Earthquake Faults Help (page 3)
The sudden slip that is an earthquake takes place after a gradual buildup of stress inside the Earth. Rocks, under enough pressure, reach a breaking point. Think of the weakest link idea, where in weakened places, increasing rock stress causes a break. As the stress in a specific area overwhelms forces holding the rocks together, the proverbial “straw that broke the camel’s back” takes place. Something cracks and an earthquake is set off.
A fault takes place along a fracture in a plane of the Earth’s crust where slip between the two sides has taken place.
A fault plane is used to represent an actual fault or a section of a fault. Faults are generally not perfectly flat, smooth planes, but they do give geologists a rough idea of the direction and orientation of a fault. The intersection of a fault plane with the Earth’s surface, along which a crack takes place, is called the fault line or surface trace . Fault lines are not always obvious on the surface. Figure 12-1 shows how faults change from a stable original position to a new post-fault position.
A fault trace’s trend is the direction it takes across the Earth’s surface. Trends are used to average out the small bends in a long fault and study its overall direction. This direction is something like a fault’s strike , but the two are different.
A fault strike is the line formed by the intersection of the fault plane with a horizontal plane. The direction of the strike is as angle off north. So when geologists talk about the direction of a certain strike they might call it a northwest-striking fault.
The forces that grind large masses of rock together cause plate margin stress and are unimaginable. That is the reason why the release of pent up seismic energy is equally powerful and damaging.
San Andreas Fault
California’s active and well-known San Andreas Fault was named in 1895, by geologist A.C. Lawson. The San Andreas Lake, about 32km south of San Francisco, is said to have given Lawson the idea for the name. However, the San Andreas Fault and its neighboring faults extend almost the entire length of California.
The San Andreas Fault is not truly a single, continuous fault, but a fault system made up of many parts. Plate movement is so common in California that earthquakes take place across the zone anywhere, anytime. The San Andreas fault system, over 1300km long, is also up to 16km deep so it is no wonder that something is happening all the time.
The average rate of motion across the San Andreas Fault Zone, during the past 3 million years is 56 mm per year. This is estimated to be about the same rate that fingernails grow. At this rate, geologists speculate that Los Angeles and San Francisco will be next to each other in about 15 million years. You will be able to live in San Francisco and go to school or work in Los Angeles in a few minutes (depending on traffic)!
Fault Size And Orientation
Faults can be found in any orientation and at any angle to horizontal (dip), but an active fault’s orientation and dip (angle) are not particularly random. These characteristics are affected by the regional stress created by tectonic movement.
Slips take place along some fault planes and angles easier than others. For example, if you are sitting on a flat table, you won’t usually slip off unless someone pushes you. However, if someone tips the table, you will slide off, because gravity is stronger than the frictional force keeping you in place. With earthquakes along fault lines, the driving force eventually overcomes the resistant force, causing slip.
Some faults don’t break through the surface anywhere along their length. They may have become covered by surface deposition of sediment, but some faults just never reach the surface. Faults may also spread into one or more folds under the surface. A fault that doesn’t reach the surface and has no surface trace is called a blind fault .
When a blind fault takes place at the surface and forms a chain of hills or a rounded scarp, it is known as a fold belt or fold bend .
A scarp is any roughly linear slope or rock face. A fault that causes a vertical offset will often create a scarp. While some scarps are formed by fault movement, they can also be created by erosion, or other means. When created by faults, scarps are good indicators of surface traces.
Active faults have a high potential for causing earthquakes. Inactive faults slipped at some point in time (causing earthquakes), but are now stuck solid. However, if the tectonic processes in an area change, it is possible for inactive faults to become reactivated.
Faults can measure from less than a meter to over a thousand kilometers in length, with corresponding widths. Large fault depths are limited by the thickness of that portion of the Earth’s crust and lithosphere. In southern California, this depth is roughly 15–25km. Most seismologists study faults that are at least a square kilometer in area, and often around a 100 square kilometers in area. Faults of this size or of a greater size crack violently enough to cause significant earthquakes. Geologists estimate there are roughly 200 faults in southern California, considered major faults, able to produce damaging earthquakes. Smaller southern California faults (thousands to millions) cause only small tremors. Large faults that only crack in a small section along their length cause minor earthquakes.
↑ fault slip area ↑ earthquake produced
It is important to remember that the size of a fault rupture is directly proportional to the size of the earthquake produced by the slip. Just remember, the bigger the fault slip area, the bigger the earthquake produced. Sometimes, however, when you look at the total surface area of a cracked fault, only a small part of the total area sees a slip. When a set of fractures is large and developed, it is known as a fault zone .
Often, you will hear geologists talk about hanging walls and footwalls. These terms refer to the orientation of a horizontal fault in relation to the Earth’s crustal position. When the crust is above the plane of the fault, it is called a hanging wall . It is said that miners could hang their lamps on the wall above in these fault areas. The footwall is the part of the Earth’s crust below a fault. Miners are said to have walked on the lower footwall side of a mined-out fault.
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.
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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