EXPLANATION of the online "Intensity Maps" activity

This page should help you familiarize yourself with the purpose and operation of the online section of this activity, involving the construction of an online intensity map. This online section uses frames and layers to allow the user to interactively create and modify the map. If your browser supports neither frames nor layers, you will not be able to use this section of the activity. (We apologize that there is no alternate page available.)

When you start the activity, you will see four frames created in your browser window. The image below shows you the main features of each frame, discussed in detail in the text that follows. Clicking on the name of a frame or feature will take you directly to a description of it. Clicking the arrow next to each text entry will bring you back to the imagemap below.

Title Frame

There is only one "active" part of the title frame, a button labelled Begin, Quit, and Done, explained below.

Begin/Quit/Done Button

This button changes during the course of the activity. At first, it is green, and labelled "Begin"; clicking it starts the actual activity (you should read the on-screen intro first). Once the activity is in progress, the button is red, and labelled "Quit"; clicking it then will take you back to the introduction page for this activity. Once you have completed the activity this button will also bring you back to the introduction page, but it will be labelled "Done" to let you know you've finished.

Text Frame

When you first load up the activity, this frame will contain a text explanation of the point of this exercise. Read it carefully, and then click the "Begin" button in the Title Frame. When you do, the Text Frame will display the first intensity report. It should contain three fields labelled Location, Modified Mercalli Intensity, and Description.


This field gives the name of the community in which this intensity report was made. Though most of these towns are not specifically labelled on the background map (in the Map Frame), a box-shaped cursor -- the current square -- will mark the whereabouts of each report's location for you.

Modified Mercalli Intensity

When the report first comes up, this field will contain a question mark, meaning that it has yet to be determined. Once you have determined the intensity at this location and made that choice with the keypad, this field will display a Roman numeral corresponding to that intensity. Clicking on the title of this field will pop up a new window containing the Modified Mercalli Intensity Scale of 1931 in its entirety.


This field will give you a brief verbal description of the types of effects that people at this location felt or noticed. You will have to assign Modified Mercalli intensity ratings based entirely upon these descriptions.

Map Frame

The Map Frame consists primarily of a background map, explained below. Also within this frame, below the map, are some controls. They are hidden at first, but are revealed as the activity progresses.

Background Map

The background map is actually made up of two layers: a transparent overlay (of towns, highways, and the ocean/coastline) and a grid of small gray squares, some 90 of which are "active" -- that is, they represent locations from which intensity reports were made. You will have the chance to "color in" these squares as you work on the activity.

Current Square

When a new entry is pulled up in the Text Frame, a square on the background map will "light up", displaying a white box, outlined in black, containing a red question mark. This square displays the location represented by the intensity report given in the Text Frame. By choosing an intensity using one of the intensity keys found on the keypad, you can assign a Modified Mercalli Intensity value for this location and "fill in" the current square with the appropriate color. This square won't officially become a filled square until you continue on to another entry using the NEXT action key. Until you do this, you can change the color of the square by clicking another one of the intensity keys, clear the location (change it back to the white, outlined box) using the CLR action key, or skip this location altogether by clicking the SKIP action key.

Filled Squares

Once you have confirmed your choice for the Modified Mercalli Intensity value at a location by clicking the NEXT action key, the current square will become a "locked" filled square, and a new current square will be chosen at random by the computer. The filled square can no longer be selected by the computer, and thus can never be revised.


Clicking on the box labelled "HELP" will open a new window containing this help page, should you need to review the features of this activity.

Epicenter Controls

After you have filled in your first 15 squares on the map, a pair of epicenter controls will appear in the space below the map. One control is a draggable set of crosshairs; the other is a button labelled "Confirm Epicenter". Instructions will appear next to these. They inform you to drag the crosshairs to a point on the map where you think the epicenter of this earthquake may be located, and then to click the button labelled "Confirm Epicenter".

You need not choose a location for the epicenter immediately. If you desire, you can continue to fill in squares until you feel more confident about your answer. But since you are not penalized for guessing incorrectly, you may wish to hazard a guess even if you're completely uncertain. When you have dragged the crosshairs into position and clicked "Confirm Epicenter", the computer will decide if your guess was close enough to be considered correct. If so, you have completed the activity. The identity of this earthquake will be revealed, as will a more detailed intensity map that you can blink-compare with your own. If your guess is too far off from the actual epicentral location to be correct, you will need to continue plotting intensities -- ten more before you are allowed to guess the epicentral location again.

Keypad Frame

This frame contains the controls you will need to create the intensity map, as well as a miniature guide to the Modified Mercalli Intensity Scale that will help you assign intensity values with speed and confidence.


The keypad is home to fifteen different keys, divided into two sets: twelve intensity keys and three action keys. Each of these fifteen keys lights up when in active mode, to let you know it is operational and ready to be clicked.

Intensity Keys

There are twelve intensity keys, labelled with Roman numerals to correspond to the twelve intenisty levels on the Modified Mercalli Intensity Scale of 1931. Each key is a different color, in a sort of rainbow scale with the red end of the spectrum indicating the most intense shaking and the blue end indicating weak shaking. White is used for intensity I, where no shaking is felt at all.

When you move the mouse pointer over an intensity key, two things happen: that key "lights up", and a brief description appears in an area above the keypad. This description is the Mercalli "cheat sheet". It condenses the Modified Mercalli rating down to a short paragraph highlighting the main details typically reported for each intensity level.

By clicking on an intensity key, you assign that intensity rating to the location of the current square. Your choice is not confirmed, however, until you click on the action key labelled NEXT. Until then, you may change the intensity rating as many times as you like using the intensity keys. You may also clear the rating or skip the location and have the computer select a new intensity report using one of the two other action keys.

Mercalli "Cheat Sheet"

When you move the mouse pointer over an intensity key, this area directly above the keypad will display a summary of the characteristic effects generally reported for the corresponding intensity value. This "cheat sheet" was designed to allow you to complete this activity without needing to refer to the full text of the Modified Mercalli Intensity Scale. You are still welcome to refer to the full scale, should you want to; just click on the words Modified Mercalli Intensity in the Text Frame to open the scale in a new browser window.

Action Keys

There are three action keys on the keypad below the intensity keys. They are labelled CLR, SKIP and NEXT.

CLR stands for "clear". When clicked, this button will remove any changes you've made to the current square, returning it to a blank state.

SKIP is pretty much self-explanatory. It skips the entry, and makes the computer bring up a new, randomly-chosen intensity report. The report you skipped may come up again in the future (in fact, it may be the very next entry the computer selects!). You may skip it again, if it does. You may skip any number of entries as often as you like, but they will not count toward the total number of reports for which you must assign values, and of course, skipped entries will do nothing to improve your map.

NEXT is the most important of the action keys. It only becomes functional once you've chosen an intensity value for the current square using one of the intensity keys. By clicking on NEXT, you confirm this assigned intensity, change the current square to a filled square, and tell the computer to select a new intensity report. You also add one to the counter that determines when you are allowed to guess the epicenter of the earthquake. The NEXT key is the only way to permanently fill in a square on the map.

If you've read through the instructions above, you should have a good idea of how to work the interactive intensity map. Click here to load this activity now! When you're finished, use the button labelled "Done!" to return to the introduction page and answer the wrap-up questions there.

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