Friday, March 23, 2007

Everyone has their Faults

Let's talk faults. There are 4 types of faults that exist in the entire world: Normal faults, Reverse faults, Strike-Slip faults and Oblique faults. Anyone that tells you otherwise is a liar, a fool, or someone who majored in humanities (both).

The first type of fault is called a Normal Fault. A little bit of history here: all the terminology that deals with fault structures came by way of English miners. England doesn’t see much seismic action now-a-days, but when these miners were digging around for coal n’ stuff, they saw lots of old fault traces in the walls of their suffocating mine shafts. The faults they typically saw looked like this:

Move your eyes from left to right. Follow the yellow layer along until it is broken by the fault. Where does that layer go? Up or down? If you said down, give yourself a gold star! The hunk of rock on the left side of the fault is called the Foot Wall, the right side of the fault (the part that moved down) is called the Hanging Wall. That’s because these poor-ass miners in England, standing in a hole directly on the fault, would hang their lamps above them. Hence, the Hanging Wall. The wall they would be standing on is called the Foot Wall because, you guessed it, their feet were on it! I’m aware that this is totally archaic nomenclature, but it’s the best anyone has come up with. If you have a better idea, please contact your local geologist. They will promptly ignore you because you are not a geologist and shouldn’t go around mucking with precedence. The next kind of fault is a Reverse fault (see above). These types of faults occur where the Earth’s crust is being mashed together. To help you visualize this, picture yourself at a Demolition Derby/Monster Truck rally. Two cars are gunning for one another and there is about to be a SICK collision. POW! They collide, but what happens? Usually one car will pop up over the other. That’s like a reverse fault. One block has to pop up over the other block as they are colliding together.

When this collision happens between two oceanic plates, or between an oceanic plate and a continental plate, we get a subduction zone. When it happens between two continental plates – like India and Eurasia, we get big-ass mountains, or OROGENIES as hoity-toity geologists like to call them.

The next kind of fault is called a Strike-Slip or Lateral fault. These faults have all their motion in the horizontal. Our favorite Strike-Slip fault is of course, the San Andreas Fault in western California. There is an analogous fault in Northern Turkey called the North Anatolian Fault. But we aren’t bombing Turkey right now, so who gives a shit what’s going on over there?!

The tricky part with strike-slip faults is determining whether they are a Right Lateral or Left Lateral. I’ll tell you how I do it:

1.) Look at your fault in map view, ie looking down on it from above.

2.) Draw two dots, one on either side of the fault. Imagine one dot is you and the other dot is a friend of yours. Wave hi to your buddy!3.) Now imagine that there is an earthquake. EARTHQUAKE, AHHH! Move your friend over a little bit according to the arrow of motion on his/her side of the fault 4.) From your perspective, your friend should have moved to the right. And from your friend’s perspective, you have moved to his/her right.
5.) You’ve got yourself a right-lateral fault, Chief!

The San Andreas Fault is a right-lateral fault. The Pacific plate is moving northwards at a rate of about 35 mm/yr. Attached to that plate is everything on the west of the red line you see on the map below. That means that in a few million years, Los Angeles will be within spitting distance of San Francisco. And spit we shall.

The last type of fault, an Oblique fault is just a combination of a Dip-Slip fault (either Reverse or Normal) and a Strike-Slip fault. A little bit of vertical motion. A little bit of horizontal motion.

Now you know all about faults! Good for you!

1 comment:

Furious said...

it was all my fault...bla ha ha ha ha! (that wasn't funny, was it?)