# Earthquakes Earthquakes and Volcanoes

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```					  Earthquakes and Volcanoes
Earthquakes occur
when rock masses
in the Earth’s
crust move
suddenly along
ruptures called
faults. The rock
movement may
occur vertically,
horizontally or in
combination.
Epicenter- the
point on the
surface
directly above
the focus

Focus – The location on a
fault where the greatest
amount of movement is.
It is far underground.
Energy travels outward from the
focus in waves.
The intensity of an earthquake
can be expressed on the Richter
Scale.
Types of Earthquake Waves

An “S Wave” (“shear” or “secondary”)
causes material to move up and down as it
goes through.
Earthquake Waves
A “P Wave (or “Primary” or
“Pressure” wave) consists of
compressions which move
straight through a material.

p & s waves animations
P waves can travel through solids, liquids
and gases.
S waves can travel only through solids.
Earthquake waves are detected on a
Seismometer.
Seismometers produce a seismograph
P waves travel faster than S waves.
The farther away from the
epicenter, the greater the
difference in arrival time of P and
S waves.
This graph lets
a person
calculate the
distance from
the epicenter,
knowing the
time interval
between
arrival of P
and S waves.
An earthquake occurred on June
23, 1946 and these were recorded
times of P and S wave arrivals at
different stations.
0   2   37

For example, the P/S time interval
in Saskatoon was 0 h, 2 min and
37 seconds.
2 minutes and
37 seconds
would
correspond to
A circle with a 1400 km radius
could be drawn on a map with
epicenter would be somewhere
on this circle.

The procedure could be repeated for
all the other stations. The point where
all 5 circles overlap would be the
epicenter of the earthquake.
In a different example, circles were drawn
around 3 seismic stations.

Eg. All 3 circles
intercept at this
point, so this
would be the
location of the
epicentre
High Risk Zones
for Earthquakes:
1. They are located near
a plate boundary.
2. There is a history of
earthquakes in the
region.
3. There are numerous
active faults.

The West Coast of
North America has
earthquakes.
Earthquakes from National
Geographic
Volcanoes

A volcano
is formed
when
magma
makes its
way to the
surface.
Three types of Volcanoes

The lava is quite liquid and flows down the sides.
These volcanoes are very large but not very steep. An
example is Hawaiian volcanoes like Kilauea
form mainly from explosive eruptions of lava. Blown violently into the air,
the erupting lava breaks apart into fragments called cinders. The fallen
cinders form a cone around the volcano's central vent.
such as Mount St. Helens, build from both explosive and
slow flowing eruptions. Layers of tephra alternating with
layers of viscous lava flows create steep-sided, often
symmetrical cones that we think of as the classic volcano
shape.
Effects of
Volcanoes

Not all
volcanoes
have all these
effects.
Earthquakes
always occur
when a
volcano erupts