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Explore how tectonic plates cause earthquakes, the different types of faults, seismic waves, earthquake measurement, and the dangers they pose. Discover how earthquakes have shaped our Earth.
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Earthquakes Essential Question: How has earthquakes shaped our Earth? How do tectonic plates cause earthquakes?
Whose Fault is it? • Faults are fractures in Earth where movement has occurred • Because earthquakes occur when two plates rub together, they are often said to occur on fault lines. • Faults occur because forces inside the Earth cause Earth’s plates to move placing stress on or near the plate edge
What are the types of Faults? • To relieve this stress, the rocks tend to bend, compress, or stretch • If the force is great enough the rocks will break. • This breaking produces an earthquake. • An earthquake is when abrupt shakings of the Earth are caused by the release of built up pressure on the Earth’s surface
What are the 3 types of Faults? • 3 types of forces act on rocks: tension, compression, and shear • Normal Fault- caused by rock above the fault moving downward in relation to the rock below the fault
What are the 3 types of Faults? • Reverse faults result from compression forces that squeeze rock. • If rock breaks from forces pushing from opposite directions, rock above a reverse fault surface is forced up and over the rock below the fault surface.
What are the 3 types of Faults? • At a strike-slip fault, rocks on either side of the fault are moving past each other without much upward or downward movement • The San Andreas Fault is the boundary between two of Earth’s plates that are moving sideways past each other.
What are the features of an Earthquake? • Seismic Waves- waves generated by an earthquake, can move the ground forward and backward, up and down, and side to side • Focus- an earthquake’s point of energy release • This is the point within Earth where the earthquake starts • Epicenter- is the point on the earth’s surface directly above the earthquake focus
What are the types of Seismic Waves that are produced? • Primary Waves (P-Waves)- cause particles in rocks to move back and forth in the same direction that the wave is traveling • Secondary Waves (S-waves)- move through Earth by causing particles in rocks to move at right angles to the direction of wave travel. • Surface Waves-are seismic waves that travel along Earth’s outer layer • These waves cause most of the destruction resulting from earthquakes
P-waves S-waves Surface waves
How do we locate an Earthquake? • The different speeds of seismic waves allow scientist to determine the epicenter • Primary waves move fastest • Secondary waves follow • Surface waves move slowest and arrive at the seismograph station last • The difference in arrival times is used to calculate the distance from the seismograph station to the earthquake epicenter
How do we locate an Earthquake? • Seismic waves from earthquakes are measured with an instrument known as aseismograph • Consists of a rotating drum of paper and a pendulum with an attached pen • The paper record of a seismic event is called a seismogram
How do we measure an earthquake? • We can measure earthquakes by its: • Intensity – a measure of the effects on an earthquake at a particular location OR • Magnitude: a measure of the strength or amount of energy released during an earthquake
How do we measure an earthquake? • The Richter scale is used to describe the strength of an earthquake and is based on the height of the lines on the seismograph
How do we measure an earthquake? • The Mercalli Scale is based on actual observations of damage
Earthquake dangers… • Most earthquake damage occurs when surface waves cause building, bridge, and roads to collapse • However, an earthquake under the ocean causes a sudden movement of the ocean floor • The movement pushes against the water, causing a powerful wave that can travel thousands of kilometers in all directions • These ocean waves caused by earthquakes are called seismic sea waves, or tsunamis