Guide: Mapping Earthquake Intensity with ArcGIS Desktop Using IDW and Spline Methods

In this guide, you’ll learn how to create an earthquake intensity map based on data from a magnitude 9.0 earthquake off Japan’s coast on March 11, 2011. Using ArcGIS Desktop 10.7 and the Spatial Analyst extension, you’ll apply Inverse Distance Weighted (IDW) and Spline interpolation techniques, tweaking settings to improve the surface representation.

Time Required: Approximately 35 minutes
Requirements:

• ArcGIS Desktop 10.7 (any license level: Basic, Standard, or Advanced)
• Spatial Analyst extension

Step 1: Obtain the Data

1. If you haven’t already, download the exercise dataset.
2. Store it in a folder such as C:\Training\GISData for easy access.

Step 2: Load the Map File

1. Open ArcMap.
2. From the Existing Maps section, select Browse For More.
3. Go to C:\Training\GISData and open the file named EarthquakeMap.mxd.
   • You’ll see a map of Japan, the earthquake’s epicenter from March 11, 2011, and dots marking seismic stations with intensity readings.

Step 3: Configure Analysis Settings

1. Click Geoprocessing in the menu and choose Environments.
2. In the settings window, adjust:
   • Output Coordinates:
     • Select Same As Display for the coordinate system.
   • Raster Analysis:
     • Choose As Specified Below for cell size and input 5000.
     • Set the mask to Boundary.
3. Hit OK to apply these settings.

Step 4: Generate an IDW Surface (Power Setting = 2)

1. Activate the Spatial Analyst extension if it’s not already on:
   • Go to Customize > Extensions, check Spatial Analyst, and close the dialog.
2. Locate and launch the IDW (Spatial Analyst) tool via the search bar.
3. Enter these settings:
   • Input Points: SeismicStations
   • Value Field: intensity
   • Cell Size: 5000
   • Output Location: C:\Training\GISData\Output\IntensityIDW.gdb\Surface1
   • Power: 2
   • Search Radius: Variable
4. Click OK to process.
5. Hide the Boundary and SeismicStations layers in the map.
6. Right-click the new Surface1 layer, select Properties:
   • Under General, rename it to IDW Power 2.
   • Under Symbology, set:
     • Renderer: Stretched
     • Color Scheme: Bright Spectrum
     • Check Invert for the stretch.
   • Click OK.

Insight:

• A power of 2 gives nearby points a lot of weight, leading to a surface with more abrupt changes.

Step 5: Apply IDW with a Lower Power (Power = 0.5)

1. Open the IDW (Spatial Analyst) tool again.
2. Use these parameters:
   • Input Points: SeismicStations
   • Value Field: intensity
   • Cell Size: 5000
   • Power: 0.5
   • Search Radius: Variable
3. Run the tool by clicking OK.
4. Rename the output to IDW Power 0.5 and style it like Step 4.

Insight:

• Dropping the power to 0.5 lessens the impact of nearby points, smoothing out the surface.

Step 6: Limit the Search Radius in IDW

1. Start the IDW (Spatial Analyst) tool once more.
2. Set the following:
   • Input Points: SeismicStations
   • Value Field: intensity
   • Output Location: C:\Training\GISData\Output\IntensityIDW.gdb\Surface3
   • Cell Size: 5000
   • Power: 0.5
   • Search Radius: Variable
   • Points to Use: 5
   • Max Distance: 50000 (50 kilometers)
3. Click OK to generate the surface.
4. Rename it to IDW 50k and apply the same styling as Step 4.

Insight:

• Limiting the radius to 50 km and using only 5 points focuses the interpolation on local data, improving detail.

Step 7: Create a Spline Surface with Default Options

1. Find and open the Spline (Spatial Analyst) tool.
2. Input these values:
   • Input Points: SeismicStations
   • Value Field: intensity
   • Cell Size: 5000
   • Keep default settings (Regularized, Weight = 0.1).
3. Click OK to run.
4. Name the result Spline Default 0.1 and format it like Step 4.

Insight:

• Spline can predict values outside the recorded range, producing a fluid, flowing surface.

Step 8: Adjust Spline with Tension Settings

1. Launch the Spline (Spatial Analyst) tool again.
2. Configure it as follows:
   • Input Points: SeismicStations
   • Value Field: intensity
   • Output Location: C:\Training\GISData\Output\IntensityIDW.gdb\SplineTen
   • Cell Size: 5000
   • Spline Type: Tension
   • Weight: 1
3. Click OK.
4. Label the output Spline Tension 1 and style it as in Step 4.

Insight:

• Tension settings reduce exaggerated peaks and dips, making the surface flatter near data points.

Step 9 (Optional): Fix Out-of-Range Values

1. Open the Raster Calculator tool from the search bar.
2. Type this expression: Con("Spline Tension 1" < 1, 1, "Spline Tension 1")

• This changes any value below 1 to 1, preserving higher values.

3. Save the output to C:\Training\GISData\Output\IntensityIDW.gdb\AdjustedIntensity.
4. Click OK and style the result like Step 4.

Insight:

• Spline might generate values below the minimum (1), so this step ensures accuracy.

Wrap-Up

1. If you skip Step 9, save your map with a unique name in C:\Training\GISData\Output.
2. Close ArcMap.

Points to Ponder

• What does earthquake intensity mean?
• It’s a gauge of how much shaking occurs at a spot due to an earthquake.
• How do IDW and Spline results differ from the station data range?
• IDW sticks to the recorded limits; Spline can go beyond them for prediction.
• What’s the effect of a high power setting (e.g., 2) in IDW?
• It sharpens the surface near data points.
• How else can you smooth an IDW surface?
• Widen the search area or include more points.