GEOG 140 Isoseimal Mapping Lab: Dr. Rodrigue

Geography 140

Introduction to Physical Geography

Lab 9: Isoseismal Mapping

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There are two different approaches to measuring an earthquake's severity: magnitude and intensity. Magnitude measures (including the Richter scale) express the energy released in an earthquake; intensity measures (the Modified Mercalli Intensity scale) express an earthquake's severity in terms of more subjective overall impacts on human beings, their assets, and the physical environment.

Way back in one of the mapping lectures, I introduced you to the concept of isolines as a value symbol used in mapping. Isolines are any line on a map connecting places having the same value of "something or other" (e.g., parallels and meridians, contour lines, isotherms, isohyets, "isodobsons"). In this lab, I'd like you to construct your own isoline map, one based on the Modified Mericalli Intensity Scale. The isoline involved is called an "isoseism" and the map you'll make is called an "isoseismal map." An isoseism encloses all places having the same level of intensity of shaking and damage, as reported by people who fill in a survey for the USGS or other earth science agency about their experiences. If you ever feel an earthquake (or learn about one nearby that you didn't feel), you can help the USGS collect intensity data on it by visiting the Did You Feel It? website and filling in the form about your experiences and giving your location at the time of the event, so they can map your intensity report.

For this lab, what you'll do is print out a copy of a map showing earthquake intensity reports in the wake of the magnitude 5.6 earthquake in North Palm Springs back on the 8th of July of 1986. Then, you'll examine the pattern of earthquake intensity reports by location and pencil in isoseisms enclosing all areas reporting a 7, a 6, a 5, a 4, a 3, and not felt (a o). If you were doing the 5-4 isoseism, for example, you would want to draw it so the 5s were inside your isoseism and the 4s were outside. As ever, it's a good idea to read through this lab first to get your bearings and then start in on the process of collecting your data, manipulating them on a map, analyzing your results, and writing them up. All you need to do this lab is a pencil and, if you're ambitious, colored pencils.

Lab 9a: Download Your Data

Click on this link to view and print a map put out by the Southern California Earthquake Center. I didn't want to just put it here because your printer might not print it all on the same page if I did.

Lab 9b: Pencil in Your Isoseisms

Now, have a look at the patterns of intensity reports (it might help to review the Modified Mercalli Intensity scale by clicking here). Start lightly penciling in isoseismic zones by enclosing all the areas around the epicenter (the star) with reports of 7. Then, try the area separating the 6s from the 5s. You will start to notice "problem children" in here: anomalous readings within a zone, such as some 4s and 6s well within an area dominated by 5s. You are trying for pretty smooth isoseismic lines, so don't feel obligated to zig your lines absurdly to cover one of these anomalies.

When you think about it, it makes sense that you'll have anomalous readings. The people calling in to report their experiences might have been in terrain where the seismic waves travel fast with lower amplitudes and, so, do less damage. Also, this is a subjective scale, dependent on people honestly and thoroughly reporting the kinds of things this scale is based on. Also, your experience of an earthquake depends (at lower intensities) on whether you are moving around or lying/sitting still or whether you're on the upper floors of a building or on the ground floor.

Anomalous readings also have a lot to do with the kind of rock and soil underneath the people calling in. Seismic waves speed up and their amplitude diminishes in solid rock that is dense and well-consolidated, so a caller might be fairly close in to the epicenter and not have so dramatic a story to tell. Seismic waves slow down, forcing their amplitude to increase, in loose, unconsolidated soil or rock materials, such as the marine and alluvial fill on valley floors. Someone calling in from such a location may be relatively far from the epicenter and yet still experience a great deal of shaking and damage.

So, you want to draw smooth isoseisms that express the general trend, not every anomalous reading. In other words, you want to draw in smooth curves, such that nearly all of the 6s are inside the line, and most of the 5s are outside the line. You are basically delineating general regions that experienced different levels of intensity.

Lab 9c: Completing the Map

Look over your map carefully at this point. Your isoseisms should form sort of a bulls-eye pattern (pretty wavy bulls-eye, to be sure), with the circle in the middle enclosing the area dominated by 7s and each band beyond that enclosing areas with progressively smaller and smaller intensity levels. None of your isoseisms should touch one another or, heavens forbid, cross one another. Make sure you see the bulls-eye pattern and make sure there are no crossing or touching isoseisms.

When you're happy with your penciled-in isoseisms, go back over them, neatly, in darker pencil or ink (committing yourself). Now, label each of the zones, but don't use the Arabic numbers: Use Roman numerals. Roman numerals are used to make sure people don't confuse an intensity reading with a magnitude reading, which uses Arabic numerals. So, 7=VII, 6=VI, 5=V, 4=IV, and 3=III. You might want to color the bands in, with a consistent color scheme that ramps up from light to dark or vice-versa or white to some dark color or maybe from some cool color to a hot color, so that your eye is immediately drawn to the middle of the map where the intensity is highest.

Lab 9d: Analysis of Your Map

Look closely at your isoseisms. You'll find that they sure as heck don't make perfect circles or ellipses: There are some concave and convex sections. Why might your isoseisms bulge away from the epicenter in some areas and toward it in others? Frame your answer in terms of earthquake waves and what happens to their speed and amplitude as they cross into different media (the lecture on " Lithospheric Plate Boundary Zones", section E.3.c.iv.). I left a hint up in Lab 9b, by the way. Once you find the hint and read the lecture, it might help if you looked at an image of regional topography, such as this one from NASA's Landsat Thematic Mapper. I've indicated the approximate location of Palm Springs and the earthquake with a red marker.

Look at the exact location of the epicenter compared to Zone VII. How would you characterize that location relative to your isoseism? What is odd about its relative location? It might help you to look at the diagram of epicenter and focus in the "Lithospheric Plate Boundary Zones" lecture, in section E.3.c.ii, about two-thirds of the way through the lecture.

The fault that produced the North Palm Springs quake dips about 45° to the north and the focus was about 10 km down. You might want to draw a cross-section of the 45° fault dip and the ground surface, with a line linking the focus 10 km down with the epicenter on the surface right on top of it and North labeled in the direction the fault dips. Given that information, about where would you expect the fault plane to intersect with the surface (north or south of the epicenter -- brownie points for using a diagram to guess how far away)? This might help you visualize and explain why the epicenter is so eccentric with respect to the isoseism bordering the VII zone.

Lab 7e: Brief Lab Report Interpreting Your Results

Now, having analyzed these issues and the evidence, write a brief report answering the following questions:

Why might the isoseisms form a wavy pattern, bending closer to the epicenter in some areas and away from it in others?
Considering fault geometry, why is the epicenter not right in the middle of the VII zone? Where does the fault plane intersect the surface, north or south of the epicenter?
For a half point of extra credit, look at your cross-section (hint: maybe use a ruler on the epicenter-to-focus distance and the epicenter-to-strike line distance) and hazard a guess as to how far away from the epicenter the fault plane would intersect the surface (the strike).

abridged and modified from a lab on earthquake measurement on the Southern California Earthquake Center web site

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