Describe the types of seismic waves produced by earthquakes, how earthquakes are ranked, and what can be done to reduce our vulnerability to earthquakes.

No two earthquakes are exactly alike. After people have been in an earthquake, they may describe “rolling” or “up-and-down” or “sideways” movement. Earthquakes generate several different types of seismic waves. The movements people experience depend on the dominant type of seismic waves passing through the ground beneath them and the type of ground underfoot. Seismic waves can be categorized by where they travel and how they move through the crust, as illustrated in Figure 14.24.

P waves are fast-traveling compressional waves that move through the body of Earth. They are always the first to arrive after an earthquake. They are soon followed by S waves, which move perpendicularly to the direction they travel through the body of Earth. L waves and R waves, which move through the crust at Earth’s surface, arrive last and produce the greatest shaking (Figure 14.25).

The seismic waves detected at the earthquake focus always reach the epicenter first and, normally, shake the ground there the most. Ground shaking usually decreases with distance from the epicenter because the crust absorbs seismic wave energy.

The instruments used to detect, measure, and record ground shaking are called seismographs (or seismometers). Before the digital era, seismographs consisted of a swinging pendulum that recorded ground shaking (Figure 14.26A). Modern electronic seismographs generate an electrical signal to measure ground shaking and are far more sensitive than traditional pendulum seismographs (Figure 14.26B).

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The amount of ground shaking caused by an earthquake depends on the earthquake’s magnitude, the distance from its focus, and the composition of the ground being shaken. Two measures are used to characterize an earthquake’s strength: (1) intensity and (2) magnitude.

Earthquake Intensity

Earthquake intensity is determined by the amount of damage an earthquake causes to physical structures. The Mercalli intensity scale (or Mercalli scale) was developed in 1902 by the Italian scientist Giuseppe Mercalli as a means to estimate the intensity of shaking. No instruments are used to rank earthquakes on the Mercalli scale; instead, the scale is subjectively based on the observed damage done to structures. Later, the Mercalli intensity scale was developed into the modified Mercalli intensity (MMI) scale. In this system, earthquakes are ranked using Roman numerals, ranging from I to XII (Table 14.1).

There is no single MMI value for a given earthquake. Instead, locations progressively farther away from the epicenter experience less shaking as the seismic waves dissipate with distance, so each location is given its own MMI value. Figure 14.27 provides a modified Mercalli intensity map of the 2010 Haiti earthquake.

The distance seismic waves travel through the crust depends in large part on the integrity of the crust. In the western United States, for example, many faults separate sections of the crust, and seismic waves do not travel as far as they would if the crust were not fractured. In the eastern United States, seismic waves tend to travel greater distances because there are fewer faults.

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Another factor that influences the intensity of an earthquake is the composition of the ground. Loose, wet sediments deposited by rivers or human-made landfills are susceptible to liquefaction. Liquefaction is the transformation of solid sediments into an unstable slurry by ground shaking. Buildings resting on top of sediments may sink during liquefaction, as Figure 14.28 shows, unless their supporting piles are anchored in more stable ground, such as bedrock.

Earthquake Magnitude

Earthquake magnitude is determined from measurements of ground movement using seismographs. More ground movement creates higher-magnitude earthquakes. Each earthquake is given a single magnitude number that indicates the maximum shaking at the epicenter. Scientists can calculate earthquake magnitude from any seismograph on Earth if its distance from the epicenter is correctly established.

In 1935, the American geologist Charles Richter developed the Richter scale to quantify earthquake magnitudes using seismographic measurements. Richter’s system had limitations that have been addressed by several newer scales. One of these is the moment magnitude scale, an earthquake ranking system based on the amount of ground movement produced.

The moment magnitude scale relies on seismographic data to quantify ground movement. The scale also uses other types of data, such as how much the fault slipped, the amplitude of the ground movement (its up-and-down and back-and-forth extent), and the physical characteristics of the rocks at the epicenter. It takes several weeks to collect data and calculate the moment magnitude scale because scientists have to go out and inspect the ground for indications of the extent of movement. Although there is no upper limit to the moment magnitude scale, no earthquake exceeding magnitude 10.0 has ever been recorded. The strongest earthquake ever recorded, which occurred in Chile in 1960, was a magnitude 9.5.

What Do Magnitude Numbers Mean?

Earthquake magnitude indicates both how much the ground shakes and how much energy is released:

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The amount of energy released by a large earthquake is phenomenal. Comparing earthquake magnitude with familiar events, or kilograms of TNT, allows us to put the power of earthquakes into perspective (Figure 14.29).

When you think about it, earthquakes in and of themselves are not much to be feared. Imagine that you are picnicking in an open, grassy field when a strong earthquake occurs. What would happen? You would first experience up-and-down movement with the arrival of P waves, then you would experience side-to-side movement with the following S waves. Then the R waves and L waves would move the ground up and down and sideways. Your drinks would spill, and you might be tossed into the air. The sensation would be disorienting and exhilarating or terrifying, depending on your perspective. But you would probably not get hurt.

The same strong earthquake, occurring in a populated area, could bring death to thousands as structures collapse, bridges fail, bricks and glass rain down from above, and gas mains burst into flames. By itself, ground movement is not the problem—the structures that fail during ground movement create the hazard (Figure 14.30).

Earthquakes are as old as Earth’s crust itself. As long as Earth’s mantle moves the crust’s plates, there will be earthquakes. So we have to learn to live with them. But what are our options?

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Saving Lives

Because many lives are lost when built structures fail during earthquakes, engineers have redesigned structures to better withstand ground shaking. In the United States, building codes require that engineers build structures in accordance with the seismic risk for the region, and older structures must be retrofitted with steel support to make them safer. These building codes have made earthquakes far less of a hazard than before. Unfortunately, many countries do not have such building codes, and their residents are at risk from the collapse of buildings in earthquakes.

Another effective means of saving lives is to give earthquake warnings. Electrons in copper wire travel far faster than seismic waves in Earth’s crust. Thus, after an earthquake occurs, an automated system of alerts can be broadcast electronically. After the 2011 Töhoku earthquake, for example, people living in Tokyo, 370 km (230 mi) from the epicenter, had 80 seconds to shut off gas mains, stop trains, and seek shelter. These actions saved many lives. The USGS is developing an earthquake warning system in Southern California. This system could give downtown Los Angeles 50 seconds of warning time if a major earthquake occurred along the nearby San Andreas Fault.

Responsibility for earthquake safety also rests with every individual who lives where earthquakes are common. Table 14.2 lists some of the important ways individuals can prepare themselves for an earthquake.

Predicting Earthquakes

Scientists cannot predict earthquakes. Many seismologists believe we will never be able to predict earthquakes because their precise timing and location are largely random. Seismologists are much better at determining long-term seismic probabilities than at making short-term predictions. For example, they know that a 6.7 earthquake has a 99% probability of happening in California within the next three decades, and they know where the probability of such an event is highest (Figure 14.31).

The seismic risk of an area is determined by considering many factors, including the seismic probability, the types of faults present, how active those faults have been in recorded history, and the number of people living near them. Picture This highlights a twist of events in which scientists were prosecuted for failing to adequately warn the public of an earthquake.

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