Very Long Period Seismicity Accompanying Increasing Shallower Activity at Cotopaxi Volcano

Cotopaxi is an andesitic stratovolcano, located in the highland region of Ecuador, which renewed its activity in April 2015, showing an increased number of volcano-tectonic (VT), long-period (LP), very long period events (VLP), and tremors. The VLP events were recorded in several episodes between 2002 and 2014, and have been interpreted as volumetric changes due to the release of gas and subsequent pressure drop and recovery in the magma intrusion. The two peaks of VLP seismicity in June 2002 and April 2015 preceded an increase of surficial activity (fumarolic increase) and the deformation data during those episodes suggested a small intrusion of magma beneath the volcano.Using polarization analysis, we found that most of these events were located at 2-3 km depth beneath the volcano summit, while the deformation data suggests the intrusion is deeper (5-10 km deep). Using tiltmeter data, Mogi point source modelling on successive periods of inflation and deflation show a significant shallowing of sources since the end of May 2015, matching the recent very large spike in SO2 emissions (~3000 t/d). From mid-February until the gas emission spikes in May 2015, Mogi source modelling has indicated inflation/deflation events at 11 to 10 km depth, having shallowed to a depth of between 8 and 7 km after the SO2 emission increase. Shallow source volumes suggested by deformation indicate values of 4 - 31x106 m3, with the most recent, most shallow inflation currently at 8x106 m3.