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Resistivity variations related to the large March 9, 1998 eruption at La Fournaise volcano inferred by continuous MT monitoring

Abstract : The 2645 m-high La Fournaise volcano, located in the Southwest of Reunion Island (Indian Ocean), is a shield basaltic volcano where effusive eruptions generally occur along long fissures starting from the summit, alongside major fractures that characterize the eruptions' dynamism and effusivity. Between 1992 and 1998, the volcano underwent a quiet period during which few earthquakes were recorded. Minor seismic activity returned after 1997 and picked up in March 1998 during the 35 h preceding the March 9 eruption. From 1996, two autonomous stations (CSV and BAV) were installed on the volcano. CSV was located inside the Enclos Fouque caldera while BAV was positioned 82 km NW of the volcano summit. Horizontal components of the electric and magnetic fields were sampled every 20 s. Continuous time-series were available from 1996 to 1999 at CSV, and from 1997 to March 1998 at BAV. Data have been processed using both single-station and remote-reference processing. Both results show apparent resistivity variations synchronous to the eruption. Time-lapse impedance estimates are computed on overlapping time windows of about two days at both stations. The only major decrease of the observed impedance coincides with the March 1998 eruption. At CSV, the resistivity started to drop about five days before the eruption, reached several local minima until April, and then slowly increased as the volcanic crisis reduced in activity. After the end of the crisis in September 1998, the apparent resistivity recovered its pre-crisis value. The time-lapse results also show variability in directionality: sharp and elongated phase tensor ellipse residuals appear during the eruption with a N105 degrees orientation, suggesting the emergence of an almost NS-striking dyke. A 1D background model built from MT soundings performed during the quiet period (1996 to February 1998) on which a 3D NS-striking dyke was added shows a good agreement with phase tensor residuals and spatial distribution of the resistivity variations observed during the eruption.
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https://hal-brgm.archives-ouvertes.fr/hal-01849829
Contributeur : Myriam Chergui <>
Soumis le : jeudi 26 juillet 2018 - 15:07:58
Dernière modification le : vendredi 18 septembre 2020 - 08:42:04

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Pierre Wawrzyniak, Jacques Zlotnicki, Pascal Sailhac, Guy Marquis. Resistivity variations related to the large March 9, 1998 eruption at La Fournaise volcano inferred by continuous MT monitoring. Journal of Volcanology and Geothermal Research, Elsevier, 2017, 347, pp.185 - 206. ⟨10.1016/j.jvolgeores.2017.09.011⟩. ⟨hal-01849829⟩

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