CHAPTER FIVE: EARTHQUAKES
SUMMARY
This chapter begins with a personal account of an earthquake and an example of using the
scientific method related to tsunami threats for the west coast of North America. The nature and
significance of earthquakes are discussed in the initial sections where much of the terminology
associated with earthquakes (e.g., epicenter, focus, dip-slip) is introduced. Earthquake
distributions are discussed in context to plate boundaries. Sections are dedicated to describing P,
S, and surface waves, their velocities and particle motion, how they are detected, and the damage
each may cause. A discussion of magnitude scales and seismic hazards follows. Lastly, tsunamis
are discussed in the context of the Sumatra disaster and potential threat to the United States.
5.2: The Science of Ghost Forests and Mega-Earthquakes (page 107)
Read this section! You should be able to answer Checkpoint 5.3 after reading this section, and
you will find questions on the test related to this hypothesis.
5.3: Faults, Earthquakes and Plate Tectonics (page 110):
Figure Above: Focus: where the initial rupture begins, this is where body waves (P-waves and
S-waves) originate when sudden movement occurs along a fault (producing an earthquake, we
feel the energy of this movement by the seismic waves). Epicenter: location on Earth's surface
directly above the focus. Body waves reach the epicenter first on Earth's surface and travel
away as surface waves (Rayleigh waves and Love waves). The type of fault depicted here is dipslip (movement mostly vertical or along the dip of the fault plane). When one side of the surface
move up relative to the other side along a fault plane it produces a fault scarp (a steplike change
in elevation).
Figure Above: three types of faults are shown. In a normal fault, the block above the fault plane
moves downward. In a reverse fault, the block above the fault plane moves upward. Normal and
reverse faults move up and down along the dip of the fault plane (dip-slip faults). Faults that
show mostly horizontal movements are known as strike-slip faults (example is the San Andreas
Fault).
5.4: Seismic Wave and Earthquake Detection (page 117):
Body waves travel pass through the Earth's interior (P and S-waves) while surface waves travel
along the Earth's surface (Rayleigh and Love waves). P-waves (compressional) move the fastest
(2.5-4 miles per second); S-waves travel slower (2-2.5 miles per second); P-waves can travel
through solid, liquid and gas; S-waves only travel through solids, so used to infer molten areas in
the Earth's interior. Surface waves travel the slowest.
Figure Above: P-wave and S-wave motions. (a) P-waves are similar to the passage of a
vibration through a slinky. The vibration occurs in the same direction that the wave travels
(produces expansion and compression). (b) S-wave motion is analogous to a vibration moving
along a rope. The vibration occurs perpendicular to the direction in which the wave travels
(produces shearing).
Figure Above: Two types of surface waves. Highways and electric transmission lines are
examples of structures made by people that can be destroyed by an earthquake. (a) Rayleigh
waves produce vertical motions in the land surface. (b) Love waves move sideways, but not
vertically. Surface waves travel the slowest and generally do the most damage!!
P-waves will arrive at a seismic station (seismograph) first, followed by S-waves and surface
waves. The farther the seismograph is from the earthquake source, the greater the time interval
between the arrival of the first P-waves and first S-waves (since S-waves travel slower). This
difference in arrival time between the P and S-waves can be used to determine the distance of the
seismograph to the earthquake source (see figure 5.17). Note that three stations are needed to
triangulate the epicenter of an earthquake as plotted on figure 5.17 b.
5.5: Measurement of Earthquakes (page 122):
Earthquake magnitude and earthquake intensity are different measurements of an earthquake.
Magnitude is the shaking and energy released during an earthquake; intensity is the effects on
people and structures. Magnitude is measured on a logarithmic scale where each division (see
Table 5.1) represents a 10-fold increase in ground motion and an approximate 32 times increase
in energy released. For example, a magnitude 8 earthquake has 10 times as much ground motion
and releases about 32 times more energy than a magnitude 7 earthquake. remember the scals is
logarithmic: so a magnitude 8 earthquake has 100 times as much ground motion and releases
about 1000 times (32x32) more energy than a magnitude 6 earthquake.
To get an idea on a more personal level for us in the valley, the Easter Sunday earthquake last
year measured 7.2 on the magnitude scale. One of the largest earthquakes known in modern
history occurred in the Indian Ocean during 2004 that meaured 9.2 on the magnitude scale (and
generated a killer tsunami). How much more ground shaking and energy release occurred during
the 2004 earthquake?? Keep in mind that the earthquake we experienced lasted less than 1
minute; it is reported that the 2004 earthquake shook for up to 10 minutes!
For the 'Modified Mercalli Scale' (measure of earthquake intensity) please see Table 5.2. Notice
that plotted intensity values (see figure 5.19) also give you a good idea where the earthquake
epicenter is located.
5.6: Earthquake Hazards (page 126):
Please read this section carefully on hazards associated with ground shaking and vertical offset
of land. Other related hazards include landslide, liquefaction and tsunami. This section will be
of great help to you when you work on the earthquake risk rubric exercise (checkpoint 5.20).
Keep in mind the factors may be physical (such as the local geology) and/or cultural (such as the
size of population centers).
A good website to learn more about earthquakes and other hazards can by found through the link
below (the United State Geological Service, hazards program). You will find a link at this site
for recent earthquakes also.
http://www.usgs.gov/hazards/
LEARNING OBJECTIVES
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Students will explain concepts related to earthquakes.
Students will identify locations most likely to experience an earthquake.
Students will define and describe different kinds of faults.
Students will explain how movements on faults trigger earthquakes.
Students will relate characteristics of seismic waves to one another.
Students will describe how scientists detect and measure the size of earthquakes.
Students will evaluate the risk that a particular location will be damaged in an earthquake.
Students will relate earthquake information to science, society, and everyday life.