Retrieving the Stress Field Within the Campi Flegrei Caldera (Southern Italy) Through an Integrated Geodetical and Seismological Approach

Abstract

We investigated the Campi Flegrei caldera using a quantitative approach to retrieve the spatial and temporal variations of the stress field. For this aim we applied a joint inversion of geodetic and seismological data to a dataset of 1,100 optical levelling measurements and 222 focal mechanisms, recorded during the bradyseismic crisis of 1982-1984. The inversion of the geodetic dataset alone, shows that the observed ground deformation is compatible with a source consisting of a planar crack, located at the centre of the caldera at a depth of about 2.56 km and a size of about 4 9 4 km. Inversion of focal mechanisms using both analytical and graphical approaches, has shown that the key features of the stress field in the area are: a nearly subvertical r 1 and a sub-horizontal, roughly NNE-SSW trending r 3. Unfortunately, the modelling of the stress fields based only upon the retrieved ground deformation source is not able to fully account for the stress pattern delineated by focal mechanism inversion. The introduction of an additional regional background field has been necessary. This field has been determined by minimizing the difference between observed slip vectors for each focal mechanism and the theoretical maximum shear stress deriving from both the volcanic (time-varying) and the regional (constant) field. The latter is responsible for a weak NNE-SSW extension, which is consistent with the field determined for the nearby Mt. Vesuvius volcano. The proposed approach accurately models observations and provides interesting hints to better understand the dynamics of the volcanic unrest and seismogenic processes at Campi Flegrei caldera. This procedure could be applied to other volcanoes experiencing active ground deformation and seismicity.

Determining the Stress Field in Active Volcanoes Using Focal Mechanisms

“…Related B values vary between 0.3 and 0.8 (Figure 8). According to D'Auria et al (2014b), this result is compatible with the presence of a varying stress field related to a source of deformation, located at about 2.7 km depth, that during inflation and deflation episodes (associated to an increased seismicity) is able to overcome the weak regional extensional stress field having a NNE-SSW trend.…”

“…Conversely, the temporal selection of data can be conveniently done in the analysis of seismological datasets where the focal parameters are well known. In volcanic environments, a spatio-temporal analysis of the stress fields represents the best way to infer about the volcano dynamics (Wyss et al, 1992;Hardebeck and Michael, 2006;D'Auria et al, 2014b;Plateaux et al, 2014). In this perspective, a supervisioned splitting of data in 3D sub-volumes represents the best solution to retrieve the spatial distribution of active coeval stress fields.…”

“…The major drawback of this technique is the difficulty in attributing to the retrieved stress field components a proper spatio-temporal collocation. Therefore heterogeneous datasets can be approached, more properly, taking into account spatial and possibly temporal variations of the stress pattern (Wyss et al, 1992;Hardebeck and Michael, 2006;D'Auria et al, 2014b). In other words, these methods take into account the spatiotemporal distribution of the data, providing as results not a single stress tensor, but a spatially (and possibly temporally) variable stress field.…”

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

“…Moreover, in many cases, the structure of the shallow plumbing system and the dynamic processes in calderas are consistent with the emplacement of sill-like magma intrusions (Okada, 1985;Amoruso et al, 2007;Onizawa et al, 2007;Jónsson, 2009;Chang et al, 2010;Woo and Kilburn, 2010;Chadwick et al, 2011;Maccaferri et al, 2011;Menand, 2011;Unglert et al, 2011;Bagnardi and Amelung, 2012;Manconi and Casu, 2012;Bagnardi et al, 2013;Corbi et al, 2015;D'Auria et al, 2015a;Richardson et al, 2015;Rivalta et al, 2015;Trasatti et al, 2015;Le Mével et al, 2016). Usually the intrusion of a sill is modeled as a pressure increase in a horizontal crack (e.g., Fialko et al, 2001;Woo and Kilburn, 2010) which causes deformations and ground uplift.…”

“…However, the stress in deep rocks cannot be easily measured, therefore it is usually unknown. We can only investigate the stress conditions through geophysical methods, e.g., we can retrieve the orientation of the stress tensor through the inversion of focal mechanisms of earthquakes (D'Auria et al, 2015a;Massa et al, 2016). Conversely, the ground deformation, that is considered as an indicator of stress in the rock, can be measured quite easily through various techniques.…”