A Physical Model of Sill Expansion to Explain the Dynamics of Unrest at Calderas with Application to Campi Flegrei

Abstract

Many calderas show remarkable unrest, which often does not culminate in eruptions (non-eruptive unrest). In this context the interpretation of the geophysical data collected by the monitoring networks is difficult. When the unrest is eruptive, a vent opening process occurs, which leads to an eruption. In calderas, vent locations typically are scattered over a large area and monogenic cones form. The resulting pattern is characterized by a wide dispersion of eruptive vents, therefore, the location of the future vent is not easily predictable. We propose an interpretation of the deformation associated to unrest and vent pattern commonly observed at calderas, based on a physical model that simulates the intrusion and the expansion of a sill. The model can explain both the uplift and any subsequent subsidence through a single process. Considering that the stress mainly controls the vent opening process, we try to gain insight on the vent opening in calderas through the study of the stress field produced by the intrusion of an expanding sill. We find that the tensile stress in the rock above the sill is concentrated at the sill edge in a ring-shaped area with radius depending on the physical properties of magma and rock, the feeding rate and the magma cooling rate. This stress field is consistent with widely dispersed eruptive vents and monogenic cone formation, which are often observed in the calderas. However, considering the mechanical properties of the elastic plate and the rheology of magma, we show that remarkable deformations may be associated with low values of stress in the rock at the top of the intrusion, thereby resulting in non-eruptive unrest. Moreover, we have found that, under the assumption of isothermal conditions, the stress values decrease over time during the intrusion process. This result may explain why the long-term unrest, in general, do not culminate in an eruption. The proposed approach concerns a general process and is applicable to many calderas. In particular, we simulate the vertical displacement as occurred in the centre of Campi Flegrei caldera during the last decades, and we obtain good agreement with the data of a leveling benchmark near the center of the caldera.

Radial interpolation of GPS and leveling data of ground deformation in a resurgent caldera: application to Campi Flegrei (Italy)

“…For example, in Berrino et al (1984) and Dvorak and Berrino (1991), the volume variation of the ground in 1980-1984 is about half of the inferred volume variation of the source, mainly because their analytical models include a significant deformation below the source. In contrast, the volume variation of the source during the minor uplifts of -2013in D'Auria et al (2015 and Giudicepietro et al (2017) is consistent with volume variation of the ground because their analytical model neglects any possible elastic deformation occurring below the source (Macedonio et al 2014;Giudicepietro et al 2016). We remark that, in general, the proportionality between the volume changes at the source and at the surface is dependent upon many factors, such as the elastic parameters and the depth of the source (e.g., Lisowski 2006).…”

Measurement of Seafloor Deformation in the Marine Sector of the Campi Flegrei Caldera (Italy)

“…During deformation episodes reported by the literature, Campi Flegrei has shown the maximum deformation in the center of the caldera, but the eruptive vents in the last 15 ka is not located in the zone of maximum uplift (Bevilacqua et al, 2015;Giudicepietro et al, 2017). Before an eruption, the radial symmetry of the deformation pattern is broken because of the intrusion of a lateral dike.…”

Insight Into Campi Flegrei Caldera Unrest Through Seismic Tremor Measurements at Pisciarelli Fumarolic Field

“…Starting from 2004, changes in seismicity (Chiodini et al, ; D'Auria et al, ; Saccorotti et al, ), deformation (Troise et al, ; Trasatti et al, ; D'Auria et al, ; De Martino et al, ; Giudicepietro et al, ; Iannaccone et al, ), and degassing activity (Cardellini et al, ; Chiodini et al, ; Chiodini et al, ; Tamburello et al, ) became evident in the Campi Flegrei caldera. These changes were the subject of scientific investigations that resulted in a remarkable production of articles over the recent years (Amoruso et al, ; De Siena et al, ; Di Luccio et al, ; Giudicepietro et al, ; Kilburn et al, ; Zaccarelli & Bianco, ; Zollo et al, ). In December 2012, the civil protection authorities raised the alert of the caldera from green level (base) to the current yellow level (attention) as consequence of a further increase in the deformation rate, seismicity and degassing (Chiodini et al, ).…”