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The Focus and Epicenter of an Earthquake

The point within Earth where rock under stress breaks is called the focus The point directly above the focus on the surface is the epicenter. The Focus and Epicenter of an Earthquake. What are Seismic Waves?.

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The Focus and Epicenter of an Earthquake

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  1. The point within Earth where rock under stress breaks is called the focus • The point directly above the focus on the surface is the epicenter The Focus and Epicenter of an Earthquake

  2. What are Seismic Waves? • Vibrations that travel through Earth carrying the energy released during an earthquake • Energy starts at the focus and moves outward • Two types: • Body waves • P and S • Surface waves

  3. Body Waves: P and S waves Undisturbed Material • Body waves • P or primary/pressure waves • fastest waves • travel through solids, liquids, or gases • Waves compress and expand like an accordian • S or secondary/shear waves • slower than P waves • travel through solids only • Move up and down as well as side to side P Waves S Waves

  4. Surface Waves: R waves • Surface Waves • Travel just below or along the ground’s surface • Slower than body waves • Surface waves are the most damaging and are especially damaging to buildings. R waves L waves L waves

  5. Seismographs record earthquake events

  6. How is an Earthquake’s Epicenter Located? Seismic wave behavior • P waves arrive first, then S waves, then surface waves • 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.

  7. Where Do Earthquakes Occur and How Often? ~80% of all earthquakes occur in the circum-Pacific belt ~15% occur in the Mediterranean-Asiatic belt ~remaining 5% occur in the interiors of plates and on spreading ridge centers • more than 150,000 quakes strong enough to be felt are recorded each year

  8. How do you locate an epicenter? The Focus and Epicenter of an Earthquake • The point within Earth where rock under stress breaks is called the Focus • The point directly above the focus on the surface is the Epicenter. epicenter Surface B A Which seismic Station will Receive the Wave first? A or B focus

  9. How do you locate an epicenter? You must have three different seismographs at three different Locations - then you can pinpoint the different times each picked up the earthquake wave. A scientist will do this in a lab. epicenter Surface B A focus

  10. How is an Earthquake’s Epicenter Located? • Three seismograph stations are needed to locate the epicenter of an earthquake • A circle where the radius equals the distance to the epicenter is drawn • The intersection of the circles locates the epicenter

  11. How is an Earthquake’s Epicenter Located? Time-distance graph shows the average travel times for P- and S-waves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves

  12. How are the Size and Strength of an Earthquake Measured? • Magnitude: the measurement of earthquake strength based on seismic waves and movement along faults

  13. Three ways of measuring earthquakes: • Mercalli Scale measures the intensity of an earthquake • Richter Scale is a rating of the size of seismic waves • Moment Magnitude Scale estimates the total energy released by an earthquake

  14. Mercalli Scale

  15. Richter Scale

  16. The Economics and Societal Impacts of EQs • Building collapse • Fire • Tsunami • Ground failure Damage in Oakland, CA, 1989

  17. Ground Shaking • The earthquake’s amplitude, how long it lasts, and damage increases in poorly consolidated rocks What are the Destructive Effects of Earthquakes?

  18. Local soil conditions: loose soil shakes more violently than the surrounding rock (house built on solid rock will shake less than a house build on sandy soil) Liquefaction: earthquakes violent shaking suddenly turns loose soil into liquid mud Aftershocks: buildings weakened by an earthquake may collapse during aftershocks Tsunamis: earthquakes on the ocean floor cause large waves that can grow to the height of a six story building Earthquake Hazards

  19. Can Earthquakes be Predicted? • Forecasting when and where earthquakes will occur and their strength is difficult. • One way seismologist forecast earthquakes in a place is to observe their past strength and frequency.

  20. Earthquake Hazard • Earthquake hazard is a measurement of how likely an area is to have damaging earthquakes in the future. An area’s earthquake-hazard level is determined by past and present seismic activity.

  21. Forecasting Earthquakes • A seismic gap is an area along a fault where relatively few earthquakes have occurred recently but where strong earthquakes are known to have occurred in the past. Ex: California The gap hypothesis states that sections of active faults that have had relatively few earthquakes are likely to be the sites of strong earthquakes in the future.

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