Earthquakes: How do they happen

Stick-Slip Behavior Fault: a fracture in the earth in which plates are moving past each other Friction is the “force” that resists the movement between the two plates Stress builds up Eventually enough stress to override the friction and the plates “slip” until they reach a point of increased friction again Earthquake is caused by the rocks fracturing once the friction is released.

Shaking before and after EQ Foreshocks may occur before the major slip event, minor cracks that are caused by the stress for the main fault. Aftershocks occur after a major earthquake and can occur for weeks or years

Elastic-rebound theory Adjacent rocks to the fault will bend and once there is a slip event, they will rebound like an elastic band to their original shape

Amount of stress Bigger the earthquake the bigger the stress Some fault movement that happen in the upper 15-20 km do not even cause EQ because not enough stress. Called Fault creep instead.

Types of Stresses Compression occurs when the rock is pressed inward. Tension occurs when the rock is stretched out Shearing occurs when the rock is pushed from 2 different opposite directions.

Hanging Wall Vs Foot Wall The hanging wall is the block of rock that moves. A footwall never moves. So, once you can determine which rock is moving, then you can determine the type of fault: normal, reverse, thrust or transform.

Normal Faults In a normal fault, the hanging wall moves DOWN relative to the foot wall. Keep in mind, an easy way to remember this:   “It is normal to fall down. It’s not normal to fall up!”

Normal faults

Reverse Fault In a reverse fault, the hanging wall moves UP in relation to the footwall. This fault is opposite of the normal fault. In special circumstances, the hanging wall will sometimes move up and over the footwall. This is called a THRUST fault.

Transform Fault This is the same idea as a transform boundary, only this time we are looking in a smaller area. The blocks of rocks are sliding past one another. As this happens, friction occurs and causes an Earthquake. Only has to be 1-4 cm to cause damage

Fold Types and Features If a rock does not break under the various types of stresses, then typically folds are created. There are two types of folds: Anticline: an upward fold in the rock. It looks like an ant hill.

Fold Types and Features  Syncline: a downward fold in the rock. It looks like a smile.