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Volcano‐Tectonic Setting of the Pisciarelli Fumarole Field, Campi Flegrei Caldera, Southern Italy: Insights Into Fluid Circulation Patterns and Hazard Scenarios

Phreatic and hydrothermal eruptions are small energetic explosive events that generally have few to no precursors and represent a considerable hazard in tourist and urban areas. At the Campi Flegrei caldera, these events have occurred at the Solfatara volcano and have likely occurred at the nearby Pisciarelli site, where the most powerful hydrothermal phenomena are located. Here, increased hydrothermal activity has caused relevant morphological changes that has led local authorities to deny access to the site. Stratigraphic, structural, and geophysical investigations have allowed us to reconstruct the volcano‐tectonic setting of the area. In particular, we have recognized a fault system and related damage zones that act as the preferred pathway for hydrothermal fluids in the caldera. At the surface, these faults control the migration and/or accumulation of deep‐seated gases into the subsoil and the formation of fumaroles and mud pools. We have recognized two main fault systems with different ages that show variable displacements. The electrical anomalies identified by electrical resistivity tomography further highlight the main fault pattern and show the interplay between volcano‐tectonic structures and fluid circulation. Host rocks and fault zones may be involved in self‐sealing processes and/or rock failure phenomena capable of modifying the fluid pathways and establishing favorable conditions, leading to overpressure and/or rapid decompression of fluids and triggering an explosive event. Furthermore, stratigraphic mapping shows fossil mud pool sediments embedded in an old debris flow located above the modern hydrothermal system. This implies that they were at a higher elevation when they formed. The morphotectonic evolution and intense rock alterations in the area could promote further landslide episodes, producing debris‐flow deposits that can cover the active area and possibly trigger hydrothermal/phreatic events.

Frequently, phreatic eruptions are generated at very shallow volcanic levels and consist of short-lived and impulsive explosions (Gallagher et al., 2020;Montanaro, Cronin, et al., 2020), spreading a mixture of acidic gases and hot pyroclastic materials on areas reaching square kilometers around the vent; this has been reconstructed, for instance, for Solfatara eruptive activity (Isaia, Vitale, Di Giuseppe, et al., 2015). It follows that the study of phreatic eruptions is mandatory for defining volcanic hazards. With this aim, crucial geological features need to be assessed. They include (1) the characterization of the main structures that determine the rise of liquids and gases in the emission zones, (2) their interconnection, which favors gas accumulation or leakage, and (3) the possible interactions of these structures with shallow water reservoirs and the hydrothermal system. All these elements, together with the geological and physical characterization of the host rocks and the geochemical definition of fluids, contribute to a full description of the volcano-tectonic framework of the fumarolic-hydrothermal emission areas. The description of these features also aids in the understanding of the evolution and the possible abrupt change within the volcanic system. Furthermore, the reconstruction of the structural architecture of these active sectors is fundamental for interpreting geochemical and deformation monitoring data and the development and management of the monitoring networks themselves.
With this in mind, we performed multidisciplinary investigations to define the structural setting of the Pisciarelli fumarolic-hydrothermal area located on the outer eastern flank of the Solfatara volcano within the Campi Flegrei caldera (Figure 1). This caldera sector hosts several fumarole vents, thermal springs, and boiling mud pools, both within the Solfatara crater and along its flanks. Furthermore, this area is characterized by a dense network of fractures and faults, which has allowed the emplacement of lava domes and the formation of an extensive hydrothermal system involved in phreatic and small hydrothermal explosions (Isaia, Vitale, Di Giuseppe, et al., 2015;Montanaro, Mayer, et al., 2017). Over the last few years, the Pisciarelli site has been characterized by significant morphological variations in the emission zone, geochemical characteristics of the fluids, and some mud emission episodes (INGV, 2020;Tamburello et al., 2019). In this study, we produced detailed volcanological and structural maps through stratigraphic and structural surveys. We performed geophysical investigations to characterize the fumarolic field structure consisting of electrical resistivity tomography (ERT) surveys. This technique allows for imaging and reconstruction of the shallow, subsoil fluid distribution, down to a depth of approximately 100 m. Furthermore, by identifying faults on the surface and their continuation at depth through the study of electrical resistivity anomaly patterns, some hypotheses on the interplay between volcano-tectonic structures and hydrothermal activity have been proposed. Structural and geophysical results may help to evaluate the present state of this active area, which is presently considered among the most hazardous within the Campi Flegrei caldera, particularly for the possible occurrence of phreatic/hydrothermal explosions (e.g., Isaia, Vitale, Di Giuseppe, et al., 2015).
In the last 15 kyr, Campi Flegrei experienced alternating uplift and subsidence episodes, which mainly affected its central sector. Generally, the uplift of the ground, reaching several tens of meters, was accompanied by seismovolcanic events that were frequently coupled with volcanism (e.g., Isaia, Vitale, Marturano, et al., 2019;Marturano et al., 2018;Vitale et al., 2019). A ground uplift of approximately 15 m preceded the Monte Nuovo eruption (Di Vito, Acocella, et al., 2016), and ground movements (called bradyseism) on the order of 1-10 m have been detected since Roman times (more than 2,000 years ago; e.g., Bellucci et al., 2006;Parascandola, 1947) and have continued to the present (Del Gaudio et al., 2010;INGV, 2020). Recent structural studies in the Campi Flegrei caldera, both inland (Isaia, Vitale, Di Giuseppe, et al., 2015;Vitale & Isaia, 2014;Vitale et al., 2019) and offshore (e.g., Natale et al., 2020), have shown that faults are located close to volcanic vents and caldera rims. These structures are associated with both eruptive and ground deformation activity (e.g., Bevilacqua, Neri, De Martino, et al., 2020). Generally, these structures show a prevalence of regional tectonic directions (NW-SE and NE-SW); however, NNE-SSW-and WNW-ESE trending faults are associated with the youngest deformational episodes within the caldera (e.g., Vitale & Isaia, 2014;Vitale et al., 2019). According to Vitale and Isaia (2014), the fault orientation is the result of the interplay between the regional tectonic field associated with the opening of the Tyrrhenian Sea (Vitale & Ciarcia, 2013) and the local volcano-tectonic deformation. The central sector is also characterized by intense degassing and hydrothermal activity (e.g., Cardellini et al., 2017;Chiodini, Caliro, et al., 2012) and by the highest values of surface fractures and fault density, particularly around the town of Pozzuoli and in the Solfatara area (Bevilacqua, Isaia, et al., 2015;Bevilacqua, Neri, De Martino, et al., 2020).

Volcano-Tectonic Setting of the Pisciarelli Fumarole Field
The Pisciarelli area is located on the eastern margin of the structural high between the Solfatara maar and the Agnano Plain ( Figure 2). The main fumaroles and mud pool (Figures 3a-3e) occur along the intersection of the NW-SE-trending ring faults of the Agnano caldera and the major ENE-WSW fault passing through the Solfatara crater ( Figure 2a). Other fumaroles, with temperatures reaching 100°C, are present, both southward (in Antiniana) and northward (in the old alum quarry) of the Pisciarelli site. These fumarolic centers are evident in CO 2 emission maps (Cardellini et al., 2017). The Pisciarelli fumarole field has recently experienced relevant changes in the CO 2 flux (of a maximum of 600 tons/d, Tamburello (Figures 4h and 4i), which was accompanied by small mud spatters around the pool (Figures 4h and 4i). After this episode, a recharge period coincided with meteoric water input, which brought the mud pool level back to the previous and present levels.

Stratigraphy
To better define the stratigraphic and structural setting, we performed a geological survey of the area and produced the geological map shown in Figures 2a and 5. The study area is characterized by diffuse hydrothermal manifestations that affect the exposed rocks with severe alteration. These rocks were emplaced during the most recent activity (in the last 5.5 kyr) of the central sector of the caldera (Figures 2a and 2c). The oldest exposed deposits are from vents located in the present Agnano Plain, with ages ranging between 4.9 and 4.55 ka (Smith et al., 2011). They are overlain by younger deposits of Solfatara and Astroni eruptions (4.28-4.25 ka). The oldest rocks show beds gently dipping toward the west, while the younger deposits unconformably overly this sequence, following the resulting paleomorphology after the Agnano-Monte Spina caldera collapse. The rocks are mainly composed of alternating coarse and fine pyroclastic materials and are often profoundly altered by secondary mineralization due to fumarolic and hydrothermal activity. Alteration processes have changed the mechanical properties of the rocks, favoring the lithification of loose sediments Mayer, Scheu, Montanaro, et al., 2016). The physicochemical variation in the rocks makes it difficult to attribute and correlate to well-known eruptions. Nonetheless, the paleosols are well-preserved, favoring deposit recognition. The stratigraphy of the area is defined by the superposition of several volcanic deposits (Figure 2c; Isaia, Vitale, Di Giuseppe, et al., 2015;Vitale et al., 2019), including, from bottom to top, the Monte Sant'Angelo (MSA, ∼4.92 ka), Paleoastroni 2 (PA2, ∼4.73 ka), Agnano-Monte Spina (AMS, ∼4.55 ka), Santa Maria delle Grazie (SMG), Olibano tephra (OLI), Solfatara (AVS, ∼4.28 ka), and Astroni (AST, ∼4.25 ka) deposits. The oldest volcanic deposits (MSA and PA2), hereafter referred to as pre-AMS, are exposed in the Pisciarelli fumarole field along a NW-SE direction and are bounded by NW-SE major faults and covered directly and unconformably by AVS deposits (Figure 6a). South of the Pisciarelli fumarole field, pre-AMS deposits are covered by AMS pyroclastics, which are in turn overlain by scoria in the SMG and OLI deposits and pyroclastic rocks of the AVS and AST deposits ( Figure 6b). The passage to the AVS deposit is marked by a muddy layer with leaves and charcoal fragments. The area is highly altered ISAIA ET AL. by hydrothermal activity, and in some localities, the alteration has affected only part of the succession ( Figure 7a). Different paleosols are present; among others, the most developed paleosol occurs on top of the AMS deposit. Close to the main fumarole and mud pool, the volcanic succession is covered by several debris flow clastic deposits (named DF1-4 in Figures 6b and 6c) that are locally well cemented ( Figure 6f) and embed a sedimentary sequence of mud rhythmites (Figures 6c-6e), which are located ca. 14 m above the current mud pool.

Deformation Structures
Fractures and faults highly deform the whole volcanic sequence in the Pisciarelli area. Syn-sedimentary deformation structures are high-angle fractures and low-to high-angle normal faults (named system S0), which are especially hosted in the AVS deposits. They include faults with associated drag folds and growth strata ( Figure 6g). In particular, where the beds show moderate to steep dips, high-angle fractures and ISAIA ET AL.
10.1029/2020TC006227 6 of 21 small-scale normal faults pervasively affect the rocks (Figure 6a). At larger scale the whole area is affected by two fault systems (S1 and S2), which are both characterized by two mutually orthogonal sets. Kinematic indicators are rarely present. However, a dominant normal dip-slip with a secondary oblique-slip movement for all faults is observed, which is consistent with what has been reported in the whole Campi Flegrei caldera (Vitale & Isaia, 2014). This feature is further supported by the evidence that several faults appear as conjugate structures with a subhorizontal intersection, suggesting dominant dip-slip kinematics. We have recognized the first system of volcano-tectonic faults (S1) as structures crosscutting the pre-AMS and AMS deposits and covered by the AVS tephra (Figures 6a, 6b, 6h, 7a, and 7b). Generally, these faults show a well-preserved fault scarp with the AVS deposit mantling it, often showing high dip values (60°-90°) (Figures 6a, 6h, 7a, and 7b). These major faults are defined by a dominant NW-SE strike generally dipping to the NE and a secondary NE-SW direction, which both usually show displacements of a few tens of meters. Within the S1 system, we have identified two principal faults, named F1a and F1b; at their intersection, the ISAIA ET AL.  mud pool area is located. The second fault system (S2) typically consists of E-W/ESE-WNW-and NNE-SSWtrending planes that cut the S1 faults and the AVS and AST deposits (Figure 7c). The dip angles for this fault system range from 70° to 90°, and the maximum displacements are a few meters. A synsedimentary NNE-SSW-trending S2 fault bounds the eastern side of mud rhythmites with a displacement of a few centimeters (Figure 6d). The structural study of mesoscale faults and fractures has provided ∼100 fault data points and ∼700 fracture data points. Poles to fault planes and corresponding rose diagrams mark the occurrence of two main directions well ( Figure 8): NW-SE for S1 ( Figure 8a) and E-W for S2 ( Figure 8b). On the other hand, fractures show a prevailing WNW-ESE direction (Figure 8c).

Resistivity Survey
Geophysical investigations are very useful to reconstruct the structural pattern in an area at depth so actively degassing and characterized by very rapid morphological and geochemical variations. One of the most suitable physical parameters for gaining information about the subsurface structure is the electrical resistivity, which is highly sensitive to lithological variations, rock alterations, and fluid content (Unsworth & Rondenay, 2013).
10.1029/2020TC006227 9 of 21   In the geoelectrical method, a primary electric current is injected into the ground between two source electrodes. This current induces an electric field between two or more measurement voltage electrodes. Transferred resistances are then determined through Ohm's law, with the depth of investigation related to the distance between the source and voltage electrodes.
Modern geoelectrical imaging techniques are essentially based on the concept of remapping the same profile with a wide range of electrode spacings (Suski et al., 2010). The use of multipolar cables permits the contemporary deployment of a great number of electrodes in the field. The resulting multitude and partial redundancy of the current paths crossing the subsurface then lend themselves to tomographic imaging (e.g., Loke & Barker, 2004). The corresponding technique is called ERT.
In the present application, three ERT surveys (A-A′, B-B′, and C-C′ in Figure 5) were performed in 2019 to reconstruct the subsurface structure of the Pisciarelli site. An Iris Syscal Pro instrument was used as a multichannel resistivimeter. The same device was employed as a power source, producing a direct current with a maximum voltage and current of 800 V and 2 A, respectively. Several tens of electrodes were deployed in the field with uniform spacing. When the source current was injected between a couple of electrodes, the induced voltages were measured in correspondence with other electrode couples up to a distance of a fixed multiple of the spacing (realizing the so-called dipole-dipole configuration). In this way, a depth of approximately 100 m from the surface was investigated. Profiles A-A′ and B-B′ (crosscutting each other and trending NNW-SSE and W-E, respectively) correspond to 360 m long survey lines. The shorter C-C′ profile was 220 m in length and crossed the mud pool and main fumaroles in the Pisciarelli area in the W-E direction. For all these profiles, a 10 m electrode spacing was adopted. The ERT data, acquired in correspondence with each profile, were separately inverted, including topography, using ERTlab3D® commercial software. The inversion procedure was based on the smoothness-constrained least squares method inversion technique (LaBrecque et al., 1995). We used Occam's inverse algorithm (LaBrecque et al., 1995;Morelli & LaBrecque, 1996) to reconstruct the electrical resistivity image from the electric potential data. In the inversion routine, the subsurface is divided into rectangular regions of constant resistivity, and the optimization method adjusts the resistivity model to iteratively reduce the difference between the calculated and measured resistivity transfer values. Occam's inversion finds the smoothest possible model whose response best fits the measured data to an a priori chi-square statistic (Constable et al., 1987;de Groot-Hedlin & Constable, 1990). The model smoothness is enforced by minimizing the differences in the log resistivity of adjacent blocks. The conjugate-gradient method was employed to solve both the forward and inverse matrix systems, and a data-error reweighting scheme was implemented to suppress the effects of data outliers (Morelli & LaBrecque, 1996). The obtained root-mean-square (RMS) error is approximately 4, and the results are compatible with the volcanic environment (e.g., Vargemezis, 2014). To support the reliability of the images, it is common to look at S, the so-called sensitivity function, which takes into account the effects on the data by infinitesimal changes in the model resistivity. The sensitivity, which was estimated for all the tomograms, shows the degree to which a change in the resistivity of a section of the subsurface influences the potential measured by the array. The higher the value that the sensitivity function is, the greater the influence of the subsurface region on the measurement. In other words, this function tells us roughly how deep we can see with the performed array. The resistivity sections for each profile are reported in Figure 9a.
The tomograms show diffuse lateral and vertical heterogeneities within a resistivity range of approximately three orders of magnitude as an expression of a complex geological setting resulting from faulting, fracturing, and hydrothermal alteration processes that acted in the last 5.5 kyr. The sections show similar electrical patterns defined by a high resistivity zone (hundreds of Ω•m, indicated as a1 in Figure 9a) lying between two conductive layers (a few Ω•m), one deeper layer (labeled as b1 in Figure 9a) and one shallower layer (labeled b2 in Figure 9a). In particular, the Pisciarelli permanent mud pool and its main fumarole area, which are located in the intersection between the A-A′ and C-C′ profiles, are characterized by the largest conductive area b1.

Mesoscale Exposed Structures
Geological and structural surveys have allowed us to draw a detailed geological map of the Pisciarelli area ( Figure 5). Tectonic and volcanological features have been characterized by paying attention to their relationships with the intense hydrothermal emission distribution in the area. The geophysical survey has helped us to better constrain the main faults and to reconstruct their extension at depth. The structural survey has allowed us to recognize a local ductile-brittle deformation system (S0) and two main fault systems (S1 and S2). S0 structures show the characteristics of soft-sediment deformation and are associated with the deposition of volcanic products (Vitale & Isaia, 2014). Usually, their attitudes are conditioned by the preexisting morphology. In contrast, the S1 and S2 fault systems are widespread in the whole study area and are associated with the volcano-tectonic activity of the Campi Flegrei caldera ). The first system (S1), showing a NW-SE main direction and normal kinematics, affected the oldest exposed volcanoclastic succession (Pre-AMS and AMS deposits). Furthermore, S1 faults are covered by AVS deposits, indicating that they were active until ∼4.28 ka, which was probably due to ground deformation preceding the Solfatara eruption, as observed elsewhere in the caldera (e.g., Vitale et al., 2019). Within the S1 system, two main faults, named F1a and F1b (Figures 5, 6a, 6b, 6h, 7a, and 7b), play a crucial role in intercepting and channeling the uprising hydrothermal fluids in the Pisciarelli area. In fact, the mud pool is located at their intersection; moreover, the main fumaroles occur along their surficial trace. Fault F1b hardly controls the morphological evolution of the head scarp receding along the western flank of the Solfatara volcano. The S2 fault system is characterized by smaller displacements (a few meters) and does not show a crucial role in controlling hydrothermal activity. Historical and ongoing events have also been accompanied by meters of ground deformation, which was likely associated with the reactivation of main faults, as already occurred during past deformation events (e.g.,  Most of the high-frequency volcano-tectonic earthquakes (with Md > 0.5; OV-INGV, 2020b) of the 1983-1984 unrest event (Figure 10a), as well as the seismic events that occurred from 2000 to 2020 (Figure 10b), including the earthquake with the largest magnitude Md of 3.3 ± 0.3 (April 26, 2020; OV-INGV, 2020a), were localized in the Solfatara area. The relation between seismic hypocenters and surficial faults is not simple to reveal. Resistivity surveys performed in this work furnish structural information at shallow depths (100 m at depth). A deeper ERT survey of the central sector of the Campi Flegrei caldera reached a depth of 500 m (Troiano, Isaia, et al., 2019), indicating the same structural trends observed on the Earth's surface. The study also showed the shallow earthquakes (hypocenters <1 km b.s.l.), which are located between Solfatara and Pisciarelli, are confined eastward by a major NW-SE S1 fault (indicated as F1c in Figure 10c). The hypocentral depths of the largest earthquakes range between 2 and 3 km (for instance, the 2000-2020 seismic events with Md > 0.5 are shown in Figure 10d). Generally, the hypocenters cluster close to the S1 faults ( Figure 10c). Considering that (1) the mapped faults are the principal pathways for the uprising magmatic fluids and (2) there is a strict geometric relation between the earthquake distribution and the surficial fault array, we suggest that the investigated faults are shallow expressions of deep-seated faults, as illustrated in Figure 10d. With this in mind, we envisage that the overpressurized fluids migrate upward, triggering fracturing, fault reactivation, and earthquakes. It is worth noting that by comparing the epicentral earthquake distributions in [1983][1984] (Figures 10a) and 2000-2020 (Figure 10b), the seismic activity moved from SW to NE, focusing in the area south of Pisciarelli. This is consistent with the fluid migration observed in the Solfatara-Pisciarelli area (Troiano, Isaia, et al., 2019;Young et al., 2020)

ERT Interpretation: Interplay Between the Gas-Water System and Structures at Depth
The geological survey indicates that the stratigraphic setting of the study area is characterized by the superposition of pyroclastic deposits of the well-known eruptions of Epochs 3A and 3B. Generally, volcanic beds are subhorizontal or weakly dipping to the west and are crosscut by high-angle normal faults with displacements reaching a few tens of meters. Most deposits are composed of fine to coarse layers of pyroclastic materials. Hence, the variation in the resistivity values resulting from the ERT investigation is related to the water and/or gas content, which is mainly due to the rock permeability. The latter is strongly controlled by the fracturing intensity and eventual sealing processes related to hydrothermal activity (i.e., mineralization).
The ERT profiles highlight the alternation of more resistive (a1 in Figure 9a) and more conductive layers (b1 and b2 in Figure 9a), which are dislocated by subvertical structures that we interpret as normal faults. Some of these structures are well exposed, such as those shown in Figures 6 and 7 (F1a-and F1b-faults). Most of these normal faults lower the volcanic succession toward the Agnano Plain (located to the east). Combining data from the ERT profiles with information on the exposed geology, we draw three cross-sections (Figure 9b). The main F1a-fault, which is present in all cross-sections, corresponds to the principal NW-SEstriking fault dislocating the pre-AMS deposits in the Pisciarelli fumarole field (Figure 6a) and continues northwestward (Figures 7a and 7b). The other main fault F1b recognized in the area, represented in the A-A′ cross-section, corresponds to the NE-SW fault in Figure 6b. This normal fault, which we interpret as the prolongation of the fault passing through the Solfatara volcano, dislocates the pre-AVS succession, lowering the SE sector of the Pisciarelli area. According to field observations, the AVS deposits mantle the F1afaults, crosscutting pre-AMS, AMS, and SMG deposits. In the A-A′ cross-section, the F1b-fault also occurs. Regarding the distribution of the more conductive and resistive bodies, we note that a large conductive zone is located under the Pisciarelli fumarole field (anomaly b1 in sections A-A′ and C-C′ of Figure 9a) and the old alum quarry and has a NW-SE trend. This feature suggests that the conductive volume follows the F1afault. We suggest that a high permeability characterizes this NW-SE-trending body due to intense fracturing and faulting. Hence, it allows gases to migrate upward. The piezometric surface, as reconstructed in the ERT profiles, is at least 20 meters higher in the Pisciarelli-old alum quarry sector than in the neighboring areas (Figure 11), where the water table depth is 30-50 m b.s.l. (Celico et al., 1992). According to Chiodini, Vandemeulebrouck, et al. (2015) and Tassi et al. (2013), these shallow fluids (b1) are mainly condensed water of upward migrating magmatic gases. This water/gas plume is similar to that reconstructed below the Fangaia mud pool within the Solfatara volcano (Byrdina et al., 2014;Isaia, Vitale, Di Giuseppe, et al., 2015). However, unlike this volcanic area, in the Pisciarelli fumarole field, condensed water does not accumulate in a limited area, such as the Solfatara maar-diatreme volcano (see the reconstruction in Troiano, Isaia, et al., 2019). We hypothesize that this water entirely flows toward the Agnano Plain, which is the nearest depressed area acting as a water collection basin (Figure 11).
In summary, the ERT survey allowed us to reconstruct the geometry of the complex array of faults present in the Pisciarelli area and identify a NW-SE directed water plume associated with the condensation of uprising magmatic gases that are strictly linked to the main volcano-tectonic structures. Moreover, the F1a-fault and, in general, the NW-SE-trending structures of the S1 fault system act as a limit for the main resistive body (a1) that is well marked in the central sector of sections A-A′ and B-B′ (Figures 9a and 9b). This anomaly, which is approximately 100-150 m wide, is likely related to gas accumulation, as also suggested by larger-scale ERT images (Troiano, Isaia, et al., 2019). Finally, the ERT investigation strengthens the interpretation of considering these faults as the primary conduits for the uprising magmatic fluids. Hence, the self-sealing process mainly affects the host rocks, focusing on the fluid flow within the fault zones. This phenomenon can cause the pore pressure to increase, thereby decreasing the fault strength and triggering rock failures and earthquakes.

Volcanic Hazard Scenario
The investigated area of Pisciarelli includes a fumarole field that has dramatically changed in the last 15 years, with the formation of very powerful fumarolic vents and a very energetic mud pool, which continuously increases its surface area and the number of boiling points. The main fumarole vents experienced an escalation in temperature. Furthermore, an increase in the CO 2 flux has been measured in the whole emission area (Cardellini et al., 2017;Tamburello et al., 2019). Mud/gas explosions have been heard from locals, while small mud ejecta close to the pool have occurred and have recently been observed. On the other hand, a close relationship exists between the tremor associated with the fumarolic activity in the Pisciarelli area and the changes in the uplift rate, swarm seismicity, and hydrothermal activity (Giudicepietro et al., 2020). These phenomena and their ongoing variations are the most significant phenomena recorded in the caldera since the current unrest started in 2005. As discussed before, this sector of the caldera is also affected by the largest number of shallow high-frequency volcano-tectonic earthquakes (0.5-2.5 km b.s.l.) and seismic swarms recorded in recent years at the Campi Flegrei caldera (La Rocca & Galluzzo, 2019). Furthermore, the increase in hydrothermal and seismic activity led authorities to prohibit public access to this area. Our investigations clearly show that the fumaroles and mud pool of Pisciarelli localize along major faults, especially at their intersection ( Figure 5). The impressive increase in fumarolic activity is likely due to increased gas flux from depth and a thermal change within the uppermost crust (Chiodini, Vandemeulebrouck, et al., 2015). These phenomena could favor fracturing and faulting reactivation and increase rock permeability. This feature is well highlighted at depth by the ERT image showing the formation of larger conductive bodies (anomaly b1 at approximately 270 m along profile A-A′ and 80 m along profile C-C′; Figure 9a) in the highly fractured zone close to the main faults (F1a and F1b in Figure 9b). These active structures facilitate the circulation of the fluids at shallow levels and/or the gas to escape as single fumarole vents. Any further enlargement of the emission area or increase in the fumarole activity (e.g., temperature and CO 2 flux) could be due to an increase in the gases from depth and/or rock fracturing. Both phenomena are strictly connected to the possible movement of magmatic fluids at depth. Particular settings, such as rocks characterized by several faults and related damage zones, could also lead to a sudden increase in heat flow and gas uprising, causing groundwater steaming and flashing that, in turn, may cause critical overpressure at shallow levels and trigger hydrothermal/phreatic explosions. Different phreatic eruptive events have occurred in the past at Campi Flegrei, which are mainly documented in the Solfatara area, including the initial phase of the Solfatara eruption, which destroyed a large lava dome complex and spread volcanic products in a wide area around the volcano (Isaia, Vitale, Di Giuseppe, et al., 2015). Lava dome intrusions and small phreatic/ phreatomagmatic explosive events characterize this caldera sector and are favored by the activity of the fault systems described above (Figure 2a). These structures are common for both the Solfatara and Pisciarelli areas and are also the site of most of the seismicity that occurred recently at Campi Flegrei; therefore, they are structures capable of generating strong (reaching M 4.2, which occurred during the 1982-1984 unrest; De Natale & Zollo, 1986) shallow (<3-4 km) earthquakes. Hence, rock failure phenomena could occur in response to energetic seismic shock, with rock fracturing in the shallow hydrothermal system, and cause sudden fluid decompression, triggering explosive events (e.g., Gallagher et al., 2020;Kennedy et al., 2020).
However, further mechanisms may cause phreatic/hydrothermal eruptions. The present state of activity in the Pisciarelli area suggests an open system accompanied by continuous degassing and morphological variations in the mud pool, which only partly have a seasonal signature . The evolution of the degassing and fluid circulation activity may cause chemical alteration and mineral precipitation and hence favor the self-sealing of pores and fractures. For example, mineralogical analyses of samples collected in the Solfatara and Pisciarelli areas reveal intense surficial solfataric alteration made of amorphous silica and alunite . The precipitation of alunite in pores and fractures due to fluid circulation leads to the formation of veins, as observed in the Pisciarelli area and other volcanoes (e.g., New Zealand; Gallagher et al., 2020;Kennedy et al., 2020). This process produces a decrease in the host rock permeability (e.g., Carlino et al., 2016) and, in turn, a pore pressure increase within the fault zones, which may trigger a hydrothermal explosion (Roman, LaFemina, et al., 2019).
Another mechanism that may cause a temporary obstruction of the fumarole vents and mud pool at Pisciarelli is the development of landslides. Field data show that a pile of mud rhythmites that is very similar to the sediment presently forming within the mud pool has been recognized approximately 14 m above it (Figures 6c-6e). This fossil mud pool deposit is embedded in debris flow deposits composed of large lithics and reworked scoriae produced during landslide events that were generated from the scarp close to the emission area. Locally, the debris is hydrothermally cemented ( Figure 6f); furthermore, the whole sequence has been subsequently eroded. Finally, it is worth noting that the fossil mud pool deposit is associated with a synsedimentary fault (Figure 6d), suggesting that its formation was synchronous with a deformation stage that affected the area. Taking into account the latter features and other geological clues, such as (1) the changing morphology of the area, including the occurrence of unstable masses located uphill, (2) the chemical alteration of the rocks, and (3) the powerful hydrothermal activity mainly localized along the fault (F1b) passing for the head scarp, we suggest that a rapid closure of the main emission zones by landslide events could occur. The above-described phenomena have been tested worldwide in other hydrothermal systems (e.g., Rott et al., 2019), suggesting a possible trigger mechanism for an explosion that occurred in the past in the Pisciarelli area, as shown in the model of Figure 12. Figure 12a represents the starting state of the Pisciarelli fumarole field characterized by landslide deposits hosting fumaroles and a mud pool. A possible development scenario envisages a renewed landslide event producing a debris flow deposit, which forms an impermeable cap above the active fumarole area (Figure 12b). This seal increases the pressure within the shallow hydrothermal system, which could act as a trigger for an explosion capable of ejecting materials around the area (Figure 12c). The explosive event moved the main vents to a lower elevation, as observed currently (Figure 12d). We suggest that this scenario could again happen in the future, especially during energetic seismic events that are important trigger mechanisms for sliding phenomena (e.g., Keefer, 2002). However, horizontal migration of fluids through complex faults and fracture patterns and eventual leakage at the surface elsewhere cannot be ruled out as a consequence of the landslide covering the mud pool.

Conclusions
In this study, we have defined in detail the volcano-tectonic setting of the Pisciarelli area, representing basic knowledge crucial for understanding any possible evolution of volcanic activity in the area. The geological and volcanological survey has allowed us to provide a new geological map of the area and relative cross-sections. Morphological, stratigraphic, and structural analyses integrated with data from geoelectrical surveys suggest that the area is characterized by two main fault systems. The first (S1) affects the oldest part of the volcanoclastic succession, showing NW-SE and NW-SE main directions, normal kinematics, and displacement of tens of meters. The second fault system (S2) crosscuts all the previous structures and the youngest part of the volcanic succession. It is characterized by WNW-ESE and NNE-SSW main directions, normal kinematics, and a few meters of displacement.
The ERT survey, which consists of three profiles crosscutting the main fumaroles and mud pool, further sheds light on the deep structure of the Pisciarelli area. Resistivity anomalies outline the preferred fluid pathways, mainly located along two main S1 faults (named F1a and F1b) and at their intersection where highly fractured rocks occur. The reconstructed structural images show a complex hydrothermal system formed by a mix of upwelling gas, condensed water, and meteoric water. At depth, part of the recent seismicity is distributed along the S1 faults, suggesting that, presently, these structures represent the main pathways for the upward migration of magmatic fluids within the Campi Flegrei system.
Field evidence also suggests that the main fumaroles and active mud pool could be obstructed by landslide deposits, as occurred in the past, causing critical overpressure in the shallow hydrothermal system and a possible explosion in the area. An increase in gas overpressure within the fault system could also be triggered by a decrease in the host rock permeability due to the self-sealing process. In contrast, an energetic seismic shock could cause large rock failure phenomena along the main faults, causing sudden fluid decompression in the shallow hydrothermal system and triggering explosive events.
The results of this multidisciplinary investigation provide new elements of knowledge on the active structures in this Campi Flegrei caldera sector, which are of fundamental importance for the forecast of future eruptive scenarios and have already experienced phreatic and hydrothermal eruptions. They further indicate how studies on the volcano-tectonic characterization of hydrothermal systems are necessary and crucial to shed light on their evolution, which recently, in several cases worldwide, rapidly evolved into eruptive phenomena causing human losses.
Although monitoring of the Pisciarelli area by the INGV-OV surveillance system is intensive, the proposed evolution scenarios indicate that further structural investigations at higher depths are necessary. In addition, highly detailed geophysical measurements reiterated in time or continuously are useful to detect any changes in the relationships between the faults and the hydrothermal fluid circulation.

Data Availability Statement
The data were collected in the framework of the INGV-DPC research agreement 2012-2021. This study does not necessarily represent the DPC official opinion and policies. The structural and electrical resistivity data used in this work are available in the following repository: R. Isaia, M. G. Di Giuseppe, J. Natale, F. D. A. Tramparulo, A. Troiano, and S. Vitale (2021). Structural and electrical resistivity tomography data sets from "Volcano-tectonic setting of the Pisciarelli fumarole field, Campi Flegrei caldera, southern Italy: Insights into fluid circulation patterns and hazard scenarios", https://doi.org/10.6084/m9.figshare.14445582.