Paolo Boncio

Defining a model of 3D seismogenic sources for Seismic Hazard Assessment applications: The case of central Apennines (Italy)

Geology-based methods for Probabilistic Seismic Hazard Assessment (PSHA) have been developing in Italy. These methods require information on the geometric, kinematic and energetic parameters of the major seismogenic faults. In this paper, we define a model of 3D seismogenic sources in the central Apennines of Italy. Our approach is mainly structural-seismotectonic: we integrate surface geology data (trace of active faults, i.e. 2D features) with seismicity and subsurface geological–geophysical data (3D approach). A fundamental step is to fix constraints on the thickness of the seismogenic layer and deep geometry of faults: we use constraints from the depth distribution of aftershock zones and background seismicity; we also use information on the structural style of the extensional deformation at crustal scale (mainly from seismic reflection data), as well as on the strength and behaviour (brittle versus plastic) of the crust by rheological profiling. Geological observations allow us to define a segmentation model consisting of major fault structures separated by first-order (kilometric scale) structural-geometric complexities considered as likely barriers to the propagation of major earthquake ruptures. Once defined the 3D fault features and the segmentation model, the step onward is the computation of the maximum magnitude of the expected earthquake (Mmax). We compare three different estimates of Mmax: (1) from association of past earthquakes to faults; (2) from 3D fault geometry and (3) from geometrical estimate ‘corrected’ by earthquake scaling laws. By integrating all the data, we define a model of seismogenic sources (seismogenic boxes), which can be directly used for regional-scale PSHA. Preliminary applications of PSHA indicate that the 3D approach may allow to hazard scenarios more realistic than those previously proposed.

Architecture and seismotectonics of a regional low‐angle normal fault zone in central Italy

Information from surface geology, subsurface geology (boreholes, seismic reflection, and refraction profiles), and seismicity are used to depict the geometry and the possible seismogenic role of the Altotiberina Fault (AF), a low-angle normal fault in central Italy. The AF extends along the inner Umbria region, for a length of ∼70 km, with an average dip of ∼30° and an horizontal displacement up to 5 km. It emerges west of the inner border of the Tiber basin and deepens beneath the Umbria-Marche carbonate fold-and-thrust belt to a depth of 12–14 km. Close to the AF surface trace, low-angle synthetic east dipping normal faults extensively outcrop, whereas high-angle antithetic west dipping normal faults prevail farther east. Integrating geological and seismologic information, it can be stated that the AF behaves as an active extensional fault zone and represents the basal detachment of the west dipping seismogenic normal faults of the Umbria-Marche region. The AF belongs to a regional NE dipping low-angle normal fault system (Etrurian Fault System (EFS)), which extends for ∼350 km from northwestern Tuscany to southern Umbria. Early preliminary considerations suggest that the EFS may play an important role in controlling active extension and related seismicity in northern central Italy.

A structural model for active extension in Central Italy

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 Colfiorito 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 Colfiorito) 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.

A lithospheric-scale seismogenic thrust in central Italy

In this paper, the relatively deep (up to 90 km) seismicity of central Italy is considered in comparison with the upper crust seismicity at the outer front of the Apennine fold-and-thrust belt system and with the major crust-scale tectonic discontinuities outlined by deep crust seismic reflection and refraction profiles. In particular, attention is given to the presence of a Plio-Quaternary SW-dipping thrust zone cutting the entire Adriatic crust and emerging at the outer front of the Apennine thrust system, along the Po Plain-Adriatic belt. This discontinuity, detectable from the CROP 03 near-vertical seismic reflection profile, but only partially highlighted in previous studies, is here named Adriatic Thrust (AT). The geometry of the AT is compared with the distribution and kinematics of the Coastal-Adriatic upper crust seismicity, of the Marche Foothill lower crust seismicity and of the Apennine deep seismicity (base of the crust and upper mantle). A good spatial correspondence between the down-dip fault geometry and the seismicity is observed. The rheological stratification of the lithosphere controls the location of the seismic activity at different depth intervals along the fault plane (0–10, 15–25, 35–45 km). The reconstructed rheological conditions do not allow to justify in terms of brittle behavior the deepest earthquakes, located at depths from 55 to 70 km within the possible down-dip intra-mantle prosecution of the AT. These events might represent either co-seismic plastic instabilities due to high strain concentrations within the AT shear zone or seismic failure of the continental Adriatic lithosphere due to a strong increase in pore fluid pressure (infiltration from the asthenosphere of hydroxil-CO2 rich volatiles). The proposed interpretation of the AT as a lithosphere-scale seismogenic fault zone, has several implications: (1) the Po Plain-Adriatic front of the Apennine thrust system is still active; (2) the Apennine deep seismicity is not associated with a SW-dipping subduction plane; (3) seismogenic deformation in association with lithospheric failure is possible.

The 1984 Abruzzo earthquake (Italy): an example of seismogenic process controlled by interaction between differently oriented synkinematic faults

The evolution of the seismogenic process associated with the Ms 5.8 Sangro Valley earthquake of May 1984 (Abruzzo, central Italy) is closely controlled by the Quaternary extensional tectonic pattern of the area. This pattern is characterised by normal faults mainly NNW striking, whose length is controlled by pre-existing Mio–Pliocene N100F10j left-lateral strike-slip fault zones. These are partly re-activated as right-lateral normal-oblique faults under the Quaternary extensional regime and behave as transfer faults. Integration of re-located aftershocks, focal mechanisms and structural features are used to explain the divergence between the alignment of aftershocks (WSW–ENE) and the direction of seismogenic fault planes defined by the focal mechanisms (NNW–SSE) of the main shock and of the largest aftershock (Ms=5.3). The faults that appear to be involved in the seismogenic process are the NNW–SSE Barrea fault and the E–W M. Greco fault. There is field evidence of finite Quaternary deformation indicating that the normal Barrea fault re-activates the M. Greco fault as right-lateral transfer fault. No surface faulting was observed during the seismic sequence. The apparently incongruent divergence between aftershocks and nodal planes may be explained by interpreting the M. Greco fault as a barrier to the propagation of earthquake rupturing. The rupture would have nucleated on the Barrea fault, migrating along-strike towards NNW. The sharp variation in direction from the Barrea to the M. Greco fault segments would have represented a structural complexity sufficient to halt the rupture and subsequent concentration of post-seismic deformation as aftershocks around the line of intersection between the two fault planes. Fault complexities, similar to those observed in the Sangro Valley, are common features of the seismic zone of the Apennines. We suggest that the zones of interaction between NW–SE and NNW–SSE Plio-Quaternary faults and nearly E–W transfer faults, extending for several kilometres in the same way as M. Greco does, might act as barriers to the along-strike propagation of rupture processes during normal faulting earthquakes. This might have strong implications on seismic hazard, especially for the extent of the maximum magnitude expected on active faults during single rupture episodes. D 2002 Elsevier Science B.V. All rights reserved.

A geological model for the Colfiorito earthquakes (September-October 1997, central Italy)

In this study, surface and subsurface geologicaldata are integrated with seismological data in orderto reconstruct a structural model for theSeptember-October 1997 Colfiorito earthquakes. Theseismic sequence is mainly controlled by two majorSW-dipping normal faults outcropping in the area (M.Pennino-M. Prefoglio and M.Civitella-Preci faults).The activated faults detach, at depth, on a commoneast-dipping low-angle normal fault, the AltotiberinaFault (AF). The AF is interpreted as the base of anactive hangingwall block which is stretching towardNE. The decrease in maximum depth of the earthquakefoci from the Colfiorito area (about 8 km) to theSellano area (about 6 km), suggested by the available seismological data, could be related to the eastward-deepening geometry of the AFdetachment. The seismic fault planes, inferred fromfocal mechanisms and aftershock distributions, arecharacterised by a moderate dip (average 40°)toward SW, which appears to be independent from thepresence of pre-existing thrust planes.

Ground deformation and source geometry of the 24 August 2016 Amatrice earthquake (Central Italy) investigated through analytical and numerical modeling of DInSAR measurements and structural-geological data

We investigate the ground deformation and source geometry of the 2016 Amatrice earthquake (Central Italy) by exploiting ALOS2 and Sentinel-1 coseismic differential interferometric synthetic aperture radar (DInSAR) measurements. They reveal two NNW-SSE striking surface deformation lobes, which could be the effect of two distinct faults or the rupture propagation of a single fault. We examine both cases through a single and a double dislocation planar source. Subsequently, we extend our analysis by applying a 3-D finite elements approach jointly exploiting DInSAR measurements and an independent, structurally constrained, 3-D fault model. This model is based on a double fault system including the two northern Gorzano and Redentore-Vettoretto faults (NGF and RVF) which merge into a single WSW dipping fault surface at the hypocentral depth (8 km). The retrieved best fit coseismic surface deformation pattern well supports the exploited structural model. The maximum displacements occur at 5–7 km depth, reaching 90 cm on the RVF footwall and 80 cm on the NGF hanging wall. The von Mises stress field confirms the retrieved seismogenic scenario.

Tectonic setting of the carbonatite-melilitite association of Italy

Abstract In this paper the Late Pleistocene carbonatite-melilitite association (CMA) of Italy is evaluated in terms of the Plio-Quaternary extensional structures in the crust and mantle lithosphere. In particular, we briefly discuss the geometry and stress field of the host geological structures, the tectonic control on the eruptive style, the thickness and composition of the lithosphere. Some implications for a passive rift geodynamic environment are also considered

The contribution of fluid geochemistry to define the structural patterns of the 2009 L’Aquila seismic source.

Field investigations performed in the epicentral area within the days following the April 6, 2009 L’Aquila earthquake (M w 6.3) allowed several researchers to detect evidence of coseismic ground rupturing. This has been found along the Paganica Fault and next to minor synthetic and antithetic structures. Although a lot of geo-structural and geophysical investigations have been recently used to characterize these structures, the role of the different fault segments – i.e. as primary or secondary faults – and their geometrical characteristics are still a matter of debate. In light of this, we have here integrated data derived from fluid geochemistry analyses carried out soon after the main-shock with field structural investigations. In particular, we compared structural data with CO 2 and CH 4 flux measurements, as well as with radon concentration measurements (for other geogas concentration see Voltattorni N., this issue). Our aim was to better define the structural features and complexities of the activated Paganica Fault. Here, we show that, in the near rupture zone, “geochemical signatures” could be a powerful method to detect earthquake activated fault segments, even if they show subtle or absent geological-geomorphological evidence and are still partially “blind”. In detail, a clear degassing zone was identified just along the San Gregorio coseismic fracture zone – i.e., the surface deformation related to the “blind” San Gregorio normal fault. Indeed, CO 2 and CH 4 flux maximum anomalies were aligned along the Northern sector of the San Gregorio fault, in the Bazzano industrial area as well as at the border of the studied area (Fossa antitethic fault crossing an anti-apenninic transverse belt). The Bazzano-San Gregorio fault area also corresponds to the depocenter of the maximum coseismic deformation highlighted by DInSAR analysis (Atzori et alii , 2009). Here, maximum radon concentration values in soil gases were also found. As a whole, these results corroborates the hypothesis of Boncio et alii (2010) who suggested that the San Gregorio fault probably represents a synthetic splay of the Paganica Fault, being thus connected with the main seismogenic fault at depth. Moreover, another maximum in CO 2 flux anomaly has been measured along the Southernmost tip of the earthquake rupture zone, close to the San Gregorio village. Minor or absent soil gas and flux anomalies were instead located along antithetic structures as the Bazzano fault, while some anomalies in CO 2 flux or radon concentration in groundwater have been found within transfer zones, such as the step-over zone between the central segment of the Paganica fault and the San Gregorio fault and in the zone which separates the Paganica fault from the i) Middle Aterno Valley-Subequana Valley and ii) Barisciano-S. Pio delle Camere-Navelli fault systems. Our results corroborate the power of fluid geochemistry in investigating the structural features of active tectonic structures, being particularly helpful in discerning blind faults. More specifically, our data suggest that the youngest fault splays, as in the case of the San Gregorio fault, may represent preferential sites for degassing.

Reply to “Comment on ‘Layered Seismogenic Source Model and Probabilistic Seismic-Hazard Analyses in Central Italy’ by B. Pace, L. Peruzza, G. Lavecchia, and P. Boncio” by W. Marzocchi

We greatly appreciate the interest in and feedback onour article. We will address the comments in the next par-agraphs, following the comment points.Our article is not a novel or new approach toquantifyingthe recurrence behavior and seismic hazard of faults in Italy.Fault recurrence behavior has been addressed previously innortheastern Italy (e.g., Papoulia and Slejko, 1992; Slejkoand Rebez, 2000), southern Italy (Peruzza