Hazard Assessment and Risk Management at San Miguel volcano, El Salvador

Abstract

The purpose of this PhD thesis is to evaluate the volcanic risk associated with San Miguel Volcano, one of the most active volcanoes of El Salvador, through assessing volcanic hazard, that is, identify how a volcanic system (i.e., an active volcano or volcanic area) has behaved in the past and then use this information to infer how it may behave in the future. This task requires a compilation of all existing geological and geophysical information concerning the eruption style of the volcanic system in question, its eruptive recurrence, the structural constraints on the opening of new vents, and the characteristics and potential extent of its main hazards. The next step is to draw up eruption scenarios and hazard maps using the information gathered of the previous stage, which will constitute the basis for estimating exposure and vulnerability analysis, the third objective of this study.

We followed a probabilistic methodology to conduct the volcanic hazard assessment and scenario simulations. Probabilistic models are widely used in volcanic hazard assessment due to: (1) the lack of precise knowledge of the physical processes governing the dynamics of most volcanic hazards; (2) the difficulties in getting complete parameterisation sets for each phenomena; (3) the normally short time and computational costs; and (4) the acceptable results that probabilistic models provide. Thus, probabilistic or stochastic volcanic hazard analyses provide probabilistic outcomes that reflect the degree of uncertainty in the simulation.

We conducted the first systematic and comprehensive long-term hazard assessment for San Miguel using available geological data, past eruption records, stratigraphic information, and volcano-structural data, as well as new information gathered from fieldwork. We obtain a susceptibility map of the volcano and highlighted the areas with the greatest likelihood of hosting future eruptive vents. We conducted two temporal analyses, one with a forecasting time window of two years using information on volcanic activity over the past 430 years (historical period), and another with a forecasting window of six months, with information from the past 16 years (monitoring period). Then we calculated the most likely scenarios for each specific time windows. Secondly, we simulated: (1) the five most likely scenarios (ashfall scenarios, shortmedium extent, and VEI 1-2); (2) other probable scenarios related to lava flows, both according to its historical record; (3) other possible scenarios related to PDCs with similar characteristics to those that occurred during its geological history; and (4) the most hazardous scenario (ashfall, lava flow, PDC) also deduced from its geological record. We also constructed a qualitative integrated volcanic hazard map through the combination of the simulated scenarios.

Finally, we made an exposure analysis of San Miguel volcano area, considering population distribution, land use, as well as the distribution of the main infrastructures of the area. Moreover, we estimated a Vulnerability Index for the hazardous areas based on the characterization of the construction materials of walls and roofs of stocks. We constructed different exposure maps for 1) Population, 2) land use, 3) road network, 4) schools, and 5) health centers. For private houses and public infrastructures, we made an estimation of the Vulnerability Index in a village where lahars are frequent. This study was developed with the aim of improving land use and the already existing emergency plans, and pretends to be the starting point for the collaboration and coordination between scientists, the national observatory (OA-MARN), and the civil protection agency of San Miguel municipality, thus helping to strength this cooperation to face future volcanic crises related to San Miguel volcano.