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Constructive Forces Part III: Earthquakes Shake, Rattle and Roll Earthquakes A. Stress Earthquake - shaking of Earth’s crust and rigid mantle caused by a release of energy 1. Causes - volcanic eruptions - Collapse of a large cavern - Meteorite.asteroid impact - Large, underwater landslides Earthquakes (contd.) 2. Elastic Rebound Theory - all materials can be stretched to a certain point without breaking elastic limit - States that earthquakes occur when stress (pressure) overcomes the force of friction between two sections of crust - A sudden release of energy and movement occurs Earthquakes (contd.) - after the movement, the pieces of crust snap back, or rebound close to their original shape Earthquakes (contd.) 3. Depth of an Earthquake Focus - point underground where an earthquake originates Epicenter - point on Earth’s surface directly above the focus - earthquake (seismic) waves travel in all directions from the focus, like ripples in a pond Earthquakes (contd.) B. Earthquake Waves - 2 main types 1. Body Waves - seismic waves that travel through Earth’s crust a. P-wave (Primary Wave) - compression (back and forth wave) - 1st wave to arrive at a seismic station - - can travel through solids, liquids, and gases Earthquakes (contd.) b. S-waves (Secondary Waves) - transverse (up and down) waves - Arrives 2nd at a seismic station - Cannot travel through liquids - Used as evidence of Earth’s liquid outer core Earthquakes (contd.) 2. Surface Waves created when p- and s-waves reach the surface a. L-waves (Love Waves) - waves move side to side - - movement of the crust is perpendicular to the direction the wave travels in Earthquakes (contd.) b. Rayleigh waves - waves travel in an elliptical a pattern, like the ripples in a calm bod y of water created when you toss a stone in the water Earthquakes (contd.) C. Locating and Measuring Earthquakes 1. Seismographs - an instrument that detects and measures seismic waves 2. Seismograms - physical record of seismic waves taken from a seismograph - the first “jump” on a seismogram indicates the arrival of a p-wave Earthquakes (contd.) 3. Determining the Distance to an Epicenter - use travel time graph (ESRT p. 10) - Shows relationship between the distance seismic waves travel and how fast they travel Earthquakes (contd.) - As the distance between a seismic station and the epicenter increase, the difference in arrival times between the two waves increases as well - if you know the difference in arrival times between the pand s-waves, you can determine the distance to the epicenter by using the graph Earthquakes (contd.) - the travel time graph can also be used along with a seismogram to determine the origin time of the earthquake (when the earthquake occurred at local time) Earthquakes (contd.) C. Locating Epicenters - need three seismograph stations triangulation Earthquakes (contd) D. Earthquake Magnitude 1. Richter scale - scale of earthquake magnitude (strength) - scale of 1 to 10 - Each value on the Richter scale is 32 times more powerful than the previous value - Established by Charles Richter in 1935 Earthquakes (contd.) 2. Moment Magnitude more acccurately desribes the amount of energy released by an earthquake - more difficult to measure than Richter magnitude, but provides a more accurate description of the energy released Earthquakes (contd.) 3. Modified Mercalli Scale - describes the level of damage caused by an earthquake - - based upon anecdotal (personal) accounts and historical records Earthquakes (contd.) E. Earthquake Waves Inside Earth 1. P- and S-wave Velocities sharp change in velocities at 2 900 km depth P-waves slow, s-waves stop Evidence of the liquid portion of Earth’s interior - partial recovery of pwaves at 5 200 km depth Earthquakes (cpntd.) 2. Moho - named for Andrija Mohorivicic - - noticed difference in wave speeds - Reasoned difference was caused by waves traveling through more dense material under the crust - 32 km under continents - 8 km under oceans Earthquakes (contd.) 3. Shadow Zone - wide belt around Earth on the side opposite the focus of the earthquake - caused by Earth’s outer core - P-waves are double refracted (bent) by outer core - S-waves can’t travel through outer core