Seismicity under a dormant volcano; unveiling active crustal faulting beneath Piton des Neiges, La Reunion

Volcanic environments are commonly associated with seismic activity. The two prominent shield volcanoes of La Reunion island, Piton des Neiges and Piton de la Fournaise, exhibit sustained seismic activity. Whereas the seismicity at Piton de la Fournaise is tied to its volcanic activity, the seismic activity beneath Piton des Neiges, which has been dormant for 27,000 yr, remains poorly understood. The occurrence of earthquakes under the north flank of Piton des Neiges, often felt by the population, led to the deployment of several seismic stations in the area since 2012. In this study, we employ template matching and double-difference relocation techniques to construct a high-resolution catalog of the region. Our results reveal that the seismicity observed under Piton des Neiges is primarily concentrated on a northeast-dipping fault located in the oceanic crust beneath the volcanic edifice. Analysis of focal mechanisms indicates that this structure operates as a reverse fault. In the vicinity of this primary fault, we also identify secondary seismicity clusters with similar orientation and focal mechanisms. The region has experienced continuous seismicity since 1999, with occasional periods of increased swarmlike activity in 2011 and 2018. These fluctuations in seismicity rate do not correlate with markers of deep magma transfers often observed before eruptions at the Piton de la Fournaise volcano. Beyond the crustal faults highlighted in this study, it is noteworthy that the majority of earthquakes in northern La Reunion island exhibit consistent reverse focal mechanisms. These observations suggest that seismic events may not be driven by deep magmatic activity but, rather, result from regional tectonic stress and edifice loading on pre-existing faults.