Fluid migrations and volcanic earthquakes from depolarized ambient noise

Abstract

Ambient noise polarizes inside low-velocity fault zones, yet the spatial and temporal resolution of polarized noise on gas-bearing fluids migrating through stressed volcanic systems is unknown. Pressurized fluids increase stress and lead to volcanic earthquakes; imaging their location in real time would be a giant leap toward forecasting eruptions and monitoring volcanic unrest. Here, we show that depolarized noise detects fluid injections and migrations leading to earthquakes inside the laterally-stressed hydrothermal systems of Campi Flegrei caldera (Southern Italy). A polarized transfer structure connects the deforming centre of the caldera to open hydrothermal vents and extensional caldera-bounding faults during periods of low seismic release. Fluids depolarize the transfer structure and pressurize the hydrothermal system, building up stress before earthquakes and migrating after seismic sequences. During sequences, fluid migration pathways connect the location of the last eruption (Monte Nuovo, 1538AD) with the part of the eastern caldera trapped between transfer and extensional structures. After recent intense seismicity (December 2019-April 2020), the transfer structure appears sealed while fluids stored in the east caldera have moved further east. Depolarized noise has the potential to monitor fluid migrations and earthquakes at stressed volcanoes quasi-instantaneously and with minimum processing.

3D structure of the Campi Flegrei caldera central sector reconstructed through short-period magnetotelluric imaging

“…In recent years, literature papers have presented many geophysical results devoted to the structures of this sector of the caldera 1,15,[17][18][19][20][21] . Among the various types of prospections, those based on electrical resistivity have already emerged as some of the most proficient sources of information aimed at understanding the role and distribution of geothermal fluids, their interactions with meteoric recharge, and the main structural lineaments, together with the effects of their circulation on the surrounding rock volumes (e.g., Isaia et al 16 ).…”