GEOL3025, Section 096Lecture #19

18 October 2007

GEOL3025, Section 096Lecture #19

18 October 2007

Dr. Lysa ChizmadiaDr. Lysa Chizmadia

Measuring Earthquake SizesMeasuring Earthquake Sizes Intensity Scale

Mercalli Intensity Scale Based on destruction of buildings

Magnitude Scales Richter Magnitude (logarithmic) Based on the Amplitude of the largest

seismic wave recorded on seismogram

Intensity Scale Mercalli Intensity Scale Based on destruction of buildings

Magnitude Scales Richter Magnitude (logarithmic) Based on the Amplitude of the largest

seismic wave recorded on seismogram

Mercalli Intensity Scale

Mercalli Intensity Scale

Richter Magnitude Scale

Richter Magnitude Scale

Earthquake DestructionEarthquake Destruction

Seismic Vibrations Tsunami Landslides & Ground Subsidence Fire

Seismic Vibrations Tsunami Landslides & Ground Subsidence Fire

Formation of TsunamisFormation of Tsunamis

Global Distribution of Earthquakes

Global Distribution of Earthquakes

Evidence for Plate TectonicsEvidence for Plate Tectonics

Chapter 12: Earth’s InteriorChapter 12: Earth’s Interior

Propagation of Seismic Waves

Propagation of Seismic Waves Velocity of seismic

waves depends on density (ρ) & elasticity of material

Velocity of waves increases with depth b/c pressure (P) makes rocks more elastic

Refraction of waves occurs when they pass from one material to another

Velocity of seismic waves depends on density (ρ) & elasticity of material

Velocity of waves increases with depth b/c pressure (P) makes rocks more elastic

Refraction of waves occurs when they pass from one material to another

P waves travel through both liquids & solids

S waves travel only through solids

P waves travel faster than S waves in all materials

P waves travel through both liquids & solids

S waves travel only through solids

P waves travel faster than S waves in all materials

P wavesP waves P waves

Compressional First to arrive

(primary) Travel through both

liquids & solids

P waves Compressional First to arrive

(primary) Travel through both

liquids & solids

S wavesS waves S waves

Shear waves Second to arrive Travel through

solids only

S waves Shear waves Second to arrive Travel through

solids only

Propagation of WavesPropagation of Waves Through Uniform

Solid Constant velocity

with depth

Through Uniform Solid

Constant velocity with depth

Through Solid of Increasing P

Increasing velocity with depth

Through Solid of Increasing P

Increasing velocity with depth

Wave Propagation Through Earth

Wave Propagation Through Earth

Because Earth has differentiated, waves travel at different speeds through the different layers

Because Earth has differentiated, waves travel at different speeds through the different layers

Layers Defined by Composition

Layers Defined by Composition

Crust: 3-70 km thick outer skin

Mantle: 70-2900 km solid silicate-rich shell

Core: Fe-rich sphere with 3486 km radius

Crust: 3-70 km thick outer skin

Mantle: 70-2900 km solid silicate-rich shell

Core: Fe-rich sphere with 3486 km radius

Layers Defined by Physical Properties (Seismic Waves)Layers Defined by Physical Properties (Seismic Waves)

Lithosphere: 3-250 km thick crust &

uppermost mantle Cold and rigid

Asthenosphere: 250-660 km lower upper

mantle Warm and soft

Mesosphere: (lower mantle) 660-2900 km lower mantle More rigid than

asthenosphere due to increased P

Lithosphere: 3-250 km thick crust &

uppermost mantle Cold and rigid

Asthenosphere: 250-660 km lower upper

mantle Warm and soft

Mesosphere: (lower mantle) 660-2900 km lower mantle More rigid than

asthenosphere due to increased P

Inner Core: 2270 km thick Liquid b/c s-waves don’t

penetrate Convection produces

Earth’s magnetic field Outer Core:

Fe-Ni metallic sphere with radius of 3486 km

Solid because of increased P

Inner Core: 2270 km thick Liquid b/c s-waves don’t

penetrate Convection produces

Earth’s magnetic field Outer Core:

Fe-Ni metallic sphere with radius of 3486 km

Solid because of increased P

Earth’s LayersEarth’s Layers

Earth’s Major BoundariesEarth’s Major Boundaries The Mohorovičič discontinuity (Moho)

Divides Crust from Mantle Seismic velocities higher in Mantle so deeper

waves arrive earlier Core-Mantle Boundary

P-wave shadow zone 105°-140° Due to high refraction of waves

S-waves do not penetrate at all Inner-Outer Core

P-waves reflected off surface of solid inner Core

P-waves faster in inner Core than in outer Core

The Mohorovičič discontinuity (Moho) Divides Crust from Mantle Seismic velocities higher in Mantle so deeper

waves arrive earlier Core-Mantle Boundary

P-wave shadow zone 105°-140° Due to high refraction of waves

S-waves do not penetrate at all Inner-Outer Core

P-waves reflected off surface of solid inner Core

P-waves faster in inner Core than in outer Core

The MohoThe

Moho

Core-Mantle

Boundary

Core-Mantle

Boundary

Core-Mantle BoundaryCore-Mantle Boundary

Discovery of Inner

Core

Discovery of Inner

Core