I. Introduction

Earthquake, shaking of the Earth’s surface caused by rapid movement of the Earth’s rocky outer layer. Earthquakes occur when energy stored within the Earth, usually in the form of strain in rocks, suddenly releases. This energy is transmitted to the surface of the Earth by earthquake waves. The study of earthquakes and the waves they create is called seismology (from the Greek seismos, “to shake”). Scientists who study earthquakes are called seismologists.

The destruction an earthquake causes depends on its magnitude and duration, or the amount of shaking that occurs. A structure’s design and the materials used in its construction also affect the amount of damage the structure incurs. Earthquakes vary from small, imperceptible shaking to large shocks felt over thousands of kilometers. Earthquakes can deform the ground, make buildings and other structures collapse, and create tsunamis (large sea waves). Lives may be lost in the resulting destruction.

Earthquakes, or seismic tremors, occur at a rate of several hundred per day around the world. A worldwide network of seismographs (machines that record movements of the Earth) detects about 1 million small earthquakes per year. Very large earthquakes, such as the 1964 Alaskan earthquake, which caused millions of dollars in damage, occur worldwide once every few years. Moderate earthquakes, such as the 1989 tremor in Loma Prieta, California, and the 1995 tremor in Kobe, Japan, occur about 20 times a year. Moderate earthquakes also cause millions of dollars in damage and can harm many people.

In the last 500 years, several million people have been killed by earthquakes around the world, including over 240,000 in the 1976 T’ang-Shan, China, earthquake. Worldwide, earthquakes have also caused severe property and structural damage. Adequate precautions, such as education, emergency planning, and constructing stronger, more flexible, safely designed structures, can limit the loss of life and decrease the damage caused by earthquakes.

II. Anatomy of an Earthquake

Seismologists examine the parts of an earthquake, such as what happens to the Earth’s surface during an earthquake, how the energy of an earthquake moves from inside the Earth to the surface, how this energy causes damage, and the slip of the fault that causes the earthquake. Faults are cracks in Earth’s crust where rocks on either side of the crack have moved. By studying the different parts and actions of earthquakes, seismologists learn more about their effects and how to predict and prepare for their ground shaking in order to reduce damage.

A. Focus and Epicenter

The point within the Earth along the rupturing geological fault where an earthquake originates is called the focus, or hypocenter. The point on the Earth’s surface directly above the focus is called the epicenter. Earthquake waves begin to radiate out from the focus and subsequently form along the fault rupture. If the focus is near the surface—between 0 and 70 km (0 and 40 mi) deep—shallow-focus earthquakes are produced. If it is intermediate or deep below the crust—between 70 and 700 km (40 and 400 mi) deep—a deep-focus earthquake will be produced. Shallow-focus earthquakes tend to be larger, and therefore more damaging, earthquakes. This is because they are closer to the surface where the rocks are stronger and build up more strain.

Seismologists know from observations that most earthquakes originate as shallow-focus earthquakes and most of them occur near plate boundaries—areas where the Earth’s crustal plates move against each other (see Plate Tectonics). Other earthquakes, including deep-focus earthquakes, can originate in subduction zones, where one tectonic plate subducts, or moves under another plate. See also Geology; Earth.

B. Faults

Stress in the Earth’s crust creates faults, resulting in earthquakes. The properties of an earthquake depend strongly on the type of fault slip, or movement along the fault, that causes the earthquake. Geologists categorize faults according to the direction of the fault slip. The surface between the two sides of a fault lies in a plane, and the direction of the plane is usually not vertical; rather it dips at an angle into the Earth. When the rock hanging over the dipping fault plane slips downward into the ground, the fault is called a normal fault. When the hanging wall slips upward in relation to the footwall, the fault is called a reverse fault. Both normal and reverse faults produce vertical displacements, or the upward movement of one side of the fault above the other side, that appear at the surface as fault scarps. Strike-slip faults are another type of fault that produce horizontal displacements, or the side by side sliding movement of the fault, such as seen along the San Andreas fault in California. Strike-slip faults are usually found along boundaries between two plates that are sliding past each other.

C. Waves

The sudden movement of rocks along a fault causes vibrations that transmit energy through the Earth in the form of waves. Waves that travel in the rocks below the surface of the Earth are called body waves, and there are two types of body waves: primary, or P, waves, and secondary, or S, waves. The S waves, also known as shearing waves, move the ground back and forth.

Earthquakes also contain surface waves that travel out from the epicenter along the surface of the Earth. Two types of these surface waves occur: Rayleigh waves, named after British physicist Lord Rayleigh, and Love waves, named after British geophysicist A. E. H. Love. Surface waves also cause damage to structures, as they shake the ground underneath the foundations of buildings and other structures.

Body waves, or P and S waves, radiate out from the rupturing fault starting at the focus of the earthquake. P waves are compression waves because the rocky material in their path moves back and forth in the same direction as the wave travels alternately compressing and expanding the rock. P waves are the fastest seismic waves; they travel in strong rock at about 6 to 7 km (about 4 mi) per second. P waves are followed by S waves, which shear, or twist, rather than compress the rock they travel through. S waves travel at about 3.5 km (about 2 mi) per second. S waves cause rocky material to move either side to side or up and down perpendicular to the direction the waves are traveling, thus shearing the rocks. Both P and S waves help seismologists to locate the focus and epicenter of an earthquake. As P and S waves move through the interior of the Earth, they are reflected and refracted, or bent, just as light waves are reflected and bent by glass. Seismologists examine this bending to determine where the earthquake originated.

On the surface of the Earth, Rayleigh waves cause rock particles to move forward, up, backward, and down in a path that contains the direction of the wave travel. This circular movement is somewhat like a piece of seaweed caught in an ocean wave, rolling in a circular path onto a beach. The second type of surface wave, the Love wave, causes rock to move horizontally, or side to side at right angles to the direction of the traveling wave, with no vertical displacements. Rayleigh and Love waves always travel slower than P waves and usually travel slower than S waves.

III. Causes

Most earthquakes are caused by the sudden slip along geologic faults. The faults slip because of movement of the Earth’s tectonic plates. This concept is called the elastic rebound theory. The rocky tectonic plates move very slowly, floating on top of a weaker rocky layer. As the plates collide with each other or slide past each other, pressure builds up within the rocky crust. Earthquakes occur when pressure within the crust increases slowly over hundreds of years and finally exceeds the strength of the rocks. Earthquakes also occur when human activities, such as the filling of reservoirs, increase stress in the Earth’s crust.

A. Elastic Rebound Theory

In 1911 American seismologist Harry Fielding Reid studied the effects of the April 1906 California earthquake. He proposed the elastic rebound theory to explain the generation of certain earthquakes that scientists now know occur in tectonic areas, usually near plate boundaries. This theory states that during an earthquake, the rocks under strain suddenly break, creating a fracture along a fault. When a fault slips, movement in the crustal rock causes vibrations. The slip changes the local strain out into the surrounding rock. The change in strain leads to aftershocks (smaller earthquakes that occur after the initial earthquake), which are produced by further slips of the main fault or adjacent faults in the strained region. The slip begins at the focus and travels along the plane of the fault, radiating waves out along the rupture surface. On each side of the fault, the rock shifts in opposite directions. The fault rupture travels in irregular steps along the fault; these sudden stops and starts of the moving rupture give rise to the vibrations that propagate as seismic waves. After the earthquake, strain begins to build again until it is greater than the forces holding the rocks together, then the fault snaps again and causes another earthquake.

B. Human Activities

Fault rupture is not the only cause of earthquakes; human activities can also be the direct or indirect cause of significant earthquakes. Injecting fluid into deep wells for waste disposal, filling reservoirs with water, and firing underground nuclear test blasts can, in limited circumstances, lead to earthquakes. These activities increase the strain within the rock near the location of the activity so that rock slips and slides along pre-existing faults more easily. While earthquakes caused by human activities may be harmful, they can also provide useful information. Prior to the Nuclear Test Ban treaty, scientists were able to analyze the travel and arrival times of P waves from known earthquakes caused by underground nuclear test blasts. Scientists used this information to study earthquake waves and determine the interior structure of the Earth.

Scientists have determined that as water level in a reservoir increases, water pressure in pores inside the rocks along local faults also increases. The increased pressure may cause the rocks to slip, generating earthquakes. Beginning in 1935, the first detailed evidence of reservoir-induced earthquakes came from the filling of Lake Mead behind Hoover Dam on the Nevada-Arizona state border. Earthquakes were rare in the area prior to construction of the dam, but seismographs registered at least 600 shallow-focus earthquakes between 1936 and 1946. Most reservoirs, however, do not cause earthquakes.

IV. Distribution

Seismologists have been monitoring the frequency and locations of earthquakes for most of the 20th century. Seismologists generally classify naturally occurring earthquakes into one of two categories: interplate and intraplate. Interplate earthquakes are the most common; they occur primarily along plate boundaries. Intraplate earthquakes occur where the crust is fracturing within a plate. Both interplate and intraplate earthquakes may be caused by tectonic or volcanic forces.

A. Tectonic Earthquakes

Tectonic earthquakes are caused by the sudden release of energy stored within the rocks along a fault. The released energy is produced by the strain on the rocks due to movement within the Earth, called tectonic deformation. The effect is like the sudden breaking and snapping back of a stretched elastic band.

B. Volcanic Earthquakes

Volcanic earthquakes occur near active volcanoes but have the same fault slip mechanism as tectonic earthquakes. Volcanic earthquakes are caused by the upward movement of magma under the volcano, which strains the rock locally and leads to an earthquake. As the fluid magma rises to the surface of the volcano, it moves and fractures rock masses and causes continuous tremors that can last up to several hours or days. Volcanic earthquakes occur in areas that are associated with volcanic eruptions, such as in the Cascade Mountain Range of the Pacific Northwest, Japan, Iceland, and at isolated hot spots such as Hawaii.

V. Locations

Seismologists use global networks of seismographic stations to accurately map the focuses of earthquakes around the world. After studying the worldwide distribution of earthquakes, the pattern of earthquake types, and the movement of the Earth’s rocky crust, scientists proposed that plate tectonics, or the shifting of the plates as they move over another weaker rocky layer, was the main underlying cause of earthquakes. The theory of plate tectonics arose from several previous geologic theories and discoveries. Scientists now use the plate tectonics theory to describe the movement of the Earth’s plates and how this movement causes earthquakes. They also use the knowledge of plate tectonics to explain the locations of earthquakes, mountain formation, and deep ocean trenches, and to predict which areas will be damaged the most by earthquakes. It is clear that major earthquakes occur most frequently in areas with features that are found at plate boundaries: high mountain ranges and deep ocean trenches. Earthquakes within plates, or intraplate tremors, are rare compared with the thousands of earthquakes that occur at plate boundaries each year, but they can be very large and damaging.

Earthquakes that occur in the area surrounding the Pacific Ocean, at the edges of the Pacific plate, are responsible for an average of 80 percent of the energy released in earthquakes worldwide. Japan is shaken by more than 1,000 tremors greater than 3.5 in magnitude each year. The western coasts of North and South America are also very active earthquake zones, with several thousand small to moderate earthquakes each year. Intraplate earthquakes are less frequent than plate boundary earthquakes, but they are still caused by the internal fracturing of rock masses. The New Madrid, Missouri, earthquakes of 1811 and 1812 were extreme examples of intraplate seismic events. Scientists estimate that the three main earthquakes of this series were about magnitude 8.0 and that there were at least 1,500 aftershocks.

VI. Effects

Ground shaking leads to landslides and other soil movement. These are the main damage-causing events that occur during an earthquake. Primary effects that can accompany an earthquake include property damage, loss of lives, fire, and tsunami waves. Secondary effects, such as economic loss, disease, and lack of food and clean water, also occur after a large earthquake.

A. Ground Shaking and Landslides

Earthquake waves make the ground move, shaking buildings and causing poorly designed or weak structures to partially or totally collapse. The ground shaking weakens soils and foundation materials under structures and causes dramatic changes in fine-grained soils. During an earthquake, water-saturated sandy soil becomes like liquid mud, an effect called liquefaction. Liquefaction causes damage as the foundation soil beneath structures and buildings weakens. Shaking may also dislodge large earth and rock masses, producing dangerous landslides, mudslides, and rock avalanches that may lead to loss of lives or further property damage.

B. Fire

Another post-earthquake threat is fire, such as the fires that happened in San Francisco after the 1906 earthquake and after the devastating 1923 Tokyo earthquake. In the 1923 earthquake, about 130,000 lives were lost in Tokyo, Yokohama, and other cities, many in firestorms fanned by high winds. The amount of damage caused by post-earthquake fire depends on the types of building materials used, whether water lines are intact, and whether natural gas mains have been broken. Ruptured gas mains may lead to numerous fires, and fire fighting cannot be effective if the water mains are not intact to transport water to the fires. Fires can be significantly reduced with pre-earthquake planning, fire-resistant building materials, enforced fire codes, and public fire drills.

C. Tsunami Waves and Flooding

Along the coasts, sea waves called tsunamis that accompany some large earthquakes centered under the ocean can cause more death and damage than ground shaking. Tsunamis are usually made up of several oceanic waves that travel out from the slipped fault and arrive one after the other on shore. They can strike without warning, often in places very distant from the epicenter of the earthquake. Tsunami waves are sometimes inaccurately referred to as tidal waves, but tidal forces do not cause them. Rather, tsunamis occur when a major fault under the ocean floor suddenly slips. The displaced rock pushes water above it like a giant paddle, producing powerful water waves at the ocean surface. The ocean waves spread out from the vicinity of the earthquake source and move across the ocean until they reach the coastline, where their height increases as they reach the continental shelf, the part of the Earth’s crust that slopes, or rises, from the ocean floor up to the land. Tsunamis wash ashore with often disastrous effects such as severe flooding, loss of lives due to drowning, and damage to property.

Earthquakes can also cause water in lakes and reservoirs to oscillate, or slosh back and forth. The water oscillations are called seiches (pronounced saysh). Seiches can cause retaining walls and dams to collapse and lead to flooding and damage downstream.

D. Disease

Catastrophic earthquakes can create a risk of widespread disease outbreaks, especially in underdeveloped countries. Damage to water supply lines, sewage lines, and hospital facilities as well as lack of housing may lead to conditions that contribute to the spread of contagious diseases, such as influenza (the flu) and other viral infections. In some instances, lack of food supplies, clean water, and heating can create serious health problems as well.

VII. Reducing Damage

Earthquakes cannot be prevented, but the damage they cause can be greatly reduced with communication strategies, proper structural design, emergency preparedness planning, education, and safer building standards. In response to the tragic loss of life and great cost of rebuilding after past earthquakes, many countries have established earthquake safety and regulatory agencies. These agencies require codes for engineers to use in order to regulate development and construction. Buildings built according to these codes survive earthquakes better and ensure that earthquake risk is reduced.

Tsunami early warning systems can prevent some damage because tsunami waves travel at a very slow speed. Seismologists immediately send out a warning when evidence of a large undersea earthquake appears on seismographs. Tsunami waves travel slower than seismic P and S waves—in the open ocean, they move about ten times slower than the speed of seismic waves in the rocks below. This gives seismologists time to issue tsunami alerts so that people at risk can evacuate the coastal area as a preventative measure to reduce related injuries or deaths. Scientists radio or telephone the information to the Tsunami Warning Center in Honolulu and other stations.

Engineers minimize earthquake damage to buildings by using flexible, reinforced materials that can withstand shaking in buildings. Since the 1960s, scientists and engineers have greatly improved earthquake-resistant designs for buildings that are compatible with modern architecture and building materials. They use computer models to predict the response of the building to ground shaking patterns and compare these patterns to actual seismic events, such as in the 1994 Northridge, California, earthquake and the 1995 Kobe, Japan, earthquake. They also analyze computer models of the motions of buildings in the most hazardous earthquake zones to predict possible damage and to suggest what reinforcement is needed. See also Engineering: Civil Engineering.

A. Structural Design

Geologists and engineers use risk assessment maps, such as geologic hazard and seismic hazard zoning maps, to understand where faults are located and how to build near them safely. Engineers use geologic hazard maps to predict the average ground motions in a particular area and apply these predicted motions during engineering design phases of major construction projects. Engineers also use risk assessment maps to avoid building on major faults or to make sure that proper earthquake bracing is added to buildings constructed in zones that are prone to strong tremors. They can also use risk assessment maps to aid in the retrofit, or reinforcement, of older structures.

In urban areas of the world, the seismic risk is greater in nonreinforced buildings made of brick, stone, or concrete blocks because they cannot resist the horizontal forces produced by large seismic waves. Fortunately, single-family timber-frame homes built under modern construction codes resist strong earthquake shaking very well. Such houses have laterally braced frames bolted to their foundations to prevent separation. Although they may suffer some damage, they are unlikely to collapse because the strength of the strongly jointed timber-frame can easily support the light loads of the roof and the upper stories even in the event of strong vertical and horizontal ground motions.

B. Emergency Preparedness Plans

Earthquake education and preparedness plans can help significantly reduce death and injury caused by earthquakes. People can take several preventative measures within their homes and at the office to reduce risk. Supports and bracing for shelves reduce the likelihood of items falling and potentially causing harm. Maintaining an earthquake survival kit in the home and at the office is also an important part of being prepared.

In the home, earthquake preparedness includes maintaining an earthquake kit and making sure that the house is structurally stable. The local chapter of the American Red Cross is a good source of information for how to assemble an earthquake kit. During an earthquake, people indoors should protect themselves from falling objects and flying glass by taking refuge under a heavy table. After an earthquake, people should move outside of buildings, assemble in open spaces, and prepare themselves for aftershocks. They should also listen for emergency bulletins on the radio, stay out of severely damaged buildings, and avoid coastal areas in the event of a tsunami.

In many countries, government emergency agencies have developed extensive earthquake response plans. In some earthquake hazardous regions, such as California, Japan, and Mexico City, modern strong motion seismographs in urban areas are now linked to a central office. Within a few minutes of an earthquake, the magnitude can be determined, the epicenter mapped, and intensity of shaking information can be distributed via radio to aid in response efforts.

VIII. Studying Earthquakes

Seismologists measure earthquakes to learn more about them and to use them for geological discovery. They measure the pattern of an earthquake with a machine called a seismograph. Using multiple seismographs around the world, they can accurately locate the epicenter of the earthquake, as well as determine its magnitude, or size, and fault slip properties.

A. Measuring Earthquakes

An analog seismograph consists of a base that is anchored into the ground so that it moves with the ground during an earthquake, and a spring or wire that suspends a weight, which remains stationary during an earthquake. In older models, the base includes a rotating roll of paper, and the stationary weight is attached to a stylus, or writing utensil, that rests on the roll of paper. During the passage of a seismic wave, the stationary weight and stylus record the motion of the jostling base and attached roll of paper. The stylus records the information of the shaking seismograph onto the paper as a seismogram. Scientists also use digital seismographs, computerized seismic monitoring systems that record seismic events. Digital seismographs use rewriteable, or multiple-use, disks to record data. They usually incorporate a clock to accurately record seismic arrival times, a printer to print out digital seismograms of the information recorded, and a power supply. Some digital seismographs are portable; seismologists can transport these devices with them to study aftershocks of a catastrophic earthquake when the networks upon which seismic monitoring stations depend have been damaged.

There are more than 1,000 seismograph stations in the world. One way that seismologists measure the size of an earthquake is by measuring the earthquake’s seismic magnitude, or the amplitude of ground shaking that occurs. Seismologists compare the measurements taken at various stations to identify the earthquake’s epicenter and determine the magnitude of the earthquake. This information is important in order to determine whether the earthquake occurred on land or in the ocean. It also helps people prepare for resulting damage or hazards such as tsunamis. When readings from a number of observatories around the world are available, the integrated system allows for rapid location of the epicenter. At least three stations are required in order to triangulate, or calculate, the epicenter. Seismologists find the epicenter by comparing the arrival times of seismic waves at the stations, thus determining the distance the waves have traveled. Seismologists then apply travel-time charts to determine the epicenter. With the present number of worldwide seismographic stations, many now providing digital signals by satellite, distant earthquakes can be located within about 10 km (6 mi) of the epicenter and about 10 to 20 km (6 to 12 mi) in focal depth. Special regional networks of seismographs can locate the local epicenters within a few kilometers.




All magnitude scales give relative numbers that have no physical units. The first widely used seismic magnitude scale was developed by the American seismologist Charles Richter in 1935. The Richter scale measures the amplitude, or height, of seismic surface waves. The scale is logarithmic, so that each successive unit of magnitude measure represents a tenfold increase in amplitude of the seismogram patterns. This is because ground displacement of earthquake waves can range from less than a millimeter to many meters. Richter adjusted for this huge range in measurements by taking the logarithm of the recorded wave heights. So, a magnitude 5 Richter measurement is ten times greater than a magnitude 4; while it is 10 x 10, or 100 times greater than a magnitude 3 measurement.

Today, seismologists prefer to use a different kind of magnitude scale, called the moment magnitude scale, to measure earthquakes. Seismologists calculate moment magnitude by measuring the seismic moment of an earthquake, or the earthquake’s strength based on a calculation of the area and the amount of displacement in the slip. The moment magnitude is obtained by multiplying these two measurements. It is more reliable for earthquakes that measure above magnitude 7 on other scales that refer only to part of the seismic waves, whereas the moment magnitude scale measures the total size. The moment magnitude of the 1906 San Francisco earthquake was 7.6; the Alaskan earthquake of 1964, about 9.0; and the 1995 Kobe, Japan, earthquake was a 7.0 moment magnitude; in comparison, the Richter magnitudes were 8.3, 9.2, and 6.8, respectively for these tremors.

Earthquake size can be measured by seismic intensity as well, a measure of the effects of an earthquake. Before the advent of seismographs, people could only judge the size of an earthquake by its effects on humans or on geological or human-made structures. Such observations are the basis of earthquake intensity scales first set up in 1873 by Italian seismologist M. S. Rossi and Swiss scientist F. A. Forel. These scales were later superseded by the Mercalli scale, created in 1902 by Italian seismologist Giuseppe Mercalli. In 1931 American seismologists H. O. Wood and Frank Neumann adapted the standards set up by Giuseppe Mercalli to California conditions and created the Modified Mercalli scale. Many seismologists around the world still use the Modified Mercalli scale to measure the size of an earthquake based on its effects. The Modified Mercalli scale rates the ground shaking by a general description of human reactions to the shaking and of structural damage that occur during a tremor. This information is gathered from local reports, damage to specific structures, landslides, and peoples’ descriptions of the damage.

B. Predicting Earthquakes

Seismologists try to predict how likely it is that an earthquake will occur, with a specified time, place, and size. Earthquake prediction also includes calculating how a strong ground motion will affect a certain area if an earthquake does occur. Scientists can use the growing catalogue of recorded earthquakes to estimate when and where strong seismic motions may occur. They map past earthquakes to help determine expected rates of repetition. Seismologists can also measure movement along major faults using global positioning satellites (GPS) to track the relative movement of the rocky crust of a few centimeters each year along faults. This information may help predict earthquakes. Even with precise instrumental measurement of past earthquakes, however, conclusions about future tremors always involve uncertainty. This means that any useful earthquake prediction must estimate the likelihood of the earthquake occurring in a particular area in a specific time interval compared with its occurrence as a chance event.

The elastic rebound theory gives a generalized way of predicting earthquakes because it states that a large earthquake cannot occur until the strain along a fault exceeds the strength holding the rock masses together. Seismologists can calculate an estimated time when the strain along the fault would be great enough to cause an earthquake. As an example, after the 1906 San Francisco earthquake, the measurements showed that in the 50 years prior to 1906, the San Andreas fault accumulated about 3.2 meters (10 feet) of displacement, or movement, at points across the fault. The maximum 1906 fault slip was 6.5 meters (21 feet), so it was suggested that 50 years x 6.5 meters/3.2 meters (21 feet/10 feet), about 100 years, would elapse before sufficient energy would again accumulate to produce a comparable earthquake.

Scientists have measured other changes along active faults to try and predict future activity. These measurements have included changes in the ability of rocks to conduct electricity, changes in ground water levels, and changes in variations in the speed at which seismic waves pass through the region of interest. None of these methods, however, has been successful in predicting earthquakes to date.

Seismologists have also developed field methods to date the years in which past earthquakes occurred. In addition to information from recorded earthquakes, scientists look into geologic history for information about earthquakes that occurred before people had instruments to measure them. This research field is called paleoseismology (paleo is Greek for “ancient”). Seismologists can determine when ancient earthquakes occurred.

C. The Earth’s Interior

Seismologists also study earthquakes to learn more about the structure of the Earth’s interior. Earthquakes provide a rare opportunity for scientists to observe how the Earth’s interior responds when an earthquake wave passes through it. Measuring depths and geologic structures within the Earth using earthquake waves is more difficult for scientists than is measuring distances on the Earth’s surface. However, seismologists have used earthquake waves to determine that there are four main regions that make up the interior of the Earth: the crust, the mantle, and the inner and outer core.

The intense study of earthquake waves began during the last decades of the 19th century, when people began placing seismographs at observatories around the world. By 1897 scientists had gathered enough seismograms from distant earthquakes to identify that P and S waves had traveled through the deep Earth. Seismologists studying these seismograms later in the late 19th and early 20th centuries discovered P wave and S wave shadow zones—areas on the opposite side of the Earth from the earthquake focus that P waves and S waves do not reach. These shadow zones showed that the waves were bouncing off some large geologic interior structures of the planet.

Seismologists used these measurements to begin interpreting the paths along which the earthquake waves traveled. In 1904 Croatian seismologist Andrija Mohorovi?ic showed that the paths of P and S waves indicated a rocky surface layer, or crust, overlying more rigid rocks below. He proposed that inside the Earth, the waves are reflected by discontinuities, chemical or structural changes of the rock. Because of his discovery, the interface between the crust and the mantle below it became known as the Mohorovi?ic, or Moho Discontinuity.
In 1906 Richard Dixon Oldham of the Geological Survey of India used the arrival times of seismic P and S waves to deduce that the Earth must have a large and distinct central core. He interpreted the interior structure by comparing the faster speed of P waves to S waves, and noting that P waves were bent by the discontinuities such as the Moho Discontinuity. In 1914 German American seismologist Beno Gutenberg used travel times of seismic waves reflected at this boundary between the mantle and the core to determine the value for the radius of the core to be about 3,500 km (about 2,200 mi). In 1936 Danish seismologist Inge Lehmann discovered a smaller center structure, the inner core of the Earth. She estimated it to have a radius of 1,216 km (755 mi) by measuring the travel times of waves produced by South Pacific earthquakes. As the waves passed through the Earth and arrived at the Danish observatory, she determined that their speed and arrival times indicated that they must have been deflected by an inner core structure. In further studies of earthquake waves, seismologists found that the outer core is liquid and the inner core is solid.

IX. Extraterrestrial Quakes

Seismic events similar to earthquakes also occur on other planets and on their satellites. Scientific missions to Earth’s moon and to Mars have provided some information related to extraterrestrial quakes. The current Galileo mission to Jupiter’s moons may provide evidence of quakes on the moons of Jupiter.

Between 1969 and 1977, scientists conducted the Passive Seismic Experiment as part of the United States Apollo Program. Astronauts set up seismograph stations at five lunar sites. Each lunar seismograph detected between 600 and 3,000 moonquakes every year, a surprising result because the Moon has no tectonic plates, active volcanoes, or ocean trench systems. Most moonquakes had magnitudes less than about 2.0 on the Richter scale. Scientists used this information to determine the interior structure of the Moon and to examine the frequency of moonquakes.

Besides the Moon and the Earth, Mars is the only other planetary body on which seismographs have been placed. The Viking 1 and 2 spacecraft carried two seismographs to Mars in 1976. Unfortunately, the instrument on Viking 1 failed to return signals to Earth. The instrument on Viking 2 operated, but in one year, only one wave motion was detected. Scientists were unable to determine the interior structure of Mars with only this single event.