Transcript Planet Earth - HCC Learning Web

Seismicity and
Earth’s
Interior: Part I
Chapter 18
Dynamic Earth
Eric H Christiansen
Major Concepts
• Seismic waves are vibrations in Earth caused by the rupture and
sudden movement of rock.
• Three types of seismic waves are produced by an earthquake shock:
• P waves, S waves, and surface waves.
• Earthquakes hazards include:
• Ground shaking, Surface faulting, Liquefaction, Tsunamis, and Landslides.
• Fires from broken gas lines and Floods
• The exact location and timing of an earthquake cannot be predicted.
However, seismic risk can be evaluated and, in areas with high risk,
preparations for future earthquakes made.
Characteristics of Earthquakes
• Earthquakes are vibrations of
Earth, caused by the rupture and
sudden movement of rocks that
have been strained beyond their
elastic limits. Three types of
seismic waves are generated by
an earthquake shock: (1)
primary waves, (2) secondary
waves, and (3) surface waves.
Earthquakes
• Earthquakes are caused by a
sudden release of energy as
rocks are broken
• The amount of energy released
determines the magnitude of
the earthquake
• Seismic waves carry the energy
away from its origin
Courtesy of DigitalGlobe, Inc
Figure 15.32B: The great Indian Ocean Tsunami of 2004
devastated shorelines and was one of the deadliest known
on Earth.
Elastic Rebound
Strain builds up in rocks in seismically active areas until
the rocks rupture or move along preexisting fractures.
Energy is released when the rocks rupture, and seismic
waves move out from the point of rupture.
Figure 18.01A: Strain builds up in rocks in
seismically active areas until the rocks
rupture or move along preexisting fractures.
Figure 18.01B: Energy is released when the
rocks rupture, and seismic waves move out
from the point of rupture.
Earthquake Geometry
• The focus is the point of initial
rupture or movement on the
fault.
• Seismic waves radiate from the
focus.
• The epicenter is the point on
Earth’s surface directly above
the focus.
Figure 18.02: The relationship between an earthquake’s
focus, its epicenter, and seismic wave fronts is depicted in
this diagram.
Types of Seismic Waves
Figure 18.03: Motion produced by various types of seismic waves can be illustrated by the distortions
they produce in a regular grid. For comparison, the motion of one type of surface wave is shown.
Locating an Earthquake
Figure 18.04: Locating the epicenter of an earthquake is
accomplished by comparing the arrival times of P waves and S
waves at three seismic stations.
Earthquake Intensity
• The intensity of an
earthquake can be magnified
in unconsolidated sediment.
• Seismographs on
unconsolidated sediment
measured much greater wave
amplitudes and durations
than seismometers on solid
bedrock. Amplification can be
by a factor of 10 or more.
Figure 18.05: The intensity of an earthquake can be magnified in
locales with unconsolidated sediment or landfill.
• The seismograms shown here
are for the same aftershock of
the 1992 Landers earthquake
in California.
Earthquake Magnitude
• Magnitude is a measure of the
energy released
• Magnitude measurements are
based on:
• Measurement of amplitude of
seismic waves
• Measurement of the amount of
energy released
• Evaluation of damage caused
• The Richter scale measures the
amplitude of seismic waves
• The Richter scale is logarithmic Each unit on the scale relates a
10 fold increase in the amplitude
of the seismic wave
• Amplitude may be related to the
energy released – 1 Richter unit
~ 30x the energy
• No longer commonly used
Moment Magnitude Scale
Magnitude
Approx Number per Year
1
Millions
2
1,300,000
3
130,000
4
13,000
5
1300
6
130
7
15
8
1
>8
1 every few years
• Moment magnitude scale
measures the amount of energy
released
• Designed to differentiate large
earthquakes
• May be used to calculate energy
of old events by slip along fault
• Most commonly used by
geologists today
Earthquake Hazards
• Earthquakes pose a significant factor killing thousands .
• Ground shaking
• Surface faulting
• Liquefaction
• Tsunamis
• Landslides.
• Fires from broken gas lines
• Floods caused by breakage of water lines or dams.
Surface Faulting
• Rupture of the Earth’s surface
• Fault scarp is produced
• Only a threat in small area
• The ground surface can rupture
along stretches as long as 10s of
km, with offsets as much as a
few meters.
Figure 07.11: Cumulative movement on the fault during
a vast period of time produced the mountain range.
Courtesy of Glenn Embree
Surface Faulting
• Rupture of the Earth’s surface
• Horizontal offsets occur on
strike-slip faults
• The ground surface ruptures can
be 100s of km long, with offsets
as much as 10 meters.
• 1971 earthquake in San
Fernando, California, laterally
displaced railroad tracks near
Los Angeles
Courtesy of U.S. Geological Survey
Figure 18.07D: Earthquake of February 9, 1971, San
Fernando, California. Southern Pacific railroad tracks near
Los Angeles were laterally displaced.
Surface Rupture and Radar Interferometry
Ground Shaking
• Wide areas experience ground
shaking, not just the area along
the fault
• Most damage is from ground
shaking
• Shaking can cause the collapse
of buildings, bridges, masonry,
freeway overpasses
• Unreinforced buildings most
susceptible
© Robert A. Eplett/FEMA News Photo
Figure 18.07B: Earthquake of January 1994, Northridge,
California. Extensive damage to freeway overpasses occurred.
Liquefaction
Figure 18.07A: Earthquake of June 16, 1964, Niigata,
Japan. Apartment houses tilted by liquefaction.
Courtesy of W. Godden Collection, NISEE-PEER, University of California, Berkeley
• Unconsolidated, water-saturated
regolith, soil, or landfill loses its
strength and behaves like a fluid
when shaken by an earthquake.
• Liquefied soils are unable to
support buildings and other
structures.
• Liquefaction most strongly affected
areas with shallow water tables.
• Geysers of wet sediment erupt
along fissures to form small mud
volcanoes.
Tsunamis
• Large (9.2 magnitude) Alaskan
earthquake of 1964.
• Shaking liquefied deltaic materials
along the coast
• Caused landslides into the ocean
• Triggered destructive tsunamis
• An overturned ship, demolished truck,
and torn-up dock strewn with logs and
scrap metal attest to the power of the
wave.
• Oil tanks ruptured and flaming
petroleum spread over the water,
igniting homes and an electrical
generation plant.
Courtesy of Woods Hole Oceanographic Institution
Figure 19.08: Open fissures along the Mid-Atlantic Ridge
were photographed from the Alvin. Hundreds of such
fissures were mapped.
Landslides
• The Northridge, California,
earthquake of January 1994
(magnitude 6.6).
• An aftershock caused landslides
and dust.
• Landslides closed roads
• Surface rupture 15 km long
• Buildings collapsed
• Ruptured highways, water and gas
lines, fires
• 50 people died and $30 billion in
property damage
Courtesy of P. Morton
Figure 18.09: The Northridge, California, earthquake of
January 1994 caused several types of damage.
Fires
• Kobe, Japan 1995, magnitude 7.2
• Shaking and liquefaction were the
main causes of damage
• Much of the city is built on artificial
landfill on a granite basement
• Nearly 5500 died mostly as a result
of building collapse
• Fires started from broken gas lines
and burned out of control because
of ruptured water lines
• Surface rupture 9 km long
Seismic Risk Map
Figure 18.06: A seismic risk map shows the likelihood that an earthquake of a certain magnitude will occur.
Earthquake Prediction
• Effective short-term
earthquake prediction could
save many lives and millions of
dollars in property damage
• But it is proving to be elusive.
• Preparation is a more
achievable goal.
Earthquake Prediction
• Six earthquakes near Parkfield,
California, happened almost as
regularly as the ticks of a clock.
• Led to a prediction that a major
earthquake would occur between
1988 and 1993 (yellow circle).
• However, the next earthquake
along the fault did not occur until
2004, showing just how difficult it
is to predict earthquakes.
Figure 18.12: Six Parkfield earthquakes between 1857 and
1966 happened almost as regularly as the ticks of a clock.
Earthquake Prediction and Seismic Gaps
Figure 18.13: Seismic gaps are important in earthquake forecasting.
• Seismic gaps are important in earthquake forecasting.
• Areas along plate margins that are not seismically active
are building up stress and may be sites of future seismic
activity
• In the gray areas, recent earthquakes relieved strainWithin
In the red areas, no large quakes occurred the 40 years
prior to 2000 and strain was building
• The 2004 Sumatran earthquake closed the gap in the
western islands of Indonesian
Earthquake Preparation: Before An Earthquake
Check for hazards inside your home.
Focus on objects that could fall on you
during the quake.
Learn how to turn off utilities at your
house. Flexible gas lines should be used
to avoid breaking. Keep an adjustable
wrench near the gas main.
Have disaster supplies on hand
(flashlight, water, food, first-aidkit,
blankets, foot protection)
Have an emergency communication
plan to reunite family members who
may be separated from one another
during the earthquake.
Courtesy of W. Godden Collection, NISEE-PEER, University of California, Berkeley
Figure 18.07A: Earthquake of June 16, 1964, Niigata,
Japan. Apartment houses tilted by liquefaction.
Earthquake Preparation: During An Earthquake
If indoors, DROP, COVER, and HOLD
under a heavy piece of furniture
positioned against an inside wall. Stay
away from windows (shattering glass)
and things that could fall on you. Never
use the elevator.
If you are outdoors, move to an open
area away from buildings, street lights,
and utility lines until the shaking stops.
If in a moving vehicle, stop as soon as
possible and stay in the car, as the shock
absorbers will absorb some of the wave
energy. Move away from buildings,
bridges, trees, and overpasses.
Courtesy of U.S. Geological Survey
Figure 18.07C: Earthquake of September 19, 1985, Mexico
City, Mexico. A 15-story reinforced concrete building
collapsed.
Earthquake Preparation: After An Earthquake
Remain calm and reassure others.
Be prepared for aftershocks, and plan where you will
take cover when they occur.
Check for injuries. Give first aid, as necessary.
Avoid broken glass.
Check for fire. Take appropriate actions and precautions.
Check gas, water, and electric lines. If damaged, shut off
service. If gas is leaking, don't use matches, flashlights,
appliances, or electric switches. Open windows, leave
building, and report to gas company.
Use telephone for emergency calls only.
Tune to the emergency broadcast station on radio or
television. Listen for emergency bulletins.
Stay out of damaged buildings
Courtesy of U.S. Geological Survey
Figure 18.07D: Earthquake of February 9, 1971, San Fernando,
California. Southern Pacific railroad tracks near Los Angeles
were laterally displaced.
Major Concepts: Part I Summary
• Seismic waves are vibrations in Earth caused by the rupture and
sudden movement of rock.
• Three types of seismic waves are produced by an earthquake shock:
• P waves, S waves, and surface waves.
• Earthquakes hazards include:
• Ground shaking, Surface faulting, Liquefaction, Tsunamis, and Landslides.
• Fires from broken gas lines and Floods
• The exact location and timing of an earthquake cannot be predicted.
However, seismic risk can be evaluated and, in areas with high risk,
preparations for future earthquakes made.
Seismicity and Earth’s
Interior: Part II
Chapter 18
Dynamic Earth
Eric H Christiansen
Major Concepts: Part II
• Most earthquakes occur along plate boundaries. Divergent plate
boundaries and transform fault boundaries produce shallow-focus
earthquakes. Convergent plate boundaries produce an inclined zone
of shallow-focus, intermediate-focus, and deep-focus earthquakes.
• The velocities at which P waves and S waves travel through Earth
indicate that Earth has a layered internal structure based on
composition—crust, mantle, and core. Based on mechanical
properties--It has a solid inner core, a liquid outer core, a weak
asthenosphere, and a rigid lithosphere.
• Plate tectonics and upwelling and downwelling plumes are the most
important manifestations of Earth’s internal convection. The magnetic
field is probably caused by convection of the molten iron core.
Earthquakes and Plate Tectonics
• The distribution of earthquakes
delineates plate boundaries.
• Shallow-focus earthquakes
coincide with the crest of the
oceanic ridge and with
transform faults between ridge
segments.
• Earthquakes at convergent plate
margins occur in a zone inclined
downward beneath the adjacent
continent or island arc.
Courtesy of P. Morin
Earth’s Seismicity
Figure 18.14: Earth’s seismicity is clearly related to plate margins.
Earthquakes and Plate Tectonics
•
•
•
•
•
Earthquake frequency correlates with plate boundaries
Divergent boundaries – narrow zone of shallow focus, low intensity quakes
Convergent boundaries
Subduction zones – shallow to deep quakes of varying intensity
Collision zones – wide zone of shallow to moderate depth quakes of
varying intensity
• Transform boundaries – shallow focus quakes that follow the pattern of
faults of varying intensity
• Intraplate –
• Infrequent seismic activity associated with incomplete rifting events or
paleo plate margins or intraplate volcanoes
Earthquakes at Divergent Plate Boundaries
• Narrow zone
• Shallow focus (less than 15 km)
low intensity quakes
• Centered on oceanic ridges
• Also occur at continental rifts
like East Africa
• Related to extensional normal
faults and intrusion of basaltic
magma
Earthquakes at Transform Plate Boundaries
• Narrow zones
• Shallow focus (<15 km) quakes
• Varying intensity
• On strike-slip faults
• Especially those that offset oceanic ridges
• Also cut continents like the San Andreas fault of California and or in
northern Turkey
1999 Izmit Turkey Earthquake
Figure 18.11: The 1999 Izmit, Turkey, earthquake occurred on a strike-slip fault system that connects two convergent plate
boundaries. The red line shows the length of the fault that actually ruptured. It was responsible for the deaths of 20,000 people.
Base map by Ken Perry, Chalk Butte, Inc.
Earthquakes at Convergent Plate Boundaries
• Subduction zones – shallow to
deep quakes
• Most are on reverse faults
Inclined zone of seismicity
defines a subducting slab of
oceanic lithosphere
• Depths range from very shallow
to as deep as 600 km
• Varying intensity
1995 Kobe Earthquake
• The cause of the Kobe
earthquake was the subduction
of the oceanic Philippine plate
beneath southern Japan.
• Because of oblique collision, part
of the displacement is taken up
by movement along a long
strike-slip fault zone.
• The main shock occurred along a
fault in this zone.
Figure 18.10: The 1995 Kobe, Japan earthquake occurred
on a strike-slip fault.
2004 Sumatra
Earthquake
Figure 18.15B: In the past,
the overriding plate may
have had a shape like this
with the accretionary
wedge on the overriding
plate extending.
Figure 18.15C: Progressive
underthrusting drug the
overriding plate to the
right during the decades
that preceded the
earthquake.
Figure 18.15D: Suddenly this
deformation or strain was
released along the low-angle
fault plane during the 2004
earthquake.
Figure 18.15A: The main earthquake is on the
southern end of the zone that ruptured during the
earthquake—marked by thousands of aftershocks.
Earthquakes at Continental Collision Zones
• Wide zone of earthquakes
• Shallow to moderate (<75 km)
depth
• Varying intensity
• Typically related to thrust faults
Figure 08.14: Earth’s seismicity is clearly related to
plate margins.
Intraplate Earthquakes
• Plate interiors experience
infrequent shallow focus
earthquakes
• Some associated with old rift
faults—New Madrid, Missouri,
and South Carolina
• Other intraplate earthquakes are
related to hotspots like Hawaii or
Yellowstone related to landslides
and the movement of magma
Figure 08.14: Earth’s seismicity is clearly related to
plate margins.
Seismic Waves as Probes of Earth’s Interior
• Seismic waves passing through
Earth are refracted in ways that
show distinct discontinuities
within Earth’s interior
• Provide evidence that Earth has
a distinctive core
• The inner core is solid and the
outer core is molten
Seismic Waves Inside Planets
Figure 18.16: Seismic waves in a
homogeneous planet would be neither
reflected nor refracted.
Figure 18.17: Seismic waves in a differentiated planet
would pass through material that gradually increases
in rigidity with depth.
S-Wave Shadow Zone
• A shadow zone of S waves
extends almost halfway around
the globe from an earthquake’s
focus.
• This can be explained if the
outer core of Earth is liquid.
• S waves cannot travel through
liquid.
Figure 18.18: The shadow zone of S waves extends
almost halfway around the globe from the earthquake’s
focus.
P-Wave Shadow Zones
Figure 18.19: A P wave shadow zone occurs in the area
between 103° and 143° from an earthquake’s focus.
• P wave shadow zone occurs in the
area between 103° and 143° from
an earthquake’s focus.
• A central core through which P
waves travel relatively slowly.
• A complex pattern appears
because of wave refraction at the
major internal boundaries
• The boundary of the core is 2900
km below the surface
• P wave velocities give us good
estimates of the density of the
crust and mantle
P-Wave Shadow Zones
• Some P waves are reflected off
the inner core boundary
• They are received in the shadow
zone as weak, indirect signals.
• This deflection shows that the
inner core is solid.
Figure 18.20: P waves are deflected by the inner core
and are received in the shadow zone as weak,
indirect signals.
Seismic Wave Velocity Discontinuities
• Seismic discontinuities
reveal the crust, mantle,
and core and show that
they have different
chemical compositions.
• Seismic studies reveal
much about the physical
nature of the interior,
revealing a solid inner
core, a liquid outer core,
a soft asthenosphere,
and a rigid lithosphere.
Figure 01.05: The internal structure of Earth consists
of layers of different composition and layers of
different physical properties.
Seismic Waves and Earth’s Internal Structure
• Seismic velocities reveal he internal
structure of Earth
• The velocity of both P waves and S waves
increases until they reach a depth of
approximately 100 km.
• Wave velocities are slow between about
100 km and about 250 km. This low-velocity
layer lies within the asthenosphere.
• Below this, the velocity of P waves and S
waves increases until a depth of about 2900
km.
• S waves do not travel through the central
part of Earth, and the velocity of the P
waves decreases marking the core.
• Another discontinuity in P wave velocity, at
a depth of 5000 km, indicates the surface of
the solid inner core.
Figure 18.21: The internal structure of Earth is
deduced from variations in the velocity of seismic
waves at depth.
Mohorovicic Discontinuity (Moho)
• First discovered by Andrija
Mohorovicic
• Between 5 and 70 km deep
• Represents the base of the crust
• Compositional change from
feldspar-rich to olivine-rich
causes change in seismic
velocities
Figure 18.21: The internal structure of Earth is deduced
from variations in the velocity of seismic waves at
depth.
Low-Velocity Zone in the Mantle
• Layer from ~100 to 250 km deep
• Seismic velocities usually
increase with depth
• But they decrease by ~ 6% in low
velocity zone
• Caused by partially molten
mantle that slows seismic waves
Figure 18.21: The internal structure of Earth is
deduced from variations in the velocity of seismic
waves at depth.
Earth’s Outer Layers
Figure 18.22: The Moho and the low velocity zone
are two important seismic discontinuities in the
upper part of the planet.
• The Moho marks the base of the
crust.
• The low velocity zone is revealed
by a drop in the velocities of both P
waves (shown here) and S waves.
• A zone of low strength in the upper
mantle between about 100 and
250 km deep.
• The low-velocity zone is contained
in the weak asthenosphere and is
very near its melting point and may
be a zone of partial melting.
Discontinuities in Middle Mantle
• Abrupt increases at:
• 440 km
• 660 km
• Metamorphic phase changes in
mantle to denser minerals at
greater depth
Figure 18.21: The internal structure of Earth is deduced
from variations in the velocity of seismic waves at
depth.
Discontinuities in Middle Mantle
• Discontinuities in seismic wave velocities
may correspond to phase changes.
• Blue line shows how P wave velocities
change with depth.
• The uppermost mantle is dominated by
olivine.
• Below about 410 km a velocity increase
implies olivine is replaced by minerals
with higher velocities—garnet and spinel.
• At greater depths, magnesium-spinel is
probably replaced by magnesiumperovskite.
• Each change increases the density and
seismic wave velocity of the mantle.
Figure 18.23: Discontinuities in seismic wave
velocities may correspond to phase changes.
Earth’s Core
• Large P-wave velocity drop at coremantle boundary at 2900 km deep
• S-waves don’t move through this
layer of molten iron
• P-wave velocity increases in solid
inner core
• Iron melts at lower temperature
than silicates of the mantl
• Inner core solid because of
continued increase in pressure
Figure 18.21: The internal structure of Earth is deduced
from variations in the velocity of seismic waves at
depth.
Earth’s Internal Structure
Figure 18.21: The internal structure of Earth is deduced from variations in the velocity of seismic waves at depth.
Two Ways of Looking at Earth
• Seismic discontinuities
reveal the crust, mantle,
and core and show that
they have different
chemical compositions.
• Seismic studies reveal
much about the physical
nature of the interior,
revealing a solid inner
core, a liquid outer core,
a soft asthenosphere,
and a rigid lithosphere.
Figure 01.05: The internal structure of Earth consists
of layers of different composition and layers of
different physical properties.
Convection Inside Earth
• Convection of the core and mantle is the most important mechanism
of heat transfer in Earth.
• Convection in the iron core probably creates the magnetic field, and
• Convection in the mantle creates mantle plumes and plate tectonics.
Convection in the Mantle
• Plate tectonics
• Subduction of dense
oceanic crust
• Spreading at ridges
• Mantle plumes
• Buoyant lower
mantle
Figure 18.25: Earth’s thermal structure and convection can be
modeled using computers to complement the observations of
seismic tomography.
Courtesy of Gary A. Glatzmaier, University of California, Santa Cruz.
The Result of Convection in Earth’s Core
Figure 18.24: Earth’s magnetic field probably
forms by convection of the outer core, which
is made of molten iron.
• Earth’s magnetic field probably
forms by convection of the outer
core, which is made of molten
iron.
• A computer model of convection
shows magnetic field lines as a
smooth dipole with blue lines
directed inward and gold field
lines directed outward.
• The field inside the core is much
more complex.
Plate Tectonics and Convection
Figure 18.25: Earth’s thermal structure and convection can be modeled
using computers to complement the observations of seismic
tomography.
• Subducted slabs may
pass through the phase
boundary at 660 km.
• Or the phase boundary
may be a temporary
barrier that is broken
down when enough
subducted material
accumulates and
flushes rapidly through
the lower mantle.
• Some plumes may be
triggered by the sinking
of the dense slabs
Seismic Tomography
Courtesy of P. Morin
Major Concepts: Part II
• Most earthquakes occur along plate boundaries. Divergent plate
boundaries and transform fault boundaries produce shallow-focus
earthquakes. Convergent plate boundaries produce an inclined zone
of shallow-focus, intermediate-focus, and deep-focus earthquakes.
• The velocities at which P waves and S waves travel through Earth
indicate that Earth has a layered internal structure based on
composition—crust, mantle, and core. Based on mechanical
properties--It has a solid inner core, a liquid outer core, a weak
asthenosphere, and a rigid lithosphere.
• Plate tectonics and upwelling and downwelling plumes are the most
important manifestations of Earth’s internal convection. The magnetic
field is probably caused by convection of the molten iron core.
Summary of the Major Concepts
• Seismic waves are vibrations in Earth caused by the rupture and
sudden movement of rock. Three types of seismic waves are
produced by an earthquake shock: (a) P waves, (b) S waves, and (c)
surface waves.
• Earthquakes hazards include: Ground shaking, surface faulting,
liquefaction, tsunamis, and landslides, fires from broken gas lines, and
floods
• The exact location and timing of an earthquake cannot be predicted.
However, seismic risk can be evaluated and, in areas with high risk,
preparations for future earthquakes made.
Summary of the Major Concepts
• Most earthquakes occur along plate boundaries. Divergent plate
boundaries and transform fault boundaries produce shallow-focus
earthquakes. Convergent plate boundaries produce an inclined zone
of shallow-focus, intermediate-focus, and deep-focus earthquakes.
• The velocities at which P waves and S waves travel through Earth
indicate that Earth has a layered internal structure based on
composition—crust, mantle, and core. Based on mechanical
properties--It has a solid inner core, a liquid outer core, a weak
asthenosphere, and a rigid lithosphere.
• Plate tectonics and upwelling and downwelling plumes are the most
important manifestations of Earth’s internal convection. The magnetic
field is probably caused by convection of the molten iron core.