Earthquake Faults Help
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)!
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