TEST THREE OUTLINES

TEST THREE OUTLINES - 6 - Earthquakes 10/25/07 We learn...

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Unformatted text preview: 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 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 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) 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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TEST THREE OUTLINES - 6 - Earthquakes 10/25/07 We learn...

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