# 5.1 The Earth Moves

# Assignment Outcomes

- Describe how the energy from breaking rocks at an earthquake epicenter travels away in waves.
- Determine where earthquakes occur using the difference in speed of waves from the seismograph

# Explore: How do you measure an earthquake?

**Step 1**: Go to the ArcGIS Online map, The Earth Moves Under Our Feet, and explore the map.

**Step 2**: Click Modify Map on the upper right of the map and then click the button, Show Contents of Map (Content). The Global Seismographic Network is displayed on the map. Remember, you can always change the basemap if it helps you better analyze the information.

Seismographs measure earthquakes by an independent mass hanging from a spring and a writing device attached to a string. When the seismograph moves, the mass tends to stay still because of inertia, and the pen marks the disturbance on a moving plot of paper attached to the floor.

**Step 3**: Click several of the seismographic locations on the map, and then click More Info when the pop-up box opens. Read what several of the seismographs are stating.

**Step 4**: Scroll down to the tabs below the map on the More Info seismograph page for one seismograph you clicked on.

**Step 5**: Click the Heliplot tab.

- Looking at the heliplot drawing, describe what a typical earthquake looks like on this type of plot.

# Explain: Do all seismic waves travel the same?

Much like you can tell how far away lightning strikes are by counting the time between seeing the strike and hearing the thunder, the same between P and S waves tells how far you are away from an earthquake.

**Step 6**: Choose one earthquake on the heliplot. Estimate the time difference between the start of the P wave and the start of the S wave. For example, if the P-wave starts at 12 minutes and the S-wave starts at 18 minutes, then the time difference is 6 minutes.

- What is the time difference for the earthquake you chose?

**Step 7**: On the ArcGIS map, click on any red station, and then click on the P-S Wave Travel Curve graph to expand it. Use this graph to estimate the distance of the earthquake from this station. Find the time difference on the y-axis, go across to the red line, then go down to find the distance. For the example above, a time difference of 6 minutes would be a distance from the epicenter of 4200 km.

- What is the distance from the epicenter for the earthquake you chose?

# Elaborate: What is used to calculate earthquake distances?

**Step 8**: Click the Caribbean Quake bookmark to zoom to the three practice seismometers to determine where an earthquake occurs.

**Step 9**: Turn on the Finding Epicenter Tools layer under Content. Click each pin, and for each site, measure the time between P and S waves.

**Step 10**: Use this time to calculate the earthquake distance from each instrument with the formula in the Explain section.

**Step 11**: Use the Measuring tool to mark some landmark at the earthquake distance that is easily recognizable.

**Step 12**: Click the Edit button, and choose the Quake Distance From Seismograph circle to stretch a circle from your seismograph to the feature you just determined. The earthquake should have occurred somewhere on this circle.

**Step 13**: Draw circles around the remaining two seismographs, and find the location of the earthquake where the circles overlap.

# Evaluate: Are you ready to be an earthquake hunter?

**Step 14**: Now, you will repeat this process in another location with another earthquake. Search the seismograph link heliplots to determine an earthquakeâ€™s location from three different seismographs.

For ease, after you find a medium-sized or massive earthquake, look for sensors within a similar region whose P and S waves are less than 10 minutes apart.

- Write a summary of how you calculated and drew the circles to locate your earthquake. Include the location of the earthquake epicenter.