Grotzinger • Jordan
Understanding Earth
Sixth Edition
Chapter 13:
EARTHQUAKES
© 2011 by W. H. Freeman and Company
Chapter 13
Earthquakes
About Earthquakes
• Earthquakes can be understood in terms
of the basic mechanisms of deformation.
• Most earthquakes occur at plate
boundaries (convergent, divergent, and
sliding).
• Earthquakes cannot yet be reliably
predicted or mitigated.
Lecture Outline
1. What is an earthquake?
2. How do we study earthquakes?
3. Earthquakes and patterns of faulting
4. Earthquake hazards and risks
5. Can earthquakes be predicted?
1. What Is an Earthquake?
● Global forces at work
● stress
● strain
● strength
1. What Is an Earthquake?
● Earthquakes occur where rocks
being stressed suddenly break
along a new or pre-existing
fault.
● Seismic waves are ground
vibrations caused by rocks
slipping along opposite sides of a
fault.
1. What Is an Earthquake?
● Why earthquakes occur
● elastic rebound
theory
● fault rupture
● epicenter
● focus
1. What Is an Earthquake?
Example of Elastic Rebound
1. What Is an Earthquake?
Fault Rupture
1. What Is an Earthquake?
● Local buildup and release of
stress
● foreshock
● aftershock
2. How Do We Study Earthquakes?
● Seismographs are machines that
record the seismic waves
generated by earthquakes.
● vertical ground movements
● horizontal ground movements
2. How Do We Study Earthquakes?
● Seismic wave types
● P waves (primary waves)
● S waves (secondary waves)
● Surface waves
2. How Do We Study Earthquakes?
● Locating the earthquake epicenter
● P- and S-wave arrival times
from at least 3 seismographs
● Graph of distance traveled
versus time elapsed
2. How Do We Study Earthquakes?
● Measuring the size of an
earthquake
● Richter magnitude
● Moment magnitude
● Shaking intensity
2. How Do We Study Earthquakes?
● Magnitude and frequency
● many small earthquakes
● few large earthquakes
2. How Do We Study Earthquakes?
Modified Mercalli Intensity Scale
Mercalli
intensity
of the
New Madrid
earthquake,
Dec. 16, 1811
2. How Do We Study Earthquakes?
● Determining fault mechanisms from
earthquake data
● pattern of ground shaking
(first motion of P waves)
● orientation of fault rupture
● direction of slip
2. How Do We Study Earthquakes?
● Fault mechanism tells us whether
the rupture was:
● normal
● reverse
● strike-slip (right- or leftlateral)
2. How Do We Study Earthquakes?
Main Types of Fault Movement
2. How Do We Study Earthquakes?
First Motion of P Waves
3. Earthquakes and Patterns of Faulting
● Earthquakes and plate
tectonics
● divergent boundaries
● transform-fault boundaries
● convergent boundaries
● intraplate earthquakes
3. Earthquakes and Patterns of Faulting
3. Earthquakes and Patterns of Faulting
3. Earthquakes and patterns of faulting:
Regional fault systems
Example: Fault system of southern
California
3. Earthquakes and Patterns of Faulting
● Earthquakes and destruction
● loss of life
● property damage
● tsunami and landslides
Earthquake damage in
Los Angeles, 1994
Earthquake damage in
Kobe, Japan, 1995
Earthquake damage in
Kashmir, 2005
Earthquake damage in
Mexico City, 1985
Tsunami effects in
Thailand, 2004
Tsunami effects in
Sumatra, 2004
4. Earthquake Hazards and Risks
● How earthquakes cause damage
● faulting and shaking
● landslides and ground failures
● tsunamis
● fires
4.Earthquake Hazards and Risks
● Reducing earthquake risks
● land-use policies
● hazard characterization and
proper emergency planning
● earthquake engineering
(proper building codes)
Example of
poor land-use
planning:
Construction
along the trace
of the
San Andreas
fault zone,
San Francisco
Example of emergency planning:
Tsunami barrier, Taro, Japan
4. Earthquake Hazards and Risks:
Planning
4. Earthquake Hazards and Risks
● Reducing earthquake risks
● mapping seismic hazards
● assessing seismic risks
4. Earthquake Hazards and Risks:
World Seismic Hazard Map
U.S. seismic hazard map
U.S. seismic risk map
5. Can Earthquakes Be Predicted?
● Earthquake forecasting
● long-term
● intermediate-term
● short-term
Thought questions for this chapter
Taking into account the possibility of false alarms,
reduction of casualties, mass hysteria, economic
depression, and other possible consequences of
earthquake prediction, do you think the objective of
predicting earthquakes should have a high priority?
Key terms and concepts
Aftershock
Building code
Earthquake
Elastic rebound theory
Epicenter
Fault mechanism
Fault slip
Focus
Foreshock
Intensity scale
Magnitude scale
P wave
Recurrence interval
S wave
Seismic hazard
Key terms and concepts
Seismic risk
Seismograph
Surface wave
Tsunami