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P1H
Smart Teach 3
Write Down
Frequency – means the number of vibrations or
waves passing in one seconds.
Humans can only hear sounds with a frequency from
20Hertz to 20,000Hertz.
Infrasound – sound too low in pitch to hear with a
frequency below 20Hertz
Ultrasound – sound too high in pitch to hear with a
frequency above 20,000Hertzs.
Transverse vs. longitudinal waves
Transverse waves
are when the
displacement is at
right angles to the
direction of the
wave.
Displacement
18/04/2016
Direction
Examples – light, other EM waves, some seismic waves
Displacement
Direction
Longitudinal waves
are when the
displacement is
parallel to the
direction of the
wave…
Examples – sound, some seismic waves
Write Down
Longitudinal waves are waves where the vibrations
are parallel to the direction the wave travels.
e.g. Infrasound, ultra sound, sound, P Seismic waves.
Before you look in more detail about
earthquakes you first need to
understand the structure of the earth
Key PointOuter core is
liquid!
•The Earth is almost a sphere. These are its main layers, starting with
the outermost:
•crust - relatively thin and rocky
•mantle - has the properties of a solid, but can flow very slowly
•outer core - made from liquid nickel and iron
•inner core - made from solid nickel and iron
Note that the radius of the core is just over half the radius of the
Earth. The core itself consists of a solid inner core and a liquid outer
core.
Tectonic Plates?
•The outermost layer of the Earth is
made up of tectonic plates.
•These plates
are slowly
moved by
convection
currents.
Write Down
Earthquakes are caused
by sudden movement of
the Earth’s Tectonic
Plates in the Crust.
The plates are moved by
the Convection Currents
in the hot mantle below.
Hot Mantle – Rising
Cold Mantle - Sinking
Convection currents can lead
to earthquakes...
•Radioactive decay takes place in the mantle, this
produces a lot of heat, which causes the mantle
to flow in convection currents. The hot rock rises
then cools, causing the plates to move. As the
plates move they slide past each other. There is
friction between the plates. Sometimes they get
stuck, the pressure and energy builds up.
Eventually they slip releasing the energy as an
earthquake.
Seismic Waves?
•These are waves that travel through the
earth when an earthquake occurs. You need to
learn the properties of the two types of
waves.
Key Point- Seismic waves
like any other waves will
refract!
Key Points
•S waves cannot pass through the liquid outer core,
but P waves can. When P waves pass from solid to
liquid, then from liquid to solid, there are sudden
changes in direction – they are reflected and
refracted. Seismic waves are also reflected and
refracted as they pass into different rock types.
Questions about Earthquakes will often ask you to
calculate time or distance a wave has travelled.
You will use the following equation to answer them:
Wave speed = distance
time
Always show all working for calculations:
1 write down the equation used.
2 substitute the numbers into the equation.
3 check the units are correct for each number, convert
them if needed to the correct units.
4 Write down the answer with its unit.
Write Down
Seismometers detect the P Seismic and S seismic
waves from Earthquakes.
P seismic waves travel faster than S waves so are
detected by Seismometers first.
The time difference between P and S seismic waves,
known as S-P time, tells us how far an Earthquake
epicentre is from a seismometer.
The larger the time difference is between P and S
seismic waves arriving the further away the Epicentre
is.
Write Down
Triangulation can be used
with 3 or more Seismometers
to locate the Epicenter of an
Earthquake.
Write Down
Seismic waves do not travel
in straight lines as they pass
through the Earth.
Some waves are reflected at
boundaries between layers.
Some waves are refracted as
they pass from layer to layer.
The pathways seismic waves take travelling through
the Earth have told us about its layered structure.