After completing this chapter, you should be able to:
- Compare and contrast the different types of seismic waves.
- Understand the different scales used to measure earthquakes, and apply them to the amount of devastation.
- Understand how different geologic materials behave during an earthquake and the resulting impact on structures.
- Explain how an earthquake epicenter is located.
- Explore the relationship between the fracking industry and seismicity.
- 13.1: Introduction
- It was the deadliest day in the history of Mt. Everest. On April 25, 2015, a 7.8 magnitude earthquake hit Nepal. This triggered an avalanche that killed 19 climbers on Mt. Everest. In Nepal, over 8,800 people died, and many more were injured and made homeless. Hundreds of aftershocks (smaller earthquakes that follow a larger earthquake) have occurred since.
- 13.2: The Epicenter, Focus, and Waves
- An earthquake is like a telegram from the Earth. It sends a message about the conditions beneath the Earth’s surface. The shaking or trembling experienced during an earthquake is the result of a rapid release of energy within the Earth, usually as a result of movement along geologic faults. Think back to the strike-slip fault from the Crustal Deformation chapter.
- 13.3: Seismology
- Earthquakes have been experienced by humans as long as humans have roamed the Earth, though most ancient cultures developed myths to explain them (including envisioning large creatures within the Earth that were moving to create the quake). The study of earthquakes, called seismology, began to take off with the development of instruments that can detect earthquakes; this instrument, called a seismograph, can measure the slightest of Earth’s vibrations.
- 13.4: Locating an Earthquake Epicenter
- During an earthquake, seismic waves are sent all over the globe. Though they may weaken with distance, seismographs are sensitive enough to still detect these waves. In order to determine the location of an earthquake epicenter, seismographs from at least three different places are needed for a particular event. In Figure 13.9, there is an example seismogram from a station that includes a minor earthquake.
- 13.5: Lab Exercise (Part A)
- You will determine the location of an earthquake epicenter using seismograms from Carrier, Oklahoma, Smith Ranch in Marlow, Oklahoma, and Bolivar Missouri available at the end of this chapter. These are actual seismograms that you will be reading, from an actual event. For each, three different readouts are given, as the seismograph measured in three different axes. You may focus on any of the three readouts for each station, as all will have the same arrival times for each wave.
- 13.6: Hazards from Earthquakes
- Earthquakes are among nature’s most destructive phenomena, and there are numerous hazards associated with them. Ground shaking itself leads to falling structures, making it the most dangerous hazard. The intensity of ground shaking depends on several factors, including the size of the earthquake, the duration of shaking, the distance from the epicenter, and the material the ground is made of. Solid bedrock will not shake much during a quake, rendering it safer than other ground materials.
- 13.7: Lab Exercise (Part B)
- Download the kml file from the USGS for Google Earth. Note that this file adds a layer of liquefaction susceptibility, with areas more likely to experience liquefaction in yellow, orange, or red. Once in Google Earth, type in San Francisco, CA. Zoom in to less than 25 miles to see the layers added and note where liquefaction is most likely, then answer the following questions. When necessary, type locations into the Search box to locate them.
- 13.8: Induced Seismicity
- The number of significant earthquakes within the central and eastern United States has climbed sharply in recent years. During the thirty-six year period between 1973 and 2008, only 21 earthquakes with a magnitude of 3.0 or greater occurred. During the 5 year period of 2009-2013, 99 earthquakes of that size occurred within the same area, with 659 earthquakes in 2014 alone and well over 800 earthquakes in 2015 just in Oklahoma.
- 13.9: Lab Exercise (Part C)
- The exercises that follow use Google Earth. Let’s start by examining the 1906 earthquake that hit Northern California. There are several links of interest here. Scroll down to the section entitled “The Northern California Earthquake, April 18, 1906” and open the link. The San Andreas Fault is ~800 miles long, located in California. In 1906, a major earthquake occurred along a portion of the fault. Scroll down and check out the Rupture Length and Slip.
- 13.10: Student Responses
- The following is a summary of the questions in this lab for ease in submitting answers online.
Thumbnail: Aerial photo of San Andreas Fault looking northwest onto the Carrizo Plain with Soda Lake visible at the upper left. (CC BY 3.0; John Wiley via wikipedia).