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19.1 Forces Within the Earth Most earthquakes are the result of movement of Earth’s crust produced by plate tectonics. Along the boundaries between two plates, rocks in the crust often resist movement. Over time stress builds up. Stress is the total force acting on crustal rocks per unit of area. When stress overcomes the strength of the rock, movement occurs along the fractures in the rock. The vibrations caused by this sudden movement are felt as an earthquake. There are three kinds of stress that act on Earth’s rocks: compression, tension and shear. Compression is stress that decreases the volume of material, tension is stress that pulls a material apart, and shear is stress that causes a material to twist. The deformation of materials in response to stress is strain. Rocks can be twisted, stretched, and squeezed, but everything has a breaking point and when the stress and strain on rock becomes to great, an earthquake happens. When stress on a rock is low, the material shows an elastic deformation. This is when the material is compressed, bent, or stretched. Like a rubber band being stretched. When the stress is released, the rubber band returns to its original shape. Plastic deformation is when stress builds up past a certain point and permanent deformation occurs even when the stress is released. Faults – A fault is a fracture or a system of fractures along which Earth moves. There are three types of faults. moves horizontally & vertically because of compression moves horizontally & vertically because of tension shear causes horizontal movement Types of seismic waves Seismic waves are vibrations produced in the ground by an earthquake. There are three types of seismic waves. Primary waves – also known as P-waves push and squeeze rocks in the direction along which the waves are traveling. The vibration passes through the ground parallel to the direction of the movement. Secondary waves also known as S-waves are named according to their arrival time. These waves move slower than P-waves, so they are the second set of waves to be felt. S-waves have a motion that causes rocks to move at right angles in relation to the direction of the waves. Surface waves are the slowest of the three types of waves and travel only along Earth’s surface. These waves can cause the ground to move both sideways and up and down. These waves cause the most destruction because they cause the most movement of the ground and take the longest time to pass. Focus – is the point below the surface of the Earth where the waves originate from. Epicenter – is the point on Earth’s surface directly above the focus.