Faults

Faults and earthquakes

Like most stories in geology, this one starts beneath the surface. As you may know, the continents we live on are parts of moving plates. Most of the action takes place where plates meet. Plates may collide, pull apart, or scrape past each other.

All the stress and strain produced by moving plates builds up in the Earth's rocky crust until it simply can't take it any more. All at once, CRACK!, the rock breaks and the two rocky blocks move in opposite directions along a more or less planar fracture surface called a fault.

The sudden movement generates an earthquake at a point called the focus. The energy from the earthquake spreads out as seismic waves in all directions. The epicenter of the earthquake is the location where seismic waves reach the surface directly above the focus.

Normal fault

We classify faults by how the two rocky blocks on either side of a fault move relative to each other. The one you see here is a normal fault. A normal fault drops rock on one side of the fault down relative to the other side. Take a look at the side that shows the fault and arrows indicating movement. See the block farthest to the right that is shaped kind of like a foot? That's the foot wall. Now look at the block on the other side of the fault. See how it's resting or hanging on top of the foot wall block? That's the hanging wall.

Now, consider this: if we hold the foot wall stationary, gravity will normally want to pull the hanging wall down, right? Faults that move the way you would expect gravity to move them normally are called normal faults! Not so hard, is it?

Take a look where the fault has ruptured the Earth surface. Notice that movement along the fault has produced an elongate cliff? That fault-generated cliff is called a fault scarp.

Reverse fault

Compare this image with the normal fault above. Along a reverse fault one rocky block is pushed up relative to rock on the other side.

Can you see the foot-shaped foot wall and the hanging wall resting or hanging above it? Think about this: if we hold the foot wall stationary, where would the hanging wall go if we reversed gravity? The hanging wall will slide upwards, right? When movement along a fault is the reverse of what you would expect with normal gravity we call them reverse faults!

Strike-slip fault

Strike-slip faults have a different type of movement than normal and reverse faults. You probably noticed that the blocks that move on either side of a reverse or normal fault slide up or down along a dipping fault surface.

The rocky blocks on either side of strike-slip faults, on the other hand, scrape along side-by-side. You can see in the illustration that the movement is horizontal and the rock layers beneath the surface haven't been moved up or down on either side of the fault.

Take a look where the fault has ruptured the Earth surface. Notice that pure strike-slip faults do not produce fault scarps. There are other tell-tale changes in the landscape that signal strike-slip faulting. As you might guess, where the two massive blocks on either side of a strike-slip fault grind against each other, rock is weakened. Streams flowing across strike-slip faults are often diverted to flow along this weakened zone.

Real-life

In real-life faulting is not such a simple picture! Usually faults do not have purely up-and-down or side-by-side movement as we described above. It's much more common to have some combination of fault movements occurring together. For example, along California's famous San Andreas strike-slip fault system, about 95% of the movement is strike-slip, but about 5% of the movement is reverse faulting in some areas!