During the 20th century two patterns have emerged. There are increasing numbers of reported hazard events in each decade and there are bigger costs (in terms of death or damage) in many events. Over geologic time, plate movements in concert with other geologic processes e.g. glacial erosion, have created some of natures most magnificent scenery. The Himalayas and the Swiss Alps are some examples. Yet violent earthquakes related to plate tectonics have caused terrible catastrophes such as the magnitude 7.7 earthquake that struck the Chinese province of Hebei in 1976 killing 800,000 people.
The earth’s rocky outer crust solidified billions of years ago soon after the earth formed. This crust is not a solid shell; it is broken up into huge thick plates that drift atop the soft, underlying mantles.
The plates are made of rock and drift all over the globe, they both move horizontally and vertically. Over long periods of time the plates also change in size.
Different types of plate boundary (e.g. oceanic v continental) and different plate motions (e.g. destructive, constructive and conservative) generate different types of hazard. The most explosive volcanic activity and the worst earthquakes occur on destructive boundaries (involve large energy levels due to the stress when one slice of crust slides under another). The most frequent volcanism occurs on constructive boundaries (when magma is emplaces along the line of the mid ocean ridge).
Volcanic hazards are complicated by the fact that volcanic areas are often attractive to settlement, thus increasing the population of the area likely to be affected, Volcanic hazards can vary with the nature of the magma. They may be associated with a range of related hazards. They include flows (e.g. Mt Pelee), lahars (Nevada del ruis), ash fall (hekla) and torrential rain and floods (Pinatubo). Volcanoes can be monitored by a variety of methods to try to predict possible eruptions but the only way to really avoid eruptions is to avoid high-risk areas and evacuate as necessary.
An earthquake is the sudden movement of the earths crust caused by the release of energy at depth. The vibrations are seismic waves. Earthquakes are very common. Unlike volcanoes, earthquakes offer no direct benefits to humans but some earthquake areas are coincidentally areas of good climate (California), good trade links (Lisbon, Portugal) or good soils (Eastern China). It is not surprising therefore that many earthquake zones occur in areas of recent population growth.
Earthquakes mainly occur at plate boundaries. They are by far the most destructive short-term natural force on earth and have plagued civilizations for millennia. Earthquake impacts include both direct and indirect effects. Direct effects include shaking and structural damage (Mexico City 1985) and indirect effects include fire (Tokyo 1923- 99,000 deaths).
Earthquake records suggest there are more and more earthquakes with bigger levels of destructiveness. The explanation is simply that human populations have expanded into earthquake risk zones and human structures are increasingly expensive and vulnerable.
Many quakes tend to be located along the ring of fire, a narrow band that coincides with the margins of the Pacific Ocean. The ring of fire is the most seismically and volcanically active zone in the world. Tectonic earthquakes beyond the ring of fire occur in a variety of geological settings.
Major earthquakes occurring along subduction zones are especially hazardous, because they can trigger tsunamis and pose a potential danger to coastal communities and islands that dot the pacific.
Humans can induce earthquakes through a variety of activities such as the filling of new reservoirs, the underground detonation of atomic explosives, or the pumping of fluids deep into the Earth through wells. In England for example there are small earthquakes caused by the collapse underground of old mine workings. If humans have caused the earthquake then it can be classified as a quasi-natural hazard as it has been initiated by humans but has still occurred within the natural environment.
Because many heavy populated areas are located near active fault zones, such as the San Andreas, millions of people have suffered personal and economic losses as a result of destructive earthquakes. Some societies are more prepared for hazards than others, some are more exposed to hazards than others and then there are those societies that amplify the risk of hazards by their activities e.g. construction.
Great earthquakes such as the 1964 Good Friday earthquake in Alaska have magnitudes of 8.0 of higher. When they occur in heavily populated areas, earthquakes can be disastrous. Many thousands can be killed and injured. This is true in LEDC countries and in the early stages of development in MEDC’s for example in California and Tokyo in the 19th and early 20th centuries. Earthquakes destroying 50,000 homes and killing 6,000 people have devastated LEDC countries such as Chile in 1960.
The eruption of volcanoes in areas such as Mount Pinatubo in the Philippines in 1991 caused great hardship for the local people and disrupted communications and infrastructure. In the same year Japan’s Unzen volcano generated destructive ash flows that swept down its slopes with high speeds. The San Francisco bay area of California is particularly at risk from earthquakes because is lies on the San Andreas Fault boundary. In 1989 an earthquake brought chaos to the city but well funded emergency management agencies meant that the damaged infrastructure was soon repaired.
In May 1980 Mount St. Helens erupted causing 400 million tons of ash and dust to be thrown into the atmosphere. Conifers were knocked down. The ash blocked out the sunlight and choked rivers, causing flood and mudflows. 61 people lost their lives even though most of the population where within ‘safe’ zones due to being evacuated.
From the Japanese words tsunami meaning harbour wave.
Less Economically developed countries
More economically developed countries.
