Faults

• In a normal fault (see animation below), the fault plane is nearly vertical. The hanging wall, the block of rock positioned above the plane, pushes down across the footwall, which is the block of rock below the plane. The footwall, in turn, pushes up against the hanging wall. These faults occur where the crust is being pulled apart, due to the pull of a divergent plate boundary.

• The fault plane in a reverse fault is also nearly vertical, but the hanging wall pushes up and the footwall pushes down. This sort of fault forms where a plate is being compressed.
• A thrust fault moves the same way as a reverse fault, but the fault line is nearly horizontal. In these faults, which are also caused by compression, the rock of the hanging wall is actually pushed up on top of the footwall. This is the sort of fault that occurs in a converging plate boundary.

• In a strike-slip fault, the blocks of rock move in opposite horizontal directions. These faults form when the crust pieces are sliding against each other, as in a transform plate boundary

In all of these types of faults, the different blocks of rock push very tightly together, creating a good deal of friction as they move. If this friction level is high enough, the two blocks become locked -- the friction keeps them from sliding against each other. When this happens, the forces in the plates continue to push the rock, increasing the pressure applied at the fault.

If the pressure increases to a high enough level, then it will overcome the force of the friction, and the blocks will suddenly snap forward. To put it another way, as the tectonic forces push on the "locked" blocks, potential energy builds. When the plates are finally moved, this built-up energy becomes kinetic. Some fault shifts create visible changes at the earth's surface, but other shifts occur in rock well under the surface, and so don't create a surface rupture.

Photo courtesy USGS Crop rows offset by a lateral strike slip fault shifting in the 1976 earthquake that shook El Progresso, Guatemala.

The initial break that creates a fault, along with these sudden, intense shifts along already formed faults, are the main sources of earthquakes. Most earthquakes occur around plate boundaries, because this is where the strain from the plate movements is felt most intensely, creating fault zones, groups of interconnected faults. In a fault zone, the release of kinetic energy at one fault may increase the stress -- the potential energy -- in a nearby fault, leading to other earthquakes. This is one of the reasons that several earthquakes may occur in an area in a short period of time.

Photo courtesy USGS Railroad tracks shifted by the 1976 Guatemala earthquake

Every now and then, earthquakes do occur in the middle of plates. In fact, one of the most powerful series of earthquakes ever recorded in the United States occurred in the middle of the North American continental plate. These earthquakes, which shook several states in 1811 and 1812, originated in Missouri. In the 1970s, scientists found the likely source of this earthquake: a 600-million-year-old fault zone buried under many layers of rock.

The vibrations of one earthquake in this series were so powerful that they actually rang church bells as far away as Boston! In the next section, we'll examine earthquake vibrations and see how they travel through the ground.