Journal of Seismology 4: 345–356, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.
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A geological model for the Colfiorito earthquakes (September-October 1997, central Italy) Paolo Boncio & Giusy Lavecchia Dipartimento di Scienze della Terra, Universit`a G. D’Annunzio, Chieti Scalo, Italy; e-mail:
[email protected],
[email protected] Received 9 March 1999; accepted in revised form 17 March 2000
Key words: earthquake, normal fault, seismic sequence, seismotectonics, Umbria-Marche Apennines
Abstract In this study, surface and subsurface geological data are integrated with seismological data in order to reconstruct a structural model for the September-October 1997 Colfiorito earthquakes. The seismic sequence is mainly controlled by two major SW-dipping normal faults outcropping in the area (M. Pennino-M. Prefoglio and M.Civitella-Preci faults). The activated faults detach, at depth, on a common east-dipping low-angle normal fault, the Altotiberina Fault (AF). The AF is interpreted as the base of an active hangingwall block which is stretching toward NE. The decrease in maximum depth of the earthquake foci from the Colfiorito area (about 8 km) to the Sellano area (about 6 km), suggested by the available seismological data, could be related to the eastward-deepening geometry of the AF detachment. The seismic fault planes, inferred from focal mechanisms and aftershock distributions, are characterised by a moderate dip (average 40◦ ) toward SW, which appears to be independent from the presence of pre-existing thrust planes.
Introduction The September-October 1997 Colfiorito earthquakes (central Italy) have been widely investigated from both seismological and geological point of views (Amato et al., 1998; Anzidei et al., 1999; Boncio and Lavecchia, 1999; Calamita et al., 1999; Cattaneo et al., this volume; Cello et al., 1998; Cinti et al., 1998; Ekstrom et al., 1998; Galli et al., 1998; Stramondo et al., 1999). In this paper, through the integrated analysis of geological, geophysical and seismological data, we attempt a structural interpretation of the seismic sequence, in the frame of a previously proposed (Boncio and Lavecchia, 2000) regional seismogenetic model. Seismotectonic framework Stress field and structural style of the Plio-Quaternary extensional tectonics The Umbria-Marche Apennines (U-M Ap.) of central Italy are affected by Plio-Quaternary extensional
deformation, with mainly SW-dipping normal and normal-oblique faults and associated intramontane basins (Figure 1). The related stress field is tensional with SW-NE trending maximum tension (Calamita and Pizzi, 1994; Lavecchia et al., 1994). An east-dipping low-angle reflector has been recently pointed out in the northern Umbria region on the basis of NVR seismic data (CROP 03 project, Barchi et al. 1998). Barchi et al. (1998) and Boncio et al. (1998) interpreted such reflector as an eastward deepening active low-angle normal fault zone named Altotiberina Fault (AF) (Figure 1). The surface expression of the AF zone consists of low-angle (average 30◦ ) synthetic splays bordering westward the Eastern Tiber basin (Brozzetti, 1995). The deep geometry of the AF has been detailed through the analysis of deep crust (CROP 03) and commercial seismic reflection profiles (Figure 2 and references therein). The geological interpretation of these profiles shows that the AF deepens eastward to a depth of 12–14 km; it has a staircase trajectory with an average dip of about 30◦ , and represents the basal detachment of the intra-Apennine
346 Umbria-Marche active stress field and seismicity
Figure 1. Schematic structural map of the Umbria-Marche region with location of Figure 6. The major extensional structures and the related mean direction of the horizontal minimum principal stress (σ 3-axis) deduced from fault slip data (the σ 1-axis is sub-vertical) are shown. The focal mechanisms refer to the CMT solutions of the largest instrumental intra-Apennine earthquakes.
Quaternary SW-dipping normal faults (Figure 3 and references therein). Seismic reflections along the AF plane suggest the presence of a thick, mature shearzone associated to the fault. The total displacement measured along the fault is about 5 km (Barchi et al., 1998), suggesting that the role played by the AF is of regional importance. An inner én échelon segment of the AF may be assumed in southern Umbria, bordering the west side of the Western Tiber basin (dashed AF line in Figure 1). This hypothesis mainly rests on surface evidences of low-angle east-dipping normal faults in the NarneseAmerina ridge area (west of Terni, Boncio et al., 1995).
The Umbria-Marche region is still undergoing active SW-NE extension as shown by the focal mechanisms of the intra-Apennine seismicity (Brozzetti and Lavecchia, 1994; Boncio et al., 1996; Frepoli et al., 1998) (Figure 1). The area to the east of the AF surface trace is characterised by a relatively intense seismic activity consisting of almost continuous microseismicity (M≤3), relatively frequent moderate magnitude (5≤M≤6) earthquakes, rare large magnitude (M = 6–7) earthquakes. Clustering of microseisms, small magnitude earthquakes (3
