Seismic Profiles of Earth’s Interior
Objective: Use seismic waves to determine the nature of
Earth’s interior layers
Scientists learn about Earth’s interior by using seismic waves or vibrations
that travel through Earth’s layers during earthquakes. There are different
types of seismic waves. P waves (Primary waves) are compressional
vibrations that travel accordion style through solids and liquids. S waves
(Secondary waves) are transverse. They travel in a snake-like fashion, but
only through solid mediums. Surface waves travel in a variety of methods,
but only along the solid crust.
Seismic waves change speed and angle as they move through different
mediums. In general, seismic waves travel faster through solids or rigid
densely packed materials.
1
Radius = 6370 km
Mantle
Crust
Liquid iron
outer core
Solid iron
inner core
http://www.seismo.unr.edu/ftp/pub/louie/class/100/interior.html
http://www.uh.edu/~jbutler/physical/chapter19.html
Distance to core is ~1/2 the radius (2900 km depth)
Core
Mantle
Crust
Sulfur (4%)
Nickel
(11%)
Iron (85%)
Density:
~11 g/cm3
Density:
~5 g/cm3
Density:
~3 g/cm3
Types of Seismic Waves
4-6 km/s
2-3 km/s
P (primary) and S (secondary) waves travel through Earth’s interior
info about the mantle and core (density, composition)
Surfaces waves (Love, Rayleigh) travel through the crust
 info about the crust
Seismic Wave Velocity
Density
(composition)
Physical State
Which rock will seismic waves travel
the fastest through: Sandstone or
Periodotite? Give a reason why.
Seismic waves travel faster through:
solids or liquids?
Reflection and Refraction
Refraction
Reflection
This is what happens to
seismic waves in the Earth.
Waves BEND or REFRACT
when they move through
different mediums.
Seismic Wave Reflection
• Used to find depth to boundaries (density discontiniuites)
P-Wave Reflections: P waves can travel through solids or liquids
P-Wave Shadow Zone
Indicates depth to the
core-mantle boundary
S-Wave Shadow Zone
Indicates that the outer
core is liquid
Seismic Velocity Profile
of Earth
A
B
D
E
liquid
F
solid
1. Which seismic waves
travel faster: S waves or P
waves?
2. Describe how velocity of
seismic waves change
from 0 – 670 km depth.
3. Give a reason for the
change in speed in the first
670 km depth.
4. How does the velocity of
seismic waves in the lower
mantle compare to that of
the upper mantle? (Where
do they travel faster?)
5. Give a reason for the
change is velocity between
the upper and lower
mantle.
6. What happens to waves
between Dand E? Why?
7. Why are there no S waves
in the outer core?
8. Which layer do waves
travel fastest and why?
The Crust & Mantle:
Close Up
A
.
B
9. How does velocity of P wave
change from A to B?
Explain why.
C
D
10. What change in
composition or structure
may account for the slow
down of waves by C?
(Hint: you may need to
Google “low-velocity
zone”)
E
F
11. What accounts for the
gradual increase in speed
from D to F?
12. Summarize how seismic
waves allow us to “see”
inside Earth.
Convection in the Mantle
subducted slabs
large-scale
convection
CMB
mantle
plumes