Preparing The Field To Avoid A Volcanic Disaster By Means Of Numerical Modeling At Tacaná Volcano

Volcanoes are often seen as agents of destruction and catastrophes when they present major eruptive activity such as continuous lava flows, sustained vertical eruptive columns of ash and gases, mudflows in the form of lahars, and pyroclastic density currents (i.e. pyroclastic flows, PFs), which are a super-hot mix of ash, rocks, and gas (>300°C) that travels at ground level at speeds of more than 100 km/h, burning and destroying everything on its way down the volcano.

However, volcanologists have managed to assess these type of phenomena by constructing volcanic hazard maps based on a) the eruptive history of the volcano (deterministic approach); b) using a probabilistic approach (statistical); or c) a combination of both. Yet, on those volcanoes that have remained for decades with little or no visible activity, or where the deposits from ancient eruptions have not been well preserved, numerical modeling becomes a handy tool in the process of volcanic hazard assessment. But what is a numerical model and how can it be used to mimic a natural phenomenon such as those developed in volcanoes?

Pyroclastic flows descend the south-eastern flank of Mayon Volcano, Philippines. Maximum height of the eruption column was 15 km above sea level, and volcanic ash fell within about 50 km toward the west. There were no casualties from the 1984 eruption because more than 73,000 people evacuated the danger zones as recommended by scientists of the Philippine Institute of Volcanology and Seismology. Image credit: C.G. Newhall/USGS, licensed under CC0

In plain words, a model will always be an abstraction of the real world in the form a conceptual drawing, a mathematical formula, an experiment, or, in a more complex way, a computer program. However, a computer program needs all these representations and all the available information from the real world to function properly. Hence, in geology, a numerical model uses mathematical equations to describe the physical conditions related to a certain phenomenon as well as other input parameters such as the topography of the terrain, or even other models which are put together on a computational program for the simulation of geological scenarios like volcanic eruptions and their related hazards.

This was the case of the analysis that we performed for the Tacaná Volcanic Complex (TVC), the southernmost active volcano in Mexico (better known as Tacaná Volcano), which is also shared by Guatemala since the international border between the two countries crosses the complex from NW to SE. For this volcano, we used the Titan2D open code (Patra et al., 2005) developed by the Geophysical Mass Flow Group of Buffalo University, N.Y., USA, for the simulation of geological mass flows over natural terrain. In particular, we focused on the simulation of PFs at the TVC in order to evaluate the development of future flows (using those generated over the past 40,000 years in the history of the volcano), in case it resumes its explosive activity. This type of event just happened in June last year at Fuego volcano in Guatemala, located ~100 km southeast from the TVC. This event produced a PF that traveled more than 13 km southwards of the volcano and buried several villages, a disaster that renewed our interest to assess these types of perils at the TVC.

The TVC is not just one volcano, but a complex system of four volcanic structures knit together and oriented NE-SW, from which at least 8 PFs have been developed in the past. The largest one, originating approximately 2000 years ago during the last explosive episode of the complex, reached a distance of ~15 km south from the volcano summit — an even a larger distance than the PF developed last year at the Fuego volcano!

Therefore, using the numerical code of Titan2D, we simulated different eruptive scenarios at the TVC, considering the possible development of events ranging from small-size PFs up to huge-magnitude events (like the one that originated 2000 years ago). The results obtained with Titan2D depict the trajectories downslope the volcano that the PFs would follow, along with the maximum velocities, distances, and depths reached for the flow deposits.

Map showing a summary of the flow paths of simulated PFs of a volume of 0.5×106 m3, considering eight source points for the flows around the summit of the TVC (Image by Rosario Vázquez).

With those results, and after a meticulous and time-consuming analysis, the next step consisted of putting all the information on a map, which showed the zones that could be affected for PFs of small, medium, and large events (i.e. low, medium and high impact). The final hazard map proposed by our work, published recently in the Journal of Volcanology and Geothermal Research, showed that in the case of a renewal of the explosive activity of the TVC (in any of the main volcanic structures that form it), the flows developed would be mostly directed toward Mexican territory (which is also the most populated in this region), affecting between 2000 and 12,000 people in a radius of ~10 km. In that map, we also showed that other types of volcanic hazards could be promoted contemporaneously with the formation of the PFs; that is, the formation of lahars, which are a mix of water and volcanic debris that form mudflows moving rapidly downslope the main drainages of the volcano.

In the case of the TVC, there are two main rivers that drain it: the Coatán and Suchiate rivers, along with other small streams that could easily transport the volcanic material deposited by the PFs downstream to the lowlands, leading to the flooding and affectation of an even larger zone. This phenomenon also occurred during the 2018 eruption of the Fuego volcano in Guatemala because the deposits left by the PFs of June began to be eroded by the heavy rainfalls that occurred a few days after the eruption, developing hot lahars that led to an even larger disaster.

Therefore, the construction of these types of maps serves as an important tool to prevent a major volcanic disaster, because if the Civil Protection authorities knew the trajectories that future PFs would take, they could design proper evacuation routes or update  emergency plans in conjunction with the local governments (in Mexico and Guatemala) in order to be prepared for a future volcanic crisis of the Tacaná volcano.

Reference:

1. Patra, A.K., Bauer, A.C., Nichita, C.C., Pitman, E.B., Sheridan, M.F., Bursik, M.I., Rupp, B., Webber, A., Stinton, A.J., Namikawa, L., Renschler, C., 2005. Parallel adaptive numerical simulation of dry avalanches over natural terrain. J. Volcanol. Geotherm. Res. 139, 1-21.