*4.3. Morphostructural Analysis and Failure Mechanism*

Faults and fractures trends have been analyzed by combining a morphostructural analysis of Google Earth orthophotos and detailed scale topographic maps with field surveys aimed at collecting structural data in seven geo-mechanical stations.

Morphostructural lineaments along the Posillipo coast mainly trend N20, N45 and N135 (Figure 11A). Moreover, the more diffuse trends are the N20 and N45, which result to be parallel to the fault that bound the Posillipo promontory towards the NW, i.e., towards the Bagnoli-Fuorigrotta plain [35] (Figure 1).

In addition, NW-SE trending faults and fractures have also been recognized in most of the structural stations along the Posillipo sea cliff, in the area between the Coroglio promontory and the Trentaremi bay (structural stations A to E in Figure 11B). The only exceptions are the structural stations F and G, near the Trentaremi bay, which also record some N-S trend. Faults and fractures measured in all the structural stations have high dip angles that range between 75◦ and 90◦.

The diffuse presence of both NE-SW and NW-SE trending faults and fractures is consistent with data from Vitale and Isaia [11], who performed detailed structural analysis in the Campi Flegrei caldera, and suggested that these trends are the prevalent ones.

Overall data from morphostructural analysis and from the structural stations point out the occurrence of a pervasive system of sub-vertical, mainly NE-SW and NW-SE trending faults and fractures that affect the tuff deposits. These systems also cause the formation of rupture surfaces from which rock-falls originate, thus conditioning the stability of the Coroglio-Trentaremi sea cliff.

**Figure 9.** Landsliding along the Coroglio-Trentaremi sea cliff. Red arrows indicate landslide bodies, whereas yellow arrows indicate detachment niches.

**Figure 10.** Beating fractures and rock-falls niches along the Coroglio-Trentaremi sea cliff. Unstable fractures, dihedrons and pinnacles set on high angle systems are well evident.

**Figure 11.** (**A**) Morphostructural lineaments (red lines) along the Coroglio-Trentaremi sea cliff plotted on Google Earth orthophoto. Labels A to G indicate the location of the structural stations. The rose diagram to the right of panel A shows the direction of all the morphostructural lineaments, the faults and fractures shown in the rose diagrams of panel B. (**B**) Rose diagrams showing the direction of the morphostructural lineaments and the direction of the faults and fractures measured in the single structural stations along the Coroglio-Trentaremi sea cliff. Roses represent the absolute number of data.

To define the failure mechanism of the tuffaceous cliff we analyzed field data from seven structural stations, whose locations are shown in Figure 11. All the structural stations highlight the occurrence of a dense net of fractures that, in many cases, allow us to identify several low-equilibrium sectors in the sea cliff. More specifically, we recognized five systems of fractures and bedding whose dip-direction and dip are listed in Table 3.

**Table 3.** List of fracture systems (K1 to K5) along the Coroglio-Trentaremi sea cliff with indication of the dip-direction (α) and dip (β). Bedding (Ks) and sea cliff front trend are also reported. Structural stations' locations are reported in Figure 11.


The intersection between fracture systems and bedding allows us to hypothesize different failure mechanisms along the Coroglio-Trentaremi sea cliff, which are listed in Table 4. To synthesize, structural analysis of the sea cliff points out a high susceptibility to landsliding of several tuffaceous blocks by means of rock-fall and toppling. Systems of fractures are 5 to 7 m spaced and intercept the bedding, thus isolating large rocky dihedrons and pinnacles whose volume may exceed some hundreds of cubic meters (Figure 10). This volume estimation is confirmed by measurements of landslide niches' dimensions, that may reach values of 500 m3, and of large blocks within the landslide bodies.

**Table 4.** Failure mechanism proposed for each structural station.

