# Unveiling the hidden source of major historical earthquakes: A multi-scale, trans-disciplinary approach to the 1456 and 1688 Sannio earthquakes (Mw 7.0, southern Italian Apennines)

Vincenzo Amato, Sabatino Ciarcia, Paolo Galli, Domenico Cicchella, Antonio Galderisi, Lorenzo Monaco, Giada Fernandez, Roberto Isaia, Sebastien Nomade, Alison Pereira, et al.

► To cite this version:

Vincenzo Amato, Sabatino Ciarcia, Paolo Galli, Domenico Cicchella, Antonio Galderisi, et al. Unveiling the hidden source of major historical earthquakes: A multi-scale, trans-disciplinary approach to the 1456 and 1688 Sannio earthquakes (Mw 7.0, southern Italian Apennines). Quaternary Science Reviews, 2025, 356, pp.109282. 10.1016/j.quascirev.2025.109282. hal-04987502

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# Unveiling the hidden source of major historical earthquakes: A multi-scale, trans-disciplinary approach to the 1456 and 1688 Sannio earthquakes (Mw 7.0, southern Italian Apennines)

Vincenzo Amato<sup>a, *, †</sup>, Sabatino Ciarcia<sup>b †</sup>, Paolo Galli<sup>c, d †</sup>, Domenico Cicchella<sup>b †</sup>, Antonio Galderisi<sup>e, d</sup>, Lorenzo Monaco<sup>f †</sup>, Giada Fernandez<sup>d †</sup>, Roberto Isaia<sup>g</sup>, Sebastien Nomade<sup>h †</sup>, Alison Pereira<sup>i</sup>, Biagio Giaccio<sup>d †</sup>

## ARTICLE INFO

Handling Editor: Dr Giovanni Zanchetta

Keywords:

Quaternary morpho-sedimentary-tectonic evolution
Tephrochronology
Active tectonics
Hidden seismogenic faults
Paleoseismology
1456-1688 Mw7 sannio earthquakes
Calore river
Southern Italian apennines

## ABSTRACT

The southern Apennine chain ranks among Europe's regions with the highest historical seismicity, yet its seismic structures remain poorly defined or completely unknown, including those of highly destructive Mw ~7.0, 1456 and 1688 Sannio earthquakes, which are examined in this study. Using a multi-scale, interdisciplinary approach - combining detailed field investigations (stratigraphic, geomorphological, structural, and paleoseismological analyses), tephrochronological and OSL dating, and reassessment of macroseismic intensity distribution from archival sources - this study identifies a possible source for these earthquakes. It corresponds to a ~45 km normal fault system composed by two main branches (Eastern Calore Fault and Western Calore Fault) with primary segments trending E-W to ESE-WNW and dipping N- to NNE. Segments extend along the southern border of both eastern and western Calore River sub-basins and are partially connected by NNW-SSE to N-S trending, east-dipping, transfer zone fault splays. Although the morphostructural evidence of Quaternary tectonic activity is unevenly expressed in both sub-basins, they share a similar Middle Pleistocene-to-Holocene morpho-sedimentary evolution, suggesting a common driving factor, ascribable to the sub-coeval activity of the fault segments delimiting both sub-basins. On a short-time scale, this study provides the first evidence of a post-14 ka occurrence of a paleoearthquake with a surface displacement ≥0.7 m, as well as of a decametric off-set affecting post-9 ka sediments. These coseismic surface ruptures have an estimated recurrence time of approximately 1400 years, with the two more recent events likely corresponding to the Mw ~7.0, 1456 and 1688 Sannio earthquakes. Further detailed paleoseismological investigations are recommended to uncover direct evidence of co-seismic displacement linked to the 1456, 1688, and earlier earthquakes.

## 1. Introduction

Identifying and characterising seismogenic faults is pivotal for assessing regional seismic hazard. In peninsular Italy, the active fault systems responsible for the most significant historical earthquakes are

predominantly located along the central-southern Apennine backbone and the Calabrian arc (e.g., Galli et al., 2008, Fig. 1A), as well as all the epicentres of the strongest earthquakes (CPTI15, 2021). However, a sharp contrast exists in the identification of seismogenic faults between Central and Southern Apennines, with the latter lacking evidence of

<footnote><sup>a</sup> Dipartimento di Bioscienze e Territorio, Università del Molise, Pesche, Isernia, Italy</footnote>

<footnote><sup>b</sup> Dipartimento di Scienze e Technologie, Università del Sannio, Benevento, Italy</footnote>

<footnote><sup>c</sup> Dipartimento della Protezione Civile Nazionale, Rome, Italy</footnote>

<footnote><sup>d</sup> Istituto di Geologia Ambientale e Geoingegneria, Consiglio Nazionale delle Ricerche, Monterotondo, Rome, Italy</footnote>

<footnote><sup>e</sup> Dipartimento di Scienze della Terra, dell'Ambiente e delle Risorse, Università di Napoli Federico II, Naples, Italy</footnote>

<footnote><sup>f</sup> Istituto di Scienze Marine, Consiglio Nazionale delle Ricerche, Sede Secondaria di Bologna, Bologna, Italy</footnote>

<footnote><sup>g</sup> Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Napoli Osservatorio Vesuviano, Naples, Italy</footnote>

<footnote><sup>h</sup> Laboratoire de Sciences du Climat et de l'Environnement, CEA, UMR 8212, UVSQ, IPSL and Université de Paris-Saclay, Gif-sur-Yvette, France</footnote>

<footnote><sup>i</sup> Université Paris-Saclay, UMR 8148, Laboratoire GEOPS, Orsay, France</footnote>

<footnote>* Corresponding author.
E-mail address: vincenzo.amato@unimol.it (V. Amato).</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Reference maps of the investigated area. A) Shaded relief map of central-southern Italy with the main faults active during Late Pleistocene–Holocene (1, Colfiorito; 2, Mt Vettore; 3, Norcia; 4, Laga Mts; 5, Upper Aterno; 6, Campo Felice; 7, Gran Sasso; 8, Fucino; 9, Mt Morrone; 10, Upper Sangro; 11, Mt Rotella-Aremogna; 12, Aquae Iuliae; 13, northern Matese; 14, Mt Marzano; 15, Caggiano; 16, Mt Pollino; 17, Rossano; 18, Crati Valley; 19, Savuto; 20, Cecita Lake; 21, Lakes; 22, Lamezia; 23, Serre; 24, Cittanova). The grey triangles line indicates the buried front of the Apennine Arc. Yellow circles, $7.2>M > 6$ earthquakes (size proportional to Mw) (a–h) GPS-velocity profiles from coast to coast in mm/year (after Galli and Peronace, 2014; original GPS data from D'Agostino, 2014). Faults from Galli (2020); earthquakes modified from CPTI15, 2021). B) Schematic geological map of the study area. C)-D) Shaded relief maps of the Eastern and Western Calore sub-basin (see squares in panel 1b) with location, stratigraphic sections and structural measurement points investigated in this study, along with the reference stratigraphic sections from literature. E) Areal distribution of the MCS site intensities of the 1456, 1688 and 1805 earthquakes, with respect to the known regional seismogenic structures of the North Matese Fault System (NMFS) and Aquae Iuliae Fault (AIF). Note the overlapping of the 1805 mesoseismic area (hanging-wall of the NMFS) with the northern mainshocks of the 1456 sequence and the overlapping of the 1688 event with the southern 1456 mainshock, both lacking potential sources. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
most of the seismogenic sources compared to the distribution of the historical epicentres (Fig. 1A). Such a gap of knowledge might be due to the structural complexity of southern Apennines, which are basically made by a buried duplex system of Mesozoic-Tertiary thrust sheets that is overlaid by a thick pile of rootless nappes derived from platform and basin depositional realms (Patacca and Scandone, 1989, 2007; Cinque et al., 1993; Vitale and Ciarcia, 2013, 2018, 2022). These nappes are mostly made by siliciclastic, terrigenous series (Fig. 1B), which are highly erodible and, therefore, not prone to preserve the morphological footprint of active tectonics (Bosi, 1975; Galli et al., 2008).

The Sannio territory (Campania Region, southern Apennines) stands out as an emblematic example of this contrast, with a very high seismicity and a poor identification of the seismogenic sources. Specifically, this area exhibits the highest historical seismicity on the Italian peninsula (CPTI15, 2021), as well as the highest seismic moment summation (Westaway, 1992), and the highest ongoing extensional rates (D'Agostino, 2014) (i.e., 3.2 mm/yr: see profile "e" in Fig. 1A). Notably, in the last six centuries this region experienced two Mw ~7.0 earthquakes with comparable mesoseismic areas: the first striking on December 5, 1456, and the second on June 5, 1688, respectively (CPTI15, 2021) (Fig. 1E). The 1456 event (Magri and Molin, 1984; Meletti et al., 1988), featuring at least three epicentral areas along the Apennine chain (Galli, 2024), with the southernmost one overlapping the mesoseismic area of the 1688 event (Fig. 1C), and the central one that overlapped the more recent 1805 earthquake (Mw 6.6). Both earthquakes resulted in collapses, extensive damage, and significant life losses across a wide territory north of the Calore River, spanning from towns of Apice to Benevento, Cerreto Sannita, and Pietraroja. Palaeoseismological and archaeoseismological data allowed attributing the central mainshock of the 1456 sequence to the rupture of the Northern Matese fault system (13 in Fig. 1A) (Galli and Galadini, 2003; Galli et al., 2002; 2017), which formed a prominent rock-fault scarp along the Mesozoic limestone slopes (e.g., Galli et al., 2017). On the contrary, the geometry and location of the seismogenic faults responsible for both the 1688 event and the southern mainshocks of 1456 in the Sannio territory remain uncertain (e.g., Di Bucci et al., 2005).

To uncover the seismogenic source of both 1688 and 1456 earthquakes, we investigated the Calore River valley sectors (i.e., Western and Eastern Calore sub-basins; WCSB and ECSB, respectively, Fig. 1E) which correspond to the region with the highest MCS (Mercalli-Cancani-Sieberg scale; Sieberg, 1932) site intensities in both events (CPTI15, 2021). We applied an integrated, trans-disciplinary and multiscale approach that allowed us to provide indications about the location and kinematics of either (i) morpho-structurally well expressed or (ii) hidden seismogenic faults, that feature the WCSB and the ECSB, respectively (Fig. 1C and D). Specifically, the WCSB local geological setting allowed performing a standard morphostructural analysis, whereas in the ECSB, where morphostructural evidence was elusive or entirely absent, active tectonics, location, and kinematics of the seismogenic faults have been inferred by examining the deformation patterns of Quaternary deposits.

Field investigations, including stratigraphical, geomorphological and structural analyses, along with paleoseismological observations of existing exposures, supported by tephrochronological and OSL age constraints, have been applied to outline the Quaternary morpho-sedimentary evolution of the poorly investigated ECSB and to improve the morphostructural and sedimentary framework of the previously studied WCSB (Magliulo, 2005; Massa et al., 2005; Di Bucci et al., 2006; Amato et al., 2018; Filocamo et al., 2021) (Fig. 1C). Furthermore, to improve the areal distribution of the MCS site intensities of the 1688 Sannio earthquake, and to better define the geometry of the seismic source, we also conducted a thorough reanalysis of the 1688 macro-seismic effects, also using previously undiscovered contemporaneous reports collected in the Naples and Vatican Archives. Similarly, we used the new macroseismic field proposed by Galli (2023) for the 1456 events.

The integration of the new data acquired for both ECSB and WCSB

with those from the literature, especially related to the WCSB (Fig. 1C), resulted in an updated morpho-sedimentary, geochronological, tectonic and macroseismic framework that sheds new light on the seismotectonic setting of the area and the potential source of the 1688 and 1456 Sannio earthquakes.

## 2. Geological and seismological setting

### 2.1. Geology of the investigated area

#### 2.1.1. Regional geology

The Campania sector of the southern Apennine chain, including the Calore River valley, is part of a Neogene fold and thrust belt generated by the tectonic superposition of several sheets made up of Meso-Cenozoic deep basin to shallow-water platform successions, since the Early Miocene (Patacca et al., 1990; Patacca and Scandone, 2007; Bonardi et al., 2009; Ciarcia and Vitale, 2024, Fig. 1B). Geodynamic processes, implying migration towards the Apulian foreland, caused coeval NE thrusting and hinterland extension related to the opening of the Tyrrhenian back-arc basin (Malinverno and Ryan, 1986; Patacca et al., 1990; Hippolyte et al., 1994; Mazzoli et al., 2000). The allochthonous units forming the orogenic structure can be grouped into three main tectonic complexes: (i) Ligurian Accretionary Complex, (ii) Apennine Platform units and (iii) Lagonegro-Molise Basin units. In this sector of the Apennines, several unconformable wedge-top Neogene deposits overlay these allochthonous units, frequently sealing the tectonic boundaries between different thrust sheets (Bonardi et al., 2009; Patacca and Scandone, 2007; Vitale and Ciarcia, 2013, 2018, 2022). From the Lower Pleistocene, this sector of the chain underwent extensional tectonics that generated high-angle fault systems, responsible for the opening of sedimentary basins and the onset of sedimentation of Quaternary fluvial and slope deposits (Casciello et al., 2006). These fault systems are mainly composed of NW-SE trending strike-slip faults and NE-SW trending normal faults acting under NW-SE extension. In the Middle Pleistocene, a rotation in the direction of extension, from NW-SE to NE-SW, caused the reactivation of previous structures, especially those NW-SE and, subordinately, E-W oriented (Caiazzo et al., 2006; Corrado et al., 1997; Di Bucci et al., 2005; Ascione et al., 2008; Vezzani et al., 2010; Lavecchia et al., 2024).

The Calore River valley, located in the axial zone of the Apennine chain, consists of two main large fluvial sectors (Western Calore basin and Eastern Calore sub-basin), both ca. E-W oriented and separated by N-S oriented narrow fluvial sector close to Benevento city (Fig. 1B). The WCSB sector developed within tectonic units of Meso-Cenozoic Apennine carbonate Platform and Lagonegro-Molise Basin domains (D'Argenio et al., 1973; Mostardini and Merlini, 1986) (Fig. 1B). The pre-Quaternary bedrock cropping out to the south and north-west consists of Mesozoic limestones and dolostones belonging to shallow water carbonate platform successions of the Matese-Camposauro Mountains, and by localised outcrops of Paleogene varicoloured marly clays and recrystallized calcareous breccias accumulated near the slope base of the carbonate platform margin, according to Scaglia Detritica-type deposits described in Camposauro Mt (Vitale et al., 2018). Upper Miocene terrigenous sediments unconformably overlay the sedimentary succession. (Fig. 1B) (Bonardi et al., 2009; Cesarano et al., 2011; Pappone et al., 2012; Vitale and Ciarcia, 2018). The Calore River has a meandering planform morphology and crosses a structurally controlled, E-W elongated valley depression, limited to the north by the gentle marginal slopes of Matese Massif, mainly made of wedge-top and fore-deep basin terrigenous deposits and deep basin Lagonegro-Molise Basin unit successions, and to the south by the steep Camposauro carbonate mountain front (Fig. 1B). Available geological data show that this sector of the chain emerged since the Pliocene because marine and transitional deposits of this age are lacking (Carannante et al., 2011).

On the other hand, the ECSB sector developed within tectono-sedimentary units of upper Zanclean marine and alluvial deposits
(ISPRA, 2009; Ciarcia et al., 2003; Ciarcia and Vitale, 2013) overlapping deep basin successions of Lagonegro-Molise Basin units (Fig. 1B). Upper Zanclean deposits are mainly constituted by alternating massive and stratified sandstones and siltstones, locally overlapped by conglomerates (ISPRA, 2009; Ciarcia and Vitale, 2013), forming gently sloping hills affected by large fluvial-denudational processes. The Lagonegro-Molise Basin units consist mainly of varicoloured clays, limestones, marls and, subordinately, arenites (ISPRA, 2009). The N-S oriented sector of the Calore River, separating WCSB and ECSB between Benevento city and Ponte village, is entrenched within the Lagonegro-Molise Basin units (Fig. 1B), and presents high-gradient valley flanks, where Quaternary deposits are entirely lacking or confined within the nowadays narrow floodplain and river channels.

### 2.1.2. Quaternary geology of the Western and Eastern Calore river valley

The WCSB has been investigated by several authors, who outlined the morpho-sedimentary Quaternary evolution also given the recent tectonics and seismicity (e.g., Magliulo, 2005; Massa et al., 2005; Di Bucci et al., 2006; Comentale, 2010; Amato et al., 2018; Filocamo et al., 2021). Conversely, the WCSB has been hitherto poorly investigated, except for field geological surveys carried out for the 1:50.000 Geological map of Italy (Benevento Sheet, ISPRA, 2009). Some geological data related to pre-Quaternary and Lower Quaternary tectono-sedimentary evolution are reported by Ciarcia et al. (2014), Pescatore et al. (1996), Amato et al. (2017) and Senatore et al. (2019).

**WCSB** - This sector is characterised by a meandering and high sinuosity course flowing in an E-W direction for ~15 km. It is bounded southward by the steep Camposauro mountain front and by the gentler slopes of the Matese carbonate massif to the north, giving rise to its asymmetrical profile (Fig. 1C). The northern flank of the valley is characterised by terrace flights, the origins and ages of which remain debated. Five orders of fluvial terraces have been identified at elevations between 50 and 250 m a.s.l. (Di Nocera et al., 1995; Magliulo, 2005; Massa et al., 2005; Di Bucci et al., 2005, 2006; Magliulo et al., 2007; Amato et al., 2018; Filocamo et al., 2021).

The highest and oldest terrace has been chronologically constrained to 560 ± 2 ka (Magliulo et al., 2007), 674 ± 10 ka (Di Bucci et al., 2006) and 648 ± 6 ka (Amato et al., 2018) by means of $^{40}$Ar/$^{39}$Ar dating of tephra layers embedded in the upper part of the succession. The second terrace level has been constrained to 97 ± 25 ka based on $^{40}$Ar/$^{39}$Ar dating of a pyroclastic layer recognized at the top of the fluvial succession (Di Bucci et al., 2006). Filocamo et al. (2021) proposed that this terrace is set against the oldest fluvial deposits through tectonic contact, while Magliulo (2005) considered it in overlapping, through an unconformity surface, over the oldest fluvial deposits. The third terrace level is referred to the 97–39 ka time span (Di Bucci et al., 2006) since the channels dissecting this terrace are filled by Campanian Ignimbrite deposits (CI; ~40 ka, De Vivo et al., 2001; Giaccio et al., 2017). The fourth terrace contains Early Neolithic materials (Magliulo et al., 2007), which constrained its age between 39 ka and <7 ka.

On the left flank, the Quaternary successions mainly consist of slope and coalescing alluvial fan deposits, generically ascribed to the Lower Pleistocene–Holocene time span (Bergomi et al., 1975; Carannante et al., 2011). The oldest slope deposits belonging to Laiano Synthem (Carannante et al., 2011) consist of slope breccias, including decimetric horizons of dark, brown paleosols which, giving the absence of tephra and/or volcaniclastic material derived from the explosive activity of peri-Tyrrenian volcanic system, which started at beginning of the Middle Pleistocene (e.g., Branca et al., 2023), was ascribed to the Lower Pleistocene. The Laiano Synthem is faulted against the Mesozoic limestones and is overlaid by alluvial fan and debris slope deposits of Middle-Upper Pleistocene age. It is locally affected by high-angle faults, mainly NW-SE and ~E-W oriented, with displacements of up to several tens of metres (Carannante et al., 2011; Amato et al., 2018).

Most of the piedmont area includes Late Pleistocene-Holocene col-luvium and alluvial fan deposits, which are telescopically arranged in

two or more generations, likely controlled by tectonics (Massa et al., 2005; Di Bucci et al., 2005, 2006; Massa and Zuppetta, 2009; Magliulo et al., 2007; Amato et al., 2018; Filocamo et al., 2021). According to the most recent investigations supported by $^{40}$Ar/$^{39}$Ar geochronology (Amato et al., 2018; Filocamo et al., 2021), there are four generations of superimposed or entrenched alluvial fan systems grown above the Laiano Synthem slope breccias. The first generation was chronologically constrained to the late Middle Pleistocene-early Upper Pleistocene, based on the presence of tephra layers interbedded in its upper part and dated with $^{40}$Ar/$^{39}$Ar at 158 ± 6 ka and 113 ± 7 ka, respectively. The second generation was referred to the Upper Pleistocene, thanks to the presence of a 48 ± 7 ka $^{40}$Ar/$^{39}$Ar dated tephra layer and a CI tephra layer (~40 ka; Giaccio et al., 2017) interbedded in the upper part. The third generation was dated to the late Upper Pleistocene and Holocene based on the presence of a tephra layer ascribed to the Neapolitan Yel-low Tuff eruption (14.9 ± 0.4 ka, Deino et al., 2004; 14.4 ± 0.5 ka, Galli et al., 2017) in its upper part. Finally, the fourth generation was referred to the historical times.

**ECSB** - This sector extends from Benevento to Apice and consists of a W-E oriented valley which hosts the Calore River, flowing meandering in a high sinuosity channel, and hosting three important tributaries: two on the right side, Ufita and Tammaro Rivers, respectively, and the Sabato River on the left side (Fig. 1D). Unlike the WCSB, ECSB lacks detailed studies, and according to the 1:50.000 Geological map (Benevento sheet), it includes four sub-synthsems within the Quaternary sedimentary succession. The oldest deposits (SFL1), referred to as Cas-tello del Lago Subsynthem of the Calore Synthem, lack radiometric dating and are attributed to the Middle Pleistocene (Chiocchini, 2007; Pescatore et al., 1996).

The following Capodimonte Subsynthem (SFL2) consists of 10-to-100 m thick fluvial gravel, forming wide depositional terraces at vari-able elevation a.s.l., generally ca. 70–80 m hanging over the present-day floodplain. This unit was also ascribed to the Middle Pleistocene (Chiocchini, 2007, Pescatore et al., 1996) and can be correlated to the 1st order of the WCSB (Amato et al., 2017; Senatore et al., 2019).

The late Middle-Upper Pleistocene Benevento Subsynthem (SFL3) outcrops in the SW sector of the Benevento city where it constitutes small fluvial terrace remnants, nowadays 30–40 m hanging over the floodplain. They are made of heterometric and polygenic rounded openwork and sandy matrix-supported gravels with local tephra layers and paleosol horizons. The fluvial succession, 10–20 m thick, is locally covered by the pyroclastic deposits of the Campanian Ignimbrite.

Further downstream, the Ufita Subsynthem (SFL4) forms large fluvial terraces at ca. 5–10 m above the floodplain. The deposits, 5–10 m thick, are made of loose heterometric gravels, mainly sandy matrix-supported volcaniclastic, with intercalations of sandy silt layers, paleo-sols and tephra deposits. Frequently, pyroclastic fallout deposits of the Pomici di Avellino eruption from Somma-Vesuvius (3.9 ka, Sevink et al., 2011) are recognized in several archaeological excavations and trenches (Amato et al., 2017; Paradiso et al., 2015).

Finally, more recent deposits, generally due to colluvial, slope and fluvial processes, are locally present.

### 2.2. Regional historical seismicity and seismotectonic

The Campanian Apennine ridge is among the highest seismic hazard regions in the Mediterranean area, both in terms of maximum magni-tude (Mw~7) and macroseismic intensity (Is 11 MCS, Mercalli-Cancani-Sieberg scale; Sieberg, 1932), and in seismic event frequency. Stretching from the northern boundary with Molise and Lazio regions (upper Vol-turno Valley) to the southern boundary with Basilicata (Vallo di Diano), the Campanian Apennine ridge hosted the epicentral area of the stron-gest Apennine seismic sequences. In a sector approximately 200 km long and generally no more than 20 km wide, over 20 destructive earth- quakes have occurred in the past Millennium, including Mw~7. Conceivably, many of these events were generated by normal or
transtensive NW-SE crustal faults, as conclusively ascertained through geological, paleoseismological or seismological evidence. In particular, the Aquae Iuliae fault (12 in Fig. 1A; Fig. 1E; Galli and Naso, 2009) was responsible for the southern mainshocks of the epochal 1349 sequence (Mw 6.8; Galli et al., 2022), the N-Matese fault (13 in Fig. 1A; Fig. 1E; Galli et al., 2017) sourced the central mainshocks of the equally epochal 1456 sequence (Galli, 2023), along with its twins in 1805 (Mw 6.7) and 280 BCE (Galli and Galadini, 2003) (Fig. 1E) whereas further south, the Mt Marzano fault (14 in Fig. 1A; Galli and Peronace, 2014) was responsible for the events of 989 CE, 1466 (Mw 6.3), 1694 (Mw 7.0), and 1980 (Mw 6.9). The Caggiano fault (15 in Fig. 1A; Galli et al., 2006; Castelli et al., 2008) has been associated with the 1561 earthquake (Mw 6.7) and the northern mainshocks of the catastrophic 1857 event (Mw 7.1).

Conversely, as mentioned in the Introduction chapter, it has not been possible to identify the seismogenic source of some destructive earthquakes in the central section of the Campanian Apennines, despite their epicentral area being located there. In particular, no reconstruction was

possible for the first, southernmost mainshock of the 1456 sequence (Mw 7.0) and for any of the events that devastated the entire Sannio and Baronia regions over a span of 40 years, between 1688 (Mw 7.1), 1702 (Mw 6.6), and 1732 (Mw 6.8) (Fig. 1A and E). As shown in Fig. 1E, the comparison amongst the highest intensity data points distribution evaluated for the 1456, 1688 and 1805 events strongly suggests that, while the 1805 event is a twin of the central mainshock of the 1456 sequence, sourced by the coseismic rupture of the N-Matese fault, the 1688 is the twin of the southernmost mainshock of the same sequence (see also Magri and Molin, 1983; Meletti et al., 1988). Their common seismogenic source should thus be searched along the north-eastern or south-western bound of their overlapping mesoseismic area.

# 3. Material and methods

## 3.1. *Geomorphological and stratigraphical investigations*

Stratigraphical, geomorphological and structural survey and field

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Pictures of the outcrop containing some of the investigated tephra layers and sand levels sampled for OSL dating (for the pictures of the other tephra layers investigated in this study, the reader is referred to Fig. 14).</figcaption></figure>
sedimentological-facies analysis were carried out on the Quaternary deposits outcropping along the ECSB so far little investigated, from Apice to Benevento (Fig. 1D). In order to get an overall and coherent framework for the Quaternary deposits and landforms of the entire Calore River valley, some spot surveys were carried out also along the WCSB, previously extensively investigated (Fig. 1C) (Magliulo, 2005; Massa et al., 2005; Amato et al., 2018; Filocamo et al., 2021), specifically at Postiglione site, (POS1 in Fig. 1C), Madonna del Roseto road (MR1, in Fig. 2A), Cesine locality (sites SOL1, SOL2 in Fig. 1C), through morpho-stratigraphic and structural analysis, and along the Camposauro massif, for structural analysis along fault planes in Mesozoic carbonates (Fig. 1C).

Geomorphological analyses were mainly focused on the identification of Quaternary fluvial landforms, using large-scale topographic maps (Carta Tecnica Regionale of Campania Region, 1992; in scale 1:5000), aerial photos and orthophotos, high resolution DTM (5 m per pixel) derived high resolution Digital Elevation Model. All mapped elements were checked via field surveys and stratigraphic data derived from boreholes (Fig. 1 and Supplementary Figure SF1).

The stratigraphical analysis was mainly focused on the definition of the morpho-stratigraphical framework of the study area, the characterization of the valley fillings and the stratigraphical features of the terraced fluvial landforms. This analysis was essentially based on field surveys and the study of 34 borehole logs provided by public institutions and private companies (Figure SF1). Stratigraphical data obtained from boreholes were compared with those derived from outcropping successions.

Sediments were characterised in terms of colour, texture, grain features (size, shape, composition), fossil content, sedimentary and diagenetic structures, according to the methods illustrated in Tucker (2011) and Miall (1984) for facies classification. Based on lithofacies, unconformities, and the presence of tephra layers and paleosols, the examined sedimentary successions were grouped into sedimentary units and subunits using the Unconformity Boundary Stratigraphic Unit method (UBSU; after Salvador, 1987) (Supplementary Table ST1).

Field surveys also allowed to identify and characterise different tectonic features that affect the investigated Quaternary successions.

## 3.2. Tephra sampling

Primary and sub-primary tephra layers, embedded in the Quaternary sedimentary successions, were sampled mainly for tephrochronology through $^{40}$Ar/$^{39}$Ar dating and geochemical fingerprinting for

correlations with dated pyroclastic layers. In the ECSB, they were recognized and sampled in the following sections: (i) Minicozzi 1 (three tephra layers, Minicozzi 1-T0 to Minicozzi 1-T2), (ii) Minicozzi 2 (one tephra, Minicozzi 2-T1), (iii) Liderlat (three tephra layers, Liderlat-T1 to Liderlat-T3), (iv) Ponte Valentino (two tephra layers, PV3a and PV3b), (v) Cancelleria (two tephra layers, Cancelleria T1 and Cancelleria T2) and (vi) Apice (one tephra, Apice-T1) (Table 1, Fig. 1D and 2). Four tephra layers in the WCSB, were also collected at Postiglione and Cesine sites (POS1 and SOL2 in Fig. 1C). At POS1 site (Di Bucci et al., 2006; Amato et al., 2018), we sampled FT-1, equivalent to the sample BM87 of Di Bucci et al. (2006) and FT-2, lying above FT-1. At the site SOL2 we collected tephra SOL2-T2 (three sub-samples: bottom, middle, and top) and the underlying SOL2-T1.

Tephra were relabeled according to their stratigraphic order and sub-basin occurrence, from the younger ECSB tephra 1 (ECSB-T1) to the older ECSB-T10, and from the younger WCSB-T1 to the older WCSB-T4. A summary of the collected samples and of the performed analyses is provided in Table 1.

## 3.3. Geochronological analyses

### 3.3.1. $^{40}$Ar/$^{39}$Ar dating

The $^{40}$Ar/$^{39}$Ar ages for tephra ECSB-T1 (Liderlat T3), ECSB-T3 (Liderlat T1), ECSB-T4 (Cancelleria T2), ECSB-T6 (Minicozzi 2 T1) and ECSB-T10 (Ponte Valentino PV3b) (Table 1; Fig. 1D and 2) were obtained at the Laboratoire des Sciences du Climat et de l'Environnement (CEA, CNRS UMR 8212, Gif-sur-Yvette, France) dating facility. Between 40 and 50 fresh and transparent K-rich feldspars were picked under a binocular for each sample. After being washed in distilled water, transparent K-feldspars (350–500 µm) without visible inclusions were handpicked under a binocular and used for dating. Beforehand, irradiated crystals were leached for 5 min in a 10% HF solution to remove potential groundmass pieces and alteration phases that could still be attached to the surface of the crystals. Between thirty and forty crystals for each sample were irradiated in the Cd-lined, in core CLICIT facility of the Oregon State University TRIGA reactor in distinct irradiations, IRR. CO-011 (1 h); IRR. CO-012 (2 h); IRR. CO-013 (2 h); IRR. CO-017 (1 h); IRR. CO-019 (1 h) for Liderlat T3, PV3b, Liderlat T1, Cancelleria T2 and Minicozzi-1 T2, respectively. Interference corrections were based on the nucleogenic production ratios given in Balbas et al. (2016). After irradiation, crystals were transferred into a copper 133 pits sample holder placed into a differential vacuum Teledyne Cetac window connected to a home designed compact extraction line. Minerals were fused one by one

<table><caption>**Table 1**
Summary information on the performed analyses of the investigated tephra layers.</caption><thead><tr><th rowspan="2">Sub-basin</th><th rowspan="2">Section</th><th rowspan="2">Coordinates</th><th colspan="2">Tephra label</th><th rowspan="2">Major elements (EMPA-WDS)</th><th rowspan="2">$^{40}$Ar/$^{39}$Ar</th></tr><tr><th>Field label</th><th>Final label</th></tr></thead><tbody><tr><td rowspan="12">Eastern</td><td rowspan="3">Liderlat</td><td>41° 8' 52.06"N</td><td>Liderlat-T3</td><td>ECSB-T1</td><td>× altered</td><td>×</td></tr><tr><td>14° 50' 25.42"E</td><td>Liderlat-T2</td><td>ECSB-T2</td><td>× altered</td><td>×</td></tr><tr><td></td><td>Liderlat-T1</td><td>ECSB-T3</td><td>×</td><td>×</td></tr><tr><td rowspan="2">Cancelleria</td><td>41° 7' 45.50"N</td><td>Cancelleria-T2</td><td>ECSB-T4</td><td>×</td><td>×</td></tr><tr><td>14° 51' 54.28"E</td><td>Cancelleria-T1</td><td>ECSB-T5</td><td>×</td><td>×</td></tr><tr><td rowspan="2">Apice</td><td>41° 8' 3.18" N</td><td rowspan="2">Apice-T1</td><td rowspan="2">ECSB-T5</td><td rowspan="2">× altered</td><td rowspan="2">×</td></tr><tr><td>14° 55' 3.52"E</td></tr><tr><td rowspan="2">Minicozzi 2</td><td>41° 9' 8.05"N</td><td rowspan="2">Minicozzi 2-T1</td><td rowspan="2">ECSB-T6</td><td rowspan="2">×</td><td rowspan="2">×</td></tr><tr><td>14° 49' 42.52"E</td></tr><tr><td rowspan="3">Minicozzi 1</td><td>41° 9' 9.55"N</td><td>Minicozzi 1-T2</td><td>ECSB-T6</td><td>×</td><td>×</td></tr><tr><td>14° 49' 47.54"E</td><td>Minicozzi 1-T1</td><td>ECSB-T7</td><td>×</td><td>×</td></tr><tr><td></td><td>Minicozzi 1-T0</td><td>ECSB-T8</td><td>×</td><td>×</td></tr><tr><td rowspan="5">Western</td><td>Ponte Valentino</td><td>41° 8' 42.06"N</td><td>PV3a</td><td>ECSB-T9</td><td>×</td><td>×</td></tr><tr><td rowspan="2">Postiglione</td><td>14° 50' 41.62"E</td><td>PV3b</td><td>ECSB-T10</td><td>× altered</td><td>×</td></tr><tr><td>41°10'49.96"N</td><td>FT-2</td><td>WCSB-T1</td><td>×</td><td>×</td></tr><tr><td rowspan="2">Cesine (SOL2)</td><td>14°31'8.51"E</td><td>FT-1 (BM87*)</td><td>WCSB-T2</td><td>×</td><td>×</td></tr><tr><td>41°11'6.76"N</td><td>SOL2-T2</td><td>WCSB-T3</td><td>×</td><td>×</td></tr><tr><td></td><td>14°31'11.05"E</td><td>SOL2-T1</td><td>WCSB-T4</td><td>×</td><td>×</td></tr></tbody></table>

<footnote>a Di Bucci et al. (2006).</footnote>
using a 100 W Teledyne Cetac CO₂ laser for 15 s at 2.5 W. Before fusion, each crystal underwent a 20 s long sweeping at 0.3 W to remove unwanted gas potentially trapped on the crystals surface and fractures. Extracted gases were firstly purified by a SAES GP 50 cold getter for 90 s and then for 230 s by two hot SAES GP 50 getters. The five Argon isotopes (i.e., ⁴⁰Ar, ³⁹Ar, ³⁸Ar, ³⁷Ar and ³⁶Ar) were measured using a multi-collector NGX 600 mass spectrometer equipped with 9 ATONA® amplifiers array and an electron multiplier. More technical specifications regarding the NGX 600 ATONA detector array are presented in detail in Cox et al. (2020). ⁴⁰Ar, ³⁹Ar, ³⁸Ar, and ³⁶Ar isotopes were collected simultaneously while the ³⁷Ar was measured in a second time. In the first run, ⁴⁰Ar, ³⁹Ar and ³⁸Ar were measured simultaneously on 3 ATONA® amplifiers and ³⁶Ar on the electron multiplier. Following this first run, the ³⁷Ar was measured alone using the electron multiplier. Each isotope measurement corresponds to 15 cycles of 20 s integration time. Peak intensity data were reduced using ArArCALC V2.4 (Koppers, 2002). Neutron fluence J factor was calculated using co-irradiated Alder Creek sanidine standard ACs-2 associated to an age of 1.1891 Ma (Niespolo et al., 2017) according to the K total decay constant of Renne et al. (2011) ($\lambda_{e,c.} = (0.5757 \pm 0.016) \times 10^{-10} \text{ yr}^{-1}$ and $\lambda^{\beta-} = (4.9548 \pm 0.013) \times 10^{-10} \text{ yr}^{-1}$). To determine this neutron flux we used 14 flux monitor crystals coming from pits framing the samples in each irradiation disk. J-values measured are 0.0002698 ± 0.00000016; 0.0005503 ± 0.00000050; 0.0005524 ± 0.00000033; 0.0002824 ± 0.00000023 and 0.0002840 ± 0.00000020 for Liderlat T3, PV3b, Liderlat T1, Cancelleria T2 and Minicozzi-1 T2, respectively. To verify the detector linearity, mass discrimination was monitored by analysis of at least 10 air shots of various beam sizes ranging from $1.0 \times 10^{-2}$ up to $5.0 \times 10^{-2}$ V (1–5 air shots). About 15 air shot analyses are performed every day. These measurements are done automatically during the nights before and after the unknown measurements. Discrimination is calculated according to the ⁴⁰Ar/³⁶Ar ratio of 298.56 (Lee et al., 2006). Procedural blank measurements were achieved after every two to three unknowns. For a typical 5 min time blank backgrounds are between 0.6 and $2.3 \times 10^{-4}$ V for ⁴⁰Ar and 20–50 cps for ³⁶Ar (about $3.0 \times 10^{-7} \times 10^{-7}$ V equivalent). Supplementary Dataset 1 (SD-1) provides full analytical data for each sample.

### 3.3.2. OSL dating

In order to estimate the minimum age for the oldest fluvial deposit associated with T1 terrace in the ECSB, a sample (PV1) of pedogenized fluvial sands was collected at the top of the Ponte Valentino succession (site Ponte Valentino in Fig. 2). The age was estimated using Optically Stimulated Luminescence (OSL) at the Gliwice Luminescence Laboratory (GLL) of the Institute of Physics of the Silesian University of Technology (Poland). Sampling was done by hammering 5-cm-diameter and 30-cm-long stainless-steel tubes horizontally into freshly cleaned sections (Fig. 2). The tubes were completely filled and wrapped with aluminium foil and adhesive tape to avoid the risk of sediment mixing and prevent further exposure to light and moisture loss during transport. The laboratory procedures for OSL age determination are illustrated in Moska et al. (2021).

### 3.4. Tephra geochemical analyses

Samples have been geochemically characterised in terms of major (>1 wt%) and minor (<1 wt%, >0.1 wt%) element composition by analysis of glass shards and micro-pumice fragments with the Electron Probe Micro Analyzer (EPMA). A summary of the analysed tephra is provided in Table 1.

Analysis on tephra ECSB-T8 to ECSB-T6, from Minicozzi 1 section, ECSB-T10 and ECSB-T9, from Ponte Valentino section, and ECSB-T5, from Apice and Cancelleria sections (Fig. 1D and 2; Table 1), have been performed with the EPMA at Istituto di Geologia Ambientale e Geologegneria, of the Italian National Research Council (CNR-IGAG), using a CAMECA SX-50 equipped with five-wavelength dispersive

spectrometers (WDS). At least 15–20 data points, required to express the full geochemical variability of the tephra layers, were obtained for each sample.

Operating conditions were set at 15 kV accelerating voltage, 15 nA beam current, and 10 µm defocused beam diameter to limit Na mobilisation and loss, and 20 s element counting time for all elements. Wollastonite (Si and Ca), corundum (Al), periclase (Mg), magnetite (Fe), rutile (Ti), orthoclase (K), jadeite (Na), potassium chloride (Cl), F-apatite (P) and metals (Mn) were used as calibration standards. The Kakanui augite and Rhyolite RLS132 glass from the United States Geological Survey and the artificial glass secondary standards from the Max Plank Institute StHs6/80-G and GOR128-G (Jochum et al., 2006) were measured before each analytical run to evaluate the accuracy of the electron-microprobe analysis.

Samples ECSB-T3 to ECSB-T1, from Liderlat section, ECSB-T4, from Cancelleria section, and WCSB-T1 to WCSB-T4, from Postiglione and Solopaca sections (POS1 and SOL-02 in Fig. 2A), were successively analysed at the Laboratory of Microanalysis and Electron Microscopy (LAM2), of CNR-IGAG, with the new JEOL JXA-iSP100 Super Probe EPMA, equipped with five WDS spectrometers and an Energy Dispersive Spectrometer (EDS). Operating conditions were set to 15 kV accelerating voltage, 10 nA beam current, and 10 µm defocused beam diameter to limit Na mobilisation. Element counting times were 20 s for all elements besides Na and Si (10 s). The following calibration standards were employed: quartz (Si), ilmenite (Ti), anorthoclase (Al), chromite (Fe), dolomite (Mg), diopside (Ca), albite (Na), orthoclase (K), apatite (P), sylvite (Cl), and metals (Mn). The artificial glass secondary standards from the Max Plank Institute ATHO-G, StHs6/80-G and GOR128-G (Jochum et al., 2006) were measured before the sample analysis to evaluate the accuracy of the electron-microprobe analysis.

Data reduction and processing were carried out using the ZAF (Z = atomic number; A = absorption; F = fluorescence) correction, whilst data normalisation and visualisation were performed in Microsoft Excel®. We adopted 93 wt% as a threshold for the measured total; analyses with total values lower than 93 wt% were discarded. All compositional data are shown as oxide weight percentages (wt%) in the Total Alkali vs Silica (TAS; Le Maitre et al., 2002) classification diagram and bi-plots diagrams, with total iron (FeO<sub>tot</sub>) expressed as FeO, and normalised to 100% on a volatile-free basis (i.e., excluding Cl) for correlation purposes. Collected data are all reported in Supplementary Dataset-2 (SD-2), along with secondary standards measured values.

### 3.5. Structural analyses

The kinematics for each single fault segment was derived from the fault plane and striae orientations analysis. The FaultKin5.2 software (Allmendinger et al., 2012) was used for the calculation of the local stress tensor through inversion of the fault and striae data. Kinematic indicators necessary for determining fault block movement were obtained from calcite shear fiber steps and other well-established indicators from brittle fault zones. The rake of the striae and kinematic indicators indicate that slickensides oriented ~ N325° predominantly exhibit dip-slip kinematics, whereas fault planes oriented ~ N115° show oblique-slip kinematics (Table 2). The kinematics of each fault segment were analysed using FaultKin 5.2 software (Allmendinger et al., 2012), based on fault plane and striae orientations. FaultKin 5.2 was also used to calculate the local stress tensor through fault-slip inversion (Bello et al., 2025 and reference therein). In this process, we assumed that slip on fault planes occurred in the direction of the resolved shear stress and that the measured data reflect a spatially and temporally uniform stress field (Angelier, 1990).

### 3.6. Macroseismic intensity distribution for the 1456 and 1688 Sannio earthquake

It is well known that the extent of a strong earthquake mesoseismic
<caption>**Table 2**</caption>

Pseudo-fault solution for all data collected on different segments. Kinematics: N, normal fault – striae pitch between 70° and 90°. NR, oblique right normal fault – striae pitch between 30° and 70°.

<table><thead><tr><th rowspan="2">Fault system</th><th rowspan="2">Fault segment</th><th rowspan="2">Acronym</th><th rowspan="2">Strike</th><th rowspan="2">Dip</th><th rowspan="2">Rake</th><th rowspan="2">Kinematics</th><th colspan="2">T-axis</th><th colspan="2">P-axis</th></tr><tr><th>Trend</th><th>Plunge</th><th>Trend</th><th>Plunge</th></tr></thead><tbody><tr><td rowspan="2">WCBF</td><td>Cesine</td><td>C</td><td>270°</td><td>80°</td><td>129°</td><td>NR</td><td>29°</td><td>25°</td><td>143°</td><td>42°</td></tr><tr><td>San Michele</td><td>SMK</td><td>323°</td><td>58°</td><td>86°</td><td>N</td><td>50°</td><td>14°</td><td>244°</td><td>76°</td></tr><tr><td rowspan="5">ACF</td><td>S. Maria in Gruptis</td><td>SMG</td><td>128°</td><td>42°</td><td>91°</td><td>NR</td><td>39°</td><td>03°</td><td>231°</td><td>87°</td></tr><tr><td>Torrecuso</td><td>T</td><td>339°</td><td>61°</td><td>88°</td><td>N</td><td>68°</td><td>16°</td><td>254°</td><td>74°</td></tr><tr><td>Torrecuso village</td><td>Tv</td><td>327°</td><td>77°</td><td>92°</td><td>N</td><td>59°</td><td>32°</td><td>234°</td><td>58°</td></tr><tr><td>Madonna del Roseto</td><td>MR</td><td>93°</td><td>66°</td><td>132°</td><td>NR</td><td>212°</td><td>11°</td><td>315°</td><td>50°</td></tr><tr><td>San Mennato</td><td>SM</td><td>115°</td><td>46°</td><td>122°</td><td>NR</td><td>47°</td><td>03°</td><td>309°</td><td>67°</td></tr><tr><td></td><td>Vitulano</td><td>V</td><td>113°</td><td>78°</td><td>115°</td><td>NR</td><td>223°</td><td>29°</td><td>355°</td><td>51°</td></tr><tr><td colspan="2">All data</td><td></td><td>315°</td><td>54°</td><td>98°</td><td>N</td><td>51°</td><td>09°</td><td>193°</td><td>79°</td></tr></tbody></table>

area (i.e., the distribution of the highest points of site intensity Is) and its azimuthal elongation strongly depend from the geometry and kinematics of its seismogenic structure. The maximum intensities are always scattered in the hanging-wall block of a seismogenic normal fault, with an elongation parallel to the fault itself and with a possible azimuthal asymmetry controlled by the direction of coseismic rupture along the plane (directivity). It is likely that the concentration of the strongest effects in the near-fault hanging wall occurs because this region experiences the largest vertical component of coseismic motion and the strongest vertical component of peak ground acceleration (e.g., Al Shawa et al., 2021). This effect has been observed in all strong extensional earthquakes in the Apennines, including those that have occurred in recent years, where instrumental and geological data have fully overlapped with macroseismic results (Galli and Galadini, 1999; Galli and Bosi, 2002; Galli and Peronace, 2014; Galli et al., 2010, 2017). In order to better constrain the geometry of the mesoseismic area of the 1456 and 1688 events, we reassess their entire macroseismic intensity distribution. For the 1456 event, we used the new intensities reported in Galli (2023), whereas for the 1688 event we elaborated a new macroseismic framework. All historical descriptive material on the effects of the 1688 earthquake has been collected (see all references in Serva, 1981; CFTI5Med, 2019), including coeval news not previously considered founded in local and State archives and libraries, and applying the Mercalli-Cancani-Sieberg intensity scale (Sieberg, 1932), we reassessed the macroseismic intensities of this event. We adopted the methodology proposed by Molin (2009), who considers the five damage levels (levels 1–5) included in almost all the macroseismic scales: light, moderate, severe, destruction, and collapse. The evaluation of the percentages of these levels in a locality (i.e., 5%, 25%, 50%, 75% and 100%) yield an immediate assessment of each MCS degree, as implicitly contained in the original scale (e.g., in Galli et al., 2012). The image maps of the intensity distribution have been obtained by gridding the original data with the algorithm Kriging provided by Golden Software Surfer 12.8 (Variogram Model Linear: slope = 1, anisotropy = 1.0, 231 rows x 457 columns. Total nodes: 10,5567) (Fig. 1E).

# 4. Results

## 4.1. Quaternary deposits and landform and structural setting

### 4.1.1. Eastern Calore sub-basin

Our study highlights the presence of remnants of an ancient fluvial unit and four orders of depositional fluvial terraces (from the oldest T1 to the youngest T4) suspended at various heights over the present floodplain of the Calore River (Fig. 3B). The main lithofacies and morpho-stratigraphical features of the fluvial deposits and terraces of the eastern and western Calore River valley are summarised in Table 3, while a more detailed description are provided in Supplementary text-1 (SText-1) and Supplementary Table 1 (ST1).

A striking feature of these terraces is their asymmetry, i.e., they are well-preserved only in the northern flank of the ECSB (orographic right

of the Calore River), while in the southern flanks (orographic left of the Calore River) only remnants of little planar surfaces and narrow ridges are exposed. Specifically, the T1 terrace (220-180 m a.s.l.) is widely documented in the right orographic side with an outcropping thickness of the subtended gravel deposits of ~50 m, but exceeding 100 m in boreholes (see 171107 log in Supplementary figure SF1). Along the left orographic side, the same unit is instead scantly represented by very thin deposits and lacking of the top depositional surface because of erosion (Table 3; Fig. 3B). A sand sample for OSL dating was collected at site Ponte Valentino (PV-1 in Fig. 3B).

The unit T2 (130–200 m a.s.l.) is widely represented by a ~20 m-thick succession along the right side, while it is thinner and only sporadically present along the left orographic side (Table 3, Fig. 3B). At Ponte Valentino outcrop (PV in Fig. 3B), two tephra layers (PV3a/ECSB-T9 and PV3b/ECSB-T10) were sampled (Table 1; Fig. 2).

Similarly, the T3 order (120–180 m a.s.l. Fig. 3B) is mainly represented on the right orographic side, with exposed thickness of the subtended deposits of ~20 m, while it is thinner (10–15 m) and less developed along the left orographic side (Table 3; Fig. 3B). Several tephra layers from T3 deposits were sampled at Cancelleria (Cancelleria T1-T2), Apice (Apice T1), Minicozzi 1–2 (Minicozzi 1 T0 to T2, Minicozzi 2 T1) and Liderlat (Liderlat T1-T2) outcrops (Table 1; Fig. 2).

The youngest T4 order (115–155 m a.s.l.) is instead equally developed and distributed on both orographic sides (Fig. 3B). The subtended deposits do not exceed 10 m in thickness and contain, or are capped by, a marker tephra sampled at site Liderlat 2 (sample Liderlat T3; Table 1, Fig. 2).

### 4.1.2. Western Calore sub-basin

**Madonna del Roseto area (Site MR1)** - The road that leads to the Madonna del Roseto sanctuary crosses the NW side of the Camposauro massif, exposing the sediment of the Late Pleistocene-Holocene third generation of alluvial fan deposits and then, at higher elevation, the Laiano breccias. At site MR1 in Fig. 1C, the uppermost part of the breccias contains several decimeter-thick, pedogenized pyroclastic levels made of strongly altered ash.

**Camposauro area** - We recognized and mapped several fault segments in the Camposauro massif, roughly organised in south-dipping and north-dipping, as reported in the interpretative structural map shown in Fig. 4. The north-dipping system borders the northern slope of the massif (i.e., western Calore Border Fault: WCBF in Fig. 4), with rock-fault scarps sporadically outcropping in the thick forested slopes of northern Camposauro massif. The south-dipping system has more field expression and consists of two en echelon faults, here called Madonna del Roseto (MR in Figs. 4, 5A and 5B) and S. Maria in Gruptis-S. Mennato (SMG-SM, Fig. 5C) faults, respectively. A summary of the overall geometry of the fault systems and the results of the structural analyses are reported in the interpretative structural map shown in Fig. 4. A detailed description of the north-dipping, WCSBF and south-dipping fault system in some selected localities of Cesine (site SOL1-SOL2 in Fig. 1C), Postiglione and Vitulano area is provided below.
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Quaternary geology and Landforms of the a) western (modified from Amato et al., 2018) and b) eastern Calore River sub-basin. Legend: CVT: Castelvetere Group; SCA: Scaglia Detritica-type deposits; FYR: Flysch Rosso Fm; ACP: Apennine Carbonate Platform; SFL₁: Castello del Lago Subsynthem; BNA: Baronia Fm.</figcaption></figure>
<table><caption>**Table 3**
Main litho-morpho-stratigraphic features of the terraced units in eastern (a) and western (b) Calore River sub-basins.</caption><thead><tr><th colspan="7">a) EASTERN CALORE SUB-BASIN</th></tr><tr><th rowspan="3">Terrace order</th><th rowspan="3">Lithofacies<sup>a</sup></th><th colspan="4">Elevation range of the depositional top surface (m)</th><th colspan="2">Max thickness of the subtended deposits (m)</th></tr><tr><th colspan="4">Orographic side</th><th colspan="2">Orographic side</th></tr><tr><th>Right</th><th>Left</th><th>Right</th><th>Left</th></tr></thead><tbody><tr><td></td><td></td><th>a.s.l.</th><th>a.r.b.<sup>b</sup></th><th>a.s.l.</th><th>a.r.b.</th><td></td><td></td></tr><tr><td>4</td><td>Gms, Sh</td><td>115-155</td><td>5-10</td><td>115-155</td><td>5-10</td><td>~10</td><td>~10</td></tr><tr><td>3</td><td>Gms, Gp, Sh</td><td>120-180</td><td>10-30</td><td>120-160</td><td>10-30</td><td>>20</td><td>&lt;15</td></tr><tr><td>2</td><td>Gm, Gp, St</td><td>130-200</td><td>20-50</td><td>195-205</td><td>45-55</td><td>~20</td><td>&lt;15</td></tr><tr><td>1</td><td>Gm, Gp</td><td>180-220</td><td>50-80</td><td>n.p.<sup>c</sup></td><td>n.p.<sup>c</sup></td><td>>100</td><td>&lt;10</td></tr></tbody><thead><tr><th colspan="7">b) WESTERN CALORE SUB-BASIN</th></tr><tr><th rowspan="3">Terrace order</th><th rowspan="3">Lithofacies<sup>a</sup></th><th colspan="4">Elevation range of the depositional top surface (m)</th><th colspan="2">Max thickness of the subtended deposits (m)</th></tr><tr><th colspan="4">Orographic side</th><th colspan="2">Orographic side</th></tr><tr><th>Right</th><th>Left</th><th>Right</th><th>Left</th></tr></thead><tbody><tr><td></td><td></td><th>a.s.l.</th><th>a.r.b.<sup>b</sup></th><th>a.s.l.</th><th>a.r.b.</th><td></td><td></td></tr><tr><td>4</td><td>Gh, St, Sp</td><td>75-50</td><td>4-10</td><td>80-60</td><td>5-10</td><td>~10</td><td>~10</td></tr><tr><td>3</td><td>Gmm, Gp, Gt</td><td>85-550</td><td>10-15</td><td>90-70</td><td>10-30</td><td>>20</td><td>&lt;15</td></tr><tr><td>2</td><td>Gmm, Gcm</td><td>150-180</td><td>20-50</td><td>n.p.<sup>c</sup></td><td>n.p.<sup>c</sup></td><td>~20</td><td>&lt;15</td></tr><tr><td>1</td><td>Gm, Gh</td><td>180-220</td><td>50-80</td><td>n.p.<sup>c</sup></td><td>n.p.<sup>c</sup></td><td>>100</td><td>&lt;10</td></tr></tbody></table>

<sup>a</sup> Lithofacies code according to Miall (1984).

<sup>b</sup> Above river bed.

<sup>c</sup> Not preserved, eroded.

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Shaded relief image of the Camposauro Massif with the interpretative structural map showing main normal fault systems affecting Middle and Late Pleistocene deposits (dashed where inferred). Ruby-red, N-dipping faults (main synthetic fault system) of Western Calore Basin Fault (WCBF; segments C, Cesine; T, Torrecuso; SMK, San Michele; CP Castelpoto). Red, S-dipping faults (main antithetic fault system; ACF; Madonna del Roseto Fault, MR and Santa Maria in Gruptis-San Mennato fault, SMG-SM, V, Vitulano tip). Yellow dots, structural stations. The light-green focal mechanism (upper right) represents the pseudo-fault-plane solution obtained by inverting all the slickenlines data collected along the different fault segments (see single stereoplots, suggesting NE-SW extension (T axis 51°). Stereoplots for single fault splay show fault planes (black cyclographs), slip vector (black arrow) and stress axes σ1, σ2, and σ3. Since we analysed two fault domains, inverting the structural data was necessary to determine the direction of the extensional stress acting on both systems. This analysis revealed that the extensional direction is the same for both domains and aligns with the current Apennine stress field. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

**Western Calore Border Fault (Cesine area, sites SOL1 and SOL2)** - At this locality (Fig. 1C), a farm-road traverses the main synthetic N270° striking WCBF system, exposing a 5–6 m high striated slickenside carved both in marine carbonate and slope deposits (Fig. 5E). Here we followed the slickenside both westward, where it enters and disappears into the siliciclastic terms of the Miocene flysch, and eastward, along the carbonate Camposauro slope, where it slightly turns to N290°. In the hanging-wall, strongly cohesive slope deposits (sparse gravels in a red-dish sandy-silty matrix) are dragged along the fault plane, tapering upward. Along strike, going westward, the slickenside is directly carved in sandy-silty slope deposits (Fig. 5E). Moving further west (site SOL2 in Fig. 1C), the fault affects a 3-m-thick succession (footwall side), made of

volcanic-rich sandy colluvial deposits and pedogenized horizons, containing two primary tephra layers (WCSB-T3 and WCSB-T4; Table 1, Fig. 5E, see description in section 4.4. And Supplementary Table ST1).

**South-dipping fault system (Antithetic Camposauro Fault, Postiglione and Vitulano area)** - At site Postiglione (Fig. 3A), a road-cut along the Provincial Road 21 Solopaca-Frasso Telesino crosses slant-wise the easternmost tip of the MR normal fault (Fig. 4), where tephra-rich slope deposits are offset and dragged against Cretaceous marine carbonates (Fig. 5A, see also Di Bucci et al., 2006; Amato et al., 2018). We followed and measured the rock-fault scarp with its slickensides for hundreds of metres in the forested hillslope, both below and above the road-cut, where it strikes N105°-120° with dips of 65°–85° (Fig. 5A). In
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Outcrops anthology of the south-dipping, antithetic Northern Camposauro fault system (A–D) and the Western Calore Basin Fault (E). A, road-cut across the Madonna del Roseto fault, offsetting Holocene slope debris against cretaceous marine carbonates. B, the same fault cutting the inner portion of the Camposauro Massif. C-D, slickensides of the S. Maria in Gruptis-S. Mennato fault. E, View looking west of the northern basal fault of the Camposauro Massif west of Solopaca (i.e., southern border fault of the western Calore basin). Here the fault scarp is carved both in the Cretaceous marine carbonates, and in the slope deposits (slope debris and colluvia), rich in late Middle Pleistocene tephra layers.</figcaption></figure>

the key-outcrop along the road-cut, the hanging-wall presents strongly tectonized Paleogene Scaglia Detritica-type varicoloured clays at the bottom, mantled by a stratified thick nail of slope deposits (carbonate gravels in abundant sandy-loamy matrix) rich in sub-primary and reworked pyroclastic material (tephra sample WCSB-T1 and WCSB-T2, Table 1, see description in section 4.4. And Supplementary text-3, SText-3), faulted against the Mesozoic marine carbonates.

Eastward to this site, the MR fault extends for about 9 km with a N110°-120° strike (Fig. 4), providing both geological-structural (i.e., rock-fault scarps, slickensides) and morphological (i.e., rectilinear fault-slopes) evidence. Eastward (Vitulano area), the MR fault overlaps the other south-dipping splays of the SMG-SM fault (striking N105°-125°) (Fig. 5C and D), in a left-stepping en echelon relationship, with a 2 km step-over. Together, these two south-dipping faults are interpreted as the antithetic Camposauro fault system (ACF in Fig. 4), reaching a length of approximately 13 km and traversing the entire footwall of the Western Calore border fault (Fig. 4).

## 4.2. Highest intensity data-point distribution for the 1688 Sannio earthquake

The 1688 earthquake ranks among the strongest and deadliest in Italian seismic history, with 10,200 casualties (AGS, 2025: 11,639, according to Balaguer, 1697) and a 2500 km² spanning heavy destruction area (9 MCS degree). The event caused extensive collapses even in Naples, the populous capital of the Viceroyalty, located over 50 km away from the epicentre, where several dozen people perished and where, according to a coeval relation, “the fall of the houses is of utmost entity, without being able to gauge their value due to the large number that have suffered” (AGS, 1688a; see details in Varriale, 2021). Jacinto de Arcayna, Agent of the Duke of Gandia in Madrid, writes that in Naples, the earthquake “ruined many buildings of the most prominent, and have brought down some convents, among which they mentioned

that of Jesus”, i.e., the most famous of Naples (AHNB, 1688).

The earthquake was preceded on February 13 and 14 by some well-felt foreshocks in Benevento and then followed by many aftershocks, often accompanied by new collapses and casualties, at least until October. The mainshock itself on June 5 at 201/2 h (Italian time; equivalent to 4:30 p.m. local time: “... las quatro y media de la tarde”: AGS, 1688a) was preceded less than half an hour earlier by another strong shock (Sarnelli, 1688). In the city of Benevento, "within the span of a quickly recited 'Pater Noster'" (ASV, 1688a), the earthquake caused the death of over 1400 of the 7500 residents, in addition to 400 foreigners (ASV, 1688b). Since it was a weekday in rural areas, the death toll was proportionally lower than in cities, as much of the population was working in the fields.

The attribution of macroseismic intensity to each locality has been completely reevaluated based on all available contemporaneous accounts, as well as other previously unknown ones, existing in the State Archive of Naples, the Vatican Secret Archive and other archives both in Spain (Varriale, 2021) and in some localities of the mesoseismic area. In particular, we acquired very useful information from a report commissioned by the Viceroy Francisco de Benavides Dávila y Corella, conde de Santiesteban to Andrea Guerrero de Torres (ASN, 1688) with the scope of evaluating the post-earthquake, best and timely intervention strategy. In this report, the localities affected by the earthquake are divided into 5 provinces (Terra di Lavoro, Principato Ultra, Contado del Molise, Capitanata, Principato Citra, today roughly matching the provinces of Caserta, Avellino, Campobasso, Foggia and Salerno, respectively) and classified into 4 intensity degrees of damage and mortality. As argued by Bruno (2021), although compressed into 4°, the Guerrero de Torres scale truly predates by two centuries the Mercalli macroseismic scale, in which, as used today, the corresponding damage levels are divided into 6° (excluding the 12th, never assigned so far). This report also includes numerous damaged localities, unmentioned in the previously known sources, allowing the extension of intensity datapoints to about 260
from the previous 215 in DBMI15 (2021). Overall, about 130 localities were assigned an intensity Is ≥ 8 MCS degree (destruction >25%), about 50 Is ≥ 9 MCS (destruction >50%), about 20 Is ≥ 10 MCS (destruction >75%), while 5 were assigned an Is 11 MCS degree, corresponding to almost total destruction, with 75% collapses and very high mortality, between 30 and 80% (e.g., Cerreto Sannita, Civitella Licinio and Pietraroja).

The intensity points were subsequently processed with a gridding algorithm that yielded the image in Fig. 6. The most heavily shaken area is approximately contained within a 50-km-long rectangular area, oriented WNW-ESE, extending north of the main course of the Calore River and with an asymmetric areal distribution of intensities, higher in the northern quadrants (northernmost Campania Region, between northern Tammaro and western Calore rivers course).

### 4.3. $^{40}$Ar/$^{39}$Ar dating

The results of all $^{40}$Ar/$^{39}$Ar dating of individual samples are presented as probability diagrams (Fig. 7). Weighted mean age uncertainties are all reported at $2\sigma$, including $\sigma$ uncertainty and were calculated using Isoplot 4.1 (Ludwig, 2003). The details of the results for each sample are described in Supplementary text-2 (SText-2).

### 4.4. Tephra lithology and major-minor element compositions

A full, detailed description of the lithostratigraphic features and major element composition of the sixteen investigated tephra in ECSB and WCSB (Table 1) are provided in Supplementary text-3 (SText-3). The eleven tephra yielding fresh glass suitable for major element composition analyses (Table 1) show a relatively wide compositional variability, which, in the TAS classification diagram, mostly range from trachyte and phonolite fields, with minor compositions falling in the tephri-phonolite and phono-tephrite fields (Fig. 8a). According to the CaO/FeO vs Cl diagram, which allows to discriminate the perityrrenian volcanic sources (Giaccio et al., 2017), the analysed tephra are mainly ascribable to the Campi Flegrei, and few should be related to the Somma-Vesuvius and Roccamonfina sources (Fig. 8b).

### 4.5. OSL results

The age of the sample PV1 (GLL label GdTL-5184) from the top of T1 terrace at Ponte Valentino site (Fig. 2) is 240 ± 20 ka (Supplementary Report SR1). This age provides chronological constraints for the uppermost interval of the fluvial deposits of the T1 terrace and a terminus post quem for the formation of the terrace T1 in the Eastern Calore sub-basin.

## 5. Discussion

### 5.1. Chronological framework of the Calore River valley Quaternary deposits

#### 5.1.1. Tephra correlation

Based on the acquired stratigraphic, geochemical, and geochronological data, the individual investigated tephra collected in ECSB and WBSB, were correlated to well-known and dated or undetermined pyroclastic units or distal tephra. A full and detailed discussion supporting the proposed correlations is provided in Supplementary text-4 (SText-4), while a synthesis is provided in Table 4 and Figs. 9 and 10.

#### 5.1.2. Age of the Quaternary deposits in the Eastern Calore sub-basin and correlation with the western sub-basin

The obtained $^{40}$Ar/$^{39}$Ar and OSL dates from the investigated tephra and sedimentary successions, as well as the inferred dates deriving from the tephra geochemical fingerprinting (Table 4, Figs. 9 and 10, see details in Supplementary SText-3 and SText-4) provide robust geochronological constraints for the ECSB terraced units, allowing their correlation with the corresponding units of the WCSB (Table 5).

**T4** - The youngest terraced unit T4 is well-constrained to the Holocene by the occurrence of the Pomici di Avellino eruption (tephra ECSB-T1) (Table 5; SText-3). It could correspond to the third generation of alluvial fan of the western sub-basin, containing the Neapolitan Yellow Tuff tephra (~14 ka) (Amato et al., 2018) and dated by $^{14}$C at ~7 ka (Magliulo, 2005) (Table 5; Fig. 11). In WCSB, the NYT of ~14 ka (WCSB-T2) and post-NYT, Campi Flegrei tephra of ~9–13 ka (WCSB-T1)

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** MCS-macroseismic intensity distribution of the June 5, 1688 earthquake. White circles, site intensities (σ proportional to $7 \le Is \le 11$ MCS). Cyan-to-purple image colours in the background suggest the areal distribution of shaking. Blue lines, regional active faults: AIF, Aquae Iuliae fault; NMFS, northern Matese fault system; UF, Ufita fault; PGF, Piedimonte-Gioia fault (dashed where inferred). Black lines, rock-fault scarps locally affecting late Upper Pleistocene deposits (carbonate Camposauro Massif), dashed where inferred (WCF, western Calore Fault; ECF, eastern Calore Fault). The southern border of the white rectangle (i.e., seismogenic box) tentatively suggests the upper side of the seismogenic structure. The smaller dotted box is the seismogenic box proposed in Di Bucci et al. (2006). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 7. $^{40}$Ar/$^{39}$Ar probability diagrams for individual layers dated by laser single crystal.</figcaption></figure>

have been recognized also in the, faulted slope-colluvial succession in the hanging-wall of the MR fault at Postiglione section (HW faulted deposits in Table 5). Specifically, WCSB-T2 constrains the base of this succession at ~14 ka, while WCSB-T1 provides a *terminus ante quem* of ~9.2 ka for the middle and upper part of the deposits (Table 5).

**T3** - The terraced unit T3 contains four primary tephra layers (ECSB-T3, ECSB-T6, ECSB-T7 and ECSB-T8) and two volcaniclastic deposits (ECSB-T4 and ECSB-T5), allowing to date it to the interval between ~41

ka and ~28 ka (Table 5), and thus correlate it with the fluvial unit 1b, containing the Campanian Ignimbrite (Filocamo et al., 2021), and the second generation of alluvial-fan, including a tephra of the WCSB dated to 48 ± 7 ka (Amato et al., 2018) (Table 5; Fig. 11).

**T2** - The age of the uppermost interval of the Unit T2 is constrained by Maddaloni and Santa Lucia tephra between ~109 ka and ~101 ka (Table 5). The general litho- and morpho-stratigraphic setting and the chronology of T2 replicate well that of the Upper Pleistocene terraced
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Representative classification diagrams for the investigated tephra in eastern and western Calore sub-basins (ECSB and WCSB, respectively). a) Total Alkali vs Silica (TAS; Le Maitre et al., 2002) classification diagram of the analysed samples from the western and eastern sub-basins of the Val Calore River. b) CaO/FeO vs Cl classification diagram (Giaccio et al., 2017a) showing fields of the peri-Tyrrhenian Quaternary Italian volcanoes. c) SiO₂ vs K₂O/Na₂O classification diagram, showing that tephra layers investigated in this study can all be classified as potassic to ultrapotassic.</figcaption></figure>

deposits outcropping along the orographic right side of the WCSB. Here, a similar fine-grained alluvial deposit containing a tephra dated to 97 ± 25 ka (Di Bucci et al., 2006) overlies the Middle Pleistocene gravel/-conglomerate terraced deposits throughout an evident unconformity surface (Magliulo, 2005). This terraced unit is chronologically equivalent to the first generation of alluvial-fan deposit outcropping on the left orographic side of the western sector of the Calore River valley, containing tephra layers dated to 113 ± 7 ka and 158 ± 6 ka (Amato et al., 2018), the latter corresponding to the Taurano Ignimbrite dated to 160.2 ± 2.0 ka (recalculated from De Vivo et al., 2001) or 159.4 ± 1.6 ka (weighted mean of proximal and distal tephra, Giaccio et al., 2019).

**T1** - The unit T1 in ECSB, the uppermost interval of which has been dated to 240 ± 20 ka, can be reliably correlated to the first order terrace of the WCSB, made of similar conglomerate/gravel lithofacies and containing, in its middle-lower stratigraphic interval, ignimbrite deposits and tephra layers dated to 560 ± 2 ka (Magliulo, 2005), 648 ± 6 ka (Amato et al., 2018) and 674 ± 10 ka (Di Bucci et al., 2006). With this regard, the terraced unit T1 can be dated to the interval ranging between ca. 680 and 240 ka (Fig. 11).

**Ancient fluvial deposits** - Finally, as far as the oldest fluvial deposits (SFL1) recognized in ECSB are concerned, based on the morpho-stratigraphic relationship with the T1 unit and the Pliocene substratum, they could be considered sub-coeval with Laiano Breccias of the WCSB, whose suggested age is Lower Pleistocene (Carannante et al., 2011). However, the recognition of pyroclastic material in the uppermost interval of the Layano breccias (see section 4.1.2.) would suggest that this unit could extend at least to the early Middle Pleistocene when the explosive activity of the peri-Tyrrhenian volcanic system started (e. g., Branca et al., 2023). Consequently, the Laiano breccias could be sub-coeval to the T1 fluvial deposits and represent the slope lithofacies of the same depositional phase.

Summarising, the results of our investigation on the ECSB shows strong analogies with the previously investigated western sector, both in terms of litho- and morpho-stratigraphic succession and chronology, allowing to combine data and proposing an overall framework for the entire Calore River valley. A summary of the correlations of the Quaternary morpho-stratigraphic units in the eastern and western Calore sub-basins is provided in Table 5 and Fig. 11.
<table><caption>**Table 4**
Summary of the correlations of the investigated tephra layers.</caption><thead><tr><th rowspan="2">Sub-basin</th><th rowspan="2">Section</th><th rowspan="2">Investigated tephra</th><th rowspan="2">Correlated tephra</th><th colspan="2">⁴⁰Ar/³⁹Ar or ¹⁴C age (ka)</th></tr><tr><th>This study</th><th>Literature</th></tr></thead><tbody><tr><td rowspan="12">Eastern</td><td rowspan="3">Liderlat</td><td>ECSB-T1</td><td>Avellino Pumices</td><td rowspan="3">3.8 ± 0.3</td><td rowspan="3">3.945 ± 0.01<sup>a</sup></td></tr><tr><td>ECSB-T2</td><td>Undetermined</td></tr><tr><td>ECSB-T3</td><td>Masseria del Monte</td></tr><tr><td rowspan="2">Cancelleria</td><td>ECSB-T4</td><td>Codola reworked</td><td rowspan="2">28.1 ± 0.5<br/>&lt;30</td><td>29.0 ± 0.8<sup>b</sup></td></tr><tr><td></td><td></td><td>29.9 ± 1.5<sup>c</sup></td></tr><tr><td rowspan="3">Apice</td><td>ECSB-T5</td><td rowspan="3">Roccamonfina-CI-Codola reworked</td><td></td><td>30.0 ± 1.5<sup>c</sup></td></tr><tr><td>ECSB-T5</td><td></td><td>30.2 ± 1.5<sup>c</sup></td></tr><tr><td></td><td></td><td>40.0 ± 0.1<sup>d</sup></td></tr><tr><td rowspan="3">Minicozzi 2</td><td>ECSB-T6</td><td rowspan="3">Campanian Ignimbrite</td><td rowspan="3">39.4 ± 0.4</td><td rowspan="3">29.9 ± 1.5<sup>c</sup></td></tr><tr><td>ECSB-T6</td><td>Pre-CI Group D/TM-18-1a-b-c</td></tr><tr><td>ECSB-T7</td><td>Pre-CI Group C/TM-18-1d</td></tr><tr><td rowspan="4">Ponte Valentino</td><td>ECSB-T8</td><td>Santa Lucia</td><td></td><td>40.1–40.3 ± 0.1<sup>e</sup></td></tr><tr><td>ECSB-T9</td><td>Maddaloni</td><td rowspan="4">109.2 ± 0.8</td><td>40.6 ± 0.1<sup>e</sup></td></tr><tr><td>ECSB-T10</td><td>Soccavo 1-Pisani 1-Pigna San Nicola</td><td>101.2 ± 0.8<sup>f</sup></td></tr><tr><td>ECSB-T2</td><td>NYT</td><td>109.3 ± 1.0<sup>f</sup></td></tr><tr><td></td><td></td><td>~13.1–9.2<sup>g</sup></td></tr><tr><td rowspan="3">Western</td><td rowspan="2">Postiglione</td><td>WCSB-T1</td><td></td><td></td><td>14.9 ± 0.4<sup>h</sup></td></tr><tr><td>WCSB-T2</td><td></td><td></td><td>14.3–13.9<sup>i</sup></td></tr><tr><td>Cesine (SOL2)</td><td>WCSB-T3</td><td>OH-DP-0725</td><td></td><td>174.4 ± 5.2<sup>j</sup></td></tr><tr><td></td><td>WCSB-T4</td><td>Vitulazio</td><td></td><td>174.5 ± 0.6<sup>k</sup></td></tr></tbody></table>

## 5.2. Quaternary and active tectonics along the Calore River valley

### 5.2.1. Long-term activity inferred from sedimentary basin architecture

While along both orographic sides of the WCSB, and especially the left one, faint structural evidence of recent tectonic activity are exposed (e.g., Di Bucci et al., 2006; Amato et al., 2018; Filocamo et al., 2021, see section 2.1.2), the ECSB lacks any morpho-structural evidence of active tectonics.

Nevertheless, besides sharing a similar history with the sedimentary and landform evolution, the WCSB and the ECSB show, at the same time, either analogies or differences in terms of fluvial basin infill architecture, which would suggest a similar tectonically-driven evolution (Fig. 12).

In the ECSB, the main depositional phase is represented by the Lower (?)-late Middle Pleistocene (680-240 ka) gravel deposits of the terraced unit T1, related to a braided-stream fluvial system (Fig. 12). This is the most significant stage of the morpho-sedimentary history of this sector of the Calore River basin, during which more than 100 m of gravel accumulated, mainly on the right orographic side of the Calore River (Fig. 12). An analogue, asymmetric architecture characterises the terraced unit T1 in the western Calore sub-basin, which extensively outcrops only along the right orographic side (Fig. 12). The left orographic side of the WCSB is instead characterised by the presence of a >200 m thick succession of the Lower Pleistocene slope deposits (Laiano Breccia, Carannante et al., 2011), which could be either older or be at least partially heteropic with the lower interval of the fluvial deposits of the T1 unit in ECSB (Fig. 12 borehole data in: http://portalesgi.isprambi.ente.it/it/categorie-db/sondaggi). The lack of similar slope deposits along the left side of the ECSB can be straightforwardly interpreted as the result of the different lithotypes of the stratigraphic-structural units forming the pre-Quaternary substratum in the left orographic side of the two sub-basins; i.e., siliciclastic, high-erodible rocks in the eastern

sub-basin, and limestone, more prone to form high reliefs and preserve morpho-structural features, in the western one. Indeed, the presence of the Camposauro limestone relief promoted the development of large detrital cones and alluvial fans only along the left side of the WCSB (Fig. 12).

In the WCSB, such a geometrical configuration can be easily interpreted as an effect of the activity of the bounding fault system of Mt. Camposauro limestone massif (Calore River Fault system of Di Bucci et al., 2006, here termed WCBF), which is structurally partly documented (Fig. 12). Therefore, by analogy, we can assume that a similar tectonic activity drove the asymmetric distribution of the T1 terraced unit along the ECSB as well (Fig. 12).

Indications on the occurrence and geometry of this inferred fault in ECSB are also provided by some borehole evidence (Fig. 12). In fact, the depth of the pre-Quaternary substratum in the borehole 171175 and 171177, few metres aside from each other (Fig. 1D), is 0 m and 40 m below ground level (b.g.l.) respectively (Fig. 3B and 12). Though more distant from each other, in the hole 16866 and 16867 (Fig. 1D), the pre-Quaternary substratum is at 80 m and 0 m b.g.l., respectively (Fig. 3B and 12). In both cases, the elevation differences could be interpreted as due to the displacement of a ESE-WNW trending and NNE dipping normal fault, here termed East Calore Fault (ECF). An estimate of the minimum vertical displacement of the Middle Pleistocene deposits, attributable to the inferred ECF, can be evaluated by considering the difference between the elevation of the base of the terraced unit T1 (~670-240 ka) on the right side, and the minimum elevation of the pre-Quaternary bedrock on the left side of the Calore River valley, which is of the order of ~100 m (Fig. 12).

The second morpho-stratigraphic event, recorded in both ECSB and WCSB, is the deposition of the late Middle-early Upper Pleistocene (158-97 ka) terraced unit T2, which unconformably lays on the T1 unit, following a phase of valley incision (Fig. 11). As seen for the T1 unit, also

<footnote>a ¹⁴C cal. Age BP (Sevink et al., 2011).</footnote>

<footnote>b ⁴⁰Ar/³⁹Ar age (Albert et al., 2019).</footnote>

<footnote>c Monticchio chronology (Wulf et al., 2004).</footnote>

<footnote>d ⁴⁰Ar/³⁹Ar age (Giaccio et al., 2017b).</footnote>

<footnote>e Monticchio interpolated age (Wutke et al., 2015) using for the CI the age of 40.0 ± 0.1 ka (Giaccio et al., 2017b).</footnote>

<footnote>f ⁴⁰Ar/³⁹Ar age (Monaco et al., 2022).</footnote>

<footnote>g Bayesian modelled cal. 14C ages BP (Smith et al., 2011).</footnote>

<footnote>h ⁴⁰Ar/³⁹Ar age (Deino et al., 2004).</footnote>

<footnote>i Bayesian modelled cal.¹⁴C ages BP (Blockley et al., 2008).</footnote>

<footnote>j Lake Ohrid chronology (Leicher et al., 2021); k⁴⁰Ar/³⁹Ar age (ISPRA Geological sheet 430 – Caserta Ovest, in preparation).</footnote>
<figure><img src="image_11.png" /><figcaption>b) VCE-T4 and VCE-T5: Codola Top (~30 ka) + CI (40.0 ± 0.1 ka) + Roccamonfina (undefined)</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>c) VCE-T6, VCE-T7, and VCE-T8: CI/TM-18 (40.0 ± 0.1 ka), pre-CI/TM-18-1a+b+c (~40.2 ka), and pre-CI/TM-18-1d (~40.6 ka)</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>d) VCE-T9: Santa Lucia/TM-24a (101.2 ± 0.8 ka)</figcaption></figure>

<figure><img src="image_14.png" /></figure>

**Fig. 9.** TAS and bi-plot diagrams compare the composition of tephra layers from this study with the literature data. Data source of EPMA-WDS: ECSB-T3, -4, -5, -6, -7, -8, and -9: this study; Masseria del Monte Tuff: Albert et al. (2015, 2019); TM-17a, TM-17b, TM-16a, TM-18-1a, TM-18-1b, TM-18-1c, TM-18-1d: Wulf et al. (2004) and Wutke et al. (2015); Campanian Ignimbrite: Smith et al. (2016); pre-CI Group C and D: Vineberg et al. (2023); Santa Lucia proximal: Monaco et al. (2022); Santa Lucia distal: Wulf et al. (2012) and Regattieri et al. (2015). Data source of chronology: ECSB-T3/MdMT: $^{40}$Ar/$^{39}$Ar age, this study; Codola (top): Monticchio chronology, Wulf et al. (2004); Campanian Ignimbrite (CI): recalculated $^{40}$Ar/$^{39}$Ar age, Giaccio et al. (2017b); TM-18-#: modified incremental Monticchio chronology, Wutke et al. (2015), Santa Lucia: $^{40}$Ar/$^{39}$Ar age, Monaco et al. (2022).

this unit is asymmetrically distributed, being documented only on the right orographic side of the Calore River valley of both sub-basin (Fig. 12). However, while along the left side of the ECSB there is no evidence of deposits dated to the 158-97 ka interval, due to the

differences in the pre-Quaternary stratigraphical-structural setting, in the WCSB they occur as alluvial fan deposits (1st generation, Magliulo et al., 2007; Amato et al., 2018), whose apron area is interrupted by a degraded fault scarp (Magliulo et al., 2007).
a) WCSB-T1: Campi Flegrei Epochs 1 and 2 (Soccavo-1, Pisani-1, Pigna San Nicola; 13.1-9.2 ka)

<figure><img src="image_15.png" /></figure>

b) WCSB-T2: Neapolitan Yellow Tuff (NYT; 14.3-13.9 ka)

<figure><img src="image_16.png" /></figure>

c) WCSB-T3: Lake Ohrid OH-DP-0725 (174.4 ± 5.2 ka)

<figure><img src="image_17.png" /></figure>

d) WCSB-T4: Vitulazio (174.5 ± 0.6 ka)

<figure><img src="image_18.png" /><figcaption>**Fig. 10.** TAS and bi-plots diagrams of the investigated tephra layers compared with literature data. The data source of EPMA-WDS: WCSB-1 and WCSB-2: this study; Pisani-1, Soccavo-1, and Pigna San Nicola-1: Smith et al. (2011); Neapolitan Yellow Tuff: Tomlinson et al. (2012) and Forni et al. (2018). Data source of chronology: NYT: Bayesian modelled cal. $^{14}$C ages BP, Blockley et al. (2008); Campi Flegrei Epochs 1 and 2: Bayesian modelled cal. $^{14}$C ages BP: Smith et al. (2011). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

Similarly to the oldest T1 unit, also the asymmetrical distribution of the T2 unit in the ECSB can be related to the activity of the inferred ECF. Along the right side of the Calore River valley, this fault allowed sediment deposition in the hanging-wall, leaving it suspended and subject to prevailing erosion of the footwall block in the right side of the valley (Fig. 12).

After a relatively short incision phase, which can be constrained between ~75 ka and ~48 ka, i.e., between the upper and lower chronological limits of terraces unit T2 and T3 (Fig. 11), the fluvial sediments of the Late Pleistocene terraces unit T3 (~48-29 ka; Table 5; Fig. 11) were deposited. Also in this case, in the ECSB, the volume of this unit is not equally distributed along the two sides of the Calore River (Table 3;
<caption>**Table 5**</caption>

Summary of the tephrochronological and OSL dating constraints for the eastern and western sectors of the Calore River valley terraced units T1, T2, T3 and T4 and other sedimentary units.

<table><thead><tr><th rowspan="2">Units</th><th rowspan="2">Tephra or OSL sample</th><th rowspan="2">Correlated tephra</th><th colspan="1">Age (ka)</th><th rowspan="2">Correlated tephra</th><th colspan="3">Associated deposits</th></tr><tr><th>ECSB Tephra</th><th>Eastern sub-basin</th><th>Western sub-basin</th><th>Combined data</th></tr></thead><tbody><tr><td rowspan="3">T4<br>HW faulted slope deposits</td><td>ECSB-T1</td><td>Avellino Pumice</td><td rowspan="3">3.8 ± 0,3<sup>a</sup></td><td>3.945 ± 0.01<sup>b</sup></td><td>3.945 ± 0.01<sup>a</sup></td><td>~14<sup>b</sup>-7<sup>i</sup></td><td>~14-4 ka</td></tr><tr><td>ECSB-T1</td><td>Soccavo 1-Pisani 1-Pigna San Nicola</td><td>~13.1-9.2 ka</td><td></td><td>~13-9</td><td>~14-9 ka</td></tr><tr><td>ECSB-T2</td><td>NYT</td><td>14.9 ± 0.4<sup>i</sup></td><td></td><td></td><td></td></tr><tr><td rowspan="7">T3</td><td>ECSB-T2</td><td>Undefined</td><td rowspan="3">28.1 ± 0.5<sup>c</sup></td><td>29.0 ± 0.8<sup>c</sup></td><td>28.1 ± 0.5</td><td></td><td></td></tr><tr><td>ECSB-T3</td><td>Masseria del Monte Tuff</td><td></td><td></td><td></td><td></td></tr><tr><td>ECSB-T4</td><td>CI-Codola</td><td>~30<sup>a</sup></td><td>29.9 ± 1.5<sup>d</sup> 30.0 ± 1.5<sup>d</sup></td><td>≤30 ka</td><td></td><td></td></tr><tr><td>ECSB-T5</td><td>Reworked</td><td></td><td>30.2 ± 1.5<sup>d</sup></td><td></td><td></td><td></td></tr><tr><td>ECSB-T6</td><td>Reworked</td><td></td><td>40.0 ± 0.1<sup>e</sup></td><td></td><td></td><td></td></tr><tr><td>ECSB-T7</td><td>Reworked</td><td></td><td>29.9 ± 1.5<sup>d</sup> 30.0 ± 1.5<sup>d</sup></td><td>≤30 ka</td><td></td><td></td></tr><tr><td>ECSB-T8</td><td>Roccamonfina-CI-Codola</td><td></td><td>30.2 ± 1.5<sup>d</sup></td><td></td><td></td><td></td></tr><tr><td rowspan="4">T2</td><td>ECSB-T9</td><td>Campanian Ignimbrite</td><td rowspan="2">39.4 ± 0.4<sup>a</sup></td><td>40.0 ± 0.1<sup>e</sup></td><td>40.0 ± 0.1</td><td></td><td></td></tr><tr><td>ECSB-T10</td><td>Pre-CI Group D/TM-18-1a-b-c</td><td>40.1-40.3 ± 0.1<sup>f</sup></td><td>40.1-40.3 ± 0.1</td><td></td><td></td></tr><tr><td>ECSB-T8</td><td>Pre-CI Group C/TM-18-1d</td><td></td><td>40.6 ± 0.1<sup>f</sup></td><td>40.6 ± 0.1</td><td></td><td></td></tr><tr><td>ECSB-T9</td><td>Santa Lucia</td><td>109.2 ± 0.8<sup>e</sup></td><td>101.2 ± 0.8<sup>g</sup></td><td>101.2 ± 0.8</td><td>~158<sup>h</sup>-113<sup>h</sup></td><td>~158-97</td></tr><tr><td rowspan="3">FW faulted slope deposits<br>T1</td><td>ECSB-T10</td><td>Maddaloni</td><td></td><td>109.3 ± 1.0<sup>g</sup></td><td>109.3 ± 1.0</td><td>97<sup>i</sup></td><td></td></tr><tr><td>WCSB-T3</td><td>OH-DP-0725</td><td></td><td>174.4 ± 5.2<sup>na</sup></td><td></td><td></td><td></td></tr><tr><td>WCSB-T4</td><td>Vitulazio</td><td></td><td>174.5 ± 0.6<sup>na</sup></td><td></td><td></td><td></td></tr><tr><td></td><td>PV1</td><td>–</td><td></td><td></td><td>240 ± 20<sup>o</sup></td><td></td><td></td></tr><tr><td></td><td>Pyroclastic deposits</td><td>–</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

Fig. 12). Along the right side of the ECSB, the unit T3 has a total thickness >20 m, with its lowermost exposed stratigraphic interval occurring at an elevation ~140 a.s.l. (e.g., Liderlat site, Fig. 3B), but likely reaching a lower elevation in subsoil. Along the left side, instead, its base outcrops at an elevation of ~150 m, exposing a maximum thickness of only ~10–15 m (Fig. 12). The minimum vertical displacement imputable to the inferred ECF during the last ~48-29 ka, can be estimated to ~20 m, representing the minimum difference in elevation of the base of the unit T3 in the right (e.g., Liderlat section) and left (e.g., Cancelleria section) orographic sides of the ECSB (Fig. 12). A further indication of the difference in volume accommodation for the fluvial-alluvial sediment deposition along the left and right sides of Calore River in ECSB, potentially ascribable to tectonic factors, is provided by the nature and stratigraphic setting of the sub-coeval tephra and volcaniclastic material deposited in T3 sediments along the orographic sides. In fact, while along the right orographic side four tephra dated between ca. 41 and 28 ka occurs as distinct primary deposits, each other well-separated by relatively thick intervals of alluvial-fluvial deposits, on the left orographic side, the sub-coeval tephra deposits occur as an undifferentiated and condensed volcaniclastic level including material

from multiple eruptions. Thus, over the same time-span of ca. 10 kyr, while in the right side, because of the tectonic subsidence, several metres of fluvial-alluvial deposits accumulated, allowing the deposition of distinct tephra layers, in the left side, due to the lack of tectonic accommodation, reworking of volcanic material from multiple eruptions, spanning at least 10 kyr, prevailed.

Summarising, the ECSB and the WCSB would seem to have undergone a similar tectonic-sedimentary evolution, driven by the Middle to Late Pleistocene activity of the Calore Fault system, either inferred (ECF, in the ECSB) or observed (WCF, in WCSB) that, except for the differences related to the inherited, pre-Quaternary structural settings, resulted in a similar morpho-sedimentary architecture in the two sub-basins (Fig. 12). Overall, the sedimentary basin geometry of both eastern and western sub-basin is that of half-grabens, with master, E-W to ESE-WNW trending and N- to NNE-dipping, normal fault systems and minor, S- to SSW-dipping antithetic faults (Fig. 13). The estimated length of the inferred ECF is ~14 km, while the observed WCF is ~22 km-long (Figs. 3 and 4).

<footnote>a $^{40}$Ar/$^{39}$Ar age (this study).</footnote>

<footnote>b $^{14}$C cal. Age BP (Sevink et al., 2011)).</footnote>

<footnote>c $^{40}$Ar/$^{39}$Ar age (Albert et al., 2019).</footnote>

<footnote>d Monticchio chronology (Wulf et al., 2004).</footnote>

<footnote>e $^{40}$Ar/$^{39}$Ar age (Giaccio et al., 2017b).</footnote>

<footnote>f Monticchio interpolated age (Wutke et al., 2015) using for the CI the age of 40.0 ± 0.1 ka (Giaccio et al., 2017b).</footnote>

<footnote>g $^{40}$Ar/$^{39}$Ar age (Monaco et al., 2022).</footnote>

<footnote>h $^{40}$Ar/$^{39}$Ar age of tephra or correlated tephra (Amato et al., 2018).</footnote>

<footnote>i $^{14}$C and $^{40}$Ar/$^{39}$Ar age (Magliulo, 2005).</footnote>

<footnote>j $^{40}$Ar/$^{39}$Ar age (Di Bucci et al., 2006).</footnote>

<footnote>k Bayesian modelled cal.$^{14}$C ages BP (Smith et al., 2011).</footnote>

<footnote>l $^{40}$Ar/$^{39}$Ar age (Deino et al., 2004).</footnote>

<footnote>m Bayesian modelled cal.$^{14}$C ages BP (Blockley et al., 2008), n Lake Ohrid chronology (Leicher et al., 2021).</footnote>

<footnote>n $^{40}$Ar/$^{39}$Ar age (ISPRA Geological sheet 430 – Caserta Ovest, in preparation).</footnote>

<footnote>o OSL dating (this study).</footnote>
<figure><img src="image_19.png" /><figcaption>**Fig. 11.** Summary of the geochronological constraints available for the main sedimentary units from the eastern and western Calore sub-basins. For details and references for literature data, see Table 5.</figcaption></figure>

### 5.2.2. Short term evidence and paleoseismic insights

One of the most striking evidence of post-Lower Holocene tectonic activity of the WCBF system has been acquired aside the main fluvial Calore basin at the NW tip of the Madonna del Roseto south-dipping fault (Fig. 3A and 13a; Postiglione site), which offers the opportunity to obtain valuable paleoseismological data (Fig. 13c-c'). The most significant feature is the presence of a faulted, 10-cm-thick sub-horizontal layer of pumices confidently identified as Neapolitan Yellow Tuff (NYT, ~14 ka; WCSB-T2; Table 5, see details in Supplementary text-4, SText-4), which is overlain by a wedge-shaped colluvium (i.e., colluvial wedge: CW in Fig. 13c'), consisting of reworked tephra, from the same WCSB-T2/NYT layer, mixed with sandy deposits and gravels.

The geometry and composition of the WCSB-T2/NYT layer suggest the occurrence of a surface faulting event that took place while the NYT tephra had recently mantled the surrounding slope. The fault rupture produced a coseismic step along the whole slope, allowing for the erosion and transport of the NYT ash from the footwall to the hanging-wall, with the formation aside the slickenside of a CW that buried and preserved the primary tephra in the downthrown side.

The faulting event, and thus the resulting earthquake, can be dated to a moment immediately following the emplacement of the NYT, around 14 ka. The height of the CW (~0.7 m) suggests a slightly higher fault offset, certainly due to a Mw > 6.5 earthquake, especially considering that the Madonna del Roseto fault is an antithetic fault of the main WCBF. Subsequently, we have direct evidence of at least one other coseismic faulting event, as not only the CW and the primary NYT layer are re-faulted but also the overlaying slope succession containing WCSB-T1 tephra, dated to about 9 ka (Table 5), is displaced by a fault-splay, approximately 20 m east of the main slickenside.

Considering that the minimum height of the cumulative rock fault scarp over the last 14 kyr is 7 m (Fig. 13c-c'), which should be added to the retreated fault scarp carved in slope deposits, and tentatively assuming that the coseismic rupture has roughly been around 0.7 m in the other past surface rupture events, it is possible that, at least 10 earthquakes of comparable magnitude have occurred since 14 ka, with an average return period of 1400 years.

We can hypothesise that the last two could be the earthquakes of 1688 and 1456 (southern mainshock). The historical account supports this hypothesis (AGS, 2025), since in one of the hamlets comprising Vitulano, the one crossed by the S. Maria in Gruptis-S. Mennato fault

system (i.e., Casale dei Fuschi), the 1688 earthquake caused a coseismic rupture of 0.4 m in width and considerable depth, displacing the entire road.

Another key site with high potential for future paleoseismological investigations is the Cesine site, located only 0.5 km from the Postiglione site (Fig. 14a). As mentioned above, in this site we have found evidence for recent displacements of both slope debris and colluvia - faulted and dragged along the rock-fault scarp in the hanging-wall - and an amazing slope sequence in the footwall hosting the Vitulazio pumice fall (WCV-T4) and another slightly younger Middle Pleistocene tephra level (VCW-T3, Fig. 13b', see Supplementary SText-3 and SText-4), all offset by the same fault. Although we have not yet obtained permission for excavating paleoseismic trenches yet, the presence of faulted late ~175 ka old deposits in the footwall and the proximity of the Madonna del Roseto fault suggest the possibility of synchronous coseismic activation of the main synthetic and antithetic faults during the same earthquakes. This would suggest that in 1688, and probably earlier, the entire northern Camposauro fault system ruptured together with the largely hidden western Calore Valley fault.

### 5.3. Tectonic model and potential seismogenic source of the Mw ~7.0 1456 and 1688 Sannio earthquakes

The striking similarities in the sedimentary basin architectures and general morpho-sedimentary settings of the western and eastern Calore River sub-basins suggest a similar and coeval tectonic evolution. Given the Holocene activity of these faults, this also implies the presence of a common seismogenic structure at depth.

Although not easily traceable on a map, the general trends of the two active structures between the localities of Castelpoto and Ponte localities roughly represents a right step-over zone between their adjoining eastern and western tips (Fig. 14a). This geometry can be thus interpreted as a transfer zone (e.g., Gupta and Scholz, 2000) that accommodates the extension between ECF and WCF, consistent with the observed distribution of the Quaternary deposits across both sub-basins. Specifically, between Castelpoto and Ponte, where the Calore River flows through a relatively narrow and deep gorge (Fig. 14a), these deposits are absent, as the valley is incised into pre-Quaternary rocks. In this area, we also observed a clockwise rotation of the faults affecting the NE corner of the Camposauro Massif (WCF system), which strike
## Eastern Calore sub-basin

<figure><img src="image_20.png" /></figure>

## Western Calore sub-basin

<figure><img src="image_21.png" /><figcaption>**Fig. 12.** Schematic cross sections (not to scale) of the eastern and western sectors of the Calore River valley.</figcaption></figure>

NNW-SSE, as shown by the structural analyses in Fig. 4. This rotation can be interpreted as a linking structure, the transfer zone between the two adjoining segments of the ECF and WCF (Fig. 14a).

The hypothesis of a transfer zone between the two master faults of WCBF and ECBF, implying a common seismogenic structure at depth, is consistent with the seismic source obtained from the highest intensity data points of both the 1456 (southern mainshock) and 1688 earthquakes (Fig. 14b). In both cases the “butterfly”-shaped intensity distribution (Figs. 6 and 14b) occupies the hanging-wall of the two proposed NNE-dipping structures that bound the southern side of the two sub-basins. This is evident in the two areas of maximum intensities focused to the NE and NW of Benevento (Paduli and Cerreto lobes in

Fig. 14b). The southern border of the seismogenic box enveloping the highest intensities (white rectangle in Fig. 6) indicates the approximate location of the two hidden or semi-hidden master faults in the ECSB and WCSB suggesting a 45-km-long, N295°- striking seismogenic box (sensu Gasperini et al., 1999). According to empirical relationships between Mw and normal fault length (Galli et al., 2008; Leonard, 2010; Thing-baijam et al., 2017; Wells and Coppersmith, 1994; Wesnousky, 2008), the associated earthquake could have a Mw~7.0, which is the value attributed to the 1688 event in the Italian seismic catalogue. For the sake of completeness, Fig. 6 presents also the trace of the seismogenic source proposed in Di Bucci et al. (2006; dotted box), which does not fit the highest intensity distribution and is too short (30 km) for a Mw 7.1
<figure><img src="image_22.png" /></figure>

**Fig. 13.** Sedimentary, tectonic and paleoseismological features along the Western Calore Border Fault (WCBF) and Madonna del Roseto antithetic fault (MR) at Postiglione and Cesine sites (SOL1 and POS1 in Fig. 2). **a)** North-south striking geological section between the Cesine and Postigione sites where the main WCBF and its footwall antithetic structure MR fault offset Late Pleistocene deposits. Both faults likely ruptured together in 1456 and 1688, generating the two Mw 7 earthquakes. **b)** View looking west of the WCBF, bounding the Camposauro massif, at Cesine locality, west to Solopaca. Here the fault scarp is carved both in the Cretaceous marine carbonates and in the slope deposits (slope debris and colluvium), rich in late Middle Pleistocene tephra layers, as shown in detail in panel **b'**. **c)** along-strike view of the Madonna del Roseto (MR) antithetic fault offsetting Late Pleistocene-Holocene slope deposits. **c')** Details of the colluvial wedge emplaced after the surface faulting occurred around ~14.1 ka. Note the primary Neapolitan Yellow Tuff (NYT) buried by the colluvial wedge that, in turn, is made by the reworked portion of the tephra itself. Both are successively re-faulted in later paleoearthquakes. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

earthquake, according to the aforementioned empirical relationships.

This structural framework is confirmed and strengthened by the
sparse evidence of surface faulting

recognized along the fault strands affecting the carbonate Campo-
sauro Massif, providing an open window into the active tectonics of
WCBF. The faulting of 175-to-9 ka slope deposits, coupled with the 1688
accounts of surface breaks along the faults, serves as compelling evi-
dence of the region's active tectonics and its significant seismicity.
Pending permission to open paleoseismological trenches, our pre-
liminary data indicates the occurrence of at least 10 paleoearthquakes
with Mw ~7.0 since 14.5 ka. The first occurred slightly after 14 ka, the
penultimate likely in 1456 and the most recent certainly in 1688.

Finally, regarding possible mutual activation of the different faults
(e.g., through Coulomb static stress transfer: CST), observing the

distribution of destructive earthquakes in the region, it seems that in
1456, the southern mainshock, released by the Calore faults discussed
here, triggered the N-Matese fault system (Bojano Basin), which caused
a second mainshock of similar magnitude. In this case, the rupture of the
first mainshock might have had a directivity towards the NW quadrants,
thereby explaining the increase in CST towards the Bojano Basin (e.g.,
Galli, 2024). Conversely, the 1688 earthquake could have triggered the
1702 earthquake (Mw 6.3), but more significantly, the 1732 Ufita
earthquake (Mw 6.6), further to the SE. In this case, the rupture direc-
tion may have been towards the SE, delaying or at least not shortening
the rupture time of the subsequent earthquake associated with the
N-Matese fault, which finally occurred in 1805 (Mw 6.6). Assuming that
stress and strain have remained constant throughout historical times,
the CST can partially explain the variable return times of the Apennine
<figure><img src="image_23.png" /><figcaption>**Fig. 14.** 3D Structural model of the eastern and western Calore Basin fault system (a) and areal distribution of the MCS site intensities of the 1456 and 1688, with respect to the East and West Calore Fault (ECF and WCF) systems (b). In white, the path of the Calore River flowing westward.</figcaption></figure>
fault-ruptures.

# 6. Summary and concluding remarks

Seismic hazard assessment requires accurate identification, tracing, and characterization of fault systems responsible for significant historical earthquakes. In the seismotectonic framework of the southern Apennines, despite the occurrence of some of the strongest earthquakes of the whole peninsula, the highly erodible lithotypes that form the mountain belt often fail to preserve the morpho-structural imprints of tectonic activity. This makes the trace of the seismogenic faults elusive and their recognition a challenge. Consequently, the seismogenic sources of many large, destructive historical earthquakes in this region remain vague or poorly defined.

To fill this knowledge gap, we have applied a multi-scale, transdisciplinary approach to the study of the eastern and western Calore River basin (ECSB and WCSB), with the aim of uncovering the unknown source of the ~Mw 7.0 Sannio earthquakes of 1456 and 1688. Our approach integrates i) detailed sedimentary facies analysis, ii) geomorphological and iii) structural analysis, iv) a review of macroseismic intensity distributions from archival sources, and v) paleoseismological observations from existing exposures. Our analyses are robustly supported by $^{40}$Ar/$^{39}$Ar and OSL dating, and tephrochronological analysis.

Despite strong morpho-structural differences, our investigations indicate striking analogies in terms of morpho-stratigraphic settings and chronology of the overall long-term morpho-sedimentary evolution of the eastern and western sectors of the Calore River basin, suggesting a common tectonic driving source.

In the ECSB, where direct morphostructural evidence of active tectonics is completely hidden, the geomorphological features of the terraced units and the general architecture of the Middle Pleistocene to Holocene fluvial basin infill - well-constrained by the acquired geochronological data - provided clear indications of recent tectonic deformation. This is attributable to the activity of a hidden, ~14 km-long and WNW-ESE trending fault system, here termed East Calore Fault (ECF), which lies roughly along the axis of the Calore River valley.

In the WCSB, where the presence of the Camposouro limestone massif facilitates the detection of morphostructural clues of recent tectonic activity, our investigations allowed us to map the discontinuous trace of a normal, active fault system over ~22 km, here termed Western Calore iFault (WCF). In addition, paleoseismological inferences in existing exposures allowed for the first time the detection of a post-14 ka coseismic surface rupture event with displacement >0.7 m and decametric offset of post-9 ka sediments. This finding suggests the occurrence of several strong Holocene earthquakes, with associated surface rupturing and an overall recurrence time of ca. 1400 years. Based on their mesoseismic areas, the two more recent earthquakes might correspond to the Mw ~7.0 Sannio events of 1456 and 1688. The envelope of the ECF and WCF is about 45 km long, with an overall geometry interpretable as a complex structural system including a transfer zone, with the link segment represented by the clockwise rotated southeastern branch of the WCF, which is fully compatible with the magnitude and MCS-macroseismic intensity distribution of the 1688 and 1456 earthquakes. The identification and precise mapping of the putative source of these large earthquakes will allow future paleoseismological investigations to be carried out, which could provide definitive evidence of coseismic surface ruptures during both earthquakes, and similar earlier events.

Finally, from a broader methodological perspective, this study highlights the need for an integrated, trans-disciplinary approach, supported by a robust geochronology, to detect the seismogenic source of large earthquakes in a morphostructurally elusive setting.

## CRediT authorship contribution statement

Vincenzo Amato: Conceptualization, Methodology, Investigation,

Project administration, Resources, Data curation, Writing – original draft, Writing – review & editing, Visualization. **Sabatino Ciarcia:** Conceptualization, Investigation, Data curation, Writing – review & editing, Visualization. **Paolo Galli:** Conceptualization, Methodology, Investigation, Data curation, Writing – original draft, Writing – review & editing, Visualization. **Domenico Cicchella:** Conceptualization, Investigation, Writing – review & editing, Funding acquisition. **Antonio Galderisi:** Methodology, Investigation, Data curation, Writing – review & editing, Visualization. **Lorenzo Monaco:** Investigation, Formal analysis, Writing – original draft, Writing – review & editing, Visualization. **Giada Fernandez:** Investigation, Data curation, Writing – review & editing, Visualization. **Roberto Isaia:** Investigation, Writing – review & editing. **Sebastien Nomade:** Investigation, Data curation, Writing – review & editing, Visualization. **Alison Pereira:** Investigation, Data curation, Visualization, Writing – review & editing. **Biagio Giacchio:** Conceptualization, Methodology, Investigation, Project administration, Resources, Data curation, Writing – original draft, Writing – review & editing, Visualization, Funding acquisition.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

This work was partially supported by the Italian MUR, PRIN 2022 (project COMET 2022; grant 2022MS9KWR, B. Giaccio coordinator) and by French National program LEFE (Les Enveloppes Fluides et l'Environnement) in the frame of the ICARE project (2022-2024, A. Pereira coordinator). We are grateful to: Bruno Comentale who shared with us his field-notes on the faults of the Camposouro Massif; to Gaia Bruno, Bruno Varriale and Luciano Frasso for sharing as archive documents on the 1688 event. Edoardo Peronace helped us in surveying the Cesine site.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.quascirev.2025.109282.

## Data availability

All data and/or code is contained within the submission.

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