Slow earthquakes

The earth's outer shell, or crust, is composed of huge blocks called plates that regularly move small distances, usually several inches each year, causing them to collide with other plates. Slow earthquakes—the movements of the earth that result from small plate movements—are barely felt but occur regularly in some areas.

Kinds of Earthquakes

Most people who think of earthquakes immediately visualize huge, destructive, earth-shattering movements of the earth's surface that last only seconds but that bring down buildings, rupture gas and water mains, crush people under tons of rubble, and often are followed by fires. When such movements occur, newspaper headlines are filled with statistics about the amount of damage they have done and about the numbers feared dead.

Among the most severe earthquakes in modern history are the one in the Kansu Province of China in 1920, which killed 180,000 people; the Japanese earthquake of 1923, which killed some 143,000 people in Tokyo and Yokohama; the 1935 earthquake in Quetta, India, which killed more than 60,000 people; the 1970 earthquake in Peru, which killed more than 60,000 people; and the colossal 1976 great Tangshan earthquake in the Hebei Province of China, which killed 240,000 people. Severe earthquakes that occur in heavily populated areas result in heavy casualties. This is particularly true of those that strike less affluent countries in which buildings are often badly constructed, resulting in their collapse when the earth shakes violently.

Another type of earthquake, often as severe as those felt on land, are deep earthquakes beneath the ocean's surface. The ocean floor has been drastically changed by such earthquakes. If they result in casualties on land, it is usually from the tsunamis, or enormous waves, that they generate. These waves can attain heights of more than 30 meters. When they hit developed and heavily populated areas of a shoreline, they can crush everything in their paths and leave behind incredible destruction and thousands of casualties.

Slow earthquakes, also called silent or quiet earthquakes, are undramatic and receive no headlines in the press. A slow earthquake is a discontinuous event that releases energy over a period of hours to months, rather than the seconds to minutes of a typical earthquake. They occur with considerable frequency, although most people are unaware of their existence even if they are in an area where considerable seismic activity is taking place. Seismographs may record their occurrence, but few people feel threatened by slow earthquakes because they do not cause the earth to tremble, books to fall from library shelves, or walls of canned goods to fall on the floors of supermarkets. Their destructive force is cumulative; it takes place over substantial periods of time, causing such minute changes on a day-to-day basis that these changes are not apparent to the naked eye.

Structure of the Earth

Regardless of which kind of earthquake one is considering, all earthquakes have similar underlying causes. To understand these causes, one must consider how the earth is constructed. The planet is composed of three basic parts. The one with which people are most familiar is the crust, the outer layer that, below the surface, consists of solid rock. The crust has an average thickness of about 32 kilometers beneath the earth's seven continents, although it is considerably thinner beneath the sea, where its thickness averages about 5 kilometers.

Underlying the crust is the mantle. It, like the crust, is composed of solid rock, but this rock is extremely hot. The mantle is thick, extending in many places more than 3,000 kilometers below the crust. Its rigid upper portion is called the lithosphere. Beneath it is a weaker area of the mantle, the asthenosphere, which, being closer to the earth's molten core, is much hotter than the lithosphere. The lithosphere may be nearly 100 kilometers thick beneath the continents and some oceanic areas, but it shrinks to just 8 to 10 kilometers in thickness beneath submerged ridges in mid-ocean.

Inward toward the earth's center from the lithosphere and asthenosphere are the two major parts of the earth's core, the liquid core and the solid core. The liquid core has a radius of about 2,300 kilometers and consists of molten iron and nickel whose temperature averages about 5,000 degrees Celsius. The solid core, which is at the earth's very center, has a radius of just under 1,300 kilometers and is composed of solid iron and nickel.

Ancient people had various quaint explanations about what caused earthquakes. The ancient Greeks thought that the titan Atlas carried the world on his shoulders and that every time he shifted the weight of this great burden, the earth moved, causing earthquakes. In India, people conceived of the earth as an object balanced on the head of an elephant riding on the back of a huge tortoise. Whenever either animal moved, an earthquake resulted. Other theories viewed the world as being carried by giant catfish, whales, or oversized gods who rode in sleds pulled by dogs. In the past, many religions viewed earthquakes as expressions of God's anger and punishment for humankind's transgressions.

Modern science has been slow to offer rational physical explanations for earthquakes. English geologist John Michell, in 1760, suggested that they are caused by the movement of subterranean rocks. Few accepted this explanation, and those who did thought that such movement was caused by gigantic explosions deep inside the earth. It took another hundred years before Robert Mallet, an engineer from Ireland, contended, in 1859, that the causes of earthquakes were strains in the earth's crust. After 1960, most seismologists accepted the theory of plate tectonics as the cause of earthquakes.

Plate Tectonics

According to the theory of plate tectonics, which has gained wide acceptance, the earth was once a solid landmass surrounded by a great sea. The planet began to cool after its fiery formation some 5 billion years ago. As it cooled, its surface cracked. Over hundreds of millions of years, parts of the once-solid landmass drifted away, forming seven large and twelve small islands, all with ragged edges. The large islands are the earth's seven continents. These islands, or tectonic plates, float and are in constant but often quite limited motion. For example, the two plates that exist on the western part of the North American continent, the Pacific plate and the North American plate, hardly move at all. The Pacific plate drifts north at the barely perceptible rate of about 5 centimeters per year. The North American plate drifts southwest at a similar rate.

Despite the slow movement of the North American plate, it has been estimated that over hundreds of millions of years, the North American continent could, through continental drift, collide with Australia. Between the Pacific and North American plates lies the San Andreas fault, a gash in the earth's surface that runs more than one-half the length of California. The movement of these plates results in slow or silent earthquakes.

When the edges of the two plates collide, however, as they did in the San Francisco earthquake of 1906 and the Northridge earthquake of 1994, the result is a major earthquake. The Northridge quake, which registered 6.8 on the Richter scale, revealed the existence of a hidden thrust fault and a horizontal fault that had previously gone undetected. The Northridge earthquake was followed by more than one thousand aftershocks as the earth beneath the area resettled. Many slow earthquakes preceded the Northridge disaster as foreshocks and followed it as aftershocks. Slow earthquakes often presage the coming of major earthquakes. As increased knowledge about slow earthquakes evolves, seismologists are beginning to understand more fully how to interpret the often subtle signals they send. The interpretation of these signals can help predict future deep earthquakes.

Signs of impending earthquakes exist—probably quite often in the form of slow earthquakes—that cause animals to react in anticipation of severe, deep earthquakes. The behavior of animals in zoos in the hours preceding a severe earthquake shows clear signs that they are disquieted and sense something, perhaps minute subterranean vibrations, that humans are not able to perceive. The fields of plate tectonics and seismology are becoming more and more sophisticated as technology produces ever more sensitive instruments for detecting seismic activity.

Types of Slow Earthquakes

Most people think of earthquakes as cataclysms in which a crack breaks through the earth's crust at a speed of several kilometers per second, causing a violent shaking of the ground, severe damage to structures, and injury to living things in the quake zone. In many parts of the world, however, the development of a crack along a fault line occurs at a speed of less than 1 meter per second, with some slips even measured in millimeters per year. Three faults in California—the Hayward, San Andreas, and Calaveras faults—demonstrate the great variety of seismic activity, ranging from the ordinary earthquakes that occur from rapidly developing breaks in the earth's crust that suddenly release waves of stored elastic-strain energy, to a variety of smaller tremors.

Among the types of earthquakes that geologists and seismologists have discovered and named are slow earthquakes, defined as having speeds of hundreds of feet per second; silent earthquakes, defined as having speeds of tens of feet per second; strain migration events, measured at speeds of centimeters per second; and creeping earthquakes, with speeds measured in millimeters per second. These varieties are not always measurable on typical seismographs. Few of them attract attention as they are taking place.

Slow earthquakes can, at times, cause rapid ruptures that produce high-frequency sound waves, but more often they take a much longer time to rupture through the earth's crust than ordinary earthquakes of comparable magnitude. Some slow earthquakes occur in oceanic transform faults, as happened on June 6, 1960, in the Chilean transform fault, which ruptured for about one hour as a series of small, barely detectable breaches.

Silent earthquakes have been so named because they are never accompanied by the high-frequency sound waves that most seismographs need to register seismic events. Some researchers have employed delicate instruments that measure tectonic strain to detect silent earthquakes. Such instruments also have revealed creeping movement of about 10 millimeters per second in parts of California's San Andreas fault. The low-frequency waves of a silent earthquake moving about 0.3 meters per minute were recorded shortly before the 1976 earthquake in Fruili, Italy, and again in 1983 before a severe earthquake hit the Japan Sea. Seismologists think that the occurrence of some slow and some silent earthquakes may be warning signs that, if heeded, could prevent substantial loss of life when an ensuing ordinary earthquake is on the brink of shattering a region. The stick-slip earthquake, with its jerky, sliding motion at a fault, usually comes after a slip with propagation speeds of 20 to 200 meters per second. Not all such slips can be detected on the typical seismographs that most geophysicists and seismologists use. Close to the earthquake, silent earthquakes can be recorded geodetically and by using strainmeters. Only digital, broadband seismographs are able to record seismic waves of such low frequencies.

Pacific Rim nations are shaken yearly by thousands of tiny earthquakes resulting from the collision of oceanic and continental plates. Some of these are slow or silent earthquakes that relieve the pressure that is built up in subterranean rocks when one tectonic plate rams into another. Severe earthquakes are the result of several years of pressure buildup, but slow and silent quakes relieve the pressure more gradually and possibly act as the safety valves that prevent the earth from experiencing more numerous major earthquakes than it does.

Slow Creep at Work

Hollister, California, about 160 kilometers southeast of San Francisco, lies close to the San Andreas fault. Studies of major fault lines near Hollister have revealed a gradual slippage beneath the earth's surface, although it has not been possible to measure this slippage with total accuracy. A winery constructed in Hollister in 1939 is located almost on top of the San Andreas fault. In 1956, the winery began to experience damage that could not be easily explained but that could no longer be ignored. Strong reinforced concrete walls and floors in one of the warehouses were gradually crumbling. None of the local people remembered any overt seismic action that could explain the phenomenon.

Finally, because of the winery's location close to the known fault line, the owners engaged seismologists to assess the situation. They found that an active branch of the fault zone ran directly below the building. They discovered that the two sides of this fault line were moving past each other at an estimated rate of 1.3 centimeters every year. Although such movement does not attract immediate attention, over fifty years the distance involved is more than 0.6 meter, which causes damage readily observable by anyone who looks at the building.

As such movement continues, structures are weakened and are finally felled by it. Because the San Andreas is a right-lateral fault, the winery's west side was steadily moving north of its east side. When cracks appeared in the floor and walls, they were patched up. Sagging walls were reinforced. Since 1956, the situation has been monitored carefully. It has been determined that the creep continues, as it surely will do in the foreseeable future.

This sort of creep is related to slow earthquakes in that it does not involve a dramatic underground upheaval that happens in a matter of minutes, although creep is thought not to be entirely gradual. It often occurs in a matter of seven to ten days at a time, after which there is a period of quiescence for weeks or months. It can, however, continue for decades and be barely detected in areas that are neither built up nor heavily populated. In 1960, recorders clocked an earthquake in Hollister in which instant creep of about 0.3 centimeter occurred. The frequency with which earthquakes occur may have an influence on periods in which seismic activity takes place. An earthquake in the area near Hollister in 1939 separated the winery's adobe walls from side walls and pulled girders from their brick moorings. Rents appeared in the ground around the winery. A major jolt in 1960 severely shook the winery, causing damage to it.

The slow creep beneath the winery and in other areas in the Hollister area is under the constant scrutiny of seismologists, who are trying to determine why slow creep occurs along some areas of the fault but is not observed in other nearby areas. A tunnel for the Los Angeles Aqueduct that was constructed in 1911 and crosses the San Andreas fault has remained intact for its entire existence, with no signs of seismic activity.

Principal Terms

asthenosphere: flexible rock in layers beneath the earth's brittle crust

fault: a deep fissure in the earth's surface along which rock moves

Richter scale: one of several scales used to measure an earthquake's magnitude

seismic wave: a wave of energy released during an earthquake

seismologist: a scientist, often a geophysicist, who specializes in studying earthquakes

tectonic plate: any one of about ten enormous pieces that form the earth's outer layer

Bibliography

Beroza, G. C., and T. H. Jordan. “Searching for Slow and Silent Earthquakes Using Free Oscillations.” Journal of Geophysical Research 95 (1990): 2485-2510. The authors relate how free oscillations, which ring like a bell, were recorded over a decade, most of them caused by large, ordinary earthquakes. In some instances, the earthquake involved was not big enough to cause free oscillation, suggesting that they were slow earthquakes. Of the 1,500 free-oscillation earthquakes, 164 were not accompanied by a recorded earthquake.

Bolt, Bruce A. Earthquakes. 5th ed. New York: W. H. Freeman, 2005. This comprehensive overview of earthquakes is easy to understand and highly informative. Its material on seismic waves and seismography is of great significance to those interested in various types of earthquakes and in where and how they occur. Bolt writes clearly and with authority in this field.

Ebert, Charles H. V. Disasters: Violence in Nature and Threats by Man. Dubuque, Iowa: Kendall/Hunt, 1988. Chapter 1, “Earthquakes,” and Chapter 4, “Tsunami Waves and Storm Surges,” should prove of particular interest to readers interested in earthquakes, although parts of other chapters also contain relevant information. Chapter 3, for example, relates how earthquakes can trigger landslides and avalanches.

Koyhama, Junji. The Complex Faulting Process of Earthquakes. New York: Springer, 2010. This well-documented, carefully researched study, though quite technical, is excellent in the scope of its coverage. Koyhama explains how various faults have developed and what courses they have taken.

Levy, Matthys, and Mario Salvadori. Why the Earth Quakes: The Story of Earthquakes and Volcanoes. New York: W. W. Norton, 1995. Written with general readers in mind, this volume is exceptionally clear. Its illustrations, both verbal and graphic, add to the accessibility of what the authors are saying. Chapter 9, which focuses on California's San Andreas fault, should be of particular interest to American readers, illustrating as it does how various forms of seismic activity can occur simultaneously along the fault.

Melbourne, Timothy I., and Frank H. Webb. “Slow but Not Quite Silent.” Science 300 (2003): 1886-1887. Provides a short overview of slow earthquakes and the discovery that such events are a result of deep tremors.

Rundle, John B., Donald L. Turcotte, and William Klein, eds. Reduction and Predictability of Natural Disasters. Reading, Mass.: Addison-Wesley, 1996. Among the nine contributions to this book on earthquakes, “Thoughts on Modeling and Prediction of Earthquakes,” by S. G. Eubanks, and “A Hierarchical Model for Precursory Seismic Activation,” by W. I. Newman, D. L. Turcotte, and A. Gabrielov, are the most relevant to the topic of slow earthquakes. The contributions to this volume, while remarkably significant, are highly specialized and may be difficult for beginners.

Schenk, Vladimir, ed. Earthquake Hazard and Risk. Dordrecht, Netherlands: Kluwer Academic Press, 1996. The twenty contributions to this volume, all of which were written by acknowledged specialists in the field, focus on the prediction and management of earthquakes. The volume covers the field thoroughly but would be more useful if it contained an index. The essays seem more specialized than most beginners can easily handle.

Walker, Sally M. Earthquakes. Minneapolis: Carolrhoda, 1996. Written with the young audience in mind, Walker's account is lively, interesting, and accurate. The illustrations are colorful and cogent. Various easily comprehended charts and tables add considerably to the substance of the book's engaging text. The glossary is of special value to readers, as is the index.

Wright, Karen. “The Silent Type.” Discover 23 (2002): 26-27. This article describes the mechanics of a slow earthquake and how it relates to the regular surface earthquakes. It also addresses the methodology used to detect slow earthquakes. Easily understood by the nonscientific reader.

Yeats, Robert, Kerry Sieh, and Clarence R. Allen. The Geology of Earthquakes. New York: Oxford University Press, 1997. This comprehensive study of the geology of earthquakes, written essentially as a textbook, is well presented and thorough. Its material on slow, silent, creeping, and strain migration occurrences, though brief, is as solid as any in the field. The writing style, even in the presentation of the more technical material, is extremely appealing.

Zebrowski, Ernest, Jr. Perils of a Restless Planet: Scientific Perspectives on Natural Disasters. New York: Cambridge University Press, 1999. Chapter 1, “Life on Earth's Crust,” and Chapter 6, “Earth in Upheaval,” are the most useful to those seeking more information about types of earthquakes. Zebrowski obviously has a strong background in ancient Greek and Roman mythology, as well as a scientist's grasp of the mechanics of earthquakes. The illustrations are well chosen, and the appendices include a great deal of technical information in charts that make it easily understandable.