Explain what causes earthquakes.

On Friday, March 11, 2011, seismographs around the world began detecting one of the largest earthquakes in recorded history, now called the 2011 Toãhoku earthquake. The shaking began at 2:46 p.m. local time. The earthquake was calculated at magnitude 9.0, a colossal event. There are more than 1 million detectable earthquakes on the planet each year, and this single 9.0 event released more energy than all of the others combined. Only four other recorded earthquakes have been larger. The earthquake focus was 32 km (20 mi) deep and 128 km (80 mi) from Sendai, on the island of Honshu, Japan. The aftershocks that followed for weeks were as powerful as magnitude 7.2.

The damage caused by the earthquake and its aftershocks was made much worse by a tsunami that reduced the low-lying coastal regions in its path to ruins (see the Human Sphere section at the beginning of this chapter to learn about tsunamis). To make matters even worse, local nuclear power plants survived the shaking, but were not designed to be flooded by salt water. After they were flooded, they leaked radiation, which traveled across the Northern Hemisphere. Bringing the damaged nuclear plants under control and stopping radiation leaks have been among the greatest challenges brought by this earthquake. As of 2014, radiation continues to leak from the Fukushima Daiichi nuclear power plant into the Pacific Ocean.

Although usually less noticeable than volcanic hazards, earthquakes are as dangerous as volcanoes, or even more so. The 2011 Toãhoku earthquake, like all earthquakes, occurred when Earth’s crust broke along a geologic fault, which is a fracture in the crust where movement and earthquakes occur (see Section 12.4).

Most earthquakes are too small to be felt by people. Only seismographs can detect them. Many of those that do shake the ground strongly occur in remote areas, such as the deep seafloor, and are harmless to people. Very rarely, a massive earthquake, such as the Toãhoku earthquake, occurs near a populated region, causing catastrophic loss of life and structural damage to the built environment.

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Three Types of Faults

There are three basic types of faults: normal faults, reverse faults, and strike-slip faults (Figure 14.19). A normal fault is a result of tensional force (extension) as two pieces of Earth’s crust, called fault blocks, are pulled apart. As a result, one fault block slips downward in relation to the other fault block. A reverse fault results from compressional force, which pushes one block upward in relation to another block. Under certain circumstances, reverse faults are also called thrust faults. A strike-slip fault occurs where one block moves horizontally in relation to another block as a result of shearing (lateral) force.

Reverse and normal faults create a fault scarp, or cliff face, that results from the vertical movement of the fault blocks. Strike-slip faults create little up or down block movement. Where strike-slip faults cross orchards, streams, roads, sidewalks, and other linear features, those features may be offset by fault movement. Left-lateral strike-slip faults occur when, from the perspective of either block, the opposite block moves to the left. Right-lateral strike-slip faults, as shown in Figure 14.20, occur when the opposite block moves to the right.

Fault scarps indicate that a normal or reverse fault is at work, and offset features indicate that a strike-slip fault is present. Like much of the western United States, California and Nevada have many fault systems with all three fault types, as shown in Figure 14.21.

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How Do Faults Generate Earthquakes?

When subjected to geologic stresses, fault blocks usually do not move smoothly past one another. Instead, friction between them causes them to stick together, and stress energy builds up in the crust. Eventually, the geologic stress exceeds the friction, the crust breaks (either along a preexisting fault or along a new fault), and the blocks move. As each block moves, the built-up stress energy is released and travels through the crust as seismic waves, resulting in an earthquake.

Elastic-rebound theory describes how fault blocks bend, break, and rebound back to their original shape as they move in relation to one another. The blocks may become stuck again from friction, then slip again in this stick-slip process. The focus is the location of initial movement along a fault during an earthquake. The epicenter is the location on the ground’s surface immediately above the focus of the earthquake and is usually the area of greatest shaking. These concepts are illustrated in Figure 14.22.

What Are Foreshocks and Aftershocks?

Small foreshock earthquakes sometimes precede large earthquakes. Foreshocks may be caused by smaller cracks developing as the deformed and stressed crust is about to fail. Going back to the bending stick analogy used in Figure 14.22, as the stick bends, small splinters of wood may form—these are the foreshocks. They may indicate that the stick is about to break—or that the rocks are about to fault. The breaking of the stick represents the main earthquake.

Very commonly, especially with large earthquake events, smaller earthquakes called aftershocks follow the main shock. Aftershocks occur because the blocks are settling into their new positions after they have been moved. Most aftershocks are much smaller than the main earthquake and occur on the same fault as the initial earthquake. Occasionally, aftershocks occur on different faults nearby.

Most earthquakes occur along plate boundaries in seismic belts. Plate boundaries give rise to earthquakes because of the interactions between moving plates that occur there. Figure 14.23 explains some major characteristics of earthquakes at different types of plate boundaries.

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