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Earthquakes are experienced as shockwaves or intense vibrations on the Earth’s surface. They are usually caused by ruptures along geological fault lines in the Earth’s crust, resulting in the sudden release of energy in the form of seismic waves. They can also be triggered by volcanic activity or human actions, such as industrial or military explosions.
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Earthquakes can occur almost anywhere in the world, but most take place along particularly active belts ranging from tens to hundreds of miles wide. An earthquake’s epicenter is the point on the Earth’s surface directly above the source or focus of the earthquake. Most earthquakes are small and cause little or no damage, but very large earthquakes, followed by a series of smaller aftershocks, can be devastating. Depending on the location of the epicenter, these earthquakes can have particularly disastrous effects on densely populated areas as well as the infrastructure that supports them, such as bridges, highways, apartment buildings, skyscrapers, and single-family homes.
Earthquakes can destroy our built-up environments and the essential systems we rely on for our lives and livelihoods. They also have the potential to cause landslides and tsunamis (giant ocean waves that can flood and destroy coastal regions), both of which can have devastating effects on people and communities. The social and economic consequences of earthquakes can be vast, and recovering from them can take many years.
Humans have come a long way in their understanding of the causes of earthquakes. At first, myths and legends explained processes beneath the Earth’s surface. Thinkers from the time of the Greek philosopher Anaxagoras (500–428 BCE) to the German canon and councillor Konrad von Megenberg (1309–1374) in the late Middle Ages believed, with slight variations, that air vapors caught in Earth’s cavities were the cause of earthquakes: thus, Thales of Miletus (c. 625–547 BCE), the founder of Ionian natural philosophy, was among the first to attribute earthquakes to the rocking of the Earth on water. The Greek philosopher Anaximenes of Miletus (585–526 BCE) thought that periods of dryness and wetness were responsible for earthquakes. Aristotle (384–322 BCE) described earthquakes as the consequence of compressed air captured in caves; his ideas were used to explain meteorological phenomena and earthquakes until the Middle Ages. Moved by the devastating earthquake at Pompeii and Herculaneum on 5 February 62 (or 63) CE, the Roman statesman and philosopher Seneca (4 BCE–65 CE) backed Aristotle’s thinking. Plinius (23–79 CE), the Roman historian and author of Historia naturalis, considered earthquakes to be underground thunderstorms.
When classical antiquity was rediscovered by the Christian Occident around 1200, significant parts of Greek ideology were merged with Christian ideas. Albertus Magnus (1193–1280), a German scientist and philosopher, supported the study of the writings of Aristotle and Arabic and Jewish commentators. His own works made an outstanding contribution to the development of the sciences. Georgius Agricola (1494–1555), a German humanist, physician, and mineralogist, believed that earthquakes were the consequence of a subterranean fire ignited by the sun. The long-lasting hypothesis of a central subterranean fire, proposed by the Greek philosopher Pythagoras (570–500 BCE), was revived in the book Mundus Subterraneus by German scholar Athanasius Kircher (1601–1680).
During the eighteenth century scientists became increasingly convinced that no natural phenomenon was unexplainable, thus an explanation for earthquakes became a challenge for scientists of the Enlightenment. The English physician William Stukeley (1687–1765) wrote in his Philosophy of Earthquakes that earthquakes were caused by electrostatic discharge between sky and Earth, like lightning.
The most catastrophic earthquake of the eighteenth century occurred in 1755, destroying Lisbon, Portugal, killing about sixty thousand people, and initiating great debate about the cause of earthquakes. The following year the German philosopher Immanuel Kant (1724–1804) proposed chemical causes for earthquakes. He rejected mystical and religious explanations and held that the cause is below our feet.
The Englishmen John Winthrop (1606–1676) and John Michell (1724–1793) began to reflect not only on the causes but also the effects of earthquakes. Winthrop, a mathematician and natural philosopher, made the important discovery that earthquakes were waves; this discovery would be revived a hundred years later. In 1760, Michell published a study in which he recognized wavelike motions of the ground. With that he anticipated the perception that would lead to an understanding of the cause of earthquakes.
Another significant step was taken by the Irish engineer Robert Mallet (1810–1881) when he began documenting worldwide earthquake occurrences. He compiled a catalog of six thousand earthquakes from which he was able to draw the most complete earthquake map of the world in 1857. The cause of earthquakes was still unknown, but Mallet’s research, which led to the understanding of the origin of mountains and continents, supplied the basic approach to answering the question. In 1912, the German meteorologist and geophysicist Alfred Wegener (1880–1930) presented his theory of continental drift, which states that parts of the Earth’s crust slowly drift atop a liquid core. Wegener hypothesized that there was a single gigantic continent (Pangaea) 200 million years ago.
Earthquakes are classified as either natural or induced. Natural earthquakes are further classified as tectonic—the most common (more than 90 percent of all earthquakes are tectonic)—volcanic (occurring in conjunction with volcanic activity), and collapse (for example, occurring in regions with caverns). Induced earthquakes are vibrations of the ground caused by human activities, such as construction of dams, mining, and nuclear explosions. For example, filling a reservoir in Koyna, India, induced a catastrophic earthquake in December 1967 that caused 177 deaths.
Most earthquakes are caused by the movement of tectonic plates, as explained by the continental drift theory of Wegener. Tectonic plates are large segments of the Earth’s lithosphere (the outer, rigid shell of the Earth that contains the crust, continents, and plates). The Earth’s surface consists of nine major plates: six continental plates (the North American, South American, Eurasian, African, Indo-Australian, and Antarctic plates) and three oceanic plates (the Pacific, Nazca, and Cocos plates). Tectonic plates move in relation to each other and along faults over the deeper interior. Faults are fractures in rock along which the two sides have been displaced relative to each other. An example is the well-known San Andreas Fault in California, which separates the Pacific plate (on which San Francisco and Los Angeles lie) from the North American plate.
When lava is upwelling at midoceanic (mid-Pacific, mid-Atlantic) ridges, rock moves slowly on either side of the ridges across the Earth’s surface. New plates are constantly created, while other plates must be absorbed at subduction zones (where the edge of one plate descends below the edge of another).
Earthquakes, volcanoes, mountain building, and subduction zones are generally explained as consequences of steady, large, horizontal surface motions. Most tectonic plates contain both dry land and ocean floor. At present, those plates containing Africa, Antarctica, North America, and South America are growing, whereas the Pacific plate is shrinking. When plates collide, mountain chains such as the Alps and Himalayas arise, accompanied by persistent earthquake activity.
Seismographs and the Richter Scale
Earthquakes are recorded by sensitive instruments called seismographs. Today’s seismographs record ground shaking over a band of frequencies and seismic amplitudes. A seismogram (the record created by a seismograph) shows the motions of the Earth’s surface caused by seismic waves across time. Earthquakes generate different kinds of seismic waves: P (primary) waves alternately compress and dilate the rock, whereas S (secondary) waves move in a shear motion, perpendicular to the direction the wave is traveling. From a seismogram, the distance and energy of an earthquake can be determined. At least three seismograms are needed to locate where an earthquake occurred. The place at which rupture commences is the focus, or hypocenter, while the point on the Earth’s surface directly above the focus of an earthquake is the epicenter. The distance between the focus and the epicenter is the focal depth of an earthquake.
The amount of energy released by an earthquake is measured and represented by its magnitude. One common type of magnitude measurement is the Richter scale, named after the U.S. seismologist Charles Francis Richter (1900–1985). The Richter scale is logarithmic, meaning the seismic energy of a magnitude 7 earthquake is one thousand times greater than that of a magnitude 5 earthquake.
The following examples from different regions provide vivid examples of the kind of devastation earthquakes can inflict on human populations.
1906: San Francisco, California
The 18 April 1906 San Francisco earthquake, with a magnitude of 7.8, remains one of the most cataclysmic in Californian history. The damaged region extended over 600 square kilometers (about 232 square miles). The earthquake was felt in most of California and parts of western Nevada and southern Oregon. The earthquake caused the longest rupture of a fault that has been observed in the contiguous United States. The displacement of the San Andreas Fault was observed over a distance of 300 kilometers (about 186 miles). The maximum intensity of XI, measured on the Modified Mercalli Intensity Scale ratings of I–XII, was based on geologic effects.
The earthquake and resulting fires took an estimated three thousand lives and caused about $524 million in property loss. The earthquake damaged buildings and structures in all parts of the city and county of San Francisco. Brick and frame houses of ordinary construction were damaged considerably or completely destroyed, and sewers and water mains were broken, including a pipeline that carried water from San Andreas Lake to San Francisco, interrupting the water supply to the city. This made it impossible to control the fires that ignited soon after the earthquake occurred, and subsequently those fires destroyed a large part of San Francisco. It was not until 1908 that San Francisco was well on the way to recovery.
1995: Hanshin-Awaji; Kobe, Japan
On 17 January 1995, the Great Hanshin-Awaji earthquake with a magnitude of 6.9 occurred directly under the industrialized urban area of Kobe, Japan, a city of about 1.5 million people. The shock occurred at a shallow depth on a fault running from Awaji Island through Kobe. Strong ground shaking lasted for about twenty seconds and caused severe damage over a large area. More than five thousand people were killed; the total cost of damage and destruction exceeded $100 billion, or about 2 percent of Japan’s gross national product. More than 150,000 buildings were ruined; highways, bridges, railroads, and subways failed; water, sewage, gas, electric power, and telephone systems were extensively damaged.
The city of Kobe—then one of the six largest container cargo ports in the world and Japan’s largest—was devastated. Its relative importance as a major hub in Asia declined over the following years, with significant enormous economic consequences. With Japan’s having invested heavily in earthquake research, people believed they would be ready for the next earthquake, but their faith was shattered deeply by the Kobe catastrophe.
2003: Bam, Iran
On 26 December 2003, an earthquake occurred below the city of Bam in the southeast of Iran, illustrating again the tragic connection between poor building quality and large numbers of victims. The earthquake had a magnitude of 6.5, and the hypocenter was only 8 kilometers (about 5 miles) below the city. The people of Bam were still sleeping when the earthquake struck. The death toll was estimated at 43,200, with more than 30,000 injured and 100,000 left homeless. The main reason for the large number of fatalities was the generally poor construction quality of buildings, 85 percent of which were damaged. Even though experts had classified the region as a highly exposed zone prior to the earthquake, many of the residences were traditional houses of mud-brick construction, with heavy roofs. Unreinforced masonry holds almost no resistance against the ground motion generated by strong earthquakes.
Preparing for Earthquakes
Increasing population density magnifies the potential damaging effects of earthquakes, especially in urban areas with high seismic activity—for example, San Francisco. For this reason, anti-seismic building codes are important. Appropriate planning and regulation of new buildings and seismic upgrading of existing buildings can safeguard most types of buildings against earthquake shocks. One obstacle to adhering to anti-seismic building codes is high cost; this is true particularly in poorer cities in the developing world, and the effects can be particularly devastating.
Mexico City; Sichuan Province, China; Haiti
The 19 September 1985 Mexico City earthquake, occurred 200 kilometers (about 124 miles) from Mexico City, but the shaking of loose sediments in the city was much stronger than at the epicenter. Nearly ten thousand people died, and the city was heavily damaged as poorly constructed buildings collapsed. The earthquake destroyed as many as 100,000 housing units and countless public buildings.
Hundreds of millions of people live in buildings that would collapse in a strong earthquake, as happened in the mountainous Sichuan Province of China in 2008, when as many as 90,000 people were killed or remain missing, with another 374,000 injured and at least 15 million displaced.
On the afternoon of 12 January 2010, an earthquake with a magnitude of 7.0 devastated parts of Haiti, a nation on the island of Hispaniola in the Caribbean; it was the strongest in the region in over two hundred years. The earthquake occurred at a fault that runs right through Haiti and is situated along the boundary between the Caribbean and North American plates; the epicenter was just 16 kilometers (10 miles) south of the capital, Port-au- Prince, whose population at the time was over 2 million. Aftershocks continued for days, including one a week later registering a magnitude of 5.9. As of late January 2010 the projected death toll ranged from 70,000 to 200,000. The severity of the earthquake was exacerbated by two factors: the depth of the quake was shallow, meaning that energy released was closer to the Earth’s surface and less able to be absorbed by the Earth’s crust; and nearly all of the buildings in Haiti were substandard construction, many cinderblock and mortar.
It is anticipated that, in the future, more catastrophes with high death tolls will occur. Owing to the rapid growth of many developing-world metropolises in highly exposed regions, such scenarios are distinctly more probable, despite the possibilities provided by modern earthquake engineering.
As of 2010, the time, location, and magnitude of earthquakes cannot be accurately predicted. Damage and casualties can be minimized, however, if builders adhere to building codes based on the seismic hazards particular to their areas.
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