The effect of collapse structures on ground deformations in calderas

Abstract

Abstract. Ground defom•ations related to unrest episodes in calderas are generally interpreted, like in other volcanic environments, in terms of increased pressure within a magma chalnber embedded in a continuous, elastic medium. h• this framework, the depth of the pressure source is inferred from the size of the deformed area. This scheme works quite xvell for individual volcanoes (for instance Hawaiian shields) where a good correspondence between inflation events and eruptive episodes has been observed. Ground deformation in calderas, however, show unusual features that are difficult to interpret within such a scheme, because considerable positive deformation (in the order of several meters) and microfracturing (i.e. intense seismicity) may occur without eruptions, altough the estimated depth for the magma source is sometime very shallow (less than 1-2 lon). It has been recently suggested that the contact zones at the border of collapsed calderas, could act as stress-strain discontinuity zones and thus bias modeling results obtained for continuous elastic media. We use both observations and theoretical modeling based on 3-D finite element techniques to show that ground deformations in collapsed calderas are strongly influenced by the caldera structure, giving a new perspective in related geological and geophysical observations in such areas.

Numerical models for ground deformation and gravity changes during volcanic unrest: simulating the hydrothermal system dynamics of a restless caldera

“…Ring faults significantly alter strain partitioning and fluid propagation and hence must be considered for the interpretation of geophysical signals (De Natale and Pingue, 1993;De Natale et al, 1997;Beauducel et al, 2004;Folch and Gottsmann, 2006;Troiano et al, 2011;Jasim et al, 2015). In this paper we explore the impact of vertical and lateral mechanical heterogeneities in the shallow crust beneath the CF, including ring faults, on monitoring signals at the surface (ground deformation and gravity changes) as a consequence of unrest caused by a perturbation of the shallow hydrothermal system.…”

Modeling of recent volcanic episodes at Phlegrean Fields (Italy): geochemical variations and ground deformation

“…Many of these authors describe the magma chamber as a pressure source embedded in a homogeneous half space, with elastic or visco-elastic properties. Later on, the effects of structural discontinuities in controlling both the magnitude and the spatial extent of ground deformation were introduced (De Natale and Pingue, 1993;De Natale et al, 1997). The role of heating and expansion of hydrothermal fluids during a bradyseismic event was accounted for in a physical model by Bonafede (1991), and later on by other authors who recognized the importance of hydrothermal circulation not only to explain some of the observed ground uplift, but also to account for the following subsidence phase (Gaeta et al, 1998;Orsi et al, 1999;De Natale et al, 2001;Castagnolo et al, 2001).…”

Modeling of Stress Changes at Mayon Volcano, Philippines

“…In addition, gravity calculations must be modeled separately, requiring an iterative inversion process. More complicated models have been proposed for the modeling of deformation (DAVIS, 1986;YANG et al, 1988;BONAFEDE, 1991;LANGBEIN et al, 1995;DE NATALE et al, 1997;FOLCH et al, 2000;TIAMPO et al, 2000;FERNA´NDEZ et al, 2001b), but the joint modeling of gravity and deformation has remained elusive, despite the obvious benefits (BONAFEDE and MAZZANTI, 1998;JENTZSCH et al, 2001b;BATTAGLIA and SEGALL, 2007).…”

“…The Mogi model has been extensively applied in modeling ground deformations in volcanic areas and has been successful primarily in explaining the vertical component of the ground deformations. However, this model often poses difficulties in simultaneously modeling all components of the observed displacements in conjunction with gravity changes (KISSLINGER, 1975;RYMER et al, 1993;RYMER, 1996), and there is a large body of evidence for ground deformations and seismicity at calderas and other volcanic areas that cannot be modeled by these purely elastic effects (e.g., EGGERS, 1987;BERRINO and CORRADO, 1991;BONAFEDE, 1991;RYMER et al, 1993;DE NATALE et al, 1997;GAETA et al, 1998;JAHR et al, 1998;WATANABE et al, 1998;JENTZSCH et al, 2001b;PRITCHARD and SIMONS, 2002).…”