• Essential Knowledge:
• What are the causes of earthquakes?
UNIT 7 (1) PLATE TECTONICS,
EARTHQUAKES AND VALCANOES
Earthquake is series of seismic waves or tremors in
the earth’s crust.
Caused when pressure builds up at edge of plates
as they collide or slide past one another.
Pressure released a huge amount of energy is
released causing the plates to shake.
Seismology-study of earthquakes.
• Earthquake vocabulary:
• Tremors-waver of seismic energy which spread
from the focus.
• Aftershocks- smaller tremors which may occur
hours or days after original earthquakes.
They can be extremely dangerous because they
may cause already damaged building to collapse.
• Focus- (Hypocenter)place deep in the earth’s
crust where earthquake begins.
• Epicenter- place on surface of earth, directly
above focus. The tremors are strongest at this
point.
• Elastic Rebound-rocks bend until the strength of
the rock is exceeded.
• Rupture occurs
& rocks quickly
rebound to an
Undeformed
shape.
Energy is released
in waves radiate
outward from fault.
• Richter scale-used to describe strength of
earthquake.
• Seismograph-device that measures strength of
earthquake.
• Richter scale is logarithmic (6 is 10X stronger than
5).
Magnitude and Energy
Magnitude
Energy
3
Explosive Power
Example
One ton of Explosives
World Trade Center
Collapse
2
1
Topple 50-meter
tree
One kilogram of
explosives
Head-on colision at
60 mph
0
Drop a car 10
meters
Half stick of dynamite
Very bad day
skydiving
-1
Impact of bullet
One gram of explosives
-2
Hammer blow
-3
Dribbling a
basketball
5. Magnitude and Intensity
Magnitude and Energy
Magnitude
Energy
Explosive Power
Example
9
U.S. Energy Use for
a month
Alaska 1964
Indonesia 2004
8
U.S. Energy Use for
a day
San Francisco, 1906
7
6
5
One Megaton
World Series
Earthquake, 1989
U.S. Energy Use for Large Thunderstorm
a minute
One Kiloton
4
3
One ton of explosives
5. Magnitude and Intensity
World Trade Center
Collapse
Seismic waves-response of material to arrival of
energy fronts released by rupture.
2 types body waves P and S
Surface waves R and L
Body Waves: P and S waves
•
Body waves
– P or primary waves
• fastest waves
• travel through solids,
liquids, or gases
• compressional wave,
material movement is
in same direction as
wave movement
– S or secondary waves
• slower than P waves
• travel through solids
only
• shear waves - move
material
perpendicular to
wave movement
Surface Waves: R and L waves
•
Surface Waves
– Travel just below or along the ground’s surface
– Slower than body waves; rolling and side-to-side
movement
– Especially damaging to buildings
How is an Earthquake’s Epicenter Located?
Seismic wave behavior
– P waves arrive first, then S waves, then L and R
– Average speeds for all these waves is known
– After an earthquake, the difference in arrival times at a seismograph
station can be used to calculate the distance from seismograph to the
epicenter.
How is an Earthquake’s Epicenter Located?
Time-distance graph
showing average
travel times for Pand S-waves. Farther
away a seismograph
is from e focus of an
earthquake, longer
interval between
arrivals of the P- and
S- waves
How is an Earthquake’s Epicenter
Located?
• Three seismograph
stations are needed
to locate epicenter
of an earthquake
• A circle where
radius equals
distance to
epicenter is drawn
• Intersection of
circles locates
epicenter
• Intensity
– subjective
measure of
kind of
damage done
& people’s
reactions to it
– isoseismal
lines identify
areas of equal
intensity
• Modified Mercalli Intensity Map
– 1994 Northridge, CA earthquake, magnitude
6.7
How are the Size and Strength of an Earthquake Measured?
• Magnitude
– Richter scale
measures total
amount of energy
released by an
earthquake;
independent of
intensity
– Amplitude of the
largest wave
produced by an
event is corrected
for distance and
assigned a value
on an open-ended
logarithmic scale
ESSENTIAL KNOWLEDGE:
What are the factors contributing to earthquake
damage, and other dangers associated with
earthquakes.
Can Earthquakes be Controlled?
• Graph showing the
relationship between the
amount of waste injected
into wells per month and
the average number of
Denver earthquakes per
month
• Some have suggested
that pumping fluids into
seismic gaps will cause
small earthquakes while
preventing large ones
Faults- fracture along which blocks of crust on
either side have moved relative to one another
parallel to the fracture.
Strike-slip-vertical fractures where blocks have
mostly moved horizontally.
Faults are classified by
kinds of movement that
occurs along them.
Strike-slip left lateral
fault.
• Strike-slip right lateral fault.
• Dip-slip faults-inclined fractures where blocks
have shifted vertically
• Dip-slip normal fault. Rock mass above an
inclined fault
moves down.
• Dip-slip reverse fault. Rock above fault moves up.
• Normal faults are caused by extension.
• Reverse faults are caused by compression.
• Reverse faults are often called thrust faults.
• Earthquake Hazards.
• Building collapse
• Landslides
• Fire
• Tsunamis
•
•
•
•
•
Most dangerous buildings adobe house
Unreinforced masonry.
Reinforces concrete.
Steel-frame.
Small wood-frame house.
• Tsunamis usually caused by submarine landslidestravel 400mph
• They pass unnoticed at sea, but cause damage on
shore.
• There is a warning network around Pacific ocean,
then can forecast arrival.
• Damage from a Tsunamis depends on direction of
travel of wave, harbor shape, bottom, and tide &
weather.
5. Magnitude and Intensity
Earthquake Fatalities Since 1800
7. Predicting earthquakes is not yet
possible
Are Earthquakes Getting More
Frequent?
7. Predicting earthquakes is not yet
possible
Some Important Earthquakes
1755 - Lisbon, Portugal
• Killed 70,000, Raised Waves in Lakes all over
Europe
• First Scientifically Studied Earthquake
1811-1812 - New Madrid, Missouri
• Felt over 2/3 of the U.S.
• Few Casualties
1886 - Charleston, South Carolina
• Felt All over East Coast, Killed Several Hundred.
• First Widely-known U.S. Earthquake
Some Important Earthquakes
1906 - San Francisco
• Killed 500 (later studies, possibly 2,500)
• First Revealed Importance of Faults
1923 – Tokyo - Killed 140,000 in firestorm
1964 - Alaska
• Killed about 200
• Wrecked Anchorage.
• Tsunamis on West Coast.
1976 - Tangshan, China
• Hit an Urban Area of Ten Million People
• Killed 650,000
• History and earthquakes
• In last 500 years more then 7 million people
have died.
• Destroyed economies and food sources.
• Lisbon Earthquake 1755 -60,000 killed –ocean
waves 30-40 feet above high tide level swamped
city –fires burned 3 days.