1. Earthquakes: Earth’s Way of Releasing Stress Chapter 8: Earthquakes

2. Where Do They Occur? Most earthquakes will occur near the edges of tectonic plates, which is where most faults are located. The largest zone of seismic activity is around the Pacific Ocean, properly named the Pacific Ring of Fire. During earthquakes, the plates move very suddenly past each other, releasing energy through the crust and mantle of the Earth.

3. Why Do We Have Quakes? As plates bump and grind with one another, deformation takes place. Deformation is the rocks response to stress being applied to it. 2 ways: 1. Plastic (or ductile): does not lead to earthquakes, but changes the shapes of structures (look at the wall on page 197, figure 2). 2. Elastic (or brittle): does lead to earthquakes, where layers of rock are stretched until it reaches a breaking point (think of stretching a rubber band!). After the rocks deform, elastic rebound returns the rock to its original shape…this is when energy is released and an earthquake occurs.

4. Faults Review 3 types of faults 3 types of faults 1. Normal: rock layers are pulled apart horizontally by tension, hanging wall down 2. Reverse: rock layers are pressed together due to compression, hanging wall up 3. Strike-slip: shear stress pushes the fault blocks in opposite directions, no hanging wall

5. How Strong are Earthquakes? The strength of an earthquake depends on the type of plate motion involved at the different faults. 1. Strike-slip faults: because of the horizontal motion of the plates, earthquakes are typically very SHALLOW and MODERATE in strength 2. Reverse faults: due to compression, earthquakes are usually very DEEP and very STRONG 3. Normal faults: due to tension pulling plates apart, the quakes are SHALLOW and WEAK

6. Seismic Energy When energy is released during an earthquake, it travels via seismic waves. Seismic waves that travel through the Earth’s interior are called body waves. 2 types: 1. Primary waves (p-waves): squeeze and stretch the rocks, can travel through solid and liquid parts of the Earth, travel fastest 2. Secondary waves (s-waves): move in a side-to-side fashion, travel at half the speed of p-waves, can only go through solid parts of Earth and NOT liquid

7. Surface Waves Surface waves travel on the Earth’s surface and cause the most damage to buildings and roads. Their movement resembles ocean waves, moving the ground up and down. They travel the slowest of all seismic waves.

8. Focus vs. Epicenter The focus is the point inside the Earth where primary and secondary waves are projected from. The focus is the point inside the Earth where primary and secondary waves are projected from. The epicenter is the point directly above the focus on the surface where surface waves are generated from. The epicenter is the point directly above the focus on the surface where surface waves are generated from.