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San Francisco Earthquakes • 1906 ~ 7.8 • 1989 – 6.9 Add to Table of contents • Earthquake/Volcano plots • Earthquake Pg. 24 Pg. 25 • What are the top 3 states for earthquake activity(3.5 or higher)? • Out of the 50 states, where do you think Nevada ranks? 1Alaska: The number of earthquakes is under reported, even though Alaska already accounts for more than 50 percent of all US earthquakes! Events in the magnitude range of 3.5 to 4.0 in the Aleutian Islands are not recorded on enough seismograph stations to be located. 2Hawaii: The earthquake count was obtained from the USGS Hawaiian Volcano Observatory (HVO). Hawaii and Nevada are essentially tied in the rates of earthquakes of magnitude 5.0 and greater. Oklahoma is now #1 Earthquakes Pg. 25 • Earthquake-sudden movement of the ground caused by the release of energy when the rocks move along a fault. • Fault-a break in rock where movement occurs. Where do most earthquakes occur? • Most earthquakes occur at plate boundaries or faults, where there is a lot of stress. • Friction prevents the plates from moving until the stress is too great and the rocks suddenly grind past each other =trembling and vibrations that are felt. • Elastic Rebound – the sudden return of the deformed rock to its undeformed shape. (Yes, draw this!) Anatomy of an earthquake • Focus-where the first movement occurs in the earth’s crust. • Epicenter-point on the earth’s surface directly above the focus. This is a map of all the known faults in the Las Vegas Valley. The brown lines are East-dipping normal faults. The yellow line is a West-dipping normal fault, and the green lines are inactive faults.