Dramatic volcanic instability revealed by InSAR

Interferometric synthetic aperture radar (InSAR) was used to measure ground deformation during explosive eruptions on 27 and 28 May 2010 at Pacaya volcano, Guatemala. Interferograms produced using spaceborne and airborne synthetic aperture radar data reveal ~3 m of along-slope movement of the southwest sector of the edifice during these eruptions. This is the largest measured slope instability witnessed in a single event at a volcano that did not result in a catastrophic landslide. This rapid and extreme movement is particularly concerning given the history of sector collapse and persistent activity at this volcano. These findings emphasize the utility of high-resolution InSAR measurements for monitoring deformation and potential catastrophic slope instability at volcanoes. INTRODUCTION The structural failure of volcanoes has been widely recognized as a pervasive and potentially devastating phenomenon, given the historic volcanic sector collapses of Bezymianny (Kamchatka, Russia, 1956), Sheveluch (Kamchatka, Russia, 1964), and Mount St. Helens (Washington, USA, 1980) (see CarrascoNúñez et al., 2011, and references therein). One way to identify regions of instability on volcanoes is to monitor deformation using geodetic techniques. Currently, deformation has been measured at 214 volcanoes (Volcano Deformation Database; globalvolcanomodel. org/gvm-task-forces/volcano-deformationdatabase/), 160 of which were measured using interferometric synthetic aperture radar (InSAR). InSAR, in which the phase of 2 or more synthetic aperture radar images are differenced to determine surface deformation, has been used to study more than 500 volcanoes worldwide since the 1990s (Biggs et al., 2014) and has high accuracy (<1 cm) over large areas (several cubic kilometers or more). Here we use interferometric data from the Advanced Land Observing Satellite (ALOS; Lband, l = 23 cm) to measure ground deformation during explosive and effusive eruptions on 27 and 28 May 2010 at Pacaya volcano, Guatemala. This was compared to Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR; L-band) aircraft and GPS data. Several possibilities for the origin of deformation of the southwest flank are considered against interferometric results and existing structural and volcanological information. BACKGROUND Pacaya is a basaltic to dacitic stratovolcano complex in Guatemala, located on the southern rim of the Amatitlán caldera 25 km south of the capital, Guatemala City (Fig. 1A). During an explosive phase between 3000 and 700 yr ago, the southwest sector of the cone failed in a major collapse, forming a debris avalanche that traveled 25 km southwest (Fig. 1B), with an estimated volume of 0.65–0.8 km3 (Vallance et al., 1995). Dating of two tephra fall deposits by Kitamura and Matías (1995) confine the age of the sector collapse within 1500–600 yr ago. Episodic activity since the 16th century rebuilt the edifice within the collapse amphitheater, creating the Cerro Chino and modern Pacaya cone. However, the scarp from the collapse is still visible on the north and northeast sides of the volcano (Figs. 1B and 1C). After a period of repose beginning in the mid-19th century, activity renewed once again in 1961 with a flank eruption that continued for one month (Eggers, 1971). In June of 1962, an ~300 × 200 m oval-shaped area subsided near the summit (Fig. 1C), and was quickly filled by erupted material during the following years (Eggers, 1971). Since 1961, eruptive activity has been very frequent, with extended periods of constant lava effusion and small explosive activity (e.g., from 2004 to 2010), producing more than 250 lava flows, intermittent Strombolian activity, and ash and gas plumes (Matías Gómez et al., 2012). Details of the May 2010 Eruptions This study focuses on eruptive events in May 2010; details are derived from reports by Guatemala’s National Institute of Seismology, Volcanology, Meteorology, and Hydrology (www. insivumeh.gob.gt), the National Coordinator for Disaster Reduction (www.conred.gob.gt), and the Global Volcanism Program (www.volcano. si.edu). On the evening of 27 May, intense lava fountaining and vigorous ejection of tephra and ballistics erupted from the summit vent. Seismicity records show an increase in real-time seismic amplitude measurement (RSAM) values during the afternoon of the same day, peaking shortly before 1800 h local time and lasting GEOLOGY, August 2015; v. 43; no. 8; p. 743–746 | Data Repository item 2015254 | doi:10.1130/G36678.1 | Published online XX Month 2015 © 2015 eological Society of A erica. For permission to copy, contact [email protected]. Figure 1. A: Location of Pacaya volcano, Guatemala, 25 km south of Guatemala City. B: Map showing locations of current populated centers and the extent of the debris avalanche deposit (modified from Kitamura and Matías, 1995) from the sector collapse dated between and 1500 and 600 yr ago and the resulting collapse scarp. Outline shows area of C and Figure 2. C: Structural map with additional features including the Cerro Chino vent, the June 1962 subsidence, the May 2010 linear collapse, and the flank vents and deposition of the May– June 2010 lava flow that erupted outside of the collapse scarp. These features are aligned in a north-northwest pattern orthogonal to the direction of flank movement. as doi:10.1130/G36678.1 Geology, published online on 10 July 2015