Mapping Earthquakes in Northern California

Table of Contents

  1. Mapping Earthquakes in Northern California
  2. Setting Up Your Workspace
  3. Downloading Data from Natural Earth
  4. Download Data from the Northern California Earthquake Data Center (NCEDC)
  5. Skill Drill: Download Data from the United States Census Bureau
  6. Adding California as a Basemap Layer
  7. Skill Drill: Adding the Humboldt County Boundary as a Basemap Layer
  8. Adding XY Data
  9. Changing the Map Projection of the Data Frame
  10. Representing Earthquake Magnitude Using Graduated Symbols
  11. Changing the Map Size and Position
  12. Creating an Inset Map
  13. Inserting A Map Title
  14. Inserting a Map Legend
  15. Inserting a North Arrow, Scale Bar, and Acknowledgments
  16. Exporting your map as a PDF file

Representing Earthquake Magnitude Using Graduated Symbols

At this point, your map displays the location of earthquake epicenters in California. You can also provide additional information to your map reader by also showing the relative magnitude of the earthquakes.

As you learned in a previous activity, one of the most powerful features of GIS software is the connection between a database and the map. Each shapefile comes with a database table. In ArcMap, you refer to the database table as an attribute table. In the Table of Contents, right-click on the earthquake layer and open the attribute table. The first time you open the attribute table, it will appear floating above the map. Click and drag your attribute table towards the bottom of ArcMap. As you drag, position your cursor over the blue arrow that appears near the bottom. When you place your cursor over the blue arrow, the attribute table will snap to the bottom of ArcMap.

Take a moment to read through the attribute table. What type of information about earthquakes are provided by the NCEDC? Some of the information, such as the date and time, will be easy to interpret. For other attributes, you may need to go back to the NCEDC website to learn more. In this step, you only need to understand the Magnitude attribute. If you will notice the field named Magnitude records a series of numbers. These numbers will determine the size of the point symbols on the map.

An image of the attribute table for earthquake data
Notice that the first row in the original CSV table became the field names. Click to view a larger sized image.

In the Table of Contents, right-click on the earthquake layer and select Properties. Navigate to the Symbology tab. On the left, choose Quantities, then Graduated symbols. Near the middle, use the drop-down menu to select Magnitude for the Value. Leave all other settings as default and click OK.

An image of the Symbology tab for earthquake data
The drop-down menu provides a list of all the number-based attributes. Click to view a larger sized image.

The earthquake data is now symbolized using different sized circles based on the magnitude value in the attribute table. Go ahead and close the attribute table.

You may also change the colors for the remaining layers as well. For this activity, I recommend choosing either a light color scheme or a dark color scheme for the basemap. Either one should have colors with low saturation, low value, or both. For example, a light color scheme might use light greys and pastels for the basemap features. A dark color scheme might use medium to dark greys instead. Your goal is to provide context without competing with your thematic data.

You will work from the bottom up, starting with the states and provinces layer. It will lay the foundation for the remainder of the map design. On the Table of Contents, click the colored rectangle under the states and provinces layer. When symbol selector opens, choose a something neutral for the Fill Color. Set the Outline Color and adjust the line weight.

An image of the earthquake data symbolized as graduated symbols
ArcMap chooses random colors. If you want to change your point symbol color, return to the Symbology tab. Click to view a larger sized image.

Repeat these steps for the remaining layers, one at a time. Try to maintain your chosen color scheme throughout the process. If necessary, use the Zoom In tool on the Layout toolbar to get a closer look at the details.