Earthquake-triggered fluid release at Campi Flegrei Caldera, Italy.

Abstract:

Bradyseismic episodes consist in vertical movements of the ground that are driven by hydrothermal and/or magmatic fluids. The Campi Flegrei Caldera near Naples, Italy, is affected by episodic ground uplifts as fast as 1 m/yr. Such deformations have previously been suggested to be caused by pulses of CO₂-rich fluids injected into the hydrothermal system or by the intrusion of magmatic bodies at shallow depths.

We show that since 1945 uplift episodes occurring at Campi Flegrei are caused by large regional earthquakes. Namely, passing body waves impose high dynamic strains at the roof of the magmatic reservoir at approximately 7 km depth resulting in the embrittlement of the magmatic carapace. This releases H₂O-CO₂ mixtures (and eventually magmas) that migrate upwards through a nominally plastic zone. After passing the brittle/ductile transition at approximately 3 km depth, the H₂O-CO₂ mixture undergoes phase separation and expands, leading to the observed rapid ground uplifts. Our model is quantitatively supported by numerical studies and statistics. The latter indicates a consistent time-lag between the occurrence of regional earthquakes and the onset of the following fluid-driven ground uplift. The corollary is that i) processes activated by dynamic triggering may become apparent long after the passage of the seismic waves and ii) hydrothermal systems are much more sensitive to external perturbations than previously thought. This study represents a valuable observation for volcanic risk mitigation plans and provides a new mechanism to explain rapid fluid flow in volcano-hydrothermal systems. Additionally, our results represent a new interpretation of one of the most studied volcano-hydrothermal system worldwide.