Recurring Geophysical Manifestations at Campi Flegrei Caldera, Italy: A Perfect Storm of Geology?

The Campi Flegrei Caldera (CFc) in Italy exhibits periodic unrest episodes characterized by slow ground deformation known as bradyseism, first described by Charles Lyell in the 19th century. These events involve substantial uplift, measuring meters each time, accompanied by delayed seismicity despite the significant accumulated strain energy, followed by a period of subsidence. Geophysical studies have typically focused on individual episodes, attributing deformation and seismicity to melt refill, often overlooking CFc's fundamental nature as a geothermal field. Since 2011, a new phase of unrest has emerged after approximately 26 years of subsidence since the last unrest in 1982-1984. By comparing these two unrest episodes, we identified recurring patterns of geophysical manifestations, including the depth and location of a low Vp/Vs anomaly, as well as seismicity that initiates at shallow depths and progressively deepens over time. By integrating site-specific hydrothermal experiments, twenty-four years of pluviometry data, and subsurface hydrodynamics, we highlight two synergistic processes within the caldera: a confined, recharging water-steam reservoir and the sealing properties of hydrothermal water forming a fibrous-rich caprock. These structures can work in concert to drive gradual deformation due to overpressure, reducing effective stress and ultimately triggering naturally induced seismicity. The rapid (isenthalpic) fluid upflow during fracturing promotes the downward migration of seismic events from the shallower caprock to the deeper reservoir.

Speaker: Tiziana Vanorio, Stanford University