Earthquake

Earthquake

Causes of earthquakes

Seismic waves

Collapse of buildings

Earthquake-triggered landslide

Liquefaction of soil

Subsidence

Tsunamis

Secondary hazards: fire, disease, famine

Historical incidence of earthquakes

Resources

An earthquake is the shaking or vibration of Earth’s surface as the result of sudden movement along a fault, the movement of molten rock within the Earth, or human activities. The terms temblor and seism are often used as synonyms for earthquake. The location of an earthquake source within the Earth is known as its focus, and the point on the Earth’s surface directly above the focus is known as the epicenter.

Earthquakes are common events. The United States Geological Survey estimates that more than three million earthquakes occur on Earth each year, which is equivalent to more than 9, 000 earthquakes per day. Virtually all of these are too small to be noticed by humans and many occur in remote areas far from seismometers. Since 1900, there has been on average about 1 magnitude 8 earthquake, 18 magnitude 7.0 to 7.9 earthquakes, 120 magnitude 6.0 to 6.9 earthquakes, and 800 magnitude 5.0 to 5.9 earthquakes on Earth each year.

Earthquakes can range in severity from small events that are imperceptible to humans to devastating shocks that level cities and kill thousands. The world’s most destructive earthquake, which occurred in China during the year 1556, killed 830, 000 people. Twenty other earthquakes in Europe, Asia, and the Middle East are known to have resulted in more than 50, 000 deaths each. The most devastating earthquake to strike the United States was the 1906 San Francisco earthquake, which killed about 3, 000 people as a result of shaking and resulting fires. Modern engineering and construction methods have significantly reduced the danger posed by earthquakes in developed countries.

In the United States, for example, only five earthquakes since 1950 have killed more than 60 people. The great Alaskan earthquake of 1964, the second largest earthquake ever recorded by seismologists, killed only 15 people. An additional 110 perished, however, in earthquake triggered tsunamis that struck coastal Alaska, Oregon, and California. Most of the fatal earthquakes occurring in the United States since 1950 have killed only one or two people, and the vast majority of earthquakes do not kill anyone.

The size of an earthquake is described by its magnitude, which reflects the amount of energy released by the temblor. There are many different ways of calculating earthquake magnitude, the most famous of which was proposed in the 1930s by the American seismologist Charles Richter (1900–1985). The Richter magnitude is the base 10 logarithm of the largest seismic wave amplitude recorded on a particular kind of seismograph located 62 mi (100 km) from the earthquake epicenter. Adjustments must be made if other kinds of seismographs are used or if they are located at a different distance from the epicenter. An earthquake of a given magnitude will produce waves 10 times as large as those from an earthquake of the next smaller magnitude. The energy released increases by a factor of about 30 from one magnitude to the next. The Richter scale is open-ended, meaning that it has no mathematical upper or lower limits. In reality, however, there are no faults on Earth large enough to produce a magnitude 10 earthquake. The two largest recorded earthquakes were the magnitude 9.5 Chilean earthquake of 1956 and the magnitude 9.2 Prince William Sound, Alaska, earthquake of 1964.

The effects of an earthquake are measured by its intensity. Unlike magnitude, earthquake intensity varies from place to place. The most common measure of intensity is the modified Mercalli scale, which ranges from an intensity of I (not felt except by a few people under especially favorable circumstances) to XII (total destruction, with objects thrown in the air and lines of sight distorted). Surveys and interviews after a large earthquake can be used to create an iso-seismic map, which shows the distribution of reported earthquake intensities. Most isoseismic maps show a distorted bull’s eye pattern of concentric rings of equal intensity area centered around the epicenter.

Causes of earthquakes

Tectonic plate movements

Some earthquakes occur in areas where the tectonic plates comprising Earth’s lithosphere move horizontally past each other along large faults or zones of faults. Examples of this type include earthquakes along the San Andreas and Hayward faults in California. Earthquakes also occur in places where a continental plate subducts an oceanic plate, for example along the western coast of South America, the northwest coast of North America (including Alaska), and in Japan. If two continental plates collide but neither is subducted, as in Europe and Asia from Spain to Vietnam, earthquakes occur as the rocks are lifted to form mountain ranges.

In other parts of the world, for example the Basin and Range physiographic province of the western United States and the East African Rift, continental plates are being stretched apart by tectonic forces. The result is that some parts of the Earth’s crust are lifted to form mountain ranges while neighboring blocks subside to form basins that collect sediment eroded from the mountains. Earthquakes can occur when movement occurs along faults developed as a result of the stretching.

Faults are planes of weakness, across which rock has moved in opposite directions, within the Earth’s crust. They can range in size from continental scale features such as the San Andreas fault in California to small features across which only a few millimeters or centimeters of movement has occurred. Tectonic plate motions increase the level of stress within Earth’s crust, which is accommodated as elastic strain energy, until the stress exceeds the strength of the fault. Then, the energy is suddenly released as the rocks on each side of the fault slip past each other to create an earthquake. This process is analogous to a rubber band snapping when it was been stretched to the breaking point. Because there is a frictional resistance to movement along faults, rapid seismic slip can generate enough heat to melt the adjacent rocks and form a glassy rock known as pseudotachylyte. In other cases, the elastic strain energy is slowly and quietly dissipated through a process known as aseismic creep.

Magma movement

Rhythmic earthquakes known as harmonic tremors, which are caused by magma and volcanic gas moving through conduits in the Earth’s crust just as air moves through a pipe organ, can foreshadow or accompany volcanic eruptions. Recent studies have also suggested that very large earthquakes, such as the magnitude 9.0 earthquake that affected the west coast of the United States in 1700, may trigger volcanic activity for several decades after their occurrence as the Earth’s crust slowly adjusts to the initial movement. Seismologists can also use earthquake activity to infer the presence of magma that has not yet erupted and formed a volcano. Swarms of small earthquakes near Socorro, New Mexico, for example have helped scientists to locate a mass of molten rock about 12 mi (20 km) beneath the Earth’s surface. Detailed measurements have shown that the surface is being lifted by about 0.08 in (2 mm) per year in that area, but there are no obvious signs that a pool of molten rock lies beneath the surface.

Human activity

Explosions, especially from underground nuclear bomb testing, can produce small earthquakes. Earthquakes caused by explosions produce vibrations different than those caused by movement along faults, and seismic monitoring is an important part of nuclear test ban treaty verification. The implosive demolition of the Kingdome, a sports stadium in Seattle, in the year 2000 produced a magnitude 2.3 earthquake. Seismologists were able to deploy seismometers before the demolition and use the manmade earthquake to learn more about the geology of the area by studying how seismic waves were reflected and refracted beneath Earth’s surface. Another well-known example of earthquakes due to human activity occurred at the Rocky Mountain Arsenal near Denver, Colorado, during the 1960s. The pressure of hazardous waste being injected deep into the Earth through disposal wells was large enough to trigger a series of earthquakes. A subsequent experiment in an oilfield near Rangely, Colorado, showed that earthquakes could be triggered at will be injecting water under pressure.

Seismic waves

Rapid slip along a fault generates waves in much the same way as does a pebble falling into a pool of water, and waves moving outward from an earthquake focus are reflected and refracted each time they encounter a different rock type. There are four different kinds of seismic waves, two of which are known as body waves and two of which are known as surface waves. Body waves travel deep through the Earth, whereas surface waves travel along the Earth’s surface and generally cause the most damage.

The two types of body waves are P-waves and S-waves. P-waves, also known as primary waves, travel the fastest of the four types. They move by alternately compressing and stretching the rock through which they pass. P-wave velocity depends on the rock type and density, but it is generally about 4 mi/s (6 km/s). S-waves, also known as secondary waves, move by shearing or moving from side to side the rock through which they pass. S-waves move more slowly than P-waves and, depending on the type of rock, have a velocity of about 2 mi/s (3 km/s).

The two types of surface waves are known as Rayleigh and Love waves. They travel more slowly than either P- or S-waves, but often cause more damage than body waves because they travel along the Earth’s surface and have a greater effect on buildings.

Seismologists can determine the epicenter of an earthquake by noting the times that seismic waves arrive at three or more different seismometers. Multiplication of the wave velocity by the travel time gives the distance to the epicenter, which is the radius of a circle with its center at the seismometer. The radii from at least three circles will intersect at a point that is Earthquake epicenter. In practice, seismologists first make a rough estimate of the epicenter and then refine their estimate as additional data become available, for example by using velocities corresponding to specific rock types rather than a general estimate.

Collapse of buildings

To construct a house or building under static conditions, the materials need only to be stacked up, attached to each other, and balanced. These kinds of buildings are not designed to accelerate rapidly and change directions like cars or airplanes. Buildings in seismically active areas, however, must be designed and built to withstand the dynamic acceleration that can occur during an earthquake. Large buildings and structures such as bridges, in particular, must be designed so that vibrations arising from earthquakes are damped and not amplified.

Because noticeable earthquakes are rare in most areas, people may not recognize that the objects and buildings around them represent potential hazards. It is not movement of the ground surface alone that kills people. Instead, deaths from earthquakes result from the collapse of buildings and falling objects in them, fires, and tsunamis. The type of construction that causes the most fatal injuries in earthquakes is unrein-forced brick, stone, or concrete buildings that tend not to be flexible and to collapse when shaken.

The most earthquake-resistant type of home is a low wooden structure that is anchored to its foundation and sheathed with thick plywood. Some of the traditional architecture of Japan approximates this shock-resistant design, including wooden buildings that are more than a thousand years old. Unfortunately, wood and paper houses can be easily ignited in the fires that are common after large earthquakes. Both unreinforced masonry and shock-resistant wood houses are used by different cultures in areas of high earthquake risk.

Active faults lie under many parts of the world that do not commonly experience earthquakes. The crust under such places as Italy, California, and Central America moves often enough that an earthquake there, although still unpredictable, is not entirely unexpected. But other populated areas, such as the East Coast and Mississippi Valley in the United States, periodically experience earthquakes just as big as those in any earthquake-prone part of the world, although far less frequently.

Earthquake-triggered landslide

Earthquakes can trigger landslides and rock falls many miles (kilometers) from their epicenters. Local governments can enact zoning regulations to prevent development in areas susceptible to landslides during earthquakes or heavy rainstorms. In other cases, potentially hazardous slopes can be excavated and regarded into a configuration that is able to resist the destabilizing effects of a large earthquake.

Seismically-triggered landslides can reshape the landscape. In 1959, an earthquake triggered a landslide that dammed the Madison River in Montana and created Hebgen Lake. To prevent this natural dam from washing out and causing catastrophic floods, the U.S. Army Corps of Engineers built an emergency spillway through the landslide material. This enabled them to control the release of the water from the new lake. Prehistoric landslides have dammed the Columbia River and could be the source of a legend of the Northwest Indians. In this legend, tribes walked across the Columbia River on a bridge of land to meet each other.

Liquefaction of soil

Seismic shaking can transform water-saturated sand into a liquid mass that will not support heavy loads such as buildings. This phenomenon, called liquefaction, causes much of the destruction associated with some earthquakes. Mexico City, for example, rests on the ancient lakebed of Lake Texcoco, which is a large basin filled with liquefiable sand and ground water. In the Mexico City earthquake of 1985, the wet sand beneath tall buildings liquefied and most of the 10, 000 people who died were in buildings that collapsed as their foundations sank into liquefied sand.

Jets of sand sometimes erupt from the ground during an earthquake. These sand geysers or mud volcanoes occur when formations of soft, wet sand is liquefied and forcefully squeezed up through cracks in the ground. Despite these names, they have no relation to real geysers or volcanoes. Although they generally cause little damage, they are indications that more widespread liquefaction may have occurred or may be possible in the next earthquake.

Subsidence

Earthquakes can cause affected areas to increase or decrease in elevation by several feet, which can in turn lead to flooding in coastal areas. Port Royal, on the south shore of Jamaica, subsided several feet in an earthquake in 1692 and suddenly disappeared as the sea rushed into the new depression. Eyewitnesses recounted the seismic destruction of the infamous pirate anchorage as follows: “… in the space of three minutes, Port-Royall, the fairest town of all the English plantations, exceeding of its riches, … was shaken and shattered to pieces, sunk into and covered, for the greater part by the sea. The earth heaved and swelled like the rolling billows, and in many places Earth crack’d, open’d and shut, with a motion quick and fast. in some of these people were swallowed up, in others they were caught by the middle, and pressed to death. The whole was attended with. the noise of falling mountains at a distance, while the sky. was turned dull and reddish, like a glowing oven.” Ships arriving later in the day found a small shattered remnant of the city that was still above the water. Charts of the Jamaican coast soon appeared printed with the words “Port Royall Sunk.” During the New Madrid (Missouri) earthquake of 1811, a large area of land subsided around the bed of the Mississippi River in west Tennessee and Kentucky. The Mississippi was observed to flow backwards as it filled the new depression and created what is now known as Reelfoot Lake. The last great earthquake in the U.S. Pacific Northwest occurred two years before Port Royal sank in 1690. In the 300 years since then, no major earthquake has released the potential energy that has been building under the crust. Geologists have found buried forests and deposits indicating that coastal areas were periodically flooded, probably as the result of major earthquakes.

Tsunamis

An earthquake can create a large wave known as a tsunami (the Japanese term) or, seismic sea wave. A tsunami is barely detectable as it moves through deep water. Where the ocean becomes shallow near the shore, however, the fast-moving tsunami becomes a large wave that rises out of the sea and strikes the shore with unstoppable force. In a small, mountain-ringed bay, a tsunami can rush hundreds of meters up a sea-facing mountainside. A wall of water forms when a large tsunami enters a shallow bay or estuary, and it can move upriver for many miles. Sometimes tsunamis are mistakenly referred to as tidal waves, because they resemble a tide-related wave called a tidal bore.

The most destructive tsunamis in history have killed tens of thousands of people, many of them located great distances from Earthquake epicenter. The tsunami produced by a 1946 earthquake in the Aleutian Islands, Alaska, killed a total of 165 people. Of that number, 159 were in Hawaii, 5 were in Alaska, and 1 was in California. Coastal towns affected by tsunamis often have no topographic barriers between them and the sea and had no warning of the impending disaster. Building a breakwater to divert a tsunami and expend its energy is sometimes an option for otherwise unprotected coastal towns.

In 2004, a 9.4 magnitude earthquake under the Indian Ocean caused large tsunamis to go ashore in India, Indonesia, and elsewhere in the region. Over a quarter of million people were killed. Most could have

KEY TERMS

Active fault— A fault where movement has been known to occur in recent geologic time.

Aftershock— A subsequent earthquake (usually smaller in magnitude) following a powerful earthquake that originates at or near the same place.

Epicenter— The location where the seismic waves of an earthquake first appear on the surface, usually almost directly above the focus.

Fault— A fracture in Earth’s crust accompanied by a displacement of one side relative to the other.

Focus— The location of the seismic event deep within Earth’s crust that causes an earthquake. Also called Earthquake’s hypocenter.

Foreshock— A small earthquake or tremor that precedes a larger earthquake shock.

Modified Mercalli scale— A scale used to evaluate earthquake intensity based on effects felt and observed by people during Earthquake.

Richter scale— A scale used to compare earthquakes based on the energy released by Earthquake.

Seismic wave— A disturbance produced by compression or distortion on or within Earth, which propagates through Earth materials; a seismic wave may be produced by natural (e.g., earthquakes) or artificial (e.g., explosions) means. P waves, S waves, and surface waves are vibrations in rock and soil that transfer the force of Earthquake from the focus into the surrounding area.

Subsidence— A sinking or lowering of Earth’s surface.

been saved by retreating to higher ground if they had had warning, or understood that withdrawal of water to an unusual. In response, a regional warning system has been installed. As of late 2006, its effectiveness had yet to be tested after a major earthquake.

Secondary hazards: fire, disease, famine

Cities depend on networks of lifeline structures to distribute water, power, and food and to remove sewage and waste. These networks, whether power lines, water mains, or roads, are easily damaged by earthquakes. Elevated freeways collapse readily, as demonstrated by a section of the San Francisco Bay Bridge in 1989 and the National Highway Number 2 in Kobe, Japan, in 1995. The combination of several networks breaking down at once multiplies the hazard to lives and property. Live power lines fall into water from broken water mains, creating an electric shock hazard. Fires may start at ruptured gas mains or chemical storage tanks, but many areas may not be accessible to fire trucks and other emergency vehicles. Even if areas are accessible, there may not be water for fire-fighting. The great fire that swept San Francisco in 1906 could not be stopped by regular firefighting methods and entire blocks of buildings had to be demolished to halt the fire. Most of the 143, 000 people killed in Tokyo and Yokohama because of the 1923 Kwanto perished in fires.

Famine and epidemic disease can quickly strike large displaced populations deprived of their usual food distribution system, sanitation services, and clean water. Furthermore, collapsed hospitals may be of no use to a stricken community that urgently needs medical services. After an earthquake, relief operations commonly offer inoculation against infectious diseases. In countries that do not have sufficient organization, trained personnel, or resources to handle an earthquake-generated refugee population, more people may die of secondary causes than the direct effects of seismic shaking. Even in the most prepared countries, the disruption of networks may prevent relief operations from working as planned. In the aftermath of the January 1995, earthquake in Kobe, Japan, plans for emergency relief made before the disaster did not work as well as planned. Local residents, wary of the danger of aftershocks, had to live outdoors in winter without food, water, or power.

Historical incidence of earthquakes

Catastrophic earthquakes happened just as often in the past as they do today. Earthquakes shattered stone-walled cities in the ancient world, sometimes hastening the demise of civilizations. Knossos, Chattusas, and Mycenae, ancient capitals of countries located in tectonically active mountain ranges, fell to pieces and were eventually deserted. Scribes have documented earthquakes in the chronicles of ancient realms. An earthquake is recorded in the Book of Zachariah, and the apostle Paul wrote that he got out of jail when the building fell apart around him in an earthquake. In the years before international news services, few people heard about distant earthquakes. Only a few handwritten accounts have survived, giving us limited knowledge of earthquakes in antiquity. Because of limited and lost data, earthquakes may seem to have been less common in ancient times. In China, home of the first seismometer, the Imperial government has recorded earthquakes for over a thousand years. Their frequency has not changed through the ages.

See also Continental drift; Mass wasting; Plate tectonics.

Resources

BOOKS

Hough, Susan Elizabeth. Earthshaking Science: What We Know (and Don’t Know) about Earthquakes. Princeton, NJ: Princeton University Press, 2004.

Hough, Susan Elizabeth. Richter’s Scale: Measure of an Earthquake, Measure of a Man. Princeton, NJ: Princeton University Press, 2006.

PERIODICALS

Cochran, Elizabeth S., et al. “Earth Tides Can Trigger Shallow Thrust Fault Earthquakes.” Nature. 306 (2004): 1164-1166.

Hirn, Alfred and Mireille Laigle. “Silent Heralds of Megathrust Earthquakes?” Nature. 305 (2004): 1917-1918.

OTHER

Spall, Henry. “NEIC: An Interview with Charles F. Richter.” July 8, 2002. <http://neic.usgs.gov/neis/seismology/people/int_richter.html> (November 8, 2002).

U.S. Geological Survey. “Earthquake Hazards Program.” 2006. <http://earthquake.usgs.gov/> (accessed October 25, 2006).

Bill Hanneberg