Seismic velocity changes prior to the 2018 eruption of Sierra Negra volcano, Galapagos Island, using coda wave interferometry

Changes in external stress state and fluid content alter the mechanical properties of an elastic medium. Hence, variations in seismic wave velocity and (crack-induced) seismic anisotropy can be used as proxies for changes in stress and possible fluid ingression. Temporal variations in seismic wave velocity have previously been monitored and observed prior to volcanic eruptions. In the absence of additional constraints related to stress or fluid changes on the volcano, these pre-eruptive changes are difficult to interpret and hence their causes are often not well understood.Sierra Negra volcano is one of the most active volcanoes on the Galapagos Islands, about 1000 km west of continental Ecuador. On 26th June 2018 at 09:15 UTC, a magnitude 5.4 earthquake occurred near the southwest caldera rim and an intense seismic swarm started around 17:15 UTC. Seismic tremor dominated at about 19:35 UTC, which marked the onset of the eruption. Both prior to and during the eruption period several families of repeating earthquakes have been identified in a very large seismicity sequence.In this study, we use the Coda Wave Interferometry (CWI) technique on repeating earthquakes to measure time-lapse changes in seismic velocity prior to the 2018 eruption.We used 3 months of continuous seismic data from different temporary broadband stations, and 1 year of data from one of six permanent stations, the VCH1 station located on the caldera rim, managed by the IGEPN.Our results show a decrease in the seismic velocities observed from June 2017 to June 2018. In particular, we see a large systematic decrease in velocity starting up to a year before the eruption. A large velocity decrease of almost 2% is detected, over the year preceding the 2018 eruption at Sierra Negra. We suggest that this is related to progressive damage of the edifice, as it continues to inflate.