Flank instability structure of Mt. Etna inferred by a magnetotelluric survey

Abstract

[1] This paper presents a magnetotelluric (MT) survey of the unstable eastern flank of Mt. Etna. We take thirty soundings along two 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 SW-NE-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 suggests by a series of listric faults reaching up to 3 km bsl, then becoming almost horizontal. Toward 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.

Dynamic feeder dyke systems in basaltic volcanoes: the exceptional example of the 1809 Etna eruption (Italy)

“…The NE Rift is also the NW margin of a wide sector of Mt. Etna involved in seaward displacement (Ruch et al, , 2013Solaro et al, 2010 and references therein), affecting an onshore area of >700 km 2 (Neri et al, 2004) and a thickness of 1-4 km (Siniscalchi et al, 2012). During the last 110 years, the NE Rift eruptions lasted 21 days on average, with 7 m 3 s −1 eruption rates.…”

“…This dramatic and sudden drop in the level of the magma would have brought the magma into contact with the main groundwater contained in the volcanic body, which is located just above the contact surface between the volcanic pile and its impermeable (clay-rich) basement. In the central zone of the NE Rift, the impermeable basement is at ∼1200-1300 m asl (Branca and Ferrara, 2012;Siniscalchi et al, 2012). Therefore we assume the level of magma fragmentation may have interacted with the aquifer when the vents shifted from ∼2400 to 1450-1325 m asl, generating phreatomagmatic (Surtseyan) activity along the higher portion of Fissure 3 (see Figure 12B).…”

“…It cannot be excluded that other shallower aquifers interacted with the dyke during its deepening and propagation downward. The position of the impermeable sedimentary basement is modified from Siniscalchi et al (2012).…”

Tilt offset associated with local seismicity: the Mt. Etna January 9, 2001 seismic swarm.

“…However, the presence of fluids in this sector of the volcano is possible; recent magnetotelluric surveys (8 km depth) on the eastern flank of Mt. Etna have shown several highconductivity zones, suggesting a diffuse presence of hydrothermal activity and fluid circulation [53]. Moreover, seismicity is characterized by multiplets that can arguably be interpreted as caused by a single persistent, and possibly non-destructive, source; this factor simply does not agree with a purely tectonic origin of these earthquakes.…”

Great challenges in volcanology: how does the volcano factory work?

“…On the one side, a higher effort should be paid in detecting shallower intrusions, enhancing the resolution of common geophysical techniques, including seismic tomography, cosmic ray muon tomography and magnetotellurics (e.g., Vanorio et al, 2005;Shinoara and Tanaka, 2012;Siniscalchi et al, 2012). On the other side, we should learn more on their causes and conditions of emplacement, as well as their likely fate (freezing vs. erupting).…”