Section 1: What is an Earthquake?

Activity #2: FAULT-RUPTURE ANALOGIES

Materials:

• the checklist for rupture criteria (below)
  
• the list of suggested rupture analogies (below)
  
• the animation of fault-rupture propagation
  

Procedure:

This activity has two parts, and you should work through them in the given order. The first part will demonstrate the weaknesses of simple fault models (like block diagrams) in depicting the process of fault rupture accurately. The second part is centered around a fairly simple animation of rupture propagation, seen by an oblique map view, that attempts to show more accurately what we should envision when we think about fault rupture. Each part is fully explained within the text below.

Part 1Testing Simple Rupture Analogies

Below is a checklist of conditions which need to be met by any accurate fault model. The fewer of these criteria met by a model, the less accurate that model is. In this part of the activity, we will be judging some simple and common analogies of fault rupture -- the kinds of models we use when thinking about what happens when a fault "breaks".

Read through the checklist below, and familiarize yourself with the characteristics of real fault rupture. Try to make sure you understand each "rule" fully before you move on to the next section. Then, when you are ready, move on to the list of suggested rupture analogies. These are simple, easy-to-visualize models, some of which you may have heard or seen used before to describe what happens along a fault in an earthquake. Go through them, one by one, and check them against the list of rupture criteria. Count how many of these characteristics, out of the six given, each model meets. Then move on and answer the questions below.

Rupture Criteria Checklist:

The slip occurs along a surface.
Neither side of the surface is "fixed" (i.e. both sides move).
The two sides of the surface remain in total contact at all times.
The slip is not instantaneous; it propagates.
The amount of slip (displacement) varies along the rupture surface.
and
The amount of slip decreases to zero at the ends of the "fault" (i.e. the ends of the fault do not move).

This last condition is somewhat optional, depending upon how you look at it. Most fault ruptures are small, and never make it near the edges of the faults they rupture. Some ruptures break the surface (if the fault is not a blind fault), however, and that ground surface could be considered an "edge" or "end" of that fault. Possibly a better way to phrase the criterion, then, is:

The amount of slip generally decreases to zero at the edges of the fault, with three exceptions:

• the edge defined by the Earth's surface (if it reaches the surface)
  
• the edge defined by the base of the Earth's crust (if it extends down that far)
  
• any edge formed by an intersection with another fault
  

Suggested Rupture Analogies:

1. sliding one block of wood against another
   
2. ripping a sheet of paper in half
   
3. peeling a piece tape off of a wall
   
4. the movement of an inchworm along a log
   
5. pushing a "warp" (a "hollow" bump) out of a rug
   
6. dragging a piece of heavy furniture across a floor
   

When you have finished "rating" all of the models above, answer the following questions:

1. Which analogy or analogies met the most checklist criteria (and was thus the most accurate model of those above)? Which analogy (or analogies) above is the least accurate model?
   

2. Can you think of any fairly simple models that meet more of the checklist criteria than any of those in the list above?
   

3. Which criterion (or criteria) was met the least by the models above -- in other words, which seems to be the toughest characteristic(s) of a fault to simulate easily and accurately?
   

Part 2The Rupture Animation

Now that you've thought through several different simplistic analogies of the fault-rupture process, it's time to look at a limited animated model of fault-rupture propagation. It is a simple, yet in many ways accurate, depiction of the propagation of fault rupture along the surface trace of a fault during an earthquake, from start to finish (to keep things clear, there is no strong shaking of the area as might occur in a real earthquake). Watch this animation carefully, now, then come back to answer the questions below (during which you may review the animation as necessary).

1. Does this animation seem to meet all the criteria in the checklist above? Go through and check each one against the animation. (Note: the "crack" in the ground is a simulation of the surface disturbance generally caused by a fault rupture, and is not a sign that the two sides of the fault are separated, with space between them.)
   

2. Imagine standing on the near side of the fault in this animation, and looking across the fault rupture at the objects (road, trees, telephone poles, etc.) on the other side. In which direction did they appear to shift?

3. There are certainly a few problems with this animation. To make the slip more obvious, the scale of this animation has been distorted -- normally, a much longer rupture would be required to produce the size of the displacements seen here. The speed is also scaled for clarity -- this rupture front moves more slowly than a true rupture. Also, better resolution (or a larger animation) would make the rupture process shown in the animation more obvious. Yet there is an even more fundamental problem with this animation -- not in terms of its accuracy, but in terms of showing what happens when a fault ruptures. What is this problem? (Hint: think in terms of what you can see versus what is really happening.)
   

Closing Notes

Hopefully this activity has helped you envision the process of fault rupture more accurately. However, you should keep in mind that simple models, though inaccurate, can be preferable to more complex simulations in many circumstances. For instance, you will find that simple block diagrams, though in some ways inaccurate, are commonly used, both in this module and other materials, to illustrate earthquakes. This is primarily because they are easy to construct and view. As long as you realize the inaccuracies and limitations of the models you are using or viewing, there is no harm in taking a more simple approach to save time and effort, or improve clarity.

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