What is an Earthquake

 Over 30,000 happen each year worldwide that are strong enough to be felt  Only about 75 major earthquakes take place each year many in remote regions Powerful Earthquake Reported in Costa Rica - YouTube

 A vibration of the Earth produced by the rapid release of energy  Caused by slippage along a break in Earth’s crust

 Focus – point within earth where the earthquake starts › Waves are produced here  Epicenter – location on the surface of Earth directly above the focus

 Faults – fractures in earth’s crust and mantle where movement has occurred

 Elastic rebound hypothesis – explanation for the release of energy stored in deformed rocks Earthquakes - Shock Waves Explained - YouTube

 Rocks are in the original position  Stress builds up due to tectonic forces › Builds energy  Energy is released › Earthquake  Land rebounds into new position How Earthquakes Work - YouTube

 Aftershocks – movements that follow a major earthquake › Sometimes destroy structures weakened by the quake

 foreshocks - Small earthquakes happening before the major quake › Can be felt days or even years before

Measuring Earthquakes

 Instruments that record earthquake waves

 2 types › Surface waves › Body waves seismic waves - YouTube

 Travel along earths outer layer  Up and down as well as side to side motion  Especially damaging to buildings  Slower than body waves

 Pass through earth’s interior  P waves – push – pull or compression waves › Faster than S waves  S waves – shake particles at right angles › Cannot pass through gases or liquids S and P waves - YouTube

 Seismographs can be used to see the difference in velocity between p and s waves  Interval between 1 st p wave arrival and 1 st s wave arrival › Longer the interval the further the distance from the earthquake  Uses 3 or more seismographs

 2 types of measurement › Intensity  Amount of shaking based on damage  qualitative › Magnitude  Calculations of energy released using seismographs  Quantitative

 Based on amplitude of largest seismic wave  Logarithmic › 10 fold increase in wave amplitude of 1 on the magnitude scale  5.0 is 10X greater than 4.0  Familiar but outdated

 Based on amount of displacement at a fault zone  More precise  Many different factors › Av. amount of movement › Area of surface break › Strength of broken rock  Most widely used because it measures amount of energy released

Destruction from earthquakes

 Depends on several factors › Intensity and duration › Nature of the material on which the structure is built › Design of the structure

 Where loosely consolidated sediments are saturated with water, stable soil can turn to a liquid during earthquakes  Buildings may settle and collapse  Underground storage tanks and sewage lines may float to the surface liquefaction

 Destructive waves › Aka tidal waves  Not actually anything to do with tides  Slab of ocean floor is displaced vertically along a fault  Or when a quake sets off an underwater landslide Japan Tsunami first wave ground footage - YouTube

 Landslides – do the greatest damage › Soil and / or rock slopes fail  Fire – gas and electrical lines may be cut › when water lines are also cut, fires cannot be stopped – San Francisco 1906

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