Tectonic stress regime in the 2003–2004 and 2012–2015 earthquake swarms in the Ubaye Valley, French Alps
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We study two earthquake swarms that occurred in the Ubaye Valley, French Alps within the past decade: the 2003–2004 earthquake swarm with the strongest shock of magnitude ML = 2.7, and the 2012–2015 earthquake swarm with the strongest shock of magnitude ML = 4.8. The 2003–2004 seismic activity clustered along a 9-km-long rupture zone at depth between 3 and 8 km. The 2012–2015 activity occurred a few kilometres to the northwest from the previous one. We applied the iterative joint inversion for stress and fault orientations developed by Vavryčuk (2014) to focal mechanisms of 74 events of the 2003–2004 swarm and of 13 strongest events of the 2012–2015 swarm. The retrieved stress regime is consistent for both seismic activities. The σ3 principal axis is nearly horizontal with azimuth of ~ 103°. The σ1 and σ2 principal axes are inclined and their stress magnitudes are similar. The active faults are optimally oriented for shear faulting with respect to tectonic stress and differ from major fault systems known from geological mapping in the region. The estimated low value of friction coefficient at the faults 0.2–0.3 supports an idea of seismic activity triggered or strongly affected by presence of fluids.
KeywordsUbaye Valley south-western Alps earthquake swarm focal mechanism stress inversion fluids
The authors thank two anonymous reviewers for their helpful reviews and Dr. François Thouvenot and the SISMALP seismic network for providing the data. The study was supported by the Grant Agency of the Czech Republic, projects P210/12/2336 and 16-19751J, and by the Slovak Foundation Grant VEGA-2/0188/15. This work was carried out thanks to the support of the long-term conceptual development research organization RVO 67985891.
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