Volcano electrical tomography unveils edifice collapse hazard linked to hydrothermal system structure and dynamics

Abstract

Catastrophic collapses of the flanks of stratovolcanoes constitute a major hazard threatening numerous lives in many countries. Although many such collapses occurred following the ascent of magma to the surface, many are not associated with magmatic reawakening but are triggered by a combination of forcing agents such as pore-fluid pressurization and/or mechanical weakening of the volcanic edifice often located above a low-strength detachment plane. The volume of altered rock available for collapse, the dynamics of the hydrothermal fluid reservoir and the geometry of incipient collapse failure planes are key parameters for edifice stability analysis and modelling that remain essentially hidden to current volcano monitoring techniques. Here we derive a high-resolution, three-dimensional electrical conductivity model of the La Soufrière de Guadeloupe volcano from extensive electrical tomography data. We identify several highly conductive regions in the lava dome that are associated to fluid saturated host-rock and preferential flow of highly acid hot fluids within the dome. We interpret this model together with the existing wealth of geological and geochemical data on the volcano to demonstrate the influence of the hydrothermal system dynamics on the hazards associated to collapse-prone altered volcanic edifices.

Three‐dimensional density structure of La Soufrière de Guadeloupe lava dome from simultaneous muon radiographies and gravity data

“…The upper, well-resolved part of this anomaly coincides with the electrically conductive region found by Rosas-Carbajal et al [2016] with the inversion of electrical resistivity tomography (ERT) data. Slices of the resulting electrical conductivity model at the same depths as in Figures 3b and 3d) are presented in Figure 4.…”

“…These values are low compared to the typical density of 2.7 g cm −3 found for andesitic rocks at La Soufrière [Komorowski et al, 2008]. Rosas-Carbajal et al, 2016], we reestimated the total ejected volume of this eruption to be 5-6 ×10 6 m 3 . Indeed, using a new analysis of data from the last (1976) phreatic eruption [cf.…”

“…This anomaly may be structurally controlled as it follows the detachment plane of an ancient collapse [Rosas-Carbajal et al, 2016]. Rosas-Carbajal et al, 2016]. In the south southwest (Figures 3c and 3d) and in the northern extreme at z = 1372 m ( Figure 3b) the high density coincides with low electrical conductivity anomalies (Figures 4a and 4b), revealing an extensive volume of unaltered dome rock.…”

“…The most extensive low-density region is found below the southern part of the summit (Figures 3a-3c), where the most presently active fumaroles are located. This region counts several deep (up to 122 m) pits, craters, and fractures that were active during the last six phreatic eruptions, as well as evidence of rock alteration [Rosas-Carbajal et al, 2016]. The low-density anomaly is well resolved since the three telescopes are oriented to cover this part of the dome where many gravity measurements were done (cf.…”

“…Horizontal slices of the 3-D electrical conductivity model byRosas-Carbajal et al [2016] at the same altitude as slices inFigures 3b and 3c. High conductivity values correspond to hydrothermally altered, hot-fluid-saturated rocks.…”