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.
There is only one "active" part of the title frame,
a button labelled Begin, Quit, and Done,
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.
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
Modified Mercalli Intensity, and
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.
The Map Frame consists primarily of a background map,
explained below. Also within this frame, below the map, are some
They are hidden at first, but are revealed as the activity progresses.
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.
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
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
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.
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.
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.
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.
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.
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
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.
Return to the first
page of Activity #1