Scientific and public responses to the ongoing volcanic crisis at Popocatépetl Volcano, Mexico: Importance of an effective hazards-warning system

Abstract

Volcanic eruptions and other potentially hazardous natural phenomena occur independently of any human actions. However, such phenomena can cause disasters when a society fails to foresee the hazardous manifestations and adopt adequate measures to reduce its vulnerability. One of the causes of such a failure is the lack of a consistent perception of the changing hazards posed by an ongoing eruption, i.e., with members of the scientific community, the Civil Protection authorities and the general public having diverging notions about what is occurring and what may happen. The problem of attaining a perception of risk as uniform as possible in a population measured in millions during an evolving eruption requires searching for communication tools that can describe—as simply as possible—the relations between the level of threat posed by the volcano, and the level of response of the authorities and the public. The hazards-warning system adopted at Popocatépetl Volcano, called the Volcanic Traffic Light Alert System (VTLAS), is a basic communications protocol that translates volcano threat into seven levels of preparedness for the emergency-management authorities, but only three levels of alert for the public (color coded green–yellow–red). The changing status of the volcano threat is represented as the most likely scenarios according to the opinions of an official scientific committee analyzing all available data. The implementation of the VTLAS was intended to reduce the possibility of ambiguous interpretations of intermediate levels by the endangered population. Although the VTLAS is imperfect and has not solved all problems involved in mass communication and decision-making during a volcanic crisis, it marks a significant advance in the management of volcanic crises in Mexico.

Introduction

Popocatépetl Volcano is located in the central Mexican Volcanic Belt (Fig. 1) within a densely populated region, with over 20 million people vulnerable to direct hazards associated with a major explosive eruption. Situated about 70 km southeast of downtown Mexico City, Popocatépetl is arguably the most dangerous volcano in the country. This 5454-m-high volcano's geologic past clearly indicates that it is capable of producing catastrophic eruptions: three Plinian events have occurred within the past 5000 years B.P., well within the period of human settlement in central Mexico (Siebe et al., 1996, Siebe and Macías, 2004). Fortunately, to date the current eruptive episode—beginning in December 1994 after being dormant for nearly six decades—has consisted of relatively minor activity, which has characterized Popocatépetl's activity since the 14th century (De la Cruz-Reyna et al., 1995). Nonetheless, given the huge population potentially at risk, together with concerns about possible escalation of eruptive activity, the management of the ongoing “volcanic crisis” at Popocatépetl (CENAPRED-UNAM, 1995) has posed, and continues to pose, a major challenge for volcanologists, national and local civil authorities, and the affected public.

The effective management of a volcanic crisis usually involves several integral components, which in most cases, may be aggregated into three main elements (De la Cruz-Reyna et al., 2000):

• Identification of the areas threatened by a given volcano, together with the definition of the probabilities that specific hazardous volcanic phenomena may occur in a given interval of time. This generates a static view of the potential hazards posed by the volcano showing unrest, most commonly represented as a hazards-zonation map.

• Geophysical, geochemical, and remote-sensing monitoring of the restless volcano—in real time to the extent possible—to document its changes in state and to assess the level of associated potential hazards. To be useful, scientific information on the volcano must be interpreted and translated in terms of hazard scenarios, including the possibility of the escalating unrest culminating in eruption, nature and size of the anticipated eruption, the extent of hazardous processes, etc. The data from volcano monitoring provide the only scientific basis for making a dynamic estimate of the probability of occurrence of specific scenarios in the short term. The reliability and usefulness of such scenarios critically depend on the quantity and quality of the monitoring data, and on the ability of the members of the scientific teams to exchange opinions, compromise ideas, and reach a consensus.

• Development and implementation of a hazards-warning system and response scheme that allow the civil authorities and vulnerable population to adopt mitigation measures according to pre-established levels of risk. An effective communication and warning system should be able to generate a similar level of awareness and perception of the changing risk among the scientific team responsible for assessing possible outcomes of the volcanic activity, the decision-making authorities, and the threatened population.

The first two elements have been extensively addressed in several recent summary works (e.g., McGuire et al., 1995, Scarpa and Tilling, 1996, Sigurdsson et al., 2000; and references cited therein); these two elements as applied specifically to Popocatépetl are the topics of most other papers in this Special Issue. Here, we focus primarily on the challenges involved in the design of a hazards-warning and response scheme that may at least perform satisfactorily in a crisis situation in which the potential risk of a major eruption could affect one of the largest concentrations of population in the world.

Popocatépetl is one of the ten most populous active volcanoes in the world and the only one of the ten in the Americas; the other nine are all located in Indonesia or Japan (Small and Naumann, 2001, Table 1). Analysis of the “Mapa de Planeación de Emergencias para el Volcán Popocatépetl” (“Map for Popocatépetl volcano emergency management of the National System of Civil Protection of Mexico”) shown in Fig. 2, as well as other sources (e.g., Macías et al., 1995a, Macías et al., 1995b, Siebe et al., 1996, Sheridan et al., 2001, Siebe and Macías, 2004), shows that the densely populated areas northeast of the volcano, including México City, the capital of the country (∼ 17,000,000 population), may be significantly affected by ashfalls. Other areas may be ashfall-prone as well. Popocatépetl is only about 45 km west of the city of Puebla, capital of the state of Puebla (∼ 1,350,000 population), which may be affected by ashfall hazard to even a greater extent under particular wind conditions.

The state of Puebla has a total population of about 5.1 million, and about 4% of that population live around the volcano and are at risk from direct effects of a major eruption. To the south of the volcano, in the state of Morelos, a similar proportion (4–5%) of its 1.56 million may also be vulnerable to direct impacts of a major eruption. In addition, Morelos state contains two large cities that may be affected by ashfall (Cuernavaca, ∼ 280,000 population and Cuautla ∼ 111,000 population); parts of the latter city lie within the paths of debris avalanches and flows (Siebe et al., 1995a). To the west of the volcano, about 1.5 to 2% of the densely populated State of Mexico (∼ 13.1 million) may also be vulnerable to direct effects of a major eruption.

In summary, ashfall hazards from a major eruption (VEI ≥ 5) may affect a population well over 20 million, and more proximal hazards (e.g., pyroclastic and debris flows) may threaten about 0.5 million. The effects of a large eruption (VEI ∼ 4–5) may be very roughly estimated to be on the order of one half of those figures, and a moderate eruption (VEI ∼ 3–4) may reduce the figures for people at risk by roughly the same proportion: about 5 million from ashfall hazards, and ∼ 0.1–0.2 million from flowage hazards. Popocatépetl's summit crater is now partially filled by post-1994 lava-dome materials, thus posing “…a new threat to populations settled in the orange zone (intermediate hazard level) because future explosions will not be contained by the crater walls” (Macías and Siebe, 2005, p. 327).