Notable earthquakes

When great earthquakes occur, most of the casualties are caused by building collapse, fire, landslides, and tsunamis. Modern concepts of plate tectonics can account for the location of most great earthquakes, and sound planning can do much to minimize damages.

Measuring Strength of Earthquakes

Two measures are used for describing the strength of earthquakes: intensity and magnitude. Intensity, generally rated on the twelve-point modified Mercalli scale, is the degree of shaking noted at a given point. Intensity depends on the distance to the focus, the local geology, and the observer. Customarily expressed in Roman numerals, intensity ranges from I (felt by only a few observers) to XII (total destruction; ground motion powerful enough to throw objects into the air).

Magnitude, which is usually expressed in terms of the Richter scale, is a measure of ground motion as measured on seismographs and is related to the total energy of an earthquake. The scale is defined so that an increase of one magnitude corresponds to a tenfold increase in ground oscillation, or approximately a thirtyfold increase in energy release. Earthquakes of magnitude 3 are often unnoticed, those of magnitude 5 produce widespread minor damage, those of magnitude 7 are considered major, and those above magnitude 8 are considered great. The greatest magnitude ever recorded as of April 2023 was the Chilean earthquake of 1960, which was measured at 9.5. It is widely believed that there is an upper limit to the Richter scale, but this is a misconception. There is no upper limit to the scale. It appears, however, that the earth's crust cannot store enough elastic energy to generate earthquakes of magnitudes greater than 10.

Occurrence of Strong Earthquakes

Clear relationships exist between plate tectonics and the occurrence of great earthquakes. The magnitude of an earthquake generally corresponds to the area of fault surface where slippage occurs. The larger the slippage area, the greater the energy required to overcome friction. Because the ocean basins have a thin crust (about 5 kilometers thick), great earthquakes are rare in the ocean basins. Most great earthquakes are associated with continental crust, which has an average thickness of 40 kilometers.

The greatest earthquakes (magnitude 8.5 and higher) occur where plates converge, such as in Japan or on the west coast of South America. In these regions, one plate dips beneath the other at a shallow angle, resulting in a very large area of fault surface. Rifts, where continental crust is pulled apart, and transcurrent faults, where one block of continental crust slides horizontally past another, also have produced earthquakes above magnitude 8. The most famous transcurrent fault is the San Andreas fault of California. A few great earthquakes have also occurred well within plates. Some are reasonably well understood. Most of the earthquakes of China and central Asia are a response to the collision of India with Asia. Others, such as the Charleston and New Madrid earthquakes in the United States, are poorly understood.

Early Earthquakes

Little is known of great earthquakes of the distant past. The casualty figures reported for ancient earthquakes are unreliable. Nevertheless, it can usually be assumed that earthquakes that devastated large areas also inflicted great casualties and destruction. Even for modern earthquakes, damage is often so great that casualty figures can only be estimated. Different sources frequently list casualty figures differing by many thousands.

Perhaps the earliest great earthquake to have a major historical impact struck the Minoan civilization on Crete about 1450 BCE. During this earthquake, it appears that all the major complexes on Crete were destroyed. This earthquake possibly was related to the catastrophic eruption of Thera (Santorini), a volcano in the Aegean Sea approximately 120 kilometers north of Crete. The exact order of events is still uncertain.

One of the first earthquakes to be described by historical accounts destroyed the Greek city of Sparta in 464 BCE, killing a reported 20,000 people. In 62 CE, the city of Pompeii in Italy was severely damaged by an earthquake. Pompeii is famous for being buried by an eruption of Vesuvius seventeen years later. An earthquake on July 21, 365, devastated Alexandria, Egypt, killing 50,000 and destroying the Pharos, or lighthouse—one of the Seven Wonders of the World.

The greatest killer earthquake in history struck Shaanxi in north-central China on January 24, 1556. In this region of China, many traditional dwellings were dug into hillsides of loess (wind-deposited silt). Collapse of these cave homes and landslides triggered by the earthquake reportedly killed 830,000. The area of devastation was so large that the death toll was certainly in the hundreds of thousands.

Modern Earthquakes

The catalog of well-known modern earthquakes begins with the Lisbon earthquake, also known as the All Saints Day earthquake, of November 1, 1755. The city of Lisbon, Portugal, was demolished by three shocks between 9:30 and 10:00 a.m., with additional major aftershocks at 11:00 a.m. and 1:00 p.m. Approximately 70,000 people were killed by building collapse, fire, and a tsunami. Considerable damage also occurred in nearby Morocco. The Lisbon earthquake is sometimes listed as one of the greatest earthquakes of all time, producing widespread destruction as far away as Algeria and having been felt as far away as the West Indies. In reality, the earthquakes in Algeria and the West Indies were separate events unrelated to the Lisbon earthquake. The earthquake produced effects far beyond the region where the shock was actually felt. Lake oscillations (seiches) were noted all over Western Europe, clocks stopped, and church bells rang. Many of these phenomena were noted and recorded carefully and these observations showed that a wavelike disturbance had traveled outward from Lisbon. The Lisbon earthquake was thus the first earthquake to be studied systematically by modern scientific methods.

One of the strongest earthquakes ever to strike New England occurred off eastern Massachusetts on November 18, 1755, just days after the All Saints Day earthquake. This event occurred just before news of the Lisbon shock reached America. The first earthquakes to be recorded in the United States were those that struck the New Madrid, Missouri, area on December 16, 1811; January 23, 1812; and February 7, 1812. These events are among the few recorded earthquakes of intensity XII, and they took place in a region not generally considered earthquake-prone. Surface effects in the epicentral area were profound. The Mississippi River was churned into turmoil, and large tracts of unstable ground were affected by surface cracks and subsidence. The shocks were felt as far away as New Orleans and caused church bells to ring in Boston. Because of the sparse population in the New Madrid area at that time, only one death was reported. The New Madrid earthquakes, despite the vast area over which they were felt, were not of extremely large magnitude: They probably had a magnitude between 7.5 and 8. In the central United States, flat-lying and uniform rock layers transmit seismic waves with high efficiency, so that an earthquake at New Madrid is felt over a much larger area than an equally powerful earthquake in a geologically complex region such as California.

On January 9, 1857, a major earthquake (probably magnitude 8) struck Southern California. At least 60 kilometers of the San Andreas fault ruptured near Fort Tejon, north of Los Angeles. A strong earthquake (possibly magnitude 7) struck Charleston, South Carolina, on August 31, 1886. The earthquake was felt over most of the east coast of the United States and killed approximately 110 people. This earthquake was the first in the United States to receive wide scientific attention.

Early Twentieth Century Earthquakes

For many Americans, the word “earthquake” is synonymous with the San Francisco earthquake of April 18, 1906. The earthquake, with a magnitude of 8.3, was officially reported to have killed about 700, but later estimates have placed the death toll as high as 2,500. In October 1989, an earthquake of magnitude 7.1, known as the Loma Prieta earthquake, would again leave the city with fatalities. The 1906 earthquake triggered fires that could not be fought because of ruptured water mains. As a result, a large area of the city was burned. From a scientific standpoint, the earthquake is important because it revealed the extent of the San Andreas fault. North of San Francisco, fence lines and roads were offset as much as 6 meters by the fault. The fault ruptured for at least 280 kilometers, possibly as much as 400 kilometers.

On September 1, 1923, an earthquake known as the Kwanto earthquake, of magnitude 8.3, destroyed much of Tokyo and Yokohama, Japan. This earthquake is notable for the devastating fire that followed it. The earthquake struck when thousands of open cooking fires were in use all over Tokyo. Traditional Japanese construction, which relies extensively on wood and bamboo, is very resistant to collapse in earthquakes but is also very combustible. The earthquake ignited thousands of fires that coalesced into a firestorm—a self-sustaining whirlwind in which updrafts above the fire draw air in from the outside and keep the fire supplied with oxygen. About 140,000 people died. Forty thousand of those who died had taken refuge in an open square and suffocated from lack of air.

Later Twentieth Century Earthquakes

A little-known earthquake (magnitude 7.9) in southeastern Alaska on July 9, 1958, is remarkable for creating the highest wave ever recorded. The earthquake triggered an avalanche into one arm of Lituya Bay, sending the water 530 meters over a ridge on the other side of the bay. Anchorage, Alaska, was damaged by a magnitude 8.3 earthquake on Good Friday, March 27, 1964. Much of the damage to Anchorage was the result of liquefaction of an unstable layer of clay a few meters below the surface. When the seismic shaking liquefied the clay, the ground above broke up, tilted, or collapsed. A tsunami, reaching up to 30 meters in height, devastated the nearby coast. Of the 131 people killed in Alaska, 122 were killed by the tsunami. The tsunami swept down the coast of North America, causing little damage in most places. At Crescent City, California, however, the bottom topography of the harbor focused the wave, which swept into the center of town, killing twelve people. Surveys of the epicentral region showed that almost 300,000 square kilometers of crust had been measurably deformed. Some points on the coast moved seaward by 20 meters; shorelines were uplifted by 15 meters in places. These motions are among the greatest ever documented for any earthquake.

A magnitude 7.7 earthquake in Peru on May 31, 1970, killed about 70,000 people, including the victims of one of the worst landslide disasters in history. The earthquake triggered a rock and ice avalanche from the summit of 6,768-meter Huascaran, the highest peak in Peru. A portion of the landslide rode over a 250-meter ridge and buried the town of Yungay, killing approximately 20,000 people. This earthquake was one of the worst earthquake disasters in the Southern Hemisphere.

The greatest earthquake disaster of the twentieth century in terms of loss of life—and the second greatest in history—took place on July 28, 1976, when a magnitude 8.2 earthquake struck Tangshan in northeastern China, an urban area with about 10 million people. According to the most widely accepted estimate, 600,000 people were killed.

One of the worst earthquakes to strike North America killed 20,000 people in Mexico City on September 19, 1985. The epicenter of the magnitude 8 earthquake was actually on the Pacific coast, some 400 kilometers from Mexico City, yet damage on the coast was light. Buildings on the coast were generally modern, well built, and with foundations on bedrock. Mexico City, in contrast, is built on an ancient lake bed. Unconsolidated sediment shakes badly in earthquakes, accounting for the great damage in Mexico City. Many modern steel-frame buildings were undamaged, while poor-grade masonry suffered badly.

On December 7, 1988, an earthquake measuring magnitude 8 killed an estimated 80,000 people in Soviet Armenia. This event was notable for its political impact, because it happened at a time when the Soviet Union appeared to be moving toward greater political openness. For the first time in many years, the Soviet Union accepted foreign relief efforts after a natural disaster and permitted foreign news coverage at a disaster scene.

On October 17, 1989, a magnitude 7.1 earthquake centered 20 miles from downtown San Francisco at Loma Prieta caused greatest damage in the San Francisco Marina District, which is built upon an unstable, water-saturated landfill. The earthquake caused widespread damage to the road system, including collapse of the I-280 Skyway, many landslides along the coastal highway, and at least sixty-three deaths. On January 17, 1994, a magnitude 6.7 earthquake on a previously unknown fault rocked Northridge, California, in the San Fernando Valley for 40 seconds. Damage was estimated at $15 to $30 billion with 63 dead, thousands injured, nine freeways destroyed, and 250 ruptured gas lines. Power was cut to 3.1 million people, and 40,000 were left without water. A magnitude 7.2 earthquake rocked Kobe, Japan, in January 1995. Although it lasted only 20 seconds, it caused more than 5,000 fatalities, 25,000 injuries, and at least $30 billion in damage.

On August 17, 1999, a magnitude 7.8 earthquake near Izmit, Turkey, 55 miles east of Istanbul lasted for 45 seconds, flattened 60,000 buildings, caused up to $6.5 billion in direct property loss, and killed more than 30,000 people. Nearly 300 aftershocks rocked the region in the next 48 hours.

Twenty-First Century Earthquakes

India experienced two destructive earthquakes in the early twenty-first century: one in Gujarat on January 26, 2001 (measuring 7.9 and resulting in 20,035 deaths), and one in Bam on December 26, 2003 (measuring 6.6 and resulting in 31,000 deaths). The earthquake that resulted in the Indian Ocean tsunami on December 26, 2004, measured 9.2 on the Richter scale and resulted in 230,000 deaths. Other twenty-first century earthquakes have occurred in Kashmir, Pakistan (October 8, 2005, M7.6, 80,000 deaths); Sichuan, China (May 12, 2009, M7.9, 69,197 deaths); and Port-Au-Prince, Haiti (Jan. 12, 2010, M7.0, 316,000 deaths).

On March 11, 2011, the Tōhoku Earthquake, also known as the Japan earthquake and tsunami of 2011, or the Great Sendai Earthquake, struck near the east coast of Honshu, Japan. The magnitude 9.0 quake was the fourth largest ever recorded in the world, and the largest to strike Japan since 1900, when instrumental recordings began. It created a deadly tsunami throughout the Pacific region, which in turn resulted in a nuclear disaster at the Fukushima (Daiichi) Nuclear Power station. As of February 2015, Japan's National Police Agency reported that the disaster was responsible for 15,890 deaths, 2,590 missing/presumed deaths, and $220 billion worth of damage in Japan alone. Data from Japan’s Fire and Disaster Management Agency (reported by CNN in March 2018) estimated the total dead or missing in the earthquake and tsunami, or dying from health issues after the disaster, to be at least 22,000. Associated costs were estimated at $300 billion.

Other deadly earthquakes in the twenty-first century include a magnitude 7.8 earthquake and ensuing avalanche in 2015 that killed more than an estimated 9,000 in Nepal. In 2017, a magnitude 7.3 earthquake on the border of Iran and Iraq was estimated to have killed more than 530 persons and caused 7,460 injuries. In 2018, Indonesia experienced a 7.5 magnitude earthquake followed by a tsunami. The combination killed more than four thousand people. In February 2023, a 7.8 magnitude earthquake devastated the border of Turkey and Syria, causing more than fifty thousand deaths in Turkey and more than seven thousand deaths in Syria, as well as billions of dollars in damage.

Strong-Motion Studies

Great earthquakes present special problems and opportunities for geologists. Because of their great energy release, earthquakes are detected clearly by instruments all over the planet; these records frequently reveal details of earth's structure that cannot be detected on the records of smaller earthquakes. The infrequency and unpredictability of great earthquakes, however, mean that instruments and observers are rarely close by when the event occurs, and instruments that are close by are often destroyed.

Ground motion during great earthquakes can be measured by special seismographs called strong-motion seismographs. Strong-motion studies require that instruments be set up in locations that might experience major earthquakes. These instruments are left in place, possibly for years. After remaining dormant for a long time, the instruments must work properly when the earthquake occurs. The need to place and periodically tend instruments that may never record an event makes strong-motion studies expensive.

It is possible to simulate the effects of earthquakes on buildings. During the planning stage, models of the proposed building can be tested on a vibrating table or through computer modeling. Existing buildings can be shaken artificially. The apparatus for testing buildings consists of a set of large, rotating, off-center weights. Sensors at critical points in the building can detect motion without subjecting the building to destructive vibrations. Corrective measures might include reinforcing weak portions of the structure or redesigning connecting wings so that they can vibrate independently.

Long-Term Seismic Studies

Short-term earthquake prediction on the lines of severe weather warning is probably not achievable in the near future. Geologists are pursuing a variety of studies aimed at assessing the long-term likelihood of great earthquakes. One obvious and low-cost approach is simply to compile all historical records of earthquakes. China and the Middle East, areas with the longest and best-written records, show variations in intensity and location of earthquakes on a time scale of centuries. The short historical record of the United States is insufficient for long-term seismic studies.

One way to extend the record of great earthquakes is to look for geological changes created by ancient events. In Japan, uplifted shorelines have been identified with specific historical earthquakes. At Pallett Creek, north of Los Angeles, trenches across the San Andreas fault have revealed evidence of earthquakes over the last 2,000 years. Each earthquake ruptured sediment layers below the then-existing ground surface. Radiocarbon dating (using radioactive carbon in the sediment as a geologic clock) establishes the age of each fault break. The average interval of great earthquakes in this area is approximately 140 years, but actual intervals have ranged from 75 to 300 years.

Earthquake Hazards

Most of the casualties from great earthquakes result from a few basic causes. Building collapse is a major cause of loss of life. Wood-frame buildings, which are flexible, and steel-frame buildings, which are very strong, are the safest kinds of buildings during earthquakes. Non-reinforced masonry and adobe (mud brick) are the most dangerous. Unfortunately, these construction styles are very common in underdeveloped nations. Fire is another major threat in urban areas. Earthquakes overturn stoves and furnaces, rupture gas lines, and create electrical short circuits. At the same time, ruptured water mains and streets blocked with rubble impede fire-fighting efforts. Earthquake-induced landslides are a hazard in mountainous areas and have caused tremendous loss of life.

Tsunamis, such as the one that originated off the coast of Japan in 2011, are a threat in coastal areas. Believed to be generated by submarine landslides, tsunamis are waves of low height and long length that travel at up to 600 kilometers per hour. Because of their breadth and low height, they are entirely unnoticed by ships at sea but can cause great damage when they reach shore, sometimes thousands of kilometers away. The 2011 Japan earthquake and tsunami caused a death in Papua, Indonesia, and another death in Klamath River, California, as well as millions of dollars in damage as far away as Hawaii, California, and Chile. Tsunamis also occurred in Indonesia in 2004 and 2018 as a result of earthquakes. Whether a tsunami causes damage depends greatly on its direction of travel, on local tide and weather conditions, and particularly on the bottom topography near shore. Tsunami warnings are routinely issued after large earthquakes.

Earthquake Myths

There are a few misconceptions about great earthquakes. After a newsworthy earthquake, people often wonder if earthquakes are becoming unusually frequent. In fact, the reverse was true in the twentieth century. There were about two earthquakes per year of magnitude 8 on the average, in contrast to an annual average of eight during the years 1896–1907. One apparent pattern is real, however. Destructive earthquakes are becoming more common. The reason is demographic rather than geologic. Many seismically active regions are in underdeveloped nations where populations, especially in cities, are growing explosively and where construction standards are often poor. The population at risk from earthquakes is steadily increasing.

There are a few geologic misconceptions about earthquakes. Earthquakes frequently cause ground subsidence in areas underlain by poorly consolidated materials, often causing cracks to open on the surface, but stories of fissures opening and engulfing people, buildings, or even entire villages are unfounded. Most of these stories are probably inspired by landslides. Earthquakes and volcanoes tend to occur in the same geologic settings, and there are some recorded cases of major earthquakes associated with the eruption of a nearby volcano. As a general rule, though, earthquakes do not trigger volcanic activity. Also, the earthquakes that accompany volcanic eruptions are generally not very large.

Principal Terms

aftershocks: earthquakes that follow a major earthquake and have nearly the same focus; they are caused by residual stresses not released by the main shock

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

fault: a fracture within the earth along which opposing masses of rock slip to produce earthquakes

focus: the area or point within the earth where an earthquake originates

intensity: the strength of shaking that an earthquake causes at a given point; intensity is generally strongest near the epicenter of an earthquake

magnitude: a measure of ground motion and energy release in an earthquake; an increase of one magnitude means roughly a thirtyfold increase in energy release

plate tectonics: the crust of the earth consists of a number of moving plates; most earthquakes occur at plate boundaries where moving plates are in contact

seismograph: an instrument for recording motion of the ground in an earthquake; most seismographs are pendulums that remain static as the ground moves

tsunami: a large sea wave caused by coastal earthquakes, probably generated by submarine landslides; not all coastal earthquakes result in tsunamis

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