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NEW CONSTRAINTS FOR FAULT SEGMENTATION, TECTONIC INVERSIONS AND EARTHQUAKE TRIGGERING MECHANISMS IN THE CENTRAL APENNINES NORMAL FAULT SYSTEM BY HIGH-RESOLUTION VELOCITY MODELSM. Buttinelli, G. Pezzo, L. Valoroso, P. De Gori, C. ChiarabbaIstituto Nazionale di Geofisica e Vulcanologia

The central Apennines (Italy) have a very complex structural architecture, mostly developed as a consequence of the overprint of different deformational phases. Such tectonic phases contributed in developing new peculiar structures as well as in reworking and segmenting previous geological structures, also repeatedly producing the conditions for episodic tectonic inversions. The reconstruction of the main geological structures and the discrimination between newly formed and reworked geologic features is therefore the key issue to unravel the seismotectonics of an area. This is also necessary because the geometry of large-scale discontinuities and their relationship with the active stress regime has a direct connection with large earthquakes nucleation, expected maximum magnitudes, aftershock occurrence and spreading, and strong implications for seismic hazard assessment in general.

We present new velocity tomograms for the normal fault system activated during the 2009 and the 2016-2018 seismic sequences, showing how the principal crustal structures are currently interfering with the active extension. Moreover, we discuss on the relationship between the general crustal arrangement even at depth and the surficial expression of interpreted structures, which is still a question of strong debate within the scientific community.

Finally, we investigate the possible interaction between the faults activated during the two sequences in terms of static stress changes induced by the nucleation of main events, proposing the fluid overpressure at depth as a viable mechanism behind the partial re-mobilization of unbroken segments of the fault system.

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Main objective. Mountain belts characterized by diffuse deformation and seismicity suggest that continental deformation is a complex process related to strong lateral variations of lithosphere structure and its rheology (e.g., Ranalli and Murphy, 1987; Ziegler et al., 1995; Cloetingh and Burov, 1996). In this wide context, the pre-existing crustal discontinuities may act as long-living lithospheric weaknesses during an orogen development, concentrating the deformation and also favoring reactivation and tectonic inversions (Pfiffner, 2017 and referenced therein).

The reactivation of inherited structures has deep implications both for compressional and extensional phases, since the favorable or unfavorable orientation of faults within the active stress regime as well as the fault length, play an important role on the fault segmentation and maximum magnitude of individual earthquakes (e.g. Cowie et al., 2012; Martínez-Garzón et al., 2015). Clear examples of positive inversions are widely documented for the Mediterranean orogens during the thrust and fold belt formation, like the central Apennines build-up phase (Coward, 1994; Mazzoli et al., 2000; Butler et al., 2004; Butler and Mazzoli, 2006; Scisciani, 2009; Di Domenica et al., 2012).

In the Apennines, the existence of Triassic inverted large-scale normal faults has been widely invoked in order to explain the surficial architecture (e.g. Calamita et al., 2003), while the distribution of those structures and their orientation are still poorly constrained at depth. Moreover, even a second set of extensional faults has been associated to the Miocene bending of the Adria paleo margin foreland domain before the Apennines compression (Scisciani et al., 2014, Bigi et al., 2011). Those features were involved and reworked by the migration of the coupled pair of extension and compression, with local tectonic inversions, even at regional scale (e.g. Barchi et al., 1998).

Seismological observations show that past decades normal faulting earthquakes clearly interfered with pre-existing thrust related structures (Chiarabba and Amato, 2003; Scognamiglio et al., 2018). Such process lies behind the complexities observed for the rupture evolution due to fault segmentation, like those observed for the recent large earthquakes of the Apennines (Tinti et al., 2016; Chiaraluce et al., 2017; Cheloni et al., 2017; Pizzi et al., 2017; Scognamiglio et al., 2018).

The enduring energy release pattern of the sequences also testifies for a complex interaction between adjacent faults and triggering phenomena, which include static and dynamic stress transfer between faults (Freed, 2005) and pore fluid pressure migration (Chiarabba et al., 2009; Di Luccio et al., 2010; Malagnini et al., 2012). Several M>5.5 normal faulting earthquakes in fact ruptured contiguous portions of the NW-trending central Apennines extensional system, such as the Mw6.1 2009 L’Aquila (Chiarabba et al., 2009; Valoroso et al., 2013), and the Mw6.1-Mw6.5 2016 Amatrice (Tinti et al., 2016) and Norcia earthquakes (Scognamiglio et al., 2018) (Fig. 1).

The aim of this contribution is to try to address how structural complexities influence the interaction between adjacent faults and triggering processes via the interpretation of new high-resolution tomograms in the area interested by the two recent sequences occurred in the central Apennines.

Main results. We investigated faulting mechanisms and interaction in a key area of the central Apennines where large earthquakes progressively propagated along a 150-km-long section of the normal faulting system (Fig. 1). The two seismic sequences occurred in 2009 and 2016-2018 allowed the computation of velocity models, whose interpretation shows the complex structural architecture of the area. Compressional features like large-scale thrust ramps still depict the main shape of the orogen, down to 15-20 km, including the whole seismogenic layer. The intense fault segmentation shown by this picture is mainly controlled by interference between inherited faults, derived from either the Mesozoic extension or the Miocene compression. A high-Vp body, that we interpreted as a Mesozoic large structural high, conditioned the compressional tectonic phase, favoring the occurrence of positive inversion of

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the bordering normal faults with different styles and rates throughout the area (section a-b in Fig. 1). Furthermore, deep normal faults and seismicity are rooted down to the base of these large and extremely high-Vp bodies.

Since contiguous segments of the normal faulting system repeatedly ruptured during past decades (i.e., the 2009 L’Aquila and 2016-2018 Amatrice earthquakes), our investigations

Fig. 1 - Upper panel: map of the study area showing the principal structural elements. Solid black lines are thrusts: Sit (Sibillini), Vnt (Val Nerina), Gst (Gran Sasso); solid red lines are normal faults: MLGf (Monti della Laga-Gorzano), Asgf (Assergi), Cptf (Capitignano), SGvf (San Giovanni), PzBf (Pizzoli-Barete), Pttf (Pettino), VtBf (Mt.Vettore-Bove system), Ncf (Norcia fault system), Lnf (Leonessa), PAGf (Paganica), MtFf (Monticchio-Fossa). Larger earthquakes occurred in 2009 and 2016-17 are shown as stars. 2009 aftershocks are represented by cyan-to-pink dots depending to depth (after Valoroso et al., 2013) while green dots (without a color code for the lower resolution of hypocentral depths) for 2016-17 aftershocks sequence (Chiaraluce et al., 2017). Deep wells are shown (A1=Antrodoco 1, V1= Varoni 1, C1= Campotosto 1). Lower panel: vertical sections of Vp model across the MLGf (a) and PAGf (b) are proposed. White lines refer to the top of the Triassic evaporites (Tev) from deep well data. Aftershocks from Valoroso et al. (2013) are shown as black dots. The main faults, drawn jointly interpreting tomograms and seismicity, are represented by solid (highly constrained) and dashed (more inferred) red lines.

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suggest a lack of mechanical continuity between the interpreted thrust ramps at depth and the shallow expression of the faults, posing questions on the actual fault dimensions and the potential maximum magnitude expected for such segments. We believe that the revaluation of seismic hazard in light of these results might represent a challenging issue for next decades. Clustering of events and static stress variations associated to large seismic events investigated in this contribution might also suggest an effective triggering mechanism among the segmented fault system.

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