The stress field beneath a quiescent stratovolcano: The case of Mount Vesuvius

Abstract

We have analyzed a focal mechanism data set for Mount Vesuvius, consisting of 197 focal mechanisms of events recorded from 1999 to 2012. Using different approaches and a comparison between observations and numerical models, we have determined the spatial variations in the stress field beneath the volcano. The main results highlight the presence of two seismogenic volumes characterized by markedly different stress patterns. The two volumes are separated by a layer where the seismic strain release shows a significant decrease. Previous studies postulated the existence, at about the same depth, of a ductile layer allowing the spreading of the Mount Vesuvius edifice. We interpreted the difference in the stress pattern within the two volumes as the effect of a mechanical decoupling caused by the aforementioned ductile layer. The stress pattern in the top volume is dominated by a reverse faulting style, which agrees with the hypothesis of a seismicity driven by the spreading process. This agrees also with the persistent character of the seismicity located within this volume. Conversely, the stress field determined for the deep volume is consistent with a background regional field locally perturbed by the effects of the topography and of heterogeneities in the volcanic structure. Since the seismicity of the deep volume shows an intermittent behavior and has shown to be linked to geochemical variations in the fumaroles of the volcano, we hypothesize that it results from the effect of fluid injection episodes, possibly of magmatic origin, perturbing the pore pressure within the hydrothermal system.

Determining the Stress Field in Active Volcanoes Using Focal Mechanisms

“…A top volume dominated by a gravitational volcanic spreading that allows the setting of extensional stress fields (low B values and sub-vertical σ 1 ) active in the analyzed sub-volumes. A bottom volume strictly related to a regional extensional background stress field in accordance with the S h derived by breakout data (Figure 6; Borgia et al, 2005;Montone et al, 2012;D'Auria et al, 2014a). The computation of the stress field using the BRTM on a regular three-dimensional grid allows the 3-D figuration of the stress fields "simultaneously" acting from the surface to about 8-km depth (Figures 5, 6).…”

“…The strain field resulting from the subsidence of the Vesuvius and the asymmetric spreading of the southern portion of the Somma edifice creates an overall NS compression (see Figure 17 in D' Auria et al, 2014a). This ongoing process is also the source of the persistent seismicity located within the top-volume (Borgia et al, 2005;D'Auria et al, 2013D'Auria et al, , 2014a; Figures 5, 6). Additionally, the low B values associated to stress inversion in the top volume confirm that the retrieved fields are strongly driven by the σ 1 .…”

“…The Mesozoic basement of the volcano is displaced by both SW-and NW-dipping normal fault systems and secondarily by NE-SW and E-W faults (Bianco et al, 1998;Ventura and Vilardo, 1999;Zollo et al, 2002;Acocella and Funiciello, 2006; Figure 4). Mesoscale faults and eruptive fractures striking NW-SE, NE-SW and ENE-WSW affect volcanic units outside the Somma caldera (Rosi et al, 1987;Andronico et al, 1995;Ventura and Vilardo, 1999;D'Auria et al, 2014a, Figure 1 for a review). Seismicity at Mt.…”

“…These two volumes appear to be separated by a volume with a markedly reduced seismic strain release, possibly corresponding to a ductile sedimentary layer buried at about 1000 m b.s.l. (Bernasconi et al, 1981;Borgia et al, 2005;D'Auria et al, 2014a).…”

“…According to previous studies, the bottomvolume seems to be deeply related to a regional background stress field slightly perturbed by local heterogeneities in the volcanic structure and by the complex topography of the volcano. The intermittent behavior of the seismic activity in the deepest volume is probably due to a relation with the dynamics of the hydrothermal system: the pore pressure within the hydrothermal system can be perturbed by episodes of fluid injection that essentially influence the stress field pattern in the bottom-volume (Figures 5, 6; Chiodini et al, 2001;D'Auria et al, 2014a). The B values associated to stress inversion in the bottom volume show that for the 1-and 3-km depth-slices the three principal stresses Ippolito et al, 1973;Di Vito et al, 1999;Lavecchia et al, 2003;Acocella and Funiciello, 2006;Milia et al, 2013;Vitale and Isaia, 2013;D'Auria et al, 2014a,b, and referencer therein. are well defined (overall, 0.3 < B < 0.5).…”