Geography 1111 Lecture 25 Notes
Geography 1111 Lecture 25 Notes GEOG 1111
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This 4 page Class Notes was uploaded by Bridget Notetaker on Wednesday October 12, 2016. The Class Notes belongs to GEOG 1111 at University of Georgia taught by Hopkins in Fall 2015. Since its upload, it has received 17 views. For similar materials see Intro to Physical Geography in Geography at University of Georgia.
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Date Created: 10/12/16
Geography 1111 Lecture 25 Notes Earthquakes: a sharp release of energy resulting in a series of elastic waves propagated through the Earth at the moment of rupture along a fault o The rupture is initiated where stress along the fault exceeds the elastic limit of the rock so that sudden movement occurs They are also associated with volcanic activity Seismic waves: the pulses of energy generated by an earthquake that pass through the Earth as shock waves o Transmission speed and direction can vary according to the temperature and density of the various layers within the planet o Types of seismic waves: P wave (primary seismic wave) is a type of seismic wave, propagated like a sound wave, of short wavelength and high frequency in which the material involved in the wave motion is alternately compressed and expanded It is thus a compression or “push” wave which moves material parallel to its direction of movement P waves can pass through all internal layers of the Earth S wave (secondary seismic wave) is a seismic wave of short wavelength and high frequency in which particles of the material vibrate at right angles to the direction in which the wave travels It is thus a shear or “shake” wave which moves material at right angles to its direction of movement S waves cannot pass through the Outer or Inner core layers The P and S waves are generated at the focus of an earthquake L wave (surface wave) is a seismic wave of long wavelength and low frequency that travels along Earth’s surface, but not through the Earth, and at slower velocities than either P or S waves L waves spread out from the epicenter of an earthquake Some basic terms: o Focus: the place of origin of an earthquake and the point from which the P and S waves originate It may be at or near the surface, or deep within the crust or upper mantle o Epicenter: the point on the Earth’s surface directly above the focus of an earthquake and the point from which the L waves spread out (originate) o Fault: is a fracture in crustal rock involving the displacement (movement) of rock on one side of the fracture in respect to the rock on the other side o Fault plane: the surface of contact along which blocks on either side of a fault move o Fault scarp: the exposed cliff-like face of a fault plane formed by faulting with a vertical displacement component How are earthquakes measured? Magnitude vs. Intensity o Magnitude: measures the total amount of energy released by an earthquake by the amount of shaking of the ground during the event as measured by a seismograph or seismometer; it is a quantitative measurement Seismograph or seismometer is an instrument that measures seismic waves and earth vibrations It is measured on the Moment Magnitude Scale (formerly the Richter Scale), and considers the amount of fault slippage, the size of the area that ruptured, and the nature of the materials that faulted in estimating the magnitude of an earthquake o Intensity is a subjective measure of the size and damage of an earthquake as equated by its impact on the human landscape (structures and activities) It is measured using the Modified Mercalli Scale, and is a qualitative measurement Some major quakes: 1812 New Madrid, MO 1886 Charleston, SC (6.7) 1906 San Francisco, Ca (8.25) 1964 Alaska (8.6) 1976 Tangshan, China (7.6) 1989 Loma Prieta, Ca (7.1) 1994 Northridge, Ca (6.7) 2002 Denali, Alaska (7.9) 2004 Sumatra, Indonesia (9.3) 2010 Chile (8.8) 2010 Haiti (7.0) 2011 Honshu, Japan (9.0) Distribution: o Earthquakes may be classified as shallow (<63 mi depth), medium (63-150 mi depth), and deep (>150 mi depth) Some quakes have been recorded as deep as 400 mi below the surface o Circum-Pacific Belt is associated with the subduction zones around the Pacific Ocean 80% of shallow earthquakes occur in this belt and the vast majority of deep earthquakes occur at subduction zones o Trans-Eurasian Belt spreads from the Mediterranean Sea eastward through SW Asia, the Himalayas, to SE Asia o Mid-oceanic ridges and other divergent plate boundary areas o Intraplate earthquakes which are found not at plate boundaries This includes those which occur in Georgia, Missouri and other areas of the U.S. away from the West Coast These may be at continental hot spots like Yellowstone National Park Types of Faults: o Tensional or Normal Faults: are those which involve the stretching of crustal rock The two pieces or blocks of crustal material are being pulled apart which causes lengthening of the crust Normal faulting occurs along a steeply inclined fault plane in which one block, the hanging wall, has moved downward in relation to the opposite block, the footwall This movement results in the fault scarp of the hanging wall block not overhanging the footwall Hanging wall: is the block or surface of rock that lies above an inclined fault plane Footwall: is the block or surface of rock beneath the dipping fault plane This type of movement is analogous to what is seen at a divergent plate boundary o Compressional or Reverse Faults: are those which involve the compression of crustal rock The two pieces or blocks of crustal material are being pushed together which causes shortening of the crust Reverse faulting occurs along a steeply inclined fault plane in which the hanging wall has moved upward in relation to the footwall and thus overrides it along the fault plane This movement results in the fault scarp of the hanging wall block overhanging the footwall Thrust fault is a compressional (reverse) fault with a very low angle to the fault plane This type of movement is analogous to what is seen at a convergent plate boundary Echelon faults are a series of nearly parallel faults, either normal, reverse or a combination which forms a landscape known as horst and graben topography Horst: is the elongate fault block that has been uplifted in relation to the blocks on either side Graben: is the elongate fault block that has been lowered in relation to the blocks on either side o Shear or Strike-slip Faults: Strike-slip faulting involves movement that has occurred parallel to the strike of the fault plane In true strike-slip faulting this no vertical displacement only horizontal displacement The movement may be described as right-lateral or left-lateral This type of movement is analogous to what is seen at a transform plate boundary Tsunami is a Japanese term referring to a seismic sea wave which is the energy passing through the water, generated by an earthquake whose epicenter is on the sea floor These may travel at speeds up to 630 mph and form crested waves in shallow water as high as 200', sometimes thousands of miles from the epicenter Ex: Alaskan quakes of 1946 and 1964 caused tsunamis in Hilo, Hawaii with 100 ft. waves; the Indonesian quake of 2004 and the Japanese (Honshu) earthquake of 2011 also formed tsunamis
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