Hazard scenarios associated with the El Chichón volcano crater lake increasing area fluctuations

In the week from 28 March to 4 April 1982, El Chichón volcano, located in south-eastern Mexico, produced the largest and deadliest eruptive episode in the modern history of that country. About a month later, a lake formed on the floor of the 1 km wide, 200 m deep crater carved on its summit by the intense explosions. Since then, irregular, yet persistent monitoring of some geophysical and geochemical parameters has unveiled different types of interaction between the lake and the underlying hydrothermal and magmatic systems. Identifying the causes of the area's non-seasonal large variations in the observed hydrogeochemical results is a critical problem for assessing volcanic hazards. A running correlation analysis of the hydrogeochemical data and the lake size variations suggested that a change in permeability of the interface between the lake and the underlying systems, probably related to the stabilization of the young lacustrine system and the decreasing magmatic influence, produced the first significant change in 1983. Other causes are proposed for the following changes, mainly related to the increasing influence of two underlying hydrothermal systems, probably fed by different aquifers. The degree of influence appears to be increasingly controlled by the seismicity around the volcano. For example, during the period 1990-2006, one M 4.0 earthquake was recorded in that area on October 9, 2002, when lake-area fluctuations began to increase. The recent mounting crater lake area variations and the increasing seismicity recorded from June to August 2025, suggest a growing degree of interaction between the lake water and the hydrothermal systems, probably through the stress and displacement changes in two fractures crossing the volcanic edifice, namely the Chichón-Catedral (NW-SE) and San Juan (E-W) faults. This could lead to an increased probability of phreatic explosions, which may be followed by lava dome emplacement on the crater floor. The coordinated management of volcanic risk between the Civil Protection System and the Advising Scientific Committee has designed a specific Traffic Light Alert System, along with an operational plan to keep the surrounding population aware and protected, considering these possibilities, as well as others of higher intensity but lower probability.