Earthquake

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Earthquake

The 1906 San Francisco earthquake
Recent events: Catastrophic earthquakes since 2000
Dangerous science: What causes earthquakes?
Aftermath: What are the effects of earthquakes on Earth and its inhabitants?
The human factor: How do people contribute to earthquake damage?
Technology connection: Measuring and predicting earthquakes
A matter of survival: How can we live through earthquakes?
For More Information

An earthquake is a sudden shifting of masses of rock beneath Earth's surface. This motion releases enormous amounts of energy and sends out shock waves that cause the ground to shake. Geologists believe that no spot in the world is completely safe from earthquakes. The great internal forces of the planet are constantly changing the shape of Earth's surface. In fact, Earth has been resounding with earthquakes for more than four billion years.

Thousands of earthquakes occur each day. While not all of these earthquakes are significant, some are powerful enough to wreck cities and kill thousands of people. In the twentieth century alone, deadly earthquakes have claimed more than a million lives.

The 1906 San Francisco earthquake

On April 18, 1906, as the first rays of dawn began to spread across the sky, residents of San Francisco, California, were violently awakened as an earthquake shook the ground beneath the city. Most present-day geologists believe the earthquake, which came in two stages, measured 8.3 on the Richter scale. Other geologists believe the magnitude of the earthquake was slightly less, measuring about 7.8. (The Richter scale measures the magnitude of an earthquake or size of ground waves generated at the earthquake's source; a magnitude of 7.0 produces major damage, and a magnitude of 8.0 produces almost total damage. Every increase of one number on the Richter scale means a ten-fold increase in magnitude.)

Minutes after the earthquake struck, fires from broken gas and electrical lines began raging through the city. They burned uncontrollably for three days. Initial reports listed the number of dead as 700 to 800, but present-day research has led historians to conclude that more than 3,000 deaths were caused directly or indirectly by the earthquake and fires. Out of a population of 400,000, more than 200,000 people were injured and more than 250,000 were left homeless. Some 28,000 buildings were destroyed. The estimated total property damage from the earthquake and three-day fire was placed at more than 400 million dollars (in 1906 dollars).

WORDS TO KNOW

aftershock:: ground shaking that occurs after the main shock of an earthquake.
asthenosphere:: region of the mantle below the lithosphere, composed of partially melted rock.
continental drift:: geologic theory that all continents were part of a single, original landmass before they slowly separated and gradually drifted apart.
convection current:: circular movement of a gas or liquid between hot and cold areas.
crust:: outermost layer of Earth, varying in thickness from 3.5 to 50 miles (5 to 80 kilometers).
epicenter:: the point on Earth's surface directly above the focus of an earthquake, where seismic waves first appear.
fault:: crack in Earth's surface where two plates or sections of the crust push and slide in opposite directions against one another.
fault creep:: slow, continuous movement of plates along a fault, allowing pressure to be released.
focus:: the underground starting place of an earthquake, also called the hypocenter.
foreshock:: ground shaking that occurs before the main shock of an earthquake.
intensity:: description of the physical damage caused by an earthquake.
liquefaction:: the transformation of water-saturated soil into a liquidlike mass, usually by the action of seismic waves.
lithosphere:: the rigid outermost region of Earth, composed of the crust and the upper part of the mantle.
magnitude:: the power of an earthquake, as recorded by a seismograph, or seismometer.
mantle:: thick, dense layer of rock that lies beneath Earth's crust. The mantle is about 1,800 miles (2,900 kilometers) thick and accounts for about 84 percent of Earth's volume.
modified Mercalli scale:: scale developed by Italian seismologist Giuseppe Mercalli to measure the intensity of an earthquake based on the amount of vibration felt by people and the extent of damage to buildings.
plate tectonics:: geologic theory that Earth's crust is composed of rigid plates that "float" toward or away from each other, either directly or indirectly, creating the major geologic features on the planet's surface.
Richter scale:: scale developed by American seismologist Charles Richter that describes the amount of energy released by an earthquake.
seismic waves:: vibrations that move outward from the focus of an earthquake, causing the ground to shake.
seismograph:: instrument used to detect and measure seismic waves. Also known as a seismometer.

A city's history is changed in one day

The area from present-day San Francisco south to Monterey Bay had been home to Native Americans for more than ten thousand years before the first Europeans arrived on the land. About forty culturally diverse native tribes (now known collectively as the Ohlone, a Miwok word meaning "western people") occupied this area when Spanish explorers and missionaries established a presidio (fortress) and a mission on the site of present-day San Francisco in 1776.

The Spanish settlement, named Yerba Buena, remained in Spanish hands until 1821, when it came under Mexican control. U.S. naval forces occupied the village in 1846 when the Mexican War (1846–48) broke out between the United States and Mexico. When the war ended in 1848, the United States gained control of the village and renamed it San Francisco.

At about the time the war ended, gold was discovered in California. The population of San Francisco was then about eight hundred. Two years later, because of the mad rush to find gold, the city's population had grown to some twenty-five thousand. California became linked to the east by the Pony Express (mail delivery system comprised of relay teams of men riding ponies between Missouri and California) in 1860, and by the first transcontinental railroad in 1869.

By the beginning of the twentieth century, San Francisco was a booming metropolis with an art museum, numerous universities, a large commercial and fishing port, and a population of about 400,000. In just fifty years, it had become one of the largest cities in the United States. On April 18, 1906, however, San Francisco's rapid growth was brought to a halt, and the city's landscape and history were forever changed.

Lies on a fault

A fault is a crack in Earth's surface where two plates (sections of the crust) meet. At the San Andreas Fault, on which the city of San Francisco is situated, the Pacific plate and the North American plate push past each other. The Pacific plate moves slowly to the northwest, while the North American plate moves equally slowly to the southeast.

The San Andreas Fault came into being about fifteen to twenty million years ago. The entire fault system forms a continuous narrow break in Earth's crust that runs more than 800 miles (1,287 kilometers) from northwest California south to the Gulf of California. The fault extends downward at least 10 miles (16 kilometers) into the planet's crust. On the surface, the fault appears as an area of crushed and broken rock measuring a few hundred feet to 1 mile (1.6 kilometer) wide.

The plates move at an average rate of about 2 inches (5 centimeters) per year. Rather than sliding smoothly past each other like oiled blocks, the plates make sudden, jerky movements. First one plate moves, then the other. Sometimes the plates lock against each other and are unable to move. Pressure builds up between them. After many years, perhaps centuries or more, the plates overcome the built-up pressure and suddenly move with great force. When that happens, a massive earthquake occurs. Such was the case in 1906 along the fault near San Francisco.

Awakening to a nightmare

Before that fateful Wednesday morning in April, the Pacific and North American plates were locked, and pressure had been building along the northernmost section of the San Andreas Fault. At 5:12 am local time, the internal forces on the fault finally broke free. Like a compressed spring that is suddenly released, the two sides of the locked fault abruptly tore the ground apart along a 290-mile (467-kilometer) stretch from San Juan Bautista to Cape Mendocino. The ground west of the fault shifted northward as much as 21 feet (6.4 meters) in places. Tremendous amounts of energy were released, and the earth shook violently. The epicenter of the earthquake—the point on Earth's surface directly above the focus, or the place where energy is first released—was near San Francisco.

The foreshock, an earthquake tremor that occurs before the larger main shock, lasted about twenty seconds. It was strong enough to be felt throughout the San Francisco Bay area. The main shock hit the area about twenty-five seconds later and lasted for almost one minute. The strong shaking, punctuated by fierce jolts, was felt from southern Oregon to southern California and as far east as Nevada.

Numerous buildings in San Francisco collapsed during the main shock. Many poorly constructed buildings situated on land that had been filled with loose stones and dirt sustained the worst damage. The newly constructed six million dollar city hall was ruined, and the Sonoma Wine Company building collapsed, spilling 15 million gallons (57 million liters) of wine.

Shaken from their slumber, many people ran into the streets dazed and horrified. Others were killed instantly or mortally wounded when bricks from nearby buildings or chimneys crashed through the roofs onto them. San Francisco Fire Department Chief Dennis T. Sullivan was one of the victims. While he was sleeping upstairs in a fire station, bricks from the California Hotel next door crashed through the roof of the station and fell on him.

Ten Deadliest Earthquakes 856-2004
Year	Place	Estimated Deaths
1556	Shanxi, China	830,000
2004	Sumatra (Indian Ocean)	280,000
1976	Tangshan, China	255,000
1138	Aleppo, Syria	230,000
856	Damghan, Iran	200,000
1920	Kansu (now Gansu), China	200,000
1927	Tsinghai, China	200,000
893	Ardabil, Iran	150,000
1293	Japan, Kanto	143,000

What people saw as they rushed onto the streets was a cityscape turned into a wasteland. American writer Jack London (1876–1916), who had been born in San Francisco and lived nearby, came into the city soon after the earthquake to witness the damage. His account appeared two weeks later in the national magazine Collier's.

London described the initial scene: "The streets were humped into ridges and depressions, and piled with the debris of fallen walls. The steel rails [of the streetcar and cable car tracks] were twisted into perpendicular and horizontal angles. The telephone and telegraph systems were disrupted. The great water mains had burst. All the shrewd contrivances and safeguards of man had been thrown out of gear by thirty seconds' twitching of the earth-crust."

Consumed by fire

Even those buildings left standing after the earthquake were not out of danger. Just minutes after the initial tremors waned, fires began to break out across the city. Downed electrical lines, toppled stoves, ruptured gas lines: all combined to set San Francisco ablaze.

Firefighters desperately sought ways to extinguish the raging fires. Hampered by a lack of water (most of the city's water mains had been ruptured), they decided to use dynamite to blow up damaged buildings in the paths of the fires. The idea was to create a firebreak by flattening the buildings and thus depriving the fire of fuel. The plan failed. Not only did the dynamited buildings fail to stop the fires, but the explosions added to the inferno.

By mid-afternoon, one huge blaze had taken over the heart of the city. It had become so large and so hot that it began to create its own wind. Jack London, looking at the burning city from a boat anchored in San Francisco Bay, described the strange, dreamlike scene: "It was dead calm. Not a flicker of wind stirred. Yet from every side wind was pouring in upon the city. East, west, north, and south, strong winds were blowing upon the doomed city. The heated air rising made an enormous suck. Thus did the fire of itself build its own colossal chimney through the atmosphere."

Most intense earthquakes in U.S. history

While the San Andreas Fault in California is well known as the source of many great earthquakes in the United States, the most intense earthquakes in the country over the past two hundred years occurred elsewhere. In the winter of 1811–12, three earthquakes centered on the New Madrid Fault in Missouri affected an area sixteen times larger than the 1906 San Francisco earthquake.

The three principal earthquakes that struck New Madrid, a city in southeastern Missouri on the banks of the Missouri River, took place on December 16, January 23, and February 7. Present-day scientists estimate that the quakes ranged in magnitude from 8.4 to 8.7 on the Richter scale. The intense ground vibrations caused by the main earthquakes and their many aftershocks bent trees until their trunks snapped, opened deep cracks in the soil, caused landslides on bluffs and low hills, created waves on the Mississippi River that overturned many boats, and changed the elevation of land in the area by as much as 20 feet (6 meters). The last, and most intense, of the three earthquakes altered the course of the Mississippi River and created a depression in the northwest Tennessee landscape that filled with river water to become Reelfoot Lake.

Fortunately, few lives were lost because the area was sparsely populated. Seismic waves and their effects, however, were felt for thousands of miles. Stone and masonry buildings as far away as 155 miles (250 kilometers) suffered severe damage. Structural damage was recorded in Pittsburgh, Washington, D.C., and coastal South Carolina. The vibrations surprised people in Chicago and Detroit and even caused church bells to ring in Boston, some 1,100 miles (1,770 kilometers) away.

The aftermath

The fires burned for three days. Sixty percent of the residential buildings in the city and the entire business district had burned. Almost 500 square blocks had been destroyed, and another thirty-two had partially burned. Overall, more than 2,590 acres (1,036 hectares), or about 4 square miles (10.4 square kilometers), had been ravaged by fire.

In the days following the earthquake and fires, hundreds of thousands of people fled San Francisco and the surrounding area. Most of those who remained were homeless but found shelter at makeshift camps set up around the city. Donated supplies poured in from all over the United States, staving off possible famine and more deaths.

Scientists predict that San Francisco may experience another massive earthquake midway through the twenty-first century. They base this estimate on data from the last 1,500 years indicating that major earthquakes occur along the San Andreas Fault about once every 150 years. Nonetheless, the area is prone to frequent, moderate-sized (yet damaging) earthquakes. The 1989 earthquake centered at Loma Prieta, 50 miles (80 kilometers) south of San Francisco, is one such example.

Recent events: Catastrophic earthquakes since 2000

Though many earthquakes occurred between the 2000 and 2005, the 2004 Indian Ocean earthquake and the 2005 Kashmir earthquake were especially destructive and deadly.

The Sumatra-Andaman earthquake of 2004

The Sumatra-Andaman earthquake, also known as the Indian Ocean earthquake, occurred on December 26, 2004. It was also one of the most powerful earthquakes on record. The event registered 9.0 on the Richter scale and was later upgraded to a magnitude between 9.1 and 9.3.

The Sumatra-Andaman earthquake struck off the coast of Indonesia, but its effects were far-reaching. The event is perhaps best remembered because it triggered a series of deadly tsunamis, or giant waves, which devastated countries that border the Indian Ocean. At the time of the earthquake, there was no reliable early warning system for Indian Ocean tsunamis, so the giant waves took people by surprise, battering coastal areas.

Casualties were especially high in Indonesia, Sri Lanka, India, and Thailand. When the waves hit, people were going about their business as usual. Tourists played on beaches, fisherman worked, and families went about their ordinary routines. People did not expect the giant waves that came ashore, destroying buildings and homes, and carrying debris like cars and construction materials farther inland. Well over 100,000 people died in Indonesia alone. The damage caused by the tsunamis displaced over a million people and caused billions of dollars worth of damage.

The destructive earthquake set records. The event created the longest fault rupture on record, creating an opening in Earth's seabed almost 800 miles long. The earthquake was also the longest-lasting on record with a duration of 500 to 600 seconds (about 10 minutes). By contrast, many small earthquakes last only one second. According to scientists, the average ground movement along the fault was at least 16.5 feet (5 meters). In some places, the ground moved as much as 20 feet.

Scientists say that the Sumatra-Andaman earthquake literally shook the entire planet. Scientists were able to detect movement from the event in every place in the world that seismic activity is measured. Thanks to new technology in earthquake measuring instruments, like global broadband seismometers, scientists were able to measure the Sumatra-Andaman event with more precision than ever.

The destructiveness of the Sumatra-Andaman earthquake created widespread human suffering. This tragedy has motivated the scientific community to develop a reliable warning system for tsunamis in the Indian Ocean.

The Kashmir earthquake of 2005

Another historic and disastrous earthquake happened on October 8, 2005, in the Kashmir region of Pakistan. This region is one of the most seismically active in the world. The 2005 earthquake had a magnitude of 7.6 on the Richter scale. It was the strongest earthquake on record in Pakistan. It was followed by a series of powerful aftershocks, measuring as much as 6.3 on the Richter scale. The epicenter of this powerful earthquake was about 60 miles north-northeast of Islamabad, Pakistan's capital.

The Kashmir earthquake was one of the most destructive of all time. In addition to causing widespread destruction, the earthquake killed almost 80,000 people. At least as many people were injured. The earthquake left three million people homeless just before the onset of the winter, which is bitterly cold and severe in the region, which includes parts of the Himalayan mountain range.

The gravest damage caused by the Kashmir earthquake was said to be in the village of Balakot, which is near the epicenter of the earthquake. Portions of the village, which is located on a hillside, literally slid down the hill. Most of the village was completely flattened by the earthquake. The residents of Balakot were among the millions displaced by the earthquake.

In addition to nearly decimating many mountain villages like Balakot, the Kashmir earthquake caused a number of landslides. These landslides cut off crucial thoroughfares across the mountains to the north of Islamabad. Because winter was coming, aid workers feared a second wave of deaths from the earthquake. They worried that people left homeless would die from exposure to the cold or from starvation. Fortunately, predictions that millions of displaced people would die turned out to be wrong. Many people were spared by a winter more mild than most. Aid agencies also found ways to transport food and supplies to isolated communities.

Still, the Kashmir quake devastated the region. Eyewitnesses reported that landslides continued well into the month of October, making mountain areas especially dangerous. Extensive farming infrastructure was destroyed, causing not only food shortages but also losses of livelihood.

Dangerous science: What causes earthquakes?

The answer to what causes earthquakes is found in both the structure of Earth's surface and the forces that rage inside the planet. The interior of Earth—from the planet's center to its surface (a distance of about 3,975 miles [6,395 kilometers])—is divided into layers, defined by their material composition. The core, at the center of the planet, is composed of a solid inner portion about 780 miles (1,300 kilometers) thick and a liquid outer portion about 1,380 miles (2,220 kilometers) thick. Surrounding the core with a depth of about 1,800 miles (2,900 kilometers) is the mantle. The crust, or surface layer surrounding the mantle, varies in thickness from 5 to 25 miles (8 to 40 kilometers).

The mantle is divided into two sections. The upper section, directly below the crust and about 40 miles (65 kilometers) thick, is solid. The section beneath it is soft, or partially melted. The crust and the rigid section of the mantle together compose what geologists call the lithosphere (pronounced LI-thuh-sphere). The soft section of the mantle is called the asthenosphere (pronounced ass-THEE-nuh-sphere).

Most geologists believe that convection currents (circular movements of fluid between hot and cold areas) in the asthenosphere are the driving force behind earthquakes and other movements on the surface of the planet. The heat energy at the center of Earth—where temperatures are estimated to exceed 9,900°F (5,480°C)—is carried to the surface by convection currents. As they near the crust, the currents cool and sink back toward the center to be heated once again. The pressure created by the action of these currents is released on Earth's surface through volcanoes and earthquakes.

Plate tectonics

Plate tectonics is the geologic theory that Earth's crust is made up of rigid plates that float on the surface of the planet. (Tectonics comes from the Greek word meaning "builder.") The plates make up the lithosphere and float on the underlying asthenosphere. There are seven major plates and several smaller ones that are in constant contact with each other. When one plate moves, it causes other plates to move. The movement of the plates toward or away from each other either creates the major geologic features, such as mountain ranges or faults, at Earth's surface.

Plate tectonics is a relatively new scientific theory. This theory is built on the idea of continental drift, introduced in the 1920s by German geophysicist Alfred Wegener (1880–1930). Wegener believed that all continents were part of a single, original landmass—a supercontinent he called Pangaea (pronounced pan-JEE-ah)—before they gradually separated and drifted apart. His concept was based on the fact that several of the planet's continents seem to fit together like pieces in a jigsaw puzzle. This is particularly apparent when examining the eastern coast of South America and the western coast of Africa. Wegener, however, could not provide a convincing argument as to what made the continents shift around Earth's surface.

Reports from the past: Prehistoric earthquakes point to the future

In July 1998, two scientists published a study in the journal Science in which they asserted that there had been at least two giant prehistoric earthquakes near present-day Los Angeles. The scientists estimated that the earthquakes occurred within the last 15,000 years on the Sierra Madre Fault, which runs 12 miles (19 kilometers) north of the city. It was previously believed that earthquakes had not occurred on that fault.

The scientists, by measuring different layers of soil in the faulted region, determined that the ground had moved more than 16.5 feet (5 meters) on average during the ancient tremors. Based on that figure, they estimated that the earthquakes had ranged in strength from 7.2 to 7.6 on the Richter scale.

The scientists made no predictions of future earthquakes, merely pointing out that it was possible for an earthquake to eventually strike the city. If an earthquake were to occur today in the same place as the prehistoric earthquakes, the scientists reasoned, strong tremors would be sent directly into the heart of Los Angeles. Such an earthquake would cause immense destruction in the heavily populated city.

That question was answered by the theory of plate tectonics. The theory states that tectonic plates, moving about Earth's surface in response to pressure beneath them, interact with each other in one of three ways: they converge (move toward one another), diverge (move away from one another), or transform (slide past one another). The boundaries where plates meet are known as plate margins.

Faults

As two rock plates slide past one another, a crack or fault develops at the plate margin. Most earthquakes occur along faults (also called fault lines). The principle types of faults are normal, reverse, and strike-slip. A normal fault forms when two plates are being pulled or stretched apart. A reverse fault, in contrast, forms when two plates are being pushed together—the compression forces one plate up and over the other. A strike-slip fault, the most common type of fault, forms when the edge of one plate grinds against the edge of another as it slips sideways. The San Andreas Fault, the best known fault in the continental United States, is a strike-slip fault.

When plates along a fault move slowly and continuously, allowing pressure to be released, the movement is called fault creep. More often, however, the lower parts of the plates move slowly and continuously along the soft portion of the mantle while the upper parts of the plates, where the surface is solid rock, remain locked in position. Such a configuration causes stress to build up in the crust. When that stress becomes greater than the forces holding the surface plates in position, the crust can rupture. The stored energy is then released through ground vibrations—better known as earthquakes. There are more than one million earthquakes a year around the planet, although most are too faint to be noticed.

Earthquake zones

On Earth, about 90 percent of earthquakes occur around the outer rim of the Pacific Ocean. This area is where the edges of the Pacific plate, the portion of the Earth's crust that lies beneath the Pacific Ocean, come in contact with several other plates. On its western side, the Pacific plate slides beneath the Eurasian and Indo-Australian plates. Along its northeastern side, the Pacific plate comes up against the North American plate. The north-central edge of the Pacific plate rides above the Nazca plate (located between the Pacific plate and the South American plate).

Not only does the outer rim of the Pacific Ocean have frequent earthquakes, but it also contains three-quarters of the world's volcanoes. For this reason, this circular area is also known as the Ring of Fire (or Circle of Fire).

Seismic waves: The motion of earthquakes

Vibrations transmitting the shock of an earthquake are called seismic (pronounced SIZE-mik) waves. These waves travel both underground and along Earth's surface in all directions, like ripples from a stone dropped in a pond. The underground area where energy is first released, triggering an earthquake, is called the focus (or hypocenter). The focus may be as far as 430 miles (700 kilometers) beneath Earth's surface. The point on the surface directly above the focus is called the epicenter.

When an earthquake occurs, two main classes of seismic waves are generated. The first class waves, called body waves, are generated below ground, from the source of the earthquake, and travel to the surface. Body waves consist of P (primary) waves and S (secondary) waves. The P waves travel fastest, up to 4 miles (6.4 kilometers) per second, and are the first waves to reach the surface. They stretch and compress the rock in their path and cause the ground to move vertically (upward). P waves release their energy to the atmosphere, resulting in the common thundering or rumbling sound associated with earthquakes.

S waves travel at about half the speed of P waves, or 2 miles (3.2 kilometers) per second. They move from side to side as well as upward, causing the ground to move horizontally as well as vertically. For this reason, S waves are far more destructive to buildings than are P waves.

S and P body waves combine near the epicenter to form surface waves or L (long) waves, which travel along Earth's surface. Although surface waves move slightly more slowly than body waves—less than 2 miles (3.2 kilometers) per second—they cause greater damage. Surface waves can set off avalanches, landslides, and tsunamis.

Seismic waves can travel great distances. For instance, in late 1811 and early 1812 a series of intense earthquakes hit the United States near New Madrid, Missouri. Vibrations could be felt more than 1,000 miles (1,600 kilometers) away. Although waves lose energy as they travel, they can still cause major destruction as they ripple outward.

Aftermath: What are the effects of earthquakes on Earth and its inhabitants?

Great earthquakes leave telltale signs in the surface of Earth, especially in the area around the epicenter. A horizontal or vertical change in the ground level can often be seen near a fault. The most dramatic change is the creation of a scarp, or cliff. Depending on the type of rock and the amount of uplift caused by the earthquake, a scarp may be up to 1 mile (1.6 kilometer) tall.

Experiment: Understanding plate tectonics

Earth's crust is made up of large tectonic plates. These plates are always moving. They move slowly, but they move with incredible force. To understand what happens when plates move into each other, try this experiment. Get a sheet of newspaper, and find a large, hard, flat surface, either a table or the floor, where you can spread the newspaper flat. Put your palms down on the newspaper about a foot apart and slowly push them together. The newspaper will crumple and fold upward in the middle. When tectonic plates fold upward, mountains are created. When they crumple together, earthquakes are the result.

Earthquakes can also produce fissures, or long cracks, in the ground. Often the ground on one side of a fissure is uplifted several inches or even several feet. In addition to being moved vertically, the ground on one side of a fissure can also be moved horizontally. One example of vast changes in ground level occurred in southern Alaska on March 27, 1964. On that date, an earthquake measuring 8.6 on the Richter scale struck the area, permanently resculpting 77,220 square miles (200,000 square kilometers) of land surface. Almost 620 miles (1,000 kilometers) of coastline from Kodiak Island to Prince William Sound was either uplifted or lowered. In some places, the vertical uplift was as much as 36 feet (11 meters); in others, the subsidence, or lowering of ground level, was as much as 7 feet (2 meters).

If a fissure occurs on a hill, it may set off a landslide. Landslides or mud slides are rapid downhill movements of soil and rock. These slides may encompass whole sections of a hill or mountain. During the landslide, the loosened soil and rock rushes down the slope, often destroying everything in its path before finally coming to rest at the bottom of the slope.

Underground structures such as springs and wells may be disturbed by earthquakes. Springs may stop flowing, either temporarily or permanently, and new springs may form. The water in wells may be muddied or its level changed. All of these effects can occur even at a great distance from the epicenter of an earthquake. An extremely intense earthquake may even force water, mud, and sand out of the ground, forming what are called earthquake fountains and sand blows.

What makes earthquakes deadly?

The damage created by an earthquake is most evident in human-made structures. An earthquake is more likely to cause injuries, fatalities, and damage the closer the epicenter is to a populated area. Earthquakes occurring in crowded cities leave behind buckled and broken sidewalks and streets, downed trees and power lines, and collapsed homes and buildings.

Pre-Renaissance artwork destroyed by earthquakes

Priceless frescoes attributed to Florentine painters Giotto (c.1266–1337) and Cimabue (c.1240–c.1302) were severely damaged when two earthquakes struck central Italy within hours of each other on September 26, 1997.

The earthquakes, which measured 5.5 and 5.6 on the Richter scale, killed eleven people and caused damage to nearly 1,500 buildings. The first earthquake struck at 2:33 am local time. Its epicenter was close to the town of Foligno in Umbria, a region dotted with historic towns. The second earthquake struck some nine hours later. Tremors from both earthquakes were felt in Rome, more than 62 miles (100 kilometers) away.

Frescoes are paintings made on damp plaster walls. When the plaster dries, the painting becomes fixed to the wall. The frescoes by Giotto and Cimabue were part of the walls and ceiling of the thirteenth-century Basilica of San Francesco in Assisi, one of early Christianity's most important shrines. A renowned tourist attraction, the basilica is famous for its twenty-eight frescoes depicting the life of Francis of Assisi (c.1182–1226), founder of the Franciscan religious order.

The first earthquake caused deep cracks in many of the precious frescoes. Just as friars and structural experts were inside inspecting this damage, the second earthquake struck, causing an even greater tragedy. Huge sections of the vaulted ceiling of the basilica collapsed, killing four people and destroying many of the frescoes.

On November 28, 1999, just over two years after the earthquakes struck, a commemorative mass was held to reopen the church. The bell tower, the vault sections, and some of the frescoes had been repaired or reattached. Restorers estimated it would take at least another two years before other damaged frescoes, including Cimabue's St. Matthew and Giotto's St. Jerome, would be reassembled.

What usually kills people in an earthquake is not the earthquake itself, but the collapse of buildings, roads, and other structures. The greatest destruction is brought about, often miles away from the epicenter of an earthquake, through the process of liquefaction (pronounced liquh-FAC-shun). When earthquake vibrations encounter layers of loosely packed soil containing water, the vibrations cause the particles of soil to lose contact with one another. This allows the water to bubble through, transforming the ground into a watery mass resembling quicksand.

Structures and roads built on this type of soil cannot be supported by this liquid mixture and thus sink or slide from their foundations.

Kobe earthquake of 1995

The largest earthquake to hit Japan in the last half of the twentieth century occurred on January 17, 1995. At 5:46 am local time, an earthquake measuring 7.2 on the Richter scale struck the region of Kobe and Osaka in south-central Japan. The region includes Kobe, Japan's sixth largest city and the world's sixth largest port, with a population of some 1.5 million. The Kobe earthquake, dubbed the Great Hanshin Earthquake by the Japanese media, killed more than 5,500 people and injured 25,000 others. More than 300,000 people were left homeless. The Kobe earthquake was the deadliest natural disaster in Japan since 1923—the year that the Great Kanto Earthquake killed about 140,000 people.

The epicenter of the Kobe earthquake was located about 20 miles (32 kilometers) south of the city of Kobe near Awaji Island. The minor strike-slip fault on which the earthquake occurred runs from Awaji Island through Kobe. The earthquake caused the ground to move 7 inches (18 centimeters) horizontally and 4 inches (10 centimeters) vertically—the greatest ground displacement ever recorded in Japan. In the twenty-four hours after the main shock, 716 aftershocks were recorded.

The Kobe earthquake damaged or destroyed one out of every five buildings in the area, totaling more than 180,000 buildings. The violent ground shaking, which lasted for about twenty seconds, caused an estimated damage of about 147 billion dollars (U.S.).

Damage occurred throughout a 62-mile (100-kilometer) radius from the epicenter. This area encompassed the cities of Kobe, Osaka, and Kyoto, with Kobe being the most severely affected. Kobe, like much of modern urban Japan trying to keep up with a growing population, was built on very soft soil and landfills. The worst damage occurred near the waterfront where severe liquefaction took place, causing industrial buildings to sink and crumble. Ninety percent of the port's 187 berths were destroyed, and most large cranes along the waterfront were either damaged or destroyed.

Major utilities were severely disrupted in Kobe and surrounding cities. At one point, almost one million households were without electrical power, another 800,000 households lacked gas, and some 250,000 telephone lines were disconnected. Severed gas lines started fires that burned for days. Approximately 150 fires broke out in Kobe and surrounding areas in the hours immediately following the earthquake. Firefighters, hampered by a lack of water because of thousands of breaks in the water line system, were ill-equipped to fight the blazes. Streets clogged with collapsed buildings and traffic congestion also blocked firefighters' efforts.

Houses built in the style of traditional Japanese architecture readily collapsed, killing their inhabitants. Heavy roofs composed of thick mud or tile fell as the thin walls supporting them buckled. Other structures throughout the city were also destroyed by the earthquake. Railway lines were twisted and bent, and train cars were thrown like toys. Elevated portions of the area's highway system collapsed or caved in. Impassable roads, bridges, and rails slowed the arrival of aid from nearby cities. In the period immediately after the earthquake, the only way of transporting emergency supplies was by bicycle or on foot.

The Kobe earthquake hit when most people were asleep. Had it come later in the day—when people filled trains, freeways, and sidewalks—the death toll would have been far higher.

Another serious consequence of earthquakes is fire. Broken gas lines, downed electrical wires, and overturned stoves and heaters start many fires (as was the case in the 1906 San Francisco earthquake). Because

water mains and streets are damaged at the same time, the water needed to put out the fires is unavailable. In a populated region, the damage caused by fires frequently exceeds that caused by the ground shaking.

A secondary effect of an earthquake that occurs under the ocean is a tsunami (pronounced tsoo-NAH-mee), also called a tidal wave. A tsunami

Turkey's earthquakes of 1999

One of the deadliest earthquakes of the twentieth century hit northwest Turkey on August 17, 1999. At 3:02 am local time, a violent tremor measuring 7.4 on the Richter scale shook the ground for forty-five seconds. The epicenter of the earthquake was located near the city of Izmit, 55 miles (88 kilometers) east of Istanbul. Almost half Turkish population lived in the area affected by the earthquake. When it was over, nearly 76,000 buildings had collapsed, an estimated 17,500 people were killed and more than 33,000 others injured. Almost 500,000 were left homeless. The Turkish government, which called the natural disaster one of the greatest in Turkish history.

Turkey lies atop some of the world's most unstable geology. The North Anatolian fault runs through the densely populated, industrial area of northern Turkey. This fault is a small slab of crust that marks the boundary between the Arabian plate to the south and the larger Eurasian plate to the north. Between the two plates lies the relatively small Turkish microplate, on which most of the country of Turkey is located. As the Arabian plate moves slowly northward, it pushes the Turkish microplate to the west relative to the Eurasian plate at a rate of about 1 inch (2.5 centimeters) per year. At times, the microplate sticks, then jerks free with great energy. This occurred on August 17.

The North Anatolian fault is more than 800 miles (1,287 kilometers) long and 10 miles (16 kilometers) deep. It is a perfect example of a strike-slip fault, where the ground moves side to side. After the August 17 earthquake, geologists measured the North Anatolian fault and found that in some places one side of the fault had shifted more than 12 feet (3.7 meters) relative to the other side.

Earthquakes are common in geologically unstable Turkey. According to historians, over the last 5,000 years major earthquakes have shaken northern Turkey about once every 175 years. Seven destructive earthquakes have occurred along the North Anatolian fault since 1939, the year in which an earthquake struck the eastern province of Erzincan and killed 30,000 people. Since then, the locations of the earthquakes along the fault have moved progressively westward.

The earthquake on August 17 was the most powerful to strike the area since 1939. Lives and buildings were destroyed up to 200 miles (322 kilometers) from the epicenter. Tremors from the earthquake were felt as far east as Ankara and across parts of the Balkans.

In central Istanbul, the majestic museums and historic mosques suffered no visible damage. In areas on the outskirts of the city, home to some twelve million people, newly built apartment complexes crumbled. In some places to the southeast of Istanbul, the destruction was nearly total.

Rescue teams representing thirty-eight international organizations and forty-two countries arrived in the country shortly after the disaster. But many areas in desperate need of help received nothing for days, forcing the living to dig through the rubble with their bare hands in search of their family members, friends, and neighbors. Smashed sewer lines and a lack of fresh water and portable toilets raised the risk of the spread of infectious diseases among the hundreds of thousands of people left homeless by the earthquake.

On November 12, an earthquake measuring 7.2 struck the northwestern town of Duzce just after nightfall. Located in a hilly region about 115 miles (185 kilometers) east of Istanbul, Duzce was on the eastern fringe of the region hit by the August 17 earthquake. Hundreds of thousands of survivors from the first disaster were still living in tents in the rainy winter weather when the new earthquake struck. The Duzce earthquake lasted for thirty seconds, cutting communication lines and crumbling the main road leading into the town. More than 5,100 people were injured and more than 750 buildings were destroyed. The death toll exceeded 700.

is set in motion by a vertical shift in the ocean floor, which pushes the water ahead of it. The vertical shift generates waves that can travel across the ocean with speeds of 250 to 500 miles (400 to 800 kilometers) per

hour. Tsunamis start out small and grow larger as they near land. It is typical for a large tsunami to measure 60 to 100 feet (18 to 30 meters) in height by the time it crashes onto land.

Earthquakes usher in year 2001

Within the first twenty-six days of 2001, major earthquakes struck two locations at opposite ends of Earth. The first quake shook the tiny Central American nation of El Salvador on January 13, and the second quake hit western India thirteen days later.

Measuring 7.6 on the Richter scale, El Salvador's earthquake killed some 700 people, injured more than 2,000 people, and left 65,000 families homeless. Damages totaled 1.5 billion dollars—about half of the gross earnings of all Salvadorans.

El Salvador's earthquake affected the capital city, San Salvador, and villages in the countryside alike. A large portion of the casualties occurred in Las Colinas, a suburb of San Salvador. A cluster of homes at the base of a hill there were buried under a wall of soil and mud that collapsed in an earthquake-triggered landslide. (The removal of trees from the hillside to make room for a coffee plantation and luxury homes was considered partially to blame for the landslide.) San Francisco Javier and San Augustin, both about 100 miles southeast of the capital, were representative of outlying towns in which virtually every building, most made of adobe walls and tin roofs, was reduced to rubble.

India's earthquake, a 7.9 on the Richter scale, had far greater consequences than El Salvador's. The death toll in India exceeded seventeen thousand, there were more than sixty thousand injuries, and one million people were left homeless. The earthquake, India's worst in half a century, occurred at about 9 o'clock in the morning on Republic Day (a national holiday honoring the Indian constitution).

The epicenter of the earthquake was below the heavily populated state of Gujarat, near the border with Pakistan. Buildings in cities throughout Gujarat, constructed without following earthquake-proof building codes, teetered and fell or collapsed in heaps. The hardest-hit city, Bhuj, lost 6,000 of its 150,000 residents and its hospital. Not a single building in Bhuj escaped without serious damage. In the town of Anjar, 400 children marching in a Republic Day parade were buried alive by debris from tumbling houses and high-rise buildings.

Throughout the region, the earthquake caused water supplies to become contaminated; destroyed roads, bridges, and railroad tracks; and downed telephone and power lines. The destruction not only made it hard for survivors to find food and water, but it made it difficult for relief workers with needed supplies to reach affected areas.

The human factor: How do people contribute to earthquake damage?

The most obvious way that people contribute to earthquake destruction is by building cities and towns within earthquake zones (areas situated on or near faults and other plate boundaries). Because scientists have identified these zones only within the last one hundred years, most cities were built with no knowledge of the danger. Complicating matters, wetlands and other moist lowland areas around many cities have been filled in with soil

to create more living space. These filled-in areas are highly prone to liquefaction and destruction during earthquakes.

Along with ground instability and the possibility of liquefaction, the design of many buildings makes them susceptible to earthquake damage. Structures that are insufficiently braced or not tightly secured to their foundations can be damaged easily by earthquakes. The materials used to construct the buildings is another factor. For instance, wood is more flexible than brick and cement; therefore, wood-frame buildings are better equipped to withstand ground motion. Brick and cement structures often fracture and collapse during large earthquakes.

Further, some scientists said that human development and coral mining created conditions that worsened the impact of the Sumatra-Andaman earthquake. That 2004 earthquake, whose epicenter was in the Indian Ocean, triggered massive tsunamis that pulverized many countries that border that ocean. Normally, a coral reef acts as a natural barrier between the land and powerful ocean waves. When a reef is compromised or destroyed, there is no barrier to protect the land. Thus, some scientists think that the tsunami that resulted from the Sumatra-Andaman earthquake was taller and more powerful in areas where reefs had been compromised.

Artificial earthquakes?

Scientists have recently begun exploring the occurrence of artificial or human-made earthquakes, such as those caused by quarry blasts or other large explosions. Another possible trigger of earthquakes is the underground detonation (for the purpose of testing) of large nuclear bombs. The tremors generated by this type of explosion are strong enough to be felt more than 100 miles away from the testing site. It is therefore plausible that such tremors could trigger the release of built-up pressure between two nearby plates, resulting in an earthquake.

The underground storage of hazardous liquid wastes is yet another problem. (Wastes deemed hazardous are those detrimental to human health or the environment because, for example, they catch fire at low temperatures, are extremely acidic, or undergo violent physical and/or chemical changes when mixed with water. Such wastes must be disposed of in accordance with strict standards developed by the Environmental Protection Agency.) Scientists believe that improper underground disposal of hazardous wastes may have caused an earthquake beneath northeastern Ohio on January 31, 1986. For the twelve years before that date, hazardous liquid wastes had been disposed of in two underground wells located almost 1.3 miles (2 kilometers) below ground. A total of about 350 million gallons (1.3 billion liters) of hazardous waste had been pumped into the wells. The pressure created by that volume may have ruptured the surrounding rock, causing cracks that reached an area where tectonic pressure had been building up. Whatever the cause, an earthquake measuring 4.9 on the Richter scale rocked this area in Ohio.

Technology connection: Measuring and predicting earthquakes

From the beginning of recorded history, people have tried to explain, predict, and measure earthquakes. Ancient people believed that the movement of various animals such as giant catfish, snakes, spiders, or turtles that lived beneath Earth's surface created earthquakes. One of the first people to attempt to explain the action of earthquakes based on natural phenomena was Greek philosopher Aristotle (384–322 bce). He believed that winds within the planet caused shaking at the surface.

The first known earthquake-measuring device was invented by Chinese scholar and poet Zhang Heng (78-139) in the second century ce. The device, now called a seismoscope, is a huge bronze vase measuring 6 feet (1.8 meters) in diameter. Eight dragon heads are sculpted around the top. In the mouth of each dragon is a small bronze ball. Directly below each dragon head sits a bronze frog. Inside the vase, each dragon head is attached to a bar, which connects to a single pendulum (a hanging object that freely swings back and forth) in the middle of the vase. The pendulum swings from the movement of even the slightest tremor. When the pendulum swings, it pulls back one of the bars. The dragon's mouth opens, and the ball drops into the open mouth of the bronze frog directly below. The frog holding the ball indicates from which direction the tremor came.

Present-day scientists measure an earthquake's power by two standards: intensity and magnitude. While the intensity of an earthquake is determined by the amount of damage caused, magnitude is measured by using seismographs, also known as seismometers, or other devices that detect ground movement. Intensity is a measure of an effect; magnitude is a measure of released energy.

Seismology

Scientists use an instrument called a seismograph, or seismometer, to measure the waves caused by an earthquake. A seismograph consists of a heavy weight or pendulum hanging over a constantly revolving drum wrapped with recording paper. Attached to the end of the pendulum, with its tip touching the paper, is a recording pen or needle. When an earthquake occurs, the seismic waves cause the pendulum to swing, and the pen records that motion on the paper. The bigger the seismic wave, the larger the swing of the pendulum, and the wider the zigzag line made by the pen on the paper. Today, the motion of the pendulum can also be recorded as digital data on a computer.

Seismographs, or seismometers, are usually placed underground in deep holes away from the artificial vibrations caused by cars, airplanes, and other modern machinery. In 1931, only 350 seismographic stations existed around the world to measure earthquakes. By 2000 there were more than 4,000 stations recording data. That data is transmitted via satellite and computer to scientific institutions around the world.

The most recent seismographic technology includes the broadband seismometer, which is able to record a broad range of frequencies. These devices have allowed scientists to collect even more specific data on recent earthquakes like the 2004 Sumatra-Andaman earthquake and the 2005 Kashmir earthquake. This electronic device confines a small mass between electrical forces. When the Earth moves, the device measures how much force is required to hold the mass steady.

Mercalli scale

In 1902 Italian seismologist Giuseppe Mercalli (1850–1914) developed a scale to compare the surface effects of earthquakes. Mercalli's measurement of earthquake intensity was based not on scientific measurement but on the damage done to buildings. Updated in 1931 by American seismologists Harry Wood and Frank Neumann, the scale (now called the modified Mercalli scale) rates the intensity of earthquakes from levels I (detected only by seismographic instruments) through XII (destruction of all buildings).

On the modified Mercalli scale, a rating of II or III describes a weak earthquake that is felt by few people. At this strength, ground vibrations are similar to the passing of a large truck. People on upper floors of buildings would most likely feel such small tremors as a slight swaying motion. Tremors of this strength may also cause hanging objects like chandeliers to swing.

Medium-strength earthquakes with a modified Mercalli rating from IV to VI are felt by most people and usually wake those who are asleep. Earthquakes of this intensity can cause walls to crack and dishes and windows to move or break. During this type of earthquake, unstable objects may fall over or drop to the floor, heavy furniture may move across the floor, and people may feel off-balance as they walk.

Earthquakes with a VII or VIII rating on the modified Mercalli scale can cause considerable damage to poorly designed and built structures. Even well-built structures may suffer moderate damage. An earthquake of this strength can cause chimneys, factory stacks, columns, and walls to

tumble, and unsecured houses to move off their foundations. The ground motion may cause wet ground to develop cracks and sand and mud to spurt up through those cracks.

Serious damage and partial-to-total collapse of buildings—even specially designed structures—is common in earthquakes rating IX or higher on the modified Mercalli scale. Underground water pipes, reservoirs, dams, and embankments break or become damaged. Railroad rails may bend, and noticeable cracks may appear in the ground. Liquefaction occurs, causing buildings and roads to sink into the ground. Landslides occur on steep slopes. At level XII of the scale, the ground ripples in waves, objects are thrown into the air, and the courses of rivers may shift.

Richter scale

In 1935 American seismologist Charles Richter (1900–1985) developed a scale to measure the magnitude of earthquakes. Richter and colleague Beno Gutenberg (1889–1960) sought a standard method of measuring and comparing earthquakes. Up to that point, the Mercalli scale was the only earthquake-measurement tool. The problem

with the Mercalli scale, however, was that it depended on nonscientific factors. It was greatly influenced by the soundness of building construction and the behaviors of the people living in the earthquake area. The Mercalli scale also made it difficult to rate earthquakes that occurred in rural areas where there were few buildings or people.

Richter and Gutenberg developed a way of measuring an earthquake's power, rather than its effects on humans or buildings. The Richter scale, ranging from 1 to 10, uses seismographic readings to compare the energy released by a specific earthquake to the energy released by other earthquakes. Each whole number increase in value on the scale indicates a ten-fold increase in the energy released and a thirty-fold increase in ground motion. Therefore, an earthquake with a value of 6.0 on the Richter scale is ten times more powerful than an earthquake with a value of 5.0, and so on up and down the scale.

Most earthquakes that are reported measure between 3.0 and 8.0 on the Richter scale. Those that have a rating less than 3.0 are not usually felt. Earthquakes measuring between 3.0 and 6.0 cause minor to moderate damage. Strong earthquakes, causing destruction in areas with large populations, measure between 6.0 and 6.9 on the scale. Earthquakes measuring 7.0 to 7.9 are major. (California is struck by earthquakes in this range about once every eighteen years.) Any earthquake that measures 8.0 or above on the Richter scale is considered a massive earthquake, one that causes widespread destruction. An earthquake of this magnitude generally occurs somewhere on the planet once every five to ten years.

Earthquake prediction

Of the more than one million earthquakes that shake the surface of Earth each year, the vast majority measure 3.4 or below on the Richter scale and cannot be felt by people. The few earthquakes that register high on the Richter scale, however, can be disastrous. Great earthquakes have accounted for some of the most dreadful natural disasters in recorded human history. In the past eight hundred years, seventeen earthquakes have caused fifty thousand or more deaths apiece.

Seismologists in many countries constantly monitor the stresses within Earth's crust. Ultrasensitive instruments placed across faults at the surface measure the slow, almost imperceptible movement of plates. Other instruments measure phenomena that sometimes precede earthquakes, including changes in tide and groundwater levels, fluctuations in the magnetic properties of rocks, and the swelling or tilting of the ground.

Earthquake rattles Washington State

At 10:54 am on February 28, 2001, northwest Washington state was rocked by its most powerful earthquake in fifty-two years. The forty-second tremor measured 6.8 on the Richter scale. It was centered between Olympia and Tacoma, some 30 miles from Seattle, on the southern border of Puget Sound. The quake produced just one fatality (one woman died of a heart attack) and approximately 235 injuries; only a handful of the injuries were serious, and none were life-threatening.

Damage caused by the earthquake was estimated at more than two billion dollars. Examples of damage included crumbled bridges, buckled roads and sidewalks, and broken glass and fallen plaster in buildings. Some of the worst destruction occurred at the seventy-four-year-old capitol building in Olympia, about 12 miles from the earthquake's epicenter. The building's dome was cracked in several places, and engineers were unsure as to whether or when the statehouse could be used again.

Geologists noted that the damage in the region was relatively minor for a quake of that magnitude. They attributed the gentleness of the earthquake to the location of its epicenter, which was more than 30 miles beneath the surface. "We were very, very lucky," stated Washington governor Gary Lock in the New York Times of March 2, 2001. "There could have been utter catastrophe had it been higher, closer to the surface."

Also deserving credit for the prevention of greater damage was the strict earthquake code for building construction in the region. Those regulations are intended to protect buildings from all but catastrophic earthquakes. Almost all buildings built or upgraded to be in compliance with the code escaped without serious damage.

For more than thirty-five years, the U.S. Geological Survey (USGS) has operated seismographic stations throughout the world. In the mid-1990s, the USGS and the Incorporated Research Institutions for Seismology (an association of more than ninety universities) developed the Global Seismographic Network (GSN). This worldwide network is composed of 128 stations in more than eighty countries on all continents. Its purpose is to make readily available high-quality data regarding earthquakes. Within minutes after data is received and recorded, scientists can analyze the information to determine the location and magnitude of any large seismic event that has occurred anywhere on the planet.

Even with this state-of-the-art equipment, scientists cannot predict the exact time and place an earthquake will occur. Earthquake researchers continue to sharpen their ability to correctly interpret the significance of seismic activity. At present, scientists rely primarily on information about past earthquakes in order to determine the likelihood of future earthquakes.

Some people look to peculiar animal behaviors as a sign of impending earthquakes. Certain animals have heightened sensitivities to electricity, magnetic fields, odors, and vibrations. In China it is said that prior to earthquakes snails come out of the ground, rats leave houses, birds fly from their nests, and dogs bark constantly. In Japan, there are reports of goldfish swimming frantically just before earthquakes. From a scientific standpoint, however, the connection between animal behavior and earthquakes has not been proven.

A matter of survival: How can we live through earthquakes?

Earthquakes can neither be stopped nor controlled, but humans can minimize the destruction earthquakes cause. A seemingly simple solution is for people to avoid living in earthquake-prone areas. This solution is not very practical, however, since many cities—some with large and growing populations—already stand in these areas.

A more realistic solution is to design structures resistant to earthquake damage. In many earthquake zones, strict building codes for new structures have been adopted. For existing buildings, internal and external braces have been added to strengthen them. Buildings have also been anchored to their foundations to keep them from slipping off during an earthquake.

Builders in earthquake zones now use metal straps or braces to help strengthen a building's (especially a tall building's) resistance to earthquakes. They use materials and designs that can absorb or withstand ground vibrations. They also put layers of rubber and steel underneath new buildings to lessen the effect of an earthquake on the building itself.

Bridges can also be constructed to withstand earthquakes. San Francisco's Golden Gate Bridge, for instance, has a flexible structure that allows it to sway but not break during most earthquakes. Cantilever bridges (type of bridge made of two sections that extend outward from banks or piers and join together in the middle), like the San Francisco-Oakland Bay Bridge, in contrast, are less flexible and more prone to collapse during earthquakes.

Personal safety

An earthquake is unpredictable and terrifying, but people caught in one can follow a few steps to help reduce personal injury. Safety experts recommend the following:

First and foremost, do not panic. The shaking of the ground is not harmful; falling objects are.
If you are indoors when an earthquake hits, stay there. Crawl beneath and hang onto a heavy desk or table. If that is not possible, move into a hallway or crouch against an inside wall. Stay away from windows, fireplaces, and heavy standing objects. Get out of the kitchen—stoves and heavy appliances make it a dangerous place. Do not run downstairs or rush outside while the house or building is shaking.
If you are outside when an earthquake hits, move quickly into an open area away from buildings, power lines, walls, or other structures that might fall.
If you are driving when an earthquake hits, move the car out of traffic as quickly and carefully as possible and stop. Do not park on or under a bridge or overpass, or under trees, light posts, power lines, signs, or anything else that might fall. Stay inside the car until the tremors stop.

If in a mountainous area, move into a clearing or open area. Be mindful of landslides, falling rocks, or other loose debris that could come rolling down a slope.

After the tremors of an earthquake subside, remember to do the following:

Check the utilities. If the smell of gas is present in the air, turn off the main gas valve and open the windows, if possible. Do not light matches or lighters. Do not turn on electrical equipment or appliances. A spark from any of these may cause an explosion and fire. Leave the house or building and report the gas leak.
Use the telephone only to report an emergency. If there is an emergency and the telephone lines are down, send someone for help.
Stay out of damaged buildings. Aftershocks (tremors that occur after the main shock of an earthquake) can topple already weakened structures.
Advance preparation for an earthquake, like for any disaster, can greatly reduce the chances of injury or death. Develop an earthquake plan at home or at school that explains the safest course of action should an earthquake strike. Keep on hand an emergency kit that includes a flashlight, a battery-powered radio, extra batteries, first-aid materials, a fire extinguisher, canned or packaged food (with a manual can opener), plastic containers filled with drinking water (allow at least one gallon or three liters per person per day), warm clothes, matches, candles, and a camp stove or barbecue with extra fuel (stored carefully).

[See AlsoTsunami ]