Anticipating and following the 2018 eruptive activity of Sierra Negra volcano

Sierra Negra is the largest and one of the most prolific volcanoes in the Galápagos islands. It has erupted on seven occasions since 1948 with an interval average of 11.3 yrs. The last two occurred in 1979 ( 1 km3 of lava) [Reynolds and Geist, 1995] and in 2005 (0.12 km3 of lava) [Yun et al., 2007; Geist et al., 2008]. The Instituto Geofisico is monitoring this volcano with a permanent bb seismic network since Sep 2013. It detected a steady increase of seismic activity at Sierra Negra from an average of 55 events/month in 2015 to an average of 1115 in Jan-May 2018. The increased seismicity led to the temporary deployment of 14 bb seismometers and 2 tiltmeters in April, 2018. Energy release also showed an important increase since 13 events with magnitudes ≥ 4.0 were detected in the period Jan-Jun 2018 before the eruption of June 26, including a ML 5.3 earthquake, registered 8 hours before the eruption onset. This event was followed about six hrs later by a swarm with LP, VT and VLP events and two hours afterward by a low-frequency tremor that indicated the onset of eruptive activity. These events allowed the Instituto Geofisico to issue a prompt notification of an impending eruptive activity to the Parque Nacional officials. Hypocentral locations have shallow foci presumably above either a horizontal magmatic reservoir or sill. Geodetic data show large, and differential vertical motions across the 10 km-wide caldera. Although most seismic events were classified as VT, large events exhibit spectral peaks in the range of LP and VLP even at the closest station. Lava emission from fissures in the northern flank was accompanied by a 1-5 Hz disharmonic tremor in Agt. Seismic activity followed an increasing uplift of the caldera floor with an inflation of 4.5 m since 2005 up to Jun 26, 2018 with a rate of 15 cm/month in 2018, based on InSAR and GPS data. After more than a month with effusive activity, it has been estimated that fresh lavas cover an area more than an order of magnitude larger than 2005 eruption. Afterwards, minor effusive activity persists from a fissure in the lower northwestern flank. Since August 23, no effusive activity was observed and a week later the Instituto Geofisico declared the end of this eruptive period. Ongoing seismic and geodetic studies provide unprecedented opportunities to test basaltic eruption models.