Magma pathway opening before the 2018 eruption at Sierra Negra volcano, Galapagos, revealed by tremor location and local earthquake tomography

Located 1000 km west of continental Ecuador, Sierra Negra is a shield volcano with a large summit caldera and is one of the most active volcanoes in the Galapagos archipelago. The 2018 eruption started on 26th June and lasted about two months. At about 17:00 UTC on the eruption day, an intense seismic swarm and relatively weak tremor were recorded on a network consisting of 11 broadband seismometers. Three hours later, tremor amplitudes increased and lava flows from multiple fissures were observed on the north and northwestern flanks of the volcano, marking the start of the eruption. To understand subsurface processes before the eruption, we isolate and locate pre-eruptive tremor signals every 20 seconds beginning three hours before the eruption using a seismic amplitude ratio method. Results show that the onset of the tremor locates to the west of the caldera. The lateral location is stable while the depth gradually reduces over the next two hours. Around 19:30 UTC, the tremor reaches the surface, at a location which matches one of the observed eruptive fissures on the west flank. A few minutes later, the tremor location starts moving towards the north of the caldera. At about 19:55 UTC, the source location becomes stationary and coincides with the location of another eruptive fissure. The location is consistent with a low-velocity body imaged by a local-earthquake body-wave tomography. The spatio-temporal evolution of the tremor may reveal the pathway of opening of a dyke and has the potential to be used as a short-term forecast of the eruption site.