Chapter 12 Earthquakes

What is an earthquake? Movements of the ground that are caused by a sudden release of energy when rocks along a fault move. Fault – break in a body of rock

Why Do Earthquakes Occur? Elastic Rebound Two blocks of crust pressed against each other at a fault are under stress but do not move because of friction As stress builds up at the fault, the crust deforms The rock fractures and then snaps back into its original shape This step causes actual earthquake.

Occurence Usually in the crust or upper mantle Those associated with volcanoes are usually weak and less damaging Most are associated with tectonic activities and typically strong and damaging over large areas.

Characterisitics Location - Focus – The point underground where movement originates - Epicenter – The point on the surface directly above the focus

Focus & Epicenter

Types of Stress Tensional stress (or extensional stress), which stretches rock; Compressional stress, which squeezes rock; and Shear stress, which result in slippage and translation.                                                     

Deformation Stages of Deformation When a rock is subjected to increasing stress it changes its shape, size or volume. Such a change in shape, size or volume is referred to as strain.  When stress is applied to rock, the rock passes through 3 successive stages of deformation. Elastic Deformation -- wherein the strain is reversible. Ductile Deformation -- wherein the strain is irreversible. Fracture -- irreversible strain wherein the material breaks.

Deformation

Deformation We can divide materials into two classes that depend on their relative behavior under stress. Brittle materials have a small to large region of elastic behavior, but only a small region of ductile behavior before they fracture. Ductile materials have a small region of elastic behavior and a large region of ductile behavior before they fracture.

Body Wave Types Primary or P-waves - Compressional waves which can travel through any material, they travel the fastest and reach the seismograph first. They travel about twice as fast as S-waves They do little damage, however, that is dependent upon the strength and proximity to the epicenter

Body Wave Types Secondary or S-waves – These are a shearing type of wave or transverse wave (side to side) which can only travel through solids. They are the second wave to arrive at a a seismograph. S-waves have a somewhat larger wave height than a P-wave.

Surface Wave Types Love or L-waves – These are generated by the interaction of S-waves at the surface. They are transverse waves that vibrate the ground in a horizontal direction perpendicular to the direction of movement P & S waves travel through the earth but L-waves follow the surface, that’s why they take longer to reach a seismograph station.

Surface Wave Types Propagation of an L-wave

Surface Wave Types Rayleigh waves – Similar to water waves in the ocean (away from the shore). The ground moves in an elliptical trajectory.

Seismic Wave Types