Transcript here - Crescent School

Earthquakes
Table of Contents
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What is it
Measuring and Predicting
Where is it
Waves
Scales
Damage
Tsunamis
• Result of plateWhat
movements
is it?
• Rock on sides of a fault (a crack or a plate
boundary) become gradually bent.
• Elastic strain energy builds up until it reaches
the breaking point.
• The rock snaps back (elastic rebound) into its
unstrained position by slipping along the
fault.
• The strain of this energy is released and the
energy moves outward as seismic shock
waves, producing an earthquake.
Anatomy of an Earthquake
What is it?
• Seismometer:
measures
intensity
Measuring
and Predicting
– Rod is anchored deep in the ground and
vibrates when a quake occurs.
– Joined to the rod is a pendulum with a pen.
• Seismograph: graph created from
seismometer.
Focus: actual location
in the earth where
Where?
earthquake occurs- depth is important.
Epicentre: point on the surface directly
above the earthquake's focus
Where do the deepest earthquakes occur? In Tranform zones? In Convergent
zones? In Divergent zones?
Waves
• The seismic waves
produced by an
earthquake radiate out from its focus.
• These waves cause the earth to vibrate in
response to the pushing and pulling forces
that are applied to them.
• Two category of waves (each with
subsets):
– Body waves: Primary(P), Secondary(S)
– Surface waves: Love(L), Rayleigh (R)
http://www.geo.mtu.edu/UPSeis/waves.html
SEISMIC WAVES: Body Waves
• Body
waves arrive first since they are the fastest. Body
waves are broken down into two types:
•Primary (P) waves, which cause the rock particles through
which they pass to shake back and forth (compression
effect) and can result in a noise similar to a sonic boom.
•Secondary (S) waves make the particles vibrate both
vertically and horizontally (shearing effect).
• Note that Body waves travel mostly downwards into the
interior of the earth and are rarely felt by humans. They
disappear as soon as they hit a gas (atmosphere).You can hear
Body waves, but it is the next set of waves that actually move
outwards along the ground – causing damage.
SEISMIC WAVES: Surface Waves
Surface waves arrive last and as their name implies they
travel along the surface of the earth. It is these waves that
cause damage. They begin with:
•Love/Long (L) waves that move the ground from side to
side, again, in a shearing effect. These are followed by:
• Rayleigh (R) waves which are similar to ocean waves.
These cause surface materials to move in a vertical circle
just as a floating object would move as a sea wave passes
under it. These waves are responsible for most of the
damage to buildings.
• The greatest shaking occurs near the centre of large
earthquakes. It is here that all the waves originate.
MEASURING THE ENERGY RELEASED
The energy released by an earthquake is measured by a Seismograph!
MEASURING THE DAMAGE
The violence of an earthquake can be measured using
two different scales:
The Richter scale - devised in 1932 by Charles
Richter
The Richter scale actually uses a numerical value. It is
important to note that this scale is logarithmic in nature
meaning that the numbers measure in factors of 10. In
other words, an earthquake measuring a 4 on the
richter scale is 10x as damaging as an earthquake that
measured 3 on the richter scale.
The Mercalli scale - devised in 1905 by a gentlemen
named Mercalli. The Mercalli scale is a visual scale
based on the destruction that occurs.
WHAT IS THE EARTH MADE OF?
• The
study of earthquakes has helped us determine what
the earth is made up of.
• Geophysicists have calculated that the earth has a
mass of 5876 million million million tonnes. (A Zillion)
• In addition, it has been determined that the earth
has an average density of 5.5, based on water having
a density of 1.
• But the rocks that we can examine near the surface
of the earth have an average density of 2.7. This suggests
that the density of the earth is higher in some parts and
lower in others.
Assumption: density increases as depth into the earth
increases.
WHAT IS THE EARTH MADE OF?
• Can
it be proven?
• In an attempt to prove this assumption , investigations have
been made by analysing earthquake waves – the ones that go
down into the earth – Body waves.
• The study of earthquakes (which is caused by a
shifting of the earth’s crust ) is referred to as Seismology.
• As mentioned earlier (a quick review follows) when a
earthquake occurs, it gives off two types of energy (shockvibration) waves:
1.
2.
Body waves - which travel through the earth
Surface waves - which travel along the earth’s surface
Short Review and some repetition:
• Body
waves (travel through the interior of the earth) and they
are also divided into two types:
1. P waves - (Primary waves)
• They are compressional (squeeze) waves, they compress
then stretch.
• They travel very quickly through the earth’s surface.
• They will go through anything: liquid, solid, gas. Their
speed increases with density.
2. S waves - (Secondary waves)
• They are slower than a P wave
• They produce a side to side movement, shaking action.
• Their speed increases with density and will only go
through solids
• Surface waves are confined to the outer crust of the earth.
These are the ones we feel when there is an earthquake and
the ones that causes all the destruction.
• There are two type of Surface waves:
1. L waves - (Love waves) they have a side to side motion.
2. R waves - (Raleigh waves) they have an up and down
motion.
You can actually see these waves go through the earth during
an earthquake. These waves move the surface of the earth like
waves moving through water.
• SO WHAT?
•By studying the movement of these waves , Seismologists
have determined the characteristics of the inside of the
earth. For instance:
• The S and P waves move with a constant speed in the
Lithosphere, but slow down in the asthenosphere. This
backs up the notion that the asthenosphere is semi-solid in
nature.
• Both waves pick up speed in the Mantle and increase their
speed as their depth in the Mantle increases.
• When these waves reach the core the S waves stop and
the P waves slow down, which suggests that the outer core
is liquid.
SO WHAT? Cont’d
• When
the P waves reach the inner core they pick up
speed tremendously, which again suggests that the inner
core is solid and quite dense.
•In theory the core of the earth should have a density of
about 15 and the material in the inner core has been
called NIFE - a combination of 10% Nickel and 90% Iron.
Damage
Damage
Damage
Tsunamis
• Giant waves generated by an underwater
disturbance that cause destruction to
coastal areas
Tsunamis
• Waves will travel outward in all directions
• Time between wave crests may be from 5
to 90 minutes
• Wave speed in the open ocean will
average 450 miles per hour.
• Heights of over 100 feet recorded
Tsunamis
• When created, it has a very long wave
length and short wave height.
• As it approaches shallow coastal waters,
length is compressed and heights are
increased
• Wave becomes unbalanced and topples
Tsunamis
• Rapid changes in the water level are an
indication of an approaching tsunami.
• Arrive as a series of successive crests
(high water) and troughs (low water)
• After a severe earthquake on November
18, 1929, in the Grand Banks of
Newfoundland
– generated a tsunami that caused
considerable damage and loss of life at
Placentia Bay, Newfoundland.
Tsunamis
The following animations were captured from the following site:
www.pbs.org/savageearth/animations/tsunami/index.html
The Tsunami of Dec. 26, 2004
The Toronto Star
Tsunamis
The red color means that the
water surface is higher than
normal, while the blue means
lower.
Tsunamis - Before – Kalutara, Sri Lanka – image from
Digitalglobe
Tsunamis - After – image from Digitalglobe
Look at the big wave – Is that Fake?
This is Phuket in Thailand
Tsunami - Banda Acech – Northern Sumatra - Before
Tsunami - Banda Acech – Northern Sumatra - After
Flash Corner:
1. Earthquake Waves
2. How a Seismograph works
Tsunamis
Youtube Videos
Tsunamis
Tsunamis
The End