|MadSci Network: Earth Sciences|
Dear Zebulon, The short answer to your question is that earthquakes occur when movement suddenly takes place along lines of weakness in the Earth’s crust called faults. Because the crust is composed of rocks that are cold and brittle, they behave rather like a piece of brittle toffee when you try to bend it: at first the toffee does not break because it is able to withstand some of the stress you place on it, but if you use enough force it will suddenly break, and the line of breakage is, in effect, a fault. Strain is the response of rocks to the stress placed upon them; an earthquake takes place when the strain in rocks in a fault exceeds a critical limit and the rocks in the fault break suddenly. The greatest source of stress placed on rocks in the crust arises from the process of plate tectonics. The crust is composed of a number of large slabs, called tectonic plates, which slowly move in response to forces applied from the mantle below. As a crustal plate moves it exerts force along its boundary with neighbouring plates, which are also moving. So it is no surprise that most earthquakes take place along the boundaries of the tectonic plates because these boundaries are the regions where rocks are being subjected to great and constant stress. There are three main types of plate boundary. The type that is most familiar is called a transcurrent boundary, where two plates are sliding past each other. Transcurrent boundaries give rise to transcurrent faults, also called strike-slip faults. The prime example is the San Andreas Fault in California and Mexico. Here the Pacific and North American plates are moving past one another, with the Pacific Plate moving north. Along many parts of the San Andreas Fault there is a constant, small amount of movement or creep, perhaps a few centimeters per year, that continually relieves strain and prevents it from building up to a critical level. In other areas, however, the San Andreas Fault tends to lock, and the strain will build up over decades or even centuries in these areas until the rocks suddenly break. Then this area of the fault moves violently, and the force released radiates out through the crust as a series of waves which we experience as an earthquake. A second type of plate boundary where many earthquakes occur is along constructive plate boundaries, where new ocean crust is being formed as two plates move apart from each other. In this type of setting the crustal forces are extensional, with the crust being pulled apart, and the resulting faults are called normal faults. Most of these constructive plate boundaries are located on the deep ocean floors along mid-ocean ridges, and although they actually give rise to numerous earthquakes most of them occur at sea, where they tend to go unnoticed except to be recorded by seismographic stations. Sometimes, however, crustal extension happens on the continents, creating rift zones such as the East African Rift, in which huge crustal blocks are downfaulted; movement on such large normal faults will cause earthquakes. The third type of plate boundary which gives rise to earthquakes is along subduction zones, where one plate dives beneath another and plunges back into the Earth’s mantle. Although geologists know where these major subduction zones are located, the faults associated with them tend to be at depth in the crust and may not appear on the surface in the obvious way that the San Andreas Fault does. The movement in this case is compressional, where one plate descends beneath another. These faults along subduction zones can give rise to enormously powerful and destructive earthquakes. In North America such a region occurs along the Pacific coast from Oregon to British Columbia, where a small crustal plate called the Juan De Fuca Plate is being subducted beneath North America along the Cascadia Subduction Zone. Earthquakes here are much less frequent than along the San Andreas Fault to the south, but are larger and more powerful when they do occur. Again, it is the tendency of rocks in a subduction-related fault zone to lock, allowing strain to build up until it is suddenly released, that gives rise to earthquakes. Although most large earthquakes take place along plate boundaries as described in the preceding paragraphs, there are two other settings where earthquakes occur. Earthquakes can occur deep within tectonic plates (intraplate earthquakes), far away from boundary regions. The strain caused in tectonic plates as they move against each other is not only released along the plate boundaries, but is transmitted through the plates into their interiors. These strains can build up over very long periods of time, centuries or thousands of years, only to be released in huge but very rare earthquakes in the interiors of plates where one might not expect them. Such earthquakes are often found subsequently to be associated with extensional faulting deep in the crust, where continental rifting may be in its early stages. A prime example is the New Madrid Fault, which gave rise in 1811 and 1812 to a series of huge earthquakes in the region of New Madrid, Missouri. In 1886 Charleston, South Carolina was struck by a major intraplate earthquake. Smaller earthquakes can also occur on old faults in plate interiors. The British Isles, which are located well away from any plate boundary, experience minor earthquakes of this sort from time to time. One occurred in 1990 in the county of Shropshire, which was felt as far away as London about 150 miles away. Earthquakes are also associated with volcanic activity. It is very common for earthquakes centred on a volcano to provide advance warning of an impending eruption, perhaps months or a few years in advance. As magma rises through the plumbing system of a volcano it forces its way up by exploiting lines of weakness in the crust, opening up cracks and fissures, and this frequently gives rise to swarms of earthquakes. Some of these can be moderately strong. It was, for example, one such earthquake that triggered the explosive eruption of Mount St. Helens in 1980, because the earthquake caused the mountainside to give way, releasing the pressure on the magma chamber inside the volcano. The cause of earthquakes is a very large subject, and I have not done more than scratch the surface. But I hope my answer has sketched out the main scenarios for you. Best wishes, David Scarboro
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