## 4 InSAR Observations  
Using the two-pass InSAR approach (e.g., [25; 26]), six interferograms of reasonably good coherence (where 50% of the study area maintains coherence larger than 0.3 in the unfiltered interferograms) were produced from satellite-based ALOS-1 SAR scenes from May 2010 to April 2011 (Table 1). NASA's Shuttle Radar Topography Mission (SRTM) 1-arc-second global digital elevation model (DEM) was used to correct for topography, which was interpolated and re-sampled to a spacing of \(\sim\)6 m. Additionally, four interferograms from 2010 to 2014 were analyzed from aerial UAVSAR, processed by the NASA Jet Propulsion Laboratory (JPL) but cropped to Pacaya's extent for this study. Interferograms were filtered and unwrapped using minimum cost flow and triangulation [27]. However, all interferograms are shown as wrapped phase, in which each fringe or color cycle (e.g., magenta-magenta), represents 11.8 cm or 11.9 cm of displacement in ALOS-1 and UAVSAR interferograms, respectively. Deformation measurements are assumed to be primarily vertical (see justification later), calculated using geometric relations between the incident angle of the radar to the surface (reported in Table 1), and the line-of-sight (LOS) displacement, or direction between  
Figure 2: UAVSAR amplitude images of Pacaya volcano (**a**) before and (**b**) after the May 2010 eruptions clearly show changes to the edifice as a result of the eruption, including the emplacement of the 5.4 km long lava flow, an increase in the summit crater size, and the linear collapse oriented NNW from the summit; Panel (**c**) is an averaged image produced from eight ALOS-1 acquisitions between 2010 and 2011.  
the SAR antenna and the point on the ground surface. UAVSAR images have nearly the opposite LOS direction from ALOS-1 images (Table 1). Fringe patterns were compared to other interferograms with larger baselines to discard any topography-related errors. As perpendicular baselines of interferograms used in this study are less than 341 m (Table 1), the altitude of ambiguity [25] is larger than \(\sim\)190 m. Hence, topography-induced errors in these interferograms can be neglected. Currently, no advanced analysis of GPS data is available for comparing the deformation measured here [22]. After the co-eruptive slope movement of the southwest flank on 27 May and 28 May (Figure 3b), flank-wide subsidence appeared in the first SAR image acquired on 31 May 2010, three days after the eruptions. This deformation is not seen in ALOS-1 interferograms before the eruptions (Figure 3a). The spatial extent of deformation is very similar to the original slope movement, marked with a dashed line in Figure 3a-f. Additionally, the pattern is similar between ALOS-1 (Figure 3c) and UAVSAR (Figure 3f) interferograms, which have nearly opposite looking geometries. This suggests that the deformation observed after the slide was likely dominated by vertical subsidence. The greatest magnitude of subsidence, \(\sim\)18 cm, occurs near the summit, which reduces along the slope to \(\sim\)2 cm at the toe of the slide. These values subsequently decrease in magnitude and area through early 2011 (Figure 3d,e). A UAVSAR interferogram that spans 26 April 2011-8 March 2013, shown with magnified fringe values in Figure 3f, shows that this area was still subsiding at slow rates at least into the spring 2011. In addition to the subsidence of the southwest flank, a localized deformation event on the cone appears after the May 2010 eruptions (Figure 4). This feature, elongated NW-SE, appears north of the summit and was contained mostly within the ancestral collapse scarp except for a portion to the east. These fringes are difficult to differentiate from the flank subsidence in the ALOS-1 interferograms immediately post-eruption (Figure 4a) but become clearer in subsequent months as flank subsidence diminishes (Figure 4b,c).