It was the World Series. People filled Candlestick Park in San Francisco to watch the Giants play the Oakland Athletics. But the game scheduled for October 17, 1989, was not to be played. The blimp, floating high above the stadium, was strategically positioned to capture the drama of the game. But instead, the blimp was dispatched to film another drama-the drama of buildings cracking and fires breaking out as underground gas mains exploded. Sportscasters became newscasters as the focus of the baseball championship changed dramatically to the streets of San Francisco. If you tuned in to watch this World Series game, you saw a major earthquake "live."

A half hour before Game 3 at Candlestick Park, the earth began to rumble. Giants outfielder Robbie Thompson said he was running sprints down the right-field line when the 7.1 quake hit. He remembers hearing all the folded-up stadium seats clicking as they rattled from the force of the temblor.

"Everyone (in the stands) was on the move and then all of a sudden, we felt a big wave underneath us," Thompson said. "Then, we saw the upper deck going one way and then seeing the glass on the VIP box shaking like crazy and light posts swaying back and forth."

The earthquake would collapse a section of the Bay Bridge, the very symbol of the 1989 World Series, bring down the two-tiered Cypress structure of Interstate 880 in Oakland and devastate portions of San Francisco's Marina District that had been built on landfill. Although Candlestick Park suffered only minor damage, commissioner Fay Vincent, seated at the 'Stick that warm autumn day, announced that the World Series would be postponed until further notice. After 10 days, the Series resumed.

Giants Manager, Roger Craig confers with MLB officials during the immediate aftermath of a 7.1 magnitude earthquake that interrupted the 1989 World Series between the Oakland A's and the San Francisco Giants. Many people saw the earthquake live as network televison was coducting a pregame report before the start of game 3.

Earthquakes are dramatic examples-and not so gentle reminders-that the Earth's crust is continually moving. In this chapter, you will learn what causes some movements of the Earth's crust and how these movements are studied. You will also learn about one of the most sudden and violent movements: earthquakes. The Earth seems so solid-its surface strong and stable. But the occurrence of enormous natural disturbances such as earthquakes indicates that perceptions about the Earth's stability often differ from reality. The surface of the Earth actually moves in ways most dramatic. One has only to see the effects of an earthquake to appreciate this fact. (Right) The damage brought about by this 1985 earthquake in Mexico City shows the awesome power of earthquake waves.

In 1906, a devastating earthquake struck San Francisco, California. Fires that broke out after the quake destroyed most of the city.(left) Buildings that normally stood straight and true were realigned by movements of the Earth

An earthquake is the shaking and trembling that results from the sudden movement of part of the Earth's crust. A familiar example will help you to understand how an earthquake behaves. When you throw a pebble into a pond, waves move outward in all directions. In a similar manner, when rocks in the Earth's crust break, earthquake waves travel through the Earth in all directions. The ground shakes and trembles. During a severe earthquake, the ground can rise and fall like waves in an ocean. The motion of the ground causes buildings, trees, and telephone poles to sway and fall. Loud noises can sometimes be heard coming from the ground.

The San Andreas Fault in California extends near the border with Mexico to the south through the city of San Francisco and continues on and off shore to the coast of northern California. The San Andreas Fault is about 960 kilometers long and 32 kilometers deep. The land to the west of the San Andreas Fault is slowly moving north. The land to the east of the fault is moving south. But the rocks along the fault do not all move at the same time. Earthquakes occur in one area and then in another. One of the worst of the disasters occurred in 1906, when movement along a small section of the San Andreas Fault caused the famous San Francisco earthquake. The San Andreas Fault extends 960 kilometers along the western edge of California. Only a small portion of the fault is visible in the aerial photograph. (right) (below) Giant sea waves called tsunamis are caused by earthquakes on the ocean floor. When tsunamis are out to sea they are far apart fast moving and low. What happens to these waves near shore?

Earthquakes also occur on the floor of the ocean. These earthquakes often produce giant sea waves called tsunamis (tSoo-NAH-meez). Tsunamis can travel at speeds of 700 to 800 kilometers per hour. As they approach the coast, tsunamis can reach heights of greater than 20 meters. To get a better idea of this height, consider the following: A 6-story building about 20 meters tall! When a tsunami strikes the coast, it can cause great damage.
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Seismic Waves

Some faults are located deep inside the Earth. Others are close to or at the Earth's surface. Most faults occur between the surface and a depth of about 70 kilometers.

The point beneath the Earth's surface where the rocks break and move is called the focus (FOH-Cuhs) of the earthquake. The focus is the underground point of origin of an earthquake. Directly above the focus, on the Earth's surface, is the epicenter (EHPuh-sehn-tuhr). Earthquake waves reach the epicenter first. During an earthquake, the most violent shaking is found at the epicenter. Earthquake waves are known as seismic (SIGHZmihk) waves. Scientists have learned much about earthquakes and the interior of the Earth by studying seismic waves.

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The Seismograph

A seismograph (SIGHZ-Muh-grahf) is an instrument that detects and measures seismic waves. Although crude seismographs were in use hundreds of years ago, the first practical seismograph was developed by John Milne in 1893. Milne's invention has remained relatively unchanged to this day.

A seismograph consists of a weight attached to a spring or wire. (See the graphic to the right.) Because the weight is not Attached directly to the Earth, it remains nearly still even when the Earth moves. A pen attached to the weight records any movement of the Earth on a sheet of paper wound around a constantly rotating drum. Because the pen is attached to the weight, it also remains nearly still when the Earth moves. But the drum moves with the Earth. When the Earth is still, the pen records a nearly straight line. When the Earth moves, the pen records a wavy line. What kind of line would be recorded during a violent earthquake? Seismologists (sighZ-MAHL-uh-jihstz), scientists who study earthquakes, can determine the strength of an earthquake by studying the height of the wavy lines recorded on the paper. The seismograph's record of waves is called a seismogram (SIGHz-muhgram). The higher the wavy lines on the seismogram are, the stronger the earthquake is.

In a seismograph, a heavy weight attached to a wire olds a pen motionless while a rotating drum moveswith the Earth.

The height of the tallest wavy lines on a seismogram is used to calculate the strength of an earthquake on the Richter scale, which was created by California seismologists Charles Richter and Beno Gutenberg in 1935. The Richter scale was an important development because it gave scientists a way to determine earthquake strength based on readings from scientific instruments (namely, the seismograph). Before that, scientists were limited to estimating an earthquake's strength based on observations of the destruction the earthquake caused and eyewitness reports-sources that are not always accurate, consistent, or reliable.

Each number on the Richter scale represents an earthquake stronger than an earthquake represented by the preceding number. Any number above 6 indicates a very destructive earthquake. As you might imagine, an earthquake assigned the number 10 would be truly devastating!

The amount of damage caused by an earthquake depends on several different factors. The earthquake's strength, the kind of rock and soil that underlies an area, the population of the area affected, the kinds of buildings in the area, and the time at which the earthquake occurs all influence how damaging a particular earthquake is.

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Predicting Earthquakes

In their study of earthquakes, scientists hope to improve the ability to accurately predict them. To be useful, earthquake prediction must be reliable and complete. The prediction must include where, when, and how strong the earthquake will be. If a strong earthquake is predicted, people can be moved from areas in danger. In 1975, Chinese scientists predicted with great accuracy that an earthquake would occur in their country. Most of the people in three areas of the country were evacuated before the earthquake struck. Many thousands of lives were saved.

If strong earthquakes could be predicted years in advance, people could better plan the growth of cities. Buildings could be reinforced to better withstand the shock waves produced by an earthquake. In some cities, attempts have already been made to construct earthquake-proof buildings. In what other ways might more accurate earthquake prediction save lives?

Seismologists have identified some warning signals that help to predict earthquakes with greater accuracy. Often changes occur in the speeds of Primary waves and Secondary waves before a major earthquake strikes. Sometimes slight changes in the tilt of the Earth's surface can be detected. Land near a fault may rise or sink slightly. The water level in wells often goes up or down. And although it sounds a bit unscientific, some scientists in China believe that changes in the behavior of certain animals might help to predict earthquakes.

Below, is a Seismic risk Map showing areas of the contiguous 48 United States where Earthquakes are likely to occur and the relative damage they are likely to cause.

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1. What is an earthquake? What is the most common cause of an earthquake?

2. What is the focus of an earthquake? What is the epicenter?

3. How does a seismograph work?

4. How is strength of an earthquake measured?

5. In 1906, San Francisco was the sight of a devastating earthquake. What is the name of the fault that San Francisco lies on?

6. Which direction is the land moving along the San Andreas Fault?

7. What are some factors that determine how much damage an earthquake may do?