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Chapter 8-Earthquakes
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-------------------------------------------------------------------------------------------------------------------------------Section 1: What Are Earthquakes?
-------------------------------------------------------------------------------------------------------------------------------1. There is more to earthquakes than just the shaking of the ground. An entire branch of Earth
science, called seismology, is devoted to the study of earthquakes.
2. Earthquakes are complex, and they present many questions for seismologists, the
scientists who study earthquakes.
Where Do Earthquakes Occur?
3. Most earthquakes take place near the edges of tectonic plates.
4. Tectonic plates move in different directions and at different speeds. As a result, numerous
features called faults exist in the Earth’s crust.
5. A fault is a break in the Earth’s crust along which blocks of the crust slide relative to one
another, resulting in an earthquake.
What Causes Earthquakes?
6. As tectonic plates move, stress increases along faults near the plates’ edges. In
response to this stress, rock in the plates deforms.
7. Deformation is the change in the shape of rock in response to the stress of bending, tilting,
and breaking of the Earth’s crust.
8. Rock along a fault deforms in mainly 2 ways.
9. Rock deforms in a plastic manner, like a piece of molded clay, or in an elastic manner, like
a rubber band.
10. Plastic deformation does not lead to earthquakes. Elastic deformation does. Like
a rubber band, rock can be stretched only so far before it breaks.
11. Elastic rebound is the sudden return of elastically deformed rock to its undeformed shape.
Elastic rebound occurs when more stress is applied to rock than the rock can withstand.
12. During elastic rebound, energy is released. Some of this energy travels as seismic waves,
which cause an earthquake.
Faults at Tectonic Plate Boundaries
13. A specific type of plate motion takes place at different tectonic plate boundaries.
14. Each type of motion creates a particular kind of fault that can produce earthquakes.
a. Transform motion occurs where two plates slip past each other, creating strike-slip
faults. Blocks of crust slide horizontally past each other.
b. Convergent motion occurs where two plates push together, creating reverse faults.
Blocks of crust that are pushed together slide along reverse faults.
c. Divergent motion occurs where two plates pull away from each other, creating
normal faults. Blocks of crust that are pulled away from each other slide along normal
faults.
How Do Earthquake Waves Travel?
15. Waves of energy that travel through the Earth away from an earthquake are called seismic
waves.
16. Seismic waves that travel along the Earth’s surface are called surface waves.
a. Surface waves are the slowest moving waves and therefore are the last to be
recorded on a seismograph-an instrument that amplifies and records small
movements of the ground.
17. Seismic waves that travel through Earth’s interior are called body waves. There are two
types of body waves: P waves and S waves.
a. P waves are seismic waves that cause particles of rock to move in a back-and-forth
direction.
1. P waves (primary waves), move the fastest and are the first to be recorded by
a seismograph.
2. They can travel through both liquid and solid mediums.
3. P waves are compression waves; they cause rock particles to move together
and apart along the direction of the waves.
b. S waves are seismic waves that cause particles of rock to move in a side-to-side
direction.
1. Secondary waves-or S waves, are the second waves to be recorded on a
seismograph.
2. Can only travel through solid material. Movement of the wave is similar to the
way a snake moves!
-------------------------------------------------------------------------------------------------------------------------------Section 2: Earthquake Measurement
-------------------------------------------------------------------------------------------------------------------------------Determining Time and Location of Earthquakes
1. Scientists use seismographs to study earthquakes.
2. When earthquake waves reach a seismograph, it creates a seismogram, a tracing of the
earthquake’s motion.
3. Seismograms are used to find an earthquake’s epicenter.
4. An epicenter is the point on the Earth’s surface directly above an earthquake’s starting point.
5. A focus is the point inside the Earth where an earthquake begins.
6. An earthquake’s epicenter is on the Earth’s surface directly above the earthquake’s focus.
7. The S-P Time Method is perhaps the simplest method by which seismologists find an
earthquake’s epicenter.
8. The difference in time that it takes P waves and S waves ( P waves travel ~ 1.7 times faster
than S waves) to arrive at a seismograph station helps scientists locate the epicenter of an
earthquake.
Measuring Earthquake Strength and Intensity
9. A measure of the strength of an earthquake is called magnitude. The Richter scale
measures the ground motion from an earthquake and adjusts for distance to find its strength.
a. For each increase of 1.0 on the Richter scale, the amplitude of the largest surface
wave is 10 times greater. In an earthquake of magnitude 7, the ground moves 10 times
as much as it moves in a quake of magnitude 6.
b. About 32 times as much energy is released for every increase of 1.0 on the scale.
An earthquake of magnitude 7 releases about 32 times the energy released by a quake
of magnitude 6.
10. Currently, seismologists use the Modified Mercalli Intensity Scale to measure earthquake
intensity. This is a numerical scale that uses Roman numerals from I to XII to describe
earthquake intensity levels.
11. Effects of Earthquakes:
a. Earthquakes are among the most powerful events on earth, and their results can be
terrifying. A severe earthquake may release energy 10,000 times as great as that of the
first atomic bomb.
b. Rock movements during an earthquake can make rivers change their course.
c. Earthquakes can trigger landslides that cause great damage and loss of life.
d. Large earthquakes beneath the ocean can create a series of huge, destructive
waves called tsunamis that flood coasts for many miles.
e. Earthquakes almost never kill people directly. Instead, many deaths and injuries
result from falling objects and the collapse of buildings, bridges, and other structures.
Fire resulting from broken gas or power lines is another major danger during a quake.
Spills of hazardous chemicals are also a concern during an earthquake.
-------------------------------------------------------------------------------------------------------------------------------Section 3: Earthquakes and Society
-------------------------------------------------------------------------------------------------------------------------------Earthquake Hazard
1. Earthquake hazard is a measurement of how likely an area is to have damaging earthquakes
in the future.
Earthquake Forecasting
2. Strength and Frequency: Earthquakes vary in strength. The strength of earthquakes is
related to how often they occur.
3. Another method of forecasting an earthquake’s strength, location, and frequency is
the gap hypothesis.
4. The gap hypothesis is based on the idea that a major earthquake is more likely to occur
along the part of an active fault where no earthquakes have occurred for a certain period of time.
5. An area along a fault where relatively few earthquakes have occurred recently but where
strong earthquakes have occurred in the past is called a seismic gap.