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Every so often, in different regions of the world, the Earth feels the need to release energy in the form of seismic waves. These waves cause a great deal of hazards as the energy is transferred through the tectonic plates and into the Earth’s crust. For those living in an area directly above where two tectonic plates meet, the experience can be quite harrowing! This area is known as a fault, or a fracture or discontinuity in a volume of rock, across which there is significant displacement. Along the line where the Earth and the fault plane meet, is what is known as a Fault Line.
Since faults do not usually consist of a single, clean fracture, geologists use the term fault zone when referring to the zone of complex deformation associated with the fault plane. The two sides of a non-vertical fault are known as the hanging wall and footwall. By definition, the hanging wall occurs above the fault and the footwall occurs below the fault. This terminology comes from mining: when working a tabular ore body, the miner stood with the footwall under his feet and with the hanging wall hanging above him.
When it comes to fault lines, three factors play a big role. These are known as the slip, heave and throw. Slip is the relative movement of geological features present on either side of the fault plane; in other words, the relative motion of the rock on each side of the fault with respect to the other side. Heave is used to measure the horizontal/vertical separation, while throw is used to measure the horizontal separation. Slip is the most important characteristic, in that it helps geologists to classify faults.
There are three categories or fault types. The first is what is known as a dip-slip fault, where the relative movement (or slip) is almost vertical. A perfect example of this is the San Andreas fault, which was responsible for the massive 1906 San Francisco Earthquake. Second, there are strike-slip faults, in which case the slip is approximately horizontal. These are generally found in mid-ocean ridges, such as the Mid-Atlantic Ridge, a 16000 km long submerged mountain chain occupying the center of the Atlantic Ocean. Lastly, there are oblique-slip faults which is a combination of the previous two, in that both vertical and horizontal slip occur. Nearly all faults will have some component of both dip-slip and strike-slip, so defining a fault as oblique requires both dip and strike components to be measurable and significant.
We have written many articles about the earthquake for Universe Today. Here’s an article about the San Andreas Fault, and here’s an article about fault lines.
We’ve also recorded related episodes of Astronomy Cast about Plate Tectonics. Listen here, Episode 142: Plate Tectonics.