Reviewing the Basics
People use a lot of words when they talk about earthquakes. Magnitude, intensity, epicenter, fault, liquefaction. What do these terms mean? This section describes how earthquakes happen, how they are measured, and what the shaking feels like.
What is an Earthquake?
An earthquake is caused by sudden slip on a fault, much like what happens when you snap your fingers. Before the snap, you push your fingers together and sideways. Because you are pushing them together, friction keeps them from moving to the side. When you push sideways hard enough to overcome friction, your fingers move suddenly and you hear a snap.
A similar process goes on in an earthquake. Forces within the Earth and the weight of the Earth's crust push the two sides of a fault together. The friction across the surface of the fault holds the rocks together so they do not slip immediately when pushed sideways. Eventually enough stress builds up and the rocks slip suddenly, releasing energy in waves that travel through the Earth and causing the shaking that we feel from an earthquake.
Just as you snap your fingers with the area between your finger tip and thumb, earthquakes happen over the area of a fault, called the rupture surface. In a small earthquake, the rupture surface may be only a small part of the entire fault, whereas during a large earthquake, the entire fault may slip. The whole fault plane does not slip at once, however. The rupture begins at a point on the fault plane called the hypocenter, which is usually deep down on the fault (see image above). The epicenter is the point on the surface directly above the hypocenter. The earthquake rupture keeps spreading until something stops it (exactly how this happens is one of the hot research topics in seismology).
What is a Fault?
Earthquakes occur on faults. A fault is a break in the Earth's crust having relative movement of the two sides. Faults can be any length, from inches to hundreds of miles. When an earthquake occurs on one of these faults, the rock on one side of the fault rapidly slips past the other. The fault surface can be vertical, horizontal, or at some angle to the Earth's surface.
Although many of the larger earthquakes rupture faults all the way to the Earth's surface, some faults do not intersect the surface. Some of these are termed blind faults because they cannot be easily identified at the surface. The ground above blind faults bends rather than breaks, and these faults may only be expressed at the surface as a warp or fold that erosion reduces to hills or a broad range front. Nevertheless, large earthquakes along blind faults can be quite destructive. The 1994 magnitude 6.7 Northridge earthquake in California occurred along a blind fault. Some faults can also be rapidly buried by sediment and be difficult to detect.
Movements on a Fault
The slip on a fault can be in any direction. We classify faults into two basic categories: strike-slip and dip-slip motion.
Strike-slip earthquakes commonly occur along vertical fault plane as one side of the fault slides horizontally past the other. If the far side of the fault shifts to the right it is termed a right-lateral fault; if it shifts to the left, it is left-lateral fault. With dip-slip earthquakes, the fault is usually at an angle with the earth's surface and the movement is up or down. For faults that make an angle with the Earth's surface, names are given for the two sides of the fault. The top side of the fault is known as the hanging wall and the bottom side is the footwall. The walls are so named by their appearance in mines: miners hung their lanterns on the hanging wall and walked on the footwall.
On a normal-slip fault, the hanging wall moves down and the footwall up; on a reverse-slip fault, the reverse happens - the hanging wall moves up and the footwall moves down. A common feature along normal-slip faults is for the hanging wall to collapse and create a secondary fault that dips into the main fault. The area downdropped between these two faults is called a graben. If a fault has both strike-slip and dip-slip movement, it is known as an oblique-slip fault.
In Nevada, a majority of the faults are normal-slip faults. During an earthquake, one side of the fault drops abruptly. The side that falls down is commonly a valley and the side that remains or is uplifted slightly is a highland or mountain. This is how much of Nevada?s picturesque mountain and valley landscape was formed.
Strike-slip faults are also very important in the state, especially in western Nevada where a majority of the earthquakes are strike-slip in nature. Small buried, blind, and undetected faults occur throughout Nevada, the most worrisome being those within basins where communities are located.
Further Reading:Gere and Shah (1984) Terra Non Firma
Association of Engineering Geologists (1998) Suggested Guidelines for Evaluating Potential Surface Fault Rupture
Tarbuck and Lutgens (1999) Earth
Yeats and others (1997) Geology of Earthquakes (technical)
Ground Rupture From Earthquakes
Earthquakes larger than magnitude 6.5 are so big that they tend to rupture the ground surface. The length and size of this surface rupture is proportionately larger with increasing magnitude. The surface rupture is usually not a clean single break, but occurs as a larger rupture surrounded by smaller breaks within about 50 feet (~15 m) of it. The surface offset usually reflects what occurs at depth. For example, a normal-slip earthquake might leave a small escarpment in the surface over the place where the hanging wall drops. Fortunately, earthquakes rarely cause features at the surface that might be characterized as gaping holes that swallow people and cows alike. A person caught out in the open next to surface faulting is likely in for an experience-of-a-lifetime, but is unlikely to be swallowed by the fault. Buildings across surface faulting fare less well. Few buildings are strong enough to resist faulting through the structure. Those across ruptures have been ripped apart or horribly distorted. It is considered good practice to avoid building across faults that might move from earthquakes, thus avoiding this potential hazard. Setting back 50 feet (~15 m) from a fault helps avoid the smaller breaks that might occur.
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