Seismicity under a dormant volcano: unveiling active crustal faulting beneath Piton des Neiges, La Réunion

Earthquakes occur regularly in the vicinity of La Réunion's two main hotspot volcanoes, Piton des Neiges and Piton de la Fournaise. While earthquakes at Piton de la Fournaise volcano are clearly linked to its volcanic activity, the seismicity beneath Piton des Neiges is not well understood. However, except during eruptive periods, we often record more seismic events at Piton des Neiges than at Piton de la Fournaise. This study aims to better capture this seismicity to understand the nature and causes of the activity beneath Piton des Neiges, a volcano that has been dormant for 27,000 years. We improve previously available seismicity catalog by using template matching, double relocation and focal mechanism determination. Results indicate that the seismicity is primarily concentrated on a northeast-dipping reverse fault located in the oceanic crust beneath the volcanic edifice. We also identify secondary seismicity clusters with the same orientation in the vicinity of the main fault. Seismicity occurs continuously since the installation of the first seismological stations in the vicinity of Piton des Neiges in 1999. Although occasional periods of increased swarm-like activity are observed in 2011 and 2018, they do not correlate with markers of deep magma transfers that are often observed prior to the eruptions of the Piton de la Fournaise. These variations in seismic activity are limited to the main reverse fault and might be associated with periods of creeping activity. Our findings suggest that the seismic activity beneath Piton des Neiges is likely caused by regional tectonic stress and edifice loading on pre-existing faults, rather than from deep magma transfers. This conclusion is supported by the presence of various reverse faults with similar orientation and the lack of correlation between seismicity fluctuations and deep magmatic activity.