatory phenomena occurred in the presence of air ions, ground gas emissions and/or meteorological instability. Due to the general difference in temperatures between the ground and the atmosphere, when gas escapes 90 001-502 volume 2 90 5-11-2014 16:53:07
from the ground it should produce atmospheric pressure differences. Pressure differences are also characteristics of meteorological perturbations. In the atmosphere they are responsible for air movements and, if air ions are present in the atmosphere, pressure differences are also responsible for air ion movements. Consequently, electromagnetic waves are emitted with ion movements. However, the CIEN recording of electric fields were relatively stable oscillatory phenomena, indicating that a stable phenomena of bounded source occurred. The possibility that ions form stable spherically symmetric charge configurations in the atmosphere has been investigated with the aim of better understanding unusual atmospheric phenomena, such as ball lightning and EQL (Singer, 1978; Tennakone, 2006). Such a model of spherically symmetric and dynamically stable structures has been proposed for the atmosphere via balancing of electrostatic force and air pressure (Tennakone, 2011); they have oscillating solutions. The above model, applied to low charge density oscillating objects, might explain why the electric field oscillations were recorded by the CIEN stations during relevant seismic activity. References Antonioli, A., Piccinini, D., Chiaraluce, L., Cocco, M., 2005. Fluid flow and seismicity pattern: Evidence from the 1997 Umbria-Marche (central Italy) seismic sequence, Geophys. Res. Lett., 32 L10311, 2005. Barchi, M., De Feyter, A., Magnani, M., Minelli, G., Pialli, G., Sotera, B., Extensional tectonics in the Northern Apennines (Italy): Evidence from the CROP03 deep seismic reflection line. Mem. Soc. Geol. Ital. 52, 528 538, 1998a. Barchi, M., Minelli, R., Pialli, G., The crop 03 profile: A synthesis of results on deep structures of the northern Apennines. Mem. Soc. Geol. Ital. 52, 383 400, 1998b. Bina A. Observatory, at www.binapg.it/, seismic data collected in 2014. Catalli, F., Cocco, M., Console, R., Chiaraluce, L., Modeling seismicity rate changes during the 1997 Umbria-Marche sequence (central Italy) through a rate - and state - dependent model, J. Geophys. Res. 113, B11301, 2008. Chiaraluce, L., Chiarabba, C., Collettini, C., Piccinini, D., Cocco, M., 2007. Architecture and mechanics of an active low-angle normal fault: Alto Tiberina Fault, Northern Apennines, Italy, J. Geophys. Res. 112, B10310, 2007. Collettini, C., Barchi M.R., A low angle normal fault in the Umbria region (central Italy): A mechanical model for the related microseismicity. Tectonophysics 359, 97 115, 2002. Dobrovolsky, I.P.; Zubkov, S.I.; Miachkin, V.I. Estimation of the size of earthquake preparation zones. Pure Appl. Geophys., 117, pp. 1025-1044, 1979. Fidani, C., Electromagnetic signals recorded by Perugia and S. Procolo (Fermo) stations before the L Aquila earthquakes, 28th GNGTS, Trieste, p. 370-373, Nov. 16-19, 2009. Fidani, C., ELF signals by Central Italy electromagnetic network in 2008-2010, 29th GNGTS, Prato, p. 175-179, Oct. 28-30, 2010. Fidani, C., The Central Italy Electromagnetic Network and the 2009 L Aquila earthquake: observed electric activity, geosciences, 1, p. 3-25, December 2011. Fidani, C., Albarello, D., Arcaleni, M., Martinelli, G., Siciliani, P. M., Tardioli, S., Vannucchi, A., Emilia earthquake: VLF transmitters and ELF signal from the Central Italy Electromagnetic Network (CIEN), 31th GNGTS, Potenza, p. 415-420, Nov. 20-22, 2012. Freund, F. T., Kulahci, I. G., Cyr, G., Ling, J., Winnick, M., Tregloan-Reed, J., Freund, M. M., Air ionization at rock surface sand pre-earthquake signals. J. Atmos. Solar-Terr. Phys., 71, 1824 1834, 2009. Marzorati, S., Massa, M., Cattaneo, M., Monachesi, G., Frapiccini, M., Very detailed seismic pattern and migration inferred from the April 2010 Pietralunga (northern Italian Apennines) micro-earthquake sequence, Tectonophysics, 610, 91 109, 2014. Miller, S., Collettini, C., Chiaraluce, L., Cocco, M., Barchi, M., Klaus, B., Aftershocks driven by a high pressure CO 2 source at depth. Nature, 427, 724 727, 2004. Mirabella, F., Brozzetti, F., Lupattelli, A., Barchi, M.R., 2011. Tectonic evolution of a low-angle extensional fault system from restored cross-sections in the Northern Apennines (Italy). Tectonics, 30, TC6002, 2011. Noir, J., Jacques, E., Be`kri, S., Adler, P.M., King, G.C.P., Fluid flow triggered migration of events in the 1989 Dobi earthquake sequence of Central Afar. Geophys. Res. Lett. 24, 2335 2338, 1997. Pialli, G., Barchi, M., Minelli, G., Results of the CROP03 deep seismic reflection profile Mem. 52. Soc. Geol. Ital. Ed., Rome, 1998. Siciliani, P. M., Fidani, C., Stoppa, F., Iezzi, G., Arcaleni, M., Tardioli, S., Marcelli, D., Electromagnetic perturbations associated with M=5, July 21, 2013, Ancona, Italy earthquake observed by CIEN, 32th GNGTS, Trieste, p. 66-70, Nov. 19-21, 2013. 91
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