Section 2: The Distribution of Earthquakes

Activity #7: LANDERS SHAKES THINGS UP

Concept: The shock generated by a large earthquake can trigger the onset of other earthquakes, even at great distance from the original hypocenter.

Materials:

Procedure:

In this activity, you will be investigating the seismicity in southern California around the time of the 1992 Landers earthquake, which at magnitude 7.3 was the largest earthquake to strike the region in 40 years. In addition to an enormous aftershock sequence, the Landers earthquake was followed by the occurrence of two earthquakes greater than magnitude 5 that were outside its aftershock zone, but near enough in time and distance to give the impression that they were somehow connected (since events of that size are uncommon).

Scientific studies into the circumstances surrounding these events and their connection to the Landers earthquake were carried out, primarily because of the hazard that "triggered" earthquakes (if real) could pose. For instance, the San Andreas fault does not run particularly close to the heart of Los Angeles, but if a major rupture on the San Andreas fault could "trigger" earthquakes on faults much closer to the city, those earthquakes could cause more damage in the city than "the Big One" on the San Andreas fault would.

Here, you will have a chance to perform your own investigation. The basic idea is simple: you will compare the seismicity in southern California before the Landers earthquake to that after the Landers earthquake. The time of this study will be limited to two weeks (14 days) on either side of the mainshock. For clarity, any seismic activity within the aftershock zone of the Landers earthquake, both before and after the mainshock, will be omitted (meaning of course, that an aftershock zone must be determined).

To make this study particularly easy and effective, a "blink-comparison" diagram will be used. This kind of diagram is used to look for differences (subtle or otherwise) between two similar sets of data. Two frames are used; they can be interchanged at will within the same field of view, so that the viewer can "blink" back and forth between them to see what changes between frames, and how. (This is commonly done in searches for faint astronomical objects -- the planet Pluto was discovered using a blink-comparison technique.)

Once you've finished with this brief introduction, start the slide show, which will explain how the comparison figure so crucial to this exercise was put together. At the end of that walk-through, you will be directed to the blink-comparison figure itself. When you've finished studying (and "blinking"!) it, return here using the link provided, and work through the questions below.

Begin the Slide Show

Questions:

  1. After looking at the blink-comparison figure for the two weeks of seismicity after the Landers earthquake relative to the two weeks before it, would you conclude that this earthquake triggered seismicity outside of its aftershock zone? Why or why not? Do you think the fact that you walked through the making of the comparison figure helped -- that is, would you be as sure of your conclusion than had the figure simply been presented, already complete?

  2. Is it possible that the apparent "triggering" is due only to the way the aftershock zone was drawn? Remember that another way to define an aftershock zone is as an area one rupture-length in radius surrounding the rupture trace. Would an aftershock zone shaped that way have made a difference in this activity? (In other words, were the triggered earthquakes all very close to the boundary of the given aftershock zone, or not?)

  3. Assuming you're not entirely sure you should be convinced about the reality of triggered seismicity after looking at a map, consider making a table of the number of events above a certain magnitude for the outlying areas of the map, before and after the Landers earthquake. (This kind of numerical analysis might make a good independent research project.) Do you think the numbers ("After" vs. "Before") would be significantly different?

  4. Did you notice the relative lack of earthquakes with very low magnitudes (M < 1.5) in the "After" frame of the comparison figure? Indeed, if you fail to consider the magnitude of the earthquakes far removed from the Landers area, it might seem that the rate of earthquakes changes little in some areas. This is a side-effect of the prolific aftershock sequence of the Landers earthquake. There were so many aftershocks generating so much "noise" that the signals from smaller earthquakes around the region were "drowned out". This imposed magnitude cut-off is especially noticeable along the San Jacinto fault zone, to the south of the aftershock zone, which showed a high rate of microseismicity in the "Before" frame. If you were to construct this figure again, do you think imposing a magnitude cut-off on the data used would help the clarity of the comparison? If so, at what magnitude would you set that cut-off?

  5. The Landers rupture (in reddish-orange on the map) occurred along a chain of faults, linked in a roughly north-south orientation. The earthquake started at one end of this chain, and propagated toward the other. Energy released by an earthquake has a tendency to be concentrated in the direction of the primary rupture propagation. Given this information along with the distribution of seismic activity shown on the blink comparison, can you guess which way the rupture propagated (north-to-south, or south-to-north)? Since ground shaking is dependent upon the energy released by an earthquake, does this support the idea that triggered earthquakes may be a response to the shaking caused by a large earthquake?

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