Transcript Earthquake Notes - Helena High School

Cornell Notes
Earthquakes
Poseidon
Earthquakes Worldwide
Should we care?
Loss of Life
and
Economic Costs
Northridge, California 1994
E:\HHS_2010\METROPOLI
TAN.WMV
Earthquakes Since 1900
Montana Earthquakes
What is an Earthquake?
Seismic Waves
http://wwwrohan.sdsu.edu/~rmellors/lab8/l8awa http://www.geo.mtu.edu/UPSeis/wa
v2.htm
ves.html
What happens to seismic wave in
Earth’s interior?
Shadow Zones
Locating &
Measuring
Earthquakes
Seismograph
Seismograms
Seismographs
Seismographs
Seismograph
station at Cornell
University
A modern seismograph station usually has three seismographs.
The first one records up-and-down motions, the second one
records side-to-side motions in a north/south direction, and the
third one records side-to-side motions in an east/west direction.
Measuring quakes.
Locating an Earthquake Epicenter Step 1- Analyzing 3 Seismograms
Step 2- Read Time-Travel Graph
Step 3 – Triangulate to find
Epicenter.
Measuring Earthquakes
– Magnitude is a quantitative measurement that
estimates the energy released.
• Richter Scale
• Moment Magnitude
– Intensity is a qualitative measurement that
evaluates the degree of earthquake shaking
perceived by individuals
• Modified Mercalli Scale
Richter Scale (Magnitude)
• Developed by Charles F. Richter in 1935.
• Measurement of ground movements.
• Each increase of one whole number
in Richter magnitude is a 10 x increase in shaking (32 x
increase in energy release).
For example, a magnitude 6 earthquake is about 10
times more powerful than a magnitude 5 earthquake.
A magnitude 7 earthquake is more than 100 times
(10x 10) more powerful than a magnitude 5
earthquake.
Magnitude
Magnitude is a measure of the amount of
energy released in an earthquake.
Reading a Nomogram
Richter Scale (Magnitude)
Richter Scale
Description
Less than 3.5
Not generally felt by humans
5.5 - 6.0
Slight damage to structures
7.0 – 7.9
Serious damage
8.0 and greater
Total destruction of nearby
communities
Moment Magnitude (Magnitude)
Moment Magnitude is the current method
used for measuring earthquakes
(especially large ones) and uses three
measurements:
1. Average area of slip along the fault;
2. Distance traveled; and
3. Considers the strength of the rock.
Example Magnitudes
Location
Date
Richter
Moment
Magnitude Magnitude
San
Francisco
1906
8.3
7.9
Alaska
1964
8.5
9.2
Alaska released at least twice as much energy because it involved
greater movements along a much larger fault plane.
Mercalli Scale (Intensity)
• Mercalli Scale (Giuseppi Mercalli
1902) measures the intensity of
a quake by using observed affects
of ground movement.
Earthquake Risk
Where in the United States is the risk for earthquakes the
greatest? Alaska and California are high risks since they are
found near plate boundaries.
The middle of the country by Missouri is also at a higher risk,
because many faults are buried deep beneath sediments
deposited by the Mississippi, Missouri, and Ohio rivers.
Damage from Earthquakes
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Ground shaking
Foundation failure
Aftershocks
Fire
Tsunamis
Ground Shaking
Ground shaking is produced by the
waves set in motion by an
earthquake’s sudden release of
energy. Some of the ground
vibrations are up-and-down, but
the largest are side-to-side
motions.
Most buildings can withstand fairly
violent up-and-down shaking;
however, few buildings can survive
violent side-to-side shaking, and as
a result many buildings collapse.
Foundation Failure
As a result of severe ground
shaking, soils under buildings may
settle or even liquefy. Liquefaction
occurs when loose soil temporarily
takes on some of the properties of a
liquid.
A building located on soil that
settles is no longer safely supported
and may collapse. For example,
buildings located on bog muds or soil
landfill can suffer severe damage or
can collapse because of foundation
failure.
Foundation Failure
Liquefaction caused the building in the front to tip nearly 22°.
The arrow points to a building that is tipped almost 70°.
Aftershocks
A large earthquake may
be followed by a smaller
earthquakes.
Aftershocks are a series
of smaller earthquakes
originating close to the
focus of the large
earthquake. Aftershocks
can be as great as 1000
per day, though the
frequency usually
diminishes quickly over
time.
Fire
One of the most
damaging effects of
an earthquake may be
the fires caused by
the rupturing of gas
lines by ground
movement.
Tsunami
Underwater
earthquakes and
landslides sometimes
cause huge ocean
waves called tsunami.
Tsunami can travel
very quickly across
large expanses of
water. Its speed
depends on the depth
of the water.
Tsunami
When a tsunami reaches
shallower water near a
shoreline, it will slow down
and increase dramatically in
height.
A tsunami warning system is
now in place for regions in
the Pacific Ocean.
Scientists monitor wave
activity at various stations
throughout the Pacific.
Tsunami Animation
Elastic Rebound Theory
Stress is released and crust is
occassionally cracked called the
elastic-rebound theory normally
occuring at faults.
Stresses:
• Plate Tectonics
• Thermal expansion and
contraction.
• Gravitational attraction
• Mineral phases
Tensional stress pulls rock away Normal faults occur when rock
from adjacent rock thinning
along one side of the fault slide
crust.
down relative to the other.
Divergent Boundary
Compressional stress
Reverse faults occur when
pushes rock together causing. rocks are pushed together and
one side of the fault moves up.
Convergent Boundary
Rocky Mountain Front
• Shearing stress pushes
Strike-slip faults occur when
rock in two different directions blocks of rock on each side of
the fault move in opposite
adjacent to one another.
directions.
TRANSFORM PLATE BOUNDARY