Precursory Signal Detected for the 2018 Sierra Negra Volcanic Eruption, Galapagos, Using Seismic Ambient Noise

The changes in physical properties leading up to a volcanic eruption provide clues to processes occurring within volcanoes and may reveal premonitory signals. The Sierra Negra shield volcano, located in the Galapagos Islands, erupted on 26 June 2018 after months of continued inflation and escalating earthquake activity. We applied ambient noise interferometry to five months of continuous seismic data from 12 broadband stations to calculate crustal shear-wave velocity changes before and during the eruption. Using the Moving Window Cross-Spectral technique and a pre-eruption stack of ambient seismic data as reference, we found a -0.27% decrease in velocity 17 days before the eruption in station-pairs that pass beneath the caldera's north-eastern sector. Sensitivity kernels suggest that the velocity changes of this precursory signal are greatest at depths corresponding to the shallow sill (similar to 2 km) beneath the wide caldera. Our results, considered in light of geodetic, seismicity, and petrological results, suggest that this velocity decrease is in part caused by dilatation from a M-L 4.8 earthquake, and degassing after a possible magma intrusion below or at the base of the sill. The precursory velocity decrease within the sill region offers an important tool for forecasting future eruptions at Sierra Negra.