earthquakes earthquakes are vibrations in the earth. earthquakes are recurring phenomena, affecting...
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- Earthquakes Earthquakes are vibrations in the earth. Earthquakes are recurring phenomena, affecting areas repeatedly. Earthquakes are waves resulting from Slip-lock motion: the release of stored elastic energy from motion along faults; Implosions: sudden volume changes in subducting oceanic crust due to changes in PT conditions and resulting changes in mineral phase Other causes like explosions and asteroid impacts. v 0030 of 'Earthquakes' by Greg Pouch at 2011- 03-23 11:59:08 LastSavedBeforeThis 2011-03-23 11:54:51
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- Earthquakes Terminology 3 Vocabulary 4 Vocabulary Processes 5 Motion 6 Measuring Earthquakes > Instrumentation 7 Measuring Earthquakes> Location Distribution 8 Distribution and Causes > Proximate 9 Distribution and Causes>Plate tectonics 10 Distribution and Causes>Plate tectonics>Details 11 Magnitude 12 Magnitude Formulas 13 Magnitude Table with Energies Products 14 Destruction 15 Tsunamis 16 What to Do During an Earthquake 17 Prediction and Control
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- Vocabulary Earthquakes are measured using devices called seismometers (the sensor) or seismographs (sensor + recorder) on records called seismograms (paper or computer file). (Same endings as with telegrams) Seismic refers to vibrations of the ground. Seismology is a branch of geophysics dealing with earthquakes, their causes, and the planetary distribution of seismic velocity (whole-earth seismology) or using artificially-induced vibrations to explore the subsurface (exploration seismology). Foreshocks occur before a major earthquake, aftershocks occur after it. When it happens, you don't know whether an earthquake is a foreshock to something bigger or just a regular earthquake.
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- Vocabulary The focus is the point where an earthquake initiates, but since an earthquake is generated by an area slipping or volume imploding, distance from the focus is not the same as distance from the source, especially for large earthquakes. (Large earthquakes are large because a large segment of fault slips, so distance from the focus is not as important as it seems at first.) The epicenter is the point on the surface above the focus. The depth is the vertical distance from the focus to the epicenter, and is harder to determine: often, an initial guess is used, and depths of 5 or 10 or 100 km should be taken cum grano salis.
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- Motion An earthquake is a pulse of motion emanating from some source, be it fault tear, implosion, explosion, asteroid impact, footsteps At least for atomic bombs where we know the source's energy and the waves' energy, relatively little energy is radiated away as elastic waves (like 1 to 10%). Types of motion Body waves travel through the body of the material (1/R 2 fall-off: energy distributed on sphere) P-waves are compressional waves, like sound in air and are the fastest. S-waves are vibrations at right angles to the direction of propagation, like light, and are second fastest. Surface waves travel along an interface, as between air and ground, or loose materials and bedrock and cause most of the damage in earthquakes. (1/R fall-off: energy distributed on circle) Rayleigh waves travel along the rock-air interface, and cause the most damage and are like water waves Love waves are transverse and travel along solid-solid boundaries, like bedrock.
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- Measuring Earthquakes > Instrumentation Seismometers have a large mass loosely coupled to the ground (loose springs) and other parts tightly coupled to the ground, so that when the ground moves (and the loose mass doesn't), part of the instrument moves relative to the other; then there is a bunch of mechanical or electrical engineering wizardry to magnify this and make it easily detectable. What is recorded directly is usually amplitude of ground displacement (linear), and can often read to micro-meter or less: what is interesting from the whole-earth geophysics point is energy, from the civil engineering point, acceleration. Both can be derived from the displacement-time relationships.
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- Measuring Earthquakes> Location P-waves always travel faster than S-waves, and the delay depends on how far away they originated. The velocity depends on the material, and this is used to find the structure of the earth and for exploration. By knowing the distance from several fixed points to the earthquake (based on travel time), you can determine the location. (The Global Positioning System works just the opposite: by knowing the travel- time from several satellites, you can determine your location.) Knowing the direction of first motion also helps determine the type of fault motion.
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- Distribution and Causes > Proximate Elastic Rebound Theory Lock-Slip motion accounts for most near-surface earthquakes Plates move relative to each other. Under certain circumstances, this movement is accommodated by no motion for a while, during which energy is stored elastically. Eventually, the rock cannot store any more energy and slips. This slipping changes the stress field nearby and causes vibration, which can cause other nearby rock near its breaking point to slip. (Examples: string instruments, grating machinery, shovel and tree root.) Sometimes, the rock does not store elastic energy, and simply creeps along. This is known as fault creep. Phase changes More than one mineral can have the same chemical composition. These minerals (polymorphs) often have different PT stability, and physical properties. Of particular relevance, high-density phases are usually favored at high pressure (LeChatlier's principle). Sometimes this phase transition is gradual, occurring in small steps. Sometimes, it happens as dramatic implosions. Olivine (common in oceanic crust) becomes unstable and changes to a higher density form at the depths of deep-focus earthquakes, and this is the probable mechanism for many deep earthquakes. (olivine->spinel at 470km, spinel- >perovskite at 600km) Volcanoes and magma movement Explosions Asteroids
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- Distribution and Causes>Plate tectonics Most earthquakes are due to plate motion and concentrate at and define plate boundaries. At mid-ocean ridges, there are small, shallow quakes on normal faults. During rifting, there can be large, shallow quakes on normal faults. Subduction zones have Shallow quakes due to fracturing from bending (Think of a subduction zone as a really big thrust fault.) Deep quakes due to phase-change implosions Collision zones have shallow quakes on thrust faults and strike-slip faults, and perhaps secondary normal and reverse faults. Transform boundaries have strike-slip faults with shallow- to intermediate- focus quakes, often very severe. Volcanic eruptions and intrusions often cause small earthquakes. Plates do not behave entirely rigidly, and there are also intra-plate seismic zones, such as New Madrid, MO, which might be driven by loading- unloading or differences in thermal expansion or
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- Distribution and Causes>Plate tectonics>Details
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- Magnitude Magnitude refers to description of the size of an earthquake. (how big?) A single number for the magnitude does not describe an earthquake very fully, but it is standard. Mercalli Intensity: This scale assigns magnitude based on the effect on structures, not the energy. Richter Scale magnitude is based on the displacement amplitude on a Wood-Anderson torsion seismometer which Richer owned. Richter magnitude can be converted to energy (see next slide) and measures seismic energy (the radiated waves) The Richter scale is logarithmic, meaning that for each increase of 1 on the scale, the property increases by a multiplicative factor. For the Richter scale, the step size for energy is SQRT(1000) ~ 32X, so a magnitude 6 earthquake has ~32X as much energy as a magnitude 5, a magnitude 7 has 1000X as much as a magnitude 5. An increase of Richter Magnitude by 0.2 means a doubling of energy (Ignore the book: the authors are confused about logarithms, and seem kind of weak on physics as a whole.) A Richter magnitude 6 earthquake has the energy equivalent to the Bikini atoll hydrogen bomb, a Richter 9 earthquake has radiated 475 megatons_TNT of energy as "elastic" waves. Both the Mercalli and Richter scale have modified versions available, and newscasters tend to not tell what exactly they're reporting.
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- Magnitude Formulas (PowerPoint doesn't deal well with superscripts and subscripts, so I'm using X_Y where I'd like to use X Y ) See http://earthquake.usgs.gov/learn/faq/?categoryID=2&faqID=33http://earthquake.usgs.gov/learn/faq/?categoryID=2&faqID=33 To compare seismic energy E in two earthquakes A and B of Magnitude M_A and M_B E_A/E_B=10 ( 1.5*(M_A-M_B ) ) =10^( 1.5*(M_A-M_B ) ) To convert an earthquake magnitude M_S (Ricter, or Moment Magnitude, which is the USGS standard way of reporting) to energy E_S in Joules E_S=10 (4.8+1.5*M_S) =10^(4.8+1.5*M_S) To convert energy in Joules to "Tons of TNT", divide by the USGS's conversion factor of "One ton of TNT has an energy of 4.2x10 9 =4.2E+09 Joules." Divide that by 1,000 for kilotons, or 1,000,000 for megatons, or 1e9 for gigatons. A magnitude 9 earthquake has an energy of 475 megatons_TNT as waves.
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- M_RichterEnergyInJoulesTonsTNTTons_TNT -3.02.0E+004.8E-10 -2.51.1E+012.7E-09 -2.06.3E+011.5E-08 -1.53.5E+028.4E-08 2.0E+034.8E-07 -0.51.1E+042.7E-06 0.06.3E+041.5E-05 0.53.5E+058.4E-05 1.02.0E+064.8E-04 1.51.1E+072.7E-030.003 2.06.3E+071.5E-020.015 2.53.5E+088.4E-020.084 3.02.0E+094.8E-010.475 3.51.1E+102.7E+002.671 4.06.3E+1
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