The sinkhole in San Antonio: A misunderstood early warning
This article presents an initial analysis of some of the precursors to the sinkhole that took place in the San Antonio neighborhood, located in Guatemala City. The cave-in led to the collapse of several homes and caused three deaths. This article was written in the context of Priority 2 of the Hyogo Framework for Action, which underscores the need to “identify, assess, and monitor disaster risks, and enhance early warning.” Specifically, this article focuses on the warning signs identified by the affected population and potential ways to systematize them and determine whether they can be incorporated into the country’s early warning system.
On the night of February 22, 2007, part of Guatemala City’s San Antonio neighborhood experienced a cave-in caused by an underground collapse. This event surprised not only the residents, but also the authorities from institutions such as the municipality, the Municipal Water Company (EMPAGUA), the National Coordinating Office for Disaster Reduction (CONRED) and the Institute of Seismology, Volcanology, Meteorology, and Hydrology (INSIVUMEH), among others. Weeks before the event, residents began to feel tremors and hear noises caused by the gradual process of soil collapsing under the surface. Later they noticed the unusual presence of insects and foul odors. According to accounts by people interviewed at a temporary emergency shelter set up by the National Police to house the hundreds of people evacuated from the surrounding area, it is likely that one day prior to the event, all insects in the area of the sinkhole had already evacuated to safer areas. Not surprisingly, dogs and other household pets also seemed to sense the severity of the situation, becoming more restless and even aggressive.
Both the noises and tremors can be attributed to chunks of earth falling apart as layers of soil gradually collapsed. However, the mere presence of tremors and noise does not necessarily indicate the immanent appearance of a sinkhole. Guatemala is situated on a graben, or tectonic wedge bordered by local fault lines that also result in local seismic activity.
Another sign identified by the neighbors of San Antonio was the unusually high presence of insects in their homes, as well as the agitated or aggressive behavior of their dogs and other household pets. Evacuated residents reported witnessing these phenomena to a greater degree in the weeks before the disaster, but did not notice their presence in the day or so before the event. The insects seen by the residents included cockroaches, ants, and earth worms. Table 2 shows the survey results with regards to the presence of insects and the behavior of household pets.
One factor that could explain the migration of insects and other animals that normally live underground is the appearance of foul odors emanating from the sewage systems. During the survey, housewives recalled odors so pungent in their homes that their spouses suggested a number of remedies to eliminate or minimize them. To complement this section of the survey, the researchers included questions related to water systems. Table 3 contains the survey results.
As seen in the table above, the foul odor appeared weeks before the event and became more intense with its approach. However, there were no variations reported in the quality or the pressure of the water. This is explained by the fact that the water flows through a system of sealed pipes and was, therefore, not affected. In addition, some people reported experiencing feelings of insecurity, anxiety, or concern, and a few reported having dreams they were unable to interpret.
Piecing together the comments expressed by the evacuated residents interviewed in the temporary emergency shelter, by the authorities and the media, we can conclude that the process of collapse began months before the cave-in took place and could have been caused by a number of factors. Nevertheless, two things stand out. Firstly, the unpleasant odors from the sewage system and their effect on insects and other animals living underground. Secondly, the gradual collapse of the dome-shaped inner cavity, which increased as landslides caused chunks of earth and rock to fall deep into the sinkhole, resulting in the tremors and noises reported by the area’s residents.
The foul odors likely began to seep into homes when the growing sinkhole was only a few meters from reaching the surface. Zero hour arrived, the surface collapsed, and several homes were devoured, taking the lives of three people.
A misunderstood early warning
In the field of early warnings for disasters arising from natural phenomena, the Platform for the Promotion of Early Warning of the United Nations, secretariat of the International Strategy for Disaster Reduction (PPEW-UN/ISDR), has indicated that early warnings and information must be provided using language that can be understood, above all, by those people who might be affected or are at risk. It is now clear that nature did in fact provide warning signs, but that they were not understood by specialists from the relevant institutions or by the residents of the affected area. Unfortunately, the evacuation order came too late for the victims. There was no previous systematized experience that would have made it possible to recognize the precursors to this event and thus allow the Municipality and CONRED to identify that the San Antonio neighborhood needed to be evacuated.
The appearance of such “environmental” precursors is not new. Ethnic groups in some remote regions still recognize and make use of these signs, but they are rarely taken into consideration in modern cities. On the Indonesian island of Simeleu, a tribal chief remembered the advice of his ancestors and recognized the warning signs of the tsunami that took place on December 24, 2004. He ordered his tribe to abandon the coastal region and flee inland before the tsunami struck. In Guatemala, the author of this article, along with Mr. Ramiro Batzin of the Sotzil Foundation, conducted research with the support of the International Labor Organization (ILO) to identify precursors associated with a number of hazards. As pointed out by Mr. Batzin, indigenous rural communities located near volcanoes or in areas prone to seismic activity have recognized the unusual presence of coyotes before volcanic eruptions and earthquakes in areas where such events take place.
Other precursors that were mentioned include: the flight of the azuacan bird in a certain direction indicating that rains are about to start or stop; unusual noon-time behavior of hens before a disaster; changes in lunar patterns associated with drought or high winds; and other similar signs that can be used as precursors to a variety of events if they are adequately systematized. Unfortunately, in many cases this knowledge is not taken into account.
Table 4 below presents some precursors that have been observed by numerous ethnic groups with respect to a variety of hazards in Guatemala. Given the crucial role of precursors in any early warning system, they must first be identified and systematized so that they can be utilized. Systematization consists of compiling historical data on the magnitude and location of the events and their correlation to precursors, in order to determine aspects such as how much time elapsed between the precursor and the actual event, the dimension or magnitude of the precursor in relation to the magnitude or intensity of the event itself, and whether or not the precursor was present in all cases of reported events. Similarly, information is compiled on the ways that the precursors were detected. The graphic on the next page includes some questions needed to complete the systematization of observed precursors. Once the precursors are identified, we must develop techniques for measuring them. In the case of the sinkhole in San Antonio, a number of precursors were observed. Limiting ourselves to only one precursor does not allow us to determine what kind of event is going to occur, its magnitude, or when it will happen.
Experience suggests that these events can be predicted with greater precision if we utilize a variety of different signs simultaneously. Tsunamis are a good example. Tremors or earthquakes alone do not necessarily indicate the occurrence of a tsunami. It is necessary to complement the observation of seismic activity with other precursors associated with changes in sea level or pressure at the ocean floor in order to confirm the occurrence of a tsunami.
Recognizing the potential of these precursors, it is imperative that technical and scientific institutions collaborate with national civil protection agencies to identify and systematize them in order to determine their relevance for early warning systems.
According to Dr. Enrique Molina, head of Geophysics at INSIVUMEH, the next step will be to identify the physical or chemical signals used by insects and animals to perceive the warning signs, and then design instruments to measure and replicate that process. As mentioned by the slogan of the Third International Conference on Early Warning that took place in Bonn, Germany in March 2003, it is time to move from concept to action.
Juan Carlos Villagrán de León
Center for Natural Disaster Research and Mitigation/Centro de Investigación y Mitigación de Desastres Naturales (CIMDEN)
Link to the entire article in Spanish published in Guatemala’s newspapers elPeriódico: