The Campotosto Linkage Fault Zone Between the 2009 and 2016 Seismic Sequences of Central Italy: Implications for Seismic Hazard Analysis
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The Campotosto linkage fault zone between the 2009 and 2016 seismic sequences of central Italy: Implications for seismic hazard analysis Emanuele Tondi1,2, Danica Jablonská1, Tiziano Volatili1, Maddalena Michele2, Stefano Mazzoli1, and Pietro Paolo Pierantoni1,† 1 School of Science and Technology, Geology Division, University of Camerino, Camerino, 62032 Macerata, Italy 2Istituto Nazionale di Geofisica e Vulcanologia, 00143 Rome, Italy ABSTRACT this fault has been considered as an active The CAFS is an interactive active fault sys- and silent structure (therefore representing a tem, extending along the central Apennines in In the last decade central Italy was struck seismic gap) able to generate an earthquake a north-south direction for a length of ∼100 km by devastating seismic sequences resulting in of Mw max = 6.5–7.0. However, the geologi- and ∼50 km of width (Cello et al., 1997). This hundreds of casualties (i.e., 2009-L′Aquila cal evidence provided with this study sug- system includes several active and capable moment magnitude [Mw] = 6.3, and gests that the MGF is of early (i.e., pre- to faults (sensu IAEA, 2010), interpreted as the 2016-Amatrice-Visso-Norcia Mw max = 6.5). syn-thrusting) origin. Therefore, the evalua- surface expression of deep seismogenic faults These seismic events were caused by two tion of the seismic hazard in the Campotosto (Barchi et al., 2000; Galadini and Galli, 2000; NW-SE–striking, SW-dipping, seismogenic area should not be based on the geometrical Tondi, 2000). Many of the scientific papers on normal faults that were modeled based on the characteristics of the outcropping MGF. This these active faults were published before the last available focal mechanisms and the seismic also generates substantial issues with earth- destructive seismic sequences (in addition to moment computed during the relative main- quake geological studies carried out prior those already mentioned, see also: Pizzi et al., shocks. The seismogenic faults responsible to the recent seismic events in central Italy. 2002; Tondi and Cello, 2003; Galadini and Galli, for the 2009-L′Aquila Mw = 6.3 (Paganica More in general, the 4-D high-resolution 2003; Boncio et al., 2004a; Tondi et al., 2009). Fault—PF) and 2016-Amatrice-Visso-Norcia image of a crustal volume hosting an active From 1997 to 2016, the entire fault system was Mw max = 6.5 (Monte Vettore Fault—MVF) linkage zone between two large seismogenic activated along its length (see Fig. 1), thus pro- are right-stepping with a negative overlap structures provides new insights into the be- viding the unique opportunity to evaluate the (i.e., underlap) located at the surface in the havior of interacting faults in the incipient seismic hazard estimated by geological and Campotosto area. This latter was affected by stages of connection. paleoseismological studies. Furthermore, the seismic swarms with magnitude ranging from latest seismic sequences have clearly demon- 5.0 to 5.5 during the 2009 seismic sequence INTRODUCTION strated the dominant role of extensional tecton- and then in 2017 (i.e., a few months later than ics in the upper crust of the central Apennines, the mainshocks related with the 2016 seismic Central Italy was struck by severe earthquakes with the main seismogenic sources dipping to sequence). along the Apennine chain, as documented by his- the southwest (e.g., Galli et al., 2018; Galderisi In this paper, the seismogenic faults re- torical sources (Rovida et al., 2019). The most and Galli, 2020). lated to the main seismic events that occurred significant earthquakes, clustering along the cen- A meaningful comparison may now be car- in the Campotosto Seismic Zone (CSZ) were tral Apennines fault system (CAFS; Cello et al., ried out considering the seismological data modeled and interpreted as a linkage fault 1997), occurred in three periods over the last provided, in particular, by the 2009-L′Aquila zone between the PF and MVF interacting millennium: in the 13th-14th and the 17th-18th (Mw = 6.3) and the 2016-Amatrice-Visso-Nor- seismogenic faults. Based on the underlap di- centuries, and then from the 1980’s to the pres- cia (Mw max = 6.5) earthquakes. Moreover, mension, the seismogenic potential of the CSZ ent (Tondi and Cello, 2003; Castelli et al., 2016; these high-resolution data, together with the geo- is in the order of Mw = 6.0, even in the case Rovida et al., 2019). The last decades witnessed logical surveys carried out immediately after the that all the faults belonging to the zone were several devastating earthquakes resulting in hun- mainshocks, allowed us to improve our knowl- activated simultaneously. This has important dreds of casualties (i.e., 1997-Colfiorito-Sellano edge on the seismotectonic setting of central implications for seismic hazard assessment moment magnitude [Mw] = 6.0; 2009-L′Aquila Italy, and on both the peculiar phenomenology in an area dominated by the occurrence of Mw = 6.3; and 2016-Amatrice-Visso-Norcia of earthquakes associated with crustal normal a major NW-SE–striking extensional struc- Mw max = 6.5; Amato et al., 1998; Chiarabba faults (Doglioni et al., 2015) and the interaction ture, i.e., the Monte Gorzano Fault (MGF). et al., 2009; Chiaraluce et al., 2011; Chiaraluce processes between active faults and earthquakes Mainly due to its geomorphologic expression, et al., 2017. These events were caused by the (Pino et al., 2019). Such interaction processes reactivation of NW-SE–striking, SW-dipping may be better understood considering the recent normal faults (Tondi et al., 2009; Pantosti and results on rupture directivity provided by Calde- Pietro Paolo Pierantoni http://orcid.org/0000- 0002-1237-4689 Boncio, 2012; Pierantoni et al., 2013; Galli et al., roni et al. (2017) for sixteen earthquakes of Mw †Corresponding author: pietropaolo.pierantoni@ 2017; Pizzi et al., 2017; Civico et al., 2018; Big- > 4.4 belonging to the 2016 Amatrice- Norcia- unicam.it. nami et al., 2019; Villani et al., 2018) (Fig. 1). Visso seismic sequences. GSA Bulletin; July/August 2021; v. 133; no. 7/8; p. 1679–1694; https://doi.org/10.1130/B35788.1; 10 figures; 1 table. published online 14 December 2020 © 2020 The Authors. Gold Open Access: 1679 This paper is published under the terms of the CC-BY license. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/133/7-8/1679/5353989/b35788.1.pdf by guest on 26 September 2021 Tondi et al. Figure 1. Map of the seismic se- quences that took place in the last decades in central Italy. Focal mechanisms refer to the related mainshocks. Capable faults and modeled seismogenic faults are also shown (CF— Colfiorito Fault; MVF—Monte Vettore Fault; PF—Paganica Fault) (Tondi et al., 2009; Pan- tosti and Boncio, 2012, Pizzi et al., 2017; Chiarabba et al., 2018; Falcucci et al., 2018). The Campotosto area represented in Figure 3 is also shown. The seismic sequences that occurred in the our knowledge on the seismotectonic setting of see also Calamita and Pizzi, 1992, 1994; Boncio last decades in central Italy (Rovida et al., 2019; central Italy, and (c) better understand the interac- et al., 2004a; Galadini, 1999; Galadini and Galli, Chiaraluce et al., 2017) permit to: (a) verify the tion processes between active faults (long-term) 2000; Mildon et al., 2017; Wedmore et al., 2017). geological and paleoseismological analyses car- and earthquakes (shorth-term) within an active Within the CAFS, the most recent seis- ried out prior to the seismic events, (b) improve fault system (i.e., the CAFS in Cello et al., 1997; mic sequence of 2016 bridged the two former 1680 Geological Society of America Bulletin, v. 133, no. 7/8 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/133/7-8/1679/5353989/b35788.1.pdf by guest on 26 September 2021 The Campotosto Seismic Zone (central Italy) epicentral areas of Colfiorito (in 1997) and eventually result in a linkage of the two faults scenario for the 2009-L′Aquila and 2016-Ama- L′Aquila (in 2009). A few days after the main- (hard-linkage). trice-Visso-Norcia seismic sequences. A critical shock of L′Aquila (in 2009) and a few months The critical nearness or spacing between reassessment of previous works is also carried after the mainshocks of Norcia (in 2016), seis- two fault tips interacting each other is of fun- out, particularly concerning the activity of the mic swarms with magnitudes ranging from 5.0 damental importance during the growth of fault major NW-SE–striking extensional structure of to 5.5 occurred in the Campotosto area, between populations. Mechanically, this critical spacing the region, i.e., the Monte Gorzano Fault (MGF). the Paganica Fault (PF) and the Monte Vettore has been related to the zone of stress perturba- The MGF is a large structure that has been Fault (MVF) (Fig. 1). The occurrence of seis- tion that occurs around faults (e.g., Ackermann considered as a Quaternary, active, capable, and mic swarms in the Campotosto area suggests and Schlische, 1997; Cowie and Roberts, 2001; silent fault (i.e., representing a seismic gap), able a strong interaction between the seismogenic Soliva et al., 2006; King and Deves, 2015). The to generate an earthquake of Mw max = 6.5–7.0 faults belonging to the CAFS (Cheloni et al., effect of such stress perturbed regions has been (Galadini and Galli, 2003; Boncio et al., 2004b; 2014; Calderoni et al., 2017; Mildon et al., explored by Willemse et al. (1996) and further Falcucci et al., 2018 and reference therein). 2017; Chiarabba et al., 2018; Pino et al., 2019), by Gupta and Scholz (2000), whose modeling However, based on new field observations and as already shown by the characteristics of the confirmed that tip propagation is enhanced or building on previous studies and available data, historical seismic sequences (e.g., multiple seis- retarded as a fault grows into the stress increase we provide an alternative model implying an ear- mic events that occurred in 1703; Rovida et al., or stress drop regions of an underlapping or lier evolution of the MGF.