79 Chapter 12 Summary

The topics covered in this chapter can be summarized as follows:

12.1 What Is an Earthquake?

An earthquake is the shaking that results when a deformed body of rock snaps back to its original shape. The rupture is initiated at a point but quickly spreads across the area of a fault, with aftershocks initiated by stress transfer. Episodic tremor and slip is a periodic slow movement, accompanied by harmonic tremors, along the middle part of a subduction zone boundary.

12.2 Measuring Earthquakes

Earthquakes produce seismic waves that can be measured by a seismograph. The amplitudes of seismic waves are used to determine the amount of energy released by an earthquake- its magnitude. For the moment magnitude scale used today, the amount of energy released by an earthquake is proportional to the size of the rupture surface, the amount of displacement, and the strength of the rock. Intensity is a measure of the amount of shaking that occurs, and damage done at locations that experience an earthquake. Intensity will vary depending on the distance to the epicentre, the depth of the earthquake, and the type of geological materials present.

12.3 Earthquakes and Plate Tectonics

Most earthquakes happen at or near plate boundaries. Along divergent and transform boundaries earthquakes are shallow (less than 30 km depth), but at convergent boundaries they can be hundreds of kilometers beneath the surface. The largest earthquakes happen when a broad segment of the locked zone of a subduction zone ruptures. Intraplate earthquakes happen away from plate boundaries. They can be caused by human activities, or renewed motion on ancient faults.

12.4 The Impacts of Earthquakes

Damage to buildings is the most serious consequence of most large earthquakes. The amount of damage is related to the type and size of buildings, how they are constructed, and the nature of the material on which they are built. Other important consequences are fires, damage to bridges and highways, slope failures, liquefaction, and tsunami.

12.5 Forecasting Earthquakes and Minimizing Impacts

There is no reliable technology for predicting earthquakes, but the probability of one happening within a certain time period can be forecast. We can minimize earthquake impacts by ensuring that the public is aware of the risk, that building codes are enforced, that existing buildings like schools and hospitals are seismically sound, and that both public and personal emergency plans are in place.

Review Questions

  1. What causes the shaking during an earthquake?
  2. What is a rupture surface, and how does the area of a rupture surface relate to earthquake magnitude?
  3. What is an aftershock and how are aftershocks related to stress transfer?
  4. Episodic slip on the middle part of the Cascadia subduction zone is thought to increase the stress on the locked zone. Why?
  5. What is the difference between the magnitude of an earthquake and its intensity?
  6. How much more energy is released by a magnitude 7 earthquake compared to a magnitude 5 earthquake?
  7. The images below show earthquake locations for three regions of ocean lithosphere. The colour scheme indicates the depths of earthquakes. a) The images show a subduction zone, a slowly spreading mid-ocean ridge, and a rapidly spreading mid-ocean ridge. Which is which? b) In the image with the subduction zone, which side is the subducting plate, and which is the overriding plate?
    Figure 12.35 Earthquakes along plate margins. Dots are colour coded according to depth. Source: Map details from Lisa Christiansen, Caltech Tectonics Observatory (2008) view source. Click the image for terms of use.
  8. Why is earthquake damage likely to be more severe for buildings built on unconsolidated sediments as opposed to on solid rock?
  9. Why are fires common during earthquakes?
  10. What type of earthquake is likely to lead to a tsunami?
  11. What did we learn about earthquake prediction from the 2004 Parkfield earthquake?
  12. What are some of the things we should know about an area in order to help minimize the impacts of an earthquake?
  13. What is the difference between earthquake prediction and forecasting earthquake probability?