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Tephra fallout hazard assessment at Tacaná volcano (Mexico)
Journal of South American Earth Sciences 91 (2019) 253–259
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Tephra fallout hazard assessment at Tacaná volcano (Mexico) a,∗
b
c
d
Rosario Vázquez , Rosanna Bonasia , Arnau Folch , José L. Arce , J. Luis Macías
T a
a
Instituto de Geofísica, Universidad Nacional Autónoma de México, Unidad Michoacán, Antigua Carretera a Pátzcuaro 8701, 58190, Morelia, Michoacán, Mexico CONACYT-Sección de Estudios de Posgrado e Investigación, Instituto Politécnico Nacional, ESIA Zacatenco, 07738, Mexico City, Mexico Barcelona Supercomputing Center-Centro Nacional de Supercomputación, Nexus II Building, Jordi Girona 29, 08034, Barcelona, Spain d Instituto de Geología, Universidad Nacional Autónoma de México, Coyoacán, 04510, Ciudad de México, Mexico b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Tacaná volcano Tephra hazard assessment FALL3D
Tacaná volcano is one of the dormant volcanoes in Mexico. Its activity is demonstrated by recent phreatic explosions occurred in 1949 and 1986. At least four Plinian to sub Plinian eruptions have been recorded in the eruptive history of this volcano, which produced pumice and ash deposits that blanketed a vast area straddling the Mexican and Guatemalan territory. Currently, there is no quantitative tephra hazard study of the volcano fallout deposits nor of the dispersion of ash in the atmosphere. In this work, we present the first probabilistic tephra hazard assessment for the Tacaná volcano. Probabilistic hazard maps were computed both, for the tephra deposit, and for critical thresholds of airborne ash concentration at different flight levels. The FALL3D numerical model was used to perform hazard maps for a Plinian scenario defined on the basis of the Sibinal Pumice eruption occurred ∼23.5 kyrs ago, which is the most studied deposit of this type in the volcano's surroundings, even when just few of their most distal outcrops are preserved. Results of this work show that at ground level, the disturbance for this type of hazard would affect mainly Guatemalan territory and the state of Chiapas in Mexico. It could cause serious repercussions on the economy of the region, which is characterized mainly by cocoa, coffee and banana plantations. The ash dispersion into the atmosphere could disturb the airspace and major airports in Mexican territory and neighboring countries. In fact, probability ash dispersal maps show that the level of affectation to the air traffic would involve the possible closure of at least 23 airports in Mexico, Guatemala, Belize, Honduras, El Salvador and Nicaragua. Another collateral damage would be the remobilization of ash by lahars considering the near 4000 mm of rain in average poured yearly in the Soconusco region that would increase the sediment load of the Coatán and Suchiate rivers, and then could produce a severe damage to the lowlands, which in the case of the Mexican territory, is the most populated one.
1. Introduction The Tacaná Volcanic Complex (TVC), also known as the Tacaná volcano, is considered as one of the dormant volcanoes in Mexico. It is the westernmost volcano of the Central America Volcanic Arc (CAVA), a WNW-oriented volcanic arc extending for about 1300 km, from the Mexico-Guatemala border to western Panama (Carr, 1984). The complex consists of four NE-SW oriented volcanic structures, named from oldest to youngest: Chichuj, Tacaná, Las Ardillas dome, and San Antonio volcano (Macías et al., 2000). Tacaná’s summit serves as marker of the international border between Mexico and Guatemala (Fig. 1). The last eruptive activity of the complex occurred in 1949 and 1986 with phreatic explosions, both took place near the summit of the volcano (Müllerried, 1951; De la Cruz-Reyna et al., 1989). Nonetheless, the TVC
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has shown evidence of large explosive and effusive eruptions during its eruptive history. Explosive activity has produced Plinian to sub-Plinian eruptions occurred at ∼29.5, ∼23.5, ∼14, and 0.85 kyrs ago (Macías et al., 2015), whose fallout deposits were distributed predominantly over Guatemalan territory. A future Plinian eruption of the complex could cause a major disaster in the region, endangering human life and damaging hundreds of thousands of dwellings in the volcano surroundings, due to the poor quality of the constructions in the zone. An eruption of this size could affect economic activities in the Soconusco region, which comprises at least 15 municipalities in the southern tip of the state of Chiapas in Mexico (INEGI, 2010), destroying the coffee, cacao, and banana plantations which are the main economic sustainment in the area. Actually, even a small magnitude explosive activity of the volcano could cause a
Corresponding author. E-mail address: [email protected] (R. Vázquez).
https://doi.org/10.1016/j.jsames.2019.02.013 Received 13 December 2018; Received in revised form 19 February 2019; Accepted 21 February 2019 Available online 22 February 2019 0895-9811/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Digital elevation model of the Tacaná Volcanic Complex, showing the four volcanic edifices, and the main localities around of the volcano.
been done to assess the hazard related to ash falls and tephra dispersal in case of a renewed explosive activity at the volcanic complex. Mercado and Rose (1992) were the first to propose a rough delimitation of the zone that could be affected by the accumulation of 1 m or more of ash fall. They also proposed the first inferred isopach map of Plinian deposits, with the most distal isopach of 10 cm, encountered about 20 km from the summit. Borjas-Hernández (2006) and Arce et al. (2012) described in detail the pumice-fall deposits related to the Plinian eruptions of ∼23.5 kyrs and outlined partial isopleth and isopach maps for the two deposits due to the lack of distal outcrops, which are difficult to observe given the tropical-weather conditions of the region. At present, the only hazard map that considers tephra fallout from the TVC is the one proposed by Mercado and Rose (1992); however, built only on photo-interpretation. This manuscript focuses on the probabilistic tephra hazard assessment of a Plinian eruptive scenario of the Tacaná volcano, defined on the basis of the Sibinal Pumice eruption (∼23.5 kyrs). The deposits of this eruption have been broadly studied and presents the most complete features of a Plinian event, required for modelling. The goal is to assess the impact that a Plinian eruption similar to the Sibinal Pumice could have on the area surrounding the volcano and on the main airports of Mexico and adjacent countries (i.e. Guatemala, Belize, Honduras). This information will serve to produce the first hazard map of the volcano and to prepare a long-term risk scenario to minimize the impact of such kind of eruption in terms of tephra deposition and dispersion of the volcanic ash.
considerable damage to the surrounding populations, like the city of Tapachula, located at ∼30 km south of the TVC, which is the most populated of the region with ∼320,000 inhabitants (INEGI, 2010). An evidence of the risk posed by an explosive activity of lesser magnitude than a Plinian eruption, was the June 2018 eruption of the Fuego volcano in Guatemala, located 155 km southeast from Tapachula, which produced pyroclastic density currents (PDCs) with the longest run-outs described in modern times for this volcano. This eruption caused 153 deaths (INACIF, 2018) and the partial destruction of three villages on the southern flank of the volcano (GVP, 2018). The harmful effects caused by the very fine ash transported by winds produced serious problems to the banana plantations around Tapachula. In the case of the TVC, its stratigraphic record shows that the development of PDCs has been recurrent in its eruptive history, along with other volcaniclastic flows, such as syn-eruptive lahars and debris avalanches (Macías et al., 2000, 2010; 2015, 2018). Yet, the most extended deposits related to the volcanic hazards of the TVC, are related to the dispersion of tephra by Plinian eruptions. Even if the main dispersal axis is not directed towards the city of Tapachula, winds can potentially transport fine ash for long distances in different directions and trigger secondary lahars due to the emplacement of pumice or fine material into the main drainages and slopes of the volcano (e.g. Coatán, Cahoacán and Suchiate rivers, among other small streams), which could bulk up the outflow and sediment load of the currents and endanger the lowlands with floods, something that has happened in ancient times (Macías et al., 2000, 2018). Despite the knowledge of this type of activity at the TVC, little has 254
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Fig. 2. Stratigraphic column of a representative outcrop of the Sibinal Pumice deposit showing the stratified (SM) and massive (MM) members separated by a lahar deposit (L) (modified from Arce et al., 2012).
2. Sibinal Pumice eruption: stratigraphy and physical characteristics
sized pumice (ca. 85 vol %), fresh light-gray lithics, hydrothermally altered lithics (totaling 5 vol %), loose crystals, and glass shards. Hydrothermal altered lithics are abundant at the base of this member (up to about 50 vol%). A maximum thickness of 255 cm was measured 13 km to the north of the TVC summit. The isopachs and isopleths show that the thickness and lithic size of the massive member is distributed to the north and northeast of the TVC (Fig. 3), with the most distal isopach (30 cm) covering an area of 330 km2. A minimum tephra volume of 1.9 km3 was estimated for the massive member (Arce et al., 2012). Based on Carey and Sparks (1986), an eruptive column height of 22 km was inferred for the massive member (Arce et al., 2012), with a peak mass discharge rate of 8.1 × 107 kg/s. The present work focuses on the massive unit, whose physical characteristics were used as a basis for the construction of the probabilistic hazard maps.
Based on several described outcrops, Arce et al. (2012) defined the distribution of the Sibinal Pumice deposit, produced by a Plinian eruption occurred ∼23.5 kyrs ago. Products were largely dispersed to the north-northeast of the TVC towards the Sibinal and Tacaná towns, the two most important settlements in the surroundings of the volcanic edifice, in Guatemalan territory (Fig. 1). The most distal Sibinal outcrop can be found at 21 km from the volcano summit; at farther distances the deposit is completely eroded because the area is subjected to intense rain during most of the year. In general, pumice fragments are highly altered, making it difficult to obtain granulometric analyses and goodquality whole-rock chemical analysis. The most proximal outcrop of the sequence is located near Vega del Volcán village, at 4 km from the summit. The absence of closer outcrops can be attributed to weathering and a thick cover of younger deposits (e.g. lava flows and pyroclastic aprons). The Sibinal Pumice sequence is composed of two members separated by a lahar deposit (Arce et al., 2012). The lower member is a stratified unit (i.e. SM unit, Fig. 2), constituted by seven lapilli-sized fall deposits, which overlies a dark-brown paleosol rich in organic material dated at 23,540 + 255/-245 yrs (Arce et al., 2012). Some layers contain a lot of accidental lithic fragments (up to 50 vol %). The total thickness of this member is 2.6 m. The upper member (MM unit, Fig. 2), is a tephra fall deposit composed of highly altered, vesicular, lapilli-
3. Hazard assessment methodology One thousand numerical simulations for the generation of hazard maps were performed using the FALL3D tephra dispersal Eulerian model (Costa et al., 2006; Folch et al., 2009), that solves a set of advection-diffusion-sedimentation (ADS) equations using a second-order Finite Difference explicit scheme. In order to solve the governing equations, the FALL3D model assumes that the main factors that control atmospheric transport and deposition of ash are: wind advection, turbulent diffusion and gravitational settling of particles. The model is able 255
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Fig. 3. Isopach and isopleth maps of the massive member of the Sibinal Pumice showing the main dispersal axis of the deposit. A) Isopach lines; B) Isopleth lines. Values of both maps are in cm and the values shown in black dots are the average of the five largest lithic clasts (modified from Arce et al., 2012).
atmospheric levels, considering different threshold values. In particular, ash fallout hazard maps considered three ground load thresholds of 1, 10 and 100 kg/m2. The first value is considered as a limit for airport closure; the second is typically considered as a threshold for impact on agriculture (Blong, 1984; Bonadonna et al., 2002); and the last value is significant for the collapse of roofs in buildings of low to medium quality. For airborne ash, two critical flight levels (FL) were taken into account: FL100 (altitude 10,000 ft, ∼3 km) that corresponds to a low level relevant to flight landing/take-off operations, and FL300 (altitude 30,000 ft, ∼9.1 km), which is relevant for the jet-cruise altitude. In this case, flight level hazard maps represent the probability for ash concentration to exceed the critical values. Two concentration threshold values were considered (2 and 0.2 mg/m3) based on the paper by Bonasia et al. (2014). Both values were adopted in 2010 during the European crisis caused by the Eyafjallajökull eruption, which caused an unprecedented closure of the European airspace (IVATF-1, 2010).
to run from local to regional scales and it is initialized with meteorological and vulcanological parameters. In this work, a computational domain spanning from 11° to 22° N and from −100° to −83° W, with a horizontal grid resolution of 0.15° (∼15 km), was considered. Time-dependent wind fields and other meteorological model inputs were extracted from sampling ERAInterim reanalysis data within a 10-year period (2008–2017), ensuring homogeneous stratified sampling along natural years (i.e. same number of runs per month). Other parameters used in the model were set as follows: - A vertical distribution of mass in the column following a Suzuki-type distribution (Suzuki, 1983) with parameters A = 4 and λ = 5. - The column height and total erupted mass were linearly sampled within a range (Table 1) consistent with the scenario described by Arce et al. (2012) for the massive member (MM) of the Sibinal Pumice deposit. Given a value for column height, the time-averaged mass eruption rate was determined using the relationship between column height and eruption rate proposed by Mastin et al. (2009). The eruption duration was fixed from mass eruption rate and sampled mass. - A Gaussian Total Grain Size Distribution (TGSD). Giving the lack of field data necessary to integrate deposit grain-size at different locations, we used the fit of Costa et al. (2016) to determine TGSD parameters from magma viscosity and plume height using the Askja 1875 phase D eruption as an analogue (both eruptions have comparable magma compositions and values of column height).
4. Results Fig. 4 shows the three ground hazard maps from the lowest to the highest threshold value (i.e. A, B and C, respectively). The general feature of these maps is the elongated distribution of the probability contours on a NE-SW direction, due to the preferential climatic trend of winds in this region. It is also notable that the extent of the area that could be prone to concentrate the different thresholds diminishes its reach with the increase in the ground load value as expected. From the three hazard maps, the map with a load of 1 kg/m2 (i.e. Fig. 4A) covers the wider area (∼250,000 km2), although with the lowest probability value (5%). Within this probability contour, not only the Mexican and Guatemalan territories would be affected, but also part of Belize; with almost all the state of Chiapas and Guatemala being blanketed by a ∼1 mm thick ash layer. The larger probability contour (80%) has a radius of ∼40 km from the volcano (∼4000 km2), which means that at least 26 main urban zones including towns like Tapachula, Motozintla, Tacaná, Sibinal, Huixtla, Malacatán, and San Marcos (i.e. more than 350,000 inhabitants), would be affected by a fine sheet of ash and would probably cause the closure of the International Airport of Tapachula. For the second ground load threshold of 10 kg/m2 (Fig. 4B), the area affected by the higher probability contour is ∼2000 km2 (i.e. ∼30 km from the volcano). Within this area the city of Tapachula and
Probabilistic hazard maps were generated from simulations varying wind data and eruptive input parameters as described below. These maps were built for ash fallout as well as for concentration at different
Table 1 Range of input parameters used for the FALL3D numerical runs. For comparison, also values of total mass and column height of the Sibinal eruption MM are reported.
Total mass (kg) Eruption duration (h) Column height (km)
Minimum
Maximum
Sibinal Pumice (MM)
2.5 × 1012 12 19
5 × 1012 24 26
4.8 × 1012 17.7 22
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contour maintains the elongated shape of previous maps (Fig. 4A and B), with NE-SW direction. Moreover, for this ground load threshold (100 kg/m2), the 80% probability contour covers an area of ∼1000 km2 in which the Tapachula airport will be affected and approximately 200 towns in both countries, including Cacahoatán, Santo Domingo, Unión Juárez, Sibinal, La Laguna and Tacaná. Fig. 5 shows the dispersal hazard maps obtained for the TVC, considering the flight levels FL100 (Fig. 5 A and B) and FL300 (Fig. 5 C and D), at the two critical ash concentrations adopted during the 2010 European aviation crisis (0.2 and 2.0 mg/m3). From these probabilistic maps, it can be observed that in both flight levels exists a very high probability (50%) that the two thresholds considered could be exceeded, and the airspace of at least 23 airports would be affected, including in addition to the International Airport of Mexico City other important national and international airports in Mexico (e.g. the airports of Acapulco, Villahermosa, and Cancún), Belize, Guatemala, El Salvador, Honduras and Nicaragua. Moreover, in almost all the cases, the highest probability contour (i.e. 90%) that either the 0.2 or the 2.0 mg/m3 threshold could be exceeded, covers a region with a radius of ∼50 km from the volcano, restricting the affected area to ∼8000 km2, an area that still includes the international airport of the city of Tapachula in Mexico, and the Coatepeque airport in Guatemala. In general, the probability contours of all the maps in Fig. 5, are elongated towards the NE-SW, which is consistent with the direction of the ash ground load hazard maps presented on Fig. 4; however, it is worth noticing that the shape of the contours for the flight level FL300 reach a bigger extension in the same direction NE-SW in both ash concentration thresholds, exceeding the considered computational domain. 5. Discussion Nowadays, numerical modelling and probability analysis have been used more often to complement hazard assessment of volcanic risk, making the analyses more reliable and comprehensive, especially in those cases where the geological record is not accessible, or is partly or completely missing. Therefore, as described by Bonadonna (2006), numerical modelling serves as a counterpart of direct observations in the field and to explore a much wider range of possible scenarios. In this work, we follow the Eruption Range Scenario method described by Bonadonna et al. (2005) and Bonadonna (2006), using the FALL3D numerical code (Costa et al., 2006; Folch et al., 2009) and considering as a basis the physical parameters of the TVC Sibinal Pumice eruption of ∼23.5 kyrs (Borjas-Hernández, 2006; Arce et al., 2012). In summary, the probabilistic tephra ground load hazard maps, considering the lowest probability contour of 5% for the 1 kg/m2 threshold, indicate that the extent of the area that could be affected (∼250,000 km2) is large enough to cover the entire state of Chiapas and most of the Guatemalan territory. As to the hazard posed by a layer of 1 mm of ash, it would result in significant damage, especially for the plantations of coffee, cacao, and banana. Historically, the volcanoes in the CAVA region and the Chiapanecan Volcanic Arc such as the El Chichón, have developed powerful Plinian eruptions dispersing ash with broad extensions just like the one presented in the hazard map of Fig. 4A (Carey and Sigurdsson, 1986; Macías et al., 2003; Bonasia et al., 2012). One example is the 1982 El Chichón eruption, occurred between March 29th and April 4th (GMT time), that generated three fallouts, pyroclastic density currents, and debris flows, blanketing an area of more than 50,000 km2 with tephra. The tephra fallout deposits of this eruption, had a major dispersion axis towards the northeast and east. Another example is the Santa María volcano eruption occurred in October 24th, 1902, which lasted 18–20 h, and produced a column of at least 28 km high (Williams and Self, 1983). Sapper (1905) calculated the 1 mm isopach of the deposit, which enclosed an area of 273,000 km2 covering the southern part of Mexico (up to Acapulco) and destroying most of Guatemala's coffee industry, as reported by the New
Fig. 4. Probability hazard maps for different ash ground load thresholds at the TVC: A) 1 kg/m2; B) 10 kg/m2; and C) 100 kg/m2. Contour colour denotes percent probability of exceeding the given threshold values. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
its airport could still be affected by a 1 cm ash fallout. Considering instead the 5% probability contour of Fig. 4B, the affected area would reach ∼36,500 km2, involving the main zone of coffee plantations in the Soconusco region, and at least four main airports: Tapachula (in the Mexican side); Huehuetenango, Coatepeque and Quetzaltenango (in Guatemala). Finally, on Fig. 4C, the 5% probability that the ash loading would overcome a threshold of 100 kg/m2 involves an area of ∼10,500 km2 between the localities of Puerto San Benito and Huehuetenango (a radius of ∼60 km from the volcano); this probability 257
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Fig. 5. Probabilistic hazard maps for two flight levels, considering two critical ash concentrations: A) FL100 with a threshold of 0.2 mg/m3; B) FL100 for 2.0 mg/m3; C) FL300 considering the threshold of 0.2 mg/m3; and D) FL300 for the 2.0 mg/m3 threshold. Contour colour denotes percent probability of exceeding the given threshold values. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
considered in our simulations, could have affectations of a similar scale as those produced by the Puyehue-Cordón Caulle eruption in 2011. This not only in regard to the ground and air transportation networks, but also to the affectation of agriculture, livestock, power lines, water distribution systems, human and animal health, farming and tourism sectors, to name the most important. These consequences have also been observed in other volcanic crises such as the 1982 of the El Chichón in Mexico (De la Cruz-Reyna and Martin Del Pozzo, 2009; Scholamacchia and Capra, 2015); the 1991 eruption of Mount Pinatubo in the Philippines (Newhall and Punongbayan, 1996); the 2008 and 2011 eruptions of Chaitén and Cordón Caulle, in Chile (Lara, 2009; Collini et al., 2013), among others. Another hazard related to the tephra fall dispersion presented in this work is the development of lahars due to the entrainment of volcanic ash into the main rivers that drains the surroundings of the complex, as happened before in the eruptive history of the TVC (Murcia, 2008; Murcia and Macías, 2009, 2014). Past evidences of syn-eruptive lahars originated during Plinian activity of the TVC is evidenced, for example, by the lahar deposit that separates the SM and MM members of the Sibinal Pumice sequence (Arce et al., 2012), and other thick deposits of ancient lahars located in the surroundings of the volcano (Murcia, 2008; Murcia and Macías, 2009, 2014). Globally, the best example of this type of hazard occurred during and after the 1991 Plinian eruption of Mount Pinatubo (Newhall and Punongbayan, 1996), whose impact continues until today. Therefore, the formation of syn- or post-eruptive lahars at the TVC should not be disregarded, considering that the total extent of the area that could be affected by the tephra fallout, as shown by the hazard maps of this work, would be broad, covering southern Mexico and Guatemala.
York Times (1902a, b). The 1902 ash was dispersed to the northwest of Santa María volcano across Guatemala into Mexico, reaching Mexico City (Williams and Self, 1983) and even the city of Colima located 1400 km northwest of the source (Sapper, 1905). In the case of the TVC, our probabilistic hazard maps have a NE-SW preferential dispersion direction of probability curves, reaching as far as Belize and Guatemala City with the 5% probability (Fig. 4A). This preferential dispersal orientation towards Guatemalan territory has been evidenced by previous eruptions of the TVC, occurred ∼23.5k yrs and ∼14k yrs ago (Arce et al., 2008, 2012). Therefore, the shape of the probability contours presented on the maps of Fig. 4 are in good agreement with the ancient deposits suggesting that the extent of the damage of a Plinian eruption of the complex could be comparable to the 1902 Santa María eruption. As a matter of fact, a similar crisis happened on June 2011, when the complex Puyehue-Cordón Caulle, located in the southern volcanic zone of the Andes in Chile erupted. This event started with a sub-Plinian eruption that lasted 27 h followed by a phase of weak columns that lasted 8 months (Bertin et al., 2013; Collini et al., 2013; Schipper et al., 2013). The finest granulometric fraction of the ash clouds generated during the first eruptive stage circumvented the southern hemisphere, passing over South Africa, south of Australia and New Zealand in about 10 days, causing a major air traffic disruption in Argentina (Collini et al., 2013; Schipper et al., 2013). In the case of the TVC Fig. 5 evidences that the 50% probability that any of the ash concentration thresholds considered to be exceeded at the different flight levels, would reach even a bigger region than the extent of the 1902 Santa María ash fallout, but less than the global transportation of ash of the Cordón Caulle event; moreover the maps on Fig. 5 show that not only the airspace of Mexico and Guatemala would be affected, but also the airspace of Belize, El Salvador, Honduras and Nicaragua. It is worth noticing that the lesser probability contours exceed the computational domain here considered, but the extension of the aerial disturbance produced by a Plinian scenario at the TVC, as the one
6. Conclusions In this work, we present the first comprehensive appraisal of tephra 258
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fallout and ash dispersion of the TVC by using a probabilistic approach. From our maps, it is easy to observe that in case of a major explosive eruption of the complex, the extent of the disturbance for this type of hazard at ground level would affect mainly the Guatemalan territory and the state of Chiapas in Mexico, with a significant damage to the harvests and agricultural sector of the region, which economically depends on the cacao, coffee and banana plantations. Another consequence, would be the generation of lahars, which could pose a major threat to populations settled along rivers. Probability ash dispersal maps show that the level of affectation to the air traffic would be even broader, involving the possible closure of at least 23 airports in Mexico, Guatemala, Belize, Honduras, El Salvador and Nicaragua, provoking a similar crisis to the one of 2010 in Europe due to the Eyjafallajökul volcano in Iceland, or the 2011 crisis in the Southern Hemisphere due to the Cordón Caulle eruption in Chile. The probabilistic hazard maps presented here would serve to the Mexican and Guatemalan authorities to upgrade their emergency plans in case of a renewed activity of the complex, as well to impulse the monitoring of the volcano and probably the creation of an aerotransport emergency guidance for the regional airports. This guidance will let them be prepared in case of a contingency of this type, not only in the case of a volcanic crisis of the TVC, but also the surrounding volcanoes at the CAVA.
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Reconocimiento geológico y evaluación preliminar de peligrosidad del Volcán Tacaná, Guatemala/México. Geofis. Int. 31, 205–237. Murcia, H.F., 2008. Depósitos de Lahar del Complejo Volcánico Tacaná y Depósitos Fluviales en el Abanico de Tapachula, Chiapas – México. PhD Thesis. Instituto de Geofisica, UNAM, pp. 136. Murcia, H.F., Macías, J.L., 2009. Registro geológico de inundaciones recurrentes e inundación del 4 de octubre de 2005 en la ciudad de Tapachula, Chiapas, México. Rev. Mex. Ciencias Geol. 26 (1), 1–17. Murcia, H., Macías, J.L., 2014. Volcaniclastic sequences at the foot of Tacaná volcano, southern México: implications for hazard assessment. Bull. Volcanol. 76 (835), 27. Müllerried, F.K.G., 1951. La Reciente Actividad del Volcán Tacaná, Estado de Chiapas, a Fines de 1949 y Principios de 1950. Informe del Instituto de Geología, UNAM, pp. 28. Newhall, C.G., Punongbayan, R.S., 1996. Fire and Mud: Eruptions and Lahars of Mount Pinatubo. University of Washington Press, Philipphines. Philipp Inst. Volcanol. Seism. Quezon City, 1126 pp. New York Times, 1902a. Volcanic Ruin in Guatemala. October 31. pp. 1. New York Times, 1902b. Thousands of Indians Killed in Guatemala. pp. 1 November 20. Sapper, K., 1905. In den Vulkangebieten Mittelamerikas und Westindiens. Schweizerbart, Stuttgart, pp. 174. Schipper, C.I., Castro, J.M., Tuffen, H., James, M.R., How, P., 2013. Shallow vent architecture during hybrid explosive-effussive activity at Cordón Caulle (Chile, 20112012): evidence from direct observations and pyroclast textures. J. Volcanol. Geoth. Res. 262, 25–37. Scholamacchia, T., Capra, L., 2015. El Chichón volcano: eruptive history. In: Scolamacchia, T., Macías, J.L. (Eds.), Active Volcanoes of Chiapas (Mexico): El Chichón and Tacaná. Active Volcanoes of the World. Springer, Berlín, Heidelberg, pp. 45–76. Suzuki, T., 1983. A theoretical model for dispersion of tephra. In: Shimozuru, D., Yokoyama, I. (Eds.), Volcanism: Physics and Tectonics. Terrapub, Tokyo, pp. 95–113. Williams, S.N., Self, S., 1983. The october 1902 plinian eruption of Santa Maria volcano, Guatemala. J. Volcanol. Geoth. Res. 16, 33–56.
Acknowledgments This work was supported by Consejo Nacional de Ciencia y Tecnología (CONACyT) project PN522 granted to J.L. Macías. We also thank to students and colleagues who shared fieldwork with us during this project and through the years. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jsames.2019.02.013. References Arce, J.L., Macías, J.L., Gardner, J.E., 2008. The ∼14 ka plinian-type eruption at Tacaná volcanic complex, Mexico-Guatemala. In: Conference Paper, AGU Fall Meeting, San Francisco. Arce, J.L., Macías, J.L., Gardner, J.E., Rangel, E., 2012. Reconstruction of the sibinal pumice, an andesitic plinian eruption at Tacaná volcanic complex, MexicoGuatemala. J. Volcanol. Geoth. Res. 217–218, 39–55. Bertin, D., Amigo, A., Delgado, R., 2013. Erupción del Cordón Caulle 2011-2012: análisis de dispersión bajo columna eruptiva débil. In: Conference Paper, XIII Congreso Geológico Chileno, Antofagasta, Chile. Blong, R.J., 1984. Volcanic Hazards. A Sourcebook on the Effects of Eruptions. Academic Press, Sydney. Bonadonna, C., 2006. Probabilistic modelling of tephra dispersal. In: In: Mader, H., Cole, S., Connor, C.B. (Eds.), Statistics in Volcanology. IAVCEI Series, vol. 1. Geol.Soc. Lon, pp. 243–259. Bonadonna, C., Macedonio, G., Sparks, R.S.J., 2002. Numerical modelling of tephra fallout associated with dome collapses and Vulcanian explosions: application to hazard assessment on Montserrat. In: In: Druitt, T.H., Kokelaar, B.P. (Eds.), The Eruption of Soufrière Hills Volcano, Montserrat, from 1995 to 1999, vol 21. Geol. Soc. Lon. Mem., pp. 517–537. Bonadonna, C., Connor, C.B., Houghton, B.F., Connor, L.J., Byrne, M., Laing, A., Hincks, T.K., 2005. Probabilistic modelling of tephra dispersal: hazard assessment of a multiphase rihyolitic eruption at Tarawera, New Zeland. J. Geophys. Res. 110 (10.1029), B03203. Bonasia, R., Costa, A., Folch, A., Macedonio, G., Capra, L., 2012. Numerical simulation of tephra transport and deposition of the 1982 El Chichón eruption and implications for hazard hassessment. J. Volcanol. Geoth. Res. 231–231, 39–49. Bonasia, R., Scaini, C., Capra, L., Nathenson, M., Siebe, C., Arana-Salinas, L., Folch, A., 2014. Long-range hazard assessment of volcanic ash dispersal for a Plinian eruptive scenario at Popocatépetl volcano (Mexico): implications for civil aviation safety. Bull. Volcanol. 76 (789), 16. Borjas-Hernández, M.L., 2006. Estratigrafía del Sector NE del Complejo Volcánico Tacaná, Chiapas, México-Guatemala. Tesis de Licenciatura, ESIA-Instituto Politécnico Nacional, pp. 128. Carey, S., Sigurdsson, H., 1986. The 1982 eruption of El Chichón volcano, Mexico (2): observation and numerical modelling of tephra-fall distribution. Bull. Volcanol. 48, 127–141.
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Journal of South American Earth Sciences journal homepage: www.elsevier.com/locate/jsames
Tephra fallout hazard assessment at Tacaná volcano (Mexico) a,∗
b
c
d
Rosario Vázquez , Rosanna Bonasia , Arnau Folch , José L. Arce , J. Luis Macías
T a
a
Instituto de Geofísica, Universidad Nacional Autónoma de México, Unidad Michoacán, Antigua Carretera a Pátzcuaro 8701, 58190, Morelia, Michoacán, Mexico CONACYT-Sección de Estudios de Posgrado e Investigación, Instituto Politécnico Nacional, ESIA Zacatenco, 07738, Mexico City, Mexico Barcelona Supercomputing Center-Centro Nacional de Supercomputación, Nexus II Building, Jordi Girona 29, 08034, Barcelona, Spain d Instituto de Geología, Universidad Nacional Autónoma de México, Coyoacán, 04510, Ciudad de México, Mexico b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Tacaná volcano Tephra hazard assessment FALL3D
Tacaná volcano is one of the dormant volcanoes in Mexico. Its activity is demonstrated by recent phreatic explosions occurred in 1949 and 1986. At least four Plinian to sub Plinian eruptions have been recorded in the eruptive history of this volcano, which produced pumice and ash deposits that blanketed a vast area straddling the Mexican and Guatemalan territory. Currently, there is no quantitative tephra hazard study of the volcano fallout deposits nor of the dispersion of ash in the atmosphere. In this work, we present the first probabilistic tephra hazard assessment for the Tacaná volcano. Probabilistic hazard maps were computed both, for the tephra deposit, and for critical thresholds of airborne ash concentration at different flight levels. The FALL3D numerical model was used to perform hazard maps for a Plinian scenario defined on the basis of the Sibinal Pumice eruption occurred ∼23.5 kyrs ago, which is the most studied deposit of this type in the volcano's surroundings, even when just few of their most distal outcrops are preserved. Results of this work show that at ground level, the disturbance for this type of hazard would affect mainly Guatemalan territory and the state of Chiapas in Mexico. It could cause serious repercussions on the economy of the region, which is characterized mainly by cocoa, coffee and banana plantations. The ash dispersion into the atmosphere could disturb the airspace and major airports in Mexican territory and neighboring countries. In fact, probability ash dispersal maps show that the level of affectation to the air traffic would involve the possible closure of at least 23 airports in Mexico, Guatemala, Belize, Honduras, El Salvador and Nicaragua. Another collateral damage would be the remobilization of ash by lahars considering the near 4000 mm of rain in average poured yearly in the Soconusco region that would increase the sediment load of the Coatán and Suchiate rivers, and then could produce a severe damage to the lowlands, which in the case of the Mexican territory, is the most populated one.
1. Introduction The Tacaná Volcanic Complex (TVC), also known as the Tacaná volcano, is considered as one of the dormant volcanoes in Mexico. It is the westernmost volcano of the Central America Volcanic Arc (CAVA), a WNW-oriented volcanic arc extending for about 1300 km, from the Mexico-Guatemala border to western Panama (Carr, 1984). The complex consists of four NE-SW oriented volcanic structures, named from oldest to youngest: Chichuj, Tacaná, Las Ardillas dome, and San Antonio volcano (Macías et al., 2000). Tacaná’s summit serves as marker of the international border between Mexico and Guatemala (Fig. 1). The last eruptive activity of the complex occurred in 1949 and 1986 with phreatic explosions, both took place near the summit of the volcano (Müllerried, 1951; De la Cruz-Reyna et al., 1989). Nonetheless, the TVC
∗
has shown evidence of large explosive and effusive eruptions during its eruptive history. Explosive activity has produced Plinian to sub-Plinian eruptions occurred at ∼29.5, ∼23.5, ∼14, and 0.85 kyrs ago (Macías et al., 2015), whose fallout deposits were distributed predominantly over Guatemalan territory. A future Plinian eruption of the complex could cause a major disaster in the region, endangering human life and damaging hundreds of thousands of dwellings in the volcano surroundings, due to the poor quality of the constructions in the zone. An eruption of this size could affect economic activities in the Soconusco region, which comprises at least 15 municipalities in the southern tip of the state of Chiapas in Mexico (INEGI, 2010), destroying the coffee, cacao, and banana plantations which are the main economic sustainment in the area. Actually, even a small magnitude explosive activity of the volcano could cause a
Corresponding author. E-mail address: [email protected] (R. Vázquez).
https://doi.org/10.1016/j.jsames.2019.02.013 Received 13 December 2018; Received in revised form 19 February 2019; Accepted 21 February 2019 Available online 22 February 2019 0895-9811/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Digital elevation model of the Tacaná Volcanic Complex, showing the four volcanic edifices, and the main localities around of the volcano.
been done to assess the hazard related to ash falls and tephra dispersal in case of a renewed explosive activity at the volcanic complex. Mercado and Rose (1992) were the first to propose a rough delimitation of the zone that could be affected by the accumulation of 1 m or more of ash fall. They also proposed the first inferred isopach map of Plinian deposits, with the most distal isopach of 10 cm, encountered about 20 km from the summit. Borjas-Hernández (2006) and Arce et al. (2012) described in detail the pumice-fall deposits related to the Plinian eruptions of ∼23.5 kyrs and outlined partial isopleth and isopach maps for the two deposits due to the lack of distal outcrops, which are difficult to observe given the tropical-weather conditions of the region. At present, the only hazard map that considers tephra fallout from the TVC is the one proposed by Mercado and Rose (1992); however, built only on photo-interpretation. This manuscript focuses on the probabilistic tephra hazard assessment of a Plinian eruptive scenario of the Tacaná volcano, defined on the basis of the Sibinal Pumice eruption (∼23.5 kyrs). The deposits of this eruption have been broadly studied and presents the most complete features of a Plinian event, required for modelling. The goal is to assess the impact that a Plinian eruption similar to the Sibinal Pumice could have on the area surrounding the volcano and on the main airports of Mexico and adjacent countries (i.e. Guatemala, Belize, Honduras). This information will serve to produce the first hazard map of the volcano and to prepare a long-term risk scenario to minimize the impact of such kind of eruption in terms of tephra deposition and dispersion of the volcanic ash.
considerable damage to the surrounding populations, like the city of Tapachula, located at ∼30 km south of the TVC, which is the most populated of the region with ∼320,000 inhabitants (INEGI, 2010). An evidence of the risk posed by an explosive activity of lesser magnitude than a Plinian eruption, was the June 2018 eruption of the Fuego volcano in Guatemala, located 155 km southeast from Tapachula, which produced pyroclastic density currents (PDCs) with the longest run-outs described in modern times for this volcano. This eruption caused 153 deaths (INACIF, 2018) and the partial destruction of three villages on the southern flank of the volcano (GVP, 2018). The harmful effects caused by the very fine ash transported by winds produced serious problems to the banana plantations around Tapachula. In the case of the TVC, its stratigraphic record shows that the development of PDCs has been recurrent in its eruptive history, along with other volcaniclastic flows, such as syn-eruptive lahars and debris avalanches (Macías et al., 2000, 2010; 2015, 2018). Yet, the most extended deposits related to the volcanic hazards of the TVC, are related to the dispersion of tephra by Plinian eruptions. Even if the main dispersal axis is not directed towards the city of Tapachula, winds can potentially transport fine ash for long distances in different directions and trigger secondary lahars due to the emplacement of pumice or fine material into the main drainages and slopes of the volcano (e.g. Coatán, Cahoacán and Suchiate rivers, among other small streams), which could bulk up the outflow and sediment load of the currents and endanger the lowlands with floods, something that has happened in ancient times (Macías et al., 2000, 2018). Despite the knowledge of this type of activity at the TVC, little has 254
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Fig. 2. Stratigraphic column of a representative outcrop of the Sibinal Pumice deposit showing the stratified (SM) and massive (MM) members separated by a lahar deposit (L) (modified from Arce et al., 2012).
2. Sibinal Pumice eruption: stratigraphy and physical characteristics
sized pumice (ca. 85 vol %), fresh light-gray lithics, hydrothermally altered lithics (totaling 5 vol %), loose crystals, and glass shards. Hydrothermal altered lithics are abundant at the base of this member (up to about 50 vol%). A maximum thickness of 255 cm was measured 13 km to the north of the TVC summit. The isopachs and isopleths show that the thickness and lithic size of the massive member is distributed to the north and northeast of the TVC (Fig. 3), with the most distal isopach (30 cm) covering an area of 330 km2. A minimum tephra volume of 1.9 km3 was estimated for the massive member (Arce et al., 2012). Based on Carey and Sparks (1986), an eruptive column height of 22 km was inferred for the massive member (Arce et al., 2012), with a peak mass discharge rate of 8.1 × 107 kg/s. The present work focuses on the massive unit, whose physical characteristics were used as a basis for the construction of the probabilistic hazard maps.
Based on several described outcrops, Arce et al. (2012) defined the distribution of the Sibinal Pumice deposit, produced by a Plinian eruption occurred ∼23.5 kyrs ago. Products were largely dispersed to the north-northeast of the TVC towards the Sibinal and Tacaná towns, the two most important settlements in the surroundings of the volcanic edifice, in Guatemalan territory (Fig. 1). The most distal Sibinal outcrop can be found at 21 km from the volcano summit; at farther distances the deposit is completely eroded because the area is subjected to intense rain during most of the year. In general, pumice fragments are highly altered, making it difficult to obtain granulometric analyses and goodquality whole-rock chemical analysis. The most proximal outcrop of the sequence is located near Vega del Volcán village, at 4 km from the summit. The absence of closer outcrops can be attributed to weathering and a thick cover of younger deposits (e.g. lava flows and pyroclastic aprons). The Sibinal Pumice sequence is composed of two members separated by a lahar deposit (Arce et al., 2012). The lower member is a stratified unit (i.e. SM unit, Fig. 2), constituted by seven lapilli-sized fall deposits, which overlies a dark-brown paleosol rich in organic material dated at 23,540 + 255/-245 yrs (Arce et al., 2012). Some layers contain a lot of accidental lithic fragments (up to 50 vol %). The total thickness of this member is 2.6 m. The upper member (MM unit, Fig. 2), is a tephra fall deposit composed of highly altered, vesicular, lapilli-
3. Hazard assessment methodology One thousand numerical simulations for the generation of hazard maps were performed using the FALL3D tephra dispersal Eulerian model (Costa et al., 2006; Folch et al., 2009), that solves a set of advection-diffusion-sedimentation (ADS) equations using a second-order Finite Difference explicit scheme. In order to solve the governing equations, the FALL3D model assumes that the main factors that control atmospheric transport and deposition of ash are: wind advection, turbulent diffusion and gravitational settling of particles. The model is able 255
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Fig. 3. Isopach and isopleth maps of the massive member of the Sibinal Pumice showing the main dispersal axis of the deposit. A) Isopach lines; B) Isopleth lines. Values of both maps are in cm and the values shown in black dots are the average of the five largest lithic clasts (modified from Arce et al., 2012).
atmospheric levels, considering different threshold values. In particular, ash fallout hazard maps considered three ground load thresholds of 1, 10 and 100 kg/m2. The first value is considered as a limit for airport closure; the second is typically considered as a threshold for impact on agriculture (Blong, 1984; Bonadonna et al., 2002); and the last value is significant for the collapse of roofs in buildings of low to medium quality. For airborne ash, two critical flight levels (FL) were taken into account: FL100 (altitude 10,000 ft, ∼3 km) that corresponds to a low level relevant to flight landing/take-off operations, and FL300 (altitude 30,000 ft, ∼9.1 km), which is relevant for the jet-cruise altitude. In this case, flight level hazard maps represent the probability for ash concentration to exceed the critical values. Two concentration threshold values were considered (2 and 0.2 mg/m3) based on the paper by Bonasia et al. (2014). Both values were adopted in 2010 during the European crisis caused by the Eyafjallajökull eruption, which caused an unprecedented closure of the European airspace (IVATF-1, 2010).
to run from local to regional scales and it is initialized with meteorological and vulcanological parameters. In this work, a computational domain spanning from 11° to 22° N and from −100° to −83° W, with a horizontal grid resolution of 0.15° (∼15 km), was considered. Time-dependent wind fields and other meteorological model inputs were extracted from sampling ERAInterim reanalysis data within a 10-year period (2008–2017), ensuring homogeneous stratified sampling along natural years (i.e. same number of runs per month). Other parameters used in the model were set as follows: - A vertical distribution of mass in the column following a Suzuki-type distribution (Suzuki, 1983) with parameters A = 4 and λ = 5. - The column height and total erupted mass were linearly sampled within a range (Table 1) consistent with the scenario described by Arce et al. (2012) for the massive member (MM) of the Sibinal Pumice deposit. Given a value for column height, the time-averaged mass eruption rate was determined using the relationship between column height and eruption rate proposed by Mastin et al. (2009). The eruption duration was fixed from mass eruption rate and sampled mass. - A Gaussian Total Grain Size Distribution (TGSD). Giving the lack of field data necessary to integrate deposit grain-size at different locations, we used the fit of Costa et al. (2016) to determine TGSD parameters from magma viscosity and plume height using the Askja 1875 phase D eruption as an analogue (both eruptions have comparable magma compositions and values of column height).
4. Results Fig. 4 shows the three ground hazard maps from the lowest to the highest threshold value (i.e. A, B and C, respectively). The general feature of these maps is the elongated distribution of the probability contours on a NE-SW direction, due to the preferential climatic trend of winds in this region. It is also notable that the extent of the area that could be prone to concentrate the different thresholds diminishes its reach with the increase in the ground load value as expected. From the three hazard maps, the map with a load of 1 kg/m2 (i.e. Fig. 4A) covers the wider area (∼250,000 km2), although with the lowest probability value (5%). Within this probability contour, not only the Mexican and Guatemalan territories would be affected, but also part of Belize; with almost all the state of Chiapas and Guatemala being blanketed by a ∼1 mm thick ash layer. The larger probability contour (80%) has a radius of ∼40 km from the volcano (∼4000 km2), which means that at least 26 main urban zones including towns like Tapachula, Motozintla, Tacaná, Sibinal, Huixtla, Malacatán, and San Marcos (i.e. more than 350,000 inhabitants), would be affected by a fine sheet of ash and would probably cause the closure of the International Airport of Tapachula. For the second ground load threshold of 10 kg/m2 (Fig. 4B), the area affected by the higher probability contour is ∼2000 km2 (i.e. ∼30 km from the volcano). Within this area the city of Tapachula and
Probabilistic hazard maps were generated from simulations varying wind data and eruptive input parameters as described below. These maps were built for ash fallout as well as for concentration at different
Table 1 Range of input parameters used for the FALL3D numerical runs. For comparison, also values of total mass and column height of the Sibinal eruption MM are reported.
Total mass (kg) Eruption duration (h) Column height (km)
Minimum
Maximum
Sibinal Pumice (MM)
2.5 × 1012 12 19
5 × 1012 24 26
4.8 × 1012 17.7 22
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contour maintains the elongated shape of previous maps (Fig. 4A and B), with NE-SW direction. Moreover, for this ground load threshold (100 kg/m2), the 80% probability contour covers an area of ∼1000 km2 in which the Tapachula airport will be affected and approximately 200 towns in both countries, including Cacahoatán, Santo Domingo, Unión Juárez, Sibinal, La Laguna and Tacaná. Fig. 5 shows the dispersal hazard maps obtained for the TVC, considering the flight levels FL100 (Fig. 5 A and B) and FL300 (Fig. 5 C and D), at the two critical ash concentrations adopted during the 2010 European aviation crisis (0.2 and 2.0 mg/m3). From these probabilistic maps, it can be observed that in both flight levels exists a very high probability (50%) that the two thresholds considered could be exceeded, and the airspace of at least 23 airports would be affected, including in addition to the International Airport of Mexico City other important national and international airports in Mexico (e.g. the airports of Acapulco, Villahermosa, and Cancún), Belize, Guatemala, El Salvador, Honduras and Nicaragua. Moreover, in almost all the cases, the highest probability contour (i.e. 90%) that either the 0.2 or the 2.0 mg/m3 threshold could be exceeded, covers a region with a radius of ∼50 km from the volcano, restricting the affected area to ∼8000 km2, an area that still includes the international airport of the city of Tapachula in Mexico, and the Coatepeque airport in Guatemala. In general, the probability contours of all the maps in Fig. 5, are elongated towards the NE-SW, which is consistent with the direction of the ash ground load hazard maps presented on Fig. 4; however, it is worth noticing that the shape of the contours for the flight level FL300 reach a bigger extension in the same direction NE-SW in both ash concentration thresholds, exceeding the considered computational domain. 5. Discussion Nowadays, numerical modelling and probability analysis have been used more often to complement hazard assessment of volcanic risk, making the analyses more reliable and comprehensive, especially in those cases where the geological record is not accessible, or is partly or completely missing. Therefore, as described by Bonadonna (2006), numerical modelling serves as a counterpart of direct observations in the field and to explore a much wider range of possible scenarios. In this work, we follow the Eruption Range Scenario method described by Bonadonna et al. (2005) and Bonadonna (2006), using the FALL3D numerical code (Costa et al., 2006; Folch et al., 2009) and considering as a basis the physical parameters of the TVC Sibinal Pumice eruption of ∼23.5 kyrs (Borjas-Hernández, 2006; Arce et al., 2012). In summary, the probabilistic tephra ground load hazard maps, considering the lowest probability contour of 5% for the 1 kg/m2 threshold, indicate that the extent of the area that could be affected (∼250,000 km2) is large enough to cover the entire state of Chiapas and most of the Guatemalan territory. As to the hazard posed by a layer of 1 mm of ash, it would result in significant damage, especially for the plantations of coffee, cacao, and banana. Historically, the volcanoes in the CAVA region and the Chiapanecan Volcanic Arc such as the El Chichón, have developed powerful Plinian eruptions dispersing ash with broad extensions just like the one presented in the hazard map of Fig. 4A (Carey and Sigurdsson, 1986; Macías et al., 2003; Bonasia et al., 2012). One example is the 1982 El Chichón eruption, occurred between March 29th and April 4th (GMT time), that generated three fallouts, pyroclastic density currents, and debris flows, blanketing an area of more than 50,000 km2 with tephra. The tephra fallout deposits of this eruption, had a major dispersion axis towards the northeast and east. Another example is the Santa María volcano eruption occurred in October 24th, 1902, which lasted 18–20 h, and produced a column of at least 28 km high (Williams and Self, 1983). Sapper (1905) calculated the 1 mm isopach of the deposit, which enclosed an area of 273,000 km2 covering the southern part of Mexico (up to Acapulco) and destroying most of Guatemala's coffee industry, as reported by the New
Fig. 4. Probability hazard maps for different ash ground load thresholds at the TVC: A) 1 kg/m2; B) 10 kg/m2; and C) 100 kg/m2. Contour colour denotes percent probability of exceeding the given threshold values. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
its airport could still be affected by a 1 cm ash fallout. Considering instead the 5% probability contour of Fig. 4B, the affected area would reach ∼36,500 km2, involving the main zone of coffee plantations in the Soconusco region, and at least four main airports: Tapachula (in the Mexican side); Huehuetenango, Coatepeque and Quetzaltenango (in Guatemala). Finally, on Fig. 4C, the 5% probability that the ash loading would overcome a threshold of 100 kg/m2 involves an area of ∼10,500 km2 between the localities of Puerto San Benito and Huehuetenango (a radius of ∼60 km from the volcano); this probability 257
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Fig. 5. Probabilistic hazard maps for two flight levels, considering two critical ash concentrations: A) FL100 with a threshold of 0.2 mg/m3; B) FL100 for 2.0 mg/m3; C) FL300 considering the threshold of 0.2 mg/m3; and D) FL300 for the 2.0 mg/m3 threshold. Contour colour denotes percent probability of exceeding the given threshold values. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
considered in our simulations, could have affectations of a similar scale as those produced by the Puyehue-Cordón Caulle eruption in 2011. This not only in regard to the ground and air transportation networks, but also to the affectation of agriculture, livestock, power lines, water distribution systems, human and animal health, farming and tourism sectors, to name the most important. These consequences have also been observed in other volcanic crises such as the 1982 of the El Chichón in Mexico (De la Cruz-Reyna and Martin Del Pozzo, 2009; Scholamacchia and Capra, 2015); the 1991 eruption of Mount Pinatubo in the Philippines (Newhall and Punongbayan, 1996); the 2008 and 2011 eruptions of Chaitén and Cordón Caulle, in Chile (Lara, 2009; Collini et al., 2013), among others. Another hazard related to the tephra fall dispersion presented in this work is the development of lahars due to the entrainment of volcanic ash into the main rivers that drains the surroundings of the complex, as happened before in the eruptive history of the TVC (Murcia, 2008; Murcia and Macías, 2009, 2014). Past evidences of syn-eruptive lahars originated during Plinian activity of the TVC is evidenced, for example, by the lahar deposit that separates the SM and MM members of the Sibinal Pumice sequence (Arce et al., 2012), and other thick deposits of ancient lahars located in the surroundings of the volcano (Murcia, 2008; Murcia and Macías, 2009, 2014). Globally, the best example of this type of hazard occurred during and after the 1991 Plinian eruption of Mount Pinatubo (Newhall and Punongbayan, 1996), whose impact continues until today. Therefore, the formation of syn- or post-eruptive lahars at the TVC should not be disregarded, considering that the total extent of the area that could be affected by the tephra fallout, as shown by the hazard maps of this work, would be broad, covering southern Mexico and Guatemala.
York Times (1902a, b). The 1902 ash was dispersed to the northwest of Santa María volcano across Guatemala into Mexico, reaching Mexico City (Williams and Self, 1983) and even the city of Colima located 1400 km northwest of the source (Sapper, 1905). In the case of the TVC, our probabilistic hazard maps have a NE-SW preferential dispersion direction of probability curves, reaching as far as Belize and Guatemala City with the 5% probability (Fig. 4A). This preferential dispersal orientation towards Guatemalan territory has been evidenced by previous eruptions of the TVC, occurred ∼23.5k yrs and ∼14k yrs ago (Arce et al., 2008, 2012). Therefore, the shape of the probability contours presented on the maps of Fig. 4 are in good agreement with the ancient deposits suggesting that the extent of the damage of a Plinian eruption of the complex could be comparable to the 1902 Santa María eruption. As a matter of fact, a similar crisis happened on June 2011, when the complex Puyehue-Cordón Caulle, located in the southern volcanic zone of the Andes in Chile erupted. This event started with a sub-Plinian eruption that lasted 27 h followed by a phase of weak columns that lasted 8 months (Bertin et al., 2013; Collini et al., 2013; Schipper et al., 2013). The finest granulometric fraction of the ash clouds generated during the first eruptive stage circumvented the southern hemisphere, passing over South Africa, south of Australia and New Zealand in about 10 days, causing a major air traffic disruption in Argentina (Collini et al., 2013; Schipper et al., 2013). In the case of the TVC Fig. 5 evidences that the 50% probability that any of the ash concentration thresholds considered to be exceeded at the different flight levels, would reach even a bigger region than the extent of the 1902 Santa María ash fallout, but less than the global transportation of ash of the Cordón Caulle event; moreover the maps on Fig. 5 show that not only the airspace of Mexico and Guatemala would be affected, but also the airspace of Belize, El Salvador, Honduras and Nicaragua. It is worth noticing that the lesser probability contours exceed the computational domain here considered, but the extension of the aerial disturbance produced by a Plinian scenario at the TVC, as the one
6. Conclusions In this work, we present the first comprehensive appraisal of tephra 258
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fallout and ash dispersion of the TVC by using a probabilistic approach. From our maps, it is easy to observe that in case of a major explosive eruption of the complex, the extent of the disturbance for this type of hazard at ground level would affect mainly the Guatemalan territory and the state of Chiapas in Mexico, with a significant damage to the harvests and agricultural sector of the region, which economically depends on the cacao, coffee and banana plantations. Another consequence, would be the generation of lahars, which could pose a major threat to populations settled along rivers. Probability ash dispersal maps show that the level of affectation to the air traffic would be even broader, involving the possible closure of at least 23 airports in Mexico, Guatemala, Belize, Honduras, El Salvador and Nicaragua, provoking a similar crisis to the one of 2010 in Europe due to the Eyjafallajökul volcano in Iceland, or the 2011 crisis in the Southern Hemisphere due to the Cordón Caulle eruption in Chile. The probabilistic hazard maps presented here would serve to the Mexican and Guatemalan authorities to upgrade their emergency plans in case of a renewed activity of the complex, as well to impulse the monitoring of the volcano and probably the creation of an aerotransport emergency guidance for the regional airports. This guidance will let them be prepared in case of a contingency of this type, not only in the case of a volcanic crisis of the TVC, but also the surrounding volcanoes at the CAVA.
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