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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.
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.
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.
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.
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.
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.
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
1. P waves (primary waves), move the fastest and are the first to be recorded by
a seismograph.
2.
3. P waves are compression waves; they cause rock particles to move together
and apart along the direction of the waves.
b. S waves
1. Secondary waves-or S waves, are the second waves to be recorded on a
seismograph.
2.
--------------------------------------------------------------------------------------------------------------------------------------------------------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.
4. An epicenter is the point on the Earth’s surface directly above an earthquake’s starting point.
5.
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. 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.
c. Earthquakes can trigger landslides that cause great damage and loss of life.
d.
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 Forecasting
2. Strength and Frequency: Earthquakes vary in strength. The strength of earthquakes is related to how often they
occur.
3.
4.
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.
Questions to Consider:
1. Where do earthquakes occur?
2. What directly causes earthquakes?
3. What is a fault?
4. Name two ways in which rock along a fault deforms in response to stress.
5. What is the difference between a seismogram and a seismograph?
6. How many seismograph stations are needed to use the S-P-time method? Why?
7. How is an earthquake’s epicenter related to its focus?
8. What are aftershocks?