Ceboruco (2280 m.a.s.l.), in the western Trans-Mexican Volcanic Belt, is considered among the most hazardous volcanoes in Mexico. Some 55,000 people and important infrastructure (e.g. hydroelectric dams; highways; railways) lie within the area covered by deposits of Holocene eruptions. A diverse activity over the past 1000 years spans from effusive (e.g. andesite lava flows, dacitic domes) to explosive (e.g. Strombolian, Vulcanian and Plinian eruptions). With a poor monitoring network, a first hazard map was published in 2019. Here we present the first probabilistic hazard maps for Ceboruco, which constitute a progression towards a more quantitative hazard assessment. We conduct a probabilistic hazard assessment using the pyBET_VH (Bayesian Event Tree for Volcanic Hazard) tool (i.e. software implementation of the event-tree scheme), which allows the user to estimate and visualize the probabilities and uncertainties associated with volcanic phenomena. pyBET_VH merges information from eruptive history, expert elicitation, and the output of other computer models to produce probabilistic hazard maps (i.e. absolute and conditional probabilities and associated uncertainties). We present the probability hazard maps for each eruptive scenario (i.e. Scenario 1 – small magnitude effusive eruption; Scenario 2 – medium magnitude effusive and/or explosive eruption (VEI<3), and Scenario 3 – large magnitude Plinian eruption) and the associated uncertainties. Such maps can be used by civil authorities and stakeholders for the purpose of crisis management as well as for long-term development strategies by visualizing the probabilities of areas around the volcano likely to be impacted by volcanic phenomena. Using pyBET_VH has advantages and disadvantages: the reliability of the output maps is directly related to the quality of the input data, but the tool allows easy estimation and visualization of the uncertainties; being an interactive tool, the user can continuously update the probability maps as new information becomes available.

Tephra fallout hazard assessment relies on accurate reconstruction of eruption source parameters (ESPs) from tephra deposits. Models of tephra transport and sedimentation from a volcanic plume use ESPs (e.g. erupted mass, column height, mass eruption rate, total grain size distribution) that characterize the processes and the properties of the plume, particles and the atmosphere. We use Tephra2, an Eulerian model of tephra dispersion that simplifies atmospheric dynamics to reconstruct ESPs from mapped deposits. Tephra2 works well in reconstructing ESPs for some deposits, however it does not account for the geometry (i.e. shape) of umbrella clouds of large explosive eruptions. Since the accumulation of particles on the ground is calculated with respect to their release point in the atmosphere, we hypothesize that a modification of Tephra2 that accounts for umbrella clouds would better explain the deposit variations observed in the field associated with some large eruptions. We developed a Python version of Tephra2 that uses the advection – diffusion equation to calculate the mass accumulation of tephra released from an umbrella cloud. We tested three different geometries (i.e. point, vertical line and horizontal disk) against field data from the deposit of the 2450 BP Pululagua (Ecuador) eruption that occurred in absence of wind. Our preliminary results indicate three important aspects of tephra modeling: i) a disk geometry characterizing an umbrella cloud fits the data better than the line and point sources, the last two being highly sensitive to the atmospheric diffusion coefficient; ii) a disk geometry is sensitive to the volume of tephra and the radius of the disk and, iii) different discretization of disk geometries show little sensitivity in deposit geometry with change in the release height, suggesting that disk radius is a more sensitive parameter in modeling large umbrella clouds than the release point or release height. Since large explosive eruptions are characterized by large laterally spreading umbrella clouds even when advected by wind and the umbrella diameter is controlled by eruption rate, as is plume height in vertical plumes,, we suggest the modeling of large deposits with alternative models of the cloud geometries is an important step in analysis of ESPs associated with mapped deposits.