Earthquakes and Epicenters
The Earth is composed of
several layers. We can
separate these layers by
physical properties OR by
composition (what they’re
made of).
Crust, Mantle, Core are
the simple separations.
The crust and the mantle
closest to the crust
together are called the
lithosphere.
The mantle beneath the
lithosphere is plastic –
meaning it flows like
toothpaste. It is hot and
the rock is molten. It is
called the
asthenosphere.
Lithosphere: 0-1000o C (cold): made up of
mostly Aluminum (Al), Silica (Si) and Oxygen
(O). Brittle.
Asthenosphere: 1000-1500oC: (warm) made up mostly of Magnesium (Mg) and Iron
(Fe). “Plastic” – flows like toothpaste.
Lower Mantle: 1500-5000 0 C (hot) - Iron
and Magnesium - rigid
Outer Core: 5000-6200 0 C: (VERY HOT) –
made up of mostly Iron and nickel – liquid.
Inner Core: 6200-7000 0 C: (VERY HOT) made up mostly of Nickel (Ni)- The inner core
is solid due to very high pressures.
Remember the ways that heat energy are transferred:
1. Radiation: heat energy transferred across an open space
2. Conduction: heat energy transferred from molecule to molecule by
touching
3. Convection! – heat energy transferred in a cycle through a fluid.
The mantle in the asthenosphere is heated from below, rises to the
surface, and then spreads out and sinks back down.
Lithosphere
Asthenosphere
Lower mantle
Outer core
Inner core
QUESTION: Where else in our studies of Earth have we seen something heat up,
rise, and spread out only to cool and sink back down?
Air currents and
weather!! Remember??
(Regents Exam is in less
than a month!!!)
Convection currents in the asthenosphere cause the lithosphere above
to crack and move. This cracks the surface of the Earth into big “puzzle
pieces”. We call them Plates, and the way that these plates move is
called plate tectonics.
Think of it like a hard-boiled egg:
The shell is the crust
the white is the mantle
and the yolk is the core.
When the lithosphere (shell) cracks
and the pieces move, they can
move in relation to each other in
three ways:
1. They can crash into each other
2. They can slide past each other
3. They can move away from each
other.
These movements take place over millions of years. Plates tend to only move
around a few centimeters per year. This movement means that the surface of
the Earth looked a lot different millions of years ago – and will look a lot
different millions of years from now!
As you can imagine, all this movement and cracking can build up tremendous
amounts of energy on the rocks of the lithosphere. When that energy is
released, the Earth shakes violently… An Earthquake!
Definitions:
Earthquake – a natural rapid shaking of the
lithosphere caused when rocks are displaced
due to the release of energy stored in the
rocks.
Focus – the place where an Earthquake
originates
Epicenter – the place on the Earth’s Surface
directly above the focus of an Earthquake
Seismic wave – energy waves given off by
an earthquake
Seismograph – an instrument used to detect
and record seismic waves
Earthquakes create waves that travel through the
earth. They are much like the waves created in the
water of a pond when a drop of rain lands….
Seismic waves travel
away from the epicenter
in all directions.
Seismic stations record movements in the lithosphere with seismographs, and
create seismograms – pictures of the waves arriving at the station.
Earthquakes shake the lithosphere in a few different ways, and because of this,
several different types of seismic waves are produced. The fastest of these waves
are called P-waves (Primary waves). P-waves move like an accordion –
stretching and tightening the lithosphere.
The next fastest waves are called S-waves (secondary waves).
These waves move up and down much like waves on the ocean.
The slowest waves are called
surface waves. There are two
types. Rayleigh and Love
waves.
These waves are responsible
for most of the damage in an
earthquake – but they arrive
after the P and S waves.
This seismograph
shows the arrival of
the P and S waves.
Notice that the Pwaves arrive first and
then the S-waves
come.
(If we waited long
enough, we’d see
surface waves next)
So – as the waves propagate (move) away from the epicenter, the P-waves keep
getting further and further from the S-Waves.
1 minute
5 minutes
10 minutes
15 minutes
Pete the P-wave rabbit.
Solomon the S-wave tortoise.
Pete is fast! He ALWAYS BEATS
SOLOMON IN A RACE.
Solomon is wise – but slow. He NEVER
BEATS PETE IN A RACE.
After 1 minute
After 5 minutes
After 10 minutes
This is the P-wave and S-wave
travel time graph from the
Earth Science Reference
Table.
Answer these questions from
the graph:
1. How long does it take a Pwave to travel 2 x 103 km?
2. How long does it take a Swave to travel the same
distance?
3. 7 minutes after an
Earthquake, how far apart
are the P and S waves?
4. How far apart are they after
13 minutes?
We use this difference in travel time
to determine the epicenter of an
Earthquake. To do this, we need
data from at least 3 seismic
stations.
Let’s say an earthquake occurs in
California. Seismographs in Delano,
Oxnard, and Yuma record P and S
waves. Each location determines how
much time went by between the
arrival of the P wave and the arrival of
the S wave. By doing this, they each
know how far the epicenter is from
them – BUT… It could be in any
direction!
But if all three locations determine
how far it is, they can create a circle
of possible locations – and where the
three circles connect – there is the
epicenter!
First, Oxnard determines their circle of
possible locations…
Then Delano…
Then Yuma…
They intersect here –
at the Epicenter!!