# Unit C: Earth Systems

## How do scientists measure the strength of earthquakes?

### 1. Get Set to Explore

#### Vocabulary

• earthquake: A violent shaking of Earth's crust, caused by the release of built-up energy along a fault.
• focus: The point below Earth's surface where the faulting that causes an earthquake occurs.
• seismic wave: Wave of energy caused by an earthquake.

#### Building Background

• Make a KWL chart on the board and ask students what they already know about earthquakes; list facts they state in the Know (K) column of the chart. Then ask students what they want to learn about earthquakes; let individuals write their ideas in the What (W) column of the chart. Go over definitions of any of the vocabulary words that students use or ask about during discussion. Leave the chart on the board, so that the class can fill in the Learn (L) column at the end of the simulation.
• Suggest that students read information about earthquakes on Student Edition pages C54–C55. Also, review definitions of any vocabulary words that did not come up in the previous discussion. Have students add to the K and L columns of the KWL chart.
• Pose the Discover! question. Encourage students to refer to the KWL chart to formulate possible answers. Let volunteers share their ideas.

### 2. Guide the Exploration

• Tell students to launch the Discover! Simulation. Direct students to move their cursor over the seismograph, the Mercalli scale, and the epicenter to learn important information about each of these. Encourage them to take notes on what they learn.
• Explain that students need to use the Richter scale to select an earthquake magnitude. Then they click Quake and watch what happens. They click on the different icons to find out how the earthquake affected different areas on the map.
• Through the simulation, students should realize that the Richter scale goes from 0 to 9 and measures the strength, or magnitude, of an earthquake. They should realize that the Mercalli scale uses Roman numerals and goes from I to X+ and measures the intensity, or surface effects, of an earthquake. With both scales, the low numbers are used for weaker, less damaging earthquakes.

### 3. Review/Assess

• Review Step 3's Wrap-Up text. Encourage students to share any observations and ask any questions that they have. Clarify concepts, as necessary. Then let students describe what they learned and fill in the L column of the KWL chart as a summary.
• Ask the Extension question: What is the benefit of having two different scales to measure earthquakes? Prompt students to review what the two different scales measure and to think about why each is important. Guide students to understand that each earthquake has a specific strength, based on the energy the quake releases under the surface of Earth. The Richter scale gives a measure of earthquake strength, which is important information about what is going on below Earth's surface. However, at the surface, an earthquake will cause different degrees of damage in different places, depending on the landforms on Earth's surface and the type of construction that is there. The Mercalli scale gives an indication of how an earthquake affects Earth's surface and the people nearby.

If time permits, present students with the following questions:

• Critical Thinking Draw Conclusions Why do seismographs continue recording activity many hours after an earthquake has occurred? Answer: Earthquakes generate several different kinds of waves that travel at different speeds through Earth's crust. Some of these waves also reflect off Earth's core and bounce back toward the surface. All of these different waves and their reflections are recorded by seismographs after the initial earthquake.
• Inquiry Skill Interpret Data Imagine an earthquake registered 8.0 on the Richter scale. What kind of damage would you expect on the Mercalli scale? What would the damage depend on? Answer: Based on the simulation, for an earthquake this strong, students should expect damage of at least IX at or near the epicenter; damage generally decreases with distance from the epicenter.

### 4. Reaching All Learners

#### Challenge

Encourage interested students to research major earthquakes, especially recent major earthquakes. Each student should make a chart that indicates date, location, magnitude, and maximum intensity. Suggest that students use the Internet or other library resources to gather information. Challenge them to include at least ten earthquakes on their charts. Let them post their charts on a class bulletin board.