Section 2: The Distribution of Earthquakes

Activity #4: AN "IN-DEPTH" LOOK AT EARTHQUAKE DISTRIBUTION

Concept: The maximum depth of seismicity is relatively shallow throughout southern California, but it does vary, and is related to crustal thickness. Those areas with deeper hypocenters tend to be associated with mountains and thrust faulting. Areas of extension are generally lacking in deep earthquakes.

Materials:

Procedure:

As with the previous activities in this section, you will be studying the distribution of earthquakes in southern California, but this time, you will be looking for patterns in that distribution with respect to the depth of earthquake hypocenters.

After completing this introduction, go to the plot titled Depth of Seismicity for 1983 - 1997. Read the caption that accompanies the image, and then study the distribution of the hypocenters, using the color scale to read the depth of each pixel. Pay more attention to the central part of the image, and less to the borders, as depth measurements for the outer points are not as reliable (many are fixed at 6.0 km for reasons explained on that page).

  1. Ignoring the areas near the borders of this image, would you say there are large-scale variations in the depth of the hypocenters shown? Or does the image look uniform, color-wise?

  2. Note the depth scale, and then the colors seen in the actual seismicity plot. What is the maximum depth indicated by this scale? (This should be roughly equivalent to the depth below which no earthquakes in this area occur.) Based on the plotted hypocenters, below what depth are earthquakes in southern California extremely rare (with only a small fraction of the visible points that color or deeper)? To what physical feature does this cut-off depth probably relate? Is it easy to rule out the possibility that southern California is a zone of major tectonic collision, based on this data alone? Why?

  3. Which areas have the highest concentrations of "deep" earthquakes (below 10 km; green and blue)? Which areas seem to be lacking in "deep" earthquakes (and thus are predominantly red, orange, and yellow)? Remember these answers (write them down, if you need to) as you move on to the next step.

Now, go to the same plot, but with a topographic background image. The roads and faults have been removed for clarity and the background has changed, but this is exactly the same data set. Study the relation of hypocentral depth to the surface topography and landforms, then return to work through the following questions:

  1. With what major feature of the area are most of the deeper hypocenters associated? (Think in terms of the San Andreas fault system.) What type of faults are commonly found in this area (i.e. what is the local tectonic setting)?

  2. What type of faults (and what tectonic setting(s)) are typically found in those areas that are lacking in deep earthquakes?

  3. Given what you know about tectonics and the results of different types of fault slip, what is the simplest difference between regions of varying hypocentral depth that could explain that variation? (Hint: think about question #2, above.)

Return to the Text