What Causes Earthquakes
 Earthquakes occur when rock beneath the earth’s surface moves
 The movement of plates creates stress in the rock, when the stress builds up the
rock breaks or changes shape releasing energy in the form of vibrations (waves)
Seismic Waves
 Seismic waves are what earthquakes produce
 They ripple out in all directions from the point where the earthquake occurred
 Waves can travel from one side of the earth to the other
 3 types of seismic waves
P waves
 Primary waves (P waves): fastest moving of the three seismic waves
 They are Longitudinal waves (waves move back and forth with compression,
think pushing a spring)
 They can travel through all parts of the earth (can show up on the direct opposite
side of where the earthquake was)
S waves
 Secondary waves: are transverse waves (up and down)
 Can not travel through the liquid part of the earth
Surface waves
 Combination of P and S waves
 These are the waves you feel during an earthquake
 Cause the most ground movement
Detecting Seismic Waves
 Seismograph records the ground movements caused by seismic waves
 Seismogram are the lines recorded by the seismograph
 Richter Scale: measures the magnitude (power) of the earthquake
Richter scale
 Richter Earthquake Magnitudes Effects
 Less than 3.5 Generally not felt, but recorded.
 3.5-5.4 Often felt, but rarely causes damage
 Under 6.0 At most slight damage to well-designed buildings. Can cause major
damage to poorly constructed buildings over small regions.
 6.1-6.9 Can be destructive in areas up to about 100 kilometers across where
people live.
 7.0-7.9 Major earthquake. Can cause serious damage over larger areas.
 8 or greater Great earthquake. Can cause serious damage in areas several hundred
kilometers across.