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.

In the past two decades, the central portion of Campi Flegrei caldera has experienced ground uplift up to 15 mm/month, with an increase of rate, magnitude and extent of the seismicity. In this work, we perform multi‐scale precise earthquake relocation of the 2014–2024 seismicity, mapping in detail activated fault zones. We relate the geometry, extent, and depth of these zones with up‐to‐date structural reconstructions of the caldera. The current seismicity is mainly driven by ground‐uplift‐induced stress concentration on pre‐existing, weaker fault zones, some of which identified for the first time. These structures are not only related to the inner caldera and dome resurgence but also to volcano‐tectonic events of the last 10 ka. The extent of imaged fault segments suggests they can accommodate ruptures up to a moment magnitude 5.1, significantly increasing seismic hazard in the area.

Silicic calderas are volcanic systems whose unrest evolution is more unpredictable than other volcano types because they often do not culminate in an eruption. Their complex structure strongly influences the post‐collapse volcano‐tectonic evolution, usually coupling volcanism and ground deformation. Among such volcanoes, the Campi Flegrei caldera (southern Italy) is one of the most studied. Significant long‐ and short‐term ground deformations characterise this restless volcano. Several studies performed on the marine‐continental succession exposed in the central sector of the Campi Flegrei caldera provided a reconstruction of ground deformation during the last 15 kyr. However, considering that over one‐third of the caldera is presently submerged beneath the Pozzuoli Gulf, a comprehensive stratigraphic on‐land‐offshore framework is still lacking. This study aims at reconstructing the offshore succession through analysis of high‐resolution single and multichannel reflection seismic profiles and correlates the resulting seismic stratigraphic framework with the stratigraphy reconstructed on‐land. Results provide new clues on the causative relations between the intra‐caldera marine and volcaniclastic sedimentation and the alternating phases of marine transgressions and regressions originated by the interplay between ground deformation and sea‐level rise. The volcano‐tectonic reconstruction, provided in this work, connects the major caldera floor movements to the large Plinian eruptions of Pomici Principali (12 ka) and Agnano Monte Spina (4.55 ka), with the onset of the first post‐caldera doming at ca. 10.5 ka. We emphasise that ground deformation is usually coupled with volcanic activity, which shows a self‐similar pattern, regardless of its scale. Thus, characterising the long‐term deformation history becomes of particular interest and relevance for hazard assessment and definition of future unrest scenarios.

We present a structural study on late Miocene-early Pliocene out-of-sequence thrusts affecting the southern Apennine orogenic belt. The analyzed structures are exposed in the Campania region (southern Italy). Here, thrusts bound the N-NE side of the carbonate ridges that form the regional mountain backbone. In several outcrops, the Mesozoic carbonates are superposed onto the unconformable wedge-top basin deposits of the upper Miocene Castelvetere Group, providing constraints to the age of the activity of this thrusting event. Moreover, a 4-km-long N-S oriented electrical resistivity tomography profile, carried out along the Caserta mountains, sheds light on the structure of this thrust system in an area where it is not exposed. Further information was carried out from a tunnel excavation that allowed us to study some secondary fault splays. The kinematic analysis of out-of-sequence major and minor structures hosted both in the hanging wall (Apennine Platform carbonates) and footwall (Castelvetere Group deposits and Lagonegro-Molise Basin units) indicates the occurrence of two superposed shortening directions, about E-W and N-S, respectively. We associated these compressive structures to an out-of-sequence thrusting event defined by frontal thrusts verging to the east and lateral ramp thrusts verging to the north and south. We related the out-of-sequence thrusting episode to the positive inversion of inherited normal faults located in the Paleozoic basement. These envelopments thrust upward to crosscut the allochthonous wedge, including, in the western zone of the chain, the upper Miocene wedge-top basin deposits.

The active Campi Flegrei caldera in southern Italy has a remarkably long history of coexistence between volcanism and human settlements, and it is famous for its peculiar slow ground movement called bradyseism, i.e. episodes of inflation and deflation of the caldera floor due to magmatic and/or hydrothermal processes. This natural phenomenon has interacted with the civilization that inhabited this strategic and fertile area, especially in Roman times, when the sinking of the coast hindered the flourishment of Puteoli and Baiae coastal towns. The drowning of a large part of Republic-early Imperial Roman coastal buildings, west of the modern Pozzuoli town, is classically used to illustrate the bradyseism activity. In this paper, we investigate the spatial variability and the role of this phenomenon, demonstrating that the caldera deflation alone cannot account for the submersion of Roman facilities in the western sector where the harbour structures of Portus Iulius and luxury villas of the Baianus Lacus presently lie beneath sea level. On the contrary, the sinking of this area is mainly the result of the activity of volcano-tectonic faults. We restored the topography to 100 BCE using archaeological and high-resolution topographic data. Results show that the several metres of vertical displacement recorded in the Baia area in the last 2100 yr were mainly produced by the activity of normal faults and secondarily by caldera deflation, the former including the long-lived Baia Fault and the younger normal faults associated with the Monte Nuovo eruption at 1538 CE. Graphical Abstract