Earthquakes

Earthquakes

Section 1. What are earthquakes?

An earthquake is what happens when two blocks
of the earth (tectonic plates usually) suddenly slip
past one another. The surface where they slip is
called the fault orfault plane. The location below
the earth’s surface where the earthquake starts is
called the hypocenter, and the location directly
above it on the surface of the earth is called the
epicenter.

In your own words, what is an earthquake “fault”?

Answer:

Multiple Choice

The location below the earth’s surface where the earthquake starts is called the _____

• epicenter
• subsurface
• underground
• hypocenter

Section 1. What are earthquakes?

Sometimes an earthquake has foreshocks. These
are smaller earthquakes that happen in the same
place as the larger earthquake that follows.

Scientists can’t tell that an earthquake is a
foreshock until the larger earthquake happens. The
largest, main earthquake is called the mainshock.

Mainshocks always have aftershocks that follow.
These are smaller earthquakes that occur
afterwards in the same place as the mainshock.
Depending on the size of the mainshock,
aftershocks can continue for weeks, months, and
even years after the mainshock!

Open Ended

Describe the difference between a foreshock, mainshock, and aftershock:

Section 2. What causes earthquakes and where do they happen?

The earth has four major layers: the
inner core, outer core, mantle and
crust. The crust and the top of the
mantle make up the thin rocky surface
of our planet.

Multiple Choice

Using the diagram,
which layer of the earth is the
thickest?

• Inner Core

• Outter Core

• Mantle

• Crust

Section 2. What causes earthquakes and where do they happen?

But the surface is not all in one piece – it is like a
puzzle which covers Earth. Not only that, but these
puzzle pieces continue to move slowly, sliding past one
another and bumping into each other.

We call these puzzle pieces tectonic plates, and the
edges of the plates are called the plate boundaries.
The plate boundaries are made up of many faults, and
most of the earthquakes around the world occur on
these faults.

Since the edges of the plates are rough, they get stuck
while the rest of the plate keeps moving. Finally, when
the plate has moved far enough, the edges unstick on
one of the faults and there is an earthquake.

Open Ended

Why do most earthquakes occur
along fault lines?

Section 3. How are earthquakes recorded?

While the edges of faults are stuck together, and the rest of
the block is moving, the energy that would normally cause
the blocks to slide past one another is being stored up.

When the force of the moving blocks finally overcomes the
friction of the jagged edges of the fault and it unsticks, all
that stored up energy is released. The energy radiates
outward from the fault in all directions in the form of seismic
waves like ripples on a pond. The seismic waves shake the
earth as they move through it, and when the waves reach the
earth’s surface, they shake the ground and anything on it, like
our houses and us!

The image to the right shows the types of seismic waves that
can occur from earthquakes.

Section 3. How are earthquakes recorded?

Earthquakes are recorded by seismographs. The recording
they make is called a seismogram. The seismograph has a
base that sets firmly in the ground, and a heavy weight that
hangs free. When an earthquake causes the ground to shake,
the base of the seismograph shakes too, but the hanging weight
does not. Instead, the spring or string that it is hanging from
absorbs all the movement. The difference in position between
the shaking part of the seismograph and the motionless part is
what is recorded.

Open Ended

What is the difference between S and P waves?

Section 3. How are earthquakes recorded?

Section 3. How are earthquakes recorded?

By looking at the amount of time between the P and S wave on
a seismogram recorded on a seismograph, scientists can tell
how far away the earthquake was from that location. However,
they can’t tell in what direction from the seismograph the
earthquake was, only how far away it was. If they draw a circle
on a map around the station using the distance, they can
create radius of where the earthquake took place.

Scientists then use a method called triangulation to determine
exactly where the earthquake was (see image). It is called
triangulation because a triangle has three sides, and it takes
three seismographs to locate an earthquake. If you draw a
circle on a map around three different seismographs where the
radius of each is the distance from that station to the
earthquake, the intersection of those three circles is the
epicenter!

Section 6. Earthquake preparedness!

Experts advise those who feel a strong earthquake find a
safe spot in the room under a sturdy table or against an
inside wall. People are advised to get on their hands and
knees and cover their head and neck with their arms and
only move if they need to get away from falling objects.

In an earthquake, it’s important to remember these DOs
and DO NOTs:

●DO drop, cover and hold on!

●DO stay away from glass, windows, outside doors
and walls

●DO NOT get in a doorway

●DO stay inside until the shaking stops

●DO NOT use the elevators.

●If outside, DO move away from buildings,
streetlights, and utility wires

Based on the information to the left,
prepare for an extreme scenario: You
are in your room when you start to feel
the house shake.

What is the first and second thing you
should do immediately to ensure your
safety?

Answer: