![]() The plates are rigid (or almost rigid) slabs of rock that comprise the crust and upper mantle of the Earth. The San Andreas Fault is a place where two tectonic plates touch, the North American and Pacific Plates. Indeed, almost none of the SAF's fault plane is vertical. When divergent, they usually open valleys on land and oceanic ridges like the Mid Atlantic Ridge.Īt plate boundaries, the fault plane is seldom vertical, i.e. When plate boundaries are convergent there is always a subduction zone. Convergent faults raise pressure ridges and mountain ranges. If the rocks move horizontally apart or together, they are called divergent or convergent, respectively. This means that if two people face each other across the fault and it moves, each person will see the other person move to the right. The SAF is a right lateral transform fault. If the motion is mostly horizontal and parallel to the fault plane, the fault is called a strike slip (or transform) fault. ![]() Dip slip faults with dips less than 45 degrees are called thrust faults. When the motion is predominantly vertical, they are called dip slip faults. Some faults are many miles long.įaults can be classified according to which of the three directions of space the rocks on either side move. Most faults are small - even microscopic - and are not important. But what is a fault? And what is the SAF?Ī fault is a planar crack in a rock along which slippage has taken place. And with many research institutions dedicated to studying such an accessible fault, the SAF has become a household name. ![]() Its notoriety comes partly from the disastrous 1906 San Francisco earthquake, but rather more importantly because it passes through California, a highly-populated state that is frequently in the news. The San Andreas Fault is the most famous fault in the world. Map of California showing the SAF, Natural Landmark plaque and the Field Guide to the San Andreas Fault. The San Andreas fault's nick name is " SAF". Tectonics Volcanoes Tsunamis California geology History Myths Organizations Links About Research One end of California may slowly drift so that it is eventually under water, but this can hardly be construed as "sinking into the ocean.Home Pictures Maps Google Map Visit the fault Earthquakes Be prepared ShakeOut! Feel it? Big One Instead, the pieces of land will move away from each other very slowly, taking millions of years to make large scale changes. The tension cannot build up to the point that one entire mass of land will shift many miles in relation to another one, so you will not see any sizable piece of land breaking away from another. But, even in a massive shift along the fault, the plates travel an incredibly short distance - a matter of feet in the most extreme shifts. If those plates are moving in different directions, it make sense that the two pieces of California will move in different directions too.Īnd this is indeed the case. After all, since the fault goes right through California, one part of the state is on the Pacific plate and one is on the North American plate. The notion that part of California will break off was likely inspired by the San Andreas fault. The Hayward Fault particularly concerns these scientists because it runs under heavily populated areas in and around Los Angeles. Many scientists estimate that there is enough tension built up along some locked California faults, that when they do finally slip, the earthquake will be extremely powerful. Then the pieces of earth suddenly "snap" into place, releasing a large amount of energy that causes earthquakes in the earth's crust. When this happens, tension builds up along the fault line until the force of movement is great enough to overcome the force of friction. If the force of friction exceeds the forces moving the earth, the two sides will become "locked," so they stop creeping. Smaller faults form in the crust material near the boundary line due to the forces of the plates pushing on each other.įriction builds up along faults because the two sides are pushed very tightly together. The San Andreas Fault in California is the piece that's on land. This boundary forms a fault line that extends under the ocean and on land along the west coast of the United States. The Pacific plate and the North American plate simply grind against each other - one creeps slowly northwest and one creeps southeast. At the boundaries between plates, a number of things can happen. If you've read How Earthquakes Work, then you know that the earth's surface is made up of large, rigid plates that slowly drift over the mantle layer below. Powerful earthquakes occur frequently along the west coast of the United States because the region is near a boundary between two tectonic plates.
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