Chapter 15

1. Chapter 15 EARTHQUAKES! BFRB Pages 215-226

2. Earthquake causes • An earthquake is the shaking of the Earth’s crust caused by a release of energy • The movement of the Earth’s plates causes most earthquakes

3. Elastic rebound theory • As Earth’s plates move past one another, friction causes the plates to get “stuck” • Stress and pressure builds up and causes the plate to become deformed (bend) • (the plates are solid, but elastic - they can change shape) • Eventually, the pressure overcomes the friction and the plates slip past one another • This sudden plate movement is the earthquake. The greater the plate movement, the larger the earthquake. • The plates are in a new location, but they go back to their original shape (elastic rebound).

6. Earthquake Depth • Earthquake depth depends on the type of plate boundary where the movement occurs • Diverging boundaries (mid-Atlantic Ridge), and transform (sliding) boundaries (San Andreas Fault) tend to have earthquakes which are closer to the surface(shallow focus) • Subduction boundaries tend to have very deep earthquakes as the subducting plate plunges deeper into the mantle(deep focus)

7. Shallow focus Intermediate focus Subducting plate Deep focus

8. The point where the actual movement of the plates takes place, and where the energy is released from is called the focus • The point on the Earth’s surface that is directly above the focus is called the epicenter • When an earthquake occurs, energy waves are released and move outward from the focus

10. Types of Faults(We’ll discuss these more when we do Geologic History) • NORMAL - one side of a fault slips down relative to another • REVERSE (& Thrust) - one side of a fault is driven up and over the other • STRIKE-SLIP – occur where plates meet evenly and slip past each other horizontally. (The angle at which a fault cuts through the earth is referred to as the strike, so a strike-slip fault happens when plates slip along the strike). • most common fault type in California

11. FAULT ANIMATIONS

12. Seismometers/ Seismographs • Earthquakes generate seismic waves which can be detected with a sensitive instrument called a seismograph aka seismometer • The actual readout of a seismometer is called a seismogram

13. 3 Types of Waves P-WAVE S-WAVE L-WAVE

14. P-Waves • COMPRESSIONAL WAVES (push) • cause the rocks in the crust to be squeezed and stretched • the first waves received by a seismograph station because they move the fastest • These waves can move through solids, liquids, and gases

17. S-Waves • SHEAR WAVES • Cause the rocks to move side to side, causing a ripping (shearing) motion • Slower than p-waves • Can only move through solids ****S waves = Side to Side, Slow, Solids

20. P & S Wave Animations

21. L-Waves • LONGITUDINAL WAVES • aka SURFACE WAVES • when P & S-waves reach the surface, they cause wave-like ripples on the surface of Earth like when you throw a rock in a pond

23. Wave Speed • Determined by the characteristics of the material that the earthquake’s wave is moving through! • The more solid the material, the faster the waves can move through it (but remember, P-waves will always move faster than S-waves)

25. Remember when we were studying the layers of the Earth and you asked me, “ If we’ve never been there, how do we know that these layers exist? Well, here’s how!!!!

26. Movement of Waves Inside the Earth • The velocity of the P-waves and S-waves increase as they travel deeper into the Earth • The boundary between the crust and the mantle is a thin layer of denser rocks. Because of this increase in density, the P & S-waves suddenly increase in velocity. • This thin, dense layer is called the Mohorovicic discontinuity or Moho

28. …Wave Movement… • When the waves reach the outer core, the P-waves slow down and the S-waves stop. • S-waves can’t travel through liquids, and P-waves move fastest through solids. • **The change in velocity of the P-Waves and ceasing of the S-waves is evidence that the outer core is liquid!!!** • After passing through the outer core, the P-waves increase in velocity as they pass through the solid inner core, then decrease as they go through the other side of the liquid outer core, then increase in velocity again when they go through the rest of the solid Earth.

29. …Wave Movement… • Seismograph stations on the opposite side of the earth from the epicenter only receive P-waves (the liquid outer core block the S-waves). • Some seismograph stations do not receive any waves – neither P nor S. • This is because the P-waves refract (bend) as they travel through the liquid outer core, and the S-waves don’t make it through at all. • This area where no waves reach is called the SHADOW ZONE • The location of the shadow zone, of course, depends on the location of the epicenter

31. Locating an Epicenter • The location of an earthquake epicenter can be found by using the measurement of seismic waves from 3 seismic stations and a P-wave/ S-wave Time Travel Graph (ESRT’s page 11) • Epicenters are found by drawing circles around each seismograph station that are the exact distance from the epicenter that was determined using the Time Travel Graph. • Where all three circles intersect is the location of the epicenter.

32. Things to Remember • P-waves always arrive before S-waves • The farther a seismograph is from the epicenter of the earthquake, the greater the time difference between the arrival of the P-waves and S-waves • Knowing the distance from the epicenter of one or two seismographs will only give you a rough area of where the earthquake occurred. To find the exact position of the epicenter, you must have three seismograph station recordings • Count ‘em…THREE • Did I mention that the number was 3 – no more, no less….

41. Earthquake Strength and Damage • Seismometers can also be used to determine the magnitude (strength) of an earthquake • The Richter Scale is used to measure the amount of energy released by an earthquake

42. Richter Scale • Rating scale from 1 to 10 • Each increase on the Richter Scale is an increase of 10 times in the strength of the earthquake • Ex. an earthquake measuring a 5 is 10 times stronger than a 4, and 100 times stronger than a 3 • Each increase is also 32 times greater in energy

44. Damage & Prediction • Essentially, the seismic waves that cause the ground to shake cause earthquake damage • The shaking, depending on severity, can cause building foundations to fail, gas and water lines to rupture, electrical disruptions, avalanches, etc • Earthquake prediction is important to determine the areas where damage is most likely to occur

45. Partial collapse of the 'Million Dollar Bridge' into the Copper River in Alaska following the 1964 earthquake. (9.2 magnitude)

46. LIQUIFACTION – The ground literally flows like a liquid due to the pressure that the earthquake waves put on the water that surrounds the particles. The water exerts enough pressure on the particles that they no longer are held together by the friction between them – the water in between the particles enables them to flow easily and no longer support the structure that is built on it.

49. This was a communications building in Spitak during the 1988 Armenian earthquake.

50. Elementary School torn apart by a landslide that occurred during the 1964 Alaska earthquake. Notice the large crack in the foreground that also was caused by the earthquake. (9.2 magnitude)