Section 1: What is an Earthquake?

Activity #12: FIND THAT FAULT SLIP!

Materials:

• the fault maps of southern California
  
• the individual fault files
  
• (optional) Find That Fault Slip! game for large screens (greater than 640 x 480)
  
• (UNDER CONSTRUCTION) fully interactive Find That Fault Slip! game for large screens
  

Procedure:

"Find That Fault Slip!" is a game that shows how understanding tectonics can help you understand faults. The point of this game is simple. Given a fault map and a basic overview of the tectonic environment of an area, you will be asked to locate a fault of a particular sense of slip. Click on the fault you think has the sense of slip you're looking for. This will take you to a page of information about that fault, which includes the sense of slip. If your choice was incorrect, use the "Back" button on your browser to return to the map, and try again. The fewer tries it takes to find the correct slip, the better. If you like, keep track of the total number of tries you use to finish the entire exercise -- you can score yourself at the end.

Specific Rules:

• If you found an oblique-slip fault with the proper component of the slip you were seeking (say you find a "left-reverse" fault when looking for "left-lateral"), you still need to find the exact sense of slip, but don't count that try -- take a "free" turn, and try again.
  
• If the slip of the fault is listed as uncertain, try again with no penalty (take a "free" turn).
  
• If the slip of the fault is listed as "primarily" the kind of slip you were looking for, this counts as a correct answer.
  
• Thrust and reverse count as the same slip, but not for the purposes of an oblique-slip fault -- if you're looking for a "thrust" and find a "right-reverse", you do not get a free turn.
  
• Some faults are visible on more than one map, but you may not use any single fault for more than one correct answer.
  
• Two "reference" faults are off-limits for choosing: the San Andreas fault (right-lateral; always shown in red), and the Garlock fault (left-lateral; always shown as a bright yellowish-green).
  
• Regardless of how many attempts it takes you to find the correct slip, score no more than a 4 on any single example.
  

Scroll down to begin, or click here to play the frames version of the game.

1. First we will look at the northwest fault map. The northern part of this area is dominated by the right-lateral motion along the San Andreas fault (shown in red). Here, the plates are sliding past one another in almost the exact orientation of this great fault -- the land on the left (west) side of the San Andreas fault is moving toward the northwest, and the land on the right (east) side of it is moving southeast. The southern part shows the western end of the Transverse Ranges, created by the compression of the "Big Bend" of the San Andreas fault. Some faults in this area also work to move material laterally out of the way of that compressional force. Among them is the Garlock fault (shown in bright yellowish-green in the lower right corner of the map), the longest left-lateral fault in southern California.
   

   Given this information, how many tries do you need to find:

   1. a right-lateral fault (other than the San Andreas fault)?
      
   2. a thrust fault?
      
   3. any fault with some left-lateral slip, oblique included (other than the Garlock fault)?
      

2. Next, we'll go to the Mojave fault map. The southern part of this map -- the area around the "Big Bend" of the San Andreas, is the site of compression and of some motion conjugate to that of the San Andreas fault (again, in red). Near the top of the map lies the left-lateral Garlock fault, also conjugate in motion to the San Andreas fault. The Garlock fault marks the northern boundary of the Mojave -- to its north, the tectonic environment changes. Throughout the middle of the map, the faults are influenced primarily by the Eastern California Shear Zone, an area marked by right-lateral slip trending southeast to northwest -- roughly parallel to that part of the San Andreas not involved in the "Big Bend" -- which connects with the San Andreas left of center just off the bottom of the map. You will note that near center at the bottom, the San Andreas fault splits up, denoted with slightly different shades of red. We will see this again on the southern fault map.
   

   Knowing this, see how many tries it takes to find:

   1. a right-lateral fault (other than the San Andreas fault).
      This should be an easy one.
      
   2. a left-lateral fault (other than the Garlock fault).
      
   3. a thrust fault.
      

3. Now, on to the Los Angeles area fault map. Los Angeles sits south of the "Big Bend" of the San Andreas. At the very top of the map, you can see the intersection of the Garlock and San Andreas faults. The action of the Garlock fault is probably responsible for the low number of faults north of this stretch of the San Andreas fault. To the south, however, things are not at all simple. Compression, in a roughly north-south direction, is the dominant force in this area, but as with the Garlock fault, some faults here also work to move material away from the collision zone. Along the bottom of this map, and in the lower left-hand corner the influence of the "Big Bend" begins to fade. One more hint before you start: the long, blue fault in the middle of the map is the steeply-dipping San Gabriel fault. Only the western section of this fault seems to show recent surface rupture.

   Keeping all this in mind, how many tries do you need to find:

   1. a left-lateral fault (other than the Garlock fault)?
      
   2. a thrust (or pure reverse) fault?
      
   3. a right-lateral fault (other than the San Andreas fault)?
      

4. The southern fault map is the next game board. The northern part of this map covers the confusing split of the San Andreas fault zone in the San Gorgonio Pass area (north and northwest of Palm Springs). The strands of the San Andreas fault (shown in different shades of red) rejoin near Indio, at the southeastern end of the Big Bend. Around this area, the effects of the Big Bend are reflected in nearby fault motions. The rejoining of the San Andreas strands occurs near the point where the Eastern California Shear Zone (which we saw on the Mojave map) branches off from the plate boundary. The rest of the map is dominated by right-lateral shearing motion of the plates in a northwest-to-southeast direction. One of the largest right-lateral faults in southern California is the San Jacinto fault, shown here in purple, with numerous branches. Note how this major right-lateral fault intersects with the San Andreas fault. How might such an intersection affect faults within the acute intersecting angle -- in other words, what kinds of stresses would result from this intersection?
   
   1. a right-lateral fault (other than a part of the San Andreas or San Jacinto fault zones)?
      
   2. a left-lateral fault?
      
   3. a zone of normal faulting.
      (Hint: Read the paragraph above, again.)
      

5. Finally, look at the Basin and Range fault map. This is a tectonic region bounded to the south by the left-lateral Garlock fault, and to the west by the Sierra Nevada fault zone, a large zone of normal faulting shown in dark blue (the Independence fault, in a brighter blue, is a part of this fault zone). The region is characterized by extension in a roughly east-west direction and by right-lateral shearing in a north-south direction. Consequently, right-lateral and normal slip dominate, often combining to yield right-normal faulting. You will note that the San Andreas is not visible on this map, and yet the right-lateral shearing transferred to this region by the Eastern California Shear Zone reveals the nature of the plate boundary a few hundred kilometers away. One last tip: paying attention to the topography can really help you on this map.
   

   With this knowledge as your guide, how many attempts will it take you to find:

   1. a pure normal fault (other than the Sierra Nevada or Independence faults)?
   2. a right-lateral (but not right-normal) fault?
   3. a left-normal fault?
   4. a thrust fault?
      (Hint: There is only one. You'll find it where a major fault ends.)
      

Congratulations! You've finished the game.
How did you do? Don't be ashamed if you were far from perfect -- some of those examples were pretty tough.

Below is a chart you can use to judge your score:

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