Linking Gravity Change at Cotopaxi Volcano with the Volcanic Processes Responsible for the 2015-2016 Period of Unrest

We investigate the link between observed temporal gravity changes with the physical processes involved in Cotopaxi’s episode of unrest of 2015-2016. In April 2015, Cotopaxi, an active stratovolcano located 50 km south of Ecuador’s capital city of Quito, began showing signs of unrest after 135 years of quiescence. The Instituto Geofísico of Ecuador reported an increase in the number of seismic events and daily SO2 flux, as well as a change in the type of seismicity (an increase in the proportion of LP and VLP events). This increase in activity peaked on August 14, 2015, with the initiation of mild eruptive activity. The eruption was characterized by phreatomagmatic explosions with ash plumes that rose up to 8000 m above the summit: ash emissions affected the nearby communities until November 2015. The activity declined after December 2015, and seismicity and SO2 emissions returned to background levels by April 2016. Gravity monitoring at Cotopaxi began in June 2015, with campaign measurements taken approximately every two months across a network of eight stations around the volcano. Gravity data was corrected for tide and free-air effects as well as instrumental and daily drift. A maximum gravity increase of 30 μGal between stations was detected before the August 2015 explosions. Gravity decreases up to 52 μGal were observed at several stations around the volcano between October 2015 and March 2016, after which they stabilized towards background levels. Preliminary calculations for a simple sphere-like source, using the largest observed gravity change, suggest the increase can be linked to mass variations up to 3.3 ± 0.5 x 1011 kg with an estimated depth of 2-3 km under the summit, and likely represent a magmatic intrusion preceding the initiation of eruptive activity.