Section 1: What is an Earthquake?


Concept: Accurate positioning of earthquake hypocenters can produce a rough "image" of a fault, allowing subsurface determination of dip. Obtaining this much data for a single fault usually requires a large aftershock sequence.



In this exercise, you will be plotting earthquake hypocenters along a cross-section in an attempt to "image" a fault deep beneath the surface. The hypocenters you will be using come from the aftershock sequence of the 1986 North Palm Springs earthquake. These "aftershocks" are simply smaller earthquakes that followed in the wake of the "mainshock" -- the largest earthquake of the bunch -- which was magnitude 5.6, a moderate-sized earthquake.

You are going to have to print out the map of hypocenters and the graphing template on separate pieces of paper to complete this exercise.

Start by looking at the map of hypocenters. On this map you will find points marked with small crosses, each with a number next to it. These points are epicenters, and the numbers next to them are depths, in kilometers, to the corresponding hypocenters of the earthquakes they represent. One point is marked with an asterisk; the depth written next to it is 10.35 -- this is the hypocenter of the North Palm Springs mainshock.

Choose at least ten (preferably several more than that) of these hypocenters to plot on the cross-section you will make. The ones you choose should vary as much as possible in depth; try not to pick any two that are closer than 0.5 km to each other, if possible. You must also plot the mainshock hypocenter on your cross-sectional view.

Now, look at your print out or copy of the graphing template. The scale on the left side measures depth, in kilometers below sea level. The approximate elevation of the surface in this area is shown by the top edge of the cross-section. Sea level (0 km depth) is also shown. A dashed line marks the depth of the mainshock at 10.35 km.

Now, look back to your map of hypocenters. Your first job in plotting a cross-section is to decide upon a plane onto which you will project the hypocenter points, forming the cross-sectional plot. The plane must be vertical, so on the map view, it will appear as a line. Think of slicing into a cake, or cutting a sandwich in half. This is effectively what are you doing when drawing your cross-sectional plane.

Because the point of this cross-section is to make an "image" of the fault responsible for the North Palm Springs earthquake, the best way to decide how to orient your plane is to look at the fault traces on the map. See how most of the fault traces run roughly parallel to each other in this area? To get a look at one of those faults edge-on, you should try and "cut" your section exactly perpendicular to this common trend of the local faults.

Your plane does not need to go through the mainshock, but you should try and run the section through roughly the center of the data you will be plotting. The exact choice of orientation and position, however, are up to you. Take your straight-edge and a pencil and lightly draw a line to represent your cross-sectional plane. Make it extend all the way to the borders of the map.

To plot your cross-section, the first thing you will need to do is to "project" all of your chosen data points onto your cross-sectional plane. To do this, go to each hypocenter and lightly draw the shortest line possible from that point to the cross-sectional line -- this will make a right angle with the cross-sectional line if done correctly. Check that with a protactor or similar tool. Once you're sure you've done a good job projecting the hypocenter, mark it strongly on the cross-sectional line with pencil or pen, but leave the pencil line connecting the projected point to the hypocenter point intact. You will need this to identify your data points. Continue with this same method of projecting the hypocenters to the cross-sectional plane until you have finished with all the points from your chosen data set.

Now look back at the graphing template for you cross-section and note how the top corners are marked A and A'. You need to mark two points on the cross-sectional line on your map with these two letters. The distance between them should be exactly the same as that between the edges of the cross-section plot itself -- provided the two figures were printed at the same scale! Check the "1 km" scale on each piece of paper to make sure they match. If so, then measure the distance between points A and A' on the cross-section and then make two new, dark marks on the cross-sectional line (short lines perpendicular to that line) that distance apart, and mark them A and A'. You should make sure that all of your projected data points will plot between them. These end marks will help you keep the orientation of the cross-section consistent while you make your plot.

Now that you have your end points marked and your data points projected, you can begin plotting the actual cross-section. First, using the end points A and A' as your guide, transfer the projected points from the cross-sectional line on your map to the top of the cross-section itself. Think of these new points as "epicenters".

Next, match each epicenter with its depth, and with the help of a ruler and the depth scale, project each of these points down its appropriate depth, as given on the map. Once you have finished this procedure for every point in your data set, your cross-section will be complete!

When you have finished your cross-section, read through and answer the questions below.

  1. Do the points (hypocenters) on your cross-section fall in a perfect line, in a "fuzzy" line, in some other pattern or in no obvious pattern at all?

  2. If you haven't already, draw a single line on your cross-section to best represent, in a linear fashion, the points you have plotted. This line should intersect the location of the mainshock hypocenter, but it does not need to exactly intersect any of the other hypocenters. Measure the angle this line makes with respect to the horizontal. This should be the dip of the fault that produced the North Palm Springs mainshock. According to your cross-section, at what angle does this fault dip? If others are working on the same activity, compare your findings with theirs. Are they similar?

  3. Project this line up to the surface on your cross-section. (This may require that you extend the boundaries of the cross-section with another sheet of paper.) Now, go back to the original cross-sectional line you drew on the map and note where that line crosses the surface traces of faults. Mark those points. Then transfer them to your cross-section in the same manner you used to create the "epicenters". Near which fault's surface trace is the fault plane you have drawn closest when it intersects the surface? (Again, compare with others if you have the opportunity.) This is likely the fault which ruptured to produce the mainshock. What reasons can you think of why this might not be a valid assumption?

  4. What reasons might there be for why the data points do not fall in a perfect line?

Were we to have used more data points (hypocenters), we would have defined a much more complete picture, of course. To see a what a much larger set of hypocenters would have done to the plot, go to the rotating fault plane animation. This is the same aftershock sequence that we used in the activity, but without any trimming. And naturally, we did not have the luxury of rotating the hypocenters around an axis. Obviously, you will notice more structure here than we revealed, but you should also note that there is an "artifact" visible. That is, something that looks like a pattern that is due to the way the data is handled. In this case, the artifact is a line (actually an edge-on plane) of earthquakes at exactly 6 kilometers depth. Most of those earthquakes were "fixed" at that depth when they were analyzed, usually to get a better epicentral location for earthquakes which were otherwise not easily located (due to small size or other problems). In Section 2, when we look at a distribution of earthquakes in southern California with respect to depth, you will see this same artifact again.

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