1EarthquakesChapter 19Earthquakes•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’sInteriorEarthquake 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 (underground) of fracture of the earthquake.•The epicenteris the point on the surface directly above the focus. Earthquake Fracture2Earthquake FractureSeismic 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 EarthBody 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 WavesSeismometers•Aseismometer is a device for measuring seismic waves from earthquakes.•A seismographis a recording seismometer.•A seismometeris just a heavy weight suspended on springs with devices to measure the movement of the ground relative to the weight.3SeismometersSeismometers•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 670km4Epicenters: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 250-670km.•Intermediate and deep earthquakes occur only at subduction zones.•No earthquake foci at depth below 670 km.Convection in solid mantleShallow and Deep EarthquakesVelocity 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)5Earthquake Terms•Earthquake•Seismic Wave•Focus•Epicenter•Body wave–P-wave–S-wave•Surface wave•Seismometer•Seismograph•Richter Scale•Subduction
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