Earthwuake hazards

Environmental Hazards in the 1990s

Environmental Hazards in the 1990s

The United Nations has declared the 1990s the International Decade for Natural Disaster Reduction (IDNDR). The unanimous passage of general Assembly resoliution was based on the belief that “natural” hazards are continuing to pose escalating costs on human societies- the so-called “natural tax”-though death, destruction, damage and disruption. This view, which was supported by statements such as:

During the past two decades, natural disasters have been responsible for about 3 milion death and have addversely affected at least 800 milion people through homelessness, disease, serios economic loss and other hardships, including immediate damages in the hundreds of billions of dollars.

Very largery reflected increasing awareness of hazard impacts brought about by developments in communications and the media.

Earthwuake hazards

Of all major natural hazards, earthquakes are distinct because of the integral role played by human construction in controlling the severity of hazard impact. Unlike the flood hazard, for example, the physical process of earthquake ground motion poses little thhreat to healthy human beings sitting out in the open. Most earthquake victims result total or partial coolapse of buildings on to them- in other words, inadequate desing and contruction in the face of an envirommental threat. The vulnerability of a

building to earthquake hazard varies considerably according to a range of factors, some scientific and technical in nature, others related more to social, economic and politicalconsideration.

The two parameters commonly used to describe the size of an earthquake are magnitude and intensity. Magnitude provides a quantitative measurement of energy released by an earthquake. It is determined by instrumental observations and calibrated using the Richter scale, which is logarithmic. For each unit increase in magnitude the amplitude of the seismic waves increases 10- fold and there is about a 30 – fold increase in the amount of energy liberated. Hence an earthquake of magnitude 8.6 releases almost one million times more energy than magnitude 4.4 event. The largest earthquakes recored to date registered 8.9 on Richter scale.

By waay of contrast to magnitude, intensivity provides a desciptive and relatively subjective assessment of the size of impact of an earthquake based on its effects. Upon humans, structures and the ground surface. A number of intensity scales exist, of which the modified mercalli (MM) scale of 1956 is widely used. This scale has twelve grades, denoted by Roman numerals, ranging from I (imperceptible) to XII (major disater).

Analysis of the location of the hundred largest cities in the world (accommodating 10 percent of gl

lobal population) relative to the active seismic zones, shows that 70 per cent of the cites are exposed to significant earthquake hazard; i.e. they can expect to experience earthquake intensities of VI (strong) or more at least once every 50 years on average. Indeed, 25 per cent of the cites can expect intensities of VIII (destructive) or more. The percentage of cities exposed, and the average severity of hazard exposure, is greather for the largest cities of the developing world than for those of the developed countries. It is important to realize that this pattern of global hazard exposure results from the natural distribution of global tectonic processes, and is the wnvironmental context against which studies of human vulnerability and response should be set.

The questions most freaquently asked of scientist after an earthquake are “why was it not predicted? and when will the next on be?”. The ultimate goal of earthquake prediction is to forecast the magnitude, location and timing of earthquakes, from the outset, it is important to recognize that although there have been significant advancements in earthquake prediction in recent years, no guaranteed method has yet been found, nor is it likely to be found in the foreseeable future. There have be

een some prediction successes, most notably the February 4, 1975, (magnitude 7.3) earthquake at Haicheng in China. This prediction was based upon seismological research, as well as data from community monitoring programmes aimend at detecting earthquake precursory phenomena in the local environment (e.g. from water levels in wells, radon gas content of groundwater, water temperatures, tiltmeters, seismographs, magnetometers and animals behaviour patterns). The people of Haicheng were first evacuated from their houses on December 20 and slept in the snow for two nights before returning indoors. They were evacuated again at 2.00 pm the following February 4, and the earthquake struck at 7. 36 pm. Undoubtedly this prediction saved thousands of lives.

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