Earth Science
Earthquake Unit
Introduction to Earthquakes: Read, Review/Lecture, Mapping Activity
Earthquakes
 Vibrations of the earths crust
 Occur when rocks shift suddenly under a fault
 When friction prevents rocks from moving the fault is said to be
locked
 Rocks come under stress
 Friction over come
 Rocks grind past each other
 Slippage causes trembling earthquakes
*Demonstrate with books, ball rolling on table
Elastic Rebound Theory
 Rocks on each side of the fault moving slowly
 If fault is locked stress on rocks increase
 Stressed past a certain point the rocks fracture
 Separate at weakest point
 Spring back to original shape or rebound
 As they fracture and slip they release energy in the form of
vibrations called seismic waves
 Release of energy causes stress in other rocks causing them to
fracture and spring back: aftershocks
Focus: area of a fault where slippage first occurs
Epicenter: point on the earth’s surface directly above the focus
*Show overhead
Focus can be:
Shallow
 Within 70 km of the earths surface
 Most damage caused here
 Demonstrate with tuning fork and water
Intermediate
 Occur at a depth of between 70 and 300 km
Deep
 Between 33 and 650 km
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By the time vibrations from intermediate focus or deep focus
earthquake reach the surface, much of their energy has been used up.
For this reason they do less damage
*Earthquakes do not usually originate any deeper than 650 km because the
high temperatures here cause rocks to act like plastics rather than
fracturing.
Earthquake Zones
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3 major earthquake zones
Pacific Ring of Fire
1. Large earthquake zone
2. Includes the west coasts of North and South America, the east
coast of Asia and the western pacific islands of the Philippines,
Indonesia, New Guinea, and New Zealand.
3. Some plates are being subducted
4. Some plates are being scraped past each other
5. Plate movements cause stress to build up in the rocks
6. They fracture and shift
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Mid Ocean Ridge
1. Oceanic crust pulling away from both sides of the ridge
2. Causes stress in the rocks along the major ocean ridges
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Eurasian-Melanesian Mountain Belt
1. Eurasian plate colliding with African and Indian plates
2. Causes mountains and earthquakes
3. What mountain range is this causing (Himalayans)
Fault Zones
 Formed at plate boundaries because of intense stress when plates
separate, collide, subduct or slide past each other
 Example San Andreas Fault: Pacific plate slipping along North
American plate
*Have students map the San Andreas Fault on their map with the plates
drawn on it and then try to say what plate interactions are causing the fault
Mapping Activity
 There is a link between the earthquake zones and plate tectonics
 Have students map the most recent earth quakes (Handout) and
compare them to the plate tectonic zones
 For each earthquake have students list which plates are responsible
for the earthquake
New
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Madrid Fault
Located in the middle of the United States
Stretches from Arkansas to Illinois
Caused the most widely felt series of earthquakes in the US
Church bells in Boston rang over 1000 miles away
Why would this earth quake be more widely felt than one in California
Scientist believe this is an ancient fault zone
North America started to split 600 million years ago
Suddenly tectonic activity stopped
Clue was the deep fault zone known as the New Madrid Fault
What could cause this fault to become active again?
The North American plate is being compresses by spreading of the
Atlantic
 Causes stress on this fault resulting in earthquakes
 There are similar faults like this in China
Study questions
1. Explain the elastic rebound theory. Accumulated stress can
cause rocks to deform until it fractures and rebounds, causing
an earthquake.
2. In earthquakes that cause the greatest damage, at what depth
would slippage most likely occur? Within 70 km of the surface
3. What four types of plate movements can cause earthquakes?
Plates colliding, plates pulling apart, plates subducting and
plates sliding past each other.
4. If an earthquake occurs in the center of Brazil, what can you
infer about the geology of that area? An ancient fault zone
deep within the crust might be located there.