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How and Where Earthquakes Occur Key Idea: Most earthquakes result from strain that builds up at plate boundaries. After completing this lesson, we’ll know: 1. 2. 3. 4. 5. The causes of earthquakes Where earthquakes are most likely to occur The elements of an earthquake How the energy of an earthquake is released and travels as waves Types of waves generated by an earthquake a) body waves (P waves, S waves) b) surface waves (Love waves, Rayleigh waves) What is an Earthquake? An earthquake is a shaking of the earth caused by a release of energy. The release of energy could be generated by many different causes, such as: The impact of a meteor The collapse of an cavern The eruption of a volcano Most earthquakes however, are generated by the strain that builds up along faults at or near boundaries between lithospheric plates. A fault is a break in the lithosphere along which movement has occurred. Examples of Faults A Strike-Slip Fault A Normal Fault The Elastic Rebound Theory When two plates are moving, friction occurs at their contact. Due to friction, strain builds up, causing plates to deform, or change shape. Eventually the strain becomes great enough to overcome friction, and the plates move suddenly, causing an earthquake. Then, the plates snap back to the shapes they had before they were deformed, but at new locations relative to each other. (This model is the elastic-rebound theory) The “Parts” of an Earthquake The “Parts” of an Earthquake The FOCUS is the point where the discharge of energy takes place. It is where the earthquake originates and it is usually many kilometers beneath the surface. The EPICENTER is the point on the Earth’s surface located directly above the focus. Focus and Epicenter How deep do earthquakes form? The depth at which an earthquake originates depends upon the type of plate boundary involved. At divergent boundaries, such as the Mid Atlantic Ridge, the epicenter is around 30 Km beneath the surface. Along transform boundaries, the depth is also several km. At subduction boundaries, where a plate plunges beneath its neighboring plate the focus can be located as deep as 700 Km. How does the energy released at the focus travel? The energy released at an earthquake travels in waves. As the waves travel beneath the surface, through the material of Earth’s body, they are called body waves. There are two kinds of body waves: 1. P waves 2. S waves P Waves The P waves, also called Primary waves, or compressional waves squeeze and stretch the rock materials as they pass through Earth. A slinky models the P waves perfectly. The S Waves The S waves are also called sheer waves, or secondary waves cause the rock material to move perpendicular to the direction in which the waves are traveling. More on P and S Waves The P waves travel faster than the S waves they can travel solids (rocks), and fluid materials, such as magma, water, and air. The S waves are slower they only travel through solid materials the denser the rock , the faster they travel. Surface Waves As the body waves reach the surface of the Earth, they become surface waves. These are the waves that produce damage during an earthquake. The two types of surface waves are 1. Love waves, which cause particles of material to move side to side; 2. Rayleigh waves cause particles of material to move in elliptical patterns. These waves are similar to the ripples at the surface of water. Surface Waves Let’s summarize: 1. 2. 3. 4. 5. 6. 7. What are some of the cause of earthquakes? Where are most earthquakes likely to occur? What is the difference between focus and epicenter? What are body waves? What is the difference between P waves and S waves? What are surface waves? What is the difference between Love waves and Rayleigh waves?