And Earth’s Structure
Earth’s Structure
Origin of the Earth
• Meteors and Asteroids
bombarded the Earth
• Gravitational compression
• Density Stratified planet
Earth’s Interior
• Core
Iron and Nickel
Inner Core - solid
Outer Core - liquid
• Mantle
Less dense than core
Iron and Magnesium silicates
Mostly solid
Upper mantle is partially molten
• Crust
– Outermost layer
– Very thin and rigid
– Continental – granite
– Density = 2.8 g/cm3
– Oceanic – basalt
– Density = 3.0 g/cm3
Evidence of Internal Structure
• Density
– calculate density of Earth
– Speculate on probable compositions
• Meteorites
– Use composition and age to
determine composition and age of
• Seismic waves
– Travel times and direction give
indication of internal structure of
Types of Seismic Waves
• P waves
– Primary waves
– Push and pull movement
– Travel fastest (~ 6 km/sec)
– Travel thru solids and liquids
• S waves
– Secondary waves
– Move side-to-side
– Slower (~ 4 km/sec)
– Travel thru solids only
Seismic Waves Through Earth
What is an Earthquake?
• The vibration of Earth produced by the rapid release
of energy
Elastic Rebound Theory
• The crust will first bend
• When the stress exceeds strength of the rock, it will “snap”
into a new position
• In the process of breaking, “seismic waves are released
Parts of an Earthquake
• Focus
–place within earth where EQ originate
• Epicenter
– Location on surface directly above the focus
• Energy released radiates in all directions from the focus
• Energy is in the form of waves – “seismic waves”
How Do We Locate an Epicenter?
• Instruments around the
world record EQ
• Record Earth movements by
stationary mass on rotating
• Use seismogram to:
– Locate an epicenter
– Determine magnitude
• Help determine the epicenter
• Measure the distance between P- and S-waves
– This is the time difference in arrival times
Locating the Epicenter
• Plot the time difference on y-axis (time interval)
• Trace plot to blue line to determine distance to epicenter
Locating an Epicenter
• Measure that distance
around the seismic station
• The epicenter may be
located anywhere on that
• A minimum of 3 seismic stations
are needed to determine the
Determining Magnitude
• Measure the amplitude
of the strongest wave
• The amplitude is the
height on paper
• Plot distance between pand s-wave
• Plot amplitude
• Connect plots to
determine magnitude
You are now a Seismologist!!
Virtual Earthquake Introduction:
Virtual Earthquake: