Download Earthquakes and earthquake hazards

Chapter 5: Earthquakes and
Earth’s Interior
Copyright © 2012 John Wiley & Sons, Inc. All rights reserved.
Learning Objectives
Earthquakes and earthquake hazards
• Explain the role of plate tectonics and elastic
rebound in the occurrence of giant earthquakes.
The science of seismology
• Describe the methods and tools of seismology.
Studying Earth’s interior
• Describe Earth’s liquid core and the ways in
which scientists study it.
A multilayered planet
• Discuss the composition of Earth’s crust, mantle,
and core.
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Megathrust earthquakes
This ancient stone monument marks the high-water
point of a past tsunami.
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Earthquakes and Earthquake Hazards
Seismology
The scientific study of
earthquakes and seismic
waves
Answers questions like: why
are earthquakes so sudden?
big ones cause catastrophic
damage? why occur in same
places?
Seismic waves are shock
waves releasing energy when
earthquake occurs.
Fig. 5.2a Offset orange groves
Fig. 5.2b Vertical displacement, Alaska, 1964
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Earthquakes and Earthquake Hazards
Earthquakes and Plate Motion
The elastic rebound theory
• Continuing stress along a fault
• Results in buildup of elastic
energy in the rocks
• Energy abruptly released
when an earthquake occurs
Case Study: Fence crosses SA fault
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Earthquakes and Earthquake Hazards
Earthquakes and Plate Motion
Case Study: Proving the Elastic Rebound Theory
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Earthquakes and Earthquake Hazards
Earthquakes and Plate Motion
Case Study: Proving the Elastic Rebound Theory
A = unstressed state
B= stress build-up
C= moment of frictional lock breaking, returning to unstressed state
© 2012 John Wiley & Sons, Inc. All rights reserved.
Earthquakes and Earthquake Hazards
Earthquake Hazards and Readiness
Primary hazards: cause damage during an earthquake
• Collapsing buildings, bridges, and other structures
• Aftershock
Secondary hazards: after-effects
• Landslides, fires, ground liquefaction, tsunamis
• Can cause more damage than original quake
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Earthquakes and Earthquake Hazards
Earthquake Hazards and Readiness
Secondary Hazards
Figure 5.3a Landslide, Huascaran, Peru
Figure 5.3b Open fissure, Santa Cruz, CA
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© 2012 John Wiley & Sons, Inc. All rights reserved.
© 2012 John Wiley & Sons, Inc. All rights reserved.
© 2012 John Wiley & Sons, Inc. All rights reserved.
Earthquakes and Earthquake Hazards
Earthquake Hazards and Readiness
Secondary Hazards
Figure 5.3c
Fire, San Francisco, California
Figure 5.3d
Ground liquification, Niigata, Japan
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http://www2.pvc.maricopa.edu/ssd/geog/outlines/GPH111/images/chap13_15/liquefaction_venez.jpg
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The Big Picture:
How tsunamis are unleashed
Figure 5.4 Process diagram
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Earthquakes and Earthquake Hazards
Earthquake Hazards and Readiness
Case Study: The Sumatra-Andaman Tsunami (2004)
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Earthquakes and Earthquake Hazards
Earthquake Prediction
Short-term prediction and early warning
• Precursor phenomena
• Foreshocks
Long-term forecasting
• Paleoseismology: The study of prehistoric
earthquakes
• Seismic gaps
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Evidence of ancient quakes
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Earthquakes and Earthquake Hazards
Earthquake Hazards and Readiness
Preparation and readiness
to earthquakes key to
reducing fatalities
• Reinforced structures
• Bolting wood-framed
buildings to foundation
• Protecting utility lines
from movement
Figure 5.5a Port au prince, Haiti
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Earthquake Readiness
Preparation and
readiness to
earthquakes key to
reducing fatalities
• Education
Figure 5.5c Schoolchildren in Japan EQ drill
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The Science of Seismology
Seismographs
Seismograph
•An instrument that detects
and measures vibrations of
Earth’s surface
•Advanced seismographs
detect vibrations 10–8 of a
centimeter
Seismogram
•The record made by a
seismograph
Figure 5.7a Ancient Chinese seismograph
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The Science of Seismology
Figure 5.7b How a seismograph works
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The Science of Seismology
Seismic Waves
Body waves
•Travels through Earth’s
interior
Surface waves
•Travels along Earth’s
surface
Focus
•Where rupture commences
and an earthquake’s energy
is first released
Figure 5.8 Seismic waves move outward in all directions
from the focus.
© 2012 John Wiley & Sons, Inc. All rights reserved.
The Science of Seismology
Seismic Waves
Compressional wave:
– Wave consisting of alternating pulses of compression
and expansion
– Can pass through any medium (solids, liquids, gases)
– P (or primary) wave
Shear wave:
– Rock is subjected to side to side or up and down
forces, perpendicular to wave’s direction of travel
– S (secondary) wave
– Not transmitted through liquids or gases
– Travel slower than P waves
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Locating Earthquakes
Figure 5.10a Typical seismogram showing P-, S-, and surface wave first-arrivals
© 2012 John Wiley & Sons, Inc. All rights reserved.
The Science of Seismology
Locating Earthquakes
Epicenter
•The point on Earth’s surface
directly above an
earthquake’s focus.
•Using seismograms from at
least three locations will
produce intersecting circles
to locate the epicenter.
Figure 5.10b Using seismograms to locate an earthquake
epicenter.
© 2012 John Wiley & Sons, Inc. All rights reserved.
The Science of Seismology
Measuring Earthquakes
The Richter magnitude scale
•A scale of earthquake intensity based on the recorded
heights, or amplitudes, of the seismic waves recorded on
a seismograph
•A logarithmic scale—a 10-fold increase in amplitude for
each unit
Moment magnitude scale
•A measure of earthquake strength that is based on the
rupture size, rock properties, and amount of
displacement on the fault surface
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The Science of Seismology
Measuring Earthquakes
Modified Mercalli Intensity scale
• Based on descriptions of what people feel, see,
and damage that has occurred.
• This scale differs with distance from the epicenter.
Moment Magnitude 7.0; Mercalli IX
Moment Magnitude 8.8; Mercalli VIII
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Measuring Earthquakes
What a Geologist Sees
Richter magnitude 6
Richter magnitude 7
Richter magnitude 8
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Studying Earth’s Interior
•We cannot directly study the Earth’s interior, so
remote sensing is a technique that is widely used
in geology.
•Seismic waves and how they travel through the
Earth is one of the best methods available.
© 2012 John Wiley & Sons, Inc. All rights reserved.
Studying Earth’s Interior
Figure 5.11a Seismic waves in Earth’s interior
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Studying Earth’s Interior
How Geologists Look into Earth’s Interior
Three things can happen to seismic waves when
they meet a boundary:
• Refraction: waves are bent as they pass from
one material to another.
• Reflection: some or all of the wave energy
bounces back.
• Absorption: some or all of the wave energy is
blocked.
© 2012 John Wiley & Sons, Inc. All rights reserved.
Studying Earth’s Interior
How Geologists Look into Earth’s Interior
P- and S-waves
• Travel differently but
consistently through the
Earth creating shadow
zones
Seismic discontinuity
• A boundary inside Earth
where the velocities of
seismic waves change
abruptly
Figure 5.11b Seismic waves in Earth’s interior
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Studying Earth’s Interior
How Geologists Look into Earth’s Interior
Seismic tomography
• Allows geologists to
image inside of Earth
using 3D technologies
Direct observation
• Drilling
• Xenoliths
Indirect observation
• Magnetism
• Density
Figure 5.14a Earth’s magnetic field
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Diamonds: Messengers from the deep
a. A special photo of
diamond surface to
identify variations in
composition
b. Oppenheimer
diamond from
S. Africa
Figure 5.13 Diamonds
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c. Kimberlite
pipe through
which a
diamond
erupts to reach
the surface
A Multilayered Planet
The Crust
•The crust is the outermost compositional layer of
the solid Earth, part of the lithosphere.
•Thickness ranges between 8 km (oceanic) and 45
km (continental).
•95% of crust is igneous or metamorphic derived
from igneous rocks.
•Mohorovcic (Moho) discontinuity marks boundary
between crust and upper mantle (diff. densities
and compositions, but similar physical
characteristics).
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A Multilayered Planet
Figure 5.15 Inside view of Earth
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A Multilayered Planet
The Mantle
•The middle compositional layer of Earth, between
the core and the crust.
•It is composed primarily of olivine and pyroxene.
•Asthenosphere: mantle where weak, ductile rock
is near melting but not molten.
•Mesosphere: part of the mantle below the
asthenosphere where rocks are stronger from
being highly compressed.
© 2012 John Wiley & Sons, Inc. All rights reserved.
A Multilayered Planet
The Mantle
Lithosphere
•The layer above the asthenosphere includes both
the crust and uppermost part of mantle.
•About 100 km thick.
•These rocks are stronger and more rigid than the
asthenosphere.
•It acts as one unit that makes up the tectonic
plates.
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A Multilayered Planet
Figure 5.16 Seismic discontinuities in the mantle
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A Multilayered Planet
The Core
Core
• Innermost layer, where
the magnetic field is
generated and much
geothermal energy
resides
• Separated into outer
core (liquid) and inner
core (solid)
• Composed of primarily
iron-nickel metal
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A Multilayered Planet
•Heat in the core is escaping very slowly up
through the surface.
•Release of heat from interior is driving energy for
plate tectonics.
http://www.gns.cri.nz/Home/Learning/Science-Topics/Earth-Energy/Geothermal-Energy/How-is-the-heat-created
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Critical Thinking
•If you were on a ship in the ocean, would you be
able to feel an earthquake that occurred below
you, on the ocean floor?
•Do you think there is no limit to the magnitude of
earthquakes?
•What kind of wave would you expect to travel
faster: a seismic wave or a tsunami wave?
© 2012 John Wiley & Sons, Inc. All rights reserved.