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  • Thesis title: Hazard assessment and risk management at San Miguel volcano, El Salvador
  • Author: Diana Jiménez de Contreras
  • Date: November, 20th, 2019; 11:00h
  • Place: Aula Magna "Carmina Virgili" Facultat de Ciències de la Terra (UB)
Thesis supervisors
Abstract

El Salvador lies on the Central America Volcanic Front and harbours around 50 major volcanic centers, of which some, such as - Santa Ana, San Salvador, Izalco and San Miguel – are currently active. San Miguel (or Chaparrastique, its original local name) is part of El Salvador Volcanic Cordillera and lies 11 km from the country's second largest city, the eponymous San Miguel, in the eastern part of the country. This 2130-m-high stratovolcano has a central crater of about 900 m in diameter, as well as several adventive cones from which abundant lava and pyroclasts have erupted during the Holocene. San Miguel has erupted at least 28 times in the past 430 years (both effusive and low-medium explosive events). Several villages and cities including San Miguel, Quelepa, Moncagua, Chinameca, San Jorge, San Rafael Oriente and El Tránsito lie near the volcano. The estimated population in the surrounding area is over 330,000 people, 60% of whom reside within the volcano's zone of influence due to the fertile nature of the volcanic soils and the cooler and more humid local microclimate at altitude.

In this context, studies focused on the reduction of the impact of future eruptions and on the improvement of resilience of the San Miguel socio-economic system should be a priority. Volcanic hazard and risk analyses pose the most useful tools for achieving that goal. Despite the great importance of these types of studies, they are scarce in the region, and the past history of the volcano and its eruption dynamics are still not well understood. Consequently, it is highly advisable to carry out volcanic hazard and risk analyses in order to improve the knowledge and awareness on the hazard potential of San Miguel volcano, at the time that these studies should be used as a guide to develop specific programs to reduce its related risk.

The purpose of this PhD thesis is to evaluate the volcanic risk associated with San Miguel Volcano, 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, short-medium 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.

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