Transcript Earthquakes and Earth’s Interior

EARTHQUAKES
Chapter 8
8.1 What is an Earthquake?
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Earthquake: vibration of Earth produced by the
rapid release of energy
Most (NOT ALL) occur at faults
most faults and stress occurs along active plate
tectonic boundaries
Focus: the point within Earth where the EQ starts; the
source of the earthquake
Epicenter: location on the surface directly above the
focus
Fault Types
Normal
Strike-Slip
Focus, Epicenter, and Fault
Causes of Earthquakes
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Elastic Rebound Hypothesis:
 Most
earthquakes are produced by the rapid release
of elastic energy stored in rock that has been subjected
to great forces
 As rock is stressed, it bends, storing elastic energy.
Once the rock is strained beyond its breaking point, it
ruptures and releases the stored energy in the form of
vibrations (seismic waves) of earthquakes
Elastic Rebound Hypothesis
Other causes of earthquakes:
Landslides, rockslides, or slumping of
rocks.
Movement of magma, gases, or rocks
associated with volcanism
Aftershocks and Foreshocks
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Foreshocks: small earthquakes before a major
earthquake
 Can
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happen days or years before the major quake
Main shock: is the main earthquake disturbance
generated at the focus
Aftershocks: movements that follow a major earthquake
 Smaller
than the major EQ
 Can sometimes destroy structures weakened by the major
earthquake
Where do earthquakes occur
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Time for Excel Practice!
Log on to computers, open up excel spreadsheet
from classroom website
Follow directions of excel spreadsheet
Do not worry about printing map, just call me over
when you are done so I can check over
WHERE ARE MOST OF THE EARTHQUAKES
OCCURRING?
Earthquake Zones
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80% of all occur in circum-Pacific belt
 Most result from convergent margin activity
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15% occur in the Mediterranean –Asiatic belt
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5% occur in the interiors of plates and on spreading ridge centers
Measuring EQs
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Seismographs: instruments that record EQ waves
Seismograms: traces of amplified, electronically
recorded ground motion made by seismographs
Earthquake Waves
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Surface Waves (L waves):
 Travel
along Earth’s outer layers
 Especially damaging to buildings
 Most destructive of the three types of waves
 Rolling and side-to-side movement (think of ocean wave
movement)
Body Waves-Travel through Earth’s
Interior
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P (primary) waves
Push-pull motion
 Compression wavesmaterial is moved in the
same direction as the
wave moves
 Fastest moving wave
 Travel through solids,
liquids, or gases
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S (secondary) waves
Slower than p waves,
faster than surface
waves
 Travel through solids only
 Move material
perpendicular (90 deg)
to wave movement
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Earthquake Waves
How is an Earthquake’s Epicenter Located?
Seismic wave behavior
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P waves arrive first, then S waves, then L
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Average speeds for all these waves is known
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After an earthquake, the difference in arrival times at a
seismograph station can be used to calculate the distance
from the seismograph to the epicenter.
Locating an Earthquake
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Earthquake Distance:
 Epicenter
is located using the difference in the arrival
times between P & S wave recordings, which are
related to distance
 We
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will use a reference table for these measurements
Earthquake Direction
 Travel-time
graphs from three or more seismographs
can be used to find the exact location of an
earthquake epicenter
 We
will practice this using a compass
Seismograph example
2:33:00
2:35:30 – 2:33:00 =
2:35:30
00:02:30
2:35:10
2:39:20 – 2:35:10 =
2:39:20
00:04:10
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We don’t know when
the EQ started.
But we know how much
time there was
between the P&S wave
arrivals.
Let’s say the difference
is 0:04:00
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We have to find a spot
on the graph where
P&S (careful!) are
separated by 4:00.
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2,600km
Then drop straight
down to see the
distance to the
epicenter.
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Now that we know the
EQ was 2,600km
away, when did it
start?
To travel 2,600km, a P-wave…
needs 5:00 minutes
1,400km
The EQ happened somewhere
on this line
Measuring Earthquakes
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Two different types of measurements: intensity and magnitude
Intensity: based on observed effects of ground shaking on
people, buildings, and natural features.
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Varies from place to place w/in disturbed region depending on the
location of the observer with respect to the EQ epicenter
Magnitude: related to the amount of seismic energy released
at the hypocenter of the earthquake
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Based on amplitude of earthquake waves recorded on instruments
Scales to Measure Earthquakes
Modified Mercalli Intensity Scale
Richter Magnitude Scale
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Seismic waves are the vibrations from
earthquakes that travel through the Earth
Recorded on seismographs
Richter Scale-1935
Base-10 logarthmic scale
Each unit=32 fold energy increase
Calculate combined horizontal amplitude
Range from 0-10
 <2.0 not felt or recorded
 6.0-6.9: strong
 7.0-7.9: major
 8.0-9.9: great
 10+: Epic (never recorded)
DOES NOT ADEQUATELY ESTIMATE THE
SIZE OF VERY LARGE EQS!!
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Developed in 1931
12 different levels
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I-not felt except by very few
II-felt only by a few at rest
III-felt noticeably by persons indoor
IV-felt indoors by many, outdoors by a few
V-felt by nearly everyone, many awakened
VI-felt by all, some heavy furniture moved
VII-damage negligible in well-designed
buildings
VIII-some chimneys broken
IX-buildings shifted off foundations
X-some well build wooden structures destroyed
XI-few structures remain standing
XII-total damage
Moment Magnitude
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Derived from the amount of displacement that occurs
along the fault zone
Most widely used measurements for EQs because it is
the only magnitude scale that estimates energy
released by earthquakes
Measures very large earthquakes
Calculated by different factors including:
Avg amt of movement along the fault (a)
 Area of the surface break (b)
 Strength of broken rock (c)
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a x b x c=measure of how much energy rock can store before it
slips and releases energy during an earthquake
Moment Magnitude Chart
Notable Earthquakes
Destruction from Earthquakes
Seismic Vibrations
The damage to buildings and other
structures from earthquake waves depends
on several factors.
 Factors include:
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 intensity
and duration of the vibrations
 nature of the material on which the structure is built
 design of the structure
Building Design
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Factors that determine structural damage
 Intensity
of EQ
 Unreinforced stone or brick buildings
 Most
serious safety threat
 Nature
of material upon which structure rests
 Design of the structure
Seismic Vibrations
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Liquefaction
 Saturated
material turns to fluid
 Underground objects may float to surface
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Occurs when loosely consolidated soils saturated
with water are shaken by EQ waves
Tsunamis
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Japanese for seismic sea waves
Causes:
 Triggered
by an EQ
 Occurs where slab of the ocean floor is displaced
vertically along a fault
 Can also occur when the vibration of a EQ sets an
underwater landslide in motion
Movement of Tsunamis
A tsunami is generated by movement of the ocean floor. The
speed of a wave moving across the ocean is related to the
ocean depth
Tsunami Warning System
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Large earthquakes are reported to Hawaii from
Pacific Seismic Stations
Although tsunamis travel quickly, there is sufficient
time to evacuate all but the area closest to the
epicenter
On average, only 1-2 destructive tsunamis
worldwide per year
On average only 1 tsunami every 10 years causes
major damage and loss of life
Predicting Earthquakes
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Short-Range Predictions
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Not successful yet
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Long-Range Forecasts
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Data can be important for
updating building codes
Probability of EQ occurring
within 30-100+ years
Scientists don’t yet
understand enough about
how and where
earthquakes will occur to
make accurate long-term
predictions.
A seismic gap is an area
along a fault where there
has not been any EQ
activity for a long period of
time
Other Dangers
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Landslides
 Results
from the violent
shaking of EQs,
causing the soil and
rock on slopes to fall
 The greatest cause of
structural damage
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Fire
 In
the San Francisco EQ
of 1906, most of the
destruction was caused
by fires that started
when gas and
electrical lines were cut
US Earthquakes 1973-2002