We have learned how to use noise produced by humans, ocean swell, and atmosphere solid-Earth interactions12,13 to illuminate the interior of magmatic and hydrothermal systems14-17. Noise data from expanding seismic networks are analyzed with novel array20 and interferometric21,22 techniques, allowing detection of volcanic processes and forecasting hazards without having to wait for earthquakes18,19. Noise polarization across dip-angle normal faults has been related to stress and variations in stiffness anisotropy1,2. However, the potential of noise polarization to illuminate pressurized fluids in volcanic systems is yet to be explored. Campi Flegrei (Southern Italy, Fig. 1a, small lower panel) is an inhabited volcanic caldera bordering Naples (the third most populous city of Italy) and the ideal location to discover this potential. The caldera is a capped11,23-26 geothermal system, where hazardous CO2-bearing fluids propagate from the primary deformation source (Figs. 1-3, black dot) to fumaroles (S) at least since 19845-11,23-26. Heating of the hydrothermal system, volcanic gas emissions at the surface5-8 and seismic release5,7,8,27,28 result from consecutive episodes of unrest, promoting a long-term accumulation of lateral stress and expanding reservoirs4,5. Accumulated stress and fluid migrations left marks across extensional faults and feeding systems at the caldera. Polarized noise can see through the overlying rocks to catch these marks. The azimuth of the horizontal polarization vector derived from ambient noise and the resultant length of its distribution (R)1,2,29,30 are used here for the first time as both imaging and diagnostic tools (Methods, 0.2-1 Hz). During periods of low seismic release31,32, they detect the hypothesized link between deep extensional and caldera-bounding faults (extensional structures) that bear regional stress33-37 north and east of the caldera (Fig. 1a, white dotted line), and a dynamic transfer structure34 that crosses its deforming centre and vents outgassing at the surface6-10 (Fig. 1a, black dotted line). At higher frequencies (1-5 Hz, Methods, Extended Data Fig. 1), regional and caldera-bounding faults disappear due to the sensitivity of noise to shallower and smaller structures13,29. High resultant lengths and polarized azimuths mark NW-SE-trending extensional faults34-37 with exceptional stability between 2009 and 2020 (Figs. 1, Methods, Extended Data Figs. 1-5). The transfer structure develops instead SW-NE (Fig. 1a), the direction of the volcanic ridge under the caldera34. When hydrothermal pressure, gas emission and seismicity increase (2018)31,32, the transfer structure depolarizes, allowing to monitor fluid migrations leading to high-duration-magnitude (Md>3) earthquakes (Figs. 1b-d).
The polarized extensional and transfer structures are a direct consequence of processes that have been consistently imaged and monitored during the last thirty-six years. The high-attenuation24 signature of the repeated injections25 that caused the strongest volcano-tectonic event recorded at the caldera (Md=4.1) appears as an unpolarized anomaly after more than three decades (Fig. 2a). The central hydrothermal system opened in the WSW-ENE direction on April 1st, 1984 (black diamonds, Fig. 2a) due to a NW-directed injection of magma9, magmatic or supercritical fluids23-25. After thirty years (2011-13), a low-velocity aseismic reservoir17,38 had expanded from the injection point (Fig. 2b, black cross). Expansion toward west and north continued until fluids had reached the western caldera-bounding faults, producing high magnitude earthquakes in 20124,5,17 (Fig 2b, western black diamonds). However, no apparent lateral expansion was visible east and south of the injection point (black cross, Fig. 2a,b). Fluids stopped at a barrier delineated by high velocities and high stresses, as shown by combined seismic and InSAR interferometric analyses10. This barrier coincides with the transfer structure that crosses the eastern sector of the Solfatara crater (Fig. 2b). Here, shear-wave-splitting anisotropy39, InSAR11, and gravity gradiometry33 identify a SN anomaly that accumulates the highest lateral stress during unrest10, producing small-magnitude earthquakes31 (Fig 2b, diamonds).
Imaging stressed fluid-filled structures
In the eastern caldera, the highest resultant lengths show azimuths consistently parallel to the NW-SE high-velocity extensional faults11,35-37 (Fig. 1). The area is wide enough to become a high-velocity waveguide for horizontally-polarized isotropic S waves generated either in the centre of the Tyrrhenian Sea12 or across the near coastline (Extended Data Fig. 7a,b). This waveguide explains azimuths parallel to the trend for both source configurations at stations with R>0.25 (Methods). Still, a far-field source12 better fits azimuths observed across the entire caldera (Extended Data Fig. 7a,b, Residuals). Far-field sources cannot explain azimuths perpendicular to the primary direction (SW-NE) of the transfer structure between 2009 and 2017 (Fig. 1a). These azimuths could be a consequence of seismic anisotropy, which tracks permanent directional signatures from the deep Earth mantle40 to hydrated subducting slabs41. If low-velocity faults are wide enough, stiffness anisotropy1,2 and trapping and reverberations42 on high-dip fault walls can polarize noise perpendicular to fault walls. Across the transfer structure, azimuths indeed develop perpendicular to high-dip fault walls (Fig. 2c) and crack anisotropy at least at Solfatara39. Yet, the transfer structure is a small high-velocity structure (Fig. 2b)17 consequence of lateral stress accumulated in the crust4,5,10. Azimuths across this structure better fit those obtained for sources generated at the near coastline12 (Extended Data Fig. 7a,b right). Near-field sources7 seem a more likely controller of azimuths than anisotropy, yet anisotropy increases polarization across similarly compressed structures21,22.
Depolarization of the 2009-2017 transfer structure is central to explain stress release and structural changes in the volcano. While the extensional trend appears consistent over time, the transfer structure only polarizes during periods of lower seismic and geochemical release31,32, when deep injections and hydrothermal recharge are sparse and rarely coupled6-8,27,31,32 (Fig. 1a). The structure is in contact with the high-attenuation24 and deforming9,10 location of deep injections (Fig. 2a, black cross). It runs along:
- the semi-circular east and north borders of a reservoir that was expanding in 2011-201317 (Fig. 2b);
- the lobe-shaped maxima of horizontal stresses observed using InSAR methods10;
- an abrupt structural variation in tidal tilting from WE to SW-NE43,44.
The dynamics associated with these geophysical responses and maps are linked to the sub-caprock migration of over-saturated CO2-bearing fluids5-11,17,19,23, adding persistent low-frequency noise and long-period events30. The high-scattering fluids rising and migrating from deep injections pervade fractures, producing local noise that progressively intensifies7 and depolarizes the transfer structure (Fig. 1b,c). In the presence of high-velocity contrasts, stations within one wavelength from such extended sources lose polarization in the heterogeneous medium (Extended Data Fig. 7a,b, right, R decrease at station ACL2). This behaviour is apparent at Solfatara in 2018, when the central and eastern unpolarized reservoirs connect (Figs. 1b). Fluids eventually outflow on metasediments11 between transfer and extensional structures. These high-attenuation24,25 sediments reduce ambient noise directionality between 0.2 and 1 Hz45 and are the most consistent unpolarized anomaly during the decade (Fig. 1).
The pre-seismic (Fig. 1c) and post-seismic (Fig. 1d) patterns show the progressive depolarization induced by fluids migrating from the injection location to: (1) the eastern sector of Solfatara and the Pisciarelli vent (S, Fig. 1), where the geochemical unrests of the last fifteen years have been monitored6,11; (2) Monte Nuovo, the location of the last eruption at Campi Flegrei (M, 1538AD). The Solfatara-Pisciarelli vents emit from 2000 to 3000 tons/day of CO2 in the atmosphere7,8. They have been consistently deforming toward the east in the last 20 years46, moving along with seismicity from the injection location25,38. Joint interpretations of resistivity, geochemistry and field data11,36 detect the plume that feeds these vents, the surrounding metasediments, and the eastern extensional faults that bind low-density metasomatized rocks11 (Fig. 2c). In the western portion of Fig. 2c, the transfer structure crosses the capped resistive plume that stores steam and gas, feeding fumaroles. Here, injections of fluids from depth6-11,23-26,47 coupled with meteoric recharge27,28,43 produce stress10 and outflow eastern liquid-bearing sediments11. Gas-bearing fluids over-pressurized the eastern caldera between 2011 and 201319 due to concurrent lateral expansion10 of the source region and saturation of the reservoirs6-8. The depolarization of the transfer structure that started in 2018 (Fig. 1b) led to the highest seismic release in thirty-six years at the caldera31,32. The area east of the Solfatara feeder was already suffering the highest horizontal stress in 2011-13 (white diamond10, Fig. 2b,c). Here, fluid injections from depth coupled with progressive permeability increases from heavy rains27,28,43 started the seismic sequence in December 201931. Stresses on the high-dip fault east of Solfatara (Figs. 2c and 3) generated two high-magnitude volcano-tectonic events7,31 after minor earthquake swarms47: a Md3.1 on December 6th, 2019 and a Md3.3 on April 26th, 2020 (white circles 1 and 2, Figs. 1c,d, 2c and 3a,b).
Monitoring stress and fluid migrations
This seismic sequence is the effect of pressurization of the hydrothermal system32 induced by lateral stress and fluid migrations, which horizontal noise polarization can monitor. The mechanical weakening of the crust15 and the corresponding depolarization of ambient noise22 after the Tohoku earthquake detect the release of stress and upward fluid migration at volcanoes hundreds of kilometres afar. In a stressed geothermal environment23-26 like Campi Flegrei, these surges appear at sharp lateral discontinuities, as caldera-bounding faults. In September 2012, fluid injections activated western caldera faults near Monte Nuovo (Fig. 2b, M, western black diamonds)9,10. The resultant lengths measured over months at the nearest station detect the permanent depolarization following the earthquakes, in analogy to interferometric analyses21,22 (Methods, Extended Data Fig. 8). Fluid migrations between western and eastern caldera were the mechanism that released stress at the end of the 1984 unrest25. Months after the 2012 swarm, it is the part of the eastern caldera compressed between transfer and extensional structures (Fig. 1a) that suffered the highest long-lasting velocity reductions (>0.1%)19. These reductions are symptomatic of the area bearing the highest concentration of pressurized fluids15,19, most likely to erupt, form new hydrothermal vents, and nucleate earthquakes48,49. The temporal patterns (Figs. 1a-d, 3a,b, Extended Data Figs. 5, 6) clarify that fluid migrations connecting western and eastern caldera coexist and possibly drive stress build-up and release through the seismic sequence. Fluids migrate under the Campi Flegrei caprock23-25, which forbids surges directly above the primary source of deformation23. After each earthquake in 2019-20 (Extended Data Fig. 9), the change in polarization is similar to that observed after the earthquakes in 2012 (Extended Data Fig. 8). It is analogue to the decrease in ambient noise polarization caused by hydrothermal fluid surges at Mount Fuji after the Tohoku earthquake22. Unlike Mount Fuji, horizontal stress was already in a critical state at Campi Flegrei due to magma degassing5-8 and supercritical fluids, pressurized under the caprock11,23.
During the pre-seismic period (Fig. 1c, 3a), after minor swarms stroke the eastern caldera7,41, the unpolarized anomaly under the Solfatara and Pisciarelli vents develops from north to south. After the Md3.1 earthquake, this anomaly expanded toward the eastern flank of the Solfatara and the Pisciarelli vents (Fig. 3b), matching the hypothesized low-gravity fluid-ascension path between the two vents33,36. During the inter-seismic period, the anomalies in the western and eastern caldera connected across the seismic pathways that released stress and closed the 1984 unrest25 (Fig. 3a, diamonds). These maps track fluids generated by the deformation source6-8 and over-pressurized in the capped system12,23-26. The fluids migrated both seismically31,32 and aseismically in 2020, pressurizing the eastern hydrothermal system until the Md3.3 released stress7. The Md3.3 sealed migration by polarizing noise across the transfer structure (Fig. 3a, rightmost panel). By May-June 2020, the eastern unpolarized anomaly was one km east of its original location. It comprised the earthquake location (compare Fig. 3a, left to right) and an area that was polarized before the sequence (Fig. 1a-c). This dislocation is the seismic signature of the persistent lateral stress leading to fluid migrations toward the eastern caldera38.
Toward monitoring with depolarized noise
Heat increase and critical degassing pressure from depth6 coupled with hydrothermal recharge27,28,30,32,43 make the area between regional extension and transfer structure (Fig. 1a,d) most likely to break in the future48,49. Once informed by thermo-hydro-mechanical simulations41, polarization parameters show a quasi-real-time monitoring potential. Recent thermo-hydro-mechanical modelling47 shows that fluids are injected at the base of faults in the east caldera between three and five days before the Md3.1, depending on injection volumes. Fig 3b and Extended Data Fig. 6 show polarization parameters measured using three hours of noise each day in these periods. After a consistent depolarization five days before the earthquake (Extended Data Fig. 6, 01/12/2019), the R increases at all the stations around the location of the Md3.1 (Fig. 3b, pre-seismic), in a manner that is consistent with an increase in compression preceding earthquakes47. After the Md3.1, the unpolarized anomaly east of Solfatara expands toward the east (Fig. 3b) with significant statistical variations at stations in the eastern caldera (Extended Data Fig. 9). Similar maps are obtained in a shorter time interval (one to three days) around the Md3.3 to account for the increase in pore pressure following the inter-seismic period47 (Fig. 3b, Extended Data Fig. 6). Two days before the Md3.3, the eastern unpolarized anomaly had focused on the earthquake location. Two days after the Md3.3, fluids had outflown the area east of the Md3.311, depolarizing the eastern extensional trend like after the Md3.1 (Fig. 3b, from left to right). These spatial and temporal relations confirm that depolarized noise can monitor deep sub-caprock23-27 migrations of fluids preceding and following higher-magnitude earthquakes.
Ambient noise polarization answers the long-standing question of how this stressed volcano feeds its hydrothermal vents and builds and releases stress. A transfer structure connects the central deforming caldera to regional extensional faults33,34, running under a caprock whose characteristics allow over-pressurization, lateral fluid migration and strong lateral deformation23. The area of major volcanic and seismic hazard48,49 is compressed between transfer and extensional systems. The opening of the transfer structure detects deep fluid migrations toward the surface. These fluids trigger changes in polarization patterns30, allowing mapping of stress build-up and release through further eastern fluid migrations. Temporal scanning of depolarized noise represents a substantial step toward instantaneous imaging of hydrothermal expansion, leading to earthquakes in stressed calderas. Polarization measurements from ambient noise interferometry21,22 require yearly recordings for stable imaging, several days of monitoring measurements, and high amounts of processing. As previously hypothesized1,2, horizontal noise polarization can achieve similar results using hours of noise and minimal processing.