Earthquakes:

Earthquakes:

Earthquakes: *Shaking of the ground generated by release of energy *Energy released by movement along planes of weakness in lithosphere -Fractures Fault Geography: *Focus: location on the fault where movement begins -Sub-surface Fault Geography: *Epicenter: location on the surface directly above earthquake focus Energy Release at Faults: *Similar to breaking a pencil Energy Release at Faults: *1. Build-up phase: stress is applied to rigid material Energy Release at Faults: *2. Pre-rupture phase: rocks around fault experience strain as stress continues to build Energy Release at Faults: *3. Rupture phase: motion along the fault occurs and energy is released Energy Release at Faults: *Energy is released in the form of vibrations that travel through and on the Earth, called seismic waves Seismic Waves: *Ripple outward from the focus in all directions Seismic Waves: *Two primary types: -Body waves: travel through Earth’s interior -Surface waves: travel along Earth’s surface Seismic Waves: *Body waves -Faster than surface waves -Primary waves (P waves) a) Fastest seismic wave b) Compressional motion Seismic Waves: *Body waves -Faster than surface waves -Shear waves (S waves) a) Second fastest seismic wave b) Vertical motion Seismic Waves: *Surface waves -Slower than body waves -Rayleigh waves (R waves) a) Orbital motion b) Responsible for most damage Seismic Waves: *Surface waves -Slower than body waves -Love waves (L waves) a) Lateral (horizontal) motion Seismic Waves: *Who cares? -Waves used to determine location of earthquakes Seismograph: used to record seismic waves Seismographic record of an earthquake: note body waves arrive first (P, then S); surface waves arrive later (L, then R) Seismic Waves: *Who cares? -Waves used to determine location of earthquakes a) Distance between arrival time of certain waves used to calculate distance to focus 3 locations can be used to identify site of focus Earthquake Hazards: *Shaking of the ground: during energy release, seismic waves vibrate solid ground Earthquake Hazards: *Aftershocks: movement along main fault may initiate activation of nearby faults after first earthquake Earthquake Hazards: *Landslides: shaking can cause slopes to become unstable Earthquake Hazards: *Changes in elevation: movement along the fault can drop or raise the ground surface “Ghost forests”: coastal forests dropped down into salt water and killed Earthquake Hazards: *Liquefaction: shaking causes loose (uncemented) sediment and soil to flow like a liquid Earthquake Hazards: *Tsunami: vertical movement along an underwater fault displaces all of the overlying water -Displaced water migrates outward as a giant wave Note: tsunamis only formed by vertical motion; not generated by strike-slip faults Earthquake Hazards: *Tsunami -Not dangerous in deep water (just a long wave) -Becomes dangerous in shallow water (long wave becomes tall wave) Thailand, 2004 Before After Earthquake Measurement: *Two metrics -Magnitude -Intensity Earthquake Measurement: *Magnitude: essentially, amount of energy released during fault rupture -Uses Richter scale Note: how useful is this? Earthquake Measurement: *Intensity: measure of the effects of earthquake on people and buildings -Uses Modified Mercalli scale Note: more useful than Richter scale Earthquake Hazard Zones: *Frequent earthquakes, but of low magnitudes/intensities *Rare earthquakes, but very powerful San Andreas Fault System, California: frequent minor earthquakes San Andreas Fault System, California: pressure doesn’t build up along fault New Madrid Fault System, Missouri/Tennessee: rare major earthquakes New Madrid Fault System, Missouri/Tennessee: pressure builds along fault for long time

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