2Earthquake causesAn earthquake is the shaking of the Earth’s crust caused by a release of energyThe movement of the Earth’s plates causes most earthquakes
3Elastic 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 anotherThis 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).
6Earthquake DepthEarthquake depth depends on the type of plate boundary where the movement occursDiverging 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)
8The point where the actual movement of the plates takes place, and where the energy is released from is called the focusThe point on the Earth’s surface that is directly above the focus is called the epicenterWhen an earthquake occurs, energy waves are released and move outward from the focus
10Types of Faults (We’ll discuss these more when we do Geologic History) NORMAL - one side of a fault slips down relative to anotherREVERSE (& Thrust) - one side of a fault is driven up and over the otherSTRIKE-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
12Seismometers/ Seismographs Earthquakes generate seismic waves which can be detected with a sensitive instrument called a seismograph aka seismometerThe actual readout of a seismometer is called a seismogram
14P-Waves COMPRESSIONAL WAVES (push) cause the rocks in the crust to be squeezed and stretchedthe first waves received by a seismograph station because they move the fastestThese waves can move through solids, liquids, and gases
23Wave SpeedDetermined 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)
25Remember 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!!!!
26Movement of Waves Inside the Earth The velocity of the P-waves and S-waves increase as they travel deeper into the EarthThe 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 ZONEThe location of the shadow zone, of course, depends on the location of the epicenter
31Locating an EpicenterThe 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.
32Things 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-wavesKnowing 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 recordingsCount ‘em…THREEDid I mention that the number was 3 – no more, no less….
41Earthquake Strength and Damage Seismometers can also be used to determine the magnitude (strength) of an earthquakeThe Richter Scale is used to measure the amount of energy released by an earthquake
42Richter Scale Rating scale from 1 to 10 Each increase on the Richter Scale is an increase of 10 times in the strength of the earthquakeEx. an earthquake measuring a 5 is 10 times stronger than a 4, and 100 times stronger than a 3Each increase is also 32 times greater in energy
44Damage & PredictionEssentially, the seismic waves that cause the ground to shake cause earthquake damageThe shaking, depending on severity, can cause building foundations to fail, gas and water lines to rupture, electrical disruptions, avalanches, etcEarthquake prediction is important to determine the areas where damage is most likely to occur
45Partial collapse of the 'Million Dollar Bridge' into the Copper River in Alaska following the 1964 earthquake. (9.2 magnitude)
46LIQUIFACTION –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.
56Emergency Preparedness Scientists use EQ data to identify places where earthquakes are likely to occurGovernment authorities then use this information to write emergency preparedness plans.These plans are for both before AND after an earthquake strikes.From you’ve seen in the previous slides and what you’ve learned about earthquakes, complete the information on the following slides.
57Emergency Preparedness What can authorities do BEFORE an EQ strikes to mitigate (reduce) the damage and deaths that an EQ may cause?
58Emergency Preparedness What can citizens do BEFORE an EQ strikes to mitigate (reduce) the damage and deaths that an EQ may cause?
59Emergency Preparedness What can authorities plan for AFTER an EQ strikes to mitigate (reduce) the damage and deaths that an EQ may cause?
60Emergency Preparedness What can citizens plan for AFTER an EQ strikes to mitigate (reduce) the damage and deaths that an EQ may cause?