Reconstructing Hydrothermal Fluid Pathways and Storage at the Solfatara Crater (Campi Flegrei, Italy) Using Seismic Scattering and Absorption

Abstract

Imaging of fluid pathways is crucial to characterize processes taking place in hydrothermal systems, a primary cause of volcanic unrest and associated hazards. The joint imaging of seismic absorption and scattering is an efficient instrument to map fluid flow at crustal scale, and specifically in volcanoes; however, this technique has so far been applied to image volcanoes and hydrothermal systems at the kilometre scale. Here, we use data from a meter-scale, active seismic survey inside the shallow structure of the Solfatara crater to obtain the first frequency-dependent near-surface scattering and absorption model of a hydrothermal system. The Solfatara crater is the place used to monitor historic unrest at Campi Flegrei caldera (Italy), a high-risk volcano under continuous surveillance due to its closeness to a densely populated area. Improving the imaging of the shallow part of this system is crucial to broaden the understanding of unrest processes that are progressively characterizing other portions of the eastern caldera. The scattering contrasts highlight the primary structural feature, a fault separating the hydrothermal plume from zones of CO2 saturation nearing fumaroles. While high-absorption anomalies mark zones of high soil temperatures and CO2 fluxes, low-absorption anomalies indicate zones of very shallow upflow and are caused by contrasts between liquid-rich and vapour-rich fluids coming from mud pools and fumaroles, respectively. All maps show a SW-NE trend in anomalies consistent with fluid-migration pathways towards the eastern fumaroles. The results provide structural constraints that clarify mechanisms of fluid migration inside the crater. The techniques offer complementary geophysical images to the interpretation of hydrothermal processes and prove that seismic attenuation measurements are suitable to map fluid pathways in heterogeneous media at a detailed scale.

Fast Changes in Seismic Attenuation of the Upper Crust due to Fracturing and Fluid Migration: The 2016–2017 Central Italy Seismic Sequence

“…In fluid-filled fault networks, high absorption marks regions with the highest concentrations of CO 2 -bearing fluids (Di Martino et al, 2022). Our absorption patterns evolve during the sequence as expected for high-absorption deep CO 2 -bearing fluids previously trapped in the fault systems (Miller et al, 2004).…”

“…The open-access code MuRAT, previously applied in volcanic (De Siena et al, 2016Gabrielli et al, 2020;Di Martino et al, 2022) and tectonic settings (Borleanu et al, 2017;Napolitano et al, 2020), is used to perform all the analyses. The code is applied to datasets spanning five time frames: the presequence (2013-2016), the AVN, and the three sub-sequences after the mainshocks (from August 24, 2016, to January 18, 2017).…”

“…This effect is dominant when measuring Q c −1 across volcanic structures, as documented along the Mount St. Helens volcano (US) where low-frequency waves allow to map the shallowest, most-heterogeneous volcanic structures, while deep feeding systems appear as high-Q c −1 anomalies at higher frequencies (De Siena et al, 2016;Gabrielli et al, 2020). Di Martino et al (2022) recently obtained absorption maps using an active seismic experiment (~100 m 2 array with meter-scale station distance) at the Solfatara volcanic crater, in Southern Italy. This study demonstrates that absorption anomalies mark zones of high soil temperatures and CO 2 -bearing fluids migrating from an extended fault, whose features were reconstructed using peak delay maps.…”

Scattering Attenuation Images of the Control of Thrusts and Fluid Overpressure on the 2016–2017 Central Italy Seismic Sequence

“…In the near field, the parameter is also affected by multiple reflections/reverberations generated by structures with strong impedance contrasts (Gabrielli et al., 2020; King et al., 2022). To calculate and map peak delay times, we used the most recent version of the open‐access code MuRAT, applied both in volcanic (De Siena et al., 2016; Di Martino, De Siena, Serlenga, et al., 2022; Reiss et al., 2022) and tectonic settings (Borleanu et al., 2017; Gabrielli et al., 2022; Napolitano et al., 2020). We define peak delay time as the lag time between the S‐wave arrival and the maximum amplitude of the envelope for each filtered seismogram (Calvet et al., 2013; Takahashi et al., 2007).…”

“…During the AVN, attenuation fluctuations detect permeability changes (dilatation and crack closure), pore pressure rise, and fluid diffusion (Malagnini et al, 2022), implying rupture driven by damage (pore pressure drop) mechanisms, fault weakening, and connectivity (Convertito et al, 2020). As seismic attenuation is equally sensitive to fracturing and fluid migration, measuring and modeling in space its attenuation mechanisms (scattering attenuation and absorption) provide valuable and additional information on the presence of medium heterogeneities and structural irregularities (e.g., cracks), either fluid-filled or dry (Di Martino, De Siena, Serlenga, et al, 2022;Napolitano et al, 2020;Reiss et al, 2022). Gabrielli et al (2022) performed a 2D mapping of peak delay time and coda attenuation tomography before and after the three mainshocks of the AVN.…”