Rend. Online Soc. Geol. It., Vol. 29 (2013), pp. 51-54, 3 figs. © Società Geologica Italiana, Roma 2013
ABSTRACT
The active deformation field acting since Early Quaternary times in the intra-Apennine area of Central Italy was constrained through a kinematic and strain analysis performed on two, independent, geological fault-slip and seismological data sets collected, and updated, for the areas struck by the two most energetic seismic sequences of the last fifteen years (Colfiorito 1997, Mw6.0 and L’Aquila 2009, Mw6.1).
The attitude and shape of the long-term (e.g. geological, since Early Quaternary times) and instantaneous (e.g. seismological) deformation fields in the Umbria-Marche Apennines were compared in order to detect similarities and/or inconsistencies. The kinematic analysis pointed out a sub-horizontal, NE-SW trending T-axes and a systematic coherence between the geological and seismological kinematic parameters, even if representing different time-scale deformations.
The strain analysis further supported the extensional character of the deformation field acting in both the investigated sectors, even if it highlighted some heterogeneities in the strain accommodation when considering individual seismogenic sources. On the whole, results pointed out the prevailing extensional character of the deformation field in Central Italy also coherent with GPS data investigations.
KEY WORDS: kinematic analysis, strain ellipsoid, active deformation field, Central Italy.
INTRODUCTION
Investigations aimed to infer the orientation and relative magnitude of the principal stress and strain axes, in tectonically active areas, are very common. As regards the Italian context, previous authors dealt with this subject at different scales, especially focusing on the active deformation pattern as derived from geological, seismological and geodetic data (Brozzetti & Lavecchia, 1994; Boncio and Lavecchia, 2000; D’Agostino et al., 2009).The overall information coherently pointed out an intra-Apennine extensional field, active since Early Pleistocene times and characterized by mean NE-SW direction of the tensional axis.
In this paper, we are aimed to further constrain, and to compare, the shape and attitude of the long-term (e.g. geological, since Early Quaternary times) and instantaneous (e.g. seismological) deformation fields in the Umbria-Marche Apennines. In particular, we focus on the epicentral areas of
the two most energetic instrumental seismic sequences of central Italy, that are the Colfiorito 1997 (Mw 6.0) and L’Aquila 2009 (Mw6.1).
A kinematic analysis of independent geological (fault-slip fielddata) and seismological (focal mechanisms) data sets was performed in order to detect similarities and/or inconsistencies. To summarize results, the shape of the geological and seismological strain ellipsoids obtained for the Colfiorito and L’Aquila fault systems, were represented using a Flinn-like diagram. The 3D geometry and segmentation pattern of the involved fault systems and the earthquake-fault associations were derived from Lavecchia et al. (2012) and Ferrarini et al. (submitted).
Active deformation field from earthquakes and faults in the
Colfiorito 1997 and L’Aquila 2009 seismic sequence epicentral areas
(central Italy)
Federica Ferrarini (a), Giusy Lavecchia (a), Rita de Nardis (a, b), Francesco Brozzetti (a) & Maria Adelaide Romano (a)
_____________________________________________________________________________________________________________________________________________________ (a) Laboratorio di Geodinamica e Sismogenesi, DiS.P.U.Ter, Università «G. d’Annunzio», Campus Universitario, 66013 Chieti Scalo, Chieti, Italy. E-mail: f.ferrarini@unich.it (c) Dipartimento della Protezione Civile, Rome, Italy.
Document type: Short note.
Manuscript history: received 23 September 2013; accepted xx Month 2013; editorial responsibility and handling by Domenico Calcaterra.
_____________________________________________________________________________________________________________________________________________________
Fig. 1- Extensional Quaternary fault alignments and major (main shocks Mw>5.0) instrumental seismic sequences in central Italy (references as in Lavecchia et al., 2012). GPS vector trends from D’Agostino et al. (2009) (DA09) are also reported.
FERRARINI ET AL.
52
SEISMOTECTONIC SETTING
The Apennine mountain belt of central Italy is cross-cut by an NW-SE striking extensional system of normal and normal-oblique, mainly southwest-dipping faults, active since Early Quaternary times (Fig.1) (Valensise & Pantosti, 2001; Lavecchia et al., 2002) and generating intra-mountain depressions (Lavecchia et al., 1994; Galadini & Galli, 2000; Boncio & Lavecchia, 2000; de Nardis et al., 2011; Romano et al., 2013). The activity of these faults is confirmed in historical times by many Mw≥5.5 seismic events (CPTI11 available at http://emidius.mi.ingv.it/CPTI11). Moreover, during the last 30 years, several Mw≥5.5 earthquakes were associated to the active fault systems outcropping from Gubbio to L’Aquila (Gubbio 1984, Mw 5.6; Gualdo Tadino 1998, Mw 5.1; Colfiorito 1997, Mw 6.0; Norcia 1979, Mw 5.9; L’Aquila 2009, Mw 6.1) (Fig. 1). Focal mechanisms highlight a normal and normal-oblique kinematics and are consistent with a NE-SW extensional field (Boncio & Lavecchia, 2000).
The Colfiorito 1997 sequence was characterized by six main shocks with 5.0≥ML<6.0 (Chiaraluce et al., 2003). It spread along a NW-SE direction (inset in Fig. 1 and 2A) activating different segments of the SW-dipping Colfiorito normal fault system: the (1) M. Pennino-M. Prefoglio, the (2) Cesi-S. Martino, the (3) M. Fema and the (4) Annifo (minor) faults.
Similarly, the L’Aquila 2009 seismic sequence spread for nearly 50 km along an average NW-SE direction (red inset in Fig. 1 and Fig. 2B). It was characterized by six shocks with 5.0≥ML<6.0 (Chiaraluce et al., 2011) activating four seismogenic sources: the south-west dipping (5) Gorzano and (6) Montereale-S. Franco, the southwest-to south dipping (7) Paganica-Stabiata and the (8) Ocre hidden east-dipping, normal faults (Lavecchia et al., 2012).
KINEMATIC ANALYSIS OF DATA
In order to constrain the attitude of the present deformation field in central Italy and to compare it with the long-term one, we performed, separately, a kinematic analysis of the structural and seismological data associated to each of the fault segments (as discussed in the previous section) activated during the Colfiorito 1997 and L’Aquila 2009 seismic sequences. In this way, we detected similarities or inconsistencies between them and constrained the regional Quaternary deformational field, also exploring the possibility of local deviations. For each fault segment, of both the Colfiorito and L’Aquila, an average pseudo- focal mechanism (blue beach-ball in Fig.2) and the related T-axis were calculated starting from an updated dataset of fault-slip data (blue circles in Figs. 2A and 2B) and applying a Bingham statistics. As well, the average focal mechanisms and the related T-axes were calculated starting from the M≥3.5 events (red beach balls in Fig. 2). Regarding the Annifo source, due to its lower energetic release, the ML≥2.4 events were considered. The Colfiorito focal solutions were extracted from Chiaraluce et al. (2004); the L’Aquila ones from Herrmann et al. (2011) integrated with those available at http://www.eas.slu.edu/eqc/eqcmt.html.
As concernig the Colfiorito fault system (Fig. 2A), the focal mechanisms derived for the M. Pennino-M. Prefoglio (1), Cesi-S. Martino (2) and M. Fema (3) individual seismogenic sources, highlighted an almost pure normal fault kinematics (Fig. 2A) with NE-SW trending T-axes Fig. 3A) coaxial with the same computed on the whole data sets (Fig. 2A1). These results are coherent with GPS vector trends (D’Agostino et al., 2009) and with the least principal stress axes of the stress tensors computed in Ferrarini et al., (submitted), reported, for comparison, in Fig. 2A1. In the case of the N-S trending Annifo source (4), we didn’t compared the two average focal mechanisms (both with strike-slip kinematics but with opposite sense of shear), since the geological one (blue-grey beach ball) does not refer to the Quaternary kinematics, but to the pre-existing N-S right-lateral strike-slip movements occurred during the Late Miocene-Pliocene compressional phase (Lavecchia et al., 1988; Ferrarini et al., submitted). Nevertheless, the observed coaxiality between the T-axes of the sinistral strike-slip solution and those computed for the other activated sources suggest its reactivation, with an inverted kinematics during the 1997 seismic sequence.
The kinematic analysis of the L’Aquila geological and seismological data sets (Fig. 2B) gave more homogeneous results. The geological and seismological average focal mechanisms, computed for both the Gorzano (5) and the Montereale (6) sources, show normal fault kinematics with near parallel T-axes (Fig. 3A). A very minor srike-slip component was obtained for the average focal mechanisms of the Paganica-Stabiata (7) and Ocre hidden source (8), whose extensional axes remained, however, sub-horizontal and NE-SW trending. Also in this case, we observed a coherence with the T-axes trend computed on the whole data sets (Fig. 2B1), with the GPS vector trends from D’Agostino et al., 2009 and with the least principal stress axes of the stress tensors computed in Ferrarini et al. (submitted).
STRAIN ANALYSIS AND ACTIVE DEFORMATION PATTERN
Starting from the previously discussed focal mechanisms (blue and red beach-balls in Fig.2), we characterized the corresponding strain ellipsoids using a Flynn-like diagram which pictures the strain ellipsoid resulting from distortion and/or dilatation of an original reference sphere. Pure flattening and pure constriction results in the plot along the x-axis and y-axis, respectively, whereas the plain strain plots along the 45°-sloping line intersecting the origin of the plot. Specifically, we exploited the T-B-P -axes of the computed average focal mechanisms (Bingham statistics) and their meaning as principal deformation axis of a strain matrix. In particular, we considered the eigenvectors and respective eigenvalues of the strain matrix corresponding to the directions and magnitudes of the infinitesimal principal shortening and extension directions. Consequently, the eigenvalues (and their relative magnitudes) also gave us information about the shape of the strain ellipsoid whose principal axes are represented by ɛ1≥ɛ2≥ɛ3. In the Flinn- like diagram of Fig. 3B, we reported the geological (red sym-
ACTIVE DEFORMATION FIELD FROM EARTHQUAKES AND FAULTS IN CENTRAL ITALY 53
bols) and seismological (blue symbols) strain ratios concerning each seismogenic source.
Regarding the strain ratios associated to the seismological data (red squares), we observed as those derived from the L’Aquila focal mechanisms (sources numbers 5, 6, 7 and 8 in Fig.2B) are all representative of plain strain; this confirming homogeneity in the kinematics of the different sources and, as a consequence, of the entire sequence.
More complex is the corresponding Colfiorito picture. The activated sources (1, 2, 4) reflected a noticeable heterogeneity in the deformation pattern. This is particularly evident for the Annifo (4) and M. Pennino-M. Prefoglio (1) sources, whose strain ratios are shifted towards the flattening strain. This results, probably deriving from the close spatial interaction, in the strain accommodation, of the above mentioned extensional sources and the Annifo strike-slip one. We omitted the seismological strain analysis of the M. Fema source since it has been associated with only one ML≥3.5 focal mechanism.
As regards both the Colfiorito and L’Aquila geological strain analysis, a slight shift of all the strain ratios (blue squares) towards the flattening strain field is observed (Fig.3). In our opinion, this might reflect a greater heterogeneity of the geological data which are distributed on a long-term period (e.g. the Quaternary) with respect to the “instantaneous” time-window , which is representative of the seismological data. The
Annifo geological strain ratio was omitted since referring to pre-Quaternary deformations.
CONCLUSIONS
The kinematic and strain analysis performed on geological and seismological data concerning two areas in central Italy, affected by recent seismic sequences (Colfiorito 1997 and L’Aquila 2009), provide further information and constraints on the active deformation pattern observable along the Apennine belt. Even if some minor heterogeneity can be observed when investigating the individual fault segments, our results highlight the persistence in the Umbria-Marche region of a NE-SW trending extensional deformation, at least since Early Quaternary times. This statement is totally supported by the coherence between the geological and seismological kinematic/strain parameters, even if representing different time-scale deformations.
Although, also recently, some authors advanced the hypothesis of an important regional role played by the strike-slip tectonics in Central Italy (Cello et al., 1997; D’Amico et al., 2012; Elter et al., 2012), our investigations point to different results which are also coherent with other investigations carried out in Central Italy such as GPS data (D’Agostino et al., 2009).
Fig. 2 – Summary of the kinematic analysis and proposed seismotectonic interpretation of the Colfiorito 1997 (A) and L’Aquila 2009 (B) areas with schematic seismogenic sources (from Ferrarini et al., submitted and Lavecchia et al., 2012, respectively), depth-contour lines and average focal mechanisms computed from geological (blue pseudo-focal mechanism) and earthquake data (red beach ball). Individual source key: 1-M. Pennino-M. Prefoglio; 2-Cesi-S. Martino;
3-M.Fema; 4-Annifo; 5- Gorzano; 6-Montereale; 7- Paganica; 8-Ocre. Stars are the epicenters of the ML≥5.0 events.The stereographic plot, in the bottom left
corner of each panel, shows the geological (G) and seismological (S) T- axes (computed on the whole data sets), and the GPS vector trends falling in the respective sectors(from D’AGOSTINO et al., 2009). The least principal stress axes (sigms3), as computed in Ferrarini et al. (submitted), is also reported for comparison. The Annifo strike-slip source and the related stress and strain parameters are drawn in green colour.
FERRARINI ET AL.
54
REFERENCES
Boncio P. & Lavecchia G. (2000) - A structural model for active extension in Central Italy. J. Geodyn., 29, 233-244. Brozzetti F. & Lavecchia G. (1994) - Seismicity and related
extensional stress field: the case of the Norcia Seismic Zone (Central Italy). Annales Tectonicae, 8, 36–57.
Chiaraluce L., Amato A., Cocco M., Chiarabba C., Selvaggi G., Di Bona M., Piccinini D., Deschamps A., Margheriti L., Courboulex F. & Ripepe M. (2004) - Complex normal faulting in the Apennines thrust-and-fold belt: The 1997 seismic sequence in central Italy. Bull. Seismol. Soc. Amer., 94, 99-116.
Chiaraluce L., Chiarabba C., De Gori P., Di Stefano R., Improta L., Piccinini D., Schlagenhauf A., Traversa P., Valoroso L. & Voisin C. (2011) - The 2009 L'Aquila (central Italy) seismic sequence. B. Geofis. Teor. Appl., 52, 367-387.
Chiaraluce L., Ellsworth W. L., Chiarabba C. & Cocco M. (2003) - Imaging the complexity of an active normal fault
system: The 1997 Colfiorito (central Italy) case study. J. Geophys. Res., B: Solid Earth, 108, 1–19.
D’Amico, S., Barbara, O., Presti D., Neri, G., Wen-Nan, W., Sandu, I., Zhu, L., and Herrmann, R. B. (2013) - Source parameters of small and moderate earthquakes in the area of the 2009 L’Aquila earthquake sequence (central Italy). Phys. Chem. Earth, 63, 77-91.
D'Agostino N., Mantenuto S., D'Anastasio E., Avallone A., Barchi M., Collettini C., Radicioni F., Stoppini A. & Fastellini G. (2009) - Contemporary crustal extension in the Umbria–Marche Apennines from regional CGPS networks and comparison between geodetic and seismic deformation. Tectonophysics, 476, 3–12.
de Nardis R., Garbin M., Lavecchia G., Pace B., Peruzza L., Piolo E., Romanelli M., Romano M. A., Visini F. & Vuan A. (2011). A temporary seismic monitoring of the Sulmona area (Abruzzo, Italy) for seismotectonic purposes. B. Geofis. Teor. Appl., 52, 651-666.
Elter F. M., Elter P., Eva C., Kraus R. K., Padovano M. & Solarino S. (2012) - An alternative model for the recent evolution of the Northern-Central Apennines (Italy). J. Geodyn., 54, 55–63.
Ferrarini F., Lavecchia G., de Nardis R. & Brozzetti F. - Fault geometry and active stress from earthquakes and field geology data analysis: the Colfiorito 1997 and L'Aquila 2009 cases (central Italy). Submit. to Pure Appl.Geophys. Galadini F. & Galli P. (2000) - Active tectonics in the Central
Apennines (Italy) - Input data for Seismic Hazard Assessment. Nat. Hazards, 22, 225-270.
Herrmann R.B., Malagnini L. & Munafò, I. (2011) - Regional Moment Tensors of the 2009 L’Aquila Earthquake Sequence. Bull. Seismol. Soc. Am., 101, 975–993.
Lavecchia G., Boncio P., Brozzetti F., Stucchi M. & Leschiutta I. (2002) - New criteria for seismotectonic zoning in Central Italy: insights from the Umbria-Marche Apennines. Boll. Soc. Geol. It., Spec. Vol., 1, 881–890.
Lavecchia G., Ferrarini F., Brozzetti F., De Nardis R., Boncio P. & Chiaraluce L. (2012) - From surface geology to aftershock analysis: Constraints on the geometry of the L’Aquila 2009 seismogenic fault system. Ital. J. Geosci., 131(3), 330-347.
Lavecchia G., Minelli G. & Pialli G. (1988) - The Umbria-Marche arcuate fold belt (Italy). Tectonophysics, 146, 125– 137.
Lavecchia, G., Brozzetti, F., Barchi, M., Menichetti, M. & Keller J.V.A. (1994) - Seismotectonic zoning in east-central Italy deduced from an analysis of the Neogene to present deformations and related stress fields. Geol. Soc. Am. Bull., 106, 1107–1120.
Romano M. A., de Nardis R., Garbin M., Peruzza L., Priolo E., Lavecchia G. & Romanelli M. (2013) - Temporary seismic monitoring of the Sulmona area (Abruzzo, Italy): quality study of microearthquake locations. Nat. Hazards Earth Sys., 13, 2727-2744.
Valensise G. & Pantosti D. (2001) - The investigation of potential earthquake sources in peninsular Italy: A review. J. Seismol., 5, 287-306.
Fig. 3– (A): T-axes computed for each seismogenic source of the Colfiorito and L’Aquila fault systems: geological data (blue circles) and focal mechanisms (red circles). (B): Flinn-like diagram showing the strain ratios of the individual seismogenic sources. Blue and red squares refer to geological and seismological data respectively. Seismogenic source key as in Fig. 2.
