Seismic clusters under the Maladeta massif in the central Pyrénées

Summary

The North Maladeta fault system, reactivated after the Alpine orogeny as a normal fault (Ortuño et al., 2008; Ortuño and Viaplana-Muzas, 2018) extends for almost 75 km across the border between Spain and France. This system is considered the most probable source of the historical M6.3 Ribagorça earthquake (1373). Within the system, the Coronas Fault is a secondary fault subparallel to the North Maladeta Fault and located in the southern slope of the Maladeta massif. Recent seismic activity in the massif has markedly increased since 2020, with previously unknown clusters of shallow earthquakes detected. Using a Template Matching approach, we verified that this apparent increase is not an artifact of network evolution, identifying additional clusters during 2014–2016, with far fewer events however. Relocation and focal mechanism analysis suggest that most of the seismicity is associated with the Coronas Fault, which appears to be less vertical and flatter than previously assumed. The seismicity is distributed on both sides of the fault, surrounding a 5–6 km wide aseismic segment. Another cluster, active since 2022 (Gerbosa cluster), likely corresponds to a previously unknown parallel fault.

Temporal correlations indicate that cluster activation coincides with periods of low surface water levels. In particular, one cluster at the Western part of the Coronas fault, beneath the most rapidly shrinking part of the Aneto glacier, reactivated in late June 2020 and 2024, suggesting a possible influence of glacier melting. In the eastern Coronas fault, the two largest clusters were triggered within days of major rainfall events recorded in the past five years, consistent with a combination of short-term infiltration processes, and longer-term surface unloading from reduced snow and water storage since 2019. These observations raise the possibility of a climate–seismicity link in the Maladeta Massif, whereby rainfall and snowmelt infiltrations, and/or surface unloading may contribute to triggering shallow seismicity. Longer-term monitoring and detailed studies are required to confirm these processes and assess their potential implications for seismic hazard in the Pyrenees.