CASE STUDY - Part A

Reducing disaster risk in Mexico City "vecindades"

Physical vulnerability assessment: identifying buildings most at risk

Background

After a major earthquake in 1985 caused over 7,000 deaths and caused extensive damage to the center of Mexico City, the Mexican authorities instigated a major program of risk reduction measures to protect the city against a recurrence of future disasters. A project funded by United Nations Development Program and executed by United Nations Centre for Human Settlements (Habitat) ran for three years, concentrating on measures for the revitalization and protection of the historic city center.¹⁸ The project attracted bilateral and multi-lateral technical assistance from USA, Japan, UK, Yugoslavia and Italy, providing consultants, equipment and material assistance.

Mitigation measures

The Mexican government (Department of the Federal District) instigated a number of disaster protection measures, which have been quite widely reported.¹⁹ These included re-zoning, proposals for decentralization and reductions in allowable densities; a revision of building codes to enforce higher standards of design against earthquake forces; a program of renovation, strengthening and reuse of historical buildings; a large-scale program of reinforcement of several hundred important buildings; and a major housing program to upgrade poor-quality and vulnerable housing in the city center.

Hazard analysis

Mexico City has suffered from flooding, due to poor drainage and seasonal rains, and a number of industrial accidents, such as gas explosions in the city center, but the major risk to the city comes from earthquakes. An earthquake the size of the one that damaged the city in 1985 could be expected roughly every eleven years. The worst threat comes from a seismic fault system on the coast (The Guerrero Gap) where seismologists fear a major earthquake is imminent.

Identifying buildings most at risk

The program instigated by the city to upgrade public housing was particularly important because of the characteristics of the damage in the 1985 earthquake. The earthquake selectively damaged taller buildings (due to the characteristics of the vibration), and the collapse of weaker structures with large numbers of occupants was responsible for the high death toll. Low-income, rented housing in the center of Mexico City - the vecindades - were thought to be particularly vulnerable to another earthquake in the future. Part of the mitigation project was to establish policies for reducing risk in the vecindades.²⁰ The first part of the project involved identifying the vecindades most at risk - i.e. those most likely to contribute to the losses in a future earthquake. Buildings were surveyed to find the characteristics of the building in the historic center and their occupancy rates - summarized in the urban maps on the next page.

Vulnerability assessment

The buildings most badly damaged in the earthquake were high-rise, (buildings of 5 to 15 stories were worst affected), designed as a simple reinforced concrete frame, and built in the 1960s or early 1970s before the seismic building code was upgraded. The statistics of damage from the earthquake identified the vulnerability of a range of building type characteristics to earthquakes of different strength likely to occur in the future. By identifying buildings with high vulnerability factors (for example finding high-rise reinforced concrete framed buildings built before 1975) the buildings most likely to be damaged in a future earthquake are identified. Where these buildings also have large numbers of residents, they will contribute both to casualties if they collapse, and to homelessness if they become uninhabitable. The buildings with highest projected future earthquake losses are graded into primary and secondary priority for attention.

The assessment of the human vulnerability of the occupants of these buildings and the opportunities for reducing risk are discussed in Part B of this case study.

Damage is almost zero below intensity V, but reaches 100% by intensity IX, at which intensity no buildings would remain undamaged. The D5 curve has a similar shape but shows that the proportion of collapsed buildings is nearly zero up to intensity VIII, but will reach about 50% at intensity X. The value of plotting in this way is that by drawing a vertical line at any level of ground shaking, the intersections with the 5 curves show the expected distribution of a sample of buildings among the 5 levels of damage. The vertical loss scale can also be interpreted as the probability of a given damage level for a single unreinforced masonry building at any particular level of ground shaking.

Losses inflicted on building stock by increasing depths of high-velocity water (e.g. tsunami)

For many purposes it is not necessary (or possible with the available data) to determine the damage distribution at each level of hazard, and only the total proportional loss is needed. The example of the vulnerability function in figure 12 for floods shows the expected losses to buildings caused by inundation by high velocity water, expressed as a proportion of the total replacement cost of the buildings. Such loss curves are useful for economic planning, but less valuable for assessing probable numbers of casualties or homeless people.