Earth Science 8.1 Earthquakes Earthquakes

 Each year more than 30,000 earthquakes happen worldwide. Most are minor and do very little damage.  Only about 100 major earthquakes happen each year on the average.  If one of these major earthquakes happens near a city or heavily populated area, the results can be very destructive

Earthquakes  An earthquake is the vibration of Earth produced by the rapid release of energy within the lithosphere.  Earthquakes are produced by slippage along a break in the lithosphere called a fault.

Earthquakes  The point within the earth where the earthquake starts is called the focus. The focus of an earthquake is located along a fault underneath the surface.  The energy released by the earthquake travels in all directions from the focus in the form of seismic waves. These waves are similar to the waves produced when a stone is dropped in a pond.

Earthquakes  When you see a news report about an earthquake, the reporter always mentions the place on the Earth’s surface where the earthquake was centered.  This place is referred to as the epicenter.  The epicenter is the location on the surface directly above the focus.

Earthquakes  The movement that occurs along faults during earthquakes is a major factor in changing the surface above.  The land above a fault can shift tens of meters in a single earthquake.  Over time, this type of movement can push up coastlines, mountains, and plateaus.

Earthquakes  The crust can move vertically or horizontally as a result of fault movements.  If the crust moves vertically, scientists say it has been uplifted. Vertical movements create a sharp edge called a fault scarp.  If the crust moves horizontally we say it has been offset or displaced.

Earthquakes  The San Andreas fault: The San Andreas fault in California is one of the most studied fault systems in the world. This fault extends about 1300 kilometers through the state and into the Pacific ocean. Studies have shown that displacement has occurred along the fault in areas as much as 200 kilometers long.

Earthquakes  The San Andreas fault: The San Andreas fault is 1300 kilometers long but segments of it 100 to 200 kilometers long tend to act individually. Some segments show a slow, gradual slip known as fault creep. Other sections regularly slip and produce small earthquakes. Some segments remain unmoved for hundreds of years and than move suddenly with explosive force producing large earthquakes,

Earthquakes  The San Andreas fault: One great earthquake along the fault was the San Francisco Earthquake of During this earthquake, the land on the western side of the fault moved 4.7 meters in relation to the land on the east.

Earthquakes  Before the San Francisco Earthquake, scientists did not understand what caused them. Studies afterward led to the development of a hypothesis that explained earthquakes.  According to the elastic rebound theory, most earthquakes are produced by the rapid release of energy stored in rock that has been subjected to great forces.  When the strength of the rock is exceeded, it suddenly breaks, releasing some of it’s energy as seismic waves.

Earthquakes  Deformation of Rock: Forces inside the earth slowly deform the rock that makes up the Earth’s crust, causing the rock to change it’s shape. As the rocks bend, they store energy, just as a wooden ruler does when you bend it. Elastic energy is the same kind of energy that is stored when you stretch a rubber band. As the rock is stretched farther, as each side of the fault moves in an opposite direction, the elastic energy builds as tension in the rock increases.

Earthquakes  Deformation of Rock: Suddenly the tension becomes too much; the rock slips at it’s weakest point,(the focus of an earthquake) releasing the energy. Like a rubber band snapping, the energy is released in a moment along the fault line. The tendency for the rock under pressure to spring back after an earthquake is called elastic rebound. The energy released from the elastic rebound moves along the fault line releasing the pent up energy as seismic waves.

Earthquakes  Aftershocks and Foreshocks: Even a major earthquake like the 1906 earthquake does not release all the energy in one moment. Aftershocks and foreshocks also release some of the fault’s stored elastic energy. An aftershock is an earthquake that occurs sometime after a major earthquake. Aftershocks may occur anywhere from hours to weeks after an earthquake. Small earthquakes called foreshocks sometimes precede major earthquakes days or even a year before a large event.

Computer Lab:  Go to the computer lab: research one of the following topics to write 3 paragraphs on.  A paragraph must be at least 5 full sentences on the topic.  DO NOT COPY, CUT, OR PASTE. Put all in your own words. Seismic waves San Andreas fault 1906 San Francisco Earthquake Seismographs Richter Scale Mercalli Scale