## Lesson 13: Earthquakes, Faults and Seismic Hazards

Ask anyone from the East Coast or Midwest what comes to mind when they think of California, and earthquakes are almost certain to rank right up there with Hollywood and Southern California beaches. This week's lesson on earthquakes is our second to explore a topic of statewide importance and, as with our discussion of water a few weeks ago, earthquakes are something that will impact almost all Californians at some time in their lives. Whether the state is struck by an earthquake that occurs here (like the 1906 San Francisco earthquake at right) or by a tsunami that comes ashore from elsewhere in the Pacific Ocean, Californians need to be prepared in order to minimize the loss of life and damage to property.

In chapter 13 Harden briefly describes how earthquakes are generated, how the various types of seismic waves they produce differ, and how we can determine the locations and strengths of earthquakes by measuring the properties of these waves. She also outlines what factors, in addition to magnitude, can influence an earthquake's destructiveness and describes the unique hazards that earthquakes are likely to pose in different parts of the state. Finally, this week's exercise will give you an opportunity to try your hand at measuring seismic waves and using your data to determine the location and magnitude of an earthquake.

## Weekly Learning Objectives

Upon successful completion of this week's lesson, a student is expected to be able to:

• Explain why longer periods of strain accumulation along a fault might be expected to produce larger earthquakes in light of the elastic rebound model in which bent rocks "snap back" to their original shapes when a fault ruptures (Figure 13-3).
• Cite at least two criteria that you could use to distinguish an active fault from an inactive fault.
• Contrast an earthquake's magnitude with its intensity, briefly describe what each of these properties measures, and explain how factors such as distance and type of surface material can influence an earthquake's intensity.
• Use measurements of P-wave and S-wave arrival times from at least three seismograms to locate the epicenter of an earthquake, and use a distance-amplitude nomogram to estimate the quake's magnitude.
• Estimate how much (1) the amplitude of ground motion at a given distance from the epicenter, and (2) the amount of energy released will differ for two earthquakes with different magnitudes. (Assume that ground motion increases by a factor of 10 and energy output by a factor of 30 for each step up on the magnitude scale.)
• List three seismic hazards that may accompany an earthquake and indicate which areas of the state would be at particular risk from each. (For example, tsunamis are threats to coastal areas.)
• Briefly describe how geologists study the record of past movements on a fault and assess the frequency with which earthquakes recur on a given fault.