2Introductory Clips How Earthquakes work Investigating Earthquakes--San Francisco
3What is an Earthquake?Earthquakes: Vibrations (seismic waves) within Earth materials are produced by the rapid release of energyEarth’s crust is in constant motion because of tectonic forcesEarth’s crust can store elastic energyWhen 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
4EarthquakesThe actual place underground where the rocks break producing vibrations is called the focusThe place on the surface directly above the focus is called the epicenter
24What causes Earthquakes? 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
25Seismic WavesOriginate at the focus and travel outward in all directionsForeshocks: small earthquakes that come before a major earthquakeAftershocks: Are adjustments in the crust after in earthquakeSmaller than main earthquake, but can cause as much or more damage. They can continue for weeks to months. Not every earthquake produces aftershocks
273 Types of seismic Waves P waves (primary waves) Compressional wave Particles move back and forth in the same direction as the waveTravels the fastestCan pass through solids and liquids (gases also)Does not cause damage
28Types of Waves S wave (secondary wave, shear wave) Particles move at right angles to the direction of the waveTravels slower than P wavesCan pass through solids onlyDoes not cause damage
29Types of Waves L wave (long wave, surface wave, ground wave) Particles move in elliptical orbitOriginates on the surface after the P and S waves go straight up from the focus and reach the surfaceThe L wave causes the damage and will be the strongest at the epicenterTravels the slowest
36Measuring Earthquakes Locating the epicenterLag time between the arrival of the P wave and the S wave to the seismograph station is converted to a distanceA 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 intersectLocating the focus: the lag-time of the L wave will determine the depth of the focus
38Earthquake DangersMost injuries and deaths are caused by falling objects and most property damage results from fires that startTsunami: seismic sea wave sometimes generated when an earthquake originates on the ocean floor
41Earthquake 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.
42Earthquake DangersLiquefaction: unconsolidated materials that are water saturated may turn to a fluid causing some underground objects such as storage tanks to float to the surfaceGround 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.
44Earthquake SafetyProtect 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 earthquakeAfter the earthquake and before the aftershocks, go outsideDo not return to the building until it has been inspected