Lesson 2: Faults are at Fault: Where Earthquakes Occur
Students plot the locations of faults and then map recent earthquakes in order to see the relationship between earthquakes and faults.
Materials / Preparation
Groups of two
Students will need familiarity with latitude and longitude prior to starting this lesson. The website, Latitude and Longitude provides a good introduction or review.
We have supplied two maps for plotting the faults: one of the San Francisco Bay Area and the other of California. You can choose to have students map the faults for just the Bay Area or all of California, depending upon your regional interests. Note that the Bay Area only map has many fewer points to plot. That makes it quicker, but a slightly less effective illustration of the fact that earthquakes occur along faults.
We have supplied a Fault Extent Student Exercise Answer Sheet.
The extension activity is strongly recommended for high school students so that students learn about the importance of the depth of earthquakes.
You may want to assign additional earthquakes from All Earthquakes in California-Nevada for students to map as homework. This page is a table showing recent earthquake activity in California. The magnitude, date, time, latitude, and longitude of the epicenter are indicated. On any given day, this table includes too many earthquakes for students to spend their time plotting, so you may want to select certain criteria so that the number of earthquakes to plot is reasonable.Note: This site can be accessed directly from the Earthquake Hazards Student Web Page.
In this lesson, students plotted epicenters on a map, but the map does not give any information about the third important dimension in earthquakes - depth. This extension activity, Revealing a Fault Plane with Hypocenters helps students gain an appreciation for the fact that earthquakes rupture along faults at depth. Students will need to know the term "hypocenter" (identical to the term "focus). The hypocenter (or focus) is the exact point where the earthquake rupture starts. This is usually deep within the earth's crust and many kilometers below the surface of the earth, and some distance away from the surface trace of the fault.
Remind students that the hypocenter is only the point where the earthquake STARTS, but a fault can rupture a very large area. This is the case for the Palm Springs earthquake. It started out at the hypocenter (indicated by an asterisk on the map), but spread out along a planar section of the fault, eventually rupturing a somewhat rectangular patch about 10 km long. Aftershocks tend to happen over an area that outlines the main shock patch.
You can see that the aftershocks, which were very small earthquakes, extend over an area about 10 km-long, parallel to the San Andreas fault. The aftershocks 'light up' (in a metaphorical sense) the patch that ruptured during the larger main shock. Notice how in the rotating fault plane animation (linked from the Revealing a Fault Plane with Hypocenters page) and on the student activity, few earthquakes occur below a certain depth (about 15 km in this case) or shallower than a certain depth (about 3 km in this case). Those limits define the depth extent of rupture during the main shock. In other words, the area that ruptured was a rectangle about 10 km long and about 12 km deep.
The angle that the earthquakes form in the cross section activity shows that the rectangular patch was not vertical, but instead slightly inclined. If you were to look at the surface exposure of the fault that slipped during this earthquake, you would also see that it intersects the surface at that same inclined angle. (Note: the depths are also related to other properties of faults called the brittle-ductile transition, but that is a substantially more advanced topic.)