Lecture 20 Mar 17 2008 Earths interior

# Lecture 20 Mar 17 - Magnitude 7.4 earthquake Turkey 1999 17,118 people died We like to describe the size of earthquakes Maximum amplitude of ground

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Magnitude 7.4 earthquake, Turkey, 1999. 17,118 people died.

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We like to describe the size of earthquakes. Maximum amplitude of ground shaking is used to assign a magnitude given in numbers from 1-10 (Richter scale). The human perception of the ground shaking is described in the qualitative Mercalli intensity scale given in values from I to XII. Magnitude 6.9 earthquake in Armenia in caused 25,000 deaths 15,000 injuries 517,000 homeless
The magnitude depends on the amplitude of ground movement caused by seismic waves. The scale is logarithmic. Two earthquakes that differ in size of ground motion by a factor of 10 and energy release by a factor of 32 differ in magnitude by 1.

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The intensity is the most useful information about where the greatest damage to structures, and therefore impact on human life, is concentrated.
P waves are analogous to sound waves in air -- they are compressional waves. They travel in a series of compressions and expansions. Particles of matter are pushed and pulled in the direction of the path of travel of the wave. P waves travel at about 6 km/s (3.7 mi/s), about 14 times the speed of sound waves in air. Slip on a fault in the direction of wave propagation would set up P waves.

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S waves are shear waves because they push material at right angles to their path of travel. Liquids and gases do not
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## This note was uploaded on 04/11/2008 for the course EVSC 280 taught by Professor Herman during the Spring '08 term at UVA.

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Lecture 20 Mar 17 - Magnitude 7.4 earthquake Turkey 1999 17,118 people died We like to describe the size of earthquakes Maximum amplitude of ground

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