Of all natural disasters, earthquakes cause the greatest amount of death and destruction. They generally occur without any warning and, depending on their intensity, can within a few seconds turn a prosperous town into a pile of rubble.
There are several causes for seismic tremors, the main cause being the movement of large continental plates (a few millimetres per year), which collide, move apart or rub against each other, building up immense tension within the rock formations, which at a certain point readjust themselves with a sudden violent motion, sending out seismic waves in all directions. Another cause is the leaking out of molten magma through faults in the earth's crust, which can happen deep beneath the sea or in the form of volcanic eruption. Quakes beneath the sea give rise to tsunamis (Japanese name for seismic sea waves), which can cause total devastation in coastal areas. Volcanic eruptions affect a comparatively small area and damage is mainly caused by molten lava and ash descending on houses and fields.
Artificial causes of earthquakes have recently resulted from the construction of dams, where the large water reservoirs exert great pressure on the earth's crust and lubricate faults, which release the pressure in seismic waves. The exploitation of oil and gas deposits disrupts the balance of pressures and thus can also lead to seismic tremors.
These causes make certain regions more prone to earthquakes than other areas, but exact forecasts of tune and intensity are not possible so far. Special measures to minimize damage to lives and property are recommended in these regions, but complete safety cannot be achieved.
Seismic waves comprise horizontal, vertical and torsional (twisting) movements acting simultaneously. Weak, non-elastic components break apart or disintegrate; elastic materials vibrate and absorb the tremors; while tough and rigid materials can remain unaffected. Destruction of buildings mainly begins with walls falling apart; the ceilings and roofs, lacking support, follow suit, burying the dwellers and property beneath them. However, far greater damage results from secondary effects of earthquakes, such as fire, landslides, damburst, epidemics, etc. A series of smaller tremors follow major earthquakes and can lead to further collapse of buildings, greatly complicating rescue work.
The greatest casualties occur where the population is poorest and houses are built with cheap, sub-standard materials and methods, on dangerous sites, such as slopes, sea shores, valleys below dams, etc. Earthquakes of comparable intensities cause far less destruction and deaths in industrialized countries and rich areas of Third World cities, than in the poor rural areas and slums of developing countries. Hence, earthquakes are often called "classquakes".
• Building sites should not be on or close to hillsides (danger of landslides, avalanches), or near the sea (risk of tsunamis); sufficient distance from neighbouring structures (danger of collapse), especially in prevailing wind direction (fire risk), and downstream from reservoirs (danger of dam-burst) should be maintained. Filled ditches and watercourses should be avoided.
• Building forms must be simple and symmetrical (both horizontally and vertically); complicated forms are possible, if subdivided into independent, simple components.
• Foundations should be of reinforced concrete, constructed on solid ground (preferably rock), maintaining uniform depths (no stepping on sloping ground) and having continuous reinforcement. On poor soils, strong slab foundations have the advantage of "floating" on seismic waves, thus avoiding damage.
• Walls should be relatively light (to lower the centre of gravity of the building and reduce the damaging effects of collapsing walls), capable of absorbing vibrations, but with rigid connections to foundations, adjoining walls and roof. Frame structures (timber, bamboo, reinforced concrete, metal) with light infill walls are most resistant to earthquakes, conventional masonry structures require a strong, continuous ring beam on top of the walls, to prevent them from falling apart.
• Openings should be small, not less than 50 cm from corners or other openings; glass panes should be avoided.
• Roofs should be as light as possible, either monolithic (with high tensile strength, eg reinforced concrete), or of strong, flexible members, firmly tied to the supporting structure; compact symmetrical shapes with spans as small as possible. Roofs must be securely fixed to the ring beam or building frame. Alternatively, roofs can be fixed to independent supports, structurally separated from the walls, which, in the event of failure, would not cause the roof also to collapse.
• Appendages (eg parapets, chimneys, water tanks), if they cannot be omitted, should be very securely fixed, to avoid their being shaken off.
• Stone, earth and clay brick walls generally perform poorly in earthquakes. Improved resistance to collapse is achieved by strengthening and reinforcing corners; ring beams are essential. Masonry walls and domes should be avoided in earthquake zones. Clay tile roofs need strong and heavy timber substructures, which are a hazard when they collapse, and the tiles tend to fall down under vibration.
• Reinforced concrete and ferrocement are ideal materials for seismic resistant constructions, if the qualities of cement, aggregate and workmanship are good, and the metal reinforcements are protected from corrosion. Concrete frames and thin shell structures are best, but heavy ceiling and roof slabs must be avoided.
• Timber and bamboo frames with light infill walls or cladding provide optimum earthquake resistance, and cause less destruction than heavier materials in case of collapse, but represent a fire hazard, which is of significance during earthquakes (due to breakage of chimneys, power and gas supply lines, etc.). Protection against biological hazards is essential to avoid weakening of the construction.
• Metal frames permit light, flexible constructions, but design and dimensioning should take into account the risk of buckling; fire protection and good resistance to corrosion are essential. Metal sheet roofs generally perform well in earthquakes.
• General precautionary measures are in all cases good workmanship and regular inspections of critical parts for maintenance and repairs; also all protective measures against fire.
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