High Resolution Spatio-Temporal Distribution of Volcano Seismicity Prior to the Sierra Negra (Galapagos Islands) Eruption of 2018

Sierra Negra volcano is a large basaltic shield volcano located in the Galápagos Islands, Ecuador. Sierra Negra is one of the most active volcanoes within the Galápagos, and a sparse background network operated by the Instituto Geofísico in Quito monitors seismic activity in near real time. Beginning in 2017, elevated signs of unrest prompted the deployment of a dense network of 14 additional broadband seismometers in 2018. Designed to probe the triggering response to dynamic stress perturbations and time-varying velocity structure, this network (IGUANA) also captured the increasing rates of seismicity in the months leading up to the eventual eruption in June 2018. Here we exploit a favorable network geometry and report on the detailed spatio-temporal evolution of primarily volcano-tectonic events. An automated detector generated initial picks that were then manually reviewed to remove spurious detections and supplement missing phases. Initial locations using hypo71 and a simple 1D velocity model were then subjected to a double-difference relocation algorithm. Differential travel times between pairs of earthquakes were generated from both handpicked phase data and waveform cross correlation. The majority of seismicity is focused along a system of faults that bound the approximately 9x7 km caldera floor. Prior to the eruption, most large (M≥4) earthquakes were of shallow depth and located beneath the southern rim. On the day of the eruption, a large Mw5.2 earthquake with thrust source mechanism and its subsequent aftershock sequence preceded a period of quiescence before a final pre-eruptive swarm of ~3 hours. Precise locations reveal a possible migration from deep to shallow depth at the start of the main pre-eruptive swarm. As the eruption progressed, overall seismicity patterns shifted towards the western and northern sectors of the caldera, in agreement with the eventual mapping of 5 eruptive fissures and independent tremor locations.