Section 3: Measuring Earthquakes


Concept: Isoseismal (intensity) maps show irregularities in the distribution of shaking caused by an earthquake. However, it is still possible to crudely locate the epicenter of an earthquake using a well-constructed intensity map.

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This activity will allow you to work through two exercises that will demonstrate the construction of intensity maps, familiarize you with the basic Modified Mercalli Intensity Scale, and give you the opportunity to build your own map online for the purpose of locating the epicenter of an earthquake. Follow the directions below for each of the two exercises in this activity. If hardware or software limitations prevent you from using one or the other, then work through the one you can, and skip the other exercise.

Exercise 1Finding Isoseisms

In this exercise, you will be using a map of southern California upon which are plotted intensities reported in the wake of the magnitude 5.6 North Palm Springs earthquake of July 8, 1986. If you haven't yet printed out a copy of this background map (or had one printed for you), do so now. The paragraphs below will then explain the fundamentals of this exercise to you.

Look at your background map of southern California. You will see dozens of tiny numbers and "o"s scattered across the state, and into adjacent areas of Nevada and Arizona. 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 for clarity's sake; just convert them to Roman numerals 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 labelled on the map; their intensity ratings are shown in small boxes.

Also shown on the map -- as a star, north of Palm Springs -- is the epicenter of the earthquake, as located by seismologists. This is the point on the Earth's surface directly above the underground source (hypocenter) of the earthquake. This won't really figure into the exercise until after you've completed your map and are considering the results.

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).

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:

Once you have completed drawing in the isoseisms on your map, and marked the regions with their appropriate intensity designations, consider 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. Consider the relation of the epicenter to the isoseisms on your map.
    1. Assuming the isoseisms had plotted as neat, circular regions, where would you have expected the epicenter of the earthquake to be located?

    2. On your map, where was the epicenter located, relative to the isoseisms you drew? Can you think reasons why this might be the case?

    3. The fault that produced this earthquake dips at about 45 degrees toward the north with the hypocenter at a depth of about 10 km, though the rupture broke nearly to the surface. Might this information be relevant to your findings? How?

  3. Compare your map to this example of a completed isoseismal map for the same data set.
    1. Does your map compare favorably?

    2. Given the nature of the intensity report distribution, does your map need to be exactly like the example to be valid? (i.e. Is there a definitive, "correct" way to draw this map?)

  4. Consider the following questions with regard to the irregularities and "outlying values" you saw on the isoseismal maps.
    1. 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?

    2. 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?

    3. Imagine the object (wall or mattress) represents the ground, the sitting, blindfold, and earplugged 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 southern California. 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)?

Exercise 2Locating an Epicenter by Creating an Intensity Map

In the first exercise, you drew isoseismal lines on an intensity map created from reports filed by people across southern California in the wake of the magnitude 5.6 North Palm Springs earthquake of July 8, 1986. The report intensity values were already assigned and matched with their locations, and the epicenter of the earthquake was already marked on your map. This time, you'll have to do both of these things yourself.

The premise for this exercise is as follows: an earthquake has occurred somewhere in southern California, and you, as the earthquake investigator, have the task of finding the epicenter of that earthquake. Your only clues are descriptions of the effects produced by the shaking at various locations all over southern California.

Fortunately, a special interactive page of dynamic HTML will do the intensity plotting for you, and the locations (with their matching intensity reports) are precompiled. You simply need to work through the program, assigning intensities based upon the reports, and then guess the location of the earthquake's epicenter based upon the intensity map you are creating. You won't need to assign an intensity value for every report the program offers; you need only assign values for 15 reports before you are allowed to guess the epicenter. If you fail to guess correctly, you will need to complete 10 more before you get the chance to guess again. There are 90 different locations available to assess, so you should have plenty of opportunities to guess the epicentral location correctly.

To guess the location of the epicenter, try and visualize a pattern of concentric intensity zones as you build your map. If you can get a small sample of values that span across a large area you may be able to see this pattern develop more quickly than if you concentrate on one area alone. Since the report locations are chosen at random by the computer, you should be given a good selection of locations from across southern California.

Once you know roughly where the area of greatest intensity (the center of the concentric-ring pattern) is located, try skipping past outlying locations and narrowing your focus to the epicentral region. The epicenter itself should be within or very near the area of maximum intensity on the map. A set of crosshairs will be provided for you to drag onto the map to the spot you think represents the epicenter. The computer will automatically evaluate your choice.

That covers the basics of the activity. To learn more about exactly how this online exercise works, read the explanation page before you start. Then try it for yourself, and see how well you do! When you're finished, return here and answer the questions below.

  1. How many attempts to locate the epicenter did you need to guess correctly, and to roughly how many reports did you assign intensity values in arriving at your final decision?

  2. Did all the reports fit neatly into one intensity category or another, or were some unclear, with characteristics of two or more different intensity ratings?

  3. How well did your filled squares match the more detailed intensity map given at the end of the exercise?

  4. Compare the detailed intensity map to a topographic or physiographic map of southern California, as in the question at the end of Exercise 1. Do you see correlations that match the analogy given in question 4 of Exercise 1?

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