Vocabulary: 1. seismology 2. deformation 3. elastic rebound 4. seismic waves 5. P waves 6. S waves What Are Earthquakes? Chapter 5 Section 1 p

The word earthquake defines itself fairly well, but there is more to earthquakes than just the shaking of the ground. In fact, an entire branch of Earth science, called seismology, is dedicated to studying earthquakes. Most earthquakes take place near the edges of tectonic plates. As a result of the movements of the plates, numerous features called faults exist in the Earth’s crust. A fault is a break in the Earth’s crust along which blocks of the crust slide relative to one another. Earthquakes occur along faults because of this sliding.

What Causes Earthquakes? p. 131 As tectonic plates push, pull, or slip past each other, stress increases along faults near the plates’ edges. In response to the stress, rock in the plates deforms. Rock along the plates can deform in two main ways: Plastic Deformation and Elastic Deformation. Plastic deformation causes the rock to bend like a piece of clay. Earthquakes do not result from plastic deformation. The road cut is next to the San Andreas Fault in southern California. The rocks in the cut have undergone deformation because of the continous motion of the fault.

Elastic deformation does lead to earthquakes. Think of elastically deformed rock as a rubber band. You can stretch a rubber band only so far before it breaks. When the rubber band breaks, it releases energy. Then the broken pieces return to their original unstretched shape. The sudden return of elastically deformed rock to its original shape is called elastic rebound. It’s like the return of the broken rubber band pieces to their unstretched shape. Elastic rebound occurs when more stress is applied to rock than the rock can withstand. During elastic rebound, energy is released. Some of this energy travels as seismic waves. These seismic waves can cause an earthquake.

TransformStrike-slip fault reverse fault Divergentnormal fault Plate Motion Major Fault Type Convergent Plate Motion and Fault Type A specific type of plate motion takes place at different tectonic plate boundaries. Each type of motion creates a particular kind of fault that can produce earthquakes.

Earthquake Zones p. 133 Earthquakes can happen both near Earth’s surface or far below it. Most earthquakes happen in the earthquake zones along tectonic plate boundaries. Earthquake zones are places where a large number of faults are located. The San Andreas Fault Zone in California is an example of an earthquake zone. Not all faults are located at tectonic plate boundaries. Sometimes, earthquakes happen along faults in the middle of tectonic plates. San Andreas Fault in California The three major tremors of the earthquakes in New Madrid, Mo, were so intense that according to reports, they altered the Mississippi River! In the late 1970’s, scientists found a series of faults deep beneath sediment deposited in the Mississippi River. The area is still seismically active, and large earthquakes may still occur.

How Do Earthquake Waves Travel? p. 134 Waves of energy that travel though the Earth are called seismic waves. Seismic waves that travel through the Earth’s interior are called body waves. There are two types of body waves: P waves and S waves. Seismic waves that travel along Earth’s surface are called surface waves. Each wave travels through Earth’s layers in a different way and at different speeds. The speed of the seismic wave depends on the kind of material it travels through. Waves that travel through solids, liquids, and gases are called P waves (pressure waves). They are the fastest seismic waves, so P waves always travel ahead of other seismic waves. They can also be called primary waves because they are always the first waves of an earthquake to be detected. If you were to tap a gelatin cube, you would notice its elastic-like movement. Tapping it changes the pressure inside the cube and deforms it for only a minute. Then it springs back to its original shape. This process is how P waves affect rock.

Rock can also be deformed from side to side. After being deformed from side to side, the rock springs back to its original position and S waves are created. S waves are shear waves. They are the second fastest seismic waves. They stretch rock side to side. Unlike P waves, they cannot travel through the parts of the Earth that are completely liquid. They are slower than P waves and always arrive later. Another name for S waves are secondary waves.

Surface Waves p. 135 Surface waves move along the Earth’s surface and produce motion mostly in the upper few kilometers of Earth’s crust. There are two types of surface waves. One type produces motion up, down and around. The other type produces back and forth motion like the motion produced by S waves. Surface waves are different from body waves in that surface waves travel more slowly and are more destructive.

Bill Nye – Earthquakes Part 1

Quiz Time! 1. What is a fault? a break in the Earth’s crust along which blocks of the crust slide relative to one another. 2. How does rock that is along a fault deform in response to a decrease in stress? It deforms in an elastic manner, like a rubber band, snapping back to its original shape. 3. _________ is the change in shape of rock due to stress. deformation

4. _________ always travel ahead of other waves. P waves 5. Seismic waves that shear rock side to side are called A. surface waves B. S waves C. P waves D. both B and C B 6. Where do earthquakes occur? in earthquake zones along tectonic plate boundaries Science Humor… What did one mountain say to the other mountain after the earthquake? “It wasn’t my fault!”