Seismicity of La Soufrière Volcano (Guadeloupe Lesser Antillles): Evidence for Shallow Fluid Sources and Tectonic influence

Abstract:

Since the last volcanic unrest occurred in 1976-77, the Guadeloupe Volcanological and Seismological Observatory (OVSG-IPGP) has recorded a systematic progressive increasing of the volcanic activity in term of seismicity and degassing, with the progressive reactivations of fumarolles. Between 1981 and 2013, this activity is characterized by a low magnitude seismicity occurred in swarm sequences. Observatory’s standard locations shows that more than 95% of seismic events is concentrated in a volume of ∼ 125 km³ in the shallow part of the volcano (5 kms ×5 kms × 5 kms), centred below the andesitic dome. Following these features, we investigated the relationship between this seismicity and the dynamic of the hydrothermal system. First, we perform an iterative travel times delays inversion to estimate the Vp/Vs ratio and study physical and rheological medium properties and his evolution. Then, we analyse the b-value behaviour before to perform an autoregressive analysis on codas of monochromatique events to identify sources of resonance phenomena. Finally, we perform a multiplet analysis by waveform cross-correlation to identify repetitive sources. Global b-value (∼ 1.33) and low Vp/Vs (∼ 1.50) resulting suggest the presence of hydrothermal fluids in a fractured and altered medium which is in good agreement with results of autoregressive analysis. Considering their temporal and spatial evolutions, we are able to estimate a storage depth of fluids mixage and we can infer a temporal overpressure in the system. Following the multiplets analysis results, we highlight a brutal dynamic change after the major local earthquake of Les Saintes (21 November 2004, M=6.3). In summary, we can infer that seismic events occurred at La Soufrière volcano are related to two different source mechanisms: (1) fracturing processes of rocks and (2) resonance of cracks or conduits filled with hydrothermal fluid with a brutal change by a stress modification within the volcano structure.