4 - Earthquakes


We have already talked about plate tectonics and how the Earth's crustal plates move at an average rate of a few centimetres each year. Along faults however, this movement is not slow and gradual but occurs as a series of jerks, both large and small. The reason that the motion is jerky along faults is because there can be a lot of friction between rocks on either side. As plates steadily move toward, away from, or past each other, stress is built up in the rocks around the fault. Eventually the built-up stress overcomes friction and several years worth of movement are released in one single event. These motions are what we feel as earthquakes. The location within the Earth where this sudden movement occurs is the source of the earthquake and is called the focus. The epicentre of the earthquake is the place on the Earth's surface directly above the focus.

Learning Objectives

Upon completion of this unit you should be able to:

1. Describe the types of seismic waves and their relative speeds and effects.
2. Explain how the epicenter of an earthquake is located.
3. Predict the relative intensities of earthquakes at each type of plate boundary.
4. Say where intraplate earthquakes occur.
5. Describe what the magnitude of an earthquake means
6. List the hazards associated with earthquakes.
7. Explain how long-term forecasting of earthquakes is carried out.
8. Describe at least one technique used in paleoseismology.
9. List the events that may signal an earthquake.
10. Describe the properties of building materials that affect building safety in the event of an earthquake.

Required Readings

Abbott, P.L., 2014. Natural Disasters, Ninth Edition:

Readings From the Old Book

Abbott, P.L., 2012. Natural Disasters, Eighth Edition:

Glossary Terms

14C Dating Plastic Deformation
Aftershock P-waves
Benioff Zone Seismic Gap
Brittle Deformation  Seismic Gap Theory
Elastic Deformation Seismic Waves
Epicentre Seismograph
Focus Strain
Foreshock S-waves
Liquefaction Thrust Fault
Logarithmic Scale Triangulation
L-waves Tsunami
Moment Magnitude