Ground Deformation at Nevado Del Ruiz (Caldas, Colombia) and its Correlation with the Present Episode of Volcanic Unrest

Nevado del Ruiz is an andesitic–dacitic stratovolcano that is part of the Ruiz–Tolima volcanic massif, a group of five ice-capped volcanoes (Ruiz, Tolima, Santa Isabel, Quindio and Machin). The modern volcano’s edifice comprises five lava domes, all within the caldera of an ancestral Ruiz volcano. Historical activity has consisted primarily of explosive eruptions from the central vent, followed by lahars. Ruiz is known for the tragic eruption and lahar of November 13, 1985 that completely destroyed the town of Armero. Analysis of seismic and deformation data identifies a complex plumbing system within the Tolima-Ruiz massif; magma accumulates in a deeper reservoir midway between Ruiz and Santa Isabel then moves along regional faults towards Ruiz. Magmatic fluids flow from the base of Ruiz into the shallower reservoir beneath the main Arenas crater that is feeding the current episode of unrest. After 8 years of quiescence, unrest at Ruiz resumed in 2009. The geodetic monitoring system (tiltmeters and permanent GPS sites) recorded aseismic deflation that continued until late 2011, consistent with spreading caused by gravitational collapse of the volcano’s edifice. Seismicity increased significantly in March 2012, together with gas and ashemissions and constant inflation. The volcano erupted in May 29 and June 30, 2012 (VEI = 3). After the eruptions, Ruiz remained in a state of unrest with high seismicity rates, inflation and intermittent emissions of water vapor, SO2 and ash until August 2015. Deformation stopped in September 2015 and a lava dome was emplaced at the bottom of the main crater. During the last months of 2015, 2016 and first months of 2017, the volcano’s unrest was characterized by seismicity (i.e., seismic tremor) with repeated emissions of gas and ash. Before the explosive eruptions of May/June 2012, deformation was consistent with the migration of small volume of magma to a shallow source located beneath the crater (depth »2 km and ΔV»0.6×106 m3). The inflation observed by GPS after the June 2012 was consistent with a deeper source, located midway between volcano Santa Isabel and Ruiz. Between June 2012 and December 2014 this source ascended from a depth of 17 to 7 km, with a volume change of approximately 1.5×106 m3. Interferograms processed by the Jet Propulsion Laboratory confirmed the location, depth and volume change of this deeper source.