Answers to Chapter 12 Review Questions

1. Rocks under stress can deform elastically. When rocks break, or a rupture occurs along an existing fault, the deformed rocks snap back to their original shape, causing vibrations. The vibrations are the shaking felt during earthquakes.

2. The rupture surface is the surface over which there is displacement of rock during an earthquake. The magnitude of an earthquake is proportional to the area of the rupture surface.

3. An aftershock is an earthquake that is triggered when a rupture from a previous earthquake has transferred too much stress to rocks, causing further ruptures.

4. Episodic slip on the middle part of the Cascadia subduction zone decreases stress within that area, but some of that stress is transferred to the locked zone along the plate boundary. This increases the amount of stress on the locked part.

5. Magnitude is the amount of energy released by an earthquake. Each earthquake has only one magnitude, although there are different ways of measuring it, and they may give slightly different results. Intensity is a measure of the amount of damage done or what people felt. Intensity varies depending on the distance to the epicentre and the type of rock or sediment underlying an area.

6. Each unit increase in magnitude corresponds to a 32x increase in energy. The difference between M5 and M7 is two units of magnitude. An earthquake that is two magnitude units larger would release 32 x 32 = 1,024 times as much energy.

7.  a) A: Slowly spreading mid-ocean ridge- earthquakes are shallow and evenly distributed; B: Subduction zone- earthquakes range in depth; C: Rapidly-spreading mid-ocean ridge- earthquakes are shallow, and occur in patches, corresponding to a greater likelihood of earthquakes where transform faults offset ridges.

b) The plate to the right of the chain of earthquakes is the subducting plate. The earthquakes increase in depth moving from right to left, indicating that the subducting slab is getting deeper farther to the left.

8. Unconsolidated sediments, especially if they are saturated with water, can lose strength when subjected to earthquake shaking. This can cause buildings to subside or tilt. Unconsolidated sediments can also amplify the vibrations of an earthquake.

9. Gas lines and electrical transmission wires are typically damaged during an earthquake, and this can lead to serious fires.

10. A large subduction-zone earthquake (greater than M7.5) can generate a tsunami because those earthquakes result in sufficient vertical displacement of the sea floor.

11. The 2004 Parkfield earthquake showed that we cannot rely on foreshocks to predict earthquakes, or on any of the many other parameters that were being carefully measured around Parkfield in the years leading up to the quake.

12. We should know about the history of past large earthquakes, the typical locations of small earthquakes, the types of geological materials beneath the surface (especially soft water-saturated sediments), the types of infrastructure that is present, and the various ways that people can be evacuated from an area or assistance can be brought in.

13. Forecasting involves estimating the risk of an earthquake happening in a region within a period of time. Prediction involves stating that an earthquake is likely to happen at a certain location on a specific day or month or year in the future. With our current state of knowledge of earthquakes, prediction is not possible.