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# Earthquake Faults Help (page 3)

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By McGraw-Hill Professional
Updated on Sep 4, 2011

## Fault Dip

A fault’s dip or dip angle is given by two measurements: angle and direction. The direction is perpendicular to the direction of the strike of the fault plane. The dip angle is the angle of intersection between the fault plane and the horizontal plane.

When recording a dip, geologists might find that a dip slants sharply toward the southwest. If a specific dip angle is given, it is written as an angle off north or south, like “35° east of south,” instead of “55° south of east.” Because the dip of a plane is always perpendicular to its strike, the dip’s direction doesn’t have to be given if the strike’s direction is noted. Commonly, either dip or strike direction is given.

Finding the strike and dip of a fault plane at the surface can be tough when faults are bound by solid rock. It is more difficult to break a new fault surface through a solid rock than to crack through a previously broken fault. Surface layers of soil and loose sediments, however, are easy to break through. A fault can split through a new and different surface each time.

Another way to find fault strike and dip beneath the surface involves finding the hypocenters of naturally occurring earthquakes and cross plotting these locations. Figure 12-2 illustrates the different parts of a fault compared to the surface.

Fig. 12-2. A fault may or may not be seen on the ground’s surface.

## Slip Rate

Plate movement forces together or pulls apart the Earth’s crust. It causes stress-related dip–slip faults. These faults are found with horizontal and vertical offsets from perpendicular. Their movement permits the crust to thicken or thin in places, while expanding or compressing a rock.

A fault’s slip rate is the speed that one side of the fault slides in relation to the other. Slip rates are commonly measured in millimeters per year (mm/yr). For example, on the west coast of the United States slip rates range from 0 to roughly 40 mm/yr, though anything over 10 mm/yr is considered speedy. Slip rates of 1–2 mm/yr are thought to be normal for a major, active fault.

It is important to remember that slip rates are averages of total slip along a certain fault over time. They don’t slowly move past each other all year long, but instead slip suddenly to a new offset position (a few centimeters apart) in one swift movement.

Slip rates are calculated by using the formula for finding average speed.

Figuring out fault slip rates allows geologists to understand potential danger from faults located near populated areas. Slip rates also provide historical and regional information about faults and their activities over time.

Practice problems of this concept can be found at: Earthquake Practice Test

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