Journal of Volcanology and Geothermal Research 387 (2019) 106667 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores Hydrothermal fluid migration due to interaction with shallow magma: Insights from gravity changes before and after the 2015 eruption of Cotopaxi volcano, Ecuador Antonina Calahorrano-Di Patre a, * , Glyn Williams-Jones a , Maurizio Battaglia b, c , Patricia Mothes d , Elizabeth Gaunt d , Jeffrey Zurek a , Mario Ruiz d , Jeffrey Witter a a Centre for Natural Hazards Research, Department of Earth Sciences, Simon Fraser University, BC, Canada b US Geological Survey, Volcano Disaster Assistance Program, Menlo Park, CA, USA c Department of Earth Sciences, Sapienza - University of Rome, Rome, Italy d Instituto Geofísico de la Escuela Politécnica Nacional, Quito, Ecuador ARTICLE INFO Article history: Received 6 May 2019 Received in revised form 28 August 2019 Accepted 28 August 2019 Available online 3 September 2019 Keywords: Cotopaxi Volcano Time-lapse gravity Hydrothermal fluid migration Volcanic unrest ABSTRACT On August 14, 2015 Cotopaxi Volcano (Ecuador) erupted with several phreatomagmatic explosions after nearly 135 years of quiescence. Unrest began in April 2015 with an increase in the number of daily seis- mic events and inflation of the flanks of the volcano. Time-lapse gravity measurements started at Cotopaxi volcano in June 2015. Although minor gravity changes were detected prior to eruptive activity, the largest gravity variations at Cotopaxi were measured between October 2015 and March 2016, when other geophys- ical parameters had reached background levels. Inverse modelling of GPS data suggests a deep intrusion prior to the eruptive activity, while inverse modelling of post-eruptive gravity changes suggests variations in the volcano hydrothermal system. Deformation, seismicity, and gravity changes are consistent with the intrusion of a deep magmatic source between April and August 2015. Part of the magma rose from depth and interacted with the hydrothermal system, causing the phreatomagmatic activity and pushing hydrothermal fluids from a deep aquifer into a shallow perched aquifer. © 2019 Elsevier B.V. All rights reserved. 1. Introduction Time-lapse gravity measurements have been employed in numer- ous settings to monitor sub-surface mass changes related to volcanic activity. Although the aim of these observations is usually to either infer the amount of new magma intruded (or extruded) from the volcanic reservoir (e.g., Rymer and Williams-Jones, 2000; Greco et al., 2012; Carbone et al., 2017) or changes in the density of an already identified magmatic intrusion (e.g., Williams-Jones and Rymer, 2002; Gottsmann et al., 2003), time-lapse gravity has also been successfully employed to monitor hydrothermal systems (e.g., Tizzani et al., 2015; Battaglia et al., 2018; Miller et al., 2018). Thus, these observations can not only help discriminate between possible causes of measured deformation and increased seismicity during unrest episodes, they can also shed light on interactions between magmatic and hydrother- mal systems in a volcano (e.g., Battaglia et al., 2006; Gottsmann et al., 2006, 2007). When Cotopaxi volcano showed signs of renewed * Corresponding author. E-mail address: acalahor@sfu.ca (A. Calahorrano-Di Patre). activity after 135 years of quiescence, gravity measurements were a valuable addition to the already extensive seismic, geochemical, and geodetic monitoring networks installed by the Instituto Geofísico de la Escuela Politécnica Nacional (IG-EPN). Cotopaxi, a glacier-clad stratovolcano located 50 km south of Quito, Ecuador, is part of the Northern Volcanic Zone in the Andean Volcanic Arc, and the result of the subduction of the Nazca Plate beneath the South American Plate (Stern, 2004)(Fig. 1). Ordoñez et al. (2013) and Andrade et al. (2005) proposed 4 different hazard scenarios based on the energy released by the eruption: Small (VEI 1–2), Moderate (2–3), Large ( 3–4), and Very Large (VEI >4). Considering Cotopaxi’s eruptive history, the main hazards for larger, distal cities are ash fall and lahars in a Large Eruption scenario (Mothes and Vallance, 2015). Up to 300,000 people currently live in the path of possible lahars, and agricultural lands that provide a large quantity of vegetables grown in Ecuador could be affected by ash fall from Cotopaxi. Cotopaxi began showing signs of unrest at the beginning of 2015, after a minor period of unrest in 2002 which did not result in eruptive activity. Hickey et al. (2015) modelled the source of the 2002 defor- mation as a shallow oblate magmatic intrusion beneath the SW flank of the volcano. This was reconciled with seismic signals described by https://doi.org/10.1016/j.jvolgeores.2019.106667 0377-0273/© 2019 Elsevier B.V. All rights reserved.