A structural vulnerability assessment of seismic hazard in a sector of Guatemala City
Francisco Eduardo Arrecis Sosa and Omar G. Flores Beltetón

Introduction

Guatemala has been severely hit by earthquakes throughout history. In 1541, an earthquake destroyed the capital, located in Almolonga Valley in the department (province) of Sucatepéquez, forcing the government to move the capital to what is now known as Antigua Guatemala. It, in turn, was destroyed in 1773. Modern Guatemala City, the capital since 1775, has been hit by severe seismic events in 1830, 1917, 1942 and 1976—with the latter earthquake alone causing 23,000 deaths, 77,000 injuries and the destruction of 258,479 homes.

With a view to reducing the impact of earthquakes in Guatemala, a project has been launched to assess the structural vulnerability of the country’s cities. Students of the Faculty of Engineering of San Carlos University who are interested in disaster reduction have been authorized to conduct these assessments as their main postgraduate project in civil engineering.

Figure 1. Map of the area chosen for the pilot study

The pilot study

The first of these assessments has been carried out in an area measuring 888,450m2 in Guatemala City’s Zone 3 (see Figure 1). The sector was chosen for the pilot study because it is one of the most densely populated in the city and has remarkable geographical, geological and socioeconomic diversity. It still has a significant number of adobe houses as well as much informal housing, and was one of the areas most severely affected by the 4 February 1976 earthquake.

The methodology employed in the project starts out with a review of existing seismic-hazard and geological risk maps. Structural assessments follow the techniques outlined in the ATC-21 manual Rapid Visual Screening of Buildings for Potential Seismic Hazards, the basis for the Guatemalan Structural and Seismic Engineering Society’s Recommended Standards for Risk Reduction and Structural Rehabilitation. These Standards are used by the Executive Coordination Secretariat of the Presidency of the Republic to assess government buildings, particularly public hospitals.

Assessment results

A computer software program, Evaluación, was created to handle and process all the data compiled by the students, making it possible to estimate the potential number of deaths, injuries and economic losses that might accrue in the event of an earthquake. In order to assess the economic losses, each structure’s current use was taken into account, as well as its surface area, building cost per unit of surface area, number of residents or users, the likelihood of the building being occupied at the time of the disaster, hazards to surrounding structures or the risks they pose, and the overall vulnerability of the building. The program can carry out these calculations and produce detailed reports. In order to ensure a consistent set of data, the same software will be used in all the other studies.

For an earthquake with ground acceleration equal or greater than 0.3g, the program foresees 2,037 deaths, 1,077 injuries and economic losses totalling over 19.4 million dollars. Out of the 1,438 structures assessed, 558 were made of non-reinforced abode—38.8 percent of the total. Some 85 percent of homes appear to have been built by the owners, and 10 percent display substandard construction techniques that render them highly vulnerable to seismic activity.

Figure 2. Risk maps based on the structural vulnerability assessment

Some caveats

While several microzoning studies have been carried out, ground response to earthquakes is still not well known in the country, so the actual impact of a seismic event might differ from the assessment’s estimations. Moreover, vulnerability is constantly changing as structures are remodelled or extended, frequently without planning permission. In addition, not enough studies have been carried out in the past to establish a correlation between the damage suffered by structures and the resultant economic losses. All these factors conspire against the predictability value of the ongoing assessments. Project backers argue, however, that this is hardly a valid reason for not attempting to carry out the assessments at all, in light of the massive losses that the next major earthquake will likely cause if no attempts are made to assess the level of risk and take appropriate preventive measures.

The future of the project

The project is already being implemented in Quetzaltenango, the second largest city in Guatemala, with the help of engineering students at San Carlos University’s Western Campus. However, greater participation will be needed, not only by other students but also by disaster reduction agencies, municipalities, health units and the general population—particularly if this initiative is to go beyond vulnerability assessment and lead to the development of effective mitigation plans.

Francisco Eduardo Arrecis Sosa and Omar G. Flores Beltetón teach
at the Faculty of Engineering of San Carlos University, Guatemala

For more information please contact:
Ing. Francisco Arrecis Sosa
Facultad de Ingeniería
Universidad de San Carlos de Guatemala
Edificio T-1, 3er. Nivel, oficina L-III-2
Ciudad Universitaria, zona 12.
fasbox@hotmail.com

Ing. Omar G. Flores B.
Facultad de Ingeniería
Universidad de San Carlos de Guatemala
Edificio T-1, 3er. Nivel, oficina L-III-2
Ciudad Universitaria, zona 12.
Telefax: (502) 476-0423
usacesem@usac.edu.gt