Chapter 19 Earthquakes Earthquakes An earthquake is a trembling or shaking of the ground caused by a sudden release of energy stored in the rocks beneath the Earth s surface Earth s Interior Earthquake Energy The stresses that cause earthquakes are applied by tectonic forces ultimately derived from convection in the Earth s interior driven by decay of naturally radioactive elements principally U Th and K The stresses cause elastic strain of the rocks of the lithosphere The elastic strain can be released by brittle fracture of the rocks The released kinetic energy radiates as seismic waves Earthquake Fracture The focus is the point The epicenter is the point on underground of fracture of the earthquake the surface directly above the focus Earthquake Fracture Earthquake Fracture Seismic waves are classified by their propagation mechanism Body waves travel through the Earth P waves are primary and fast S waves are secondary and slower Surface waves travel on the Earth s surface Surface waves are slower than either P or S waves but may have large amplitudes near the epicenter Body Waves P and S waves travel through the Earth Body Waves P waves propagate by compression like sound waves P waves travel through solids liquids or gasses S waves are secondary or shear waves and propagate by shear of a solid rock medium S waves only travel through solids Surface Waves Seismometers A seismometer is a device for measuring seismic waves from earthquakes A seismograph is a recording seismometer A seismometer is just a heavy weight suspended on springs with devices to measure the movement of the ground relative to the weight Seismometers Seismometers P and S waves travel at different velocities in the Earth The time difference between the P and S arrivals can be used to measure the distance to the earthquake Three seismic stations can then be used to determine the location of the earthquake Richter Scale Open ended scale no Maximum Logarithmic base 10 measure of ground motion at epicenter Magnitude 6 quake gives 10 time more ground motion than a Magnitude 5 Scale is roughly base 30 logarithmic in total energy Epicenters Where do Earthquakes Occur At Plate Boundaries In the uppermost 100 km Occasionally they occur elsewhere Occasionally as deep as 670km Epicenters Where do Earthquakes Occur Depths of Earthquakes Most earthquake foci are shallow 100km Intermediate depth quakes occur at depth of 100 250km Deep focus earthquakes occur at 250670km Intermediate and deep earthquakes occur only at subduction zones No earthquake foci at depth below 670 km Convection in solid mantle Shallow and Deep Earthquakes Velocity Structure and Earthquakes at N Tonga Vander Hilst Nature 1995 Predicting Earthquakes Stresses build over years or centuries Fracture initiation occurs in a few seconds Few if any precursors The best we can do is identify active zones Build for earthquakes in active zones Building codes Motion sensors Stop trains and shut off gas Earthquake Terms Earthquake Seismic Wave Focus Epicenter Body wave P wave S wave Surface wave Seismometer Seismograph Richter Scale Subduction Zone
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