Chapter 12

The place where the earthquake is generated is called the hypocenter or focus. Most earthquakes occur at depths of less than 100 km (60 mi), some occur as shallow as several kilometers, and some subduction-zone earthquakes occur as deep as 700 km (430 mi). The epicenter is the point on Earth’s surface directly above where the earthquake occurs (directly above the hypocenter). If the seismic event happens on the surface, such as during a humancaused surface explosion, then the epicenter and hypocenter are the same.

In a normal fault, the rocks above the fault (the hanging wall) move down with respect to rocks below the fault (the footwall). The crust is stretched horizontally, so earthquakes related to normal faults are most common along divergent plate boundaries, such as oceanic spreading centers, and in continental rifts.

Many large earthquakes are generated along reverse faults, especially the gently dipping variety called thrust faults. In thrust and reverse faults, the hanging wall moves up with respect to the footwall. Such faults are formed by compressional forces, like those associated with subduction zones and continental collisions.

In strike-slip faults, the two sides of the fault slip horizontally past each other. This can generate large earthquakes. Most strike-slip faults are near vertical, but some have moderate dips. The largest strike-slip faults are transform plate boundaries, like the San Andreas fault in California.

Volcanoes generate seismic waves and cause the ground to shake through several processes. An explosive volcanic eruption causes compression, transmitting energy as seismic waves (shown here with yellow lines). Volcanism can be accompanied by faulting and associated earthquakes. Volcanoes add tremendous weight to the crust, and this loading can lead to faulting and earthquakes. The fault shown here, which caused an earthquake at depth, has faulted down the volcano relative to its surroundings. Many volcanoes have steep, unstable slopes underlain by rocks altered and weakened by hot water. The flanks of such volcanoes can fall apart catastrophically, causing landslides that shake the ground as they break away and travel down the flank of the volcano. Numerous small earthquakes also occur as the rocks break, prior to the actual landslide. As magma moves beneath a volcano, it can push rocks out of the way, causing earthquakes. Magma can push rocks sideways or open space by fracturing adjacent rocks and uplifting the earth’s surface. The emplacement of magma can cause a series of small and distinctive earthquakes, called volcanic tremors. All types of magma-related earthquakes are closely monitored by geologists and seismologists (scientists who study earthquakes), because they can signal an impending eruption.

Catastrophic landslides, whether on land or beneath water, cause ground shaking. On the Big Island of Hawaii, lava flows form new crust that can become unstable and suddenly collapse into the ocean. Seismometers at the nearby Hawaii Volcanoes National Park often record seismic waves caused by such landslides and by fractures opening up on land in response to the sliding of the land toward the sea.

Mine blasts and nuclear explosions compress Earth’s surface, producing seismic waves measurable by distant seismic instruments. Monitoring compliance with nuclear test-ban treaties is done in part using a worldwide array of seismic instruments. These instruments recorded a nuclear bomb exploded by India in 1998. Seismic waves generated by a blast such as this are more abrupt than those caused by a natural earthquake.

An active strike-slip fault has modified the appearance of a landscape for hundreds of thousands of years, causing a linear trough along the fault. Some segments of streams follow the fault. At the time shown here, the strength of the fault is greater than the tectonic forces working to slide the blocks past each other. The rocks strain and flex, but the stresses are not great enough to make the rocks break. Friction along the fault helps keep it from moving. With time, stress increases along the fault.In response, the rocks may deform elastically, changing shape slightly without breaking. The fault might not be obvious at the surface because it is beneath the stream or covered with loose rocks, sand, and soil. One clue that the fault exists is its expression on the landscape.

Over time, stress along the fault (represented by the yellow arrows) becomes so great that it exceeds the fault’s ability to resist it. As a result, the fault slips and the rocks on opposite sides of the fault rapidly move past each other. A large earthquake occurs (at the orange dot on the front of the block), generating seismic waves (not shown) that radiate outward from the fault. At this point the rocks were no longer strong enough and there was not sufficient friction along the fault surface to prevent movement. The built-up stress will be relieved almost instantly as the fault slips.

With the stress partially relieved, the rocks next to the fault relax by elastic processes and largely return to their original, unstrained shape. The movement that has occurred along the fault, however, is permanent. It is not elastic and is recorded by a new break in the topography. After the earthquake, stress again begins to slowly build up along the fault. The new, subtle break along the straight part of the stream is a clue that something happened here. The cycle of stress buildup and release will continue. In this way, the rock strains elastically before the earthquake, ruptures during the earthquake, and mostly returns to its original shape afterwards. This sequence is called stick-slip behavior because the fault sticks (does not move) and then slips.

When a fault slips, it relieves some of the stress on the fault, causing the stress levels to suddenly drop. Gradually, the stress rebuilds until it exceeds the strength of the rock or the ability of friction to keep the fault from slipping. When the amount of stress equals the strength of the fault, the fault slips, and the stress immediately decreases to the original level. In this manner, the amount of stress on a fault forms a zigzag pattern on the graph. It increases gradually (sloping line), and then decreases abruptly (vertical line) when an earthquake occurs. This process is called the earthquake cycle, and is one explanation for why some faults apparently produce earthquakes of a similar size. The average time between repeating earthquakes is called the recurrence interval.

Most earthquakes occur in narrow belts that coincide with plate boundaries. The belt of earthquakes north of Iceland marks a divergent plate boundary along a mid-ocean ridge. Between the belts of earthquakes are some large regions with relatively few earthquakes, like the northern part of Europe. A seismically active zone stretches along the southern part of Europe and continues eastward into Asia. This activity follows a series of mostly convergent boundaries, including continental collisions, that are occurring from the Mediterranean Sea to Tibet. A diff use zone of seismic activity cuts across eastern Africa, following the East African Rift, a region of elevated topography, active volcanism, and faulted blocks. This region is a continental rift within Africa. Mid-ocean ridges, such as the one south of Africa, only have shallow earthquakes (only yellow dots on this map). In these locations, rifting and spreading of two oceanic plates produces faulting and magmatic activity, both of which cause earthquakes. Large regions of the ocean lack significant seismicity because they are not near a plate boundary. Some seismicity beneath the oceans occurs away from plate boundaries and is mostly related to volcanic activity or to minor faulting that accompanies cooling and subsidence of the oceanic lithosphere. Seismicity is concentrated in the western Pacific, with the main zones of seismicity being associated with oceanic trenches and volcanic islands near Tonga, Java, the Philippines, and Japan. These zones run parallel to oceanic trenches and mark subduction zones. Worldwide, approximately 90% of significant earthquakes occur along subduction zones. Subduction zones have shallow, intermediate-depth, and deep earthquakes, with deep and intermediatedepth earthquakes being common only along subduction zones. Note that there is a consistent pattern of shallow earthquakes close to the trench and progressively deeper earthquakes farther away.

Seafloor spreading forms new oceanic lithosphere that is very hot and thin. Stress levels increase downward in Earth, but in mid-ocean ridges the rocks in the lithosphere get very hot at a shallow depth, too hot to fracture (they flow instead). As a result, earthquakes along mid-ocean ridges are relatively small and shallow, with hypocenters less than about 20 km (12 mi) deep. Many earthquakes occur along the axis of the mid-ocean ridge, where spreading and slip along normal faults downdrop blocks along the narrow rift. Numerous small earthquakes also occur due to intrusion of magma into fissures. As the newly created plate moves away from the ridge, it cools, subsides, and bends. The stress caused by the bending forms steep faults, which are associated with relatively small earthquakes. Strike-slip earthquakes occur along transform faults that link adjacent segments of the spreading center. Largely because of the typically thin lithosphere, earthquakes along these oceanic transform faults are small and shallow.

As the oceanic plate moves toward the trench, it is bent and stressed, causing earthquakes in front of the trench. Larger earthquakes occur in the accretionary prism as material is scraped off the downgoing plate. Shearing within the prism causes slip and earthquakes along numerous thrust faults. Large earthquakes occur along the entire contact between the subducting plate and the overriding plate. The plate boundary is a huge thrust fault called a megathrust. Earthquakes along megathrusts are among the most damaging and deadly of all earthquakes. During large earthquakes, the megathrust can rupture upward all the way to the seafloor, displacing the seafloor and unleashing destructive waves in a tsunami. Earthquakes can also occur within the overriding plate due to movement of magma and from volcanic eruptions. Compressive stresses associated with plate convergence can cause thrust faulting behind the magmatic arc. The downgoing oceanic plate is relatively cold, and so continues to produce earthquakes from shearing along the boundary, from downward-pulling forces on the sinking slab, and from abrupt changes in mineralogy. Subduction zones are typically the only place in the world producing deep earthquakes, as deep as 700 km (430 mi). Below 700 km, the plate is too hot to behave brittlely or to cause earthquakes.

Large thrust faults form near the plate boundary in both the overriding plate and underthrusting plate (not shown), causing large but shallow earthquakes. These earthquakes can be deadly in populated areas, such as India, Nepal, Pakistan, and Iran. Large, deadly earthquakes are produced along the plate boundary, or megathrust, between the two continental plates. Thrust faults also form within both continental plates, causing moderately large earthquakes. The immense stresses associated with a collision can reactivate older faults within the interior of either continent, as is presently occurring in Tibet and China. Strike-slip faults and normal faults may be generated as entire regions are stressed by the collision zone or are shoved or sheared out of the way. Any oceanic plate material that was subducted prior to the collision is detached, so actual subduction and associated earthquakes stop. A few deep earthquakes have resulted from the sinking motion of such detached slabs.

Continental rifts generally produce normal faults, whether the rift is a plate boun dary or is within a continental plate. The normal faults downdrop fault blocks into the rift, causing normal-fault earthquakes. Such earthquakes are typically moderate in size. A transform fault can cut through a continent, moving one piece of crust past another. The strike-slip motion causes earthquakes that are mostly shallower than 20 to 30 km (10 to 20 mi), but some of these strike-slip earthquakes can be quite large. The San Andreas fault of California is the bestknown example of a continental transform fault, but large, destructive earthquakes also occur along continental transform faults in Turkey, Pakistan, Nicaragua (Central America). Intrusion of magma (shown here in red) within a plate can cause small earthquakes as the magma moves and creates openings in the rock. Moving magma can produce distinctive earthquakes, which are unlike those produced by movement along faults. Heat from the magma can substantially weaken the crust, causing even more rifting and seismic activity. Preexisting faults in the crust can readjust and move as the continental plate becomes older and is subjected to new stresses, such as from distant plate boundaries. Reactivation of these structures can occur in the interior of a plate and produce large earthquakes, like those in Missouri in 1811.

To describe seismic waves, we begin by defining waves in general. Most waves are a series of repeating crests and troughs. Whether moving through the ocean or through rocks, waves can travel, or propagate, for long distances. However, the material within the wave barely moves. Sound waves travel through the air and thin walls, but the wall does not move much. Think of a seismic wave as a pulse of energy moving through a nearly stationary material.

Fault movement generates seismic waves. Most earthquakes occur at depth, so they first produce waves that travel through the Earth as body waves. The waves propagate (move outward) as shown by the circles around the earthquake.

When body waves reach Earth’s surface, some energy is transformed into new waves that only travel on the surface (surface waves). It is easier to visualize processes on the surface of Earth than within it, so we begin by discussing surface waves, of which there are two kinds.

The first type of surface wave is a horizontal surface wave, in which material vibrates horizontally and shuffles side to side. The motion of the material is perpendicular to the direction in which the wave travels.

The second type of surface wave is a vertical surface wave. It is similar to an ocean wave, in that material moves up and down in an elliptical path. These earthquake waves propagate in the direction of the large arrows, or perpendicular to the crests of the waves.

Body waves travel through Earth and are of two main varieties. The primary wave, also called the P-wave, compresses the rock in the same direction it propagates. It is like a sound wave, which compresses the air through which it travels. P-waves can travel through solids and liquids because these materials can be compressed and then released. The P-wave is the fastest seismic wave, traveling through rocks at 6 to 14 km/s depending on the properties of the rock. For comparison, sound waves in air travel at an average of 0.3 km/s; P-waves are more than 20 times faster.

Body waves travel through Earth and are of two main varieties. The primary wave, also called the P-wave, compresses the rock in the same direction it propagates. It is like a sound wave, which compresses the air through which it travels. P-waves can travel through solids and liquids because these materials can be compressed and then released. The P-wave is the fastest seismic wave, traveling through rocks at 6 to 14 km/s depending on the properties of the rock. For comparison, sound waves in air travel at an average of 0.3 km/s; P-waves are more than 20 times faster.

S-waves cannot travel through liquids because liquids are not rigid (they cannot be sheared). If an area of Earth’s interior does not allow S-waves to pass, then it may be molten. S-waves are slower than P-waves, travelling through rocks at about 3.6 km/s.

A seismometer detects and records the ground motion during earthquakes. A large mass is suspended from a wire. It resists motion during earthquakes. A large mass is suspended from a wire. It resists motion during earthquakes. The mass hangs from a frame that in turn is attached to the ground. When the ground shakes, the frame shakes too, but the suspended mass resists moving because of inertia. As the ground and frame move under the mass, a pen attached to the mass marks a roll of slowly rotating recording paper. As a result, the pen draws a line that records the ground movement over time. This device only records ground movement parallel to the red arrows, so it only records a single direction or single component of motion. A modern seismic detector, called a seismograph, contains three seismometers oriented 90° from each other to record three components of motion (north-south, east-west, and up-down). From these three components, seismologists can determine the source and strength of the seismic signal. Seismologists place seismographs away from human noise and vibration and bury them to reduce wind noise. Seismic waves (in yellow) can come from any direction.

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Records from at least three stations are compared when calculating an earthquake location. Ordinarily, records from many stations are used in an automated computer-based process. P-waves travel faster than S-waves, and so reach a seismic station some time before the S-wave arrives. The time interval between arrival of the P-wave and S-wave is called the P-S interval. The farther a station is from the earthquake, the longer the P-S interval will be. Identifying the arrival of the P-wave and S-wave on these graphs is not always easy, but it can be done by seismologists or by computer. The three seismograms show diff erences in the P-S interval. Based on the P-S intervals, ISCO, which has the shortest P-S interval, is the closest station to the earthquake, followed by DUG and WUAZ.

The amplitude of S-waves decreases as a wave propagates. We plot the relationship between distance, earthquake magnitude, and S-wave amplitude on a graph called a nomograph. For each seismic station, we draw a line connecting the distance and amplitude of the S-wave. The earthquake’s magnitude is where each line crosses the center column. These three lines for the 2005 Colorado earthquake all agree, and yield a 4.1 local magnitude (Ml). Magnitude is a logarithmic scale, so a one unit increase in magnitude represents a tenfold increase in ground motion.

The Modified Mercalli Intensity Scale, abbreviated as MMI, describes the eff ects of shaking in everyday terms. A value of “I” reflects a barely felt earthquake. A value of “XII” indicates complete destruction of buildings, with visible surface waves throwing objects into the air. A series of very large earthquakes in 1811 and 1812 shook Missouri, Arkansas, Tennessee, and the surrounding areas. Shaking was felt over a wide region. The magnitudes on this map, numbered from III to XI, indicate what people in different areas felt and saw when the earthquake happened.

III: Felt strongly by persons indoors, especially on upper floors of buildings.

V. Felt by nearly everyone; many awakened. Some dishes and windows broken. Unstable objects overturned.

VI. Felt by all, many frightened. Some heavy furniture moved. Some plaster on walls and ceilings cracks and falls. Damage slight.

X and XI. Some well-built wooden structures destroyed. Most masonry and frame structures destroyed, along with foundations. Bridges destroyed. Rails bent. Damage extensive.

X and XI. Some well-built wooden structures destroyed. Most masonry and frame structures destroyed, along with foundations. Bridges destroyed. Rails bent. Damage extensive.

From such maps of intensity, the earthquake is generally near the bulls-eye in the center of the worst damage, but other factors, such as the type of soil, locally influence the intensity.

Mountainous regions that undergo ground shaking may experience landslides, rock falls, and other earth movements. The ground can rupture along parts of the fault that slip during an earthquake, or from shaking of unconsolidated materials. The fault scarp and other cracks can destroy buildings and roads. Damage to structures from shaking depends on the type of construction. Concrete and masonry structures are rigid and do not flex easily. Thus, they are more susceptible to damage than wood or steel structures, which are more flexible. In this area, a flexible metal bridge in the center of the city survived the earthquake. Horizontal motions tend to be more damaging to buildings than vertical ones, because buildings are naturally designed to withstand vertical stresses. A concrete bridge farther downstream was too rigid and collapsed. Furthermore, it was built upon delta sediments that did not provide a firm foundation against shaking. In general, loose, unconsolidated sediment is subject to more intense shaking than solid bedrock. A tsunami is a giant wave that can rapidly travel across the ocean. An earthquake that occurs undersea or along coastal areas can generate a tsunami, which can cause damage along shorelines thousands of kilometers away. Aftershocks are smaller earthquakes that occur after the main earthquake, but in the same area. Aftershocks occur because the main earthquake changes the stress around the hypocenter, and the crust adjusts to this change with more faulting. Aftershocks are very dangerous because they can collapse structures already damaged by the main shock. Aftershocks after a tsunami can cause widespread panic. Ground shaking of uncon solidated, water-satur ated sediment causes grains to lose grain-tograin contact. When this happens, the material loses most of its strength and begins to flow, a process called liquefaction. This can destroy anything built on top. Historically, most deaths from earthquakes are due to collapse of poorly constructed houses and buildings, such as ones composed of mud, loosely connected blocks, and earthen walls. Even modern reinforced concrete, like that in freeways and bridges, can fail.

Fire is one of the main causes of destruction after an earthquake. Natural gas lines may rupture, causing explosions and fires. The problem is compounded if water lines also break during the earthquake, limiting the amount of water available to extinguish fires. Flooding may occur due to failure of dams as a result of ground rupturing, subsidence, or liquefaction. Near Los Angeles, 80,000 people were evacuated because of damage to nearby dams during the 1971 San Fernando earthquake (Mw 6.7). Earthquakes may cause both uplift and subsidence of the land surface by more than 10 m (30 ft). Subsidence accompanied the 2004 Sumatra earthquake, causing areas that had been dry land before the earthquake to become inundated by seawater, flooding buildings and trees. Subsidence and uplift can occur during or after an earthquake.

Earthquake hazard maps show zones of potential earthquake damage. Near Salt Lake City, Utah, the risk is greatest (reds) near active normal faults along the Wasatch Front, the mountain front east of the city. Living away from the fault is less risky. Some utility companies and hospitals have computer ized warning systems that are notified of impending earthquakes by seismic equipment. The system will automatically shut down gas systems (to avoid fire) and turn on back-up generators to prevent loss of electrical power. Earthquakes have diff erent periods, durations, and vertical and horizontal ground motion. This makes it difficult to design earthquake-proof buildings. Some rest on sturdy wheels or have shock absorbers that allow the building to shake less than the underlying ground.

A magnitude 9.2 (Mw) earthquake, one of the three or four largest earthquakes ever recorded, struck southern Alaska in 1964. It killed 128 people, triggered landslides, and collapsed parts of downtown Anchorage and nearby neighborhoods. This event was caused by thrust faults associated with the Aleutian Islands subduction zone. Most deaths and much damage were from a tsunami generated when a huge area of the seafloor was uplifted.

A huge earthquake occurred when 470 km (290 mi) of the San Andreas fault ruptured near San Francisco. The earthquake was likely a magnitude 7.7 to 7.8 (Mw) although not directly measured on seismometers. The earthquake ruptured the surface, leaving behind a series of cracks and open fissures. Within San Francisco, ground shaking destroyed most of the brick and mortar buildings. More than 3,000 people were killed and much of the city was devastated by fires that broke out after the earthquake.

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