Earthquake hazards

Over the past four thousand years, about 13 million persons have died as a result of earthquake activity, and an unknown amount of property destruction has occurred as well.

Earthquake Occurrence

Earthquakes are the rapid motion and vibrations caused by movement of the ground along a fracture in a rock or along a fault. Movement occurs when rocks are unable to store any more stress, at which time they reach their breaking point, release energy, and create an earthquake. The point of origin of an earthquake below the surface where its energy is released is known as the focus. The focus can be located at either a shallow or a deep depth. The point on the surface of the earth directly above the focus is called the epicenter; it is the spot frequently cited by the news media as the location of the earthquake.

Throughout the earth's surface, numerous faults and fault systems exist. Larger faults are usually confined to specific areas. Earthquakes and faults are not hazardous in themselves, but they can become hazardous when they directly endanger humans and their immediate environment. Each year, the earth is subjected to at least 1 million earthquakes. Only a few, however, are strong enough to cause major structural damage or result in casualties. The major hazards directly created by an earthquake are ground shaking, ground rupture, and tsunamis. The major indirect hazards are fires, floods, building collapse, disruption of public services, and psychological effects.

Ground Shaking

Ground shaking occurs as energy released by the earthquake reaches the surface and causes the materials through which it passes to vibrate. The intensity of these vibrations and of the shaking at the surface depends on several factors: the amount of energy released, the depth of the focus, and the type of material through which the energy is moving. The closer the focus is to the surface, the more powerful the earthquake. Also, the denser the material, the more the vibrations will be felt. More vibrations result in stronger ground motion. There are a few documented cases in which very strong ground motion caused parked cars to bounce along the road, trees to become uprooted and snap, and the surface of the land to move in rippling waves. Yet, damage to open, uninhabited land is usually minimal.

The amount of damage to buildings subjected to strong ground motion is controlled by many complex and interacting factors: the buildings' method of construction, the types of building material used, the depth of the bedrock, the distance from the epicenter, and the duration and intensity of the shaking. During an earthquake, buildings constructed on thin, firm soil and solid bedrock fare much better than those constructed on thick, soft soil and deep bedrock. However, if the shaking's duration is great, even the most well-constructed building is likely to be destroyed. Such a situation was observed in the earthquake that struck Mexico City in 1985, in which more than 1,000 buildings were destroyed and 10,000 persons were killed. This city was built on ancient lakebed deposits of sand and silt that rapidly lost rigidity as a result of intense shaking. This caused tall buildings in the city to collapse vertically, one floor on top of the other.

There are four other important elements that determine the amount of destruction: the degree of compaction of the soil or bedrock on which the buildings' foundations are resting, the amount of water saturation of this material, its overall chemical composition, and the buildings' physical structure. If construction took place on or within solid bedrock, then the structures would move as a unit and would suffer much less damage. Some buildings may be able to withstand severe shaking for a few seconds, although prolonged shaking will completely destroy them. Ground shaking in the 1964 Alaskan earthquake lasted for about four minutes, causing major damage to the sturdiest of buildings. In contrast, a particular building may easily withstand the effects of shaking but be destroyed by other factors. In Soviet Armenia during December 1988, about 25,000 people were killed because of the effects of multiple aftershocks that shook apart poorly designed structures and buildings that were designed to withstand a lesser degree of ground motion. Moreover, very strong ground motion can knock a building completely off its bedrock foundation, rendering it unusable, and buildings may also fall prey to other types of ground failure triggered by an earthquake.

Ground Failure

Ground failure includes landslides, avalanches, fault scarps, fissuring, subsidence, uplift, creep, sand boils, and liquefaction. Areas such as mountain valleys and regions surrounding an ocean bay can be subjected to these kinds of failure, since they usually consist of recently deposited, fine-grained materials that have not yet been completely compacted or have variable degrees of groundwater saturation.

Landslides occur when unstable soil and rock move rapidly downslope under the influence of gravity. Landslides regularly occur as a result of an earthquake. They commonly occur on steep slopes but can also move down gentle inclines. An avalanche is similar to a landslide but consists of snow and ice mixed with rock and soil. In either case, masses of material move with great rapidity and force, sometimes filling, burying, or excavating the land along its path. Some earthquake-induced avalanches have been clocked at velocities of more than 320 kilometers per hour (or nearly 200 miles per hour). The Tadzhik Soviet Socialist Republic, in late January 1988, was hit with an earthquake of a 5.4 magnitude on the Richter scale that shook the ground for almost 40 seconds, unleashing a massive landslide that was 8 kilometers wide. It buried the nearby village of Okuli-Bolo with mud to a depth of 15 meters, killing between 600 and 1,000 people.

Fault scarps are created when a fault intersects the surface of the earth and large chunks of the ground are uplifted or dropped. Within these chunks, deep ground cracks known as fissures appear. Despite what has been portrayed in movies, there is no chance that the earth will open, close, and “swallow” anything during an earthquake. Deep ground cracks commonly remain open, since the forces that created them operate only in one direction. Sometimes, however, animals fall into these cracks and appear to have been swallowed. In many places, buildings, roads, and other structures are constructed across an old fault scarp, and they undergo extensive structural damage from vertical or horizontal ground displacement. The largest measured vertical displacement along a fault scarp is 15 meters; the largest horizontal displacement that occurred at one time is 6 meters. The rate of displacement is variable, however, and some faults can show a slow but accumulated horizontal displacement of several kilometers.

The subsidence or depression of the surface of the land may occur when underground fluids such as oil or water are removed or drained by a nearby fault; the land sinks, creating water-filled sag ponds. This process occurs over a long period and does not result in casualties or injuries. Another slow form of ground failure is earthquake creep, which occurs more or less continuously along a fault. Creep is really an earthquake in slow motion; stored energy is gradually released in the form of very small earthquakes, or microearthquakes, causing the land to move in opposite horizontal directions. It results in the slow bending and breaking of underground pipes and railroad tracks and causes concrete building foundations to crack. The Hayward fault near San Francisco Bay, California, is a prime example.

Under the worst conditions—a low degree of compaction, thick and fine-grained sandy or silty soil, a high degree of water saturation, and intense ground shaking—solid land loses its cohesiveness and strength. It then begins to liquefy and flow by a process known as liquefaction. Tall buildings slowly move as if they were built on shifting sand. An unusual, localized ground failure event related to liquefaction is a sand boil. Rapid ground motion can cause a pressurized mixture of sand and concentrated groundwater to make its way toward the surface and create a small volcano-like mound of sand that spouts sandy water.

Tsunamis

A spectacular and extremely hazardous coastal event is the seismic sea wave, or tsunami (sometimes incorrectly called a “tidal wave”). Tsunamis are usually produced by undersea earthquakes; the sea floor undergoes rapid vertical motion along one or more active faults and energy is transmitted directly from solid rock into the seawater. Tsunamis can also originate with massive undersea landslides or the eruption of an oceanic volcano. The eruption of the volcanic island of Krakatau in 1883 created a series of waves more than 40 meters high that drowned an estimated 36,000 persons who lived along the low-lying coastal areas of Java and Sumatra. The great Indian Ocean earthquake on December 26, 2004, off the coast of Sumatra, produced a tsunami with waves up to 30 meters that killed more than 230,000 people in 14 countries, inundating coastal lands around most of the Indian Ocean. The earthquake had a magnitude of 9.2 and was caused by undersea subduction. The exact cause of all these large waves is not completely understood by geologists and oceanographers.

Regardless of their cause, tsunami waves begin to radiate away from their point of origin in a manner similar to when a stone is tossed into a quiet body of water. Generally, tsunamis have a wavelength, or distance between successive wave crests, of greater than 161 kilometers and move at a velocity of more than 966 kilometers per hour. However, these large and fast-moving walls of water are not observable as such in the deep ocean, becoming visible only as they enter shallow water. Here the trough of the wave encounters the sea floor and begins to slow down, allowing the crest to build in size. At this point, water is rapidly sucked out of inland bays and harbors to feed the increasing mass as the wave comes roaring into the mainland, drowning people and smashing buildings. In some cases, tsunamis have traveled as far inland as 3 kilometers.

For example, after tossing trains and fishing boats almost 1 kilometer inland, the seismic sea waves produced from the 1964 Alaskan earthquake traveled south many thousands of kilometers as far as Crescent City, California, where local surfers decided to challenge the 6-meter-high waves that resulted from the event. The east coast of Japan, the Hawaiian Islands, the west coast of the United States, Alaska, Chile, Peru, and most other Pacific coastal regions have suffered damage from these powerful waves. They are rare in the Atlantic Ocean. The last major Atlantic tsunami occurred in 1755, when an offshore earthquake (the All Saints Day earthquake) generated a series of waves that hit the coast of Portugal, killing an estimated 14,000 persons.

The 2011 Tōhoku earthquake (also known as the Great East Japan Earthquake) off the northeast coast of Japan resulted in a tsunami that killed more than 15,000 people and destroyed the city of Sendai and its surroundings. The disaster also caused a nuclear accident at the Fukushima Daiichi power facility. Though tsunamis have regularly occurred throughout the world since, none have caused similar levels of damage.

Not as spectacular, but similar to tsunamis, are seiches. Although they also have several origins, they can be produced by an earthquake. Seiches are small, oscillating waves that may travel for several hours, back and forth within the enclosed basin of a lake or reservoir, sometimes causing flooding and minor structural damage to nearby buildings.

Indirect Hazards

One of the most deadly indirect hazards from an earthquake is fire. Fire has claimed the most number of victims and caused more property damage than all the direct hazards combined. In 1906, the San Francisco earthquake, better known as the San Francisco fire, killed 500 people, destroyed 25,000 buildings, and burned 12.2 square kilometers of the city. Fires are often started by the sparking of downed electrical wires, which can result in the ignition of ruptured gas lines. Such fires are difficult, if not impossible, to control, since water pressure in hydrants may be low or nonexistent because of the breakage of underground water pipes. Flooding is another indirect hazard of earthquakes. Although the risk of flooding is usually minimal in a seismically active area, the potential failure of large concrete or earthen dams and reservoirs poses a great threat to nearby life and property. During the 1971 San Fernando earthquake in Southern California, for example, the lower part of the Van Norman earthen dam partially gave way, threatening the 80,000 people who lived in the surrounding area. If the shaking had continued for another minute, it would have been disastrous for the local community.

The danger of being trapped in a collapsing building during an earthquake is real. No building is “earthquake proof,” but construction innovations aimed at making existing buildings more secure continue to be developed. Most concrete and steel-reinforced buildings built on solid bedrock and most one- or two-story wooden frame houses suffer little or no damage in an earthquake, provided that the ground motion is not too protracted or severe. Generally, older buildings suffer much more damage than newer ones. If you are in a building while an earthquake occurs, one of the safest places to be is in a doorway.

Structural Design

Although earthquakes cannot be prevented, engineers and city planners want the dangers they pose to be either eliminated or reduced. Modern buildings are designed to withstand a certain amount of ground shaking. As structural design in construction has improved, the number of people killed in earthquakes has decreased.

However, it is very difficult to predict the effect of ground motion on a building design. To design a more flexible building, engineers perform tests on scale models using simulated or actual samples of the local bedrock and the construction materials. In the state of California, for example, legislation requires the removal of overhanging ledges and the reinforcement of key structural supports in older buildings, bridges, and highway overpasses. The installation of numerous cutoff valves on gas, water, and sewage lines can help localize and minimize utility disruptions, while aboveground pipes, roads, and power lines built across active faults are designed to anticipate fault motion.

Geologists have the primary responsibility for the gathering of geologic information needed for the accurate assessment of the seismic risk for an area. This information can be obtained by the drafting of a geologic map of a region that includes an accurate tracing of all known faults and fault-derived topographic features, such as scarps and sag ponds, and the identity of the rock types involved. These maps help geologists to predict where an earthquake is most likely to occur. However, they cannot predict the earthquake's intensity, frequency, or time of occurrence without further data. Geologic information may also be gained through studies of the movement history of existing faults, the determining of their relative ages, the monitoring of current fault motion, and the detection of previously unknown faults. Once the information is assimilated, an estimate of a possible earthquake's magnitude on the Richter scale, its epicenter, its intensity on the modified Mercalli scale, and the amount of horizontal and vertical ground movement can be made.

If the geologic data indicate that an area may suffer an unacceptable amount of destruction, alternative land-use policies for that area are often adopted. Such land-use policies involve the establishment of a fault hazard easement, whereby construction is restricted to a certain minimum distance from the nearest fault trace or fault zone. Geologic hazard zoning also identifies areas affected by past landslides, floods, and seismic sea waves. In many regions, much urban or industrial development already exists in hazardous areas, simply because the danger was not recognized at the time of its construction.

As the recurrence interval between large earthquakes is very long or poorly known, the largest potential hazards exist in areas that have suffered little or no seismic activity. Local governments in areas such as New York City, South Carolina, and Missouri have given little thought to earthquake disaster planning. The New Madrid, Missouri, earthquake of 1811-1812 was felt over a sparsely populated area of 2.6 million square kilometers; large sections of the ground were uplifted, others sank, deep cracks appeared in the ground and bells were caused to ring in church towers as far away as Boston. It was the most powerful earthquake ever to strike the eastern half of the United States, and it occurred in an area that was thought to be earthquake-free. If a major earthquake were to strike an East Coast city today, the property damage and loss of life would be tremendous.

Humankind's Role in Earthquakes

As the population of the world continues to grow and people compete for living space, more and more areas once considered hazard-free because of their lack of development are becoming inhabited, increasing the potential that damage will be suffered from movements of the earth. Therefore, research in the area of earthquake control and prediction is growing more important.

Humans' ability to trigger an earthquake was discovered during the early 1970s at the Rocky Mountain Arsenal in Denver, Colorado. There, liquid wastes were disposed of by high-pressure injection into wells that were drilled to 3,600 meters below the surface. These liquids reduced the pressure along deep faults, allowing them to slip and causing an increase in the number of minor earthquakes in the area. When the pumping stopped, the number of earthquakes decreased, and when pumping was continued, the tremors began again. A careful study proved that the number of recorded earthquakes was statistically higher than that which would normally be expected. Human-made earthquakes were also created through underground nuclear explosions and the filling of water reservoirs behind major dams. Some geologists believe that these processes may relieve the pressure along faults and help to prevent a major earthquake from occurring.

The ability to predict an earthquake's time and place is based on the quality of geologic data available for a given area. Geologists look at historic evidence of a fault's movement and at its current activity. The problem lies in the brevity of the record-keeping period. The science of seismology is relatively new. The first seismometer was built around 1889 and data have been collected on some faults for less than one hundred years. Many more faults have been discovered since that time, and not enough information is available to give a reliable estimate of their seismic activity.

Most earthquakes are preceded by warning signs. Some of these indicators are local ground swelling, an increase in the number and frequency of minor tremors, an increase in the amount of radioactive radon gas in water wells, and unusual animal activity. The problem is that not all earthquakes have these precursors and that precursors are not always reliable. Moreover, new hazards are created by the prediction of an earthquake. A short-term prediction may cause panic, which could result in major traffic jams, riots, and looting. Long-term predictions may cause property values to drop, disrupt tourism, and cause the gradual abandonment or economic depression of cities thought to be at risk of being impacted by a seismic event. In addition, incorrect earthquake predictions would impact public trust and may result in major lawsuits arising from injuries or damages from the evacuation of an area forecasted to be at risk.

Principal Terms

creep: the very slow downhill movement of soil and rock

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

fault: a fracture or zone of breakage in a rock mass that shows movement or displacement

focus: the point below the surface of the ground where the earthquake originates and its energy is released

intensity: an arbitrary measure of an earthquake's effect on people and buildings, based on the modified Mercalli scale

landslide: the rapid downhill movement of soil and rock

liquefaction: the loss in cohesiveness of water-saturated soil as a result of ground shaking caused by an earthquake

magnitude: a measure of the amount of energy released by an earthquake, based on the Richter scale

subsidence: the sinking of the surface of the land

tsunami: a seismic sea wave created by an undersea earthquake, a violent volcanic eruption, or a landslide at sea

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"What Causes Earthquakes?" National Geographic, 27 Jan. 2025, www.nationalgeographic.com/environment/article/earthquakes. Accessed 10 Feb. 2025.