**1. Introduction**

Volcán de Colima is one of the most active volcanoes in Mexico (Figure 1a) [1–3]. Before 2015, the last major volcanic crisis occurred in 2004–2005 and was characterized by several episodes of dome growth and collapse accompanied by the emplacement of block-and-ash flow (BAF) deposits that reached up to 7 km from the volcano's summit [4,5]. The July 2015 eruption represented an extraordinary episode; a fast-growing dome collapsed and generated two major BAFs on the 10th and

11th of July and a maximum runout of 10.5 km [6,7]. No similar runout for BAFs has even been observed in previous dome collapse events or in the stratigraphic record. The only BAFs that have reached similar distances correspond to the deposits associated with the Soufriére-type eruptive phase that preceded the 1913 Plinian eruption [8]. The unexpected 2015 scenario provides evidence supporting the need to revise hazard assessments associated with pyroclastic flows from dome collapses (i.e., [5,9]). Rigorous estimations of the volumes and inundated areas of the lava flows and pyroclastic flow deposits are thus fundamental, especially for numerical model calibration.

**Figure 1.** (**a**) Sketch map of the Trans-Mexican Volcanic Belt (TMVB) showing the location of the Colima Volcanic Complex (CVC). (**b**) Picture of the cone taken in April 2015 in which the summit dome is clearly visible. (**c**) Panoramic view of the cone after the 10–11 July 2015 eruption. Note the channel eroded by the pyroclastic flow that was subsequently filled by a lava flow. (**d**) Picture of the southern flank of the volcano in which the lava flows emplaced in 2016 are clearly visible.

Satellite remote sensing data, such as those collected from synthetic aperture radar (SAR) and optical, multi-, hyperspectral and thermal images, have been largely used for near real-time data acquisition regarding ongoing eruptive activity to define the locations of eruptive events, lava flow inundated areas, volumes and discharge rates [10–12], which are key factors for hazard assessment and numerical simulations [13,14]. Examples can be found from Hawaii, the Etna volcano, the lava fields in Iceland [15,16] and, more recently, the Sinabung volcano (Sumatra), for which the dome growth and pyroclastic flow deposit distributions and their magnitudes have been defined for the 2013–2015 eruptive crises [17]. Thermal remote sensing represents an optimal tool for lava flow discharge-rate estimation relative to optical products for which cloud coverage frequently impedes object observation, and is generally the same for volcanoes in tropical environments. Additionally, the lava discharge rates retrieved using thermal images agree well with those measured with ground-based methods, as recently demonstrated for the 2014–2015 eruption at Holuhraun, Iceland [18]. Finally, in recent years, SAR interferometry radar techniques (InSAR) and time series analysis have been used to identify deformation areas and surficial changes linked to lava flow deposits after major eruptions to determine their distributions, extrusion rates and thicknesses [19–22].

A broad variety of remote sensing data are freely available, including Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), Earth Observing-1 (EO-1) Advanced Land Imager (ALI), Landsat and Copernicus constellation (i.e., Sentinel 1 A-B and Sentinel 2) data. All those data are provided at a medium spatial resolution (between approximately 10 and 30 m) including in the thermal bands (i.e., the ASTER-TIR channel, 90 m ground resolution). In Mexico, Satellite Pour l'Observation de la Terre (SPOT) constellation data have been freely distributed since 2004 under an agreement between academic institutions and the Station of Telemetry Reception in Mexico (Estación de Recepción México-ERMEX), which is under the command of the Office for National Defense. SPOT data utilizes stereo and tri-stereoscopic capabilities to generate digital surface models (DSMs) with very high resolutions that allow for the identification of topographic changes after a major volcanic eruption. Thus, accessing the SPOT data represents an invaluable opportunity for performing multitemporal remote sensing analyses of natural phenomena at more detailed scales in Mexico.

In this paper, we assess the capabilities of stereo and tri-stereo SPOT6/7 images as well as EO-1 (ALI) images for the mapping of lava flows and pyroclastic flow deposits emplaced during the 2014–2016 eruptive phase of Volcán de Colima, including their volume estimations. We also assess the usefulness of the data for complementing the interpretation of the eruptive mechanism and the volume of the magma involved and for improving the calibration of numerical modeling of lava flows and granular flows. This work represents the first effort to apply remote sensing to the reconstruction of an eruption of an active Mexican volcano.

#### **2. Chronology of the Eruptive Phases Prior to, During and After the 10–11 July 2015 Climatic Event**

After a period of a relative calm since the last dome destruction period in early 2013, in May 2014, renewed effusive activity formed a lava flow on the W flank (WLF) of the Volcán de Colima during the extrusion of a new dome that, in late September, overspilled the crater with the emplacement of a lava flow on the SW slope of the volcano (SWLF) that advanced 2.2 km from the summit. On November 21, 2014, an ash plume rose ~7 km above the crater, and from this plume, a 3-km long pyroclastic flow descended along the San Antonio ravine [23]. In January 2015, several explosions caused the partial destruction of the dome and generated a 3-km ash plume that dispersed ash towards the NE [24]. In February 2015, the volcano produced several gas-and-ash plumes per day that rose to altitudes of 5.5–7.3 km (above sea level) [25]. Similar activity continued through the following months. In May, a thermal anomaly was detected inside the crater [26] and suggested a new dome extrusion phase that, by the beginning of June, was overflowing the crater (Figure 1b). In early July, two main lava flows were rapidly advancing (a few hundred m); one was on the northern flank, and the other (the larger of the two) was on the southern slope. During the days of July 7–9, gradual increases in the frequency of ash plumes, rock falls and small BAFs were observed [27]. By July 10, a lava flow had reached ~700 m down the southern flank (Figure 2a).

On 10 July, without any detected precursory activity, the summit dome collapsed, which generated a pyroclastic flow that emplaced along the Montegrande ravine up to approximately 8 km from the volcano summit. On the morning of 11 July, a second event, which was probably associated with the collapse of a new, fast-growing dome, emplaced a pyroclastic flow that traveled up to 10.5 km from the volcano along the same ravine and caused important damage to the vegetation alongside the channel (Figure 2b) [6,7,28,29]. This second pyroclastic flow promoted the partial failure and erosion of the upper portion of the crater leaving a v-shaped scarp (Figure 1c). The same day, a new lava flow began to flow out from this scarp, and by August 6th, it reached a distance of 2.5 km from the summit [6]. During the next months, the activity was characterized by small explosions and the emplacement of pyroclastic flows with a maximum runout of 2.5 km. In September 2016, a new lava flow filled the v-shaped scarp that was formed during the dome collapse activity and flowed along the southern flank to reach a distance of 2.2 km in late October (Figure 1d) [30,31]. Since then, the volcano has decreased its activity, which has included only sporadic explosions. Based on the above reconstruction, the last

eruptive phase at Volcán de Colima began at the end of 2014, culminated in July 2015 and closed after the emplacement of the September 2016 lava flow.

**Figure 2.** (**a**) Earth Observing-1 (EO-1) Advanced Land Imager (ALI) RGB composite (bands: 7, 4, 2, spatial resolution: 30 m) image taken on 10 July 2015 a few hours before the dome collapse occurred at 20:17 LT (Local Time). Incandescence is clearly visible on the southern sector of the cone (yellow-reddish colored area). (**b**) 2015-Satellite Pour l'Observation de la Terre (SPOT) composite image (bands: 4, 3, 2, spatial resolution 6 m) acquired on 25 July 2015 in which the area affected by the emplacement of the pyroclastic flows is clearly visible (gray area). (Map Projection: Universal Transverse Mercator—UTM).
