Earthquakes: General Features and Effects

1. Chapter 8 Earthquakes

2. General Features • Vibration of Earth produced by a sudden release of energy • Associated with movements along faults • Faults – fractures in Earth where movement has occurred. • Focus – the point within the earth where the earthquake starts • Epicenter – location on the surface directly above the focus.

3. Mechanism for Earthquakes • First explained by H. Reid in Early 1900s • Rocks “spring back” – elastic rebound • Vibrations occur as rock elastically returns to its original shape • Often preceded by foreshocks and followed by aftershocks.

4. Elastic Rebound Hypothesis • The springing back of the rock into its original place • The rock behaves much like a stretched rubber band does when it is released. • Most earthquakes are produced by the rapid release of elastic energy stored in rock that has been subjected to great forces. When the strength of the rock is exceeded it suddenly breaks, causing vibrations

5. Aftershocks and Foreshocks • The movements that follow a major earthquake often produce smaller earthquakes called aftershocks. • Usually much weaker • Can still cause major damage • Small earthquakes before a major earthquake can also occur. They are called foreshocks

6. Measuring Earthquakes • The study of earthquake waves is called seismology. • Seismographs are instruments that record earthquake waves. • Seismographs amplify and electronically record ground motion producing a trace, called a seismogram

7. Earthquake Waves • The energy from an earthquake spreads outward as waves in all directions from the focus. • There are two kinds of waves produced by an earthquake • Surface waves • Body waves

8. Surface Waves • Seismic waves that travel along Earth’s outer layer. • The motion is complex • Travel along the ground and cause the ground to move. • Like an ocean wave • Move up and down as well as side to side • Destructive

9. Body Waves • Travel through the interior • Identified as either P waves or S waves depending on how they travel • P waves are push-pull waves • S waves shake the particles at right angles to their direction of travel

10. Locating an Earthquake • The difference in velocities of P and S waves provides a way to locate the epicenter • The greater the interval the greater the distance to the earthquake source.

11. Earthquake direction • Travel- time graphs from three or more seismographs can be used to find the exact location of an earthquake epicenter. • We draw circles on the globe and where the circles intersect is the epicenter

12. Earthquake Zones • About 95% of the major earthquakes occur in a few narrow zone

13. Measuring Earthquakes • scientist use two different types of measurements to describe the size of an earthquake – intensity and magnitude • Intensity is a measure of the amount of earthquake shaking at a given location based on the amount of damage • Magnitude measures size of seismic waves

14. Richter Scale • Outdated scale for measuring magnitude of earthquakes • Based on amplitude of the largest wave • Only useful for small, shallow earthquakes within about 500km of the epicenter

15. Moment Magnitude • More precise means of measuring earthquakes • Derived from the amount of displacement that occurs along a fault zone • Most widely used measurement for earthquakes because it is the only magnitude scale that estimates the energy released

16. Damage • The damage to buildings and other structures from earthquake waves depends on several factors • Intensity • Duration of the vibrations • Nature of the material on which the structure is built • Design of the structure

17. Tsunamis • Seismic Sea waves • Triggered by an earthquake occurs where a slab of the ocean floor is displaced vertically along a fault. • A tsunami also can occur when the vibration of a quake sets an underwater landslide into motion

18. Other dangers • Landslides • With many earthquakes the greatest damage to structures is from landslides and ground subsidence • Fire