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| Sunday, 24 October 2004 |
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Junior Observer |  |
| News Business Features Editorial
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When the earth moves One of the most frightening and destructive natural phenomena is an earthquake. Although about 8,000 earthquakes are reported to occur around the world every day, some of them can hardly be felt while we can expect on average, one big earthquake each year.
At other times, the plates are locked together, unable to release the accumulating energy. When the accumulated energy grows strong enough, the plates break free. If the earthquake occurs in a populated area, it may cause many deaths and injuries and extensive property damage. The scientific study of earthquakes is comparatively new. Until the 18th century, few factual descriptions of earthquakes were recorded. The earliest documented earthquake occurred in China in 1177BC. The Earth is formed of several layers that have very different physical and chemical properties. The outer layer, which averages about 70km in thickness, consists of about a dozen large, irregularly shaped plates that slide over, under and past each other on top of the partly molten inner layer. Most earthquakes occur at the boundaries where the plates meet. The three types of plate boundaries are spreading zones, transform faults, and subduction zones. At spreading zones, molten rock rises, pushing two plates apart and adding new material at their edges. Transform faults are found where plates slide past one another. Subduction zones are found where one plate overrides, or subducts, another, pushing it downward into the mantle where it melts. Earthquakes can also occur within plates, although plate-boundary earthquakes are much more common. Less than 10 per cent of all earthquakes occur within plate interiors. As plates continue to move and plate boundaries change over geologic time, weakened boundary regions become part of the interiors of the plates. These zones of weakness within the continents can cause earthquakes in response to stresses that originate at the edges of the plate or in the deeper crust. Most destructive quakes are caused by dislocations of the crust. The crust may first bend and then, when the stress exceeds the strength of the rocks, break and "snap" to a new position. In the process of breaking, vibrations called "seismic waves" are generated. These waves travel outward from the source of the earthquake along the surface and through the earth at varying speeds depending on the material through which they move. These vibrations cause the entire planet to quiver. A fault is a fracture in the Earth's crust along which two blocks of the crust have slipped with respect to each other. Faults are divided into three main groups, depending on how they move. Normal faults occur in response to pulling or tension; the overlying block moves down the dip of the fault plane. Thrust (reverse) faults occur in response to squeezing or compression; the overlying block moves up the dip of the fault plane. Strike-slip (lateral) faults occur in response to either type of stress; the blocks move horizontally past one another. Most faulting along spreading zones is normal, along subduction zones is thrust, and along transform faults is strike-slip. The focal depth of an earthquake is the depth from the Earth's surface to the region where an earthquake's energy originates (the focus). Earthquakes with focal depths from the surface to about 70km are classified as shallow. Earthquakes with focal depths from 70 to 300kms are classified as intermediate. The focus of deep earthquakes may reach depths of more than 700km. The focuses of most earthquakes are concentrated in the crust and upper mantle. The epicentre of an earthquake is the point on the Earth's surface directly above the focus. The location of an earthquake is commonly described by the geographic position of its epicentre and by its focal depth. Earthquakes beneath the ocean floor sometimes generate immense sea waves or tsunamis. These waves travel across the ocean at speeds as great as 960km per hour and may be 15m high or higher by the time they reach the shore. Liquefaction, which happens when loosely packed, water-logged sediments lose their strength in response to strong shaking, causes major damage during earthquakes. Landslides triggered by earthquakes often cause more destruction than the earthquakes themselves. ***** Moderate earthquake in Bali Indonesia (Reuters) An earthquake shook Indonesia's tourist isle of Bali recently, but there was no news of casualties or damage. "It was only around 20 seconds, small quakes," said a Reuters cameraman in Denpasar, Bali's capital, 960 km east of Jakarta.An earthquake measuring 5.0 on the Richter scale hit Bali. The epicentre is 150 m south of Denpasar (in the sea). An earthquake of a magnitude between 5 and 5.9 is considered moderate, according to the U.S. Geological Survey. ****** Measuring earthquakes The vibrations produced by earthquakes are detected, recorded, and measured by instruments called seismographs. The zig-zag line made by a seismograph, called a "seismogram," reflects the changing intensity of the vibrations by responding to the motion of the ground surface beneath the instrument. From this data, scientists can determine the time, the epicentre, the focal depth, and the type of faulting of an earthquake and can estimate how much energy was released. The magnitude of an earthquake, usually expressed by the Richter Scale, is a measure of the amplitude of the seismic waves. The intensity, as expressed by the Modified Mercalli Scale, is a subjective measure that describes how strong a shock was felt at a particular location. The Richter Scale, named after Dr. Charles F. Richter of the California Institute of Technology, is the best known scale for measuring the magnitude of earthquakes. A recording of 7, for example, indicates a disturbance with ground motion 10 times as large as a recording of 6. A quake of magnitude 2 is the smallest quake normally felt by people. Earthquakes with a Richter value of 6 or more are commonly considered major; great earthquakes have magnitudes of 8 or more on the Richter scale. |
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An earthquake is a sudden movement of the earth, caused by the abrupt
release of accumulated strain. For millions of years, the forces of plate
tectonics have shaped the earth, as the huge plates that form the earth's
surface slowly move over, under, and past each other. Sometimes the
movement is gradual.
