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The 2008 Eruption of Chaitén Volcano, Chile, as Viewed From ALOS, and the Importance of Complementary Satellite Looks

by Charles Wicks, United States Geological Survey

Figure 1: Index map and examples of two, approximately, 2-year interferograms spanning the Chaitén eruption. The red triangle in the inset map shows the location of Chaitén Volcano. a) An ascending interferogram. The black line labeled LOFZ shows the location of the main strand of the Liquiñe-Ofqui fault zone. The colorscale maps the colors into changes in range distance between the satellite and the ground. The dashed, white-filled circular area labeled Ch is Chaitén Caldera. The larger dashed, white-filled area labeled M is the approximate outline of Michimahuida Caldera. The red arrow labeled Az shows the flight direction of the ALOS satellite. The red arrow labeled LOS shows the look direction from the satellite to the ground. b) A descending interferogram. c) Schematic diagram showing geometry of the collapsing body relative to satellite geometry.

A rare explosive rhyolite eruption occurred on 2 May 2008 at Chaitén Volcano in Chile, a type of eruption that had not been seen since the 1912 eruption of Novarupta in Alaska. Chaitén had not erupted for thousands of years before the 2008 eruption and it was not monitored prior to the eruption. The eruption occurred with only 2 days of detected precursory activity and the nearly crystal-free rhyolitic magma has been shown to have migrated from greater than 5-km depth to the surface in less than about 4 hours (Castro and Dingwell, 2009). Although Chaitén was not part of an ongoing ground-based monitoring network, it was a part of the Japan Aerospace Exploration Agency’s (JAXA) ongoing mission for Advanced Land Observing Satellite-Phased Array L-band Synthetic Aperture Radar (ALOS-PALSAR) data acquisition. These data have enabled modeling of the deformation related to this rare rhyolite eruption. From this model, one can infer that the magma reached the surface quickly via diking, local faults controlled the emplacement of a rhyolite reservoir beneath Chaitén, and the source of the magma is probably beneath Michimahuida Volcano approximately 15-km east of Chaitén (Figure 1).

Although the study area is heavily vegetated, the approximately 2-year ascending- and descending- mode interferograms (Figure 1) that span the eruption, and subsequent rhyolite dome emplacement, clearly show the related surface deformation. The descending-mode interferogram displays a broad area of movement away from the satellite that encompasses the entire Michimahuida-Chaitén volcanic complex, but the deforming area is restricted to a small area west of Chaitén in the ascending-mode interferogram. The radar beams from both modes intersect the Earth’s surface at an angle of about 39° from vertical. Consideration of the radar geometry, shown schematically in Figure 1c, shows the main deformation source responsible for the broad-deformation signal should be a sill-like collapsing body that dips to the East-Northeast with a dip angle that is nearly complementary to the radar-incidence angle in the ascending geometry. Modeling of the interferograms favors a three-body deformation source (Figure 2): 1) a deep dipping, sill-like body (approximately 10 km beneath Chaitén) that we interpret to be a reservoir of rhyolitic magma, 2) an inflated dike that reaches from the rhyolite reservoir to the surface at Chaitén, and 3) a shallow lathlike collapsing conduit that extends vertically from the rhyolite reservoir to Chaitén.

Figure 2: Three deformation sources found by modeling the interferograms in Figure 1. The red triangle in each panel shows the location of Morro Vilcún, an undated rhyolite plug. a) Map view of the deformation sources. The bold red line is the surface trace of the dike, the short bold white line (on the west side of Chaitén) is the surface trace of the lath-like conduit, and the blue-dashed line shows where the modeled plane containing the rhyolite reservoir intersects the surface. The location of cross-section A-A’ in Figure 2c is shown with the East-Northeast trending black line. b) A 3-D perspective of the deformation sources viewed from the Northeast. c) A schematic cross-section through A-A’ showing the location of the three deformation sources (the dike is red) and Michimahuida Volcano (M), Chaitén Volcano (Ch), and Morro Vilcún.

Although the ascending ALOS-PALSAR data were available soon after the eruption, it was not until the descending data was acquired by JAXA in 2010 that the picture of deformation became complete. Before descending data were available over Chaitén, the model that best fit the ascending ALOS data (Figure 1a) was a narrow, steeply dipping, collapsing rectangular dike (also in Fournier, et al., 2010), similar to the lath-like conduit part of the model in Figure 2. Since nearly one km3 of material was erupted, more surface deformation than can be seen in the ascending interferogram (Figure 1a) was expected, and indeed the descending-mode interferogram (Figure 1b) shows a much different deformation field that is more appropriate to the size of the eruption. This study shows the deformation source related to an eruption and, thus the inferred-magmatic system can be complex and illustrates the need for multiple, complementary satellite looks to study volcano deformation in future SAR-satellite missions. For more information about this study, see Wicks, et al., 2011 (http://volcanoes.usgs.gov/activity/methods/insar/research_results.php).

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