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• W2116467235 abstract "Subsurface volume and pressure increases triggering surface inflation at active calderas are generally deduced by inverting ground-deformation time-series using isotropic and homogeneous half-space models (IHM). These models represent simplified mathematical analogues of the mechanical behaviour of the Earth’s crust. Using three-dimensional numerical modelling, we show that lateral discontinuities such as intracalderaor caldera-ring-faults can significantly amplify and distort the ground deformation pattern during unrest. As a consequence, data inversions using IHMs, which do not consider lateral discontinuities, can provide erroneous results on causative source parameters. We also find that the degree of amplification and distortion in the form of abrupt changes in displacement/distance gradients in proximity to faults is dependent on source geometry. Prolate bodies represent a particularly critical geometry for which pressure increases may be overestimated by a factor of up to three. Our 3D analysis suggests that amplification effects can be much larger than predicted by earlier 2D models. We validate theoretical results by applying our model to investigate the effect of boundary faults and source geometries on the displacement field during ground uplift at the restless calderas of Campi Flegrei (Italy) and Sierra Negra (Galapagos Islands, Ecuador). Based on the discrepancy in results from IHMs and our numerical model, we argue that employing IHMs for inversion of ground displacement and gravity time-series may in some cases lead to a biased assessment of hazards associated with ground uplift. Volcanic calderas are (sub)circular surface depressions of up to tens of kilometres in diameter and of up to several hundred metres in topographic change from rim to floor. Calderas are thought to be formed by slip along boundary faults as a result of gravitational collapse of overlying rocks into an emptying reservoir during large-scale volcanic eruptions (Lipman 2000). Many calderas undergo episodes of unrest associated with seismicity and/or ground deformation (Newhall & Dzurisin 1988). Periods of ground inflation are of particular interest for volcanic hazard assessment. Inflation is generally interpreted to result from a subsurface volume and pressure increase, which may be caused by a number of processes. Among those, replenishment of magma into an existing reservoir is a key candidate (Battaglia et al. 1999), and also a potential trigger for volcanic eruptions (Murphy et al. 1998). Alternatively, volume/pressure increases within a subsurface hydrothermal system may equally well account for significant ground deformation (Bonafede & Mazzanti 1998). Unrest culminating in the renewal of volcanic activity at a caldera in a densely populated area, such as, for example, Campi Flegrei (Italy), will cause significant socio-economic disruption for local communities and beyond, and it is therefore important to assess the nature of the source that causes the unrest, in order to assess hazards and mitigate risks. A widely applied technique to quantify subsurface volume/pressure changes is the inversion of ground deformation data time-series. Point (Mogi 1958; McTigue 1987) or extended (Davis 1986; McTigue 1987; Yang et al. 1988; Fialko et al. 2001) pressure sources embedded within an elastic or anelastic medium (Dragoni & Magnanensi 1989; Bonafede 1990; Hofton et al. 1995; Fernandez et al. 1997) under certain assumptions will permit analytical solutions. Simplified physical models that have analytical solutions (e.g. the Mogi model) are extremely popular and enormously facilitate the solution of the inverse problem. However, there is evidence that surface deformation at active calderas is 110 A. FOLCH & J. GOTTSMANN likely to be strongly influenced by the presence of faults and discontinuities (De Natale & Pingue 1993; De Natale et al. 1997; Beauducel et al. 2004), which are not considered by isotropic and homogeneous half-space models (IHM). One may conclude that the spatial distribution of the deformation field at caldera-type volcanoes is limited by the presence of caldera boundary faults. The motivation for this study lies in our recognition of the often unrealistic parameters obtained from inversion for ‘simple’ analytical models, even though their synthetic deformation data show an excellent fit to the observed data. In this framework, the omission of lateral medium discontinuities in the inverse problem may cause significant misfits leading to unrealistic values for source parameters such as the associated pressure change. This paper is organized into two parts: the first part provides a parametric study of the influence of faults on the surface deformation pattern. Results can be viewed as an extension of previous studies (De Natale & Pingue 1993) to 3D geometries. In the second part, we investigate the interaction of caldera boundary faults with the displacement field at two recently uplifting caldera volcanoes, the Campi Flegrei Caldera, Italy, and the Sierra Negra Caldera, Ecuador. Our results have important consequences for geodetic data inversion and its contribution to hazard assessment and risk mitigation in areas undergoing ground deformation during caldera unrest. Caldera faults and modelling framework We will limit our investigation to ground uplift resulting from a subsurface volume/pressure increase. From the perspective of hazard assessment, this scenario has the greatest potential to provide unrealistic results for subsurface kinetics from analytical data inversion if oversimplifications in the modelling framework are not accounted for. In this study we are concerned with a volume/pressure increase beneath the caldera floor, which is bounded by either ringfaults that represent the structural margins of a caldera (Gudmundsson et al. 1997; Acocella et al. 2000) or by intracaldera structural discontinuities that dissect the floor into individual blocks (Orsi et al. 1996). In either case, field and geophysical investigations suggest that fault inclination is generally subvertical to vertical (Mori & McKee 1987; Orsi et al. 1996; Prejean et al. 2002). For this study the physical phenomenon responsible for the pressure increase in the source is irrelevant, and can be of either magmatic and/or hydrothermal origin. Consider the scenario depicted in Figure 1. A source of dilatation undergoing pressure changes is embedded in a fractured medium. The model geometry can be characterized in terms of two dimensionless scaling quantities, e and jf. The former is the ratio of source depth d (measured from the surface to the top of the source) to its horizontal extension a (e=d/a). Depending on Fig. 1. Sketch of the model. A pressure source of characteristic horizontal extension a and depth d is embedded within a fractured linear elastic medium. Ring-faults (or intracaldera structural discontinuities) of averaged length Lf and dip angle a are located at a distance rf from the source centre. Faults may start at variable depths and extend downwards to the top of the pressure source. 111 FAULTS AND GROUND UPLIFT AT CALDERAS the geometry being considered, a represents the source radius (spherical source), the semi-major axis (oblate spheroid), or the semi-minor axes (prolate spheroid). The latter dimensionless number is expressed as the ratio of the distance between the centre of the source and the tip of the fault rf to a (jf=rf /a). We will assume axial symmetry – a reasonable hypothesis in the context of collapse calderas – and a flat free surface located at z=d=0. Equations of linear elasticity have been solved numerically via a finite-element method with nodal implementation (Codina & Folch 2004) for different source geometries and combinations of the scaling parameters, fault dip angle a, and fault lengths Lf. The fault lengths are chosen such that they extend from various depths downwards to a penetration depth equal to d; i.e. they extend to depths equal to that of the top of the source. As a consequence, we can explore the effect of the presence of caldera-fill successions overlying the fractured medium on the deformation field. Boundary conditions imposed at faults are zero shear strength (corresponding to null friction) and the non-overlapping condition (De Natale & Pingue 1993). The effect of the presence of faults on the intracaldera deformation field is compared to the predictions of the maximum displacements (Uz for vertical and Ur for horizontal) obtained from applying an IHM. The purpose of this normalization is twofold. First, it allows an immediate estimation of the error incurred by applying an IHM to model mechanical behaviour in faulted environments. Second, results become independent of source overpressure and the elastic properties of the medium. Standard values for rigidity m=10 GPa and Poisson’s ratio n=0.25 have been assumed in the simulations." @default.
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• W2116467235 title "Faults and ground uplift at active calderas" @default.
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