[2]. ): Numerous factors have been proposed as possible precursory signals that an earthquake is imminent including changes in low magnitude seismic activity in the weeks and months preceding a major quake, changes to groundwater levels, radon and other gases in groundwater wells, changes in the electrical resistivity of the crust (related to changes in groundwater distribution in rock as it begins forming microcracks immediately prior to an earthquake, changes in seismic wave velocity in the crust surrounding a fault that is beginning to fail prior to an earthquake, even strange behavior by animals just prior to a major earthquake. Moment magnitude is also recorded on a logarithmic scale on which an increase of one represents a 32 times increase in the stress energy released. Surface Waves - travel along the Earth's surface. The ground shakes as the stress energy is released and the rocks lurch to their new position in a matter of seconds. The Mercalli intensity of historic earthquakes can then be compared to the Mercalli intensity of modern earthquakes that have Richter and/or moment magnitudes. Damage Factors: - Loose, unconsolidated sediments, and especially saturated sediments experience stronger ground motions in an earthquake as compared to solid bedrock. They are calculated after determining the fault throw (distance the fault moved), the area broken on the fault (depth and length), and the rigidity of the fault rocks (the springiness of the rocks). Water lines also break making it difficult to fight the fires. This is known as the S-P interval. Earthquake Prediction (? Strings, chains, and cables are rigid under tension but may collapse under compression. An earthquake is the shaking and vibration of the Earth's crust due to movement of the Earth's plates (plate tectonics). distance: P and S waves travel at known velocities through the Earth. Earthquakes . The magnitude of an Earthquake is measured on the well-known Richter Scale. Some will record an initial "up" (compression) on the seismogram indicating that the crust moved toward that direction and others will record an initial "down" (dilation) indicating that the crust moved away from that direction. - Strong earthquakes that offset the seafloor produce tsunamis, which are not normally discernible from a ship at sea with their very long wavelengths, but as theses very rapidly moving waves come into shallow water approaching the shoreline they slow greatly, getting steeper and much higher innundating low-lying coasts. Seismic waves travel outward from the portion of the fault that broke, like expanding ripples from a pebble dropped in still water. The type of fault motion causing an earthquake can be determined by analyzing the first sense of motion (compression or dilation) recorded on seismograms in all directions and various distances from the earthquake. Epicenter: the point on Earth’s surface directly above the focus of an earthquake. Corrections? Then, sudden slippage of rock along a fault occurs. Those stresses (compression, tension, shear) build up in the crust until the stress exceeds the strength of the rock or the friction along a preexisting fault. http://physics.taskermilward.org.uk/KS5/html/waves.htm, http://hyperphysics.phy-astr.gsu.edu/hbase/waves/seismic.html, https://wiki.seg.org/index.php?title=Compressional_wave&oldid=38387, Problems in Exploration Seismology & their Solutions, the Creative Commons Attribution-ShareAlike 3.0 Unported License (CC-BY-SA). The Richter magnitude is recorded on a logarithmic scale on which an increase of one represents a ten times increase in amplitude. moment magnitude: Moment magnitudes measure the amount of stress energy released in an earthquake. In the latter, compression is measured against the system’s volume at the standard pressure to which an organism is subjected—e.g., the pressure of the atmosphere at sea level is the standard, or reference, for most land animals, but the standard for deep-sea fishes and similar specialized forms is the normal pressure of their environment. When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and cause the shaking that we feel.. The Richter magnitude of an earthquake can be easily determined from the S-P interval combined with the maximum motion recorded on the seismogram. Compressional waves are also known as a longitudinal waves because of the way in which they travel through a medium. To determine the Richter magnitude of an earthquake, seismologists need to know the distance to the earthquake and the amplitude of the surface wave at the recording site. Circles with the appropriate radius are then drawn around each station. Air can be viewed as being composed of layers analogous to such coils, with a sound wave propagating as layers of air “push” and “pull” at one another much like…. See more. A mass of rock formed when a large body of magma cools inside the crust . Mercalli intensity scale: In order to better understand the long term behavior of a given fault it is necessary to go back to written records of earthquakes that occurred before seismic recording began. Once determined from a seismograph record, the S-P interval and amplitude are used to mathematically solve for the magnitude, or they may be plotted on a graph called a nomogram to yield a visual solution for the magnitude. Blind Thrust Fault Animation. This type of faulting is common in areas of compression, such as regions where one plate is being subducted under another as in Japan. While the frequency of earthquakes is much greater in areas around tectonic plate boundaries, where the stresses build up quickly, there are locales in the middle of plates, far from active faulting where stresses nevertheless build up on ancient faults potentially leading to major earthquakes (for example, the New Madrid fault zone on the Mississippi River).
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