What is an Earthquake? An earthquake is the shaking and trembling that results from the movement of rock beneath Earth’s surface.
Earthquake -- cont’d Powerful forces cause the Earth’s plates to squeeze rocks together and pull it in different directions. This type of force is called stress. Stress adds energy to rocks. When this stored up energy changes the shape of rocks or breaks the crust, it is called deformation.
Types of Stress Causing Deformation There are three kinds of stress that cause deformation: Shearing: pushes a mass of rock in two opposite directions. Tension: pulls on the crust and stretches the rock. Compression: squeezes the rock until it folds or breaks. Shearing, tension, compression work over millions of years to change the shape and volume of rock.
Types of Faults There are three types of faults: Strike-slip fault: rocks on either side of the fault slide past each other sideways with little up or down motion. Caused by shearing.
Types of Faults -- cont’d Normal fault: one block of rock lies above the fault while the other block lies below it. The block above is called the hanging wall and the block below it is called the footwall. Normal faults are caused by tension.
Types of Faults -- cont’d Reverse Faults: the hanging wall slides upward past the footwall. Reverse faults are caused by compression.
Movement Along the Fault The amount of movement depends on how much friction there is between them. Low friction means the blocks slide constantly without sticking. High friction is when the blocks lock together. When the amount of stress is greater than the amount of friction, the blocks will suddenly unlock and cause an earthquake.
Fault-Block Mountain Formed when normal faults uplift a block of rock. A fold is a bend in rock that forms when compression shortens and thickens part of the crust. A fold that bends upward in an arch is an anticline. A fold that bends downward in the middle to form a bowl is a syncline. A plateau is formed when a fault pushes up a large flat block of rock. A plateau is a large area of the flat land elevated high above sea level.