Earthquakes
Chapter 19
Crustal Movement:
Stress – The total force acting on crustal rocks per
unit of area.
3 TYPES OF STRESS:
Strain – The deformation of
materials in response to stress.
Types of Tectonic
Stress
• Tensional
Stress- pulls
outward
(extensional
stress)
• Compressional
Stress- pushes
inward
• Shearing-
pulls
in two opposite
directions
Deformation
Elastic vs. Plastic
Caused when a material is
compressed, bent or stretched.
Caused when a material is
permanently deformed.
Similar to stretching a rubber
band by pulling on the ends.
Similar to pulling a rubber
band until it actually breaks.
• The
point at which Earth’s crust has reached its elastic
limit is when permanent deformation occurs.
Folds (elastic deformation) – There
are 2 types of bends that can occurs in
Earth’s crust.
Anticline folds up
like a capital “A”
Syncline “sinks”
Faults (plastic deformation) – breaks
in the Earth’s crust along which
Earth’s crust moves.
Faults produce 2 sides (walls):
Foot wall –
The side of the
fault which is
thicker at the
bottom (you can
“walk” up it)
Hanging wall –
The side of the
fault which is
thicker at the
top (you can
“hang” from it)
TYPES OF FAULTS
Normal Fault – Tension pulls rock apart, forcing the
footwall up and the hanging wall down.
Normal Fault
Animation
Normal Fault
Stress =
Tension
Normal Fault Examples
Left: Fault scarp near Hebgen Lake, Montana, after the
magnitude 7.1 earthquake of August 18, 1959, shows a
displacement of 5.5 to 6.0 m.
Right: This section of the normal fault scarp was produced
by the earthquake of October 28, 1983, at Borah Peak,
Idaho.
Left: Dixie Valley-Fairview Peaks, Nevada earthquake
December 16, 1954
TYPES OF FAULTS
Reverse Fault – Compression causes the hanging wall
to move up and the footwall to move down.
Reverse Fault
Animation
Reverse Fault
Stress =
Compression
Before and after pictures of the 2,000 yr.
Reverse Fault Example – Collision Zones Below:
old citadel in the city of Bam, the largest mudBam, Iran 2003
brick structure in the world.
Map showing the
Alpine-Himalayan
Belt.
Earthquakes in Iran and
neighboring regions (e.g.,
Turkey and Afghanistan)
are closely connected to
their position within the
active Alpine-Himalayan
belt near the convergence of
the Arabian and Eurasian
plate.
The Mw 6.5 Bam
Earthquake destroyed
nearly 80% of the adobe
buildings killing more than
26,000 people.
Thrust Faults - Reverse Faults
•
Reverse faults result from compressional
stresses along convergent boundaries.
•
The hanging wall block has moved up relative
to the footwall block.
•
A Thrust Fault is a special case of a reverse
fault where the dip of the fault is less than 15 o
•
It is a reverse fault at a very low angle.
•
There are two types of converging plate
boundaries.
1.Subduction boundaries
2.Collision boundaries
Left: Thrust Fault
TYPES OF FAULTS
Strike-Slip Fault – Shearing forces two parts of
Earth’s crust past each other with NO vertical
movement.
Strike-Slip
Animation
Strike-Slip
Fault
Stress =
Shearing
Strike-Slip Faults - Transform Faults
•
Strike-slip faults result from shear stresses acting on the lithosphere along transform
boundaries.
•
Horizontal motion can be right lateral or left lateral.
•
Earthquakes along these boundaries tend to be shallow focus with depths usually less than
about 100 km. Richter magnitudes can be large.
Reverse, Normal, or Strike-Slip Fault?
Normal Fault
Tension
Hanging Wall Sinks Down
Reverse, Normal, or Strike-Slip Fault?
Reverse
Compression
Hanging wall is pushed up
Reverse, Normal, or Strike-Slip Fault?
Strike-Slip
Shearing
No Vertical Movement
Earthquakes Defined
Earthquakes are vibrations of the earth
caused by the rupture and sudden
movement of rocks that have been
strained (deformed) beyond their elastic
limit. The forces that cause deformation
and the build-up of strain energy in the
rock are referred to as stresses.
Earthquakes occur along faults. Faults
are fractures in the lithosphere where
regions of rock move past each other
(displaced).
Inside Earthquakes Video
•
The focus is the point on the
fault where rupture occurs and
the location from which seismic
waves are released.
•
The epicenter is the point on
the earth’s surface directly
above the focus.
•
When the fault ruptures, waves
of energy called seismic waves
spread out in all directions.
Earthquake Terminology
21
Earthquake Seismic
Waves
•
Body waves travel through the
interior (body) of the earth as
they leave the focus. They
include P-waves and S-waves.
•
Surface waves travel parallel
to the earth’s surface. They are
the slowest and most damaging.
They include Love and
Rayleigh Waves.
Interior seismic waves – These waves
travel through the inner layers of Earth.
Primary Waves
Secondary Waves
-Travel twice as fast
as S-Waves.
-Travel half as fast as
P-Waves.
-Compressional
Movement: rocks are
squeezed and pushed
in the same direction
as the wave moves.
-Rolling Movement:
rocks shifted up and
down perpendicular to
the direction of the
wave.
-Move similar to a
slinky that is
stretched and then
released.
-Move similar to a
rope that is being
shaken up and down.
COMPARE P WAVES AND S WAVES
Shadow Zones – Areas on Earth where no seismic waves are recorded.
-P waves can travel
through all of the
layers (solid, liquid
and plastic), but they
are deflected in
different directions as
they move from one
layer to another.
-S waves can travel
through solid and
plastic layers, but not
liquid layers (the outer
core).
Shadow Zones
Surface Waves
Surface waves travel parallel to the earth’s surface. They are the slowest,
involve the greatest ground motion, and are therefore most damaging.
They include:
•
Love Waves - complex, horizontal motion
•
Rayleigh Waves - Rolling or elliptical motion.
-When combined, these waves
move the ground up and down
and side to side simultaneously.
-They cause the most damage
because the create the most
ground movement and take the
longest time to pass an area.
Distance from the Epicenter
Seismic waves attenuate with distance.
Locating the epicenter of an Earthquake: Triangulation
Scientists use seismic
data received from at
least 3 stations to
triangulate (the same
process GPS uses) the
epicenter of an
earthquake.
We will explore this
process more in lab.
Seismogram
Seismogram - the record of an
earthquake as recorded by a
seismograph. It is a plot of
vibrations versus time.
Refer to pages 45-46.
How do we measure earthquakes?
Richter Scale
Mercalli Scale
-Created by Charles Richter in 1934.
-This scale measures earthquakes
based on the largest seismic wave
recorded during a quake.
-The original scale used to measure
quakes. Created in 1902 by
Giuseppe Mercalli.
-Considered less accurate, because
it uses eyewitness observations of
damage to estimate the intensity of
the quake.
-The scale is from 0-10. Every whole
number up on the scale represents an
increase in energy of ten times more.
MERCALLI SCALE
Modified Mercalli Scale vs. The Richter Scale
31
Moment Magnitude Scale
**This is the most accurate scale used today**
-Introduced in 1979.
-Measures earthquake strength based on the amount of energy released by calculating size of
the fault, amount of movement, and type of rock (stiffness).
-The moment magnitude scale is preferred over the Richter scale by seismologists because it
is more accurate.
-When hearing about an earthquake on the news, this is typically the final, most accurate
magnitude given.
Earthquake
Richter Scale
Moment Magnitude
New Madrid, MO, 1812
8.7
8.1
San Francisco, CA 1906
8.3
7.7
Prince William, AK 1964
8.4
9.2
Northridge, CA,1994
6.4
6.7
Earthquake Depth
Shallow
earthquakes cause
the most damage.
Intermediate
earthquakes cause
moderate damage
Deep earthquakes
cause the least
damage.
Types of Faults
The majority of earthquakes (90%) are caused by rocks rupturing in response to tectonic
stresses at active plate margins.
Subduction Boundaries
•
At subduction boundaries there is a continuum of stress along the subducting plate.
Shallow focus earthquakes can be generated near the trench, but focal depths can reach
down to 700 km as earthquakes are generated along the subducting plate.
•
Rocks are strong under compression and can store large amounts of strain energy
before they rupture. Therefore, these earthquakes can be very powerful.
 1960 Southern Chili = 9.5
 1964 Alaska = 9.2
.
Reverse Fault Example –
Subduction Zones
Chile 1960
On May 22, 1960 the
largest earthquake on
record struck the coast of
Chile with a Mw of 9.5.
The earthquake ruptured
along a 1,000 km length
of the subduction zone.
In Chile, the earthquake
and the tsunami that
followed took more than
2,000 lives. From Chile
the tsunami radiated
outward, killing 61
people in Hawaii and 122
in Japan.
Left: Stuck to the subducting
plate, the overriding plate gets
squeezed.
Right: An earthquake along a subduction
zone happens when the leading edge of
the overriding plate breaks free and
springs seaward, raising the sea floor
and the water above it. This uplift
starts a tsunami.
Collision Boundaries
•
At collision boundaries two plates of continental lithosphere collide resulting in foldthrust mountain belts.
•
Earthquakes occur due to the thrust faulting and range in depth from shallow to about
Example: The
Himalayas from
the collision of
India with Asia
Transform Fault
Example The San
Andreas Fault System
The San Andreas Fault is the main strand of a zone of
parallel faults resulting from interaction between the
relative northwest movement of the Pacific Plate compared
with the North American Plate.
Surface Faulting and Ground Rupture
Land Uplift and Subsidence
Ground rupture and surface faulting occur due to horizontal or
vertical displacement of faults that break the surface. Damage
can result from the ground shifting upward (called uplift) or
downward (called subsidence).
Above: The photo shows a fault scarp
-- a cliff created by movement along a
fault. This scarp formed during the
1992 Landers, CA, quake.
Left: The 1999 Taiwan earthquake caused ground
movement over a fault rupture of 50 miles. The photo
shows a fresh scarp cutting across the running track
at a local high school.
Liquefaction
Liquefaction is a quicksand like condition that occurs in water-saturated soil and rock. The
shaking of earthquake waves causes the soil or rock to turn into a weak, fluid-like mass.
Structures built on areas that liquefy may fall over or sink.
The figure below shows how liquefaction can occur. Shaking of
water-saturated soils causes the particles to settle, driving the
water out from between the particles and forcing it upward,
thus liquefying the areas above.
Above: Buildings in Niigata, Japan, fell over
when the sediments below them liquefied
during the 1964 earthquake
Landslides
Vibration of water saturated sediment can force water into pore spaces between sediment
grains reducing friction and permitting the mass to slide down slope.
Turnnagin
Heights,Alaska,1964
Famous Earthquakes
1) Anchorage, Alaska - March 27, 1964
-This is the most powerful U.S. earthquake in recorded history.
-It registered as a 9.2 on the Richter scale.
-The rumbling lasted nearly 5 minutes.
-Ground fissures, collapsing buildings, and tsunami killed over
130 people.
2) San Francisco, California – 1906 (7.7 on Richter)
-Significant earthquake in that it was one of the first
instances where scientists were able
to record information
on seismic waves.
-Caused approximately $400,000,000 of
damage (at the
time)
-Several eye-witness accounts refer to the movement of
the roads to be similar to the undulation of waves in the
ocean.
-Fires raged on for days, and S.F. had to be rebuilt.
Famous Earthquakes
3) Indian Ocean – December 26, 2004
-9.2/9.3 magnitude earthquake, making it one of the most
powerful
ever recorded on Earth.
-The infamous tsunami that ensued killed over 200,000 people.
-Released an amount of energy 1502 times more powerful than
the Hiroshima atomic bomb.
4) Chile – May 22, 1960
-The most powerful earthquake ever officially recorded at a
magnitude of 9.5.
-The damage is estimated to be at over 3 billion dollars, and
nearly 6,000 killed.
-Once the red slab that is “stuck” finally
gives way, this is when the earthquake
occurs. The recoil of the plate causes
the energy to be released.
Recent Earthquakes:
Haiti-7.0 on 1/12/10, 52 aftershocks, over 250,000 dead and
1,800,000 homeless. Quake was created by a blind thrust fault
with a very shallow quake and poor constructed homes lead to
high deaths
Chile (2/27/10) 8.8 (MM) shook central Chile creating a
tsunami. Lasted 3 minutes and was ranked as 8th largest in the
world and killed 802 people.
Japan- 3/11/11, 8.9 shallow quake created a 23-30 foot tsunami
which spread over 1500 miles, around 11,000 dead at this time
but numbers will increase, nuclear crisis still impacts the soils
and water/ocean of Japan.
http://www.google.com/imgres?imgurl=http://www.japan-tsunami2011.com/images/japan_earthquake_1846151c.jpg&imgrefurl=http://www.japan-tsunami2011.com/&usg=__XwcXEmd6hcfdbHzaSjBGbCyx7kQ=&h=287&w=460&sz=38&hl=en&start=15&zo
om=1&um=1&itbs=1&tbnid=1HhweKiZ7Q3
Largest Quakes in History
1. May 22, 1960, Chile (9.5) The world's most powerful earthquake left 4,485 people dead
and injured and 2 million homeless and killed a further 170 people as the tsunami hit
the coasts of Japan and the Philippines.
2. March 27, 1964, a magnitude 9.2 hit Prince Island Sound, Alaska. The earthquake
caused landslides in Anchorage and raised parts of outlying islands by as much as 11
meters. The 67meter high tsunami killed another 128 people.
3. December 26, 2004, a magnitude 9.1 hit Sumatra, Indonesia. (The Tsunami of the
Impossible) The deadliest tsunami ever spread to 14 countries killing 230,000
thousand people.
4. November 4, 1952 a magnitude 9.0 hit Kamchatka, Russia, known as the USSR. A
volcanic peninsula was the epicenter but the tsunami hit Hawaii. No deaths occurred
but it was 3000 miles from the epicenter.
Largest Quakes in US History
Largest Quakes Since 1900
Tsunami
A series of waves that
occurs when the ocean is
disturbed by earthquakes,
underwater explosions or
impacts.
12/26/04- Southeast Asia
experienced a large scale
earthquake along the Ring
of Fire that created a
series of tsunamis that
killed 225,000 people and
displaced over 1.2 million
people! The tsunami
slammed into the coasts of
several nations within two
hours after the quake.
tsunami animation