GNGTS 2013 - Atti del 32° Convegno Nazionale

By comparing and cross-matching the results obtained in the three trenches, we are able to constrain very well the age of the last surface rupture event which has occurred contemporary or slightly after 80-130 AD (1s calibrated age), and before 130-220 AD (1s calibrated age). In turn, due to a stratigraphical hiatus spanning between the High Middle Age (post 9 th cent. AD) and the Modern Age (before 17 th cent.), we cannot evaluate the entire coseismic surficial slip of this event. Therefore, the measured vertical offset of 0.4 m has to be regarded as a minimum estimate. A previous event might be occurred considerably after 5 ka, but slightly before 4150-3980 BP (2s calibrated age). Before this, other two events have likely occurred post 9440-9140 BP, although we are not able to provide more precise interval. In the whole, by considering the age of the colluvial units in the hanging wall, we can estimate a minimum vertical slip rate for this splay of 0.44 mm/yr during the Holocene. This value is in the same order of the one that can be calculated by measuring the vertical offset of the top surface of the oldest fan at the Malepasso Valley mouth, and also in other places along the foothill of Colle Ferrano, near our trenching site. Indeed, as we have measured a cumulated vertical offset of 11-12 m in several profiles across the main fault (see methodology in Galli et al. , 2013), plus other 3 m across a secondary splay in the hanging wall, if we do consider an age younger than 36 ka for the top-surface of the oldest fan, the minimum vertical slip rate is ca. 0.43 mm/yr. From all the above it emerges that the Mount Morrone fault system ruptured the last time ca. 1850-1900 years ago, and again ca. 2000 years before. Then, it likely ruptured two more times between 5 kyr and 9 kyr BP. This would yield a rough, average recurrence time close to 2 kyr, which is also the elapsed time since the last earthquake. As far as the last earthquake is concerned, it is worth noting that the very narrow time- span defined by the AMS dating of post-quem , ad-quem , and ante-quem terms in the trenches fits with the period (half of 2 nd century AD) in which Galadini and Galli (2001) have placed the earthquake that destroyed the Roman Sulmo and the neighbouring settlements (i.e., Interpromium=San Clemente a Casauria; Hercules Curinus sanctuary; Cansano; Raiano; Fig. 3 – View looking SE of trench 1 (net 1 m). The footwall (left side) is made up by well-organized gravels deposited during the final activity of the oldest cycle of the Santopadre alluvial fan (post 36 ky BP). The hanging wall (right side) hosts different generations of Late Holocene slope colluvia (9 ka to Present), beside colluvial and tectonic wedges. Note the fault plane, here striking N110°. 57 GNGTS 2013 S essione 1.1