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Abstract
Flank instability and collapse are observed at many volcanoes. Among
these, Mt. Etna is characterized by the spreading of its eastern and
southern flanks. The eastern spreading area is bordered to the north
by the E–W-trending Pernicana Fault System (PFS). During the 2002–2003
Etna eruption, ground fracturing along the PFS migrated eastward from
the NE Rift, to as far as the 18 km distant coastline. The deformation
consisted of dextral en-echelon segments, with sinistral and normal
kinematics. Both of these components of displacement were one order
of magnitude larger (~1 m) in the western, previously known, portion
of the PFS with respect to the newly surveyed (~9 km long) eastern section
(~0.1 m). This eastern section is located along a pre-existing, but
previously unknown, fault, where displaced man-made structures give
overall slip rates (1–1.9 cm/year), only slightly lower than those
calculated for the western portion (1.4–2.3 cm/year). After an
initial rapid motion during the first days of the 2002–2003 eruption,
movement of the western portion of the PFS decreased dramatically, while
parts of the eastern portion continued to move. These data suggest a
model of spreading of the eastern flank of Etna along the PFS, characterized
by eruptions along the NE Rift, instantaneous, short-lived, meter-scale
displacements along the western PFS and more long-lived centimeter-scale
displacements along the eastern PFS. The surface deformation then migrated
southwards, reactivating, one after the other, the NNW–SSE-trending
Timpe and Trecastagni faults, with displacements of ~0.1 and ~0.04 m,
respectively. These structures, along with the PFS, mark the boundaries
of two adjacent blocks, moving at different times and rates. The new
extent of the PFS and previous activity over its full length indicate
that the sliding eastern flank extends well below the Ionian Sea. The
clustering of seismic activity above 4 km b.s.l. during the eruption
suggests a deep décollement for the moving mass. The collected
data thus suggests a significant movement (volume >1,100 km3) of
the eastern flank of Etna, both on-shore and off-shore.
Keywords:
Volcano spreading - Fracturing - Mt. Etna - Pernicana Fault System -
NE Rift
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