Earthquakes

6.1 Earthquakes and Plate Boundaries When rocks can no longer change shape—the fault breaks and slips, causing earthquakes. The rupture and sudden movement of rocks along a fault. Majority of earthquakes occur in Earth’s crust. Part of the energy released from earthquakes spreads as complex waves. 5

6.1 Earthquakes and Plate Boundaries Focus & Epicenter The focus is the location on the fault where an earthquake begins. The closer the focus is to the surface, the stronger the shaking will be. The point on Earth’s surface directly above the focus is the epicenter. 4

Plate Boundaries and Earthquakes 6.1 Earthquakes and Plate Boundaries Plate Boundaries and Earthquakes Lithospheric plates interact at different plate boundaries and produce earthquakes. Earthquake size and depth and fault type depend on the type of plate boundary. 3

Plate Boundaries and Earthquakes (cont.) 6.1 Earthquakes and Plate Boundaries Plate Boundaries and Earthquakes (cont.) 2

Earthquakes Away from Plate Boundaries 6.1 Earthquakes and Plate Boundaries Earthquakes Away from Plate Boundaries Not all earthquakes happen at plate boundaries. New Madris Earthquakes of 1911 Millions of years ago, a long zone of intense faulting was formed when the crust began to pull apart, but did not break completely. Today, the crust is being compressed, or squeezed together. (p. 246) 1

Seismic Waves

6.2 Earthquakes and Seismic Waves Waves of energy that are produced at the focus of an earthquake. Waves move outward from the focus in all directions. 3 main types of seismic waves. 4

1. Primary Waves (P-waves) 6.2 Earthquakes and Seismic Waves 1. Primary Waves (P-waves) Particles vibrate in same direction as wave Fastest seismic wave First to be detected by seismographs Travel through solids and fluids 3

2. Secondary Waves (S-waves) Particles vibrate perpendicular to wave Slower than P-waves Detected after the p-wave Travels through solids only 2

1 3. Surface Waves Slowest seismic wave Side-to-side swaying or rolling motion Cause the most damage 1

Measuring Earthquakes

Measuring Earthquakes Scientists determine size of earthquakes by measuring how much the rock slips along the fault. They also analyze the heights of the seismic waves, which indicate how much energy is released by an earthquake. 4

3 Seismograph Records size, direction, and the movement time of ground 6.3 Measuring Earthquakes Seismograph Records size, direction, and the movement time of ground Records the arrival times of the P- and S-waves 3

2 Seismogram Record of the seismic waves 6.3 Measuring Earthquakes Seismogram Record of the seismic waves Used to calculate the size and locations of earthquakes 2

6.3 Measuring Earthquakes Reading a Seismogram Wave heights indicate the amount of ground motion for each type of wave. Difference between the arrival times of P-waves and S-waves determines the distance of the seismograph from the epicenter. (p.260) 1

Locating an Epicenter

4 Locating an Epicenter Triangulation is used to locate the epicenter. 6.3 Measuring Earthquakes Locating an Epicenter Triangulation is used to locate the epicenter. This method is based on the speeds of the seismic waves. At least three seismographs must record the distances. 4

1. Find the arrival time differences. 6.3 Measuring Earthquakes 1. Find the arrival time differences. 3

2. Find the difference from the epicenter. 6.3 Measuring Earthquakes 2. Find the difference from the epicenter. 2

3. Plot the distance on a map. (p. 263) 6.3 Measuring Earthquakes 3. Plot the distance on a map. (p. 263) The epicenter is where the 3 circles overlap. 1

Earthquake Scales

Measuring Earthquake Size Magnitude measures the amount of energy released by an earthquake. Magnitude scale is based on record of height of ground motion. Most values range from 0–9. 4

Richter Scale First magnitude scale that used mechanical seismographs Good for earthquakes between 3.0-7.0 only. Not used today 3

Moment Magnitude Scale 6.3 Measuring Earthquakes Moment Magnitude Scale Used today More accurate scale for measuring earthquake size. Based on the amount of energy released during an earthquake. Uses electronic seismographs 2

6.3 Measuring Earthquakes Earthquake Intensity Intensity refers to the amount of shaking and damage caused. Intensity varies depending on: Size of earthquake Distance from epicenter Type of ground surface Construction of building Mercalli Scale measures intensity Usually, the maximum intensity is found near the epicenter. (p. 264) 1

Predicting Earthquakes

Predicting Earthquakes 6.3 Measuring Earthquakes Predicting Earthquakes At this time, geologists cannot predict earthquakes. Geologists can, however, determine the seismic risk by locating active faults and where past earthquakes have occurred. Geologists create seismic risk maps. 2

Avoiding Earthquake Hazards (p.274) 6.4 Earthquake Hazards and Safety Avoiding Earthquake Hazards (p.274) 1