Surface Rupturing at Mt. Etna Volcano: Consequences on Environment and Human Activities
by
Domenico Berti
APAT (Italian Agency for Environment Protection and Technical Services), via Vitaliano Brancati 48, 00144 Roma
Coauthors: E. Brustia, L. Ferreli, E. Vittori, A.M. Michetti, D. Bella, F. Filetti

Surprisingly enough, the most active volcano of the Mediterranean sea also hosts a wealth of evidence of early human settlements, which, from the Bronze age to now, not only have endured the volcanic violence, but have grown to populated cities (the largest being Catania, that was cut in two by a large lava flow in 1669 and destroyed by the 1693 earthquake). So, Etna is a privileged area to analyze the relation between natural violence and human reaction to it. In addition to volcanic eruptions, Etna is affected by active ground ruptures in its eastern, most inhabited, side facing the sea.

Ground displacement may result from volcano-related movements due to dyke-induced rifting and flank instability sliding. Likewise, purely tectonic faulting certainly plays a significant role. Surface faulting shows very different features and is associated to a variety of processes. The occurrence at Etna of both coseismic and aseismic surface rupture events is well known, also along the same fault. Commonly, coseismic surface faulting is here associated with very shallow (H < 5 Km), moderate earthquakes, which occur with short recurrence timing (tens of years). Such earthquakes produce macroseismic intensities reaching even the IX-X grade MSK along elongated, narrow zones including significant surface faulting, with end-to-end rupture length up to 5-7 Km and vertical offsets up to 90 cm. It is very clear the striking difference in the relations between magnitude and surface rupture length or the coseismic offsets for these earthquakes. Fault creeping affects extensively most of the faults of the volcano, with modern (last 200 years) creep rates ranging from 0.5 to 2.3 cm/year. Surface rupturing, either coseismic or due to creep, produces in the area disastrous damage, especially when crucial lifelines cross it.

In this note, we describe paleoseismological investigations recently undertaken along the Moscarello, Pernicana and S. Leonardello faults, among the most active faults affecting the Etna volcano, and their primary and secondary effects on the environment and human activities.

The Moscarello fault is one of the most active faults of the Timpe system, in term of slip rates and seismicity. During the past two centuries, this fault has generated four seismic events (in 1855, 1865, 1911, and 1971) with intensities of VII-VIII to X MSK. All these events were accompanied by surface ruptures up to 6 km long and with maximum dip-slip displacements between 25 and 90 cm. During the earthquakes the surface ruptures produced collapse of rural houses. Exploratory trenches evidenced a vertical component of the Holocene slip rate on the order of 2 to 3 mm/year, in good agreement with the short-term value inferred from the historical ruptures.

In the northeast side of the volcano, the Timpe System interrupts to the north against the arch-shaped Provenzana-Pernicana-Fiumefreddo fault system, trending east-west to west-northwest-east-southeast for ca. 18 km. This fault system crosses the volcanic edifice from the central crater to the sea. This very active system plays an important role in the geodynamic and morphologic evolution of the area, and it is interpreted by some authors as the northern boundary of the unstable eastern flank of the volcano, sliding toward the sea. Its kinematics is essentially left-lateral, with local extensional components lowering the southern and eastern sides. Slip occurs as a result of both coseismic rupture and continuous or episodic creep. The slip rate, > 2 cm/year, is the highest known in the Etna volcanic area, deforming roads, houses, the Messina-Catania tollway and a gas pipeline. Paleoseismological studies in the central segment of the system, corresponding to the Pernicana fault, revealed a vertical slip rate higher than 2.45 cm/year for the last 1.0 ka and aseismic movements along the fault planes.

One more morphologically outstanding fault of the Timpe System is the mostly normal San Leonardello Fault, with its up to 30 meters high scarps cutting the middle-lower slopes of Etna.

Based on the Late-Pleistocene-Holocene offsets, the authors estimate a slip rate of 1.25-2.5 mm/year. In historical times the fault's behaviour has been not uniform along strike. Coseismic ruptures have characterized the northern and central parts (e.g. earthquakes of 1881, 1920, 1950 and 1989) never exceeding intensity VIII MCS. The southern sector has moved mostly by creep. The fault activity, similarly to the other active faults in the area, has caused, and still causes, significant distortions to the communication routes and to buildings. Close to the main fault trace, exploratory trenches were excavated across a secondary graben, above which there is now a rapidly developing industrial settlement. The trenches have allowed to recognize and describe recent secondary ground deformation, very likely linked to the main fault, posing a significant hazard to the buildings resting on it.

Rich is the evidence testifying of the lost of fundamental understanding of the Nature by man in this delicate environment. As pointed out in many other situations, it is plain that the too fast economic growth of the last century is the main responsible for this new ignorance, for which not even disasters appear to be a sufficient cure.

Date received: July 26, 2005

Copyright © 2005 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caqy-68.