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TEST THREE OUTLINES

TEST THREE OUTLINES - 6Earthquakes ,onghastlydiagramsof...

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6 - Earthquakes 10/25/07 “We learn geology in the morning after the earthquake, on ghastly diagrams of  cloven mountains, upheaven plains, and the dry bed of the sea” – Ralph Waldo  Emerson  USGS estimates about 1 million quakes annually Most earthquakes are concentrated along plate boundaries Stress and Strain Stress: force exerted per unit area Strain: deformation caused by stress  o compressive   inward o tensile   opposite directions o shear   two different directions = tumbling Rock Strength: ability to withstand stress before rupture  EARTHQUAKE: GROUND-SHAKING CASUED BY THE SUDDEN RELEASE  OF STRAIN ENERGY CAUSED BY ROCK RUPTURE (FAULTING)   All  earthquakes occur on faults! Earthquake Cycles Drop in strain after event, takes time to rebuild Four Stages: (cycle can take hundreds to thousands of years) o Inactivity o Increase seismicity, accumulated strain o Foreshocks
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o Major earthquake, aftershocks  Earthquake Focus and Epicenter Seismic waves propagate outwards from the focus  Epicenter, point on surface, directly above the focus of the earthquake Fault plain = As rupture grows bigger and grows up, it releases energy of  strain and releases it in all directions in form of seismic waves Seismic Waves P-Waves o Travels through all material o Faster in dense material S-Wave o Travels only through solids   dissipates in gases or liquids  o ½  the speed of P-Wave Surface Waves o Seismic waves that intersect with the surface o Travel much slower o Can cause more damage to buildings  Frequency As waves move out from the focus, high frequencies attenuated (lose their  power and disappear fast)
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High F = low building vibrates Low F = high building vibrates  Measuring Earthquakes Seismograph or seismometer Amplitude of seismic waves: ground vibration First arrival of seismic waves: o Time of earthquake o Distance to epicenter  Wave Speeds   As distance to the focus increases, so does the time gap  between the arrival of the first P-Wave and the first S-Wave An earthquakes occurs 500 km. from a seismometer.  The fist P-Wave arrives  after 70 seconds, the first S-Wave arrives in 150 seconds. Locating Epicenter Locating an epicenter depends on the different velocities of P and S Waves Compare P and S wave arrival times from a minimum of 3 seismic stations A circle with a radius = distance to the epicenter is drawn around each station Triangulation!
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