Getting the Inside Story

At the start of the twentieth century, the discovery of radioactivity was not the only development to help geologists unravel the history of the planet. Scientists who were studying earthquakes at this time were finding out just how important these shocking events could be.

Earthquakes have been recorded in history for thousands of years. When an earthquake strikes, the ground suddenly starts shaking without any warning and then stops just as quickly. Most earthquakes last less than a minute, but the damage they do can take a lifetime to repair. While some earthquakes are quite violent, most are pretty tame. In fact, hundreds of earthquakes occur every day that are not even felt. But those quakes that are felt, the "big ones," are the ones that worry scientists and engineers. The worst damage from these earthquakes often comes when their vibrations cause buildings to collapse, trapping and killing the people inside.

In ancient times, most people believed that earthquakes were a sign from God, who was angry with people. They believed that the only way to stop an earthquake was to repent or offer sacrifices. As time went on, scientists started realizing that earthquakes did not happen everywhere but seemed to occur only in certain are called S-waves (secondary waves), which are also called "shear waves." P-waves can move through liquids like water and magma, but S-waves can only move through solid rock.

By the early twentieth century, seismologists had learned a great deal about earthquakes from studying their waves. They discovered that some earthquakes take place deep underground, while others are fairly shallow. They named the point where Earth actually shifts the focus. The point on the ground surface directly above the focus of an earthquake was called the epicenter. By recording the difference in arrival times between P- and S-waves, scientists could calculate the distance to an earthquake epicenter from the seismograph that recorded the waves. Using data from three different seismographs allowed them to pinpoint the location of the epicenter.

One of the most important discoveries made by seismologists was the fact that seismic waves, like light and sound waves, could bend and bounce when they hit rock with different properties. By plotting the way that waves moved through the Earth, seismologists could get an idea what the rocks looked like below the surface without having to dig or drill. This was the key that geologists were looking for. At the start of the twentieth century, geologists finally could piece together the inner structure of the planet.

By the early part of the twentieth century, geologists had set up hundreds of seismographs around the world. These devices were so sensitive, they could detect the vibrations from large earthquakes on the opposite side of the Earth. As they recorded the arrival times of seismic waves from different earthquake events, seismologists soon discovered a problem. Based on earlier studies of how fast these waves traveled through rocks, the times they were getting were way off the predicted numbers. If Earth was a solid, uniform planet made up of the same basic rock that is found near the surface, the travel times for the waves should be very consistent. The only explanation for these variations in speed was that Earth was not a solid, uniform planet.

By the mid-1900s, a number of geologists, including Charles Richter (for whom the Richter Scale is named) and Beno Gutenberg in the United States and Harold Jeffreys in England, were tackling

Figure 5.2 Scientists are able to locate an earthquake's epicenter using data from three different seismic stations. Each center determines a radius where the epicenter might be located. The intersection of the three centers' radii is the location of the epicenter.

the problem. They found that Earth is really made up of different layers. The heaviest, densest materials lie at the center of the planet, and lighter materials are near the surface. In 1952, an American geologist named Albert Francis Birch published a paper that put all these pieces together.

By studying many lines of evidence, Birch concluded that Earth is really made up of four distinct layers. At the center of the planet is a dense, solid inner core made of mostly iron and nickel. This inner core is surrounded by a second outer core that is made of a liquid metal. As mentioned earlier, it is the interaction of these charles richter invents a scale

When it comes to earthquakes, many people have heard of the Richter Scale but few know much about the man who developed it, Charles Francis Richter. Charles Richter was born in 1900 on a farm in Ohio. After his parents divorced, he and his mother moved to California, a part of the world known for its many earthquakes. In 1920, he graduated from Stanford University, eventually receiving his Ph.D. in theoretical physics from the California Institute of Technology (Cal Tech) in 1928. A year earlier, Richter had started working at the Carnegie Institution of Washington's Seismological Laboratory in Pasadena, California, where he met Beno Gutenberg, a seismologist who worked at the lab at the time.

Richter's research centered on the actual physics involved in earthquakes. He and Gutenberg measured the energy output of local earthquakes to see if they could come up with a way of classifying them. At the time, the only way of ranking earthquakes was a system developed by an Italian scientist named Giuseppe Mercalli back in 1902. The Mercalli scale was based on observations of damage to buildings at different locations near an earthquake epicenter. However, the problem with this scale was that the same earthquake could give very different Mercalli measurements at two different locations. What two parts of the core that is thought to produce Earth's magnetic field.

Surrounding the core is the bulk of the planet called the mantle. The mantle is thought to be composed of dense rocks that are under such high pressure that they actually flow like liquids over long periods of time. Finally, sitting on top of the mantle is a relatively thin, brittle layer of rock called the crust. In 1910, a Croatian seismologist named Andrija Mohorovicic used the refraction, or bending, of seismic waves to discover the crust-mantle bound-

was needed was a standard way to measure the energy output of the earthquake itself.

After cataloging the seismograms of dozens of earthquakes, Richter and Gutenberg came up with an idea. By using the height of the S-waves recorded on a seismogram, they were able to come up with a way to measure the intensity of an earthquake. Richter developed a logarithmic magnitude scale where each whole number on the scale would represent a 10-fold increase in the amplitude of height of the wave recorded on a seismogram. Richter and Gutenberg published their work in 1935, but even though both men shared in the development of the scale, only Richter's name was given to it.

In 1936, Richter was asked to head up the newly formed Seismological Laboratory at the California Institute of Technology where he also started teaching. He would remain at Cal Tech for the next 35 years, teaching and conducting research. During this time, Richter wrote several classic textbooks dealing with earthquakes, including Elementary Seismology, which was published in 1958 and is still used today. After he retired in 1970, Richter continued to work as a media consultant. He died in 1985, four days after his 85th birthday. His name will always be remembered every time an earthquake is measured.

ary and, in the process, helped explain where earthquakes come from.

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