How and Where Do Earthquakes Occur? Causes of Earthquakes How Do Earthquakes Occur? Vocabulary • • • • • • • • stress strain fault primary wave secondary wave surface wave Focus epicenter 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. 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. 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. 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. 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. How Do Earthquakes Occur? 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. How Do Earthquakes Occur? Seismic Body Waves How Do Earthquakes Occur? Seismic Surface Waves • Surface waves are seismic waves that travel along Earth’s surface. When Pwaves and S-waves reach Earth’s surface, they become surface waves called Love waves and Rayleigh waves. How Do Earthquakes Occur? Seismic Surface Waves 10.1 Section Review 1. Explain where earthquakes are most likely to originate and why they originate in these places. 2. Describe the difference between the focus of an earthquake and the epicenter of an earthquake. 3. Draw and label a diagram illustrating two types of surface waves. CRITICAL THINKING 4. 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. 5. 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? 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. How Do Earthquakes Occur? How Do Earthquakes Occur? 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.