Earthquakes and Seismic Waves
2-1
What is an Earthquake?
• Shaking and trembling that results from the
movement of rock beneath Earth’s surface
• Occur when stress along a fault increases so
much that the rock breaks
• Releases an enormous amount of stored
energy
• Most begin in lithosphere
Parts of an earthquake
• Focus-where earthquake starts beneath the
surface
• Epicenter-point on surface directly above
focus
Seismic waves
• Move like ripples on pond
• Carry energy from earthquake away from
focus, through Earth’s interior, and across the
surface
• Includes P waves, S waves, and surface waves
Types of Seismic Waves
Mercalli Scale
• Rates level of damage
• 12 step scale
• Same earthquake can
have different ratings in
different locations
Richter
• Rating of earthquakes magnitude based on size of
seismic waves
• Scale of 10
• Measured with seismograph
• Good for nearby earthquakes
**Each point increase in magnitude represents a
release of 32x more energy
Moment Magnitude
• Estimates total energy released
• Rates earthquakes near and far
• Looks at:
– kinds of seismic waves and how strong they were
– how much the fault moved
– strength of the rocks that broke
• What we usually hear on news
Locating Epicenter
• Measure difference
between the arrival of P and
S waves
• The farther away the
earthquake, the greater the
time difference between
the arrival of P and S waves
• Draw circles from data from
at least 3 different
seismographs
• The radius of each circle is
the distance from
seismograph to epicenter