Determining the Intensity

Materials / Preparation

This activity can be done online or by printing out the activity and photocopying for students ahead of time.
Review the Seismology Exercises and scroll down to section two where the text begins: "On October 17, 1989……." Students will only be doing section two in this activity.
If you are including the extension, students will need copies of Where's My Epicenter and the two shake intensity maps with Modified Mercalli Intensity values: New Madrid Earthquake and Southern California (be sure to white-out epicenter)

Grouping

Full class and then groups of two to four students

Teacher tips

For your information, the website How Geologic Materials Affect Shaking Intensity goes over how geological materials affect shaking. Section two of the Seismological Exercises provides a geolgic map of the area around Oakland, California.
The key points needed for this activity are:

Holocene mud is unconsolidated material
Quaternary alluvium is sediment from rivers that is not well compacted
Franciscan Formation is the hardest material in this area.

We strongly recommend including the extension below, at least for high school students. It could be done as homework, following the second activity, but probably best as a classroom exercise. Before presenting the extension, preview the assignment and associated resources:

Procedures

Begin the class with a review of the previous activity. Project (as overhead or on computer) the comparison between the shaking intensities of the 1906 and 1989 earthquakes for the Bay Area, as shown on the Comparison of 1906 and 1989 earthquakes website.
NOTE: This site can be accessed directly from the Earthquake Hazards Student Web Page.

a. Show the 1989 Bay Area View ShakeMap (intensity map) and ask students where the highest intensity shaking occurred (Answer: near the earthquake epicenter, indicated by the star). Highlight the bulls-eye-like pattern where the strongest shaking occurs closest to the earthquake and it gets weaker as you move further away. Tell students that the Loma Prieta earthquake ruptured a patch of fault about 40 km long (us the scale bar on the map, about the distance from the star to the northern edge of San Jose) and show them how big this would be on the map.

b. Now, switch to the 1906 Bay Area View map . Point out that the black line running along the coast in the 1906 map shows the length of the fault that ruptured during that earthquake. That means shaking was generated all along that black line. For example, ask students which city is closer to the rupturing fault, Vallejo or Gilroy? (Answer: Gilroy is closest to the fault, even though it is further from the point where the earthquake started – its epicenter). Which had stronger shaking? (Answer: Gilroy, because it is closer to the fault). Now, ask students where the strongest shaking on the map appears to be (Answer: Santa Rosa). Is it closest to the fault? Why would it shake stronger?

c. If you have time, you can show students photos of damage from
Note that the Marin County location is located right next to the fault, but experienced less damage than Santa Rosa, which is relatively far from the fault. Ask students, "why?" Similarly, buildings near the San Francisco waterfront in 1989 near the Marina district (USGS Photo Archive: Collapsed Apartment Building) suffered similar damage to buildings in 1906 (Earthquake Engineering Online Archive: San Francisco Waterfront), despite the fact that the area was far from the earthquake in 1989 but close in 1906. Have students brainstorm why this area is "cursed."
Inform students that in the next activity they will learn why Santa Rosa was so strongly damaged in 1906 and why certain areas have stronger shaking than others, even though they are far from the fault. Inform them that at the end of the lesson, they will be writing a statement that tells what factors determine the amount of the shaking an area will experience.
Explain about the three different types of rock formation they will encounter in the next portion of the lesson: Holocene mud, Quaternary alluvium, and Franciscan Formation. See teacher tip.
Have students go to Seismological Exercises and scroll down to section two where the text begins: "On October 17, 1989……." Have students read the first two paragraphs.

a. Discuss with class what the rock formations are, i.e.:
- Holocene mud is unconsolidated material;
- Quaternary alluvium is not well-compacted sediments from rivers;
- Franciscan Formation is the hardest material in this area.
b. Additional photos of the Loma Prieta earthquake can be found by choosing Loma Prieta California Earthuake 1989 in the USGS Photo Library, Earthquakes section and on the USGS site: The October 17, 1989, Loma Prieta, California, Earthquake - Selected Photographs.
Now have students finish reading section two of the Seismological Exercises and have them study the seismic data and map. Have them interpret this data and write answers to the questions in the activity.
Note: At this point, get students away from the computers (power off or physically move away from computers).
Have students write a one-sentence summary of all the factors that affect shaking intensity during an earthquake. Write the following question on the board to focus the students, "What influences the amount of shaking a location experiences during an earthquake?" A simple correct answer would be: "The intensity of shaking I would feel in an earthquake depends on the magnitude of the earthquake, the distance I am from the fault, and the type of the rock or soil beneath my feet."
Have them share their summary statements. Have students post their statements on large paper. As a class look at the statements. Are they similar? If not, why not? Students should discover that the intensity is determined by:
- Magnitude (amount of energy released), the higher the magnitude the greater the intensity
- Distance, the closer the location is to the epicenter the greater the intensity
- Rock Type, loose soils (rocks) experience stronger shaking,

Teacher Note: We strongly recommend including the extension below, at least for high school students. It could be done as homework, but probably best as a classroom exercise.

Extensions

We recommend a slight modification to the Measuring Earthquakes activity in order to make the assignment more exciting. Have the students determine the epicenter of an earthquake purely from intensity data. This is how scientists determine the magnitude and location of past earthquakes. An excellent example is the 1811-1812 New Madrid Earthquake sequence (which was compiled primarily based on newspaper accounts such as the excerpts shown here: Contemporary Newspaper Accounts of Mississippi Valley Earthquakes of 1811-1812).

To adapt the activity from Measuring Earthquakes:

Print out a copy of the Where's my Epicenter activity for each student. Have students follow the directions and answer the questions.
Print out and copy shake intensity maps with Modified Mercalli Intensity values for each student.
- New Madrid Earthquake (New Madrid Earthquake)
- Southern California (Carefully white out the black star this map before photocopying )
  
  Examples of Isoseismal Maps to show students as examples of what their end-product should look like:
  - Example 1: Isoseismal Map: 1925 Charlevoix-Kamouraska Earthquake
  - Example 2: Isoseismal Map: 1988 Tennant Creek Earthquake
  - Example 3: Isoseismal Map: 1997 Punta Gorda Earthquake
Print out copies of physiographic maps for question 4 of the activity.
- Physiographic map of Continental United States Use this map for New Madrid Earthquake (Print in Landscape orientation).
- Physiographic map of Southern California
Close with class discussion.
- Refer to the answer sheet for the Where's My Epicenter questions.
- Completed Isoseismal Map for New Madrid Earthquake . This image is part of the New Madrid Seismic Zone website.
- Completed Isoseismal Map for Southern California

Further extension

Students can explore the shaking intensity of past earthquakes online. Classes and individual students can also fill out a report (Did You Feel It?) when they feel an earthquake in their area.