Seismic waves and the shadow zone
 Sudden
release of energy in the Earth’s
crust and creates seismic waves.
• Occurs naturally or human induced.
 Faulting: most
common cause of earthquakes
 Focus: Point below the surface where rocks
break and move (underground point of origin)
 Epicenter: Directly above the focus at surface
(most violent shaking occurs here)
 Three
types of seismic waves
• 1. Primary (P waves)
• 2. Secondary (S waves)
• 3. Surface (L waves)
P
•
•
•
•
waves
S
•
•
•
•
waves
Fastest
Travel through solid, liquid, and gas
Push pull waves
Moves through the core and faster through dense
material
Arrive after P waves (slower)
Travel only through solids
Particles move side to side
Shake waves
L
waves
• Slowest
• Originate at the surface (epicenter)
• Move up and down like ocean waves
• Cause the most damage


A shadow zone is an area of the earth that is
"shadowed" by the planet's internal structure, in that it
doesn't receive seismic waves.
Seismic waves will bend and reflect at the interfaces
between different materials
• P waves will travel and refract through both fluid and solid
materials. S waves, however, cannot travel through fluids like air
or water.
 Seismographs
waves
 Seismograms
record earthquake
show:
•
Amplitude of seismic waves (how much
rock moves or vibrates)
•
Distance to the epicenter
•
Earthquake direction
•
P & S waves used to calculate magnitude of earthquake
•
Amplitude = height of wave (how much the rock moves; size of
vibration)
P
& S (body waves) move through earth & arrive first
 Exact
location can be known by using
three seismic stations or triangulation.

The time interval between the arrival of the first
P wave and S wave determines distance to
epicenter.
• The longer the “lag” the farther away the epicenter is.
 Intensity
Scales: measure the degree of
shaking and amount of DAMAGE done
• Modified Mercalli
 Magnitude
scale: relies on calculations of
estimated energy released by seismic
waves.
• Richter Scale
• Moment Magnitude scale
 Increases by a factor of 10