Name: ________________________________ Class: _____________ Date: _________
Adapted from: http://www.data.scec.org/Module/s3act01.html
Goal:
In this exercise, you will be using a map of shaking intensity to determine the location of an earthquake epicenter. This is exactly how geologists learn about ancient earthquakes that occurred before we had seismometers.
Background;
Look at your map. You will see dozens of tiny numbers scattered across the state. Each of these represents an intensity rating on the Modified Mercalli Intensity Scale of 1931.
(Studying the Modified Mercalli Intensity Scale is not required for this exercise, but it will give you a better idea of what the numbers on the map really mean in terms of what a person would feel at that location.) The intensities are shown as Arabic numerals (1, 2, 3,
4, etc…) for clarity's sake; just convert them to Roman numerals (I, II, III, IV, etc…) to understand where they fit on the Modified Mercalli scale.
The intensity values were assigned to the specific locations on the map by researchers who collected reports from hundreds of individuals across the area. Each person's experience was translated into a numerical intensity rating using the Modified Mercalli
Intensity Scale. In places where multiple reports were gathered from the same location, the results were averaged to find the best overall intensity value for that site. Some of the towns from which reports were gathered are labeled on the map; their intensity ratings are shown in small boxes. Here is an example of an intensity report from a newspaper in
Saint Louis on December 28, 1811:
"The concussions of the Earthquake which commenced at two o'clock on Monday morning still continue. We have experienced five severe shocks which split two brick Houses and damaged five brick chimneys in this place."
( http://www.eas.slu.edu/Earthquake_Center/SEISMICITY/Nuttli.1973/nuttli-73-app.html
)
The object of this exercise is to create an isoseismal map from the intensity data by drawing isoseisms on the map. Isoseisms are contour lines that enclose areas of a particular intensity, as measured (in this case, at least) by the Modified Mercalli Intensity
Scale. You can also think of them as the boundaries between areas of "adjacent" intensity
(intensities that differ by only one level on the intensity scale used).
Here is an example of an isoseismal map for an earthquake in Canada.
( http://www.seismo.nrcan.gc.ca/significant_eq/charlevoix/1925/i mages/1925CharlevoixIsoseismal.jpg
)
DLESE Teaching Boxes | October 2005

Procedure
On the background map, then, draw curves that enclose areas of similar intensity, as given by the numerical values representing the reports of witnesses. Start with the highest intensity reported -- intensity VII -- and work down the scale. Intensities II and III can be grouped into the same category (because reports of intensity II are rare), and intensity I
(not felt, shown as "o"s on the map) does not need an enclosing isoseismal contour.
Leave the areas where the earthquake was not reported felt blank. If possible, shade in the enclosed isoseismal areas with different colors, and label them.
Some tips to keep in mind when drawing your isoseismal lines:
• There's no need to extend the isoseisms beyond the shoreline out into the ocean. You can do this, if you like, but you should dash these lines to indicate that they are speculative. The same applies to those lines that may extend to areas where no reports were gathered in other countries (such as Mexico) or where nobody lived at the time of the earthquake.
• Beware of "outlying" values. Sometimes special circumstances make for big changes in the way an earthquake is perceived at a given location. Certain values on the map may seem "out of place" -- a "5" surrounded by "3"s, for example. It's generally best to ignore such oddball values and draw your isoseisms to fit the overall trend. Don't worry about catching every tiny variation in the intensity distribution.
• Remember that outlying values can be too high or too low. People within the intensity IV zone of an earthquake can easily miss it entirely if they are moving about at the time, for instance.
Questions:
Once you have completed drawing in the isoseisms on your map, and marked the regions with their appropriate intensity designations, answer the questions below:
1.
Did the isoseisms you drew form neat elliptical or circular regions, or were the boundaries more complex curves, with convex and concave segments on the same isoseism?
2.
Based upon the shapes and patterns of your isoseisms, mark where you think the earthquake epicenter was for this earthquake. How sure are you of this position?
Be specific – use the map scale to answer the question, "How far away, in kilometers, could the epicenter be from your best guess?"
3.
Consider the following questions with regard to the irregularities and "outlying values" you saw on the isoseismal maps. a.
If someone, blindfolded and wearing earplugs, were sitting down on one end of a short, low, concrete wall, and you came along and sat down on the other end (about 6 feet away), do you think they would notice?
DLESE Teaching Boxes | October 2005

b.
Imagine the same scenario as above, but replace the concrete wall with a waterbed mattress, or even a regular spring-coil mattress. Do you think they'd be more likely notice than before? c.
Imagine the object (wall or mattress) represents the ground, the sitting, blindfold, and ear plugged person represents a building, and your act of sitting down represents an earthquake. Given your results from the thought-experiment above, which building would you expect to experience greater shaking: one built upon hard bedrock (e.g. granite), or one built upon soft alluvium (silt and gravel)?
4.
Compare the map you made to a topographic or physiographic map of the same area. Do you see any correlations between terrain (mountains, valleys, etc.) and the irregularities in your isoseisms, and if so, do these correlations fit your assessment from the analogy above (where mountains=bedrock and valleys=alluvium)?
DLESE Teaching Boxes | October 2005