Predicting Wave Damage

Materials / Preparation

• Copies of Seismic Wave Activity Sheet for each student

Grouping

Whole group discussions

Teacher tips

Recommend using these sites as demonstrations using an LCD projector to facilitate whole group discussions

The Earthquakes Along the Hayward Fault animations are very good. There are 3 different versions of each mpeg movie with different file sizes for downloading. In some web browsers, the movies will play once when you first click on the links, but will look jumpy. Hitting the space bar will replay the movie at a fast speed in many programs. However, you can view the movie more slowly by clicking the cursor on a point of the fault and then pushing the left or right arrow many times to move through the movie slowly one frame at a time. This takes practice, so give it a try before doing it in class.

The downloadable software program Seismic Waves is excellent. Students select from nine major earthquakes and then watch a visualization of the progression of each type of seismic wave along with the recording on the seismogram. In order to use this site, you must download and install it using the Seismic Waves Set Up program. This is a large (4.8MB) file and takes a few moments to download, but it is worth it. Also, this program only works for Windows machines (not Mac).

CAUTION: For security reasons, some schools require that the teacher obtain permission before downloading programs to school computers. For more information and the readme file about the program, visit Alan Jones' website.

Procedures

1. Remind students of the predictions that they made resulting from the previous day's activities. Then let them know that they will be exploring several sites that show animations of these waves and how they might impact structures and landscapes.
   
2. Pass out the Seismic Wave Activity Sheet, which will instruct the students through each of the activities.
   
3. Demonstrate the Earthquakes Laboratory web page so that students are able to visualize the damage caused by these waves. Show the animations several times so that students understand what is happening. Have them record what they see on the Activity Sheet – 1 a-c.
   Teacher tip: Note that the statement "Typically, it is the surface waves that do the most damage during an earthquake, especially at distances far from the epicenter" is not entirely true. At distances far from the earthquake epicenter, surface waves have the highest amplitudes and longest duration. For those areas, they will therefore cause the most damage.
   
   However, as students will see in Lesson 5, most damage happens closer to faults where S-waves that arrive earlier than surface waves have higher amplitudes and therefore cause the most damage. Surface waves do cause a lot of damage at larger distances, especially in tall buildings (see the third paragraph of Earthquake Effects & Experiences for a discussion of how the frequency of waves is important in determining damage). In other words, the relationship between earthquake waves and damage is a bit complex. However, there are several things that are generally true that you can teach students:
   • P-waves are of lower amplitude and therefore do not generally cause much damage. Also, P-waves are often called "compressional waves" because their direction of motion results in compressing and expanding material. Buildings are designed to handle this compressive direction of motion (building foundations are exposed to compression from the weight of the building above them, so a little more compression is not usually a problem).
   • Both S-waves and surface waves are higher amplitude than P-waves and have a shear component to them. Side-to-side motion from "shear" waves can cause twisting, and buildings are not typically as resistant to twisting. The twisting jolt of the initial S-wave arrival is often enough to weaken structures. If they don't collapse from the S-wave, the surface wave with its longer duration and typically high amplitude can finish the building off.
     
4. Visit Earthquakes along the Hayward Fault to observe how waves travel away from the epicenter of an earthquake. Note: This site can be accessed directly from the Earthquake Hazards Student Web Page. Begin by clicking on the "South to North" MPEG movie. The movie shows the San Francisco Bay Area, outlined by the white line. The red line is the Hayward fault (part of the San Andreas fault system), and the box shows the area included in this computer simulation. The colors show seismic energy being emitted from points along the fault. Show the animation several times. Have students describe what they are seeing on the Activity Sheet – 2a. Ask students to share what they wrote so that you can guide the discussion to highlight the following points:
   
   a) Remember that earthquakes do not just happen at a single point but occur when a long section of a fault slips and releases accumulated strain. You can think of fault rupture during an earthquake like a zipper unzipping. In the South to North scenario movie on this web site, the earthquake begins at the southern end (epicenter at the south end) and rupture occurs progressively towards the north. Seismic waves get generated at a point as long as the fault is slipping at that point, which typically lasts less than 10 seconds (this would be analogous to the amount of time it takes to unzip a single set of zipper teeth). However, once one spot stops moving, other parts of the fault continue to slip and continue to produce seismic waves.
   
   b) Note how the waves keep spreading out from the rupture front as it unzips. For earthquakes, this unzipping occurs at about 2-3 km/s. If this movie were to run in real time, it would take about 30 seconds for the entire 70-km-long Hayward fault to slip.
   
   c) Note that some areas continue to shake long after the fault stops moving. These areas (the Santa Clara Valley near San Jose in the south and the San Pablo Bay in the north) are valleys or basins filled with softer soil and rock. The waves cause these basins to ring like a bell (resonate) long after the fault stops moving. Students will investigate this effect in more detail in Lesson 6. Students might also notice that different sections of the map ring in different scenarios (North-South v. South-North v. Bilateral). This is an effect called directivity, which will not be covered in this lesson.
   
   d) Have students answer 2b on the Activity Sheet, which asks them to compare how the earthquake might impact the South Bay vs. the North Bay.
   
5. Demonstrate the North to South rupture and the Bilateral rupture and have students make comparisons.
   
6. If students have time, the software program, Seismic Waves (this is the link to the set up program) is excellent for observing seismic wave activity. You can select from nine major earthquakes and then watch a visualization of the progression of each type of seismic wave along with the recording on the seismogram. In order to use this site, you must download and install it using the Seismic Waves Set Up program. This is a large (4.8MB) file and takes a few moments to download, but it is worth it. Also, this program only works for Windows machines (not Mac). SEE CAUTION ABOVE about installing software
   
   This program demonstrates the concept that seismic waves are produced by earthquakes and spread out from source region. Tiny amounts of shaking can be recorded by sensitive seismometers around the world. That's right, a large earthquake in California can be recorded by an instrument in China, even though the shaking is too small for people to feel. This program shows the waves spreading out from the earthquake source region.
   
   Have students notice how P waves travel fastest, S waves a bit slower, and surface waves slower than that. Notice how the surface waves only travel through the surface while P and S waves travel throughout the earth's interior. Watch the individual seismograms and notice how the P waves are the smallest amplitude, the S waves are the next highest amplitude, and surface waves that arrive last are the highest amplitude at stations far from the earthquake around the globe.
   
   Note to teacher: Unfortunately, this program reinforces the misconception that earthquakes initiate at a single point. Be sure to remind your students that this is not true — earthquakes rupture faults that have a length and width that can be up to hundreds of kilometers long. Consequently, seismic waves come from this entire rupture area. For the sake of animation, this program just draws the shaking emanating from a single point, the earthquake focus (point of initiation).
   
   Note: Because this website requires an installation, it is not listed on the Earthquake Hazards Student Web Page.

Resources used

Earthquakes Laboratory
http://www-rohan.sdsu.edu/~rmellors/lab8/l8maineq.htm#rwave

Earthquake Effects & Experiences
http://www.teachingboxes.org/catalog.jsp?id=TBOXR-000-000-000-111

Earthquakes Along the Hayward Fault
http://www.teachingboxes.org/earthquakes/resources/haywardFaultAnimations.jsp

Seismic Waves (a software program)
http://www.teachingboxes.org/catalog.jsp?id=TBOXR-000-000-000-109

Alan Jones' website
http://www.teachingboxes.org/catalog.jsp?id=SERC-NAGT-000-000-000-774