SLIP RATES vs. SEISMICITY RATESMaterials:
Procedure:
In this activity, you will study two different maps of seismicity in southern California. Each will compare the distribution of seismicity over a fixed period of time (hence, a rate) relative to the slip rates of the faults in the region. (You may wish to review slip rates before beginning, though this is not required. Simply keep in mind that slip rates and seismicity rates are measured in different units.) Your goal in examining each map is to assess whether any notable correlations exist between the slip rates of faults in southern California and the distribution of seismicity associated with them. Each map is covered by its own exercise, and no off-line materials are required. As each exercise is self-explanatory and self-contained, you may begin when ready.
Exercise 1 
All Seismicity: 1983 - 1997
You will start with a map of southern California upon which are projected every earthquake epicenter recorded in this area from 1983 to 1997. Each epicenter, regardless of magnitude or depth, is plotted as a single pixel. In some areas, earthquakes have occurred frequently enough that epicenters may overlap or be adjacent to each other, forming a solid "mass" of epicenters, rather than resolvable, individual pixels. Just remember that every pixel of color on this map represents at least one earthquake.
The color of the epicenters on this map is dependent upon the slip rate of whichever major fault's surface trace is nearest to that epicenter. The fault traces and slip rates used were taken directly from this slip rate map (which was used in Activity #13 of Section 1). The color scale for slip rates is given in the upper right-hand corner of the map.
Because epicenters are colored according to the nearest surface trace, the earthquakes they represent did not necessarily occur along the major fault zones with which they are matched. Keep in mind that this map is not meant to be exact, but is instead supposed to provide a rough overview of the corrleation between the slip rates of major faults and the seismicity rates most closely associated with them.
As you study this
color-coded seismicity map for
earthquakes of all magnitudes, pay attention to the colors associated
with the areas of highest seismicity, as well as to the amount of seismicity
associated with the faults of highest slip rate. Then come back to this
page and work through the questions below. You're free to go back
to consult the map as often as you need to.
Go now to the seismicity map and get some first impressions of the patterns there, and return when you're ready.
In general, what color(s) seem to dominate the map? What slip rate does this color represent?
After looking at this map, would you say that slip rate and seismicity rate are strongly correlated, somewhat correlated, or completely lacking in correlation?
Consider the amount of the map's area
covered by faults with low slip rates (2 mm/yr or less), as compared to
the area covered by faults with higher slip rates (refer to the
original
slip rate map
if you need to). Translate these areas into a ratio. Now do the same
for the number of pixels corresponding to low slip rate faults versus
the number of pixels corresponding to faults of higher slip rate. How
do these ratios compare? Does this comparison in any way affect your
opinion about the amount of correlation between slip rates and seismicity
rates? What simple factor in the presentation of this data (i.e. the map)
might skew this kind of rough calculation?
Exercise 2 Large Earthquakes since 1892
In Exercise 1, you studied a map of all earthquakes (any magnitude) recorded in southern California between 1983 and 1997, colored according to the slip rate of the major fault with the nearest surface trace. In this exercise, you will study a map constructed using criteria similar to those above, but each will be modified somewhat.
First, this map will reach back to the year 1892. Second, it will
show the epicenters of only those earthquakes with a magnitude of 6.0
or greater. These epicenters will be represented by filled circles,
scaled according to magnitude -- not by single pixels. Finally,
each circle (epicenter) will be colored according to the slip rate
of its causative fault (the one responsible for generating the earthquake,
not simply the one with the nearest surface trace). The colored fault
traces will be shown on the background map, and the slip-rate scale given
in the upper right-hand corner, as before.
As with the previous map, you should pay most attention to the
slip rates associated with the areas of highest activity as well as
to the seismicity rates associated with the high slip rates.
Go now to the color-coded seismicity map
of large earthquakes
As cautioned in Exercise 1, you should bear in mind that faults with low slip rates are more common than those with high slip rates. For a numerical explanation of why that's important. think not in terms of slip rates, but in terms of recurrence intervals, which are, in some part, dependent upon slip rate. Imagine the following hypothetical situation:
You are watching 110 fault segments of the exact same length, all capable of producing a large earthquake (indeed, the rupture of any would be exactly the same size as any other). You watch them all for 100 years. Of these 110 faults, 100 have a recurrence interval of 1000 years, and 10 of them have a recurrence interval of 200 years. Would you expect to see more large earthquakes along the numerous faults with slow repeat times, or on the fewer faults with fast repeat times?
On average, you should see one-tenth of the slow faults rupture (because 100 years divided by 1000 years equals 0.1), while one-half of the fast faults will rupture in those 100 years. But since you started with 100 slow faults, and 10 fast faults, that would make 10 large earthquakes on slow faults and 5 large earthquakes on fast faults, or a ratio of 2 slow-fault ruptures to every 1 on a fast fault.
Look at the map again. Can you estimate the ratio of the total length of fault segments with high slip rates (those colored yellow to red) versus those with lower slip rates? (Imagine lining up all the fault traces of each category end-to-end, and then comparing these two lengths.) Now, count those earthquake epicenters colored yellow to red, and those of all other colors. What is this ratio?
In light of the example above, would you say that there is more correlation between fault slip rates and the rates of large earthquakes than there was with earthquakes of all magnitudes? Was your opinion of the correlation different before this was pointed out?
Despite the areas of high activity along faults with high slip rates, were there also faults (or large sections thereof) with very high slip rates that were totally lacking in major ruptures? What might this mean?
There are four earthquakes plotted on
this map that were greater than magnitude 7.
These are the four largest circles plotted; can you find them?
Are any of them yellow, orange, or red? Give one reason why this finding
may not be significant. (Hint: think back to the first exercise
of Activity #6.)
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