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GEOL: CHAPTER 8
Earthquakes
and Earth’s Interior
Learning Outcomes
LO1: Explain Elastic Rebound Theory
LO2: Describe seismology
LO3: Identify where earthquakes occur,
and how often
LO4: Identify different seismic waves
LO5: Discuss how earthquakes are
located
Learning Outcomes, cont.
LO6: Explain how the strength of an
earthquake is measured
LO7: Describe the destructive effects of
earthquakes
LO8: Discuss earthquake prediction
methods
LO9: Discuss earthquake control methods
Learning Outcomes, cont.
LO10: Describe Earth's interior
LO11: Examine Earth's core
LO12: Examine Earth's mantle
LO13: Describe Earth's internal heat
LO14: Examine earth's crust
Earthquakes
• Earthquake: shaking or trembling of the
ground caused by the sudden release of
energy, usually as a result of faulting,
which involves the displacement of
rocks along fractures
• Aftershocks: from continued
adjustments along a fault; usually
smaller than the initial quake
Elastic Rebound Theory
• Rocks undergoing deformation bend
and store energy
• When strength of rock is exceeded, they
rupture and release energy – the
earthquake
• Rocks rebound to original, undeformed
shape
Seismology
• Seismology: the study of earthquakes
• Seismic waves: energy from
earthquakes
• Seismographs: detect, record, and
measure earthquakes
• Seismogram: record from a
seismograph
• Earthquakes occur along faults, where
movement is stored as energy in rocks
• Most faults related to plate movements
Focus and Epicenter
• Focus: point where energy is first
released
• Epicenter: point on surface above focus
• Shallow-focus: 0-70 km below surface
• Intermediate focus: 70-300 km below
surface
• Deep-focus: >300 km below surface
• 90% less than 100 km below surface
Earthquakes and
Plate Boundaries
• Divergent and transform boundaries:
always shallow-focus
• Convergent boundaries:
– shallow-, intermediate-, and deep-focus
– Beniorr-Wadati zones: foci along
subducted plate
Earthquake Epicenters and Plate Boundaries This map of earthquake
epicenters shows that most earthquakes occur within seismic zones that
correspond closely to plate boundaries. Approximately 80% of earthquakes
occur within the circum-Pacific belt, 15% within the Mediterranean–Asiatic
belt, and the remaining 5% within plate interiors and along oceanic
spreading ridges. The dots represent earthquake epicenters and are
divided into shallow-, intermediate-, and deep-focus earthquakes. Along
with many shallow-focus earthquakes, nearly all intermediate- and deepfocus earthquakes occur along convergent plate boundaries.
Benioff Zones Focal depth increases in a well-defined zone
that dips approximately 45 degrees beneath the Tonga
volcanic arc in the South Pacific. Dipping seismic zones are
called Benioff or Benioff–Wadati zones.
Major Earthquake Regions
• Plate boundaries: convergent,
divergent, and transform
• 80% in circum-Pacific belt
• 15% in Mediterranean-Asian belt
• 5% in plate interiors and ocean
spreading ridges
• Intraplate: from compression of plate
along margins
Seismic Waves
• All waves generated by an earthquake
• Body waves
– P-waves
– S-waves
– Travel faster through less dense, more
elastic rocks
• Surface waves
– R-waves
– L-waves
P-Waves
•
•
•
•
Primary waves
Fastest seismic waves
Travel through solids, liquids, and gases
Compressional/push-pull: expand and
compress material, like sound waves
S-Waves
•
•
•
•
Secondary waves
Slower than P-waves
Travel only through solids
Shear waves: move material
perpendicular to direction of wave
movement
• Create shear stresses
Undisturbed material
Surface
Undisturbed
material
Primary wave (P-wave)
Direction of wave movement
Wavelength
Secondary wave (S-wave)
Focus
Stepped Art
Fig. 8-7, p. 156
Surface Waves
• Travel at or just below the surface
• Slower than body waves
• R-waves (Rayleigh waves)
– Particles move in elliptical path, like water
waves
• L-waves (Love waves)
– Faster than R-waves
– Particles move back forth in horizontal
plane perpendicular to direction of travel
Undisturbed material
Rayleigh wave (R-wave)
Rayleigh wave
Love wave
Love wave (L-wave)
Stepped Art
Fig. 8-8, p. 157
Locating an Earthquake
• P-wave and S-wave average speeds are
known
• Time-distance graphs: difference in arrival
time of the 2 waves vs. distance from focus
• The farther the waves travel, the greater
the P-S time interval
• Epicenter can be determined when the P-S
time intervals of at least three seismic
stations are known
Earthquake Intensity
• Subjective measure of earthquake
damage and human reaction
• Modified Mercalli Intensity Scale
• Maps with intensity zones
Earthquake Intensity, cont.
•
Factors that affect earthquake
intensity
1.
2.
3.
4.
5.
6.
7.
Size of earthquake
Distance from epicenter
Focal depth
Population density
Geology of area
Building construction
Duration of shaking
Earthquake Magnitude
• Quantitative measure: amount of energy
released
• Richter Magnitude Scale: total amount of
energy released at earthquake source
• Measure amplitude of largest seismic
wave
• Logarithmic: each whole-number increase
is a 10-fold increase in amplitude, but a
30-fold increase in energy
Earthquake Magnitude, cont.
• Richter Magnitude Scale underestimates
energy of very large quakes
– Only measures peak energy, not duration
• Seismic-moment magnitude scale:
– Strength of rocks
– Area of fault rupture
– Amount of movement of rocks adjacent to fault
• 1964 Alaska earthquake:
– 8.6 Richter
– 9.2 seismic-moment
Earthquake Destruction
• Deaths, injuries, property damage:
– Work or school hours
– Population density
– Magnitude
– Duration
– Distance from epicenter
– Type of structures
– Local geological characteristics
Earthquake Destruction, cont.
• Destructive effects:
– Ground shaking
– Fire
– Seismic sea waves (tsunamis)
– Landslides
– Disruption of services
– Panic and psychological shock
Ground Shaking
• Magnitude and distance
• Underlying materials
– Poorly consolidated and water-saturated
materials experience stronger S-waves
– Liquefaction: water-saturated sediments
behave as a fluid
• Poor building materials: adobe, mud,
brick, poorly built concrete
• Most common cause of fatalities/injuries
Fire
• Common in urban areas
• 1906 San Francisco earthquake
– Severed electrical and gas lines
– Fires spread throughout city
– Water mains ruptured, so couldn’t put out
fires
• 1923 Tokyo earthquake
– 71% of houses burned
Tsunami
• Indian Ocean: 12/26/2004; 9.0
• Seismic sea wave, not tidal wave
• Caused by:
– Submarine earthquakes
– Submarine volcanoes
– Submarine landslides
• Can travel across entire oceans
Tsunami, cont.
•
•
•
•
Travel at ~600 mph
Wave height less than 1 meter
Wave length of several hundred miles
Shallow water: wave slows and wave
height increases
• 1946 Hilo tsunami: 16.5 m high
Tsunami, cont.
• Prior: sea withdraws, exposing the
seafloor
• Pacific Tsunami Early Warning System
– Seismographs
– Instruments that detect seismic sea waves
• No warning system in the Indian Ocean
Ground Failure
•
•
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Earthquake-triggered landslides
Very dangerous in mountain regions
Cause many deaths and much damage
1959 Madison Canyon slide
1970: Peru earthquake triggered
avalanche that killed 66,000 people
Earthquake Prediction
•
Successful prediction must include:
1. Time frame
2. Location
3. Strength
•
•
•
•
Successful predictions are rare
Successful predictions would save
lives
Seismic hazard maps help
U.S., China, Japan, Russia have
programs
Earthquake Precursors
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•
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Plotting locations of earthquakes
Locate seismic gaps on fault
Slight changes in elevation
Tilting of surface
Water level fluctuations
Changes in Earth’s magnetic field
Electrical resistance of ground
Earthquake Control
• Prevention unlikely
• But may be able to gradually release
energy stored in rocks
• Geologists can potentially inject liquids
into locked segments and seismic gaps
of faults to release small quakes; but
could also cause a big quake
Earth’s Interior
•
•
•
•
Crust
Mantle
Outer core
Inner core
Seismic Waves
and Earth’s Interior
• P-wave and S-wave velocity determined
by density and elasticity of material
• S-waves don’t travel through liquids
• Seismic waves change velocity and
direction when enter material with
different density or elasticity (refraction)
Seismic Waves
and Earth’s Interior, cont.
• Some waves are reflected
• Calculate depths of boundaries
• Discontinuity: significant change in
materials or their properties
The Core
• P-wave velocity decreases at a depth of
2,900 km: core-mantle discontinuity
• P-wave shadow zone
• Weak P-wave energy does penetrate
the shadow zone: from solid inner core
• S-wave shadow zone: shows the outer
core is liquid, because S-waves can’t
travel through liquids
Core Density and Composition
•
•
•
•
16.4% Earth volume
~33% of mass
Outer core: 9.9 to 12.2 g/cm3
Earth center: pressure 3.5 million times
of surface
• Outer core: iron, sulfur, silicon, oxygen,
nickel, potassium
• Inner core: iron and nickel
Earth’s Mantle
• Moho: discontinuity about 30 km deep
• Asthenosphere:
– 100-250 km deep
– P- and S-waves slow down
– Plastic
– Magma generation
– Lithospheric plates ride across it
• 3.3 to 5.7 g/cm3; probably periodotite
Seismic Discontinuity Andrija Mohorovičić studied seismic
waves and detected a seismic discontinuity at a depth of
about 30 km. The deeper, faster seismic waves arrive at
seismic stations first, even though they travel farther. This
discontinuity, now known as the Moho, is between the crust
and mantle.
Earth’s Internal Heat
• Geothermal gradient: 25ºC/km
– Greater in active volcanic regions
• Most heat generated by radioactive
decay
• Regions of equilibrium temperature
• Base of crust: 800ºC to 1200ºC
• Core-mantle boundary: 2,500ºC 5,000ºC
Continental Crust
• Granitic composition
• 2.5 to 3.0 g/cm3; average = 2.7 g/cm3
• 20 to 90 km thick; average = 35 km
thick
• Thickest under large mountain ranges
Oceanic Crust
•
•
•
•
Gabbro overlain by basalt
Average density = 3.0 g/cm3
5-10 km thick
Thinnest at spreading ridges