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The September–October 1997 seismic sequence in the Umbria–Marche regions of Central Italy (main shocks on September 26, Mw 5.7 and 6.0, and on October 14, Mw 5.6) left significant ground effects, which were mainly concentrated in the Colfiorito intermountain basin. These effects included surface faulting, ground cracks and settlements, rock falls, slides, hydrological and gas anomalies. The distribution and size of ground effects has proved useful for (1) defining the epicentral area and the location of the causative fault; (2) complementing the intensity pattern from damage distribution (this can be very useful in poorly inhabited zones); (3) integrating or testing the intensity assessment of many historical events, in order to obtain a better evaluation of the magnitude from intensity data. Of special interest was the observation of surface ruptures generated along segments of a system of normal faults already mapped as capable, with end-to-end lengths of 12 km and maximum displacements of 8 cm. Many pieces of evidence confirm that coseismic slip was not a secondary, gravity-induced, phenomenon, but had a tectonic origin. Detailed descriptions of surface faulting for moderate earthquakes are not common, being easily missed or misinterpreted; however, in this paper we emphasize that surface faulting due to the 1997 event can be used to infer the threshold magnitude for surface faulting in Central Apennines, allowing to calibrate palaeoearthquake size from fault offsets as seen in trench investigations.

Paleoseismology, the study of past earthquakes based on their geological record in the stratigraphy and landscape, is a successful newly developing field of research. The application of fault trench studies in volcanic environments is one of the youngest branches of paleoseismology. In this paper, we present the results of the first exploratory trenches excavated at Mt. Etna 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 geomorphology and the seismicity of the active fault in the Etna region consistently show very high rates of tectonic activity, the Holocene cumulative throw and slip-rates, along with the nature (coseismic vs. creeping fault slip), dimension and timing of the displacement events, are still poorly constrained. 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 volcanic units at the Fondo Macchia basin, in the central sector of this fault, indicate 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. According to the historical sources and instrumental observations, the Moscarello fault ruptured four times in the last 150 years during shallow (H 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 last 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 we know from recent historical observations that several parallel scarps may rupture coseismically at Fondo Macchia. Thus, the relevant deformation rate documented for the modern period might be likely extended back in the past 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 also significantly higher than for the Apennines normal faults, typically showing slip-rates lower than 1 mm/yr. The agriculturally reworked trench hangingwall stratigraphy did not allow to recognize individual displacement events. Nevertheless, the sedimentary structures observed in the trench footwall strongly suggest that, as for the last 150–200 years of detailed historical record, fault behavior at Fondo Macchia is governed by coseismic surface displacement rather than fault creep. This research confirms that paleoseismology techniques can be effectively applied also in active volcanic environments, 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 extensively applied.