**2. Geological and Geomorphological Setting of the Campi Flegrei**

The Campi Flegrei is a volcanic area placed at the border of the Campana Plain, a wide tectonic depression occurring along the Tyrrhenian, inner, flank of the Southern Apennines ([28] and reference therein). The Southern Apennines are a NE-vergent fold and thrust belt formed by the collision of the Eurasian and African plates [29,30]. Tectonic depressions along the inner sector of the mountain belt include large peri-Tyrrhenian grabens formed because of the extensional tectonics due to the opening of the Tyrrhenian back-arc basin [31,32]. Among these grabens, the Campania Plain is the largest one. The sedimentary filling of the Campania plain consists of ~3000 m of marine, transitional, and continental deposits, with abundant volcaniclastic deposits produced by both Vesuvius and the Campi Flegrei [28,33,34]. The study area falls within the Campi Flegrei (Figure 1), which is a volcanic field placed in a resurgent caldera (Figure 2).

**Figure 1.** Geolithological map of the Campi Flegrei (modified from [10]). Faults are derived from the Geological Sheet 447—Napoli of the Geological Map of Italy at scale 1:50,000, CARG Project [35]. Dashed rectangle indicates location of the Coroglio-Trentaremi sea cliff shown in Figure 3.

> The time at which volcanic activity in the Campi Flegrei began is unknown and the oldest outcropping volcanic units are ~60 ka old [36]. The caldera, which is quasi-circular and whose diameter is ~8 km long (Figure 2), formed because of collapses during the two strongest eruptions, which are related to the Campanian Ignimbrite (hereinafter CI, ~39 ka old [37–39]) and the Neapolitan Yellow Tuff (hereinafter NYT, ~15 ka old [12]). The CI eruption emplaced ~300 km<sup>3</sup> of pyroclastic fall and flow deposits [40], which have been covered by younger volcanoclastic units. The CI outcrops only in a few sectors of the Campi Flegrei, e.g., at the base of the Camaldoli hill, at Cuma and at the base of the Monte di Procida hill (Figure 1). The NYT eruption extruded at least 40 km<sup>3</sup> of pyroclastic fall and flow deposits that accumulated at the caldera boundaries, thus forming the slopes that limit the entire Campi Flegrei (Figure 1) [10]. Moreover, the hills bounding the Campi Flegrei caldera are mainly formed by the NYT; the thickness of these may exceed several tens of meters, they are blanketed by younger pyroclastic fall deposits, and their age is constrained at 15 ka [35]. Volcanic activity after the NYT eruption has been confined within the caldera and consisted of hydromagmatic phenomena with occasional Plinian and effusive events (which are evidenced by the diffuse presence of post-NYT fall deposits and by local outcrops of lavas). Consequently, several monogenic vents formed, including tuff

rings, tuff cones, cinder cones and lava domes (Figure 2) [41–45]. The last volcanic event in the Campi Flegrei occurred in 1538 AD with the formation of Monte Nuovo volcano [46,47] (Figures 1 and 2).

**Figure 2.** Geomorphological map of the Campi Flegrei (modified from [10]).

The Campi Flegrei's history is also characterized, besides volcanic activity, by ground motions in the form of episodes of uplift and subsidence during bradyseismic crises [48,49]. These episodes range from several meters to several tens of meters and continue nowadays (Figure 2) [50–53]. Ground motions are testified to, above all, by the raised marine terrace of La Starza (Figure 2), near Pozzuoli [43,50,51,54], and by submerged archaeological ruins near Baia, Nisida, La Gaiola and Castel dell'Ovo [19,51,52,55,56] (Figure 2).

The complex volcanic history of the Campi Flegrei has influenced its geomorphological setting, which results in a hilly landscape mainly modelled by slope, volcanic and coastal processes. Elevation within the Campi Flegrei spans from 0 m a.s.l. to 460 m a.s.l., with the highest peak corresponding with the Camaldoli hill (Figure 2). Slope processes include landslides affecting the volcanic slopes that often cause severe damage to people and buildings [57]. Volcanic landforms are clustered within the caldera's inner slope and consist of tuff rings, tuff cones, cinder cones and lava domes formed by volcanic events in the last 15 ka ([10] and reference therein).

Coastal landforms consist of both sandy and rocky coasts. Sandy coasts occur near the main alluvial plains, e.g., the Sebeto and the Fuorigrotta-Bagnoli plain (Figure 2) and suffered man-made intervention and strong urbanization. Sandy beaches are also present to the west of the Campi Flegrei where they still preserve remnants of beach-ridge and pass, towards the inner side, to coastal lakes (e.g., Fusaro lake, Miseno lake and Lucrino lake; Figure 2). Rocky coasts form high and steep sea cliffs occurring near Miseno Cape and around Posillipo. Sea cliffs are cut in the pre-NYT pyroclastics, in the NYT and in the post-NYT pyroclastics. In addition, sea cliffs are dissected by a dense network of faults and fractures, mainly NE-SW and NW-SE trending [11], which may act as predisposing factors for rock falls. Sea cliff instability is also enhanced by diffuse extraction of tuff deposits, since Roman times, which results in anthropic cavities at the base of most of the sea cliffs [10]. Rocky coasts include our study area, which is a sector of the Posillipo coastline, between the Coroglio sea cliff, to the west, and the Trentaremi bay and La Gaiola islet, to the east (Figures 2 and 3).
