Volcano-tectonic interactions revealed by inversion of focal mechanisms: stress field insight around and beneath the Vatnajökull ice cap in Iceland

Abstract

Volcano-tectonic processes in the central part of Iceland, covered by the Vatnajökull glacier, are investigated by inversion of focal mechanisms. Working on a large catalog of focal mechanisms determined by the Icelandic Meteorological Office (IMO), we used a damped regional-scale stress inversion method to obtain an insight of kilometric variations of the stress field. To evaluate the resolution and the stability of this stress field solution, we computed checkerboard tests, stress field models and error propagation tests. Stress field models showed a continuous stress regime between normal and strike-slip faulting, associated with a high stress shape ratio (i.e., σ 1 ≈ σ 2 ). Two main directions of σ hmin were evidenced: the first one was in agreement with the regional spreading direction of Iceland and the second one was deviated, being almost perpendicular to the first one. The deviated stress direction is sustained through the 20 year time-span of the study around the Bárðarbunga and Grimsvötn central volcanoes while the spreading direction remains predominant around the Hamarinn volcano. This result supports the hypothesis that this volcano lacks collapse caldera and shares a fissure swarm with the larger Bárðarbunga volcano. On a smaller temporal scale, during the 1996 volcanic crisis, a bimodal distribution of σ hmin showed two opposite strike-slip regimes where the deviated direction dominated. Because these two states of stress T1 and T2 show stress regimes away from the Andersonian positions, P, B, and T axes, the rapid flip between these two regimes may be associated with the progressive melt intrusion of a dyke.

Determining the Stress Field in Active Volcanoes Using Focal Mechanisms

“…The proposed processing approach of FPS datasets in active volcanic areas is suitable to be applied to a wide range of contexts. Finally, the results presented in this work suggest that these methods could be useful also as a near real-time monitoring tool to characterize spatial and temporal variations in the stress field linked to a volcanic unrest (e.g., Toda et al, 2002;Plateaux et al, 2014). A final consideration concerns the relevance of the stress-inversion approach based on the integration and/or the joint inversion of seismological and geodetical datasets.…”

“…In volcanic environments, a spatio-temporal analysis of the stress fields represents a valuable approach to infer the volcano dynamics (Wyss et al, 1992;Hardebeck and Michael, 2006;Gudmundsson et al, 2009;Plateaux et al, 2014;Costa and Marti, 2016). Stress field variations play a key role in driving magmas and/or fluids migration.…”

“…The relevance of this point becomes larger considering the seismological datasets collected in volcanic areas. Here, the retrieving of stress field spatial variations represents a crucial task: sensible variation in stress axes attitude can characterize stress field active in contiguous volumes (e.g., D'Auria et al, 2014a,b;Plateaux et al, 2014). For this reason, unsupervisioned stress inversions performed on an unselected dataset can be quite useless.…”

“…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.…”

“…These procedures allow the reconstruction of the reduced stress field that carries information about the stress orientation and the ratios between principal stresses. Inversion of focal mechanism datasets became a common task in studies aimed at constrain seismotectonic stress field in active tectonic areas (CareyGailhardis and Mercier, 1992;Gillard and Wyss, 1995;Hardebeck and Hauksson, 2001;Massa, 2003;Angelier and Baruah, 2009;De Matteis et al, 2012) and more recently also in volcanic environments (Wyss et al, 1992;Cocina et al, 1997;D'Auria et al, 2014a,b;Plateaux et al, 2014). A wide range of graphical and analytical methods were proposed by authors in last decades (e.g., Angelier and Mechler, 1977;Gephart and Forsyth, 1984;Carey-Gailhardis and Mercier, 1987;Lisle, 1987Lisle, , 1988Michael, 1987;Angelier, 1990Angelier, , 2002Rivera and Cisternas, 1990;D'Auria and Massa, 2015).…”