Natural Hazards: Causes and Effects - Disaster Management Center ...
Natural Hazards: Causes and Effects - Disaster Management Center ... Natural Hazards: Causes and Effects - Disaster Management Center ...
ocean becomes a devastating 30-meter-high wave moving at 50 kilometers per hour when it reaches shore. (See chapter 3 on tsunamis.) The risk of fire immediately after an earthquake is often high because of broken electrical lines and gas mains. In recent years, officials in most of the world’s major cities have installed devices that shut these services down automatically if an earthquake strikes. Yet the threat still exists in many of the smaller cities and the squatter settlements of the larger cities where open fires are used for cooking. 8 Finally, disturbance of the subsoil causes changes in the course of groundwater flows. This can cause abrupt changes in the level of the water table and sudden drying up of surface springs. 9 Behavior of Buildings during Earthquakes As the vibrations and waves continue to move through the earth, buildings on the earth’s surface are set in motion. Each building responds differently, depending on its construction. When the waves strike, the earth begins to move backward and forward along the same line. The lower part of a building on the earth’s surface immediately moves with the earth. The upper portion, however, initially remains at rest; thus the building is stretched out of shape. Gradually the upper portion tries to catch up with the bottom, but as it does so, the earth moves in the other direction, causing a “whiplash” effect, speeding up the top of the building and creating a vibration known as resonance. The resonance can cause structural failure in itself, or adjacent buildings having different response characteristics because different building materials can vibrate out of phase and pound each other to pieces. The walls of buildings without adequate lateral bracing frequently fall outward, leaving the upper floors or roof to collapse into the inside of the structure. 10 (See Fig. 2.6) Earthquake Forecasting The study of regional seismicity and the outline of seismic zones make it possible, within the framework of historical incidence and global tectonics, to predict the regions in which earthquakes will occur; the real problem in prediction is to be able to specify in advance the exact place, day (as precisely as possible) and magnitude of a future earthquake. Only recently has a strictly scientific approach been applied to the short-term prediction of earthquakes. The first successes now achieved by certain research workers give reason to hope that such forecasting will be possible in the fairly near future. It would then be possible, thanks to two kinds of prediction, to adopt a preventive strategy that might greatly reduce human and material losses. Long-term forecasting could be used: • for determining the optimum structures of existing buildings; • for encouraging local authorities to issue new regulations on building and land use and, in particular, for improving the choice of sites for new human settlements; • for launching campaigns to inform and educate the population on safety rules and general preventive measures; • for drawing up relief plans; • for improving response capabilities. Short-term predictions, on the other hand, would make it possible: • to mobilize relief in the event of a disaster; • to implement procedures for evacuating endangered buildings and dangerous areas (fire risk); • to shut down certain dangerous industries (nuclear reactors, electric power stations, oil and gas pipelines, etc.); • to evacuate low-lying coastal areas liable to be swept by tsunamis
How an earthquake damages a house Figure 2.6
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How an earthquake damages a house<br />
Figure 2.6
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