First study of fault trench stratigraphy at Mt. Etna volcano, Southern Italy: understanding Holocene surface faulting along the Moscarello fault

Paleoseismology, the study of past earthquakes based on their geological re cord in the stratigraphy and landscape, is a successful newly developing fi eld of research, The application of fault trench studies in volcanic enviro nments is one of the youngest branches of paleoseismology. In this paper, w e present the results of the first exploratory trenches excavated at Mt, Et na in Sicily, the largest European volcano. Modern surface faulting at Etna is a very well known feature, which poses significant hazard to the local community, both in terms of ground displacement of essential lifelines and ground shacking from frequent damaging earthquakes. However, while the geom orphology and the seismicity of the active fault in the Etna region consist ently show very high rates of tectonic activity, the Holocene cumulative th row and slip-rates, along with the nature (coseismic vs, creeping fault sli p), dimension and timing of the displacement events, are still poorly const rained. For this purpose? we selected as a sample area the Moscarello fault , one of the most outstanding segment of the Timpe system of active normal faults in the volcano's lower eastern flank. Displaced landforms and volcan ic units at the Fondo Macchia basin, in the central sector of this fault, i ndicate some hundreds of meters of vertical offset in the last ca. 80 kyr, with a long-term slip-rate substantially higher than 1.5-2.0 mm/yr. Accordi ng to the historical sources and instrumental observations, the Moscarello fault ruptured four times in the last 150 years during shallow (H < 5 km) a nd moderate magnitude (M < 4.8) earthquakes. These events were associated w ith severe damage in a narrow epicentral area (macroseismic intensities up to the IX-X grade of the MSK scale) and extensive surface faulting tend-to- end rupture length up to 6 km, vertical offsets up to 90 cm). This clearly indicates very high modern rates of deformation along this fault. We conduc ted trench investigations at the Fondo Macchia site, in a point where eyewi tnesses observed ca. 20 cm of coseismic vertical displacement after the Apr il 21, 1971, M-s = 3.7, earthquake. The excavated sections provided direct stratigraphic evidence for a vertical slip-rate of 1.4-2.7 mm/yr in the las t ca. 6 kyr. This should be regarded as a minimum slip-rate for the central section of the fault. We explored a single scarp at a single site, while w e know from recent historical observations that several parallel scarps may rupture coseismically at Fondo Macchia. Thus, the relevant deformation rat e documented for the modern period might be likely extended back in the pas t to a time-span of some thousands of years at least. As expected, for such a volcanic environment, the activity rates of the Moscarello fault are als o significantly higher than for the Apennines normal faults, typically show ing slip-rates lower than 1 mm/yr. The agriculturally reworked trench hangi ngwall stratigraphy did not allow to recognize individual displacement even ts. Nevertheless, the sedimentary structures observed in the trench footwal l strongly suggest that, as for the last 150-200 years of detailed historic al record, fault behavior at Fondo Macchia is governed by coseismic surface displacement rather than fault creep. This research confirms that paleosei smology techniques can be effectively applied also in active volcanic envir onments, typically characterized by rheology and, consequently, seismicity and fault dynamics very different from those of other tectonic environments in which paleoseismology has been firstly developed and is today extensive ly applied. (C) 1999 Elsevier Science Ltd. All rights reserved.