105_08-quakes_part_1 - Earthquakes & Earth’s...

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Unformatted text preview: Earthquakes & Earth’s Interior Earthquakes Earthquake Earthquake – trembling or shaking of the ground caused by a sudden release of energy in the rocks beneath the Earth’s surface. Chapter 8 Colombia, January 1999 Sequence of Earthquake generation Foreshock Foreshock - small, creep Main Main Earthquake Aftershock Aftershock - smaller, numerous earthquakes that can continue for up to 1-2 yrs Origin of Earthquakes: elastic rebound Explains 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 – Energy is released in waves Origin of Earthquakes: elastic rebound theory Origin of Earthquakes: elastic rebound 1 Origin of Earthquakes: elastic rebound Origin of Earthquakes: elastic rebound Seismic Waves – Rocks can deform only so much before they break. – When rocks break, energy is released in the form of seismic seismic Seismic waves waves Seismic Waves Seismology Seismology - the study of earthquakes Seismograph Seismograph (seismometer) - machine that records the seismic waves Seismogram Seismogram - the record of the seismic waves Earthquakes Focus Focus - point of origin of the energy release, usually <100 km deep Epicenter Epicenter - the spot on the surface of the Earth directly above the focus 2 Earthquakes Focus/Epicenter Focus/Epicenter Where Do Earthquakes Occur and How Often? more more than 150,000 quakes strong enough to be felt are recorded each year Earthquake Zones Earthquake Zones Convergent plate boundaries – Shallow or deep, 20-700 km 80% 80% occur along the Pacific Rim Convergent Convergent plate boundaries Transform Transform plate boundaries Divergent Divergent plate boundaries Intraplate Intraplate earthquakes Divergent plate boundaries – Shallow, 20 km Transform plate boundaries – San Andreas Fault – < 80 km Intraplate earthquakes – New Madrid Fault – < 50 km Earthquake Zones Earthquake Zones Benioff zone 1959 publication Benioff zone: quakes along the subducting plate 3 Where do Earthquakes Occur and How Often? Transform plate boundary: San Andreas Fault, California Earthquake Risk - California North American Plate Damage in Oakland, CA, 1989 Pacific Plate Earthquake Risk - California Orange trees, Imperial Valley, California, 1950 Earthquake Risk - Turkey Transform plate boundary Converging plate boundary: Japan 4 Earthquake Zones in the US Seismic Waves Two Two types: – Body waves: travel through the Earth’s interior, outward from focus P and S and Seismic Waves Body Body waves – P or primary waves – Push waves – (Phirst, Phastest) Fastest, 4Fastest, 4-7 km/s Compressional Compressional waves Travel Travel through solids, liquids, and gasesgases- parallel to wave propagation – Surface waves: travel along the Earth’s surface, outward from epicenter L and R and – S or secondary waves – Shake, shear waves Slower, 2Slower, 2-5 km/s Travel Travel only through solidssolidsperpendicular to wave propagation Seismic Waves Body Body waves – P or primary waves Seismic Waves Surface Surface waves – slowest waves – L or Love waves Move Move side to side Like Like horizontal S waves Travel Travel through solids- perpendicular to solidswave propagation – S or secondary waves – R or Rayleigh waves Rolling Rolling motion, similar to ocean waves Travel Travel only through solids 5 Seismic Waves Locating and Measuring an Earthquake Basis: Law of Inertia Objects at rest tend to stay at rest and objects in motion tend to remain in motion unless either is acted upon by an outside force. Seismograph Seismograph Seismograph Seismograph vs Seismogram Seismograph Seismogram Seismogram Seismogram: recorded by seismograph 6 How is an Earthquake’s Epicenter Located? Difference in arrival time between P and S waves How is an Earthquake’s Epicenter Located? Seismic Seismic wave behavior – 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. 5 minutes TimeTime-distance graph showing the average travel times for P- and S Pwaves. How is an Earthquake’s Epicenter Located? The PThe P-S time interval grows with increasing distance from focus Measuring Measuring the time difference is used to determine distance from focus 5 min 5 minutes 5 min 5 min 5 min How does it work? Where are you??? 625 miles from Boise How does it work? Where are you??? 625 mi from Boise 650 mi from Minneapolis 7 How does it work? Where are you??? 625 mi from Boise 650 mi from Minneapolis 615 mi from Tucson How is an Earthquake’s Epicenter Located? Three Three seismograph stations A circle with radius circle of the distance to the epicenter is drawn Epicenter Epicenter is at intersection of circles How is an Earthquake’s Epicenter Located? Earthquake Scales Mercalli Mercalli Scale - Guiseppe Mercalli 1902 – Based on the amount of damage caused by the earthquake = measuring intensity intensity – Eyewitness report of damage to human structures – 1 least damage to 12 major damage Shortcomings: Shortcomings: – – – – Only in inhabited regions Depends on distance from epicenter Depends on nature of surface material Depends on building design Earthquake Scales Mercalli Mercalli Scale - Example: – V Felt by nearly everyone, many awakened. Some dishes, windows, etc., broken, a few instances of cracked plaster. – VI Felt by all, many frightened and run outdoors. Some heavy furniture moved. Damages slight. – IX Damage considerable in specially designed structures. Buildings shifted off foundations. Ground cracked. Underground pipes broken. – XI Few, if any (masonry) structures remain standing. Bridges destroyed. Underground pipelines completely out of service. Modified Modified Mercalli Intensity Map – 1994 Northridge, CA earthquake, magnitude 6.7 8 ...
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