Chapter 5 Lesson 4 Earthquakes
Vocabulary
1.
Earthquake
2.
Focus
3.
Epicenter
4.
Magnitude
5.
Tsunami
NOTES
What is an earthquake?
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An earthquake is a sudden movement of Earth’s crust.
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The rock on both sides a fault is pushed and pulled by forces in the crust.
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Usually rocks on both sides of a fault are stuck together. o When they suddenly slip, an earthquake occurs.
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Slipping along a fault begins beneath Earth’s surface. o Most earthquakes occur at depths of less than 80km (50 Miles). o Can occur as deep as 644km (400 miles).
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Focus (see vocab) o Waves of energy ripple outward from the focus. o When they reach Earth’s surface, the waves spread out from a point directly above the focus. (Epicenter)
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People first feel the ground shaking at the epicenter of the quake.

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Waves move like ripples on a pond.
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Amount of damage depends partly on the amount of energy released at the earthquake’s focus. o More energy=more damage.
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Earthquakes happen along plate boundaries because of pressure from the movement of the plates pushes on nearby faults.
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Most earthquakes in the U.S in these states. o Alaska, Oregon, Washington, and California.
What waves do earthquakes make?
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A seismometer is an instrument that detects and measures waves produced by an earthquake.
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Earthquakes produce three different kinds of waves o Primary Wave (P waves)-fastest; pass through solid and liquid layers of Earth. Move back and forth as rocks squeeze together and spread apart. o Secondary Waves (S waves)-half as fast as P waves; Pass through solid layer only. Move up and down. o Surface Wave (Lg waves)-slowest waves; travel like ripples across a pond; cause most damage.
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Seismometers can be used to pinpoint the location of an earthquake.
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The difference in time between the arrival of P and S waves at a seismometer station gives the distance to the epicenter of the quake.
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To pinpoint the exact location, scientists need data from at least three stations. o This data is used to draw 3 circles on a map. o The point where the 3 circles intersect is the epicenter of the earthquake.
How is an earthquake’s energy measured?
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The Richter scale measures the magnitude at the epicenter. o Rates earthquakes from weakest to strongest, starting from 1. o Each larger whole number indicates an earthquake that releases about 30 times more energy than the previous whole number.

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The Mercalli Scale measures what people felt and what happened to objects at a specific location. o Rated in Roman numerals from I to XII, where XII is most severe.
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Aftershocks are earthquakes with lesser magnitudes.
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Earthquakes with larger magnitudes have more aftershocks. o After shocks will also have larger magnitudes.
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When an earthquake occurs underneath the ocean, a wave of water is pushed upward and outward in all directions. o Speeds of 311 to 622 mph
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Out at sea the energy is spread through deep water and the wave is not very tall.
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As the wave reaches shallow water near shore, the same amount of energy is spread through much less water and the height of the wave increases.
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Tsunami is a Japanese word that means “wave in port.”
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Tsunamis usually occur after underwater earthquakes that are great than 6.5 on the Richter scale.
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When a tsunami occurs, a warning is sent out to the areas at risk.
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A global tsunami warning system is being planned to detect future tsunamis.
How can people prepare?
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If caught in an earthquake, seek a safe place and be aware of the dangers.
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Damage to buildings is reduced by placing layers of rubber and steel between a building and its foundation. o These layers cushion up-and-down motion but still allow the building to move from side to side as the ground moves. o Building will sway, but is much less likely to be damaged.
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Secure objects that might fall (EX: to a wall).
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Identify safe spots to use as shelter, like a sturdy table or kitchen counter.

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Predicting earthquakes is very difficult. o Some can be predicted, and when they can people have time to seek safe shelter.
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Predicting a tsunami is easier because it follows an earthquake that can be felt using seismometers.
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People warned of a coming tsunami can seek shelter on high ground away from shorelines.