1. Chapter 19: Earthquakes

2. What are Earthquakes? Natural vibrations of the ground caused by movement in fractures in Earth’s crust or sometimes volcanic eruptions Most occur because of movements along faults Irregular surfaces in rocks snag and lock causing stress to build in the rocks When rocks meet their elastic limit they break causing an Earthquake

4. What is Stress and Strain Fractures form when stress exceeds the strength of the rock involved Strain occurs in response to stress and causes the material to deform Three types of stress Compression – squeezes Tension – pulls apart Shear – Causes twisting

5. What Is a Fault A fracture or system of fractures along which movement occurs 3 types of faults – Reverse fault – Normal fault – Strike slip fault

6. What are seismic waves? Vibrations of the ground during an earthquake P and S waves are called body waves because they pass through Earths interior 3 types of seismic waves – Primary (P) – squeeze and pull rocks along the same direction the wave travels – Secondary (S) – cause rocks to move at right angels to the direction of the waves – Surface - cause up-and- down and side –to-side as they pass through rocks

7. What is the focus and epicenter of an earthquake The focus of an earthquake is the point of failure of rocks at the depth where an earthquake originates The epicenter of an earthquake is the point on earth directly above the focus

8. Seismology is the study of earthquake waves Seismometers or seismographs are sensitive instruments that detect and record the vibrations of an earthquake A seismogram is the record produced by the seismometer The different speeds of the P and S waves are used to determine the epicenter of an earthquake

9. Clues to Earths Interior Seismic waves are reflected and refracted as they strike different materials – P waves travel through the mantel fairly straight – P waves are refracted or bent at Earths core – S waves do not enter Earths core because they do not travel through liquid

10. Clues to Earths Interior Analysis of these waves has enables scientist to determine the structure and composition of Earths interior – Disappearance of S waves leads scientist to believe Earth’s outer core must be liquid – Refraction of P waves indicates the inner core is solid

11. Seismology and Earths Interior

12. Measuring and locating Earthquakes Magnitude is a measure of the energy released during a earthquake and can be measured on the Richter Scale. Intensity is a measure of damage caused by a quake and is measured by the Modified Mercalli Scale – Uses roman numerals – Higher the number the greater the damage

13. Measuring and locating Earthquakes Data from at least 3 seismic stations are needed to determine an earthquakes epicenter Most Earthquakes occur in areas associated with plate boundaries called seismic belts

14. Earthquakes and Society Earthquakes can cause structural damage, landslides, soil liquefaction, fissures, fault scarps, uplift or subsidence and tsunamis Tsunamis is a large destructive ocean wave caused by an underwater earthquake.

15. Tsunami 2004

16. Probability of an Earthquake The probability that an area will have an earthquake is based on the history of a region and the rate of strain that builds on rocks