On 17 October 1989, an earthquake measuring 7.1 on the Richter scale rocked the United States city of San Francisco. The Loma Prieta earthquake, as it became known, killed 68 people and brought more than 24,000 homes crashing down. This was certainly a disaster, but a relatively minor one compared to what hit Izmit, Turkey on 17 August 1999. That earthquake, registering 7.4 on the Richter scale, took the lives of at least 17,100 people and flattened 300,000 homes. Why would two earthquakes of similar magnitude have such different impacts? Timing had something to do with the difference in casualties – the San Francisco earthquake occurred in the afternoon, when relatively few people were at home. In Izmit, many people were sleeping and could not escape their collapsing apartments. But the main reason was that San Francisco was more prepared. In San Francisco, most houses, offices, sports stadiums, roads and bridges had been built to resist earthquakes, and emergency services were ready to lend assistance throughout the city. In Izmit, many buildings had been poorly constructed, and it was mostly these that collapsed. There was no real emergency plan: thousands of people made homeless by the quake had nowhere to go and little access to medical services. Diseases such as typhoid and hepatitis quickly became killers. The lessons from these two examples are clear: long-term planning can greatly reduce the impact of earthquakes, and it is most needed in developing countries.
Developed versus developing A quick glance at death rates shows the massive fault line that separates developed and developing countries in their capacity to withstand earthquakes. Clearly, developing countries need to make improvements in their preparedness for earthquakes and other disasters.
Counteracting earthquakes Perhaps the most obvious way of reducing the effects of earthquakes is to establish urban centres away from earthquake-prone areas, but this is much easier said than done. Some of the world's great cities – Tokyo, Yokohama, Jakarta, Los Angeles, Mexico City and Santiago, to name only a few – are sited right on the edge of tectonic plates, where earthquakes are most likely (Box 1: What is an earthquake?). Moving them somewhere safer isn't feasible. Huge reductions in the effects of earthquakes are still possible in such cities – if they are prepared for the worst. Improved building standards help enormously. The main reason for the high death tolls in the recent earthquakes in Iran and Pakistan was the failure of buildings. In Bam, located about 1000 kilometres southeast of Tehran in Iran, most of the houses in the city were built with mud bricks: when these collapsed, thousands of people suffocated to death. In the Kashmiri–Pakistan disaster, an estimated 60 per cent of buildings in urban areas were made of un-reinforced solid concrete block masonry and more than 60 per cent of these collapsed, crushing and burying their inhabitants. Effects of earthquakes The effects of earthquakes can be classified as either direct or secondary. Direct effects are those caused by the shaking and deformation of the ground: the most dangerous consequence is the collapse of buildings, bridges and elevated roadways. The secondary effects of earthquakes might include tsunamis, which can devastate large areas of low-lying areas, as well as fire, landslides and avalanches. The Indian Ocean tsunami is a dramatic example of a secondary effect: more recently, another earthquake-triggered tsunami in July 2006 claimed more than 500 lives in Java. A small earth tremor may have contributed to a landslide in the Philippines in February 2006 that killed more than a thousand people. How do earthquakes affect buildings? The direct effects of earthquakes can damage buildings in several ways. They can cause the ground underneath to fail, thereby undermining foundations. This is particularly likely on unstable land, such as in areas that have been reclaimed from the sea. The huge Minato Mirai development in Yokohama, Japan, for example, is built on land reclaimed from Yokohama Bay. This land is expected to liquefy – turn to mud – in the event of a large-magnitude earthquake. To help counter this, the foundations of the buildings built there go through the landfill and are anchored firmly to the basement rock beneath. Earthquakes can also rock a building to the point at which it collapses. All buildings vibrate at a natural frequency. This frequency varies from building to building, depending on characteristics such as the design and the construction materials used, but typically it's high in small buildings, such as most houses, and lower in taller buildings. Earthquakes cause most damage to a building when the frequency of the ground movement caused by the earthquake is similar to the building's natural frequency. When this happens the two are said to be in resonance, which means that the shaking caused by the earthquake complements and intensifies the natural shaking of the building. If they were not in resonance, the natural shaking of the building would tend to counteract the shaking of the earthquake. Perhaps the most striking example of this effect was seen in a 1985 earthquake in Mexico City, which was particularly severe on buildings about 20 stories high, while smaller and taller buildings tended to survive. Better buildings Knowledge of earthquake-resistant design has increased dramatically in recent decades – although many of the principles have been known for centuries (Box 2: Cultural monuments). The United States, which has the research capacity and a significant earthquake risk, has led the way. Thousands of structures throughout the country have been fitted with instruments to record the responses of those structures to earthquakes and other disturbances. This and other information has been used to improve building codes with the aim of ensuring that all new buildings are capable of surviving major earthquakes. Older buildings have been retrofitted to improve their earthquake resistance. But improved building codes are useless if they are not enforced. Techniques Engineers have developed many techniques for reducing the impacts of earthquakes on buildings and other structures. For example, 'damping' devices that act like car shock-absorbers can be installed to counteract the resonance effect: they absorb some of the kinetic energy of the earthquake and turn it into heat energy. During a high-intensity earthquake, buildings will 'deform', which means that the materials from which they are constructed are bent and twisted as force is applied. The ability of a structure to accommodate large deformations without a significant loss of strength is known as ductility. Buildings built with ductile materials, such as steel and reinforced concrete, are better able to withstand the extreme forces of an earthquake than non-ductile or brittle materials, such as unreinforced masonry. Another technique (among many) that engineers recommend to increase earthquake resistance is the use of bracing frames to help counteract the lateral forces imposed by seismic waves. Tall buildings should also be wide at the base and have most of their weight in the lower floors. Japan's tallest building, the Landmark Tower, is a good example of this. But better information doesn't always produce better results. Many recently constructed houses in San Francisco's expensive Marina District collapsed during the Loma Prieta earthquake because they were built on reclaimed land. Large sections of elevated concrete freeway collapsed during the Great Hanshin earthquake, even though they were supposed to be earthquake-resistant. Planning to plan ahead Despite some notable failures, most developed countries generally have good systems for the design of building codes and monitoring their implementation. Developing countries are often not so well organised or resourced and often have great difficulty coping in the aftermath of disasters such as earthquakes, droughts, hurricanes and floods. Large numbers of people are suddenly without shelter and the systems of food and water supply and waste disposal break down: the result can be famine, malnutrition and disease. The process of development can be set back for years, even decades, meaning that poverty is perpetuated and the population remains vulnerable to further disasters.
Better planning and construction need not cost huge sums of money. In some of the towns affected by India's Gujarat earthquake, authorities were caught in a bind. They felt they could not allow reconstruction without putting in place proper plans and establishing building standards, so the reconstruction process was slow to start. Some villagers – with the aid of local assistance agencies – took matters into their own hands, strengthening the traditional mud, stick and grass houses, called 'bungas', by adding cement to the mud. This low-cost solution might have been put into effect sooner if planners had already thought of it. In other areas, villagers distrusted the new concrete houses built on their behalf and chose to continue to live in tents. A simple lesson can be learned from such experiences. Planning is best done prior to a disaster, so that when the disaster strikes, communities are prepared. Not only will they cope better with the initial shock and immediate after-effects, they will also be able to start rebuilding much sooner – not only their houses and other infrastructure, but also their lives. Introducing effective planning, and implementing the plans, might take a shake-up, but it could save hundreds of thousands of lives in the future. Boxes Related Nova topics: Calculating the threat of tsunami Looking for clues to our mineral wealth
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Posted October 2006.
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