# EPSC 185 Lecture 8: Lecture 8

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23 Jun 2018
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E-types of waves:
-Earthquakes generate
-Body waves (travel through the earth)
—> P and S waves
-Surface waves (travel along the surface)
—> Rayleigh and love waves
Body waves: Primary waves
-Primary waves, or P waves, travel through solid, liquid, and gas
-They are alternately compressional and expansive
-Their speeds are ~ 5km/s
-Rock vibrates parallel to the direction of wave propagation
Body waves: Shear waves
-Shear, or S waves, travel through solids
-They push material at right angles to their travel path
-Their speeds are 2-3km/s
-Rock vibrates perpendicular to the direction of wave propagation
Surface waves: Love
-Surface waves, such as love waves, are restricted to earth’s surface
-They cause sideways shaking of the ground
-Their speed is slightly less than S waves
-Love waves are the most dangerous waves
Surface waves: Rayleigh
-Rayleigh waves are similar to love waves
-But instead of causing shaking, they produce rolling morton of the ground
-Behave like ocean waves
A simple seismogram
-A seismogram is the graphical representation of earth movement
-The P wave is registered ﬁrst, then the S wave, then the S-P interval, the difference between S
and S-P interval is the amplitude
F-sizes of earthquakes:
-The size of an earthquake is the main factor in its destructiveness
-We will look at two ways to estimate size:
—> Richter magnitudes
—> Mercalli index
-Mercalli intensity
-The modiﬁed mercalli intensity scale is used to assign a measure of destructiveness to
an earthquake (degree of damage caused)
-It is qualitative and based upon observed effects on people and damage to buildings
-Mercalli I: very weak, not felt by people
-Mercalli XII: total destruction
Richter magnitudes:
-The richter magnitude measures the maximum amplitude of ground shaking (vibrational
energy)
-It is a logarithmic scale
-1 richter unit difference is x 10 for ground motion and x 33 for energy
-Globally, small earthquakes are more frequent than large:
- ~ 800,000/yr for events of magnitude 2.0-3.4
-While an event of magnitude 8 occurs once every 5-10 years
G-estimating the epicentre of an earthquake
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## Document Summary

Primary waves, or p waves, travel through solid, liquid, and gas. Rock vibrates parallel to the direction of wave propagation. They push material at right angles to their travel path. Rock vibrates perpendicular to the direction of wave propagation. Surface waves, such as love waves, are restricted to earth"s surface. Their speed is slightly less than s waves. But instead of causing shaking, they produce rolling morton of the ground. A seismogram is the graphical representation of earth movement. The p wave is registered rst, then the s wave, then the s-p interval, the difference between s and s-p interval is the amplitude. The size of an earthquake is the main factor in its destructiveness. We will look at two ways to estimate size: The modi ed mercalli intensity scale is used to assign a measure of destructiveness to an earthquake (degree of damage caused) It is qualitative and based upon observed effects on people and damage to buildings.

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