Fluid-driven seismic swarms in the Gripp valley (Haute-Pyrénées, France)

Abstract

SUMMARY An unusual seismic activity has recently occurred in the Gripp valley, located in the central part of the French Pyrenees. Since spring 2020, two new swarms appeared, clearly outside the usual location of the seismicity in this area. On 20 September 2020, almost concomitantly with the activation of the second seismic swarm, a hole suddenly opened in the bed of a local river, the Adour de Payolle. This hole drained the water from the river, which dried up over 500 m. We follow and study the spatial and temporal evolution of these clusters, using four temporary stations deployed a few days or months after the beginning of the crisis to complete the regional network. These additional data lead to the construction of a comprehensive catalogue of more than 4900 earthquakes, using both a template matching approach and a deep-learning based phase picking method to complete and improve the initial catalogue available from the French seismological agency. This allows highlighting a slow and clear migration of the seismicity during 1 yr. Precise absolute and relative event locations reveal a dipping faulting structure, confirmed by the focal mechanism estimated for the highest magnitude event of the sequence (ML 3). We propose to explain the observed migration of the seismicity by deep fluids going up through a newly discovered faulting structure.