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Earthquakes Global Earthquake Locations Earthquakes • Shaking of earth due to movement of rocks along a fault. • Rocks under stress accumulate strain energy over time. • When stress exceeds strength of rocks, rock breaks. • Strain energy is released as seismic waves. The longer that energy is stored up and is maintained without release, the more likely that a strong earthquake will occur. Types of Stress • Tension • Compression • Shearing Kinds of Faults • Normal Fault – caused by a tension or pulling apart force • Reverse Fault – caused by a compression or pushing force • Strike-slip Fault – caused by a shearing force when plates move past each other What is the Elastic Rebound Theory? • Explains how energy is stored in rocks – Rocks bend until the strength of the rock is exceeded – Rupture occurs and the rocks quickly rebound to an undeformed shape What is the Elastic Rebound Theory? • Explains how energy is stored in rocks – Energy is released in waves that radiate outward from the fault Earthquakes Types of seismic waves 1. Body waves -- travel through interior 2. Surface waves -- travel on surface of earth Primary or “P” Wave Secondary or “S” Wave Body Waves: P and S waves – P or primary waves • fastest waves • travel through solids, liquids, or gases • compressional wave, material movement is in the same direction as wave movement – S or secondary waves • slower than P waves • travel through solids only • shear waves - move material perpendicular to wave movement Surface Waves: R and L waves • Surface Waves – Travel just below or along the ground’s surface – Slower than body waves; rolling and side-to-side movement – Especially damaging to buildings Types of Seismographs Seismogram Printout How is an Earthquake’s Epicenter Located? – P waves arrive first, then S waves, then L and R – Average speeds for all these waves is known – After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter. Determining the location of an earthquake 1. First, distance to earthquake is determined. 2. Seismographs record seismic waves 3. From seismograph record called the seismogram, measure time delay between P & S wave arrival 4. Use travel time curve to determine distance to earthquake as function of P-S time delay Determining the location of an earthquake 1. Now we know distance waves traveled, but we don't know the direction from which they came. 2. We must repeat the activity for each of at least three (3) stations to triangulate a point (epicenter of quake). 3. Plot a circle around seismograph location; radius of circle is the distance to the quake. 4. Quake occurred somewhere along that circle. 5. Do the same thing for at least 3 seismograph stations; circles intersect at epicenter. Thus, point is triangulated and epicenter is located. Focus and Epicenter of Earthquake Time-Travel Curve Triangulation of 3 stations to locate earthquake epicenter Determining the magnitude of an earthquake • Magnitude -- measure of energy released during earthquake. • There are several different ways to measure magnitude. • Most common magnitude measure is Richter Magnitude, named for the renowned seismologist, Charles Richter. Richter Magnitude • Measure amplitude of largest S wave on seismograph record. Richter Magnitude • Measure amplitude of largest S wave on seismograph record. • Take into account distance between seismograph & epicenter. Richter Scale • Logarithmic numerical (NOT a physical) scale • Increasing one whole unit on Richter Scale represents 10 times greater magnitude. • Going up one whole unit on Richter Scale represents about a 30 times greater release of energy. • Mercalli Scale is used to express damage Intensity • Intensity refers to the amount of damage done in an earthquake • Mercalli Scale is used to express damage Hazards associated with Quakes • Shaking: • Frequency of shaking differs for different seismic waves. • High frequency body waves shake low buildings more. • Low frequency surface waves shake high buildings more. • Intensity of shaking also depends on type of subsurface material. • Unconsolidated materials amplify shaking more than rocks do. • Fine-grained, sensitive materials can lose strength when shaken. They lose strength by liquefaction. • Buildings respond differently to shaking depending on construction styles, materials • Wood -- more flexible, holds up well • Earthen materials -- very vulnerable to shaking. Hazards associated with Quakes • Ground displacement: Ground surface may shift during an earthquake (esp. if focus is shallow). • Tsunamis (NOT called tidal waves) Tsunamis are huge waves generated by earthquakes undersea or below coastal areas. If earthquake displaces sea surface, wave is generated that can grow as it moves over sea surface. • Fires Usually occurs from shifting of subsurface utilities (gas lines) Tsunami Movement Tsunami Movement: ~600 mph in deep water ~250 mph in medium depth water ~35 mph in shallow water Earthquake Prediction (?) How can scientists predict an earthquake? Currently, that is not possible. Future technology will monitor subsurface seismic waves and periodic shifting indicative of future slippage. Tracking organic movement is also a source of future study. Earthquake Hazard Potential Map Parkfield, CA “Earthquake Capital of the World” World’s Largest Earthquake: 1964 Anchorage, Alaska Registered 8.6 on Richter Scale Key Terminology Seismic waves Body waves Surface waves Primary (“P) waves Secondary (“S”) waves Love (“L”) waves Seismograph Seismogram Focus Epicenter Time-travel curve Magnitude Intensity Richter Scale Mercalli Scale Logarithmic Liquefaction Tsunami