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Today we’ll introduce earthquake Richter magnitude. I believe almost everybody

has heard of the Richter scale but only a few people know what it means.

Part 3 - How can I map the magnitude of the earthquakes? 

Color Classification of a GIS Map

One of the most powerful tools you have for mapping

your data is the ability to use colors to represent the

data according to your classification. This requires

careful thought on your part as you make choices in

how to display the data in order for your map to be

useful to other people.  

                      For this example, we will color code the earthquake

                      data you added in the previous exercise.

Earthquakes magnitude:

Earthquakes magnitude is primarily measured on the Richter scale.  It was

invented by Charles F. Richter in 1934.  The Richter magnitude is calculated

from the amplitude (or height) of the largest seismic wave released by an

earthquake, no matter what type of wave was the strongest.

Table of Richter Magnitude Scale

Source:  http://www.em.gov.bc.ca/Mining/Geolsurv/Surficial/quake/eq2.htm 

1.     First, right click on the “Quak2000.txt” layer and choose Properties from

    the drop down menu.

Choosing Layer Properties

2.    Choose the Symbols tab.

3.    Select Graduated Symbols from the Draw features using option menu and

     Mag (Magnitude) from the Field option menu.

4.    When you are finished, click .

Classifying the Layer Data

You will choose to use graduated colors based on the magnitude of each individual

earthquake.  Lighter colors will show earthquakes with a lower magnitude while

darker colors will show earthquakes with a higher magnitude.  This way, instead of

just a bunch of dots with the same color, you will also be able to tell how strong

the earthquakes are on your GIS map.

Near the middle part of the dialog box, notice the Color choice. The color let you

select the shades of color used for graduated symbols. In many situations, a

particular color scheme is appropriate. For example, blue would be a good choice

to show cold weather on weather maps while green might be useful for vegetation

maps.

Try a different color by clicking on the Start and End option menus to choose

your own preferred colors.  Be sure to click  to see the changes.  When you

are satisfied with the color, close the Properties dialog box by clicking on the

 button.

This map now shows the location of earthquakes in 2000, with the darker colors

representing the quakes with greater magnitudes:

GIS Map Showing Location, Number, and Color-Classified Magnitude of Earthquakes

Notice that the lighter colors (representing lower-magnitude quakes) tend to be

clustered around the western part of the United States and in Europe. Much of

the science of seismology and plate tectonics involves noticing and investigating

spatial patterns like this.  Your GIS map made these observations possible.  See

how powerful of a tool GIS can be!  

Now, we’ll use the ArcExplorer to investigate the locations of the “smaller” quakes.

1.     First, we need to “thin out” the quake data set to show only the smaller

    quakes.  We are sort of trying to ignore the rest of them.

      Note: To show only part of a layer, first make sure it is the “active layer”    

        (the “active layer” appears “raised up” in the list of layers on the left of  

        your ArcExplorer window).   Make sure the “quak2000” layer is “active” by   

        clicking on the layer name once.

2.    Select the Query Builder   tool from the Tool Bar.

         We will use the Query Builder to display only those quakes with a magnitude      

         of between 3.5 and 4.2.  That is because in the Richter Magnitude Scale in

         the table above, magnitudes of 3.5-4.2 represent the typical intensity at        

         an epicenter II – III.

3.    In the Query Builder dialog box, double click on the field [Mag].

4.    Click once on the Greater Than Or Equal To button .

5.    Type in the number 3.5.

6.    Click  in the Query Builder.

7.    Double click on the field [Mag].

8.    Click once on the Less Than Or Equal To button .

9.    Type in the number 4.2.

10.  Click  to execute your selection.

Query Builder Used to Sub-Select Data 

Your map now shows only the locations of recorded earthquakes with a magnitude

of between 3.5 and 4.2.  All of the other data are ignored for now and not selected.

GIS Map Showing Results of a Sub-Selection Query 

It might be interesting to zoom in on an area using the Zoom In  or Zoom Out  tools.

Go ahead and try. 

Remember, you can return to the “Full Extent” of the entire world map by

clicking on the Zoom to Full Extent button  .  This allows you to see

everything on the entire map after you have zoomed in or out. 

Do you recognize the area shown in the following map?

Zoomed in Display of GIS Map Showing Individual Earthquakes 

Woops!  Looks like we ran out of time again.  I need to say goodbye here.

Next time, if you watch TV or listen to the radio with you family or friends and

hear something about earthquakes, you can explain earthquake magnitude and

the Richter scale to them.  They will probably be so impressed after hearing

your explanation that they will call you Dr. Earthquake.  

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