|
|
|
| Nature
Gallery (Plate Tectonics)
Earthquakes |
|
| Earthquakes occur when the Earth’s crust moves suddenly along a fault. Rock under great strain ruptures and releases energy in the form of seismic waves. However, most earthquakes are so slight that they are not felt by humans. Some quakes create vibrations resembling those caused by the passing of a heavy truck. The vibrations of major earthquakes can be catastrophically destructive, having the ability to level entire cities in seconds. | |
| History of Earthquake Study | |
| Since ancient times,
people living in earthquake-prone areas have been preoccupied with
earthquakes. Some of the ancient Greek philosophers attributed them to
subterranean winds;
others blamed them on fires deep in the Earth. Around AD
130, the Chinese scholar Zhang Heng reasoned that waves must ripple
through the Earth from the source of an earthquake. He constructed an
elaborate bronze vessel to record the passage of such waves and delicately
balanced eight balls in the mouths of eight dragons placed around the
circumference of the vessel. A passing seismic wave would cause one or
more of the balls to drop.
In the early 20th century, the Russian seismologist Prince Boris Golitzyn invented the modern seismograph. His device, using a magnetic pendulum suspended between the poles of an electromagnet, ushered in the modern era of earthquake research. |
|
| Kinds and Locations of Earthquakes | |
| Scientists recognize three general
classes of earthquakes: tectonic, volcanic,
and artificially produced. The tectonic variety is by far the most
devastating, and such quakes pose particular difficulties for scientists
trying to develop ways to predict them.
According to the theory of plate tectonics, the ultimate cause of tectonic earthquakes is stresses set up by movements of the dozen or so major and minor plates that make up the Earth’s crust. Most tectonic quakes occur at the boundaries of these plates, in zones where one plate slides past or beneath another. |
|
| Earthquakes resulting
from subduction
account for nearly half of the world's destructive seismic events and
three-quarters of the Earth's seismic energy. These are concentrated along
the Ring of Fire, a narrow band about 38,600 kilometres (24,000 miles)
long, which coincides with the margins of the Pacific
Ocean. Crustal rupture in such earthquakes tends to occur far
below the Earth's surface, at depths of up to 645 kilometres (400 miles).
Tectonic earthquakes beyond the Ring of Fire occur in a variety of geological settings. Mid-ocean ridges are the sites of numerous such events of moderate intensity that take place at relatively shallow depths. Humans rarely feel these quakes. Such earthquakes account for only about 5 per cent of the Earth's seismic energy, but the instruments of the worldwide network of seismological stations record them daily. Tectonic earthquakes also occur in a zone stretching from the Mediterranean and Caspian seas to the Himalayas, and ending in the Bay of Bengal. Within this zone, which releases about 15 per cent of the Earth's seismic energy, continental land masses riding on the Eurasian, African, and Indo-Australian plates are forced together to produce high, young mountain chains. The resulting earthquakes, which occur at shallow to intermediate depths, have devastated areas of Portugal, Algeria, Morocco, Italy, Greece, Iran, India, FYRO Macedonia, Turkey, and other countries partly or entirely on the Balkan Peninsula. |
|
| One other category of tectonic earthquakes includes the infrequent but large and destructive ones that occur in areas far removed from other forms of tectonic activity. Prime examples of these so-called mid-plate earthquakes are three massive tremors that shook the central United States region around New Madrid, Missouri, in 1811 and 1812. Powerful enough to be felt 1,600 kilometres (1,000 miles) away, these shocks produced movements that rerouted the course of the Mississippi River. Geologists believe that the New Madrid earthquakes are symptomatic of forces tearing apart the Earth’s crust—forces such as those that created Africa’s Great Rift Valley. | |
| Of the two classes of
non-tectonic earthquakes, those of volcanic origin are seldom large or
destructive. They are of interest chiefly because they herald impending
volcanic eruptions, as they did in the weeks preceding the eruption in
1980 of Mount
St Helens in the northwest United
States. Such earthquakes originate as magma
works its way upwards, filling the chambers beneath a volcano. As the
flanks and summit
of the volcano swell, swarms of small earthquakes signal the rupture of
stressed rocks. On the island
of Hawaii,
seismographs register as many as 1,000 small quakes a day before an
eruption occurs.
Humans may contribute to the cause of earthquakes
through a variety of activities such as filling new reservoirs,
detonating underground atomic explosives, or pumping fluids deep into the
ground through wells.
For example, in 1962 Denver, Colorado,
in the Earthquake Effects |
|
| Earthquakes produce various adverse effects to the inhabitants of seismically active regions. They can cause great loss of life by destroying structures such as buildings, bridges, and dams. Earthquakes can also trigger devastating landslides. Massive fires caused by the rupture of gas and electrical lines have damaged or destroyed many cities. | |
| Another destructive effect of an earthquake is the generation of a so-called tidal wave. This type of wave is caused by sub-sea tremors, not tides, so it is more properly called a seismic sea wave or (its Japanese name) tsunami. These towering walls of water have struck populated coastlines, destroying entire towns. Sanriku, Japan, a town with a population of 20,000, suffered such a devastating fate in 1896. | |
| Liquefaction of soils
is another seismic hazard. When subjected to the shock waves of an
earthquake, soil used in landfill may lose virtually all its bearing
strength and become similar to quicksand. Buildings have literally been
swallowed up by these materials.
After a major earthquake, there may be a series of further tremors, some of them severe enough to cause additional damage. These tremors are called aftershocks. Richter Scale |
|
| Seismologists have
devised several scales of measurement to describe earthquakes
quantitatively. One is the Richter scale—named after the US seismologist
Charles Francis Richter (1900–1985)—which measures the energy released
at the focus of a quake. It is a logarithmic scale that runs from 1 to 9:
a magnitude 7 earthquake is 10 times more powerful than a magnitude 6
earthquake, 100 times more powerful than one of magnitude 5, 1,000 times
more powerful than one of magnitude 4, and so on. About 800 earthquakes of
magnitudes 5 to 6 occur annually worldwide, in comparison with about
50,000 earthquakes of magnitudes 3 to 4, and only about one earthquake of
magnitudes 8 to 9.
Theoretically, the Richter scale is an open-ended one, but until 1979 an earthquake of magnitude 8.5 was thought to be the most powerful possible. Since then, however, improvements in seismic measuring techniques have enabled seismologists to refine the scale, and they now consider 9.5 to be the practical limit. Devastating Earthquakes |
|
| Historical records of
earthquakes before the mid-18th century are generally lacking or
unreliable. However, reasonably trustworthy records do exist for the
following ancient earthquakes: one off the coast of Greece in 425 BC
that created the island of Évvoia,
one that destroyed the city of Ephesus in Asia
Minor in AD 17, one that levelled
much of Pompeii
in 63, and those that partially destroyed Rome
in 476 and Constantinople (now Istanbul)
in 557 and again in 936. Severe earthquakes struck England
in 1318, Naples
in 1456, and Lisbon
in 1531.
The 1556 earthquake in Shaanxi Province of China,
which killed about 800,000 people, was one of the greatest natural
disasters in history. In 1693, an earthquake in Sicily
resulted in a loss of approximately 60,000 lives. In the early 18th
century the Japanese city of Edo
(the site of modern Tokyo) was destroyed, with the loss of some 200,000
lives. In 1755 the city of In North America, the series of earthquakes that struck southeastern Missouri in 1811 and 1812 was probably the most powerful experienced in the United States in modern history. The most famous US earthquake is the one that shook the area of San Francisco in 1906, causing extensive damage and resulting in the loss of 700 lives. Among the most recent earthquakes was the one on 17 January 1995 that severely damaged Kobe, Japan, killing more than 4,000 people and leaving over 275,000 people homeless. The earthquake measured 7.2 on the Richter scale and lasted 20 seconds. On 28 May 1995 an earthquake measuring 7.5 on the Richter scale struck Neftegorsk, Russia, an oil-producing town on Sakhalin Island in the far eastern part of the country. The earthquake caused great destruction, killing more than 2,000 people and demolishing blocks of flats. In China’s Yunnan Province, near Lijiang, a tremor of magnitude 7.0 struck on 3 February 1996, killing more than 300 people, seriously injuring another 3,800, and damaging or destroying an estimated 830,000 homes. Preparing for Earthquakes |
|
| Countries in
earthquake-prone areas, such as Japan,
have placed great emphasis on researching and implementing
state-of-the-art building construction that will be able to withstand
earthquakes. Communities have established and rehearsed detailed emergency
procedures. Yet even Tokyo,
despite being among the best-prepared nations to deal with earthquakes in
the world, is vulnerable to serious damage and heavy loss of life.
Tokyo’s problems include: soft soil in some locales that may easily
liquefy; a large number of old, buildings of weak construction; narrow
streets that would be rendered impassable after earthquake damage; and
highly flammable refineries in the industrial
areas.
Earthquake Prediction |
|
| Attempts to predict when
and where earthquakes will occur have met with some success in recent
years. China, Japan,
Russia,
and the United
States are the countries most actively supporting such
research. In 1975 the Chinese predicted an earthquake of magnitude 7.3 at Haicheng,
evacuating 90,000 residents only two days before it destroyed or damaged
90 per cent of the city’s buildings. One of the clues that led to this
prediction was a chain of low-magnitude tremors, called foreshocks, that
had begun about five years earlier in the area.
Other potential clues being studied are the tilting or bulging of the land surface and changes in the Earth’s magnetic field, in the water-levels of wells, and even in the behaviour of animals. A new method currently being studied involves measuring the buildup of stress in the Earth’s crust. Most predictions are only rough estimates, but as advancements are made in seismology and plate tectonics, the accuracy of predictions will improve, leading to earlier warnings and fewer deaths. |
|
|
|
