A three‐dimensional Q_P imaging of the shallowest subsurface of Campi Flegrei offshore caldera, southern Italy

Abstract

To improve the knowledge of the shallowest subsurface of Campi Flegrei caldera, a 3‐D P wave attenuation tomography of the area was performed. We analyzed about 18,000 active seismic traces, which provided a data set of 11,873 Δt* measurements, e.g., the differential travel times to quality factor ratios. These were inverted through an adapted tomographic inversion procedure. The 3‐D tomographic images reveal an average QP about 70, interpreted as water‐saturated volcanic and marine sediments. An arc‐like, low‐QP structure at 0.5–1 km depths was interpreted as a densely fractured, fluid‐saturated rock volume, well matching the buried rim of Campi Flegrei caldera. The spatial distribution of high‐ and low‐QP bodies in the inner caldera is correlated with low‐Vp values and may reflect either the differences in the percentage of fluid saturation of sediments or the presence of vapor state fluids beneath fumarole manifestations.

Space‐weighted seismic attenuation mapping of the aseismic source of Campi Flegrei 1983–1984 unrest

“…In our interpretation, it is thus the attenuation signature of hydrothermal fluids migrating through fractures and faults [ Piochi et al , ] and today mixed with meteoric waters [ Petrillo et al , ] offshore Mount Nuovo. The same volumes are today visible as the densely fractured, fluid‐saturated buried rim of the caldera down to 1 km depth [ Serlenga et al , ]. Seismicity shows how fluids migrate from the caldera center, entering the Campi Flegrei upper hydrothermal systems in 1984 [ Battaglia et al , ; Petrillo et al , ].…”

High Resolution Attenuation Images From Active Seismic Data: The Case Study of Solfatara Volcano (Southern Italy)

“…In particular, spectral ratio method is a very useful technique in processing and analysis of signals generated by natural seismic sources. Due to some complications related to the application of this method (Sams and Goldberg, 1990;Serlenga et al, 2016) and since an active seismic dataset was used for our purposes, we proceeded in a different way. Actually, in this case, the source contribution is known.…”

“…However, due to the geological heterogeneity of volcanic media, a simple description of the subsoil in terms of elastic properties may be not sufficient for a thorough understanding of volcanic dynamics. The body wave quality factor Q, describing the anelastic attenuation of the medium, is more sensitive to the rock's physical properties such as temperature, porosity, permeability, and degree of fluid saturation than elastic parameters (Sanders et al, 1995;de Lorenzo et al, 2001;Serlenga et al, 2016;Amoroso et al, 2017). Therefore, especially for volcanic areas, it is very useful to integrate the information provided by elastic imaging with that coming from an anelastic attenuation model.…”

“…The inversion can be performed by using, for example, a tomographic method. This strategy is usually adopted when the velocity model is known and, consequently, the ray-paths are fixed (Schurr et al, 2003;Martinez-Arevalo et al, 2005, Serlenga et al, 2016Amoroso et al, 2017).…”

“…It is well known that the initial guess affects the final solution retrieved by a linearized inversion method (Kissling et al, 1994). In the anelastic attenuation tomography, a homogenous attenuation model is commonly used as the initial model (De Gori et al, 2005;Bisrat et al, 2013;Serlenga et al, 2016). Thus, an intermediate step between the t * estimations and their inversion is required to obtain a reference Q model to be used as the starting point for the inversion.…”