53 Earthquakes-Mid

Earthquakes FOCUS Question How do earthquakes change Earth's surface?

Shaking Things Up It was the third game of the 1989 World Series. More than sixty thousand baseball fans packed Candlestick Park in San Francisco, California. But the fans got more action than they bargained for when the entire stadium shook for fifteen seconds. It was an earthquake! The game was called off. Luckily, the fans were unharmed. People in other parts of the city were not so fortunate. Although you may not feel the ground shaking, an earthquake is probably happening someplace on Earth right now. An earthquake is the shaking that occurs when enormous rocks in Earth's crust suddenly move and energy is released.

Fault Formation Earth's crust—its outermost layer—is made up of many distinct pieces called tectonic (tek-TAH-nik) plates. These plates sit on top of a layer called the mantle, which is not as rigid and can flow. Currents in the mantle cause the plates to move slowly and continuously. This movement of Earth's plates is known as plate tectonics. Tectonic plates meet at plate boundaries, where they can separate, collide, or slide past one another. At plate boundaries, huge slabs of rock push and pull, and pressure builds up within them. If enough pressure builds up in the rocks, they break and form a fault— a crack in Earth's crust Most faults occur along plate boundaries, but they can also form in the middle of plates. Where there are faults, there are often earthquakes.

Earthquake Alert An earthquake begins when the land along a fault suddenly moves. The focus of the earthquake is the underground location where the land first moves. Waves of energy, called seismic (SIZE-mik) waves, radiate outward from the focus like ripples of water on a lake. This release of energy causes the ground to shake. The epicenter of the quake is the point on Earth's surface directly above the focus. It's where the strongest shaking occurs. During an earthquake, rocks on either side of a fault move in opposite direction—supward, downward, or sideways. This motion can abruptly change Earth's surface. Do You Know? Earthquakes can change Earth's rotation. A massive earthquake near Indonesia in 2004 made Earth spin a bit faster and slightly shortened the length of one day.

Changing Earth's Surface Earthquakes change the surface and put people and structures at risk. During an earthquake, the displacement of rock can be small, moving Earth's surface less than a meter. But rock can also move several kilometers. A cliff, called a scarp, can form when an earthquake causes one side of a fault to push upward. Landforms such as valleys and ridges become offset (not lined up) where one side slides past the other. Normally, soil acts like a solid and can support the heavy weight of landforms and structures. But shaking makes some soil behave like a liquid. This process is called liquefaction. Liquefied soil can trigger landslides and mudslides. Bridges, buildings, and roads built on the loosened soil can collapse.

Measuring Up More than a million earthquakes happen around the world each year. Most are so minor that you wouldn't even feel them, while others are massive. One way to measure and compare the strength, or magnitude, of earthquakes is to use the Richter scale. On this scale, larger numbers indicate more powerful earthquakes. Each higher number on the scale represents an increase of about thirty-one times more energy. Scientists also use the moment magnitude scale (below). This scale is based on the total energy recorded for an earthquake. As with the Richter scale the higher the number, the larger the earthquake and the more energy it has.