READ: Earthquakes

1. Types of Geologic Stress

When people have too much stress they may break. What happens if a rock gets too much stress?

Lithospheric plates do a lot of moving on Earth's surface. Slabs of lithosphere smash into each other. They move sideways past each other along faults. Where conditions are right, magma rises through solid rock. It's no wonder that rocks experience stress! Rocks respond differently to different types of stress and under different conditions.

Types of Stress

Stress is the force applied to a rock. Plates experience stress when they collide, move apart, or slide past each other. Plates moving on a rounded surface experience stress. Stress happens to rocks on a smaller scale, too. Local movements can cause stress in rocks.

There are four types of stresses that affect rocks:

• Confining stress comes from the weight of all the overlying rock. This weight pushes down on a deeply buried rock. The rock is being pushed in from all sides, which compresses it. The rock will not deform because there is no place for it to move.
• Compression stress squeezes rocks together. Compression causes rocks to fold or fracture (figure below). When two cars collide, compression causes them to crumple. Compression is the most common stress at convergent plate boundaries.

• Tension stress pulls rocks apart. Tension causes rocks to lengthen or break apart. Tension is the major type of stress found at divergent plate boundaries.
• Shear stress happens when forces slide past each other in opposite directions (figure below). This is the most common stress found at transform plate boundaries.

Responses to Stress

The amount of stress on a rock may be greater than the rock’s strength. In that case, the rock will undergo strain or deformation (figure below). Deep within the Earth, the pressure is very great. A rock behaves like a stretched rubber band. When the stress stops, the rock goes back to its original shape. If more stress is applied to the rock, it bends and flows. It does not return to its original shape. Near the surface, if the stress continues, the rock will fracture (rupture) and break.

2. Earthquakes

What causes the greatest damage in an earthquake?

This photo shows the Mission District of San Francisco burning after the 1906 earthquake. The greatest damage in earthquakes is usually not from the ground shaking. The greatest damage is caused by the effects of that shaking. In this earthquake, the shaking broke the gas mains and the water pipes. When the gas caught fire, there was no way to put it out. Fire causes the greatest damage in many earthquakes.

Earthquake!

An earthquake is sudden ground movement. This movement is caused by the sudden release of the energy stored in rocks. An earthquake happens when so much stress builds up in the rocks that the rocks break. An earthquake’s energy is transmitted by seismic waves.

Causes of Earthquakes

Almost all earthquakes occur at plate boundaries. All types of plate boundaries have earthquakes. Convection within the Earth causes the plates to move. As the plates move, stresses build. When the stresses build too much, the rocks break. The break releases the energy that was stored in the rocks. The sudden release of energy is an earthquake. During an earthquake the rocks usually move several centimeters. Rarely, they may move as much as a few meters. Elastic rebound theory describes how earthquakes occur (figure below).

Focus and Epicenter

The two important words used to describe an earthquake's location are focus and epicenter.

Focus

The point where the rock ruptures is the earthquake’s focus. The focus is below the Earth’s surface. A shallow earthquake has a focus less than 70 kilometers (45 miles). An intermediate-focus earthquake has a focus between 70 and 300 kilometers (45 to 200 miles). A deep-focus earthquake is greater than 300 kilometers (200 miles). About 75% of earthquakes have a focus in the top 10 to 15 kilometers (6 to 9 miles) of the crust. Shallow earthquakes cause the most damage. This is because the focus is near the Earth's surface, where people live.

Epicenter

The area just above the focus, on the land surface, is the earthquake’s epicenter (figure below). The towns or cities near the epicenter will be strongly affected by the earthquake.

3. Seismic Waves from Earthquakes

Are seismic waves like ocean waves?

Yes, in some ways. Ocean waves travel at the interface between the sea surface and the atmosphere. They have all the features that all waves have. Some seismic waves also travel at an interface. Which ones?

Seismic Waves

Seismic waves are the energy from earthquakes. Seismic waves move outward in all directions away from their source. Each type of seismic wave travels at different speeds in different materials. All seismic waves travel through rock, but not all travel through liquid or gas. Geologists study seismic waves to learn about earthquakes and the Earth’s interior.

Wave Structure

Seismic waves are just one type of wave. Sound and light also travel in waves. Every wave has a high point called a crest and a low point called a trough. The height of a wave from the center line to its crest is its amplitude. The horizontal distance between waves from crest to crest (or trough to trough) is its wavelength (figure below).

Types of Seismic Waves

There are two major types of seismic waves. Body waves travel through the Earth’s interior.Surface waves travel along the ground surface. In an earthquake, body waves are responsible for the sharp jolts. Surface waves are responsible for the rolling motions that do most of the damage in an earthquake.

Body Waves

Primary waves (P-waves) and secondary waves (S-waves) are the two types of body waves (figure below). Body waves move at different speeds through different materials.

P-waves are faster. They travel at about 6 to 7 kilometers (about 4 miles) per second. Primary waves are so named because they are the first waves to reach a seismometer. P-waves squeeze and release rocks as they travel. The material returns to its original size and shape after the P-wave goes by. For this reason, P-waves are not the most damaging earthquake waves. P-waves travel through solids, liquids, and gases.

S-waves are slower than P-waves. They are the second waves to reach a seismometer. S-waves move up and down. They change the rock’s shape as they travel. S-waves are about half as fast as P-waves, at about 3.5 km (2 miles) per second. S-waves can only move through solids. This is because liquids and gases don’t resist changing shape.

Surface Waves

Surface waves travel along the ground outward from an earthquake’s epicenter. Surface waves are the slowest of all seismic waves. They travel at 2.5 km (1.5 miles) per second. There are two types of surface waves. Love waves move side-to-side, much like a snake.Rayleigh waves produce a rolling motion as they move up and backward (figure above). Surface waves cause objects to fall and rise. They also cause objects to sway back and forth. These motions cause damage to rigid structures during an earthquake.

CK-12 Foundation, Earth Science. http://creativecommons.org/licenses/by-nc-sa/3.0/

4. Anatomy of an Earthquake

This image by KQED is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

5. Vocabulary and Summary

Vocabulary

1. compression: Stresses that push toward each other; this causes a decrease in the space that a rock can take up.

2. confining stress: Stress from the weight of material above a buried object; this reduces volume the rock is in.

3. deformation: Strain; the change of shape that a rock undergoes when it has been altered by stresses.

4. fracture: Break in rock caused by stresses; this happens with or without the movement of material.

5. shear: Parallel stresses that move past each other in opposite directions.

6. strain: Deformation in a rock when the stress exceeds the rock's internal strength.

7. stress: Force per unit area in a rock.

8. tension: Stresses that pull material in opposite directions.

9. earthquake: Ground shaking caused by the release of energy stored in rocks.

10. elastic rebound theory: How earthquakes are generated. Stresses cause strain to build up in rocks until they can no longer bend elastically. The rocks break and cause an earthquake.

11. epicenter: Point on the Earth's surface directly above the focus of the earthquake.

12. focus: Point where rocks rupture to cause an earthquake.

13. amplitude: Height of a wave; this can be measured from a center line to the top of the crest, or to the bottom of the trough.

14. body wave: Type of seismic wave that travels through the body of a planet; body waves include primary waves and secondary waves.

15. crest: Highest point of a wave.

16. Love wave: Surface wave that has a side-to-side motion, much like a slithering snake.

17. primary wave (P-wave): Fastest type of body wave, capable of traveling through solids, liquids, and gases.

18. Rayleigh wave: Surface wave that has a rolling motion.

19. secondary wave (S-wave): Slower moving, transverse body wave that can only travel through solids.

20. surface wave: Seismic wave that travels around the ground surface; the two types are Love and Rayleigh waves.

21. trough: Lowest point of a wave.

22. wavelength: Horizontal distance between two waves, as measured from crest to crest or trough to trough.

Summary

• Stress is the force applied to an object. Stresses can be confining, compression, tension, or shear.
• Rocks under stress may show strain or deformation. Deformation can be elastic or plastic, or the rock may fracture.
• Rocks respond to stress differently under different conditions.
• A sudden release of energy stored in rocks causes an earthquake.
• The focus is where the rocks rupture. The epicenter is the point on the ground directly above the focus.
• Most earthquakes are shallow. Shallow earthquakes do the most damage.
• Body waves travel through the body of a planet. Surface waves travel along the surface.
• There are two types of body waves: P-waves travel fastest and through solids, liquids, and gases; S-waves only travel through solids.
• Surface waves are the slowest, but they do the most damage in an earthquake.

CK-12 Foundation, Earth Science. http://creativecommons.org/licenses/by-nc-sa/3.0/