Earthquake (Encyclopedia of Science)
An earthquake is an unpredictable event in which masses of rock shift below Earth's surface, releasing enormous amounts of energy and sending out shock waves that sometimes cause the ground to shake dramatically. Not all earthquakes are enormous, but they can become one of Earth's most destructive forces.
Causes of earthquakes
Earth's crust is composed of many huge, rocky plates known as tectonic plates. These plates constantly move slowly across the surface of Earth, bumping into each other, overrunning each other, and pulling away from each other. When the strain produced by these movements increases beyond a certain level, the pent-up energy ruptures the crust and creates a fracture known as a fault. The released pressure also causes the ground-shaking vibrations associated with an earthquake.
The motion of earthquakes: Seismic waves
The vibrations transmitting the shock of an earthquake are called seismic waves (pronounced SIZE-mik). These waves travel outward in all directions, like ripples from a stone dropped in a pond. The area where energy is first released to cause an earthquake is called the focus. The focus lies underground at a shallow, intermediate, or deep depthown to about 430 miles (700 kilometers). The epicenter is the point on Earth's surface directly above the focus.
(The entire section is 1098 words.)
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Earthquake (World of Earth Science)
An earthquake is a geological event inside the earth that generates strong vibrations. When the vibrations reach the surface, the earth shakes, often causing damage to natural and manmade objects, and sometimes killing and injuring people and destroying their property. Earthquakes can occur for a variety of reasons; however, the most common source of earthquakes is movement along a fault.
Some earthquakes occur when tectonic plates, large sections of Earth's crust and upper mantle, move past each other. Earthquakes along the San Andreas and Hayward faults in California occur because of this. Earthquakes also occur if one plate overruns another, as on the western coast of South America, the northwest coast of North America, and in Japan. If plates collide but neither is overrun, as they do crossing Europe and Asia from Spain to Vietnam, earthquakes result as the rocks at the abutting plates compress into high mountain ranges. In all three of these settings, earthquakes result from movement along faults.
A fault block may also move due to gravity, sinking between other fault blocks that surround and support it. Sinking fault blocks and the mountains that surround them form a distinctive topography of basins and mountain ranges. This type of fault block configuration is typified by the North American Basin and Range topographic province. In such places, elevation losses by the valleys as they sink between the mountains are accompanied by tremors or earthquakes. Another kind of mountain range rises because of an active thrust fault. Tectonic compression (tectonic, meaning having to do with the forces that deform the rocks of planets) shoves the range up the active thrust fault, which acts like a natural ramp.
Molten rock called magma moves beneath but relatively close to the earth's surface in volcanically active regions. Earthquakes sometimes accompany volcanic eruptions as huge masses of magma move underground.
Nuclear bombs exploding underground cause small local earthquakes, which can be felt by people standing within a few miles of the test site. The earthquakes caused by nuclear bombs are tiny compared to natural earthquakes; but they have a distinctive "sound," and their location can be pinpointed. This is how nuclear weapons testing in one country can be monitored by other countries around the world.
Earth is covered by a crust of solid rock, which is broken into numerous plates that move around on the surface, bumping, overrunning, and pulling away from each other. One kind of boundary between rocks within a plate, as well as at the edges of the plates, is a fault. Faults are large-scale breaks in Earth's crust, in which the rock on one side of the fault has been moved relative to the rock on the other side of the fault by tectonic forces. Fault blocks are giant pieces of crust that are separated from the rocks around them by faults.
When the forces pushing on fault blocks cannot move one block past the other, potential energy is stored up in the fault zone. This is the same potential energy that resides in a giant boulder when it is poised, motionless, at the top of a steep slope. If something happens to overcome the friction holding the boulder in place, its potential energy will convert into kinetic energy as it thunders down the slope. In the fault zone, the potential energy builds up until the friction that sticks the fault blocks together is overcome. Then, in seconds, all the potential energy built up over the years turns to kinetic energy as the rocks surge past each other.
The vibrations of a fault block on the move can be detected by delicate instruments (seismometers and seismographs) in rocks on the other side of the world. Although this happens on a grand scale, it is remarkably like pushing on a stuck window or sliding door. Friction holds the window or door tight in its tracks. After enough force is applied to over-come the friction, the window or door jerks open.
Some fault blocks are stable and no longer experience the forces that moved them in the first place. The fault blocks that face each other across an active fault, however, are still influenced by tectonic forces in the ever-moving crust. They grind past each other along the fault as they move in different directions.
Fault blocks can move in a variety of ways, and these movements define the different types of faults. In a vertical fault, one block moves upward relative to the other. At the surface of the earth, a vertical fault forms a cliff, known as a fault scarp. The sheer eastern face of the Sierra Nevada mountain range is a fault scarp. In most vertical faults, the fault scarp is not truly vertical, and one of the fault blocks "hangs" over the other. This upper block is called the hanging wall and the lower block, the foot wall.
In horizontal faults, the blocks slide past one another without either block being lifted. In this case, the objects on the two sides of the fault simply slide past one another; for example, a road that straddles the fault might be offset by a number of feet. Complex faults display movements with both vertical and horizontal displacements.
Any one of the following fault types can generate an earthquake:
- Normal fault vertical fault in which the hanging wall moves down compared to the foot wall.
- Reverse fault vertical fault in which the hanging wall moves up in elevation relative to the foot wall.
- Thrust fault low-angle (less than 30°) reverse fault, similar to an inclined floor or ramp. The lower fault block is the ramp itself, and the upper fault block is gradually shoved up the ramp. The "ramp" may be shallow, steep, or even curved, but the motion of the upper fault block is always in an upward direction. A thrust fault caused the January 1994 Northridge earthquake near Los Angeles, California.
- Strike-slip (or transform) fault fault along which one fault block moves horizontally (sideways), past another fault block, like opposing lanes of traffic. The San Andreas fault in Northern California is one of the best known of this type.
When a falling rock splashes into a motionless pool of water, waves move out from the point of impact. These waves appear at the interface of water and air as circular ripples. However, the waves occur below the surface, too, traveling down into the water in a spherical pattern. In rock, as in water, a wave-causing event makes not one wave, but a number of waves, moving out from their source, one after another, like an expanding bubble.
Tectonic forces shift bodies of rock inside the earth, perhaps displacing a mountain range several feet in a few seconds, and they generate tremendous vibrations called seismic waves. The earthquake's focus (also called the hypocenter) is the point (usually deep in the subsurface) where the sudden sliding of one rock mass along a fault releases the stored potential energy of the fault zone. The first shock wave emerges at the surface at a point typically directly above the focus; this surface point is called the epicenter. Seismometers detect seismic waves that reach the surface. Seismographs (devices that record seismic phenomena) record the times of arrival for each group of vibrations on a seismogram (either a paper document or digital data).
Like surfaces in an echoing room that reflect or absorb sound, the boundaries of rock types within the earth change or block the direction of movement of seismic waves. Waves moving out from the earthquake's focus in an ever-expanding sphere become distorted, bent, and reflected. Seismologists (geologists who study seismic phenomena) analyze the distorted patterns made by seismic waves and search through the data for clues about the earth's internal structure.
Different kinds of earthquake-generated waves, moving at their own speeds, arrive at the surface in a particular order. The successive waves that arrive at a single site are called a wave train. Seismologists compare information about wave trains that are recorded as they pass through a number of data-collecting sites after an earthquake. By comparing data from three recording stations, they can pinpoint the map location (epicenter) and depth within the earth's surface (focus or hypocenter) of the earthquake.
These are the most important types of seismic waves:
- P-waveshe fastest waves, these compress or stretch the rock in their path through Earth, moving at about 4 mi (6.4 km) per second.
- S-wavess they move through Earth, these waves shift the rock in their path up and down and side to side, moving at about 2 mi (3.2 km) per second.
- Rayleigh waves and Love waveshese two types of "surface waves" are named after seismologists. Moving at less than 2 mi (3.2 km) per second, they lag behind P-waves and S-waves but cause the most damage. Rayleigh waves cause the ground surface in their path to ripple with little waves. Love waves move in a zigzag along the ground and can wrench buildings from side to side.
The relative size of earthquakes is measured by the Richter scale, which measures the energy an earthquake releases. Each whole number increase in value on the Richter scale indicates a 10-fold increase in the energy released and a
thirty-fold increase in ground motion. An earthquake measuring 8 on the Richter scale is ten times more powerful, therefore, than an earthquake with a Richter Magnitude of 7, which is ten times more powerful than an earthquake with a magnitude of 6. Another scalehe Modified Mercalli Scale uses observations of damage (like fallen chimneys) or people's assessments of effects (like mild or severe ground shaking) to describe the intensity of a quake.
Violent shaking changes water bearing sand into a liquid-like mass that will not support heavy loads, such as buildings. This phenomenon, called liquefaction, causes much of the destruction associated with an earthquake in liquefaction-prone areas. Downtown Mexico City rests on the old lakebed of Lake Texcoco, which is a large basin filled with liquefiable sand and ground water. In the Mexico City earthquake of 1985, the wet sand beneath tall buildings turned to slurry, as if the buildings stood on the surface of vibrating gelatin in a huge bowl. Most of the 10,000 people who died as a result of that earthquake were in buildings that collapsed as their foundations sank into liquefied sand.
In the sudden rearrangement of fault blocks in the earth's crust that cause an earthquake, the land surface on the dropped-down side of the fault can fall or subside in elevation by several feet. On a populated coastline, this can wipe out a city. Port Royal, on the south shore of Jamaica, subsided several feet in an earthquake in 1692 and suddenly disappeared as the sea rushed into the new depression. Eyewitnesses recounted the seismic destruction of the infamous pirate anchorage, as follows: "n the space of three minutes, Port-Royall, the fairest town of all the English plantations, exceeding of its riches,as shaken and shattered to pieces, sunk into and covered, for the greater part by the seahe earth heaved and swelled like the rolling billows, and in many places the earth crack'd, open'd and shut, with a motion quick and fastn some of these people were swallowed up, in others they were caught by the middle, and pressed to deathhe whole was attended withhe noise of falling mountains at a distance, while the skyas turned dull and reddish, like a glowing oven." Ships arriving later in the day found a small shattered remnant of the city that was still above the water. Charts of the Jamaican coast soon appeared printed with the words Port Royall Sunk.
In the New Madrid (Missouri) earthquake of 1811, a large area of land subsided around the bed of the Mississippi River in west Tennessee and Kentucky. The Mississippi was observed to flow backwards as it filled the new depression, to create what is now known as Reelfoot Lake.
Cities depend on networks of so-called "lifeline structures" to distribute water, power, and food and to remove sewage and waste. These networks, whether power lines, water mains, or roads, are easily damaged by earthquakes. Elevated freeways collapse readily, as demonstrated by a section of the San Francisco Bay Bridge in 1989 and the National Highway Number 2 in Kobe, Japan, in 1995. The combination of several networks breaking down at once multiplies the hazard to lives and property. Live power lines fall into water from broken water mains, creating a deadly electric shock hazard. Fires may start at ruptured gas mains or chemical storage tanks. Although emergency services are needed more than ever, many areas may not be accessible to fire trucks and other emergency vehicles. If the water mains are broken, there will be no pressure at the fire hydrants, and the firefighters' hoses are useless. The great fire that swept San Francisco in 1906 could not be stopped by regular firefighting methods. Only dynamiting entire blocks of buildings halted the fire's progress. Both Tokyo and Yokohama burned after the Kwanto earthquake struck Japan in 1923, and 143,000 people died, mostly in the fire.
Popular doomsayers excite uncomprehending fear by saying that earthquakes happen more frequently now than in earlier times. It is true that more people than ever are at risk from earthquakes, but this is because the world's population grows larger every year, and more people are living in earthquake-prone areas.
Today, sensitive seismometers "hear" every noteworthy earth-shaking event, recording it on a seismogram. Seismometers detect earthquake activity around the world, and data from all these instruments are available on the Internet within minutes of the earthquake. News agencies can report the event the same day. People have ready access to information about every earthquake that happens anywhere on Earth. And the earth experiences a lot of earthquakeshe planet never ceases to vibrate with tectonic forces, although the majority of them are not strong enough to be detected except with instruments. Earth has been resounding with earthquakes for more than 4 billion years. Earthquakes are a way of knowing that the planet beneath us is still experiencing normal operating conditions, full of heat and kinetic energy.
Ultrasensitive instruments placed across faults at the surface can measure the slow, almost imperceptible movement of fault blocks, which tell of great potential energy stored at the fault boundary. In some areas, foreshocks (small earthquakes that precede a larger event) may help seismologists predict the larger event. In other areas, where seismologists believe seismic activity should be occurring but is not, this seismic gap may be used instead to predict an inevitable large-scale earthquake.
Other instruments measure additional fault-zone phenomena that seem to be related to earthquakes. The rate at which radon gas issues from rocks near faults has been observed to change before an earthquake. The properties of the rocks themselves (such as their ability to conduct electricity) have been observed to change, as the tectonic force exerted on them slowly alters the rocks of the fault zone between earthquakes. Peculiar animal behavior has been reported before many earthquakes, and research into this phenomenon is a legitimate area of scientific inquiry, even though no definite answers have been found.
Techniques of studying earthquakes from space are also being explored. Scientists have found that ground displacements cause waves in the air that travel into the ionosphere and disturb electron densities. By using the network of satellites and ground stations that are part of the global positioning system (GPS), data about the ionosphere that is already being collected by these satellites can be used to understand the energy releases from earthquakes, which may help in their prediction.
Scientists have presumed that tides do not have any influence on or direct relationship to earthquakes. New studies show that tides may sometimes trigger earthquakes on faults where strain has been accumulating; tidal pull during new or full moons has been discounted by studies of over 13,000 earthquakes of which only 95 occurred during these episodes of tidal stress. Attention is also being directed toward the types of rock underlying areas of earthquake activity to see if rock types dampen (lessen the effects) or magnify earthquake motions.
Seismologists must make a hard choice when their data interpretations suggest an earthquake is about to happen. If they fail to warn people of danger they strongly suspect is imminent, many might die needlessly. But, if people are evacuated from a potentially dangerous area and no earthquake occurs, the public will lose confidence in such warnings and might not heed them the next time.
As more is discovered about how and why earthquakes occur, that knowledge can be used to prevent the conditions that allow earthquakes to cause harm. The most effective way to minimize the hazards of earthquakes is to build new buildings or retrofit old ones to withstand the short, high-speed acceleration of earthquake shocks.
See also Convergent plate boundary; Earth, interior structure; Faults and fractures; Mid-plate earthquakes; Plate tectonics; Tsunami