GEOL-1010 Lecture Notes - Lecture 7: Indian Ocean, Longwave, Epicenter

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5 Feb 2018
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Lecture 7: Earthquakes
Geology in the News
‘Lost Continent” found in the Indian ocean
Nera Mauritius
Some Mauritius rocks recently dated are much older than the island
Most of it is submerged or buried
Part One- Introduction to Earthquakes
Why do We Care
Why do some earthquakes cause more damage?
Is there any way to predict the Earthquake?
What Causes Earthquakes
Motion along faults
o Crack in rock and the rocks around are in motion
o This motion causes earthquakes
Time 2: Stress friction
o Faults aren’t constantly moving; it takes a lot of force to get them to move
o The force builds up strain within the rock
o Elastic Deformation- bending or breaking (pic on slide 6)
o Stress> friction
o When they move=earthquake
o As long as that tectonic force is working in the area you can have earthquakes on
that fault
How Frequent are Earthquakes
Small ones are quite common
Ex: ca 1million mag 2 quakes each year
o 100 magnitude 2 earth quakes happen every hour
o we don’t hear about them often because they don’t usually cause any damage
o Don’t cause a lot of motion or damage so some people don’t notice
How Powerful Are Large Earthquakes
Compared to come other things: graph on slide 9
Some are very powerful which is why they cause widespread damage
The Point of Movement
Focus- The actual point right on the fault line where the motion occurred. It doesn’t mean
the entire fault line moved, just a section. Usually some distance below ground
Epicenter- the point along the ground surface directly above the focus
These are NOT the same thing!
Important for emergency procedures- which towns/ cities will need the most help
o Doesn’t tie into where the motion occurred, just where to focus relief efforts
The foucus is usually very shallow (2-20 km below the surface)
Movements Before and After
Foreshocks- motions that occur before the main earthquake. Small movements where the
rocks are adjusting and trying to relieve some pressure
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Aftershocks- smaller earthquakes that happen after the main earthquake
o The main earthquake doesn’t usually release all of the pent of energy, so
aftershocks release what is left
o Usually happen away from the focus- rocks in other areas have to adjust after the
earthquake since they could have a lot of pressure suddenly
Pushing rolling chair on the stage example: put a little pressure on the chair and not much
happened, lots of pressure on the chair it moved across the stage. When he pushed the
first chair it hit a second chair. Pushing first chair was the earthquake, pushing the second
chair was the aftershock
Sometimes are just as powerful as the main earthquake
Part Two: Seismic Waves
There are 3 different types of Seismic Waves produced
1. P (Primary) waves-
a. They are compressional in nature; when it moves through material, all it does is
cause it to expand then contract. Isn’t shaking around or moving side to side- just
expanding and contracting
b. Don’t cause a lot of the motion we feel during earthquakes
c. Think of a slinky! When you stretch it out really tight and then push inward you
can see the expand and contract going through the slinky. (hit the end)
d. These are the fastest, they are the first ones that affect the area
e. Speed= 5km/ second
f. Can move through any kind of material
i. Speed may change, but the waves keep on going
2. S (secondary, shear) waves-
a. Secondary- second fastest
b. Shear- cause a shearing motion in the material that they pass through
c. Up and down motion
d. Responsible for a lot of the ground shaking
e. Speed= 2,3,or 4 km/ second
f. Will NOT move through fluid materials
g. If it is too ductile the S-waves will not pass through it
h. S wave shadow zone: area where the S waves aren’t registered
i. Since s-waves cant pass through fluid, so they can’t pass through the outer
core since it is very ductile
3. L (long, surface waves)-
a. Don’t worry about subtypes
b. Long wave
c. Surface waves- don’t go down in the layers like primary and secondary
d. Slowest type of wave
e. Motion is more complex
f. Shape thing vertically but also laterally, side to side and up and down
g. Don’t need to know speed
Part Three- Measurement and Detection
Seismometer (aka seismograph, but that term is outdated so like don’t use it)
3 Myths to debunk
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