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Earth Science
When the Earth Hiccups

Earth Science
Earth Science

When the Earth Hiccups
Print When the Earth Hiccups Reading Comprehension

Reading Level
     edHelper's suggested reading level:   grades 8 to 9
     Flesch-Kincaid grade level:   8.19

     challenging words:    hiccupping, rayleigh, slippage, undeformed, determined, subducting, subducts, epicenter, mesosphere, seismologists, seismic, better, intermediate, divergent, mid-ocean, subduction
     content words:    Andrija Mohorovicic, New Madrid, South Carolina, Mississippi River, North America

When the Earth Hiccups
By Trista L. Pollard

1     Occasionally the Earth moves right underneath our feet. In some areas of our world, it moves enough to cause major damage. In other areas, the Earth merely hiccups. Earthquakes, according to seismologists, are movements of the ground caused by bursts of released energy. This energy is released when rocks move along faults. It is when these rocks are under stress that they suddenly move. Remember, faults are breaks in the bodies of rocks. Within these faults, the rocks slide in relation to other rocks.
2     So why do we have earthquakes? As rocks are positioned along faults, they are pressed together very tightly. In this situation, the friction between the rocks keeps them from moving past each other. This occurs when there is a locked fault. The fault remains locked until the stress becomes too great and slippage occurs. The newly unlocked rocks move past each other suddenly. Earthquakes occur because the slippage causes trembling and vibrating.
3     When you think of rocks, the word "elastic" does not come to mind. However, elastic rebound may also be a reason for earthquakes. Elastic rebound occurs when elastically deformed rocks suddenly return to their undeformed shape. On each side of the fault, rocks move slowly, and once the fault becomes locked, the stress increases. Eventually the stress reaches a point where the rocks cannot remain locked. At the point of stress, the rocks fracture; at their weakest points the rocks separate and rebound to their undeformed shapes.
4     Scientists have defined different parts of an earthquake. The focus of an earthquake is the point located below the Earth's surface along a fault. This is the point where an earthquake's first motion occurs. An epicenter is the point on the surface that is directly above the focus. As you can imagine, the depth of an earthquake's focus can vary. However, scientists have found that about 90% of continental earthquakes contain foci that are shallow. A shallow focus occurs within 70 kilometers of the surface. Intermediate foci earthquakes occur from 70 kilometers to 300 kilometers below the surface. Shallow and intermediate foci earthquakes usually occur away from subduction zones and closer to plate boundaries. Unlike earthquakes with shallow foci, intermediate and deep earthquakes tend to cause less damage once the energy reaches the surface. This is due to the greater distance the energy travels before reaching the surface. As it travels, it radiates outward and dissipates.
5     The energy produced by earthquakes is referred to as seismic waves. Energy in the form of vibrations is released as the rocks slip into their new positions along the fault. Once the energy is released from the focus, it travels outward in all directions through surrounding rock. If you could see the energy as it moves, it would resemble the waves you would see when a drop of rain falls into a puddle. Seismic waves come in two main varieties. Body waves travel right through the surrounding rock near and beyond the focus. Surface waves travel along the surface of rocks near and beyond the focus. Both waves travel to the surface at different speeds and, therefore, produce different types of movement in the Earth's crust.

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