Earthquakes occur when tectonic forces cause rocks underground to break, releasing seismic waves. The focus is where the rocks break, and the epicenter is the point directly above on the surface. Different types of faults are caused by tension, compression, and shear forces. Earthquakes are measured by their magnitude using the Richter scale and by their intensity of shaking. Locating the epicenter involves measuring seismic wave arrival times at different stations. Earthquake dangers include falling objects, fires, tsunamis, liquefaction, and landslides.

1. Earthquakes

2. How Earthquakes work Investigating Earthquakes--San Francisco Introductory Clips

3. Earthquakes : Vibrations (seismic waves) within Earth materials are produced by the rapid release of energy Earth’s crust is in constant motion because of tectonic forces Earth’s crust can store elastic energy When forces exceed the elastic limits and structural strength of the rocks, the rocks will break and/or move producing vibrations that travel outward in all directions What is an Earthquake?

4. The actual place underground where the rocks break producing vibrations is called the focus The place on the surface directly above the focus is called the epicenter Earthquakes

6. Tension Force : stretching or pulling force Makes a normal fault What types of forces are created?

7. Normal Fault

8. Compression Force : force pushing something together Makes a reverse fault What types of forces are created?

9. Reverse Fault

11. Shear Force : a system of forces that operates against a body from different sides Makes a strike-slip fault What types of forces are created?

12. Strike-Slip Fault

13. Movement along faults : occurs when the energy exceeds the friction holding the sides of the fault together and is suddenly released . Movement of magma (volcanic) Volcanic eruptions What causes Earthquakes?

14. Originate at the focus and travel outward in all directions Foreshocks : small earthquakes that come before a major earthquake Aftershocks : Are adjustments in the crust after in earthquake Smaller than main earthquake, but can cause as much or more damage. They can continue for weeks to months . Not every earthquake produces aftershocks Seismic Waves

15. Seismic Waves

16. P waves (primary waves) Compressional wave Particles move back and forth in the same direction as the wave Travels the fastest Can pass through solids and liquids (gases also) Does not cause damage 3 Types of seismic Waves

17. S wave (secondary wave, shear wave) Particles move at right angles to the direction of the wave Travels slower than P waves Can pass through solids only Does not cause damage Types of Waves

18. L wave (long wave, surface wave, ground wave) Particles move in elliptical orbit Originates on the surface after the P and S waves go straight up from the focus and reach the surface The L wave causes the damage and will be the strongest at the epicenter Travels the slowest Types of Waves

20. How do we Measure Earthquakes? Earthquake waves are recorded by a seismograph and the recording of waves on paper is called seismogram

21. How do we Measure Earthquakes? Intensity – a measure of the effects on an earthquake at a particular location Magnitude : a measure of the strength or amount of energy released during an earthquake

22. Richter Scale : Measures the amplitude of earthquake waves on seismograms Scale from 1-10 Each number is 10 times the amplitude of the number below How do we Measure Earthquakes?

25. Locating the epicenter Lag time between the arrival of the P wave and the S wave to the seismograph station is converted to a distance A circle with a radius that equals the distance is drawn around the station. Two stations can narrow down the location to two places where the two circles intersect Locating the focus : the lag-time of the L wave will determine the depth of the focus Measuring Earthquakes

27. Earthquake Dangers Most injuries and deaths are caused by falling objects and most property damage results from fires that start Tsunami : seismic sea wave sometimes generated when an earthquake originates on the ocean floor

29. Tsunami—December 2004

30. Earthquake Dangers Seiche : rhythmic sloshing of small bodies of water A seiche is the sloshing of a closed body of water from earthquake shaking. Swimming pools often have seiches during earthquakes.

31. Liquefaction : unconsolidated materials that are water saturated may turn to a fluid causing some underground objects such as storage tanks to float to the surface Earthquake Dangers Ground fissures caused by liquefaction near the mouth of the Pajaro River in California during the 1989 Loma Prieta earthquake. When the surface of the ground oscillates, wet, sandy, and muddy soils can flow like a liquid. This is liquefaction. You can liquefy wet sand at the beach by pumping it up and down with your feet. Photo courtesy of the Loma Prieta Collection, Earthquake Engineering Research Center, UC Berkeley.

32. Landslides Earthquake Dangers

33. Earthquake Safety Protect yourself from falling objects ( GET UNDER SOMETHING ) or stand in a hallway or doorway (watch out for a swinging door) Do not try to go outside during the earthquake After the earthquake and before the aftershocks, go outside Do not return to the building until it has been inspected