A structural model for active extension in Central Italy

A structural model for active extension in Central Italy

Paolo Boncio*, Giusy Lavecchia

Dipartimento di Scienze della Terra, Universita` G. DAnnunzio, 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 andseismological data. At regional scale, the distribution and character of the seismicity appear to bemainly controlled by a low-angle east-dipping normal fault (Altotiberina fault, AF). The latter is thelower boundary of an active, continuously deforming hangingwall block moving toward NE. Moderatemagnitude earthquakes (4

associated intramontane basins, are the main expression of the extensional strain field at thesurface (Fig. 1) (Calamita and Pizzi, 1994; Lavecchia et al., 1994). A regional east-dipping low-angle normal fault zone (Altotiberina Fault, AF), which represents the basal detachment to theSW-dipping faults, is clearly imaged by the CROP-03 NVR seismic profile (Figs. 2a and 3) andby commercial seismic reflection profiles (Barchi et al., 1998). It may be also deduced byinterpretation of seismic refraction data (DSS 78, Ponziani et al., 1995) (Fig. 2b). The AFcrops out west of the Tiber Basin and extends along strike up to 5060 km, deepening with a

Fig. 1. Structural map of the Umbria-Marche region with major extensional structures and related mean directionof the horizontal minimum principal stress s3-axis) deduced from fault slip data (the s1-axis is sub-vertical). Thetraces of the geological sections refer to Fig. 7.

P. Boncio, G. Lavecchia / Journal of Geodynamics 29 (2000) 233244234

staircase trajectory to a depth of about 1214 km beneath the Umbria fold-and-thrust belt.The fault average dip is about 308 and the total displacement is about as 45 km. Low-angle,east-dipping, synthetic normal faults outcrop near the AF surface trace (Brozzetti, 1995), west-dipping antithetic high-angle structures prevail further east within the Apennine mountainchain (e.g. Gubbio, Colfiorito and Norcia normal faults).Fault kinematic and tensorial analysis in a number of selected sites show that the Umbria-

Marche extensional deformations are consistently related to a tensional stress field with SW-NE trending maximum tension (Fig. 1) (Lavecchia et al., 1994). This stress field is still active asshown 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 improveknowledge of the structural processes which control the earthquake faulting and to constrainthe seismogenic potential of Apennine normal faults. With this aim, this paper proposes a 3Dstructural model for active extension in the Umbria-Marche region.

Fig. 2. Geological cross-sections through the Umbria-Marche Apennines along the CROP-03 NVR seismic profile(section a, after Barchi et al., 1998) and along the DSS 78 profile (section b, after Boncio et al., 1998; DSS data

from Ponziani et al., 1995).

P. Boncio, G. Lavecchia / Journal of Geodynamics 29 (2000) 233244 235

2. Seismotectonic setting

2.1. Regional seismicity

The Umbria-Marche region is characterised by a well-documented historical andinstrumental seismicity, mainly confined within the upper part of the crust (

MayJune 1987 database is not time-dependent. The epicentral distribution of themicroseismicity remarks that depicted by historical seismicity: the zone located west of theTiber Valley is almost aseismic, the Apennine area is highly seismic. The hypocentraldistribution (sections a and b of Fig. 5) delineates a wedge-shaped seismogenetic volume whosebase, that progressively deepens towards ENE, fits in well with the AF fault plane.

2.3. Active stress field

Focal mechanisms of major events (Gubbio, Colfiorito and Norcia), related aftershocks(Gubbio and Norcia) and spread microseismicity (MayJune 1987), analysed through theapplication of stress inversion techniques (Careys method), allow us to define the geometry ofthe active stress tensor in the Umbria-Marche region (Brozzetti and Lavecchia, 1994; Boncio etal., 1996; Boncio, 1998) (Fig. 6). The Norcia, Gubbio and Colfiorito major events as well as amajor group of microearthquakes show fault motions compatible with a tensional state ofstress characterised by a NE-trending sub-horizontal s3-axis. This tensor, which is similar to

Fig. 4. Historical seismicity with epicentral intensity (I ) greater than VI on the MCS scale (Camassi and Stucchi,1996); the numbers in the map refer to the year of occurrence of the largest historical earthquakes (IrIX). Starsand relative numbers indicate the epicentres of the largest instrumental earthquakes (19801997).


Fig. 5. Spread microseismicity in the Umbria-Marche Apennines and relative depth distribution. Microseismic dataare from a temporary survey (MayJune 1987) and from the 19911995 recording of the Umbria local network(after Deschamps et al., 1989; Boncio et al., 1998). Data from the first database have been plotted in the northernsection (GubbioFabriano area); data from the second database have been plotted in the southern section (Massa

M.ColfioritoNorcia area). Both databases indicate a seismogenic, continuously deforming, rock-volume with awedge-shaped east-deepening geometry.

Fig. 6. Active state of stress in the Apennines of Central Italy (after Brozzetti and Lavecchia, 1994; Boncio et al.,1996). The focal mechanisms refer to the CMT solutions of the largest instrumental Apennine-earthquakes. Thestress tensors (Tr, Tt, Tc) were calculated applying the Careys method to the MayJune 1987 spread microseismicity(a) and to the aftershock sequences of the Gubbio 1984 (b) and Norcia 1979 (c) earthquakes. The regional stress

tensor (Tr) is in agreement with the kinematics of the largest earthquakes and of the Plio-Quaternary extensionalstructures; Tt and Tc represent local tensional and compressional stress tensors coaxial with the Tr tensor.

the Plio-Quaternary stress field deduced from fault kinematics, may be consideredrepresentative of the mean regional present-day state of stress. The remaining focalmechanisms appear to be compatible with tensional (Tt) and/or compressional (Tc) stresstensors dierent from, but co-axial with, the regional one (Tr). The observed complexity of themicroseismicity can be modelled by adding to the regional tensional state of stress local pushesdue to the interaction of fault-bounded blocks moving within a pre-fractured rock-body(Mercier and Carey, 1989).

3. Three examples of Apennine normal faulting earthquakes

The seismotectonic features of the three major earthquakes (5

The bottom of the aftershock volume is not deeper than 67 km in the Gubbio zone, it isslightly deeper in the Colfiorito zone (about 78 km) and deeps to 1112 km in the Norciazone. The progressive increase in depth of the hypocenters from the westernmost sectors(Gubbio) to the easternmost ones (Norcia) appears to be well related to the eastwarddeepening of the AF basal detachment.

4. The seismogenetic model

The upper crust seismicity recorded in the last decades in the Umbria-Marche Apenninesshows that the extensional strain and stress fields have not significantly changed since Plio-Pleistocene times. Judging from the geometrical relationships between seismological andgeological (surface and sub-surface) data, the structural style of the crustal extension remainedunchanged as well.We propose a model for active extension in the Umbria-Marche region which in agreement

with the observed seismotectonic features might be helpful in determining the seismogenicpotential of the Apennine active faults (Fig. 8). We propose that the low-angle east-dippingAltotiberina normal fault (AF) and its antithetic west-dipping structures have been able todrive the distribution of the crustal seismicity. In particular, the AF discontinuity is believed toplay a fundamental role in separating an active seismic hanging-wall block, moving towardsNE, from an almost aseismic footwall block. An almost continuous microseismic activity(M< 3) is spread within the AF hangingwall rocks.Relatively frequent small to moderate earthquakes (4

Gubbio fault, detachment zone at about 67 km) with respect to the easternmost Apenninesector were the detachment is deeper (e.g., Norcia faults, detachment at about 1112 km).Concerning the seismotectonic interpretation of the largest historical Central Apennines

earthquakes I X, M 6:7, Camassi and Stucchi, 1996), a number of dierent hypothesismay be erected. The most conservative theory is that they are simply due to closely cumulateeects of several smaller magnitude events; alternatively, they can be considered to be discreterupture episodes on large fault segments.Following the second hypothesis, a minimum rupture area of about 400 km2 would be

admitted. In the case of the Norcia 1328 and 1703 earthquakes, the seismogenetic rupture areacan be related to west-dipping normal faults, such as the Nottoria-Preci fault which hasappropriate dimension (maximum along-strike length of about 30 km and down dip length upto 1214 km). Alternatively, the AF east-dipping normal fault, which extends up to 5060 kmalong strike, would be considered. The east-dipping seismogenetic structure should be restrictedto the relatively high-angle down-dip segment (average dip of 408 and length of about 10 km)

Fig. 8. Proposed structural model for earthquake normal faulting in Central Italy.


that cross-cuts the high-velocity crystalline basement (grey layer in Fig. 2b) beneath the weakphyllitic basement. In the case of the Camerino 1279 and Gualdo Tadino 1751 earthquakes, aseismogenetic area of about 400 km2 cannot be easily related to west-dipping normal faults,whereas a seismogenetic role of the AF seems a reasonable working hypothesis.In conclusion, we think that the frequency and distribution of Apennine extensional

seismicity within the upper crust is mainly controlled by the geometry of the geologicalstructures. Only subordinately it reflects rheological changes with depth. The definition of astructural model is, therefore, fundamental in order to better understand and predict theprocesses which control the nucleation and evolution of earthquake faulting.

Acknowledgements

The Authors are grateful to R. Scarpa (University of LAquila) and to G. De Luca (ServizioSismico Nazionale) who provided the phase readings of the Gubbio 1984 and Norcia 1979aftershock sequences. This study was supported by COFINZIAMENTO MURST 1997 andCNR-GNDT 1998 grants (resp. G. Lavecchia).

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A structural model for active extension in Central Italy

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