Activity #3 -- What is an Earthquake?

Section 1: What is an Earthquake?

Activity #3: RUPTURE MODELS

Concept: It is possible, with good data, to "view" a fault rupture, despite the fact that the rupture surface is entirely underground.

Materials:

software for playing MPEG movies
the Northridge rupture model
the Landers rupture and energy models

Procedure:

In this activity, you will be watching three "movies" based on actual data from fault ruptures -- indeed, from the two largest southern California earthquakes in the 1990s: Landers (1992) and Northridge (1994). In Section 3, will we learn more about how such data is collected and analyzed, but for now, your job is simply to watch the animations and think through a few questions regarding them. When answering the questions, you are encouraged to go back and review the animations as needed.

Northridge EarthquakeMagnitude 6.7January 17, 1994

On a separate page, you will find the Northridge rupture model. After reading this introduction, link there, and follow the directions to watch the MPEG movie of the rupture, as modelled from seismic data.

The rectangular area used as the background of the animation can be thought of as, roughly, the "plane" of the Northridge thrust fault, which ruptured on January 17, 1994, with "up" at top. The borders of this rectangle are sharper and probably somewhat smaller than the area which ruptured. There is a scale along each axis which measures the distance spanned. Note that the space between tick marks is not the same on the vertical and horizontal scales, even though each tick mark represents 1 kilometer on each scale. This is because the fault is not vertical -- it is inclined at a fairly shallow angle to the horizontal (to the southwest). The view you are looking at is from an angle, so that the fault plane is perpendicular to your line of sight (with northwest on the left, and southeast to the right). The top of the rectangle is truncated at 6 km beneath the surface. This is roughly the depth at which the Northridge Thrust is cut off (overrun) by the Santa Susana fault zone. Thus, this fault does not reach the surface; it is a blind fault, and there was no actual surface rupture during the Northridge earthquake. The bottom of the rectangle is truncated at 20 km beneath the surface. This is approximately the lower limit of the crust (where rupture can occur) in this area.

Two more things to keep in mind: there will be a color scale and a clock on the right side of the animation. The scale will let you judge the amount of slip shown in each frame. The clock shows the elapsed time of the rupture; it may not correlate at all with how long the animation takes to play.

Go now to the Northridge rupture model page, if you haven't already, and watch the animation. Then come back to answer the questions below.

Think back to (or look back at) the checklist of fault-rupture criteria given in Activity #2 of this section. Does this animation of the Northridge rupture seem to obey those criteria?
In the rupture animation from Activity #2, we saw rupture propagating at the surface in the form of a point moving along a line. What form does the rupture "front" take here? (In other words, what does it resemble and how does it move?)
From the first frame, estimate the depth of the Northridge hypocenter. The rupture propagated as a wavefront from the bottom and southeast (right) side of the area shown, in a manner upward and to the northwest (left). Had the depth been more shallow, and not so near the base of the crust, might there have been more downward propagation of the rupture? Watch the very beginning of the animation carefully to help you decide.
Divide the distance covered by the rupture wavefront by the time it took to travel this distance, to estimate the average velocity of rupture propagation. This is a fairly standard value for such a rupture.
The maximum slip of this rupture was over 3 meters, but what would you estimate the average slip to be?
According to the clock, how long did the Northridge rupture last?
What is the size, in square kilometers, of the area shown in the animation? While this may not be the same as the size of the fault area ruptured, the answer, and that of the previous question, will give you a basis of comparison when watching the Landers earthquake rupture, below.

Landers EarthquakeMagnitude 7.3June 28, 1992

The next movie you will watch is a model of the slip during the 1992 Landers earthquake. On the page of Landers rupture models, you will find two links to MPEG movies. We will only focus on the top one, a model of the slip during the Landers rupture, in this activity. The lower example shows the kinetic energy distribution, something we will look at in later sections.

This animation is similar to the Northridge animation you just watched. Again, there are scales on the sides indicating depth and distance, a color scale indicating slip amounts, and a clock for gauging the duration of the rupture. This animation, however, begins with a brief description of how the model is set up. A map view of the surface traces of the faults ruptured is shown, and the manner in which fault planes were chosen to model this rupture, as well as the location of the hypocenter on each view, map and cross-section, is shown in a quick animation.

There are some other important differences between these two rupture animations to which you should pay attention. Probably the first thing you will notice is the proportions of the rectangle in which the animation occurs. This is a result of the fact that the Landers earthquake propagated laterally along several nearly vertical strike-slip faults, whereas the Northridge earthquake occurred on a single thrust fault at a shallow angle to the horizontal, and propagated in a far more limited and somewhat more vertical manner.

Notice, too, that this first, still frame (before you click to see the animation) is a diagram of the total slip of the Landers rupture. There was no such diagram for the Northridge model; the first frame of that animation was used as the "introductory" still image. Note the color scale used for slip -- the scale uses the same colors as before (Northridge), but they stand for a larger range of slip. The same colors are also used for the animation of the Landers rupture, but again, the scale covers a different range of slip values. Pay close attention to these color scales.

Also, note the difference in the scale of length and depth. This time, the top of the rectangle is at a depth of 0.0 km; in other words, here the animation extends right to the surface of the Earth. The maximum depth is only 15 km, because this is the maximum depth at which seismicity occurs in this area.

Finally, be wary of the difference in the rate of this animation. In the Northridge animation, there were at least two frames per second of rupture. Here, the rate is only one frame per second of rupture. Keep an eye on the clock during at least one run-through to get a better feel for the rate at which this rupture occurred.

You are now fully prepared to watch the animation. Go to the Landers rupture model page now, if you haven't already, study the first image, then click on it and watch carefully... several times if necessary. When you're ready, come back to answer the questions below. (You are welcome to go back and watch the animation again, as needed.)

The first still image you saw was a diagram of total slip. Go back and look at this diagram, if necessary, then estimate the average slip of this rupture. What was the maximum slip of the rupture, and what was the largest amount of slip that could be seen at the surface?
Did this rupture front seem to travel in a wave-like manner similar to the Northridge rupture front? What seems to be the key to the differences in how these two rupture propagations look? Using the technique you used to find the rupture-front velocity of the Northridge earthquake, estimate the velocity of the Landers rupture front. How does this compare with the Northridge velocity?
Does the rupture seem to travel with a constant velocity, or are there velocity changes? (You may not be able to answer this, if the MPEG does not play at a constant speed, itself.)
How long, according to the clock, did the Landers rupture last?
How large is the area shown in this animation?
How do these values (slip, duration, and area) compare with those of the Northridge rupture?

Of course, the magnitude of the Landers earthquake is larger than that of the Northridge earthquake, but many people fail to grasp just how much larger the Landers earthquake was, primarily because we tend to think about large earthquakes not in terms of seismology, but in terms of disaster -- damage, injury, and loss of life. Indeed, the Northridge earthquake far surpassed the Landers earthquake in these areas, but mainly because of the difference in geography. Had Landers been located near a large population center, public opinion about the relative importance of these two earthquakes would be quite different.

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