The following statements summarize and describe many of the key terms and concepts presented
in the chapter.
- Earthquakes are vibrations of Earth produced by the rapid release of energy from rocks that rupture because they have been subjected to stresses beyond their limit. This energy, which takes the form of waves, radiates in all directions from the earthquake's source, called the focus. The movements that produce most earthquakes occur along large fractures, called faults, that are associated with plate boundaries.
- Two main groups of seismic waves are generated during an earthquake: (1) surface waves, which travel along the outer layer of Earth; and (2) body waves, which travel through Earth's interior. Body waves are further divided into primary, or P, waves, which push (compress) and pull (expand) rocks in the direction the wave is traveling, and secondary, or S, waves, which shake the particles in rock at right angles to their direction of travel. P waves can travel through solids, liquids, and gases. Fluids (gases and liquids) will not transmit S waves. In any solid material, P waves travel about 1.7 times faster than do S waves.
- The location on Earth's surface directly above the focus of an earthquake is the epicenter. An epicenter is determined using the difference in velocities of P and S waves.
- There is a close correlation between earthquake epicenters and plate boundaries. The principal earthquake epicenter zones are along the outer margin of the Pacific Ocean, known as the circum-Pacific belt and through the world's oceans along the oceanic ridge system.
- Seismologists use two fundamentally different measures to describe the size of an earthquakeintensity and magnitude. Intensity is a measure of the degree of ground shaking at a given locale based on the amount of damage. The Modified Mercalli Intensity Scale uses damage to buildings in California to estimate the intensity of ground shaking for a local earthquake. Magnitude is calculated from seismic records and estimates the amount of energy released at the source of an earthquake. Using the Richter scale, the magnitude of an earthquake is estimated by measuring the amplitude (maximum displacement) of the largest seismic wave recorded. A logarithmic scale is used to express magnitude, in which a tenfold increase in ground shaking corresponds to an increase of 1 on the magnitude scale. Moment magnitude is currently used to estimate the size of moderate and large earthquakes. It is calculated using the average displacement of the fault, the area of the fault surface, and the sheer strength of the faulted rock.
- The most obvious factors that determine the amount of destruction accompanying an earthquake are the magnitude of the earthquake and the proximity of the quake to a populated area. Structural damage attributable to earthquake vibrations depends on several factors, including (1) intensity, (2) duration of the vibrations, (3) nature of the material upon which the structure rests, and (4) the design of the structure. Secondary effects of earthquakes include tsunamis, landslides, ground subsidence, and fire.
- Substantial research to predict earthquakes is under way in Japan, the United States, China, and Russiacountries where earthquake risk is high. No consistent method of short-range prediction has yet been devised. Long-range forecasts are based on the premise that earthquakes are repetitive or cyclical. Seismologists study the history of earthquakes for patterns, so their occurrences might be predicted.
- As indicated by the behavior of P and S waves as they travel through Earth, the four major zones of Earth's interior are the (1) crust (the very thin outer layer), (2) mantle (a rocky layer located below the crust with a thickness of 2885 kilometers), (3) outer core (a layer about 2270 kilometers thick, which exhibits the characteristics of a mobile liquid), and (4) inner core (a solid metallic sphere with a radius of about 1216 kilometers).
- The continental crust is primarily made of granitic rocks, while the oceanic crust is of basaltic composition. Ultramafic rocks, such as peridotite, are thought to make up the mantle. The core is composed mainly of iron and nickel.
- The crust and uppermost mantle form Earth's cool rigid outer shell called the lithosphere. Beneath the lithosphere lies a soft, relatively weak layer of the mantle known as the asthenosphere.