During July 10th–11th 2015, Volcán de Colima, Mexico, underwent its most intense eruptive phase since its Subplinian–Plinian 1913 AD eruption. Production of scoria coincident with elevated fumarolic activity and SO2 flux indicate a significant switch of upper-conduit dynamics compared with the preceding decades of dome building and vulcanian explosions. A marked increase in rockfall events and degassing activity was observed on the 8th and 9th of July. On the 10th at 20:16 h (Local time = UTM − 6 h) a partial collapse of the dome generated a series of pyroclastic density currents (PDCs) that lasted 52 min and reached 9.1 km to the south of the volcano. The PDCs were mostly channelized by the Montegrande and San Antonio ravines, and produced a deposit with an estimated volume of 2.4 × 106 m3. Nearly 16 h after the first collapse, a second and larger collapse occurred which lasted 1 h 47 min. This second collapse produced a series of PDCs along the same ravines, reaching a distance of 10.3 km. The total volume calculated for the PDCs of the second event is 8.0 × 106 m3. Including associated ashfall deposits, the two episodes produced a total of 14.2 × 106 m3 of fragmentary material. The collapses formed an amphitheater-shaped crater open towards the south. We propose that the dome collapse was triggered by arrival of gas-rich magma to the upper conduit, which then boiled-over and sustained the PDCs. A juvenile scoria sample selected from the second partial dome collapse contains hornblende, yet at an order of magnitude less abundant (0.2%) than that of 1913, and exhibits reaction rims, whereas the 1913 hornblende is unreacted. At present there is no compelling petrologic evidence for imminent end-cycle activity observed at Volcán de Colima

Long-term time series of volcanic plumes composition constitute valuable indicators of the evolution of the magmatic and volcanic systems. We present here a 4-years long time series of molecular ratios of HF/HCl, HCl/SO 2 , SiF 4 /SO 2 , HF/SiF 4 measured in the Popocatépetl ′ s volcanic plume using ground-based solar absorption FTIR spectroscopy. The instrument, based in the Altzomoni NDACC (Network for the Detection of Atmospheric Composition Change) station, facing the Popocatépetl volcano, provides an unrivaled precision. The computed mean and standard deviation of the HF/HCl and HCl/SO 2 ratios for this period were found to be 0.24 ± 0.03 and 0.11 ± 0.03, respectively. SiF 4 was detected in three occasions and the SiF 4 /SO 2 ratios ranged between (1.9 ± 0.5) × 10 −3 and (9.9 ± 0.4) × 10 −3 . The HBr/HCl and HBr/SO 2 ratios remained below their detection limits (1.25 × 10 −4 and 1.25 × 10 −5 , respectively), given that a part of the HBr has already been converted to other bromine species (e.g., BrO, Br 2 ) a few kilometers downwind of the crater. Combining our time series with satellite SO 2 fluxes and seismic data, we explain the significant long-term HCl/SO 2 variations by changes in the conduit and edifice permeabilities, impacting the deep and shallow degassing processes. The high temporal resolution of the data also allows capturing the variation of the volcanic plume composition preceding and induced by a common moderate explosion at Popocatépetl volcano. We interpret the observed variations of the HCl/SO 2 ratio during the explosion in terms of changes in the contribution of the deep/shallow degassing. We additionally report the detection of an increase of SiF 4 after the explosion, likely explained by in-plume HF-ash interaction. During this event, SiF 4 /HCl vs. HF/HCl was found to have a linear relation with a slope of −1/4, which implies a conservation of fluorine.

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The ever-increasing population living near active volcanoes highlights the need for the implementation of effective risk reduction measures to save lives and reduce the impact of volcanic unrest and eruptions. To help identify volcanic systems associated with potential high risk and prioritize risk reduction strategies, we introduce a new Volcanic Risk Ranking (VRR) methodology that integrates hazard, exposure, and vulnerability as factors that increase risk, and resilience as a factor that reduces risk. Here we present a description of the methodology using Mexican volcanoes as a case study, while a regional application to Latin American volcanoes is presented in a companion paper (Guimarães et al., submitted). With respect to existing strategies, the proposed VRR methodology expands the parameters associated with hazard and exposure and includes the analysis of 4 dimensions of vulnerability (physical, systemic, social, economic) and of resilience. In particular, we propose 41 parameters to be analyzed, including 9 hazard parameters, 9 exposure parameters, 10 vulnerability parameters and 13 resilience parameters. Since the number of parameters evaluated for each risk factor is different, they are normalized to have the same weight based on dedicated sensitivity analyses. In order to best illustrate the methodology, the proposed VRR is here applied to 13 Mexican volcanoes and compared with other approaches. We found that the volcanoes associated with the highest combination of hazard, exposure and vulnerability (3-factor VRR) for this geographic area are Tacaná and El Chichón regardless of the analyzed time window of eruption occurrence (i.e., <1 and <10 ka). Nonetheless, the volcanoes with eruption <1 ka that require the most urgent actions as associated with no or few resilience measures in place are Michoacán-Guanajuato Volcanic Field and San Martín Tuxtla (4-factor VRR); the top volcanoes in the 4-factor VRR with eruption <10 ka are Michoacán-Guanajuato Volcanic Field and Las Cumbres.

We present a Risk Atlas of Mexico City based on a Geographical Information System (RA-GIS). We identified the prevalent social risk to the more relevant hazards in Mexico City (CDMX): earthquakes, volcanic eruptions, floods, landslides, forest fires, and land subsidence. A total of 274 shape-file maps were generated in this project. Seismic hazard was estimated for return periods (RP) of 20, 125, 250, and 475 years. Three areas in central and northwestern CDMX were identified along the Younger Chichinautzin Monogenetic Volcanic Field with a high probability of forming a new volcano. Subsidence is concentrated to the east and southeast of CDMX, where subsidence rates are among the highest worldwide. Flooding events were estimated for RP of 2, 5, 10, 50, and 100 years, and most of them are concentrated in the central and northern sectors of the city. During the dry season (December–April), southern CDMX has very high probability of forest fire occurrence. There is high susceptibility of landslides on the west and southwest of the city. The goals of this RA-GIS are to provide a tool to the local and federal authorities and all organizations responsible for disaster prevention and mitigation to: (1) improve the knowledge of the potential physical and social impact of local hazards; (2) provide elements for disaster prevention, mitigation, preparedness, and response; (3) benefit decision-makers with robust risk data; (4) provide information for land-use planning; and (5) support further research to reduce the impact of disasters caused by natural phenomena.

Volcanic Risk Ranking (VRR) methods have been developed worldwide as a way to hierarchize the volcanic systems and help target strategies for risk reduction. Such hierarchization is especially important in areas characterized by a large number of active volcanoes but limited resources. This is the case of Latin America, where large populations live nearby almost 300 active volcanoes. Here we assess the volcanic systems in Latin America with at least one eruption in the last 1,000 years based on the VRR strategy presented in a companion paper that accounts for the 4 main risk factors: hazard, exposure, vulnerability and resilience. Our results reveal that, among the 123 volcanoes analyzed, Santiaguito, Tacaná and Fuego are those with the highest score in the 3-factor VRR (H×E×V), while Ecuador, Marchena and Santiago are among the systems with the lowest score. Bárcena and Pinta score zero as there is no exposure. Although vulnerability significantly contributes to the VRR score, hazard and exposure are the main factors that define the risk of Latin American volcanic systems in the proposed 3-factor VRR, while resilience contributes to its reduction in the proposed 4-factor VRR strategy. In this regard, Arenal, Copahue, Villarrica, Ubinas, Irazú and Poás are the systems with the highest number of risk reduction strategies in place. Atitlán, Almolonga and Tecuamburro are the volcanic systems with the highest score in the 4-factor VRR [(H×E×V)/(Res+1)], combining moderate hazard, exposure and vulnerability and low resilience; Bárcena, Pinta, Ecuador, Marchena and Santiago receive the lowest scores due to no or low exposure. Santiaguito, Tacaná, El Chichón and Ceboruco are characterized by high scores in the 3-factor VRR and also stand out as some of those with few risk reduction strategies implemented; thus they have intermediate to high scores also in the 4-factor VRR. Recognizing that hazard is difficult to mitigate and reducing exposure may depend on hardly feasible relocation of infrastructure and already established communities, we emphasize that measures to reduce vulnerability and increase resilience should be promoted (e.g., creating redundancy/accessibility to infrastructure, carrying out risk assessment studies, implementing early warning systems, developing emergency plans and promoting educational activities).

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