Abstract

PAVON, Fernando  and  CONTRERAS, Diana. Zoning of the Threat by Dispersion and Fall of Ash From The Guagua Pichincha Volcano, Ecuador. Invest. Geog [online]. 2021, n.105, e60315.  Epub Nov 01, 2021. ISSN 2448-7279.  https://doi.org/10.14350/rig.60315.

Volcanic activity in the equator is concentrated in the continental volcanic arc of the central-north portion of the Andes resulting from the subduction of the Nazca plate underneath the South American plate. This produces a high risk, evidenced in recent decades as an increased volcanic activity and a high impact on the population - especially those in poverty and extreme poverty - and on the country’s economy. The Guagua Pichincha volcano developed in three periods during the late Pleistocene and Holocene; together with the Rucu Pichincha, it forms the Pichincha volcanic complex. This complex recorded its latest eruption between 1999 and 2001 through the Crystal Dome with vulcanian-subplinian explosions. Guagua Pichincha is considered the third most dangerous volcano in continental Ecuador, posing a threat mainly to vulnerable rural population settled in areas close to the volcano and dedicated to agriculture and livestock raising. This research has used general parameters such as height of the eruptive column, duration of the eruption, volume of magma, and a statistical distribution of wind profiles. We obtained four eruption events that are consistent with events that occurred in the past recorded by several authors. Using a statistical analysis, we modeled 120 numerical simulations per event with the Ash 3D code. Using algorithms produced with digital tools, the dispersal and fall of ash was defined under a 75% probability of occurrence for the period 2008 - 2017, for zoning volcanic threat through the eruptive scenarios suggested. Census data of the 15 indicators (eight for vulnerability and seven for resilience) for the year 2010 were processed separately. As a result, we obtained a cover with parochial disaggregation at country level using the weights assigned in the Index for Risk Management (INFORM) methodology. Following this procedure, we estimated the magnitudes of rural-urban population and land-use categories that would be affected by using spatial analysis and automation processes through Phyton programming. Impact scenarios would have different effects according to the maximum expected thickness values estimated. The lower limit of the range is 5 mm assuming the most frequent scenario within the yearly eruptive cycle, with a volcanic explosiveness index (VEI) of 2; the upper limit is 150 mm of ash accumulation in a scenario with a VEI5, which occurs every 25 eruptive cycles and has been recorded every 2,500-21,250 years; the latter involves a significant increase in the number of villages, urban areas and, land-use categories affected. The maps obtained show the following. 1) For the VEI2 (minor) event, some 23,435 hectares dedicated to agriculture would be directly affected by the different ash thicknesses, representing one parish with moderate impact and seven parishes with low impact; for this scenario, the experience derived from the latest activity event recorded may be used to quantify the potential impact of the Guagua Pichincha volcano in the country. 2) A VEI3 (moderate) event, which occurs with a frequency between one and two events within an eruptive cycle, would result in 1,101,847 ha of farming land directly affected by ash fall, with thicknesses reaching 45 mm; 20 parishes would have a high impact, 119 parishes a moderate impact, and 78 parishes a low impact. 3) For a major event (VEI4) of the Guagua Pichincha volcano, which occurs every three to five eruptive cycles, recorded every 300-4,250 years, the expected ash thicknesses would range between 0.01 mm and 90 mm, affecting 1,130,405 ha of farmland; 98 parishes would suffer moderate impact and 66 low impact; a total of 4,240,559 inhabitants would be directly affected. 4) For scenario VEI5, 1,899,861 ha of farming land would be directly affected, including 33 parishes with a high impact, 235 parishes with moderate impact and 139 with a low impact. A VEI5 event would affect 30% of the population of Ecuador.

Keywords : Guagua Pichincha; ash deposits; threat zoning; GIS; Ecuador.

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