Earthquake distribution

Seismologists have been monitoring global earthquake activity for approximately one century. These studies have led to an understanding of earthquake frequency and distribution that have contributed dramatically to the confirmation of plate tectonics theory.

Plate Tectonics

Although seismic instruments can record them from virtually anywhere on the globe, earthquakes occur primarily along active tectonic regions of the earth's crust where mountain building, folding, and faulting are occurring. More temporal and often less severe earthquakes also accompany volcanic activity.

The key to understanding earthquakes lies in the powerful theory of plate tectonics. The earth is far from a geologically “dead” world like its moon. The earth's crust is broken into several large slabs of crust, or lithospheric plates, and convection currents caused by the planet's internal heat drive the plates into motion, like a bunch of small rafts crowded onto the surface of a boiling pot of viscous jelly. At the mid-ocean ridges, new sea floor is created by magmatic eruptions—pushing two plates away from each other. These divergent boundaries are characterized by the Mid-Atlantic Ridge, where the North American and European continents (riding on the lithospheric plates) are moving away from each other. Along the Alpine belt, two continents are literally crashing into each other as Africa is pushing into and subducting under the Eurasian plate. The subduction zone is marked by a complex series of transform and thrust faults, which give the region its high seismicity.

Earthquakes are predominantly distributed along plate boundaries. Another converging boundary is found along the Hindu Kush and Himalayan mountain ranges, where the subcontinent of India is slowly crashing into and thrusting under the huge Eurasian plate at the rate of some 5 centimeters per year. This ongoing collision has created the world's highest mountains and makes this area of the world an earthquake-prone region.

Seismic Belts

By mapping earthquake epicenters, scientists are able to map the seismicity (earthquake activity as a function of time) of the planet. Most earthquakes occur along three main belts: the Mid-Atlantic Ridge system; the Alpine Tethys belt, which extends from the Mediterranean Sea through Turkey and Armenia all the way into Asia, where it merges with the third main belt; and the infamous circum-Pacific “Ring of Fire.” The least threatening of these is the mid-ocean ridge system such as that found in the Atlantic Ocean, along which new ocean crust is being created. As the sea floor spreads from the volcanic activity occurring at the spreading ridges, earthquakes occur along transform faults that bound the offset ridges. Although population centers are sparse along the mid-ocean ridges, Iceland, the Azores, and other small mid-Atlantic islands are regions of potential quake hazard. Owing to the steady rate of spreading at a few centimeters annually, earthquakes occurring along the ridge offsets tend to be frequent and of relatively low magnitude.

A far more dangerous region of earthquake activity is the Alpine Tethys belt, which extends across southern Europe and Asia. A listing of only a few of the major earthquakes along this belt reads like a litany of destruction and human suffering: Persia in 1505; Calabria in southern Italy in 1509, 1783, and 1832; Lisbon in 1755; the Neapolitan in Italy in 1857; numerous quakes in recent decades in Yugoslavia, Romania, Greece, and Turkey; and the 1988 disaster in Soviet Armenia. Volcanic activity also occurs in the region, with notable examples including Mounts Etna and Vesuvius and the island of Thera.

Perhaps the most seismically active region of the world lies on the eastern end of the Mediterranean-Himalayan belt. Stretching across Tibet and into China, this colossal zone of high seismicity threatens all who live along its 4,000-kilometer length. More than a dozen earthquakes of Richter magnitude 8.0 or greater have caused well in excess of a million human lives to be lost in this notoriously seismic region. It is believed that the Shaanxi region earthquake of 1556, which resulted in an estimated 830,000 casualties, was one of the worst earthquakes in history. The Gansu earthquake of 1920 caused approximately 200,000 casualties.

“Ring of Fire”

The trans-Asian earthquake belt passes through Burma and Indonesia, ending in the southern Philippines. This transitional region marks the border of the earth's greatest seismic belt—the circum-Pacific, or “Ring of Fire.” A region of complex plate interactions, the Pacific Rim is no stranger to earthquakes and volcanoes. Perhaps no region characterizes the circum-Pacific belt better than the islands of Japan. In geologic terms, the Japanese islands are an island arc, formed by a subduction zone off the coast of the country's landmass. As the sea floor spreads from the ridge systems, it collides with the Asian continental plate. The dense, water-soaked sea floor is subducted at a deep ocean trench. As the oceanic plate descends, the slab grinds and shudders in resistance before finally being swallowed by the mantle. Accounting for 90 percent of the world's earthquakes, trench subduction zones have a seismic fingerprint of ever-deepening quakes that can cause very severe shocks.

Sea floor earthquakes can cause tsunamis (sometimes incorrectly called “tidal waves”) along the coastline. A 7.7-magnitude shock hit the Oga Peninsula in 1983 and brought on a tsunami that caused extensive damage. The Great Tokyo earthquake (magnitude 8.2) and fire of 1923 caused 143,000 casualties. The Fukui earthquake of 1948 killed more than 3,700 people. The 9.2-magnitude Indian Ocean earthquake on December 26, 2004, off the coast of Sumatra, produced a tsunami that killed more than 230,000 people in 14 countries. The Great East Japan Earthquake of 2011 and the tsunami it caused resulted in widespread destruction throughout northeastern Japan and killed an estimated 20,400 people. All of these events serve as stark testimonies to the danger of living near active plate subduction zones. In addition to the trench quakes, the volcanic islands are crisscrossed by numerous faults. Thousands of quakes have been recorded by Japanese historians, dating to well before the birth of Christ.

To the north and east along the circum-Pacific belt, the Aleutian Island arc reaches into the North American continent in Alaska. A complex system of faults and a subduction zone off the coast make Alaska a region of dangerous seismicity. In 1964, one of the most severe earthquakes ever recorded (magnitude 8.6) struck near the port of Valdez and generated a series of tsunamis that wracked the coast. On Alaska's southern coast is the Fairweather fault, a transform fracture on which a 1958 temblor shook loose 90 million tons of rock, which cascaded into Lituya Bay, raising a wave exceeding 500 meters in height. The Fairweather fault is a northern extension of a system of transform faults (so named because the fault is transformed into a ridge or trench at its ends) that includes the most famous of all—the San Andreas fault.

San Andreas and Alpine Faults

Extending from the Mendocino fracture zone 700 miles south to Mexico, the San Andreas and its attendant system of faults make the state of California an earthquake-prone territory. A unique type of plate boundary, the San Andreas represents a fracture line along which the oceanic Pacific plate is slowly but inexorably sliding north with respect to the North American continent at a rate of roughly 5 centimeters per year. In places where the fault is displacing smoothly, small tremors regularly shake the landscape. Yet, in regions where the fault is believed locked, major quakes are impending, placing the large population centers of San Francisco and Los Angeles at risk. On October 17, 1989, a strong earthquake shook the San Francisco Bay area, causing extensive damage, 63 deaths, and more than 3,750 injuries. In addition to the San Andreas, the Hayward fault in the northern California and a multitude of faults in southern California further increase the state's seismicity. The Whittier, San Fernando, and Inglewood-Newport faults are among those that threaten the city of Los Angeles.

As seismically active as California is, its seismicity pales by comparison with Latin America. The 1985 Mexico City earthquake (magnitude 8.1) was a grim reminder of the subduction zone off the coast of western Mexico. Central America is in a particularly precarious position. It is sandwiched between the Cocos, North American, South American, and Caribbean plates, and, therefore, is a zone of active volcanoes and numerous severe, deep-trench earthquakes. El Salvador, Guatemala, and Nicaragua are among the nations in greatest earthquake danger. As the Nazca plate bumps into South America's plate, the oceanic plate is subducted and the melting slab has caused the volcanism and massive uplifting of the towering Andes mountain range. While the eastern part of South America is seismically quiet, the west coastal regions of Peru and Chile are known to experience severe quakes.

The circum-Pacific belt continues along the South Pacific through New Zealand. Analogous to the San Andreas region, New Zealand is regularly shaken by tremors occurring along the Alpine fault. Continuing up through Indonesia, the ring of earthquake and volcanic activity completes its loop.

Volcanic Activity

Other earthquake regions of note include the American Northwest's Cascade volcanic chain, where a series of tremors often indicate pending eruptions. The 1980 explosive eruption of Mount St. Helens in Washington was caused by an earthquake that triggered a landslide, initiating the lateral blast, or nuée ardente.

In California, the San Andreas is not the only region of earthquake activity. An area with an explosive volcanic past in recent geologic times, the Mammoth-Mono Lake region was hit by four magnitude 6.0 temblors in 1980, occurring along the northern perimeter of the Long Valley caldera. South of this site, the Sierra Nevada mountain range continues to undergo periodic spasms of uplift, like the one that caused the Owens Valley quake of 1852.

In the Caribbean, earthquakes and volcanic activity are an ever-present threat along the borders of the Caribbean plate. This seismic belt is actually an extension of the Pacific belt, although it lies on the Atlantic side of the Americas. Examples of sites experiencing major shocks and activity include Port Royale, which plunged 50 feet underwater following a major earthquake in 1692, and Mount Pelée, which destroyed the town of Martinique with a nuée ardente in 1902. Regions of hot spot volcanism are also zones of especially high seismicity. The Hawaiian Islands lie on top of a mantle plume of magma, and the same forces that built the island chain are working on the main island of Hawaii today. A similar region lies beneath the North American continent, the site of Yellowstone National Park in Wyoming, Montana, and Idaho. Both Hawaii and Yellowstone are earthquake-prone regions.

Continental Interiors

For the most part, continental interiors, especially the Precambrian metamorphic basement and its thin veneer of sedimentary rocks that make up the craton, are regions of low seismicity. Such regions include parts of the United States and Canada in the Great Lakes region, virtually all of South America except for the Pacific coast and Andes belt, and most of Africa.

While continental interiors are seismically quiet compared with the active plate margins, there are exceptional regions. Although most earthquakes in the United States take place west of the Rockies, the Mississippi Valley has been the site of some of the most severe earthquakes ever, the New Madrid quakes of 1811-1812. New England and South Carolina have also experienced powerful shocks in the past. The western two-thirds of Africa is seismically inactive, but the East African rift valley is a zone of earthquake and volcanic activity where a geologically new plate boundary is rifting the eastern edge of the continent apart. Curiously, the only continent on earth that is seismically quiet is Antarctica.

Early Records

In a sense, the study of where earthquakes occur traces back to the roots of western and eastern culture roughly four thousand years ago in Mesopotamia and in Asia. The Bible's Old Testament and other ancient Middle Eastern documents are filled with accounts of earthquakes toppling cities. The most complete historical records of seismic activity are, appropriately enough, those of Japan and China. Chinese earthquake records date back thirty centuries, with exhaustive accounts of tragic earthquakes striking the Asian mainland. Japan, which experiences up to one thousand noticeable shocks per year, has been keeping detailed earthquake records on Tokyo's tremors since 818 CE.

Modern scientific views on earthquake distribution perhaps began in response to the tragic All Saints Day earthquake and tsunamis that wrecked Lisbon in 1755. While a horrified western civilization reeled at the scope of the disaster, which killed many at Lisbon's numerous downtown churches, one of the more insightful minds of the scientific revolution looked at the event more objectively. Immanuel Kant advised that learning about where and why earthquakes occur was a more reasoned approach than blaming the disaster on divine causes.

Global Seismic Study

Some one hundred years after Lisbon's fateful quake, Irishman Robert Mallet published a study of the Neapolitan earthquake of 1857 in which he produced a seismographic map of the world that, with the exception of the mid-ocean ridge systems, remains an accurate seismic diagram. Teaching in seismically active Japan, British geology professor John Milne invented the modern seismograph. By the time of the 1906 quake in San Francisco, global seismic observatories were in place; they recorded the jolts in California. As the distance to an epicenter (the surface site above the actual fracture or focus of the quake) could now be determined, a set of three properly placed seismographs, Milne reasoned, could pinpoint an epicenter anywhere on the globe.

Seismic waves generated by an earthquake produce different types of waves. The primary wave, or P wave, is compressional, while the secondary or S wave is sheering as it travels through the earth. Quakes also produce surface waves, which cause the shaking motions that occur during the most destructive part of a seismic event. By measuring the ratio of the arrival time of the primary and secondary waves and the size or amplitude of the waves on the seismograph recording, Charles Richter, in 1935, was able to establish a scale for measuring the energy released in a quake, its magnitude. Richter and his colleague at the California Institute of Technology, Beno Gutenberg, published high-quality maps of worldwide earthquake distribution in 1954.

In the 1950s and 1960s, the United States helped to organize enough of the world's seismic observatories to establish a global monitoring network called the World Wide Standardized Seismograph Network. Data from decades of shocks recorded by the network and oceanographic research vessels led to the revolutionary theory of plate tectonics and its acceptance by the vast majority of earth scientists.

Identifying Regions of Seismic Hazard

Although plate tectonics theory and plate boundaries are invoked to explain the vast majority of earthquakes, scientists are still puzzled by earthquakes that occur far from active margins. Examples are the Mississippi Valley region and Charleston, South Carolina, which shook violently in 1886. In these regions, seismologists are alarmed by public perception that the land east of the Rockies is “solid bedrock.” The most plausible cause of the Mississippi Valley quake activity is the enormous weight of sediments the great river system has deposited on a weak part of the continental crust. South Carolina is a region riddled with faults, yet it is far from an active plate margin. Seismologists use historical accounts and recent seismic records to predict regions of earthquake hazard. One theory of seismic hazard involves identifying regions where earthquakes have not occurred along active fault regions. Such seismically quiet regions are called “seismic gaps” and represent regions of accumulated strain along which a major rupture may be anticipated.

Using seismicity data, seismologists produce maps that indicate seismic hazard. Not only useful for scientific purposes, such maps help public agencies to create building codes and other earthquake prevention methods appropriate to the earthquake hazards of the region. Studies conducted by the United States Geologic Survey and the National Oceanic and Atmospheric Administration have concluded that the areas of San Francisco and Los Angeles, California; Salt Lake City and Ogden, Utah; Puget Sound, Washington; Hawaii; St. Louis-Memphis, Tennessee; Anchorage and Fairbanks, Alaska; Boston, Massachusetts; Buffalo, New York; and Charleston, South Carolina, are at greatest seismic risk in the United States. The worldwide network of seismic stations make digital records which enable computers to analyze the seismic waves more swiftly, thereby improving observation of the planet's moving plates.

Dangers to Densely Populated Regions

Approximately once every 30 seconds, a million times each year, the earth's crust shivers. Most of these tremors are perceptible only by sensitive instruments, but more than three thousand are strong enough to be felt by those nearby. Roughly twenty quakes a year are strong enough to do catastrophic damage to populated areas. By coincidence, some of the earth's most active seismic regions are also among its most densely populated. The lands bordering the Mediterranean Sea and Pacific Rim, the mountainous Middle East, India, China, and Japan are all familiar with the havoc of a major shock. In China alone, the death toll from earthquakes in recorded history exceeds 13 million.

All told, millions of people have been killed by seismic activity, with untold loss of property through the half-tick of geologic time comprising human history. Seismologists still lack the capability of precise prediction of earthquakes, but areas of high seismicity and the ominous seismic gaps warn of quake hazard. In cities such as Tokyo, Los Angeles, San Francisco, and Anchorage, citizens must be prepared for the next big quake, which is as sure to come as the slow but steady movement of the earth's crustal plates continues.

Principal Terms

epicenter: the point on the earth's surface directly above an earthquake's focus

focus: also known as the hypocenter, the actual place of rupture inside the earth's crust

P wave: the primary or fastest wave traveling away from a seismic event through the rock, consisting of a series of compressions and expansions of the earth material

plate boundary: a region where the earth's crustal plates meet, as a converging (subduction zone), diverging (mid-ocean ridge), transform fault, or collisional interaction

S wave: the secondary seismic wave, traveling more slowly than the P wave and consisting of elastic vibrations transverse to the direction of travel; S waves cannot propagate in a liquid medium

seismic belt: a region of relatively high seismicity, globally distributed; seismic belts mark regions of plate interactions

seismic wave: an elastic wave in the earth usually generated by an earthquake source or explosion

seismicity: the occurrence of earthquakes as a function of location and time

seismograph: an instrument used for recording the motions of the earth's surface caused by seismic waves, as a function of time

subduction zone: a dipping ocean plate descending into the earth away from an ocean trench

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