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Earthquakes and Volcanoes Earthquakes What are earthquakes? Imagine bending a stick until it breaks. When the stick snaps, it vibrates, releasing energy. Earthquakes release energy in a similar way. Earthquakes are the vibrations in the ground that result from movement along breaks in Earth’s lithosphere. These breaks are called faults. Why do rocks move along a fault? The forces that move tectonic plates also push and pull rocks along a fault. If these forces become large enough, the blocks of rock on either side of the fault can move past each other. The rocks might move vertically—up or down—or horizontally—sideways. Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. The size of an earthquake depends on the amount of force applied to the fault. The greater the force applied to a fault, the greater the chance of a large and destructive earthquake occurring. Earthquakes can cause billions of dollars in damage. Injuries and fatalities often occur during earthquakes. Earthquakes are common in the state of California. In 1994, the Northridge earthquake along the San Andreas Fault in California caused $20 billion in damage. Where do earthquakes occur? Few earthquakes occur in the middle of a continent. Most earthquakes occur in the oceans and along the edges of continents where tectonic plates meet. Earthquakes and Plate Boundaries Stress builds up along plate boundaries. Earthquakes result from the buildup and release of this stress along the active plate boundaries. The deepest and strongest earthquakes occur along convergent plate boundaries. At a convergent plate boundary, plates collide. The denser oceanic plate subducts, or drops down, into the mantle. These earthquakes release great amounts of energy. Shallow earthquakes commonly occur where plates separate along a divergent plate boundary or along a transform plate boundary. Earthquakes occur at varying depths where continents collide. Continental collisions form large, deformed mountain ranges. Rock Deformation Force, or pressure, applied along plate boundaries can cause a body of rock to bend and change shape. This is called rock deformation. Over time, the rocks can break and move. Faults Types of Faults Fault Name Location Movement Strike-slip transform plate boundaries Two blocks of rock slide horizontally past each other in opposite directions. Normal divergent plate boundaries Forces pull two blocks of rock apart. One block drops down relative to the other. Reverse convergent plate boundaries Forces push two blocks of rock together. One block moves up relative to the other. Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. A fault is a break in Earth’s lithosphere where one block of rock moves toward, away from, or past another. When rocks move in any direction along a fault, an earthquake occurs. The table below describes three types of faults. The forces applied to a fault determine the direction the rocks move. Earthquake Focus and Epicenter When rocks move along a fault, they release energy. Energy that travels as vibrations on and in Earth is called seismic waves. Seismic waves originate where rocks first move along the fault, at a location inside Earth called the focus. Earthquakes can occur anywhere between Earth’s surface and depths greater than 600 km. In a news report, you might hear a reporter identify the earthquake’s epicenter. The epicenter is the location on Earth’s surface directly above an earthquake’s focus. Seismic Waves During an earthquake, there is a rapid release of energy along a fault. This release of energy produces seismic waves. The waves travel outward in all directions through rock, much like ripples in water. As the waves travel, they transfer energy through the ground and produce the motion associated with an earthquake. The energy released is strongest near the epicenter. As seismic waves move away from the epicenter, their energy and intensity decrease. The greater the distance from an earthquake’s epicenter, the less the ground moves. Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. Types of Seismic Waves During an earthquake, particles in the ground can move back and forth or up and down. Particles can also move in an elliptical motion parallel to the direction the seismic wave travels. Scientists use wave motion, wave speed, and the type of material the wave travels through to classify seismic waves. The three types of seismic waves are primary waves, secondary waves, and surface waves. Primary waves, also called P-waves, cause particles in the ground to move in a push-pull motion, similar to a coiled spring. P-waves move faster than any other seismic waves. They are the first waves detected and recorded after an earthquake. Secondary waves, also called S-waves, cause particles to move up and down at right angles relative to the direction that the wave travels. These waves move like a coiled spring when it is shaken side-to-side and up and down at the same time. Surface waves cause particles in the ground to move up and down in a rolling motion, similar to ocean waves. Surface waves travel only on Earth’s surface. P-waves and S-waves can travel through Earth’s interior. However, scientists have discovered that S-waves cannot travel through liquid. Properties of Seismic Waves Seismic Waves Description Primary waves (P-waves) • Cause rock particles to vibrate in same direction as waves travel • Fastest seismic waves • First waves that seismometers detect • Travel through solids and liquids Secondary waves (S-waves) • Cause rock particles to vibrate perpendicular to direction that waves travel • Slower than P-waves; faster than surface waves • Second waves that seismometers detect • Travel only through solids Surface waves • Cause rock particles to move in a rolling or elliptical motion in the same direction that waves travel • Slowest seismic waves • Cause the most damage at Earth’s surface Mapping Earth’s Interior Inner and Outer Core Seismologists discovered that S-waves cannot travel through the outer core. This discovery proved that Earth’s outer core is liquid, unlike its solid inner core. By analyzing the speed of P-waves, seismologists also discovered that the inner and outer cores are mostly iron and nickel. The Mantle Seismologists have used seismic waves to model convection currents in the mantle. The speeds of seismic waves depend on the temperature, pressure, and chemistry of the rocks that the waves travel through. Seismic waves tend to travel slower as they move through hot material, such as in areas of the mantle below mid-ocean ridges. Seismic waves travel faster in cooler areas of the mantle near subduction zones. Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. Scientists who study earthquakes are called seismologists (size MAH luh justs). They use properties of seismic waves, as described in the table above, to map Earth’s interior. P-waves and S-waves change speed and direction as they travel through different materials. Seismologists measure the speed and direction of waves as they move through Earth at different depths. Using these measurements, seismologists can determine the materials that make up Earth’s layers. Locating an Earthquake’s Epicenter An instrument called a seismometer (size MAH muh ter) measures and records ground motion and can be used to determine the distance seismic waves travel. A seismometer records ground motion as a seismogram, a graphical illustration of seismic waves. Seismologists use a method called triangulation to locate an earthquake’s epicenter. This method uses the speeds and travel times of seismic waves to determine the distance to the earthquake’s epicenter from at least three seismometers at different locations. 1. Find the arrival time difference. First, scientists determine the number of seconds between the arrival of the first P-wave and the first S-wave on the seismogram. This time difference is called lag time. Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. 2. Find the distance to the epicenter. Next, seismologists plot the lag time against distance on a graph. This reveals the distance of the epicenter from the seismograph’s location. 3. Plot the distance on a map. Seismologists determine the distance of the epicenter to seismographs in at least three different locations. The map below shows circles around the locations of three seismometer stations. The distance from each station to its circle measures the distance from that station to the earthquake’s epicenter. The epicenter must lie somewhere on the circle around each station. Only one point lies on all three circles. The point where the three circles intersect is the epicenter. Station 1 Epicenter Station 3 Station 2 Determining Earthquake Magnitude The Richter magnitude scale uses the amount of ground motion at a given distance from an earthquake to determine magnitude. Each increase of one unit on the Richter scale represents 10 times the amount of ground motion. For example, a magnitude 8 earthquake produces 10 times greater shaking than a magnitude 7 earthquake does and 100 times greater shaking than a magnitude 6 earthquake does (10 × 10). The moment magnitude scale measures the total amount of energy released by an earthquake. For each increase of one unit on the scale, an earthquake releases 31.5 times more energy. For example, a magnitude 8 earthquake releases more than 992 times the amount of energy than that of a magnitude 6 earthquake (31.5 × 31.5). Describing Earthquake Intensity Another way to measure and describe an earthquake is to examine the amount of damage that results from the shaking. The Modified Mercalli scale measures the intensity of an earthquake based on descriptions of its effects on people and structures. The scale, shown below, ranges from I, an earthquake that people do not feel, to XII, an earthquake that destroys everything. The higher the number is, the greater the effects. Modified Mercalli Scale I Not felt except under unusual conditions. II Felt by few people; suspended objects swing. III Most noticeable indoors; strong vibrations. IV Felt by many people indoors but few outdoors; dishes rattle. V Felt by nearly everyone; dishes break. VI Felt by all; furniture shifts. VII Everyone runs outdoors; some chimneys break. VIII Chimneys, smokestacks, and walls fall. IX Great damage occurs; buildings shift off of foundations. X Most ordinary structures are destroyed; landslides occur. XI Few structures remain standing; bridges are destroyed. XII Damage is total. Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. An area’s geology also influences earthquake damage. The shaking produces more damage in areas covered by loose sediment than it does in places built on solid bedrock. Earthquake Risk In the United States, the highest risk of earthquakes occurs near tectonic plate boundaries of the western states. The transform plate boundary in California and the convergent plate boundaries in Oregon, Washington, and Alaska have the highest earthquake risks. However, not all earthquakes occur near plate boundaries. Some of the largest earthquakes in the United States have occurred far from plate boundaries. High-energy, destructive earthquakes are not very common. Only about ten earthquakes with a magnitude greater than 7.0 occur worldwide each year. Earthquakes with magnitudes greater than 9.0, such as the Indian Ocean earthquake in 2004, are rare. Seismologists evaluate risk in several ways because earthquakes threaten people’s lives and property. They study the probability that an earthquake will occur in an area. Seismologists study past earthquake activity, the geology around a fault, the population density, and the building design in an area. Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc. Engineers use these risk assessments to design buildings that can withstand the shaking during an earthquake. City and state government officials use risk assessments to help plan and prepare for future earthquakes.