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Full, constrained and stochastic source inversions support evidence for volumetric changes during the Basel earthquake sequence

Guilhem, Aurélie ; Walter, Fabian

In: Swiss Journal of Geosciences, 2015, vol. 108, no. 2-3, p. 361-377

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    Titre
    • Full, constrained and stochastic source inversions support evidence for volumetric changes during the Basel earthquake sequence
    Auteur
    • Guilhem, Aurélie. CEA, DAM, DIF, 91297, Arpajon, France
    • Walter, Fabian. Swiss Seismological Service, ETH Zurich, Sonneggstrasse 5, 8092, Zurich, Switzerland
    Type de document
    • Postprint
    Langue
    • Anglais
    Publié dans
    • Swiss Journal of Geosciences, 2015, vol. 108, no. 2-3, p. 361-377. Springer Basel
    Autre version électronique
    Identifiant OAI-PMH
    • oai:doc.rero.ch:332762
    Summary
    Co-seismic volumetric changes are often interpreted as tensile fracturing in response to fluid injection during geothermal reservoir stimulation. Such volumetric changes manifest themselves as isotropic moment tensor components, which may thus constitute a measure for hydraulic stimulation efficiency. Recent analyses found significant isotropic moments of M2+ earthquakes during the 2006 hydraulic stimulation of a geothermal reservoir in Basel, Switzerland. The results contradicted first-motion focal mechanisms, which are in close agreement with shear sources without volumetric changes. Here we revisit the magnitude 1.7+ Basel events with full and stochastic moment tensor inversions in order to provide additional and/or supporting evidences for the occurrence of volumetric sources, if any. We furthermore apply purely deviatoric, and superimposed tensile and shear fault mechanisms, which we believe are meaningful constraints for fluid-induced earthquakes. As a result, we only find a single earthquake with statistically significant volumetric faulting. Spatial and temporal patterns of fluid-induced sources therefore have to be taken as indicative only, even though they suggest a clear relationship between fluid injections and fault mechanisms. On the other hand, we confirm that most inverted moment tensors (including the statistically significant one) show some inconsistencies with first motion focal mechanisms. We suggest that this is a manifestation of a more complicated fault geometry, which none of our moment tensor constraints can describe.