A great number of studies concerning the existence of instability phenomena as eastward sliding movements, in the eastern flank of Etna (Italy) have been performed. Actually, although the existence of these phenomena is overt, clear results about the presence in depth of décollement surfaces are still lacking. Multi-disciplinary information must be put together in order to better constrain modeling. A magnetotelluric (MT) derived 2D resistivity models recovered along two profiles (N-S and NW-SE-striking) located in the eastern flank of Etna and a well-localized seismicity data set belonging to the period July 2001- December 2006 are herein jointly analysed. The comparison between the seismicity data set and the MT results, both strike analysis and the resistivity models, was performed. Detailed MT strike and dimensionality analyses reveal consistent, but period dependent, strike directions, indicating a change in the geoelectrical strike with period, that means, giving their direct proportionality, with depth. In the period ranges 0.01-1 s and 4-100 s the geoelectric strike angle is oriented W-E and almost SW-NE, respectively. A comparison with seismological data indicates that shallower MT strike is consistent with the EW clustered seismicity along the Pernicana Fault System. Projection of the seismic activity located within a 2 km wide band, centered on the two MT resistivity models, strengths the existence of two different processes affecting the eastern flank at the same time, but at different depth ranges. The boundary between them, is marked by a resistivity increasing, that is interpreted as the top of the Etna basement. Within the second depth range (3-7 km bsl) along the MT profile, a detailed analysis of the relative seismic events recovered a low seismicity level at 4.5 0.3 km b.s.l.. A 3D inspection of the seismic dataset shows that this level of reduced seismicity clearly originates from the northern wall of VdB and extends to NE. This result suggests that the deeper rupture process characterizing the north-eastern flank of Etna may occurs on multiple planes and the level of reduced seismicity could play a key role in the instability processes involving the eastern flank of the volcano.

The area hit by the M(w) 6.3, mainshock of April 6, 2009 (L'Aquila, central Italy) has been recently well investigated in terms of surface and shallow data, but so far lacks deep structural information. In order to fill this gap, Magnetotelluric (MT) and Deep Electrical Resistivity Tomography (DERT) techniques were applied in the middle Aterno Valley, which is the epicentral area of the earthquake. Both methods allowed us to hypothesize on the thickness of the different lithosomes and their geometric relationships, putting in evidence also the possible trace, at depth, of the Paganica fault, i.e., one of the segments of the seismogenic structure responsible for the 2009 earthquake (i.e., Paganica-San Demetrio Fault System).

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 Mw 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 ML≥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 Mw 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.

The Atlas Mountains in Morocco are considered as type examples of intracontinental mountain chains, with high topography that contrasts with moderate crustal shortening and thickening. Whereas recent geological studies and geodynamic modelling suggest the existence of dynamic topography to explain this apparent contradiction, there is a lack of modern geophysical data at the crustal scale to corroborate this hypothesis. To address this deficiency, magnetotelluric data were recently acquired that image the electrical resistivity distribution of the crust from the Middle Atlas to the Anti-Atlas, crossing the tabular Moulouya plain and the High Atlas. All tectonic units show different, distinct and unique electrical signatures throughout the crust reflecting the tectonic history of development of each one. In the upper crust, electrical resistivity values and geometries can be associated to sediment sequences in the Moulouya and Anti-Atlas and to crustal scale fault systems in the High Atlas developed likely during Cenozoic times. In the lower crust, the low resistivity anomaly found below the Moulouya plain, together with other geophysical (low velocity anomaly, lack of earthquakes and minimum Bouguer anomaly) and geochemical (Neogene-Quaternary intraplate alkaline volcanic fields) evidences, infer the existence of a small degree of partial melt at the base of the crust. Resistivity values suggest a partial melt fraction of the order of 2-8%. The low resistivity anomaly found below the Anti-Atlas may be associated with a relict subduction of Precambrian oceanic sediments, or to precipitated minerals during the release of fluids from the mantle during the accretion of the Anti-Atlas to the West African Supercontinent during the Panafrican orogeny (ca. 685 Ma).

This paper presents a magnetotelluric (MT) survey of the unstable eastern flank of Mt Etna. We take thirty soundings along profiles oriented in the N-S and NW-SE directions, and from these data recover two 2D resistivity models of the subsurface. Both models reveal three major layers in a resistive-conductive-resistive sequence, the deepest extending to 14 km bsl. The shallow layer corresponds to the volcanic cover, and the intermediate conductive layer corresponds to underlying sediments segmented by faults. These two electrical units are cut by ~ E-W-striking faults. The third layer (basement) is interpreted as mainly pertinent to the Apennine-Maghrebian Chain associated with ~ SE-NW-striking regional faults. The detailed shapes of the resistivity profiles clearly show that the NE Rift is shallow-rooted (~0-1 km bsl), thus presumably fed by lateral dikes from the central volcano conduit. The NW-SE profile is characterized by a series of listric faults reaching up to 3 km bsl, then becoming almost horizontal. Towards the SE, the resistive basement dramatically dips (from ~3 km to ~10 km bsl), in correspondence with the Timpe Fault System. Several high-conductivity zones close to the main faults suggest the presence of hydrothermal activity and fluid circulation that could enhance flank instability. Our results provide new findings about the geometry of the unstable Etna flank and its relation to faults and subsurface structures.

Since 2007, a permanent magnetotelluric (MT) monitoring station has been working in the seismic area of the Agri Valley (Basilicata region, southern Italy) in order to investigate the stability of the MT transfer function. The station was installed in a rural area near the supposed seismogenic fault of the strong earthquake (Mw = 6.9) that struck the Agri Valley in 1857. Analysing about 4 yr of MT data characterized by a low seismic activity, the long-term systematic variations of robust single station MT transfer function estimates were observed in two different sounding period ranges. First, a significant seasonal component of variability for short periods was noted; these short periods were up to 16 s and were linked to variations in wetting/drying of soil moisture in the shallower layers. Second, a connection between the monitored estimates and global geomagnetic activity, Ap index, was found, particularly in the [20–100 s] period range. Analysing remote reference results and tipper estimates in shorter monitoring window, it was shown that such effect cannot be explained by a local or incoherent noise, and a large-scale coherent source should be claimed. We show that this effect is subtle because it produces smooth estimates, satisfying the dispersion relationship between apparent resistivity and phase, with small error bars. As the global geomagnetic activity level increases, robust estimators, like the median value, can be considered as a representative of the estimates due to the natural source, and they tend to stabilize when the Ap index approaches 10. It is also worth noting that our monitored time window includes the recent global minimum of solar activity which occurred in 2009, thus enhancing the estimate dependence on the Ap index.