Geologic Faults

By far the most famous fault, in the US at least, is the San Andreas Fault in California. As a kid on the other side of the country, I always pictured it as one giant chasm that went on forever - similar to what you might see in a movie. Of course, I later learned what the San Andreas Fault really is and also that faults are not really giant gaps in the ground as the movies might have you believe. In fact, most faults, such as the one pictured above, are much smaller, sometimes even microscopic. Nor is California the only state with faults and earthquakes. Over half of the states in the US have at least a moderate earthquake risk, which can be seen on earthquake hazard maps published by the USGS.

Simply put, a fault is a crack in rock on which slip can occur. The rocks on top can either slip downward (normal fault), upward (reverse fault), or move side-to-side (transform fault). Can you figure out what type of fault is pictured? Hint: try to match the strata (or rock layers) on each side to see which way the rocks on top moved. This fault is far from California; it's located at the Seneca Stone quarry in the Finger Lakes region of New York. There is a big difference between this fault and the San Andreas.

Faults form for different reasons. The San Andreas Fault is a major fault because it lies at a plate boundary. The Earth's crust is made of multiple large pieces called tectonic plates. These plates "float" on the mantle and are slowly moving (a few inches per year). At plate boundaries, three main things can happen: one plate can move underneath the other, the plates can move apart, or they can move past each other sideways. Of course, they aren't moving continuously. Friction can cause the plates to get stuck on each other at the boundary. However, stress builds up if the plates are stuck. When the stress becomes large enough to overcome friction, the plates will slip and release the energy that had been stored. This energy is mostly released as waves in the Earth that we call earthquakes.

However, many faults don't occur on plate boundaries. Central New York, for one, is far from a plate boundary. Volcanic processes, such as magma intrusion and gas release, can create faults or enlarge cracks. Colliding plates create stress and strain throughout the plate, not just at the boundary. This can cause intra-plate faulting and create fold mountains. Old, brittle rocks tend to break more easily and will be more likely to have faults and cracks. For our New York fault, we can look at the geologic history of the region. The Finger Lakes were created by glaciers. When glaciers form, their weight causes the crust to sink - similar to what happens when you sit on a mattress. Eventually, the glaciers receded which allowed the crust to rise back up. The stress and strain associated with the crust's bending can create cracks in the rock. This would be a good guess to the origin of the fault in the picture.

Many faults are hidden beneath the surface. We only find them when they become active (i.e. move and create earthquakes) or when we dig into the ground for things like rock quarries, mining, and roads. At this moment, you could be sitting right above a fault and not even know it. Don't worry too much though. Few faults are large enough or have enough energy built up to create a major earthquake. The areas that do have to worry are well known and generally well prepared. Make sure to check out the earthquake hazard map to see how likely an earthquake is to occur near your house.



References and Further Information
San Andreas Fault - by David Lynch
Hazard Mapping Images and Data - by the US Geological Survey (USGS)
United States Geological Survey
Fault Motion - by the Incorporated Research Institutions for Seismology (IRIS)
Plate Boundaries - by Maggi Glasscoe at NASA's JPL
Fold Mountain - by National Geographic
Earthquake Facts & Earthquake Fantasy - by USGS