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Lecture 3

EARTH 108 Lecture Notes - Lecture 3: Peak Ground Acceleration, Seismic Wave, Seismogram

Earth and Environmental Sciences
Course Code
Eric Hetland

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Lecture 3: When Earth Attacks, the science behind natural disasters
Lecture outline
1) radiation of earthquake energy
2) earthquake magnitude
a) Richter magnitude (measure of shaking)
b) moment magnitude (measure of energy)
c) intensity magnitude (measure of damage)
3) peak ground acceleration & PAGER
4) east vs. west intensity
Earthquake slip
• earthquakes occur on fractures in the crust called faults
• earthquakes relieve accumulated stress due to continual plate motion
• earthquake magnitude is the area of the earthquake x the total offset in the
•earthquakes usually take about 10-240 seconds
Earthquake shaking
• earthquake energy is radiated from the fault as seismic waves
• epicenter: the location on Earth of the focus (or start) of the earthquake
• seimographs measure the seismic waves
• record of the movement at the surface is a seismogram
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Seismic waves
• there are two types of seismic waves:
-body waves travel through the earth
1. P-wave (primary, compressional)
2. S-wave (secondary, shear)
-surface waves travel along the surface
• most damage from surface waves
• all travel at different speeds
-P-waves: 5-8 km/sec
-S-waves: 3-5 km/sec
-Love waves: 3-4.5 km/sec
-Rayleigh waves: 2-5 km/sec
Earthquake size
We can quantify earthquake size by:
• the amplitude of the seismogram
• the energy released in the earthquake
• the intensity of the ground shaking
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