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EARTHQUAKE 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 • EQs happen along cracks in the ground called FAULTS 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 elastic-they can change shape) • Eventually, the pressure overcomes the friction and the plates slip past one another • The 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 (rebound). Earthquake depth • Earthquake depth depends on the type of plate boundary where the movement occurs • Diverging boundaries (mid-Atlantic Ridge), and 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 plate plunges into the mantle (deep focus) Shallow focus Intermediate focus Subducting plate Deep focus • 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 Types of faults • 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 FAULT ANIMATIONS seismographs • Earthquakes generate seismic waves which can be detected with a sensitive instrument called a seismograph 3 types of waves P-WAVE S-WAVE L-WAVE P-WAVES • PRIMARY 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 S-waves • SECONDARY WAVES • SHEAR WAVES • Cause the rocks to SNAKE, causing a ripping (shearing) motion • Slower than p-waves • Can only move through solids ****S waves = Snake-like, Slow, Solids P & S Wave Animations L-waves • LONGITUDINAL WAVES • SURFACE WAVES • when P & S-waves reach the surface, they cause waves like ripples on the water when you throw a rock in a pond Wave speed • Determined by the characteristics of the object the earthquakes 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 Swaves) 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 increase in velocity. • This layer is called the Mohorovicic discontinuity or Moho …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 easiest through solids. • **The change in velocity 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 to the other side of the Earth …Wave movement… • Seismograph stations on the opposite side of the earth from the epicenter only receive P-waves • Some seismograph stations do not receive any waves. • This is because the P-waves bend as they travel through the liquid outer core, and the S-waves don’t make it through • This area where no waves reach is called the SHADOW ZONE • The location of the shadow zone depends on the location of the epicenter 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 – (Reference Tables pg 11) • Epicenters are found by drawing circles around each seismograph station that are the exact distance that was determined using the Time Travel Graph. • Where all three circles meet is the location of the epicenter (LAB) 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 Earthquake Strength and Damage • Seismographs 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 Richter Scale • rating scale from 1 to 10 • Each increase on the Richter Scale is an increase of 10 times in size 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 Damage & prediction • ground shaking causes 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 areas where damage is likely to occur – This allows emergency services to develop evacuation plans, building codes to build structures to withstand an earthquake, etc Partial collapse of the 'Million Dollar Bridge' into the Copper River in Alaska following the 1964 earthquake. (9.2 magnitude) Evidence of LIQUIFACTION – The ground literally becomes liquid due to the amazing force of the earthquake waves. Happens in areas that have clay and silt soils. This was a communications building in Spitak during the 1988 Armenian earthquake. 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) Epicenter of the Manhattan earthquake on 17 January 2001 is plotted on an aerial photograph US Earthquakes 1973 - 2002