Earthquakes Reading Comprehension Lesson

What Causes Earthquakes?

Earthquakes cause the earth to shake! Earthquakes are caused by movement along a fault line under the Earth’s crust. You do not have to be right under a fault line to feel an earthquake — you could be miles away and still feel the ground rattling. Most earthquakes only last for about one minute. They cannot be predicted before they happen.

The Earth’s crust is made up of large sections called tectonic plates that fit together like puzzle pieces and move very slowly over millions of years. Fault lines form where two plates meet. When the pressure between two plates builds up and they suddenly slip past each other, energy is released as seismic waves — waves of vibration that travel through the ground in all directions from the epicenter (the point on the surface directly above where the earthquake started).

Scientists measure the strength of earthquakes using the Richter Scale. An earthquake measuring 4.3 or less usually causes no damage and may not even be felt. A 4.4–4.8 magnitude quake can break glasses and cause small amounts of damage. A 5.5–6.1 quake can damage homes — furniture may break and walls may crack. An earthquake measuring 6.6 or higher causes severe damage: trees fall, homes collapse, and the ground itself may crack open. The strongest earthquakes ever recorded have measured above 9.0 on the Richter Scale.

Famous California Earthquakes and Safety Tips

California lies along several major fault lines, including the famous San Andreas Fault, and experiences thousands of earthquakes each year — most of them too small to feel. On January 17, 1994, a magnitude 6.7 earthquake struck near Northridge, California. It caused $15 billion in damages, 9,000 injuries, and 57 deaths. On October 17, 1989, a 6.9 magnitude earthquake struck during the World Series baseball game in San Francisco, lasting only 15 seconds but causing $6 billion in damage and 62 deaths.

Aftershocks are smaller earthquakes that occur after the main earthquake, sometimes for days or weeks. They can add to the damage caused by the main quake and are especially dangerous for buildings that are already weakened. When scientists monitor aftershock patterns, they can sometimes learn more about where fault lines are located and how deep they run underground.

If an earthquake strikes, the safest thing to do is Drop, Cover, and Hold On. Drop to your hands and knees. Take cover under a sturdy table or desk, or against an interior wall away from windows. Hold on until the shaking stops. Once the shaking is over, check yourself and others for injuries before moving. Stay away from damaged buildings, broken glass, and downed power lines. If you smell gas, leave the building and call the gas company. Always have an emergency kit and a family meeting place planned in advance.

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