Journal of Geodynamics 29 (2000) 233±244
A structural model for active extension in Central Italy Paolo Boncio*, Giusy Lavecchia Dipartimento di Scienze della Terra, UniversitaÁ ``G. D'Annunzio'', Chieti Scalo, Italy
Abstract This work presents a structural model for earthquake faulting in the Umbria-Marche Apennines (Central Italy). The model is derived by an integrated analysis of geological, geophysical and seismological data. At regional scale, the distribution and character of the seismicity appear to be mainly controlled by a low-angle east-dipping normal fault (Altotiberina fault, AF). The latter is the lower boundary of an active, continuously deforming hangingwall block moving toward NE. Moderate magnitude earthquakes (4 < M < 6), such as the Norcia 1979 (M = 5.9), the Gubbio 1984 (M = 5.2) and the Col®orito 1997 (Mmax = 5.9), occur within the active hangingwall block and are related to the activity of major west-dipping normal faults detaching on the AF. The geometry of the deep seismogenic structures is listric (as in the case of Col®orito) or more complex, because of local reactivation of pre-existing low-angle thrust (e.g. Gubbio) or high-angle strike-slip faults (e.g. Norcia). For all the analysed earthquakes the rupture nucleation is located at the base of the aftershock volumes, near the line of intersection between the SW-dipping normal faults and the east-dipping AF basal detachment. The progressive increase in depth of the earthquake foci from the north±west (e.g. Gubbio, 6±7 km) to the south±east (e.g. Norcia, 11±12 km) appears to be related to the eastward deepening of the basal detachment. These seismotectonic features are relevant for determining the seismogenic potential of the Apennine active faults, which depends not only on the length of the faults, but also on the depth of the detachment zone as well. Published by Elsevier Science Ltd.
1. Introduction and Plio-Quaternary tectonic context The Umbria-Marche Apennines (Central Italy) are aected by intense Plio-Quaternary extensional deformations. Mainly SW-dipping normal and normal±oblique faults, with * Corresponding author. E-mail address:
[email protected] (P. Boncio). 0264-3707/00/$ - see front matter Published by Elsevier Science Ltd. PII: S 0 2 6 4 - 3 7 0 7 ( 9 9 ) 0 0 0 5 0 - 2
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associated intramontane basins, are the main expression of the extensional strain ®eld at the surface (Fig. 1) (Calamita and Pizzi, 1994; Lavecchia et al., 1994). A regional east-dipping lowangle normal fault zone (Altotiberina Fault, AF), which represents the basal detachment to the SW-dipping faults, is clearly imaged by the CROP-03 NVR seismic pro®le (Figs. 2a and 3) and by commercial seismic re¯ection pro®les (Barchi et al., 1998). It may be also deduced by interpretation of seismic refraction data (DSS '78, Ponziani et al., 1995) (Fig. 2b). The AF crops out west of the Tiber Basin and extends along strike up to 50±60 km, deepening with a
Fig. 1. Structural map of the Umbria-Marche region with major extensional structures and related mean direction of the horizontal minimum principal stress
s3-axis) deduced from fault slip data (the s1-axis is sub-vertical). The traces of the geological sections refer to Fig. 7.
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Fig. 2. Geological cross-sections through the Umbria-Marche Apennines along the CROP-03 NVR seismic pro®le (section a, after Barchi et al., 1998) and along the DSS '78 pro®le (section b, after Boncio et al., 1998; DSS data from Ponziani et al., 1995).
staircase trajectory to a depth of about 12±14 km beneath the Umbria fold-and-thrust belt. The fault average dip is about 308 and the total displacement is about as 4±5 km. Low-angle, east-dipping, synthetic normal faults outcrop near the AF surface trace (Brozzetti, 1995), westdipping antithetic high-angle structures prevail further east within the Apennine mountain chain (e.g. Gubbio, Col®orito and Norcia normal faults). Fault kinematic and tensorial analysis in a number of selected sites show that the UmbriaMarche extensional deformations are consistently related to a tensional stress ®eld with SWNE trending maximum tension (Fig. 1) (Lavecchia et al., 1994). This stress ®eld is still active as shown by major recent earthquakes and by microseismic activity (Brozzetti and Lavecchia, 1994; Boncio et al., 1996). The Umbria-Marche region, which has been extensively investigated from a structural, geophysical and seismological point of view, represents a key-area in order to improve knowledge of the structural processes which control the earthquake faulting and to constrain the seismogenic potential of Apennine normal faults. With this aim, this paper proposes a 3D structural model for active extension in the Umbria-Marche region.
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Fig. 3. Seismic image of the Altotiberina low-angle normal fault (arrows) on the CROP-03 seismic pro®le (stack), vertical axis is two-way travel-time in seconds (from Barchi et al., 1998). The depth-converted section is given in Fig. 2a.
2. Seismotectonic setting 2.1. Regional seismicity The Umbria-Marche region is characterised by a well-documented historical and instrumental seismicity, mainly con®ned within the upper part of the crust (
