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close this bookMitigation of Disasters in Health Facilities: Volume 3: Architectural Issues (PAHO)
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View the documentHazard and seismic risk

Hazard and seismic risk

Earthquakes consist of sudden releases of energy due to stresses that have accumulated for years in parts of the earth's crust. The main causes of stress in the crust are found in the forces pulling at its component parts (the tectonic plates), which are countered by opposing forces in adjacent plates. Not much is known about these forces, but it is thought that they are due to either the high temperatures inside the earth, or to the force of gravity. Earthquakes originated in this way are usually of intermediate depth or deep-seated.

The forces generated in the tectonic plates in turn produce cracks in the plates themselves, which are known as geological faults. Forces derived from tectonic activity can then arise within those faults and tend to move a sector of the fault, generating contrary forces in the opposite sector. This is the origin of the process of accumulation of displacement energy. Earthquakes caused by active geological faults are generally shallow or of intermediate depth and are consequently very dangerous.

The usual ways to measure an earthquake are related to their strength, their location, and their surface manifestations in cities or sites of interest. The energy or strength of an earthquake is measured as its magnitude, a simple numerical scale developed by Charles Richter.

Measurement of the magnitude, as well as the identification of the site at which the phenomenon occurred (epicenter) is carried out using seismographs. As such, the magnitude is a measure of the earthquake at the point at which energy was released. In places far away from the event, such energy is attenuated due to the cushioning effect of the rocks through which the seismic waves travel. It is for this reason that it is more desirable to measure the effect on sites of interest in terms of ground motion. This measurement, carried out by means of accelerometers, usually records ground movement in the three spatial directions, in terms of its acceleration, since this information tells us about the ground velocity and ground displacement.

Ground motion is, accordingly, a function of the magnitude of the earthquake, its distance from the point at which energy was released, and of the properties of attenuation of that energy associated with the geological province in which the earthquake occurs. Studies of seismic hazard seek to establish, for each site of interest, an earthquake unlikely to be exceeded in a period that is considered adequate as the average life of the building or buildings to be constructed, on the basis of available information on the seismic sources that might affect that site (3).

In addition to the factors already mentioned, the following can also influence the impact of an earthquake in cities:

· The amplification of seismic waves by the soils. This fact is currently the object of much attention on the part of researchers, since the energy unleashed in earthquakes can be greatly amplified depending on the characteristics of the soils which support the buildings in cities. Earthquakes occurring far from a city and which are practically insignificant on hard or rocky soils are amplified destructively when the seismic waves encounter soft soils, usually lacustrine.

· Liquefaction. In certain cases, especially in that of saturated sandy soils of uniform gradation, liquefaction of the soil can occur, a phenomenon that consists in the sudden sinking of the soil because of the increase in the pressure of the water contained in the soil when a seismic vibration occurs. It can be catastrophic.

· Mass land movements. Mountainous land can suffer landslides or collapses as a consequence of the seismic thrust of the earth. Sometimes the mass movements do not occur immediately after an earthquake, but after several hours or days.

· Ground settlement. This can occur with loose soils, or with soils supported by layers of soils that have undergone liquefaction, etc.

· Tsunamis or tidal waves. Ocean waves generated by seismic activity on the ocean floor can cause floods in coastal areas and may affect areas located thousand of kilometers from the earthquake epicenter.

· Indirect hazards. The force of the earthquake can cause cracks in dams, which can aggravate the effects of the disaster downstream from reservoirs, or contamination caused by damage to industrial plants, such as leaks of gases or dangerous substances, explosions and fires.

Most of the damage caused by earthquakes is due to the strong movements of the earth. Strong earthquakes have been felt in areas up to five million square kilometers. For this reason, engineering decisions are normally made on the basis of evaluations of large movements, expressed in terms of the maximum acceleration to be expected for ground movement in each site.

Central and South America, especially on the Pacific coast, are areas prone to earthquakes and present a high level of seismic hazard. Major earthquakes have occurred on the border between Costa Rica and Panama (measuring 8.3 on the Richter scale; 1904), on the border between Colombia and Ecuador (8.4 on the Richter scale; 1960), in Peru (8.6 on the Richter scale; 1942), to the north of Santo Domingo, Dominican Republic (8.1 on the Richter scale; 1946) and in Chile (8.4 on the Richter scale; 1960). In general, all the countries of Latin America present some degree of seismic hazard given that earthquakes have occurred in many provinces that may be not recalled as being particularly strong but did indeed frequently cause large-scale catastrophes and damage. Approximately 100,000 inhabitants of this region have died as a consequence of earthquakes during the 20th century, and 50,000 as a consequence of volcanic eruptions; the number of injuries far exceeds the number of deaths (4,5).

Hospitals and health installations in general are exposed elements that can suffer serious damage as a consequence of the occurrence of strong earthquakes. Since the seismic risk to health installations can be very high, it is necessary to construct any new building with a level of seismic resistance in accordance to the seismic hazard in its area. It is also necessary to evaluate the seismic vulnerability of existing buildings, in order to identify their weaknesses and to design and carry out the alterations or retrofittings that may be necessary (6).

Table 1 shows a list of hospitals that have suffered very serious damage or structural collapse as a consequence of earthquakes.

TABLE 1. SELECTED HOSPITALS DAMAGED BY EARTHQUAKES IN THE REGION OF THE
AMERICAS

HOSPITAL

COUNTRY

EARTHQUAKE

Kern Hospital

U.S.A.

Kern County, 1952

Hospital Traumatologico

Chile

Chile, 1960

Hospital Valdivia

Chile

Chile, 1960

Elmendorf Hospital

U.S.A.

Alaska, 1964

Santa Cruz Hospital

U.S.A.

San Fernando, 1971

Olive View Hospital

U.S.A.

San Fernando, 1971

Veterans Administration Hospital

U.S.A.

San Fernando, 1971

Seguro Social

Nicaragua

Managua, 1972

Hospital Escalante Padilla

Costa Rica

San Isidro, 1983

Hospital Juarez

Mexico

Mexico, 1985

Centro Medico

Mexico

Mexico, 1985

Hospital Bloom

El Salvador

San Salvador, 1986

Hospital San Rafael

Costa Rica

Piedras Negras, 1990