earthquakes physical geology 11/e, chapter 16. earthquakes and plate tectonics at divergent...

Download Earthquakes Physical Geology 11/e, Chapter 16. Earthquakes and Plate Tectonics At divergent boundaries, –tensional forces produce shallow-focus quakes

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  • Slide 1
  • Earthquakes Physical Geology 11/e, Chapter 16
  • Slide 2
  • Earthquakes and Plate Tectonics At divergent boundaries, tensional forces produce shallow-focus quakes on normal faults At transform boundaries, shear forces produce shallow-focus quakes along strike-slip faults At convergent boundaries, compression produces shallow- to deep-focus quakes along reverse faults
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  • World Earthquake Distribution Nearly all intermediate- and deep-focus earthquakes occur in Benioff zones inclined seismic activity associated with descending oceanic plate at subduction zones) Most earthquakes occur at < 100 km depth Earthquakes can occur 100-670 km below the surface
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  • Causes of Earthquakes Elastic rebound theory - earthquakes are a sudden release of strain progressively stored in rocks that bend until they finally break and move along a fault
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  • Seismic Waves Seismic waves originate at the focus (or hypocenter) - the point of initial breakage and movement along a fault Epicenter - point on Earths surface directly above the focus (used for 2-D map)
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  • Types of Seismic Waves Two types of seismic waves are produced during earthquakes Body waves - travel outward from the focus in all directions through Earths interior Surface waves - travel along Earths surface away from the epicenter
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  • Body Waves P wave - compressional (longitudinal or push-pull) body wave in which rock vibrates back and forth parallel to the direction of wave propagation Fast (4 to 7 kms/sec) wave that is the first or primary wave to arrive at recording station following earthquake Pass through solids and fluids P-P-P
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  • Body Waves S wave - shearing (transverse) body wave in which rock vibrates back and forth perpendicular to the direction of wave propagation Slower (2 to 5 km/sec) wave that is the secondary wave to arrive at recording station following earthquake Pass through solids only (cant travel through the outer core) S-S-S
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  • Surface Waves Slowest type of seismic waves produced by earthquakes Love waves - side-to-side motion of the ground surface Cant travel through fluids Rayleigh waves - ground to moves in an elliptical path opposite the direction of wave motion Extremely destructive to buildings
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  • Measuring Earthquakes Seismometers - used to measure seismic waves Seismographs - recording devices used to produce a permanent record of the motion detected by seismometers
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  • Locating Earthquakes P- and S-waves leave earthquake focus at the same time P-wave gets farther and farther ahead of the S-wave with distance and time from the earthquake
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  • Locating Earthquakes Plotting distances from 3 stations on a map, as circles with radii equaling the distance from the quake, locates earthquake epicenter Depth of focus beneath Earths surface can also be determined Shallow focus 0 - 70 km deep Intermediate focus 70 - 350 km deep Deep focus 350 - 670 km deep
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  • Size of earthquakes measured in two ways - intensity and magnitude Magnitude is a quantitative measure of the amount of energy released by an EQ Richter scale Logarithmic scale Amplitude increased 10 times for every one step up scale How much worse is a mag.8 compared to a 6 EQ? Measuring the Size of Earthquakes
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  • Moment magnitude - more objective measure of energy released by a major earthquake Uses rock strength, surface area of fault rupture, and amount of movement Smaller earthquakes are more common than larger ones Measuring the Size of Earthquakes
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  • Intensity - a measure of the effects an earthquake produces (on both structures and people) Modified Mercalli scale Qualitative Multiple values for one EQ Amount of shaking controlled by amount of energy released (magnitude), distance from the focus, soil type Useful for assessing historical EQs
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  • Earthquake Risk Large seismic risks or hazards exist around New Madrid, Missouri Seismic risk determined based on the assumption that large future earthquakes will occur where they have occurred in the past
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  • Effects of Earthquakes Liquefaction occurs when water-saturated soil or sediment sloshes like a liquid during a quake
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  • Tsunami Tsunami (seismic sea waves) - very large sea waves caused by sudden upward or downward movement of the sea floor during submarine earthquakes
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  • Earthquake Prediction and Seismic Risk Accurate and consistent short- term earthquake prediction not yet possible Three methods assist in determining probability that an earthquake will occur: Measurement of changes in rock properties due to buildup of strain magnetism, electrical resistivity, seismic velocity, porosity, etc. Studies of the slip rate along fault zones Paleoseismology studies
  • Slide 21
  • Earths Interior and Geophysical Properties Physical Geology 11/e, Chapter 17
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  • Evidence from Seismic Waves Seismic waves or vibrations from a large earthquake (or underground nuclear test) will pass through the entire Earth Affected by the properties of Earth materials, Density and state (solid or liquid) may result in changes in seismic wave velocity, reflection, refraction, or both reflection and refraction. Allow geologists to see into the Earth
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  • Evidence from Seismic Waves Seismic reflection - the return of some waves to the surface after bouncing off a rock layer boundary Sharp boundary between two materials of different densities will reflect waves
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  • Evidence from Seismic Waves Seismic refraction - bending of seismic waves as they pass from one material to another with different seismic velocities V2 > V1 = upward refraction V2 < V1 = downward refraction Seismic velocity (and density) of the mantle increases uniformly with depth Curved ray paths result because the wavefronts are continuously refracted
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  • Earths Internal Structure The crust, mantle and core, the three main layers within the Earth, have been determined based on seismic evidence.
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  • The Crust The crust is the outer layer of rock that forms a thin skin on Earths surface Seismic wave studies indicate that the crust is thinner and denser beneath the oceans than on the continents Seismic wave velocities are different in oceanic (7 km/sec) vs. continental (~6 km/sec) crustal rocks Indicate different compositions Oceanic crust is mafic Primarily of basalt and gabbro (basaltic and gabbroic) Continental crust is felsic Average composition is similar to granite and rhyolite (granitic and rhyolitic)
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  • Crust-Mantle Boundary The crust-mantle boundary, called the Mohorovii discontinuity or Moho, is marked by a jump in seismic wave velocity.
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  • The Mantle The mantle is a thick shell of dense rock that separates the crust above from the core below Seismic wave studies indicate the mantle, like the crust, is made of solid rock with only isolated pockets of magma Higher seismic wave velocity (8 km/sec) of mantle vs. crustal rocks indicate denser, ultramafic composition Crust + upper mantle = lithosphere, the brittle outer shell of the Earth that makes up the tectonic plates Lithosphere averages 70 km thick beneath oceans and 125-250 km thick beneath continents Beneath the lithosphere, seismic wave speeds abruptly decrease in a plastic low-velocity zone called the asthenosphere
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  • The Core-Mantle Boundary Core-mantle boundary (D layer) is marked by great changes in seismic velocity, density and temperature P-wave velocitites dramatically decrease Ultralow-velocity zone (ULVZ) Due to hot core melting lowermost mantle or reacting chemically to form iron silicate sediments
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  • The Core The core is the metallic central zone of the Earth Subdivided into a liquid outer core and solid inner core Seismic wave studies have provided primary evidence for existence and nature of Earths core
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  • The Outer Core Seismic shadow zones Specific areas on the opposite side of the Earth from large earthquakes do not receive seismic waves S-wave shadow zone (103 from epicenter) suggests outer core is liquid Liquids have no shear strength P-wave shadow zone (103-142 from epicenter) explained by refraction of waves encountering core-mantle boundary Decreased velocity causes wavefront to be refracted downward
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  • The Inner Core Careful observations of P-wave refraction patterns indicate that inner core is solid Core composition inferred from its calculated density, physical and electro-magnetic properties, and composition of meteorites Iron metal (liquid in outer core and solid in inner core) best fits observed properties Iron is the only metal common in meteorites
  • Slide 33
  • Isostasy Isostatic adjustment - rising or sinking of crustal blocks to achieve isostatic balance Crust will rise when large mass is rapidly removed from the surface, as at end of ice ages Rise of crust after ice sheet removal is called crustal rebound Rebound still occurring in northern Can


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