Volcanic hazard and public response: The Popocatepetl Volcano, Mexico, and the communities: A case study of early prevention

Alejandro Rivera Dominguez
Puebla, Mexico.
E-mail. ccl2@siu.cen.buap.mx

Since December 21, 1997, the andesitic giant volcano, Popocatepetl, is in an eruptive state. The volcano is situated between Mexico city and Puebla city, 70 and 44 km away from the active crater, respectively. Both cities had been affected by strong ash emissions, but the main problem is of a socio-cultural nature. The region consists of an ancient set of Indian settlements with very poor economy and a low educational profile. Some towns are as close as 11 km away from the crater and have experienced ash rain at least 46 times. The topography is difficult and communication is poor. This situation requires a special approach.

At the University Centre for Disaster Prevention of Puebla University, seismologists work together with volcanologists, anthropologists, sociologists and mathematicians. This multidisciplinary work aims at providing constant information for the population using broadcasts made on commercial systems and local TV channels. The only solution for the evacuation of 217 towns with around 300,000 people is an attitude change within this culture to a new awareness of the nature of the Popocatepetl volcano, traditionally a sacred symbol for the inhabitants of central Mexico. We now observe a slow change in perception as a result of the joint work of inhabitants and research workers resulting in creative interaction between the traditional, magic-based, cultural perception and a more rational scientific approach. This is a good example of collaboration between civil authorities, researchers and the public.

Since 1994, the Popocatepetl volcano shows some typical seismic precursors such as A Type volcanic tremors, sometimes lasting for 2-3 hours, due to fluid movement into the volcano structure. In addition, AB seismic signals have been observed which relate to mixed types of tremors due to rock fracturing. However, volcanic tremors are naturally stochastic and not a determinant condition for pending ash emission. The formation of a small lava domus from the crater bottom has been observed since 29 April, 1996. The domus releases gas and steam as an indication of internal volcanic pressure increases. A total of 24 shock waves reached Puebla City, and small earthquakes of a very high frequency were felt in the city buildings. Unfortunately, with only a few minutes in advance, it becomes impossible to advise the public of impending ash cloud emission. Therefore, when ash plume danger is present then the advisory service (via radio and fax) starts immediately. In less than 10 minutes, radio broadcasts begin to transmit precise instructions and an explanation of the phenomena. A joint scientific and government committee convenes immediately and evaluates the real risks taking topographic, demographic and meteorological data into account. The constant information flow alleviates the social information problem, but in far-off towns, the radio broadcast is not received. In these cases (24 towns), a telephone-satellite system is operated by the government and universities. In the towns, the authorities use church bells to alarm the population who have already been provided with instructions on evacuation which also take into account a pre-Hispanic volcanic map and evacuation routes. In those towns very near to the crater (Xalitzintla Town 11 km, San Pedro 10 km) where the problem of mud flow (Lahar) demands special care, the people must climb a slope to the north and wait for new instructions. The Army also participates with emergency equipment, weight transports, jeeps and general assistance in an evacuation, but under Mexican law, natural phenomena and its effect on the population is considered to be a civil problem. The Army is only called in for total emergencies. In general, the public perception has changed with reiterative information, but the low educational profile makes it very hard to educate and instruct people.

Currently the National Centre for Disaster Prevention in Mexico City and the University Centre for Disaster Prevention, are operating 11 short and long period triaxial seismometers, two inclinometers, COSPEC determination of SO2 contents, one direct TV channel, satellite watching and frequent helicopter fly-overs. In future, two magnetometers will be situated on the east slope. All telemetric data acquisition systems are situated in Mexico City.