Earthquakes Waves & Seismograms

1. EarthquakesWaves & Seismograms Lecture prepared by Mr. B

2. Topics Elastic rebound theory Seismic waves Seismograph and seismogram Finding distance to an earthquake Locating an earthquake Distribution of earthquakes Depth of earthquakes Earthquake intensity Earthquake magnitude

3. Earth In Cross Section

4. Under the Mantle, it’s like a Lava Lamp

5. Turkey, 1999

6. Can Earthquakes be Predicted? Earthquake Precursors • changes in elevation or tilting of land surface • fluctuations in groundwater levels • magnetic field • electrical resistance of the ground • Release of gases

7. Can Earthquakes be Predicted? Earthquake Prediction Programs • include laboratory and field studies of rocks before, during, and after earthquakes • monitor activity along major faults • produce risk assessments

8. Elastic Rebound Theory Rocks bend under stress while storing elastic energy. When the strain in the rocks exceeds their strength, breaking will occur along the fault. Stored elastic energy is released as the earthquake. Rocks“snap back”, or rebound to their original condition.

9. Spread of the Seismic Waves

10. Types of Earthquake (Seismic) Waves • Body Waves • P-Waves (primary waves) • S-Waves (secondary waves) • Surface Waves - L – Love Waves - R – Raleigh Waves (surface, vertical)

11. P-Waves

12. P Wave Animation

13. S-Waves

14. S Wave Animation

15. Body Waves: P and S waves • Body 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

16. 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

17. The Poor House 

18. Seismograph (Horizontal)

19. Seismograph (Vertical)

22. Distribution of Quakes A direct result of Continental Drift

23. Earthquake Risk

24. The Continental Plates

25. Profile of Subduction Zone

29. Tsunami

30. Tsunami

31. Seismogram

32. How is an Earthquake’s Epicenter Located? 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.

33. How is an Earthquake’s Epicenter Located? Time-distance graph showing 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

34. Locating the Earthquake

35. 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

36. How are the Size and Strength of an Earthquake Measured? • Intensity • subjective measure of the kind of damage done and people’s reactions to it • isoseismal lines identify areas of equal intensity • Modified Mercalli Intensity Map • 1994 Northridge, CA earthquake, magnitude 6.7

37. Mercalli Scale of Earthquake Intensity • Advantages: • No high-tech instruments are required. • Disadvantages: • Damage depends on geologic materials and type of structures in area • Damage varies with distance from epicenter • Subjective - different people may view damage and effects very differently

38. I. People do not feel any Earth movement. The Goofy Mercalli Scale II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings. III. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock. XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent. XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.

39. How are the Size and Strength of an Earthquake Measured? • Magnitude • Richter scale measures total amount of energy released by an earthquake; independent of intensity • Amplitude of the largest wave produced by an event is corrected for distance and assigned a value on an open-ended logarithmic scale

40. Richter Scale Math • Increase by 1 whole number means a 10X increase in the Magnitude of the quake • For every increase on the Richter Scale, the amount of energy released increases 30X Compare a 5.0 to a 7.0 quake 7.0 has 10 X 10 = 100 times greater magnitude 7.0 has 30 X 30 = 900 times more energy! Compared to a 1.0 quake, a 7.0 has: 10 X 10 X 10 X 10 X 10 X 10 = 1 000 000 greater strength And 30 X 30 X 30 X 30 X 30 X 30 = 729 000 000 more energy

41. Earthquake Magnitude aand Worldwide Occurrence Magnitude Number/Year Less than 2.0 600,000 2.0-2.9 300,000 3.0-3.9 49,000 4.0-4.9 6,200 5.0-5.9 800 6.0-6.9 266 7.0-7.9 18 Greater than 8,0 1

42. Earthquake intensity Earthquake magnitude Richter Scale Mercalli Scale Elastic rebound Fault Focus P-wave Seismic wave Seismogram Seismograph S-wave Tsunami Terminology Used in the Study of Earthquakes