Magma migration below Tancítaro and Paricutin volcanoes revealed by seismology

Summary In this study, we analyze the 2020 seismic swarm that lasted two months and occurred between the Tancítaro and the Paricutin volcanoes in the Michoacán Guanajuato Volcanic Field, Mexico. We developed a new method to automatically detect and locate about 100,000 earthquakes, enabling us to track the magma migration through narrow dykes. Additionally, we reveal the presence of two magma reservoirs from two seismic noise tomography results. The first reservoir is located from 8 km to 20 km below sea level and beneath the Tancítaro volcano and probably corresponds to a complex network of dykes and sills. This crustal reservoir is fed by a mantle reservoir with a wide horizontal extension between 35 and 50 km below sea level. The seismic swarm initiated beneath the Tancítaro summit in the lower portion of the crustal magma reservoir. At this stage, the seismicity migration was mainly horizontal, which we interpret as its response to the higher normal stress caused by the gravitational load of Tancítaro. Once the magma was displaced laterally from beneath Tancítaro, magma migration became more vertical. The swarm reached the upper portion of the crustal magma reservoir but did not escape it. We also reveal the effect of a distant but strong tectonic earthquake on the seismic swarm. Before its occurrence, magma migration followed several paths; afterwards, it became more focused along a single path. Finally, after the swarm, we observed a second type of seismicity called post-swarm seismicity, with a lower earthquake rate but with higher magnitudes. The hypocenters were diffuse and horizontally centered on the previous swarm location. Furthermore, some earthquakes were aligned along shallow faults, generating a high seismic risk to the different Tancítaro nearby localities.