Earthquakes

Know this topic well? Help others and get FREE products!

Earthquakes are the result of rocks breaking under stress. The constant movement of the tectonic plates that cover the earth's thin outer crust, or lithosphere, causes stress to build up in the rocks beneath the earth's surface. Rocks adjust to this stress over time by folding, but occasionally the stress is released, creating seismic or energy waves that cause the sudden and sometimes disastrous vibrations known as earthquakes. Earthquakes occur throughout the world, but mainly along narrow belts marking the boundaries of the earth's major tectonic plates. Ninety percent of all earthquakes take place at these plate boundaries, and about two-thirds of all large earthquakes are located in the circum-Pacific belt. This tectonic collision zone is one of the most active in the world and stretches from the Aleutians in the North Pacific through Japan, Taiwan, the Philippines, and Indonesia to the Himalayas in northern India. Although there are hundreds of earthquakes in this belt each year, few are felt by the population. However, there have been more than sixty large earthquakes registering 7.0 or higher on the Richter scale in the lastquarter century, the largest of which released twenty times more energy than the annual energy consumption of the United States. Significant property damage and loss of life can result from earthquakes and the secondary hazards associated with these events, especially when they occur near urban centers or areas of high population density.

Earthquakes occurring at weak points in the interior of plates, away from the major plate boundaries, are more difficult to explain. They do not occur according to well-defined patterns like those at plate boundaries. Scientists have not yet been able to model their occurrence so that forecasts of time, place, and size of future earthquakes could be made. China, for example, experiences numerous intraplate earthquakes hundreds of miles toward the interior of the country. Since 1990 eight major intraplate quakes of 5.2 to 7.1 on the Richter scale have been recorded in Yunnan, Hebei, Sichuan, and Menglian provinces in China.

The vibrations, or seismic waves, from an earthquake travel outward in all directions from the fault and, if the earthquake is large enough, are recorded around the world on special measuring instruments called seismographs. A seismograph measures the amplitude of the energy (or seismic waves) released; the measurement is expressed on the Richter scale. The most severe earthquakes recorded to date have not exceeded 9.0 on this scale, although there may be hundreds of low-magnitude quakes measuring 3 or 4 in Asia alone in a single year. Energy release and earthquake magnitude can be poor guides to the hazardous impact of earthquakes, however, because the duration of ground shaking is not accounted for in this scale. The extent of that hazard depends on many other factors, such as distance from the epicenter, rock and soil conditions, population density, and types of building construction.

Although ground shaking is the primary earthquake hazard and the cause of much structural damage and human injury, there are also important secondary hazards. Liquefaction, which happens when loosely packed, waterlogged sediments lose their strength in response to strong shaking, can damage structural foundations of buildings and highways, cause buildings to settle or tilt, and produce submarine avalanches that can destroy harbor facilities and break underwater telecommunication cables. Landslides and rock and snow avalanches can also result from earthquakes. Landslides triggered by earthquakes often cause more destruction than the quakes themselves. Since l964 landslides resulting from large-magnitude earthquakes in Japan have accounted for more than half of all earthquake-related deaths. The most characteristic secondary earthquake-related hazard is the seismic sea wave, or tsunami. Most tsunamis result from tectonic displacement of the seabed associated with large shallow-focus earthquakes under the oceans, but can also be caused by exploding volcanic islands. These waves may reach 100 feet or higher and can travel across the ocean at speeds as great as l50 kilometers per hour. The 1883 volcanic eruption of Krakatau Island in Indonesia generated a tsunami that killed more than thirty-six thousand people in nearby Java and Sumatra. More than 370 tsunamis were observed in the Pacific from 1900 to 1980; Japan is particularly vulnerable to these events, especially on the eastern margins of the large island of Honshu.

More than one-half of the most destructive earthquakes in the twentieth century, those causing five thousand or more deaths, have taken place in Asia. The three largest of these occurred in China, each with more than 200,000 deaths. The 1923 Tokyo-Yokohama earthquake resulted in 143,000 deaths. Indonesia, located astride the boundaries of the Indo-Australian and Eurasian plates, records hundreds of quakes each year; between 1994 and 2000 twelve registered 4.7 to 7.6 on the Richter scale. India had at least five major quakes of 6.2 to 8.4 magnitude in the twentieth century, each resulting in tens of thousands dead and injured. When earthquakes take place near urban centers or more densely settled areas, the impact can be catastrophic. The 1995 Kobe, Japan, earthquake, for example, was measured at a relatively moderate level of 6.9 but caused significant damage and loss of life because it took place in a densely populated port and industrial city. More than 5,500 people were killed and more than 300,000 left homeless due to shock waves, fires, and the collapse of housing built to withstand tropical cyclones but not earthquakes. A similarly disastrous earthquake registering 7.6 on the Richter scale hit northwestern Taiwan in June 2000, causing millions of dollars in property damage, killing 2,100 people, and injuring more than 8,000.

Programs for earthquake disaster preparedness and management in countries throughout the Asia Pacific region are quite varied. The 1990s saw a shift away from local disaster relief activities to increased preparedness and preventive strategies in how regional governments and disaster management organizations respond to earthquake hazards. Enhanced preparedness measures should be augmented with preventive strategies to mitigate earthquake hazards, link rehabilitation with sustainable human development, and focus on communitybased disaster management (CBDM) approaches. For CBDM approaches to gain wider support and be more effective, however, there must be greater acceptance by national governments throughout the region. A number of international agencies, regional disaster training groups, and private philanthropic and nongovernmental organizations are increasingly engaged in discussing and planning new earthquake and disaster management strategies in this part of the world. Groups that are involved include the Asian Disaster Preparedness Center, the ASEAN Earthquake Information Center, the United Nations Development Program, the United Nations High Commission for Refugees, CARE, and Save the Children, among others. Information and lessons learned from other earthquake-prone areas of the world help to improve earthquake forecasting, disaster preparedness, and mitigation for this region of the world. This level of international collaboration is an encouraging development for the millions of people who are vulnerable to the social and economic consequences of earthquakes in Asia.

James Hafner

Merrits, Dorothy, Andrew Dew Wet, and Kristen Menking. (1998) Environmental Geology: An Earth System Science Approach. New York: W. H. Freeman and Company.

Smith, Kenneth. (1996) Environmental Hazards: Assessing Risk and Reducing Disaster. London and New York: Routledge.

Strahler, Alan, and Arthur Strahler. (1994) Introducing Physical Geography. New York: John Wiley & Sons, Inc.

This complete Earthquakes contains 1,211 words. This article contains 1,400 words (approx. 5 pages at 300 words per page).