Complex seismicity patterns accompanying the 2021 volcanic eruption at La Palma, Canary Islands, Spain

A moderate seismicity accompanied the dike intrusion which preceded the 2021 volcanic eruption at La Palma, Canary Islands, Spain. Nevertheless, the largest magnitudes were recorded during the eruption, from September 19th to December 13th, 2021. This volcanotectonic activity accompanied the upward magma transfer to feed the eruption and provides important clues to the understand the feeding system geometry, as we are dealing with the first fully monitored eruption in the island. Seismicity during the eruption displayed a stable bimodal spatial distribution, with hypocenters clustering at two, well separated depth intervals. A shallower seismic cluster was active beneath the central area of Cumbre Vieja  ~10-14 km depth, starting by September 27, just after a short quiescence of about 3 hours in the tremor signal and with peaks of intensification rates in mid and late November. A deeper and larger cluster (~33-39 km) extended further to the Northeast. Here, the activity started with some delay on October 5th and the cluster was mostly active over October and November 2021, reaching a peak magnitude mbLg 5.1 November 19th, 2021, the largest earthquake of the whole seismic sequence. In this study, we use a variety of seismological methods to resolve hypocentral and centroid location at the two clusters, as well as full moment tensors for 156 earthquakes, including largest ones at each cluster. The hypocentral relocation of 7150 earthquakes reconstructs the geometry of the active seismogenic structures, resolving small-scale details within each of the two clusters. The centroid moment tensor inversion resolves different families of moment tensors in each cluster including earthquakes with almost reversed focal mechanism that respond to local stress perturbations introduced by the magma rise through a complex path and multiple magmatic reservoirs. The source studies are complemented by a temporal analysis of the families based on waveform characterization, which allows to reconstruct the timeline of the magma transfer and seismogenic processes. Our seismological analysis provides details of seismicity accompanying the volcanic unrest at La Palma and documents the evolution of seismogenic processes in response to the rise of magma batches through the complex plumbing system.