The occurrence of earthquakes preceding or during volcanic eruptions has been known since ancient times. The first-time earthquakes associated with a volcano eruption in scientific literature were in the description of the Vesuvius eruption in 79 AD by Pliny the Younger. At that time, neither the origin of earthquakes, nor why they occurred in volcanic environments were known. Nowadays, we know that most earthquakes associated with volcanoes are related to the movement of magma and fluids under the surface. Before eruptions, the magma and fluids that have been accumulated in reservoirs at depth start to ascend to the surface forcing their way up through shallow underground fractures and passageways. As they advance opening this magmatic plumbing system, they will cause the surrounding wall-rocks to break creating earthquakes. This fact mostly occurs on long-dormant volcanoes which typically exhibit more pronounced seismicity changes prior to eruptions. Once the eruption has started, the withdrawal of magma accumulated in the different reservoirs and the possible ascent of new magma from depth can also trigger earthquakes in the surrounding rock. 

 

For many centuries, the number of earthquakes felt by the population was the only information that scientists had about the seismicity associated with historical eruptions. Thanks to the great advances in technology and instrumentation, it is now possible to use accurate sensors to detect smaller earthquakes. With an appropriate seismic network in the vicinity of a volcano, it is possible to obtain the location of these earthquakes beneath the volcanic building and infer their "focal mechanism", which provides information on the geometry of the activated fractures. In favourable circumstances, estimation, if the earthquake rupture was accompanied by an increase or decrease in volume, is possible, for example in response to opening and closing of cracks. 

 

Therefore, the analyses of seismic waveforms allow us to track magma and volcanic fluids during a volcanic reactivation indirectly. Monitoring the evolution of volcanic unrest using seismological data only, however, faces important challenges. First, seismicity does not occur uniformly within the volcano, and it is not unusual to find some "aseismic” zones, where pressure, temperature and structural properties inhibit the occurrence of earthquakes. Second, the interpretation of seismicity may be difficult, potentially being explained by different scenarios. For these reasons, it is important to combine the analysis of seismicity with other observations, such as ground deformation, gas emission or gravity variations. A joint interpretation of different data helps the correct interpretation of the processes happening beneath a volcano and the development of conceptual models that can be used to improve early warning and volcano monitoring. 

 

Our research focuses on the reactivation of La Palma, Canary Islands (Spain) and the volcanic eruption that began on September 19, 2021. This eruption had a great media impact due to its consequences for the local population, with more than 1,600 buildings destroyed by lava flows and more than 7,000 people evacuated. Abundant seismicity, moderate in magnitude but widely felt, started a week before the eruption onset and continued throughout the eruption keeping the population on edge. 13 earthquakes reached a maximum intensity of IV-V (European Macroseismic Scale-98) and a few of them were also felt at neighbouring islands. 

 

This eruption was the first fully monitored on the island and little was known about this volcano’s feeding system before the eruption began. We analysed seismic data over nearly 5 years, from the first signs of reactivation in 2017 until the end of the eruption in December 2021. We relocated the whole series (8488 earthquakes) and obtained the focal mechanisms of the largest 156 earthquakes. Seismicity locations revealed the presence of magma accumulation at two different depth levels, in the lower crust and the upper mantle respectively, forming a complex magma storage system. The focal mechanism results provide the key to understand its behaviour during the eruption. In particular, the peculiar observation of rotated focal mechanisms for earthquakes located at very close distance provided evidence for the progressive depletion of the two reservoirs during the eruption, which is also supported for the shallowest one by deformation data. Based on our findings, we have developed a new conceptual model of the magma feeding and reservoir system beneath the island. 

 

In our work, we took advantage of the unprecedented dataset to improve our understanding of both the long-term precursor activity, with the progressive destabilisation of the plumbing system and the short-term instability of the magmatic plumbing system.