April 28. Fault types

There are three basic types of faults, which you probably know are breaks in rocks in the earth – breaks with offset and movement along them. The three types represent the three ways earth materials can move relative to each other. They can pull apart, they can collide, or they can slide past each other. 

When rocks in the earth on scales of miles or many miles are pulled apart, eventually something has to give. If the rocks were plastic, like silly putty or caramel candy, they might stretch and thin, but in the upper crust of the earth, most rocks are not plastic. They’re brittle, and when you pull them apart, eventually they will snap. One piece will probably drop down – or appear to have dropped down –  relative to the other along a break, a fault. The part that slides down is above the other part, and above the inclined fault between them. Faults like that, where one block slides down relative to the other, are called normal faults.
 
Normal faults aren’t really any more “normal” than other kinds of faults, but the name comes from the way miners referred to fault blocks. In a mine, when an ore body was cut by a fault, miners called the two blocks the footwall – the block below the fault, which they were standing on – and the hanging wall, above the fault and hanging over their heads. That was the “normal” way of things, with the footwall down and the hanging wall up. When the miners encountered faults with the opposite sense of relative offset, with the footwall up and the hanging wall down, they called them reverse faults.

So, normal faults usually come from extension, pulling apart. A good example in the United States is the Wasatch Fault in Utah at Salt Lake City. The mountains, to the east, are uplifted relative to the Salt Lake Basin to the west, which has been faulted down. Most of the western half of Utah and most of Nevada are undergoing extension, which has caused many blocks to go up and down relative to each other.

When we talked about the Caledonian Orogeny a few days ago, we talked about colliding continents and other terranes, and it’s easy to see how collisions would push a mountain range up. It’s like two big pieces of furniture on a carpet and you’re pushing the furniture – the continents – together. The carpet between them is likely to be squeezed from a flat carpet to a rippled surface, and eventually the carpet ripples, which are like folds in rocks, might even get pushed up and over each other. If the carpet was something more brittle, it would break.

The faults that result from compression, squeezing the earth between two blocks, are usually the up-and-over type, which the miners called reverse faults. They could be at any angle, and today geologists have two general terms for them. High angle reverse faults are just reverse faults, but if one package of rocks was pushed up and over older rocks at a low angle, we usually call those thrust faults. It’s a little subjective as to when you say reverse fault or thrust fault, but they are basically the same thing, caused by the same mechanism – compression.

The third kind of fault occurs where two separate blocks of the earth slide past each other, and we call that a strike-slip fault. Strike is the orientation of anything in the earth with respect to the geography of the surface, so you might have a bed or a fault or something else that is oriented north-south, or east-west, or any other angle relative to geographic coordinates. And the two blocks slide along the strike of the fault.

Probably the most famous strike-slip fault in the United States is the San Andreas fault, which moved to cause the great earthquake we mentioned on its anniversary on April 18. But there are many strike-slip faults around the world.

That’s your lesson in geological jargon for today.

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Today is Eugene Shoemaker’s birthday. He was born in 1928 in Los Angeles, and he pioneered the field of planetary geology. He was involved in many American space missions, and co-discovered the comet Shoemaker-Levy in 1993, which gave scientists the first look at a planetary impact when the comet collided with Jupiter in 1994.

—Richard I. Gibson

Drawings by Richard Gibson
Fault types from USGS
Animated normal and reverse faults (USGS)