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(Earthquakes-The Rolling Earth) 

(Lesson #4) 

The San Andreas Fault

The San Andreas fault runs through the state of California from the Mexican border northward to the city of San Francisco. The cities of San Diego and Los Angeles lay on west side of the fault, which is actually the Pacific Plate. San Francisco lays on the east side of the fault, which is the North American Plate. (See map #1)  
The North American Plate is a very large plate which consists of the west side of the Atlantic Ocean, Greenland, and all of the North American continent except the part of California west of the San Andreas fault. This plate is moving west at about 1 inch per year. The Pacific Plate is also a very large plate that is moving north northwest at an astonishing rate of over 3 inches per year. Many earthquakes are recorded on the San Andreas fault each year as these two plates slide by each other. Another interesting fact is that Los Angeles which is about 375 miles southeast of San Francisco today is moving north toward San Francisco. In about 100 million years Los Angeles will actually be north of San Francisco if the plates keep moving at the same speed and direction that they are today. (See map #2)
The San Andreas fault has been studied thoroughly because of it's location so close to many major cities and universities. The San Andreas fault is located directly on the surface of the Earth which makes it easy to view and study. It can be seen riding in a car, walking on a hike, and an even better view can be seen from an airplane. The effects of the movement on the fault line are seen in crooked or offset fences, roadways, stream beds and railroad tracks.
The San Andreas fault is a strike-slip fault.  Strike-slip faults are not collision faults like subduction zones and they are not separating plate zones like mid-ocean ridges. A strike-slip fault is a plate boundary where two plates slide past each other. See diagram 1 below. Diagram 1 (Courtesy of FEMA) 


A tsunami is a large water wave that is formed by a volcanic eruption, underwater earthquake, landslide, or a hurricane. Tsunamis are the most dangerous of all the wave types. Tsunamis have the potential to reach heights of 120 feet (Krakatoa's eruption of 1883) and speeds of over 500 miles per hour. When the wave is far out at sea it can go unnoticed because almost all of the wave stays under the surface of the ocean. When the waves reaches the shallow water of the coastline the height of the wave grows to its fullest and then crashes down on the land.  
Tsunamis have killed thousands of people. When Krakatoa erupted in 1883 it sent out a tsunami that killed 36,000 people. The great Alaskan earthquake in 1964 sent out a tsunami that killed over 60 people in Hilo, Hawaii.  


Thought and Discussion Questions 

  1. Describe in your own words what a fault is.


  2. How is a strike-slip boundary different from a collision and separation boundary?


  3. What is a tsunami?


  4. How does a tsunami form?