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.

Concepts and learning outcomes	Students will understand that: Earthquakes occur along faults at plate boundaries. Earthquakes occur along patches of planar faults – they are not just a single point but have lengths and widths.
Time requirements	One or two 50 minute periods, depending upon the use of the extension, which is recommended for high school students.
Vocabulary	Epicenter, fault, hypocenter (extension only), longitude, latitude, magnitude
Background for teachers	In the activity, you will notice that a lot of the earthquakes are located near but not on the major faults as we have mapped them, and some occur extremely far from our mapped faults. There are two reasons for this: The map is oversimplified; it only shows the major faults and draws them as straight lines. In reality, there are dozens of faults that come in all sizes from a few centimeters to hundreds of kilometers long. Each of these faults has slight curves, bends, and steps. For a more complete map, please see the map of faults for California and an index to the contiguous 48 states, (from the USGS) where each of the tiny red lines represents a known and mapped fault (you can click to zoom in). Faults are 3-dimensional features that have area (a length and a width).The lines on a map only show the location where faults intersect the earth's surface. This is a lot like looking at the edge of a book – every page appears to be a single line (like the red lines of fault maps). However, you know that when you open the book, each page has an area where there is writing. The same is true with faults. Just looking at the line on the earth's surface doesn't tell you anything about the size or orientation of the fault below the surface. It could be vertical like a book on a shelf, or it could be tilted one way or the other. If the fault is tilted, the epicenters of earthquakes initiating along the fault at depth will show up a few kilometers away from where that fault hits the surface, even though the earthquake is rupturing along the fault at depth.

Materials / Preparation

Computers with internet access
Copies of the following for each student:
- Fault Extent Student Exercise that contains data and instructions for use
- Maps for plotting faults and earthquakes: San Francisco Bay Area, and California
Earthquake Hazards Student Web Page

Grouping

Groups of two

Teacher tips

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.

Procedures

Review students' homework assignment from Lesson 1. Conduct a short class discussion to find out what students learned from the people they talked to who had experienced an earthquake. If possible compare experiences from the same earthquakes (10 minute discussion)

Explain to students that over the next few days, they will be studying earthquakes and their impacts in greater detail. As a closing activity, they will be given five specific addresses in California and their challenge will be to decide at which address they would prefer to live in terms of seismic safety.

If you want, you can read aloud the introductory paragraph from that closing activity:

You have been transferred to the San Francisco Bay Area, which is prone to earthquakes. Your boss has selected five possible sites for you to purchase a house. Obviously, you would prefer to purchase a home in a safe and livable place where there would be less possibility of earthquake damage and where there are other amenities. Your job will be to select the best place. Explain to students that the research they will be doing in the next few days will help them to make their decisions.

Brainstorm with students about where earthquakes occur. If the students mention faults, ask them what a fault is and how faults relate to earthquakes. Following the activity, students should have a clear understanding of the relationship between faults and earthquakes..
Hand out copies of the Fault Extent Student Exercise and the maps (Bay area, California) to each student. Review the information and instructions. Have students work in groups of two to complete the mapping exercise and answer the questions.
Once all students are finished, review their answers and then project the Maps of Recent Earthquake Activity in California-Nevada website to confirm their conclusion – that earthquakes occur along faults. This site can be accessed directly from the Earthquake Hazards Student Web Page.

Point out that the maps of recent earthquake activity changes daily and all of the squares represent recent earthquakes, color-coded as to when they occurred. The bigger the square, the greater the magnitude of the earthquake. Have students explore the site a bit. Then ask, have there been any earthquakes within the past hour? The past day? For schools in the Bay Area, have students identify the closest fault to the school. Have any earthquakes occurred on that fault recently?

Homework

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.

Extension

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.)

Note: This site can be accessed directly from the Earthquake Hazards Student Web Page.