Chapter 9. Earthquakes
Overview of Earthquakes
Adapted by Sean W. Lacey & Joyce M. McBeth (2018) University of Saskatchewan from Deline B, Harris R, & Tefend K. (2015) “Laboratory Manual for Introductory Geology”. First Edition. Chapter 13 “Earthquakes” by Randa Harris, CC BY-SA 4.0. View Source. Last edited: 8 Jan 2020
9.2 THE EPICENTER, FOCUS, AND WAVES
An earthquake is a like a telegram from the Earth. It sends a message about 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 (weak planes in the Earth’s crust). In the chapter on geologic structures, we introduced you to faults. In a strike-slip fault like the San Andreas fault in California, USA, rocks on either side of the fault slide past each other. As they move in opposite directions, the rocks become deformed, bend slightly, and build up pressure. Eventually they will reach a breaking point. Once the strength of the rock has been exceeded, the rocks will snap back to their normal shape, releasing the stored energy as an earthquake. The more energy that has been stored, the larger the earthquake. In the chapter on geologic structures we also introduced you to a diagram illustrating the relationship between stress and strain. When rocks are under too great of a stress, the strength of the rock is exceeded, and the rocks undergo brittle failure (the earthquake).
Earthquakes originate at a point called the focus (plural foci). From this point, energy travels outward in different types of waves. The place on the Earth’s surface directly above the focus is called the epicenter (Figure 9.2). Earthquake foci may be shallow (less than 70 km from Earth’s surface) to deep (greater than 300 km deep), though shallow to intermediate depths are much more common. Earthquakes are more frequent along active tectonic plate boundaries (95 % of all earthquakes occur along plate boundaries), and their depth is related to the type of plate boundary. Shallow focus earthquakes tending to occur at divergent and transform plate boundaries. Convergent plate boundaries have a range of earthquake depths, with the earthquakes moving from shallow to intermediate to deep focus as the subducting plate moves deeper under the overriding plate. The earthquakes associated with convergent boundaries occur along the Wadati-Benioff zone (or Benioff zone) an area of dipping seismicity along the subducting plate (Figure 9.3).
As an earthquake occurs, two different types of waves are produced; body waves, so termed because they travel through the body of the Earth, and surface waves that travel along the Earth’s surface (Figure 9.4). There are two types of body waves. P-waves, or primary waves, are compressional waves that move back and forth, similar to the action of an accordion or a slinky. As the P waves pass through the Earth’s crust, the rock is compressed and stret