Earthquakes
Ch.6
Faults
• Pg. 86
• Faults shift due to stress from surrounding
rock.
6.1 Earthquakes and Plate
Tectonics
• Earthquakes – vibrations of the earth’s
crust
• -caused by the shift in a rock because of
stress in along a fault
• Locked - Friction holds the fault together
for awhile
• 1. Elastic rebound theory
• 2. Major Eathquake zones
Elastic rebound theory
• The rock under stress springs back to its original shape after separating at
weakest point of the fault
• The rocks on each side of a fault are moving slowly.
• Stress builds until the rocks fracture at the weakest point.
• Which causes the rocks to break and spring back to their original position.
• If the rocks are “locked” stressed builds.
• Seismic waves – energy released in vibrations as rocks slip into new
positions
• causing the ground to shake.
• Aftershocks – small tremors
• Focus – area of slippage first (shallow underground) within 4 miles
• Epicenter point on the surface above the focus
• Lots of Energy and damage to surface with shallow focus
•
Aftershocks p.1
• After the initial breakage of rock causing
an earthquake, other areas around it give
off smaller waves called aftershocks.
Focus of an Earthquake p.2
• The area along a fault where the slippage
first occurred.
Epicenter p.2
• The point on the earth’s surface directly
above the focus.
Focus Depths p. 2
• 90 % of all earthquakes happen near the
surface.
• Less than 70 km down.
• Why do you think this is?
Answer
• The deeper you go down in the earth the
hotter it is.
• Making the rock like playdough.
• They mush together a faults and don’t
snap causing vibrations.
Major Earthquake zones
• Pacific Ring of Fire
• Midocean ridge
• Eurasian-Melanesian belt
• Fault zones
Earthquake zones
Major Earthquake Zones p.3
1. Pacific Ring of fire
- see page 101
- plates are subducted – causing stress resulting in
earthquakes.
2. Along mid-ocean ridges
- divergent plates push outwards and
creates stress.
3. Eurasian-Melanesian Mountain Belt
- Convergent plates – they hit and cause
stress
Why Then Does Missouri and
Tennessee Have Earthquakes?
• They are not on a plate boundary?
1970 they found an ancient fault buried
under sediment.
Fault formed 600 million years ago.
1812 big quake
Homework
• page 102 # 1-4
• Draw an earthquake. Label:
1. Epicenter
2. Focus
3. Pick a fault type and use it
4. Draw the seismic waves
6.2 Recording Earthquakes
• Seismograph – 3 electric sensing devices
• 1 vertical
• 2 horizontal
• Mechanical energy to electric on magnetic
tape
• 1. types of seismic waves
• 2. Locating an earthquake
• 3. earthquake measurement
1. types of seismic waves
• P waves – primary waves, fastest, 1st
recorded, solids and liquids, more ridgid
the rock - faster
• S waves – secondary waves, only solid
material, moves at right angles, doesn’t
make through the core to other side of
earth
• Surface waves – slowest moving causing
the ground to rise and fall
2. Locating an earthquake
• 3 plots
• P waves
• S waves
• Surface waves
3. earthquake measurement
• Magnitude – ground motion or amount of
Energy released
• Richter scale
• Microquakes – less than 2.5 magnitude
• Mercalli scale – expresses the intensity of
an earthquake, or the amount of damages
it causes I - XII
Earthquake 20 min.
Seismic Waves p.4
• When an earthquake occurs energy in the
form of seismic waves is given off.
These waves are what causes the rocks to
be stressed, break, and snap back. (ERT)
Seismograph
3 Types of Seismic Waves p.5
1. Primary waves or P waves
- move the fastest
- are the first to be recorded by a
seismograph
- can go through solids and liquids
- the more rigid the material the faster a
P wave can travel
Types of Seismic Waves cont..p.6
2. S waves
- Can travel only through solids
- can not go through the liquid outer core
3. Surface Waves
- when P or S waves reach the surface (crust)
-causes the surface to rise and fall
- really destructive when traveling through loose
soil
Locating Earthquakes
• Like thunder and lightning
• P waves are 1.7 times faster than S
waves.
• They record how far apart the P and S
waves are recorded – tells them how far
away the epicenter is located.
• 3 seismograph stations needed
- pg 104 - 3 circles
Measuring Earthquakes
• Magnitude – the measure of the energy
released by an earthquake.
- also the amount of ground movement
Scales Used to Measure p.7
Earthquakes
• Richter scale – a number describing how
big the earthquake is
7 or above – major earthquake - wide
spread damage
6-7 moderate earthquake
2.5 – 6 minor earthquake
under 2.5 – called a microquake – not
felt by people
Mercalli Scale p.8
• Measures the intensity or amount of
damage an earthquake caused
• Uses roman numerals I through XII
I not felt by people
X total destruction
Homework
• Page 105 # 1-5
Earthquake Damage
6.3
Injuries/Death p.9
• Most injury and death is not from the
ground shaking it is from
buildings/structures collapsing.
Other Problems from Earthquakes
p.9
• Fire from broken gas lines
• Dams break
Buildings Collapse p.9
• Buildings on loose soil are effected more
- the ground acts like jello and shakes
more violently
• Tall buildings sway, collapse, or tip over as
the ground shakes
Earthquake safety
• Being in an open field or area is safest
• If in your house stay away from shelving
and windows.
• Stand inside a doorway
Why would this be the safest spot?
Buildings and safety
• In Tokyo and the bigger cities in California
build their buildings that can “sway”
without falling.
• The base of the building is the key.
Irregular shapes / rubber type material
Absorbs seismic waves =less damage
Earthquake Warning/Predictions
• They are hard to predict which means we
can’t warn people to get to safety
• Possible warnings
Right before an earthquake some animals
will act restless or nervous.
Scientific Warning/Predictions
• Scientists can look at seismic gaps.
These are areas along faults that have
been locked and are experiencing stress.
Which means it can snap!
Page 111 figure 6-7
Homework
• 6.3 Review 1-5 pg. 111
• Review pp. 112-113 1-14
• Testbank 1-24
Tsunamis
Tsunamis p.10
• A major earthquake with an epicenter on
the ocean floor that causes a giant wave.
• The ocean crust breaks and along the fault
it causes a rise or fall in the sea floor.
When it rises or falls the water gets
pushed up in the form of a wave.
Video- click on it.
2004 Tsunami
• The 2004 Indian Ocean earthquake, was an undersea earthquake
that occurred at 00:58:53 UTC (07:58:53 local time) December 26,
2004, with an epicentre off the west coast of Sumatra, Indonesia.
The earthquake triggered a series of devastating tsunamis along the
coasts of most landmasses bordering the Indian Ocean, killing more
than 225,000 people in eleven countries, and inundating coastal
communities with waves up to 30 meters (100 feet). This was the
ninth-deadliest natural disaster in modern history. Indonesia, Sri
Lanka, India, Thailand, and Myanmar were hardest hit.
• With a magnitude of between 9.1 and 9.3, it is the second largest
earthquake ever recorded on a seismograph. This earthquake had
the longest duration of faulting ever observed, between 8.3 and 10
minutes. It caused the entire planet to vibrate as much as 1 cm (0.5
inches)[1] and triggered other earthquakes as far away as Alaska.[2]
• The disaster is known by the scientific community as the Great
Sumatra-Andaman earthquake,[3] and is also known as the Asian
Tsunami and the Boxing Day Tsunami. The tsunami occurred
exactly one year after the 2003 Bam earthquake and exactly two
years before the 2006 Hengchun earthquake.
• The plight of the many affected people and countries prompted a
widespread humanitarian response. In all, the worldwide community
donated more than $7 billion (2004 U.S. dollars) in humanitarian aid.
Homework
• Page 112-113
#1-14
Critical thinking
#1-4
Application #1,2