How and Where Do Earthquakes Occur?

1. How and Where Do Earthquakes Occur? Causes of Earthquakes

2. How Do Earthquakes Occur? Vocabulary • stress • strain • fault • primary wave • secondary wave • surface wave • Focus • epicenter

3. How Do Earthquakes Occur? What Is an Earthquake? • Earthquakes are natural vibrations of the ground caused by movement along fractures in Earth’s crust, or sometimes, by volcanic eruptions. • Most earthquakes occur along plate boundaries. • More than a million earthquakes occur each year.

4. How Do Earthquakes Occur? What Causes Earthquakes? • Most earthquakes occur when rocks fracture, or break, deep within Earth. • Fractures form when stress exceeds the strength of the rocks involved. • Stress is the forces per unit area acting on a material. • Strain is the deformation of materials in response to stress.

5. How Do Earthquakes Occur? Stress and Strain • There is a distinct relationship between stress and strain that can be plotted as a stress-strain curve. • A stress-strain curve usually has two segments: a straight segment and a curved segment. • Low stresses produce the straight segment, which represents the elastic strain of a material. • If the elastic strain is reduced to zero, the deformation disappears.

7. How Do Earthquakes Occur? Faults • A fault is the fracture or system of fractures along which movement occurs. • The surface along which the movement takes places is called the fault plane.

8. The Focus of the Earthquake • The place underground where the break first occurs is the focus of the earthquake. • The epicenter is the location at Earth’s surface just above the focus. • When the vibrations reach the surface , we feel them as an earthquake, first at the epicenter and then at greater distances from the epicenter.

10. How Do Earthquakes Occur?

11. How Do Earthquakes Occur? Earthquake Waves • When an earthquake occurs, its energy radiates in waves away from the focus as shown in the previous figure. • Seismic waves that travel from the focus through the Earth’s body are called body waves. • Every earthquake produces two types of body waves, called P-waves and S-waves: • P-waves or primary waves travel the fastest. • S-waves or secondary waves travel more slowly than p-waves and vibrate in all directions perpendicular to the direction of travel.

12. How Do Earthquakes Occur? Seismic Body Waves

13. How Do Earthquakes Occur? Seismic Surface Waves • Surface waves are seismic waves that travel along Earth’s surface. When P-waves and S-waves reach Earth’s surface, they become surface waves called Love waves and Rayleigh waves.

14. How Do Earthquakes Occur? Seismic Surface Waves

15. 10.1 Section Review • Explain where earthquakes are most likely to originate and why they originate in these places. • Describe the difference between the focus of an earthquake and the epicenter of an earthquake. • Draw and label a diagram illustrating two types of surface waves. CRITICAL THINKING • Compare and contrast body waves and surface waves. Explain how the depth of an earthquake’s focus might determine the extent of the damage it causes. • MATHEMATICS Suppose an earthquake’s P waves travel at an average speed of 6 kilometers per second, and its S waves travel at an average speed of 3.4 kilometers per second. How long will it take the P waves to reach a recording station that is 60 kilometers from the focus? How long after the P waves will the S waves reach the same station?

16. How Do Earthquakes Occur? Locating the Epicenter • A seismograph records the magnitude of the earthquake and the time the seismic waves arrive. • Seismologist use the difference in the speeds of the P and S waves to locate the epicenter of earthquakes.

17. How Do Earthquakes Occur?

18. How Do Earthquakes Occur?

23. Types of Faults • There are three basic types of faults: • Reverse faults are fractures that form as a result of horizontal compression. • Normal faults are fractures caused by horizontal tension. • Strike-slip faults are fractures caused by horizontal shear.