# Earthquakes by CF2084S

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Earthquakes
Objectives
Given information on Earthquakes, you will be able to
describe, in writing:

a) what an earthquake is;

b) what the elastic rebound theory is;

c) how earthquakes are measured;

d) how earthquakes are related to plate tectonic theory;

e) hazards associated with earthquakes; and

f) the significance of studying earthquakes.
Earthquakes
Definition:
An earthquake (also known as a tremor or
temblor) is the result of a sudden release
of energy in the Earth's crust that creates
seismic waves.
Earthquakes
Elastic Rebound Theory
• explanation for how energy is spread during earthquakes

• plates/blocks of earth shift on opposite sides of a fault

• these are subjected to force (Time 1)

• they accumulate energy (Time 2)

• they slowly deform until their internal strength is exceeded

• a sudden movement occurs along the fault

• accumulated energy is released

• rocks snap back to their original undeformed shape (Time 3)
Earthquakes
Measurement

1. Magnitude scale
a) measures energy released;
b) an earthquake one number higher is approximately 30
times more powerful (e.g. a 7.0 vs. a 6.0); (the scale is
logarithmic)
d) an earthquake one number higher produces
approximately 10 times more ground shaking
e) examples = Moment Magnitude, Richter
Earthquakes
Measurement (cont.)

2. Mercalli Intensity scale
a) measures intensity or damage of an earthquake;
b) quantifies the effects of an earthquake on the Earth's
surface, humans, objects of nature, and man-made
structures
d) scale ranges from I through XII, with I denoting a
weak earthquake and XII one that causes almost
complete destruction
e) based on the amount of resulting physical damage
f) data is gathered from individuals who have
experienced the quake
Earthquakes
Plate Tectonics
• Earthquakes are NOT randomly distributed
across the globe
• They are concentrated along narrow zones
• Most earthquakes (~80% or more) occur along
plate boundaries
• Interior of plates are mostly earthquake free
• Earthquakes define plate boundaries
• Some also occur at Hot Spots
Earthquakes
Hazards
1. Ground motion
2. Liquefaction
3. Landslides & mass wasting
4. Tsunamis
Ground Motion

Ground motion simulation map from the 1906 SF Earthquake
Liquefaction
Landslides & Mass Wasting
Tsunamis
Earthquakes
Significance
• It is important to study earthquakes due to
the loss of life and damage to property
they can cause
• Human population continues to grow and
many cities are cited along plate
boundaries
• Education is the key to being prepared for
earthquakes
Summary
• Earthquakes occur due to the elastic rebound
theory
• Earthquakes are measured using the magnitude
scale
• Earthquakes occur along plate boundaries
• Earthquakes cause ground motion, tsunamis,
mass wasting, and liquefaction
• Earthquake hazards may cause loss of life and
damage to property

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