Transcript Unit 6-3: Measuring an Earthquake

Unit 7-3:
Measuring an
Earthquake
Earthquake Magnitude
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In addition to locating epicenters,
seismographs are useful in determining
another factor of an earthquake.
Magnitude:
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The strength of the shaking that the quake
produces.
The most widely used and recognized
magnitude scale is the Richter Scale.
Earthquake Magnitude
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Richter Scale
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Developed in the 1940s by Charles F. Richter.
It measures the amount of energy released by an
earthquake.
Every one increase of one magnitude number on
the scale means that the earthquake’s strength
increased by 10.
So a magnitude 2 earthquake is 10x more
powerful than a magnitude 1.
Earthquake Magnitude
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That is why magnitude is usually expressed
as the magnitude number with a decimal after
it (ex. 2.2).
This also means that a 7.0 earthquake is
100x more powerful than a 5.0 earthquake!
This is also why a 1-3 magnitude earthquake
is largely ignorable, while a 4-5 can cause
massive damage.
Richter Scale
Richter
magnitudes
Description
Earthquake effects
Frequency of
occurrence
Less than 2.0
Micro
Microearthquakes, not felt.
About 8,000 per day
Generally not felt, but recorded.
About 1,000 per day
Often felt, but rarely causes damage.
49,000 per year (est.)
2.0-2.9
Minor
3.0-3.9
4.0-4.9
Light
Noticeable shaking of indoor items, rattling noises. Significant damage unlikely.
6,200 per year (est.)
5.0-5.9
Moderate
Can cause major damage to poorly constructed buildings over small regions. At
most slight damage to well-designed buildings.
800 per year
6.0-6.9
Strong
Can be destructive in areas up to about 160 kilometres (100 mi) across in
populated areas.
120 per year
7.0-7.9
Major
Can cause serious damage over larger areas.
18 per year
Can cause serious damage in areas several hundred miles across.
1 per year
8.0-8.9
Great
9.0-9.9
10.0+
Epic
Devastating in areas several thousand miles across.
Never recorded.
1 per 20 years
Extremely rare
(Unknown)
Earthquake Damage
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There are two major reasons why building
collapse during an earthquake:
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Ground shaking.
Foundation failure.
Remember, earthquakes don’t kill people.
The damage caused by the earthquake
(falling buildings, flooding, fire, etc.) kills
people.
Earthquake Damage
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Ground Shaking:
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The result of the waves of energy released by the
earthquake.
The way the earth shakes is similar to the way a
bell shakes when it is struck.
The waves move in different directions depending
on the type of wave.
The buildings have to move with the earth as it
shifts.
Earthquake Damage
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Ground Shaking:
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Most buildings can withstand large vertical
shaking without major damage.
However, when the S waves come by, and the
earth moves back and forth,
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Most buildings cannot survive horizontal movement.
This is because the different parts of the building
stretch, compress, and then break apart.
Earthquake Damage
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Foundation Failure:
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If the soil under the building settles, then the
foundation is no longer supported.
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This causes the foundation to break and the building
to collapse.
If the soil conditions are right, then liquefaction
may occur.
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Liquefaction: When the shaking of the earth makes
the solid dirt and soil act as if it were a liquid.
Earthquake Damage
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Foundation Failure:
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Having a solid foundation is of critical importance.
Buildings build upon solid rock experience very
little damage.
Buildings built on filled in bogs, soft fill, or any
other soft material suffer severe damage.
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Most collapse completely.
Earthquake Prediction
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In order to an earthquake prediction to be
successful, it must correctly forecast three
facts:
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The place where the earthquake (epicenter) will
occur.
When the earthquake will occur.
The magnitude of the earthquake.
But how do we correctly forecast these?
Earthquake Predictions
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It has been discovered that P waves slow
down by about 10%-15% for a period of time
before an earthquake occurs.
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This period of time, however, may be a few days,
or a few years.
They also discovered that the earthquake occurs
shortly after the P waves return to normal speed.
Earthquake Predictions
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It was also noted that the longer the P waves
were below normal, the stronger the
earthquake.
Although not a perfect system, it has helped
areas such as California, China, and Russia
prepare for earthquakes.
Earthquake Prediction
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In some areas, a slight uplift in elevation has
preceded an earthquake.
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In Japan, an uplift was noted for ten years before
a 7.5 earthquake struck.
In California, Palmdale Bulge has been closely
monitored for changes that may indicate an
earthquake.
Earthquake Prediction
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Other methods:
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A decrease in electrical resistance in the ground
may indicate an earthquake.
An increase in radon in well water seems to
correlate with earthquakes.
By studying these observable changes,
scientists hope to one day accurately predict
earthquakes, reduce casualties and lower
property damage.