Part 3: Analyze Data with a GIS

Step 1 –
Map Earthquake Data Using Latitude and Longitude Coordinates

Small add event theme box

Choose View>Add Event Theme... to map the individual earthquakes of each table (i.e. Sig_Big_eq.txt and Last_year_eq.txt) by latitude and longitude coordinates.






  1. Choose View>Add Event Theme... to map the individual earthquakes of last year.
    Mapping last year earthquakes by latitude and longitude
  2. Select last year's table to plot the data using latitude and longitude coordinates. Then click OK.
    Add last year earthquakes using latlong coordinates
  3. Choose View>Add Event Theme... to map the significant and big earthquakes.
    Adding data using latitude and longitude coordinates
  4. Select the Sig_Big_eq.txt table to plot the data using latitude and longitude coordinates. Then click OK.
    Plotting signiifcant and big earthquakes through histroy

Step 2 –
Turn Themes On and Off, Adjust Their Colors, and Rearrange Their Positions to Compare Earthquake Distributions

  1. When ArcVoyager maps a theme, the program randomly selects a display color. Check to see that the two new point themes (Last_year_eq.txt and Sig_Big_eq.txt) are displayed in contrasting colors. If necessary, change the color of these themes by making the theme active and choosing Theme>Edit Legend.... When the Legend Editor opens, double click the symbol to edit it. Click on the paint brush to access the Color Palette. Once you have selected a new color, be sure to click Apply in the Legend Editor dialog box.
  2. As you look at your map, notice how an active theme has a raised border around it. When you activate a theme, you instruct ArcVoyager GIS to pay attention to it. Understanding the difference between a theme that is turned on and one that is active can save you time and help prevent frustration. A theme may be on or off as well as active or not. Themes that are on have a checked box while themes that are active appear raised above the others. More than one theme can be on and more than one theme can be active.
    To change the way a theme's symbol is displayed, first make it active. The theme should be raised above the others in the Table of Contents. Then choose Theme>Edit Legend...
    In the screen shot below, four themes are on and two are off. One theme is active and that theme is not currently on.

    Edit the legend of a theme

    When the Legend Editor opens, double click the symbol to edit it. Click on the paint brush to access the Color Palette.
    Open the Legend Editor to edit the symbol

    Once you have selected a new color, be sure to click Apply in the Legend Editor dialog box.
    Click the paint brush to change the color
  3. Turn on the theme containing last year's earthquakes. This theme maps the locations of earthquakes with a magnitude of 4.0 or greater. Click the Open Theme Table Button that opens the table of a theme button to find out how many earthquakes occurred in the past year.
  4. Turn on the theme containing last year's earthquakes. Click the Open Theme Table button Button that opens the table of a theme .
    Last year's earthquake theme turned on

    Look beneath the table buttons to find out how many earthquakes occurred in the past year. (If you chose a different year, your number may vary from the circled one below.)
    Open the table of a theme


    - What do you notice about the distribution of these earthquakes?
    Is the distibution of earthquakes random or do you see any patterns?
    The distribution of earthquakes coincides with the boundaries between tectonic plates.
  5. Turn off last year's earthquakes and turn on the Sig_Big_eq.txt theme. This theme maps "big" earthquakes ranging from the earliest known to the most recent.

  6. Big earthquake theme turned on


    - How does the distribution of these earthquakes compare to the others?
    Take a look at the Mid-Atlantic Ridge.
    Big earthquakes tend to be concentrated in places where tectonic plates are colliding, rather than pulling apart. Concentrations of big earthquakes surround the Pacific Ocean in a pattern often described as the ring of fire.
  7. Turn these themes on and off to get a sense of how they differ.

  8. Two earthquake themes turned on


  9. Click and drag the Plate boundaries theme (Bounds.shp), sliding it above the earthquake themes. Take a screen shot to record the patterns you see.

  10. Plate boundaries moved up in the Table of Contents


    - What type of plate boundary is associated with earthquakes with a magnitude of 7.0 or greater?
    The plate boundaries are where the plates are coming together (magenta: convergent boundaries), spreading apart (green: divergent boundaries) or moving horizontally past one another (blue: transform boundaries). The yellow represents an unknown boundary type.
    Big earthquakes occur most frequently at convergent plate boundaries.
  11. Before continuing onto the next step, make sure you have turned off last year's earthquakes along with the plate boundaries theme.

Step 3 –
Spatially Query the "Big" Earthquakes to Characterize Their Distribution

As you discovered, most "big" earthquakes are concentrated at convergent plate boundaries. It is possible to characterize a risk zone using a GIS. For example, a spatial query allows us to find out how many magnitude 7.0 or greater earthquakes have occurred within a specified distance of a plate boundary.

  1. First, set the scale to miles by choosing View >Properties... and selecting decimal degrees as the Map Units and miles as the Distance Units. Click OK.
  2. Choose View >Properties...
    Changing the scale of the view with view properties

    Set the Map Units to decimal degrees and the Distance Units to miles. Click OK.
    Setting the distance and map units


  3. Then, select the convergent plate boundaries by querying the plate boundaries theme. Click the Bounds.shp theme once to make it active, but making sure you do not turn the theme on by clicking it's checkbox. Then click the Query Builder Query button to build queries button. Type the following expression into the input box:( [Margin_typ] = "Convergent" ) and click New Set.

  4. Query the plate boundaries theme to show convergent plate boundaries


  5. Next, turn on the significant earthquake (Sig_Big_eq.txt) theme and make it active by clicking once on its title. Perform a spatial query by choosing Theme>Select by Theme....

  6. Theme_Select_By_Theme menu shown


  7. Enter the following information into the menus: Select features of active themes that Are Within a Distance Of the selected features of Bounds.shp. Enter 100 miles as the selection distance.

  8. Select_By_Theme dialog box for spatial query


  9. Observe the earthquakes on the map that are highlighted in yellow.
  10. Click the Open Theme Table Button that opens the table of a theme button to find out how many earthquakes were selected.
  11. - What percentage of big earthquakes occur within 100 miles of a convergent plate boundary? Within 200 miles?
    Divide the number of earthquakes selected by the total number of earthquakes. Repeat the query to figure the percentage of big earthquakes that are 200 miles from a plate boundary.
    567 of 1208, which equals 47% of big earthquakes occur within 100 miles of a convergent plate boundary.
  12. Make both the plate boundaries and the significant earthquakes theme active, by holding down the shift key and clicking on their names. Then, click the Clear Selected Features Button that clears the selected features of a theme button to remove all selections before continuing onto the next step.

Step 4 –
Query Earthquake Data and Zoom In and Out to Look for Patterns

  1. Click once on the title of the Sig_Big_eq.txt theme to make it active. Choose Edit>Copy Themes..., Edit>Paste to duplicate this theme.

  2. Copy and paste themes in a view


  3. Change the color and or shape of the symbols for both themes so that they contrast.
  4. Make the top significant and big earthquake theme active and query it to display only the earthquakes that have taken place since 1900. Choose Theme>Properties.... Then, click the Query Builder Query button to build queries button and type the following expression into the input box: ( [Year] >= 1900 ). Click OK.
  5. Choose Theme>Properties...
    Theme properties options

    Click the Query Builder Query button to build queries button and type the following expression into the input box: ( [Year] >= 1900 ). Click OK.
    Query that defines the theme


  6. Use the Zoom In Zoom In button zooms in for a closer look tool and the Zoom Out Button zooms out tool to inspect areas where big earthquakes occurred in the past, but where they have not occuured in the last 100 years. Look for areas where a big earthquake may be "overdue." These areas will show up on the map as places where the earthquakes in the last 100 years do not cover up the remaining earthquakes. Look for places where the color of your bottom significant and big earthquake theme is visible.
  7. To find out the names of countries as you zoom in, make the countries theme active and then click on a country with the Label labels features of themes tool.

  8. Zoom in on Cuba region of big earthquakes


  9. Continue to search for other "overdue" areas by sumbiting another query showing earthquakes that have happened in the last 75 or 50 years.

Step 5 –
Label Earthquakes by Date to Analyze Frequencies

One of the ways that Earth scientists try to determine the likelihood of an earthquake striking in the future is by estimating their past frequency of occurence. For example, if there have been four magnitude 7.0 or greater earthquakes in an area in a 200 year period, then this frequency is one in 50 years. Consider how long it has been since the last big Earthquake struck any specific area, as you try to decide where you might expect the next big Earthquake to occur.

  1. Make the bottom significant and big earthquake theme active set the label field to year. Choose Theme>Properties... and select Text Labels. Set the Label Field to Year. Click OK.

  2. Specifying the label field in theme properties


  3. Use the Zoom In Zoom In button zooms in for a closer look tool to return to an area you investigated in Step 4. Click on individual earthquakes with the Label labels features of themes tool to label all the earthquakes in an area by year.

  4. Zoom to area of earthquake activity


  5. Select all the earthquakes in the area by clicking and dragging over them with the Select Features tool. Click the Open Theme Table Button that opens the table of a theme button to view the records of the earthquakes in the theme. Click once on the Promote button to move the selected records to the top of the list.
  6. Select all the earthquakes in the area by clicking and dragging over them with the Select Features tool.
    Select earthquakes using the Select Features tool

    Selected earthquakes appear highlighted in yellow. All the earthquakes in this view have been selected. However, the earthquakes from the last 100 years, shown with red triangles, do not appear yellow, only because that theme is covering up the lower theme.
    Earthquakes highlighted in yellow using the Select Features tool

    Click the Open Theme Table Button that opens the table of a theme button to view the records of the earthquakes in the theme.
    Open the theme table of the significant earthquakes theme

    Click once on the Promote button to move the selected records to the top of the list.
    Promote selected records to the top of the theme table list


  7. Closely examine the attribute data. Calculate frequencies of occurrence for big earthquakes in various areas on Earth by determining a time interval for earthquake events and counting the number of earthquakes occurring during that interval.
  8. The earliest earthquake within the selected records occurred in 1691 and the most recent, in 1946. By subtracting these dates, you get an interval of 255 years. When counting the number of earthquake events, do not include earthquakes that took place within from one to three years of each other. Consider them to be part of the same earthquake event. So, for this data there were six earthquake events during the 255 year interval. These were: 1691, 1787, 1842, 1867, 1916-1918, and 1943-1946. 255 divided by 6 gives a frequency of 1 in every 42.5 years. Since the last "big" earthquake in the area took place in 1946, another one would have been due by 1988. You could consider this area to be "overdue" for a big quake.
    Earthquake records for the Dominican Republic


  9. You may want to turn on last year's earthquake theme to check for recent activity or return to Part 1 to get additional data from the USGS Earthquake Seach site to overlay and compare.
  10. Take screen shots of any maps you make.

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