Paleosismologia della Piana del Fucino (Italia centrale)

Abstract

The tectonic evolution of the Fucino Plain (649-720 m a.s.l.), one of the major intermontane depressions of the Apennine chain, started during the Pliocene, as Plio-Pleistocene sediments unconformably cover the older marine deposits. The Pleistocene tectonic activity was responsible for the formation of a NW-SE trending half-graben. The major Quaternary faults, characterized by mainly extensional kinematics, are located along the eastern (NW-SE trending) and northern (NE-SW trending) borders of the basin. The Fucino Plain is one of the few areas in Italy where surface faulting unequivocally occurred during historical times (1915 earthquake). Previous works indicate that coseismic displacements occurred along different faults during the 1915 earthquake (M =6.7; more than 30,000 casualties) and that displacements occurred along these faults during events preceding the 1915 one. In 1992-1996, paleoseis-mological techniques were used at 11 different sites along 4 faults (Marsicana Hwy. fault system, San Benedetto dei Marsi-Gioia dei Marsi, Trasacco and Luco dei Marsi faults) by analysing exposures in trenches ad-hoc excavated across the different faults or in excavations made for non-scientific purposes (such as the laying of an aqueduct or a gas pipeline). Detailed paleoseismological research was carried out along the Trasacco fault, a minor fault located in the most internal area of the basin. In fact the Fucino Plain was a lake until 1875 (when it was drained for agricultural purposes), therefore the study of the Trasacco fault may provide extensive data due to almost continuous lacustrine deposition within the central part of the basin. Datings of the different sedimentary successions affected by Upper Pleistocene and Holocene displacements were obtained using radiocarbon and thermoluminescence techniques and archaeological datings on pottery fragments. In the central and south-western part of the basin the long excavation made for the laying of an aqueduct and the walls of the present main drainage channel (trending E-W) permitted the reconstruction of the Upper Pleistocene-Holocene lacustrine succession and detailed stratigraphic correlations between different sites. The studied trench-walls and outcrops are extensively illustrated in twenty text-figures. A summary of the different displacement events detected in the studied 11 sites is reported in the text as a table. Ten surface faulting events were detected in the last 33,000 years, 7 of which occurred during the Holocene. Datings of the different paleoseismic events indicate that at least the most recent events simultaneously affected the stud-ied faults. The time interval between successive surface faulting events ranges from 1100-1400 years (the difference between the 1915 event and the so-called High Middle Age Event) to 1500-1700 years (or 2000-2600 years if we consider the calibrated 14C ages; time period between the so-called Bronze Age Event and the Second Neolithic Event). Therefore, available data indicate that high magnitude events are characterized by recurrence intervals larger than 1000 years. The slip rate was calculated for the Marsicana Hwy fault (for the last 20,000 years), the San Benedetto dei Marsi-Gioia dei Marsi fault (for the last 10,000 years) and the Trasacco fault (for the last 7000 years), by taking into account their vertical offset. The inferred value for each fault is about 0.4 mm/yr. This value decreases along the Trasacco fault from south to north as a result of the decreasing offset. Considering that the 1915 earthquake is the only strong event known to have occurred in the area in the last 1000 years, the recognition of paleo-earthquake effects represents an important step in the understanding of the seismic behaviour of the region. The comparison with the previous works show that new data strongly improve the paleoseismological knowledge of the studied area; moreover, these data give important constraints for the definition of the geometry and behaviour of the Fucino Plain seismogenic structure during time.