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Transcript
What is an earthquake? An earthquake is a shaking of the earth caused by a fracture (crack) in the earth. This means the crust of the earth has broken. The vibration may be as mild as a sound like thunder in the distance to a violent rocking of the ground beneath you and falling buildings. When the earthquake occurs, the breaking sends seismic waves out from the location of the quake. What is a seismic wave? A seismic wave is a wave of energy that radiates in all directions from the point of origin (focus) of an earthquake. The epicenter is the point on Earth’s surface directly above the focus. What are the types of seismic waves? Some waves travel through the body of the Earth. There are two types of these waves: P-Waves S-Waves Some waves travel along the surface of the Earth. These are the waves that cause the damage associated with Earthquakes. They are known as: L-Waves Properties of Body Waves P-Wave Properties (Primary Waves) : Compressional Waves- motion of particles is parallel to the direction of wave motion Push-pull waves Passes through solids, liquids and gases. Arrive phirst because they have the greatest velocity (phastest). Properties of Body Waves S-Wave Properties (Secondary Waves): Shear waves- particles move perpendicular to the direction of motion. Up-and-down waves Can travel through SOLIDS ONLY! Arrive at seismic stations second, after P-Waves, so they are slower. How do the velocities of seismic waves differ? When traveling through the same material, p-waves travel at a greater velocity than swaves. As a result, a seismograph will record the p-wave arrival before the arrival of the s-waves. How are body waves used by scientists? Scientists make inferences about Earth’s interior based upon the behavior of seismic waves as they travel through the Earth. The Outer Core of Earth is presumed to be liquid because SWaves “disappear” in this region. Shadow Zones Between 102° and 143° of the epicenter of an earthquake there is a shadow zone where neither P-Waves nor SWaves are received by seismograph stations. This is due to refraction of P-Waves out of this region, and the absorption of S-Waves in the outer core. How are seismic waves measured? Instruments called seismographs are used to measure earthquake waves. A seismogram is the record of seismic activity recorded by a seismograph. Measuring Earthquake Intensity The Modified Mercalli Scale is based upon the damage inflicted by an earthquake. This scale ranges from I to XII. Can you think of a problem with using only this scale to define earthquake intensity? Measuring Earthquake Intensity The Richter Scale is a magnitude scale used to describe the amount of energy released by an earthquake. The scale generally ranges from 0-9, but has no true upper limit. Each step up in the magnitude scale indicates an increase in energy released by 32X over the previous step. Today in earthquake history Magnitude 9.0 NEAR THE EAST COAST OF HONSHU, JAPAN Friday, March 11, 2011 at 05:46:23 UTC Globally, Japan’s is the 4th largest earthquake since 1900. Great (M > 8) Earthquakes Since 1900 9.6 Chile 1960 9.4 Alaska 1964 9.2 Magnitude Sumatra 2004 Russia 1952 9 8.8 Japan 2011 Ecuador 1906 Chile 2010 Alaska 1965 8.6 8.4 8.2 8 7.8 1900 1920 1940 1960 Year 1980 2000 2020