Spatio-temporal velocity variations observed during the pre-eruptive episode of La Palma eruption inferred from ambient noise interferometry

On September 19th, 2021, a volcanic eruption began on La Palma, resulting in a significant social, economic and scientific impact. Earthquakes were first recorded in 2017, however, the pre-eruptive unrest started on Sept. 11st, 2021, only a few days before the eruption. It was characterized by a seismic sequence with hypocenters located at a depth of less than 10 km and ground deformation that reached more than 20 cm in the vertical component of the GPS. Surprisingly, this episode was very short, however, given the large amount of scientific instrumentation (seismometers, GPS, etc.) operated by the Instituto Volcanológico de Canarias (INVOLCAN) and other scientific institutions, the entire pre-eruptive episode has been accurately monitored, and the civil protection authorities were notified about the development of the volcanic unrest in advance. One of the techniques that have shown great potential in volcanic monitoring is ambient noise interferometry. This method consists in estimating the relative velocity variations using empirical Green’s functions retrieved through the cross-correlations of ambient noise signals. In this study, we applied this technique to the data recorded by six broadband seismic stations that allowed us to estimate spatio-temporal relative velocity variation during the week preceding the eruption.

The overall pattern of the pre-eruptive seismicity shows a progressive westward and upward migration of the hypocenters. However, five days before the eruption, we observed the occurrence of shallow (< 5 km) low magnitude earthquakes, whose hypocenters were detached from the main seismic cluster. At the same time, seismic interferometry detected a decrease in the seismic velocity in the region where such hypocenters were located. Therefore, we interpret those earthquakes as the effect of triggering caused by hydrothermal fluids released by the ascending magma and reaching shallow depths faster than magma.

Furthermore, a couple of days before the eruption, an even more significant reduction in relative velocity variation was observed, possibly corresponding to the rapid magmatic upward intrusion process, which led to the volcanic eruption.