EARTHFORCE i n   t h e   c r u s t

 Go outside, find a flat piece of ground, and stand perfectly still. You won't be able to notice it, but the ground beneath your feet is moving. Imagine that you are balancing on a raft on a pond, gently drifting. If you live on the North American continent, the ground beneath you is one giant raft drifting westward very slowly, about 2.5 centimeters (one inch) per year. Get a ruler and think about that. Imagine that you move just one inch during an entire year. That seems pretty slow, but, considering the size of the continent, the force of that movement can be huge. If a bicycle moved an inch and bumped into you, it wouldn't really hurt. If a car moved an inch and bumped into you, it might make a bruise. If a school bus moved an inch, the bump might break a bone. If a tractor trailer bumped into you, you'd notice. Scientists think that the entire crust of the Earth is broken into big pieces called plates. All of the plates are drifting on the liquid molten magma of the mantle beneath the crust. Even though they're drifting very slowly, when they bump into each other, the force is huge. People who live near the edges of a plate notice the bumping. Most often, the earth beneath them quakes and shakes as the plates bump and rub. Scientists call this action plate tectonics. Over millions of years, plate tectonics has changed the appearance of the Earth's crust. Besides the crustal plates drifting and moving to different locations, the pushing and pulling between plates causes mountains and valleys to form. Scientists think that, long ago, the crustal plate of India collided with the huge crustal plate of Asia to form the Himalayas, the highest mountains in the world. While crustal rock is solid, it does have some elastic properties. With enough force and heat, rock can bend and change shape. When the two plates bumped into each other, rock along the edges pushed upward. Even today, scientists think that the Indian plate is still colliding with the Asian plate, moving about five centimeters (two inches) per year, and causing the highest mountains to rise even higher. Most of the plates meet under the oceans where the force of their pushing and pulling may sometimes not even be noticed. Some plates, like the Indian and Asian plates, meet on dry land. The Himalayan mountains were formed when the two plates pushed together. The Pacific and North America plates also meet on dry land. These two giant crustal plates are not pushing together, however. They are scraping against each other. The Pacific plate is drifting Northward, scraping along the edge of the North American plate. The result of this tremendous force is the San Andreas fault, which cuts vertically through the state of California for nearly 1,125 kilometers (700 miles). Scientists think that the San Andreas fault may be one hundred million years old. The crack in the crust between the plates is at least thirty-two kilometers (twenty miles) deep. For many years, scientists have used the fault to study the crustal plates, properties of rock, and the core of the Earth. The San Andreas fault is a dangerous laboratory, however. In some places, the force of the plate scraping has broken roads, relocated streams, and lowered mountains. The biggest danger, of course, is from the quaking. The crustal plates are pushing and pulling causing motion. The mantle beneath the crust is pushing and pulling causing motion. All of that force builds up energy. When the energy gets released, the crust vibrates. Each month, scientists observe several small earthquakes along the San Andreas fault. Most of them are barely noticeable. Once in a while, however, the energy really builds and releases a tremendous vibration. Earthquakes are a natural part of the Earth's geology and are not necessarily dangerous. In the past 150 years, a tiny blip in the lifespan of the fault, Americans have moved to California and built structures near the fault. During that time, many lives have been lost due to quakes along the fault. In 1906, for example, the crust vibrated strongly for forty-five seconds. In the history of the fault, the 1906 quake was minor. However, in nearby San Francisco, a young city was destroyed. Five hundred people died—not from the quake, but from the falling buildings and raging fires that resulted. Buildings tumbled to the ground, breaking streets below. Woodstoves and oil lamps ignited the rubble. The city burned for three days. In spite of the destruction and potential for future earthquakes, the citizens of San Francisco stayed and rebuilt their city. Today, buildings are "earthquake- resistant." Emergency procedures are ready and waiting. Yet, in 1989, another quake rocked San Francisco, killing sixty- two people, destroying buildings, and igniting fires across the city. Clearly, there is no way to be prepared completely for violent vibrations in the crust. Learn where the earth quakes most often so that you know the greatest danger zones. Everyday, the EARTHFORCE is quaking somewhere. Browse these websites daily to see where. Current Quakings Today's Earthquake Activity Around the World Up-to-the-Minute Southern California Earthquake Map Maps and Lists of Current Earthquake Activity Explore EARTHFORCE and earthquakes by browsing these selected websites. Plate Tectonics Earth's Interior This Dynamic Earth: The Story of Plate Tectonics The ABC's of Plate Tectonics The Formation of Pangaea: The Making of a Supercontinent Plate Tectonics: The Cause of Earthquakes Earthquake Science Earthquake Fact and Follies What's Shakin'? Earthquake ABC: A Child's View of Earthquake Facts and Feelings What is Richter Magnitude? Seismic Deformation Every Place Has Its Faults Earthquake Quiz Teacher Lessons Earthquakes: Musical Plates Interactives Virtual Earthquake Java Earthquake Globe Java Earthquake Elastic Rebound Animation History Earthquakes by Twain, London, Darwin, and Muir A Brief History of Seismology to 1910 San Francisco Earthquake 1906 Earthquake Engineering Slide Information System Preparedness Putting Down Roots in Earthquake Country