1) Earthquakes are caused by the release of energy along faults in the Earth's crust due to the buildup of pressure. 2) There are three main types of seismic waves that radiate out from the earthquake focus: primary (P) waves, secondary (S) waves, and surface waves. 3) Earthquakes are measured using the Richter Scale, which quantifies the amount of energy released by the earthquake. The strongest earthquake ever recorded was magnitude 9.5.

1. STANDARD 8-3.8
EARTHQUAKES

2. The definition of an earthquake is…
vibrations that cause the breaking of
rocks.
These vibrations move in all directions
through the earth. They begin at a point
along a fault.

3. EARTHQUAKES
• Forces___ and __Stresses__ (8-3.7– tension, compression, and
shearing) along faults can build up as blocks of rock are pushed
(compression or shearing) or pulled apart (tension). If the
__pressure___ or stress becomes too great, the rock breaks at a
weak point along the fault and ___energy_____ is released

4. The earth’s crust is constantly
experiencing pressure from forces
within and around it. This pressure
builds up over time, and eventually
causes the crust to break. This
becomes a fault.
Let’s experience it…

5. Faults are divided into three main groups:
Normal fault - when two plates are moving apart
and one side of the fracture moves below the
other; (caused by tension forces!)
Reverse fault - when two plates collide and one
side of the fracture moves on top of another;
(caused by compression forces!!)
Strike-slip - when two plates slide past each
other. (caused by shear forces!)

6. An earthquake begins along a fault (a
crack in the earth’s surface) at a point
called the focus.
Directly above the focus is a point on
the earth’s surface called the epicenter.

7. PARTS OF THE EARTHQUAKE
• The energy spreads outward in all directions as
vibrations called ____seismic waves_____.
• The _focus____ of the earthquake is the point
in the crust, or mantle, where energy is
released.
• The _epicenter___ is the point on Earth’s
surface directly above the focus; energy that
reaches the surface is greatest at this point.

8. FOCUS – POINT INSIDE THE EARTH WHERE
AN EARTHQUAKE BEGINS
EPICENTER – POINT ON EARTH’S SURFACE
ABOVE FOCUS

9. WHAT CAUSES EARTHQUAKES?
• Used to describe both sudden slip on a fault, and the resulting
ground shaking and radiated seismic energy caused by the slip
• Caused by volcanic or magmatic activity,
• Caused by other sudden stress changes in the earth.

10. WHAT CAUSES EARTHQUAKES?
• Tectonic plates move past each other causing stress. Stress causes
the rock to deform

11. Cause of Earthquakes
WHAT CAUSES AN EARTHQUAKE?
• An aftershock is a small earthquake that
follows the main earthquake.
• A foreshock is a small earthquake that often
precedes a major earthquake.
 Aftershocks and Foreshocks

12. Earthquakes can also cause landslides, sudden
eruptions as in the case of a hot lava flow from a
volcano or giant waves called tsunamis. Sometimes new
land mass are also formed. Such earthquakes are
attributed with the creation of the greatest undersea
mountain range and the longest land mountain range.

13. WHERE DO EARTHQUAKES OCCUR AND HOW OFTEN?
~80% of all earthquakes occur in the circum-
Pacific belt
• most of these result from convergent margin
activity
• ~15% occur in the Mediterranean-Asiatic belt
• remaining 5% occur in the interiors of plates
and on spreading ridge centers
• more than 150,000 quakes strong enough to
be felt are recorded each year

14. SEISMIC WAVES
• _SEISMIC WAVES___ are waves generated by
an earthquake that travel through the Earth.
These waves can cause the ground to move
forward, backward, up, down, and even to
ripple. Seismic Waves are generated at the
__SAME___ time but move in different ways,
and at different speeds.

15. 3 TYPES OF SEISMIC WAVES
•PRIMARY (P) WAVES
•SECONDARY (S) WAVES
•SURFACE WAVES

16. Earthquake Waves
8.2 MEASURING EARTHQUAKES
 Body Waves
• P waves
• Identified as P waves or S waves
- Have the greatest velocity of all earthquake waves
- Are push-pull waves that push (compress) and pull (expand) in
the direction that the waves travel
- Travel through solids, liquids, and gases

17. PRIMARY (P) WAVE
• Move out from the earthquake focus, the
point where the energy is released
• Travel the fastest of the three waves
• Move through solid and liquid layers of
Earth (it also can move in gas)
• Push and pull rock creating a back-and-forth
motion in the direction the wave is moving
(longitudinal wave)

18. Earthquake Waves
8.2 MEASURING EARTHQUAKES
 Body Waves
• S waves
- Seismic waves that travel along Earth’s outer
layer
- Slower velocity than P waves
- Shake particles at right angles to the direction
that they travel
- Travel only through solids
 A seismogram shows all three types of
seismic waves—surface waves, P
waves, and S waves.

19. SECONDARY WAVES (S
WAVES)
• Move out from the earthquake focus
• Move slower than primary waves
• Can only move through solid rock
• Move at right angles to primary waves
causing rocks to move up and down and side
to side (transverse wave)

20. COMPARING SEISMIC WAVES

21. SURFACE WAVES
• Form when P and S waves reach the surface
• Can cause the ground to shake making rock
sway from side to side and roll like an ocean
wave
• These waves cause the most destruction
• They move back and forth and in a rolling
motion along the surface
• They release all of the energy of the
earthquake

22. SURFACE WAVES: R AND L WAVES
• Surface Waves
• Travel just below or along the ground’s surface
• Slower than body waves; rolling and side-to-side
movement
• Especially damaging to buildings

23. SURFACE WAVES
• Move along the Earth’s surface
•Produces motion in the upper crust
• Motion can be up and down
• Motion can be around
• Motion can be back and forth
• Travel more slowly than S and P
waves
• More destructive

24. Earthquake Waves
MEASURING EARTHQUAKES
 Seismographs are instruments that
record earthquake waves.
 Seismograms are traces of amplified,
electronically recorded ground motion
made by seismographs.

25. Earthquakes are measured using the Richter
Scale. The strongest earthquake ever
measured was a 9.5 on the Richter Scale. This
is a measurement of the amount of energy
released from the earthquake.

26. Measuring Earthquakes
8.2 MEASURING EARTHQUAKES
 Historically, scientists have used two
different types of measurements to
describe the size of an earthquake
—intensity and magnitude.
 Richter Scale
• The _Richter Scale expresses the magnitude of
an Earthquake and measures the energy released.
The scale goes from 1 to 10 .
• Based on the amplitude of the largest seismic
wave

27. Determining the magnitude of an earthquake
Magnitude -- measure of energy released during earthquake.
There are several different ways to measure magnitude.
Most common magnitude measure is Richter Scale, named for
the renowned seismologist, Charles Richter.
Richter Magnitude
• Measure amplitude of largest S wave on seismograph record.
• Take into account distance between seismograph & epicenter.
Intensity
• Intensity refers to the amount of damage done in an
earthquake

28. • Intensity
• subjective measure of the
kind of damage done and
people’s reactions to it

29. HOW ARE EARTHQUAKES MEASURED?
RICHTER SCALE

30. HOMEWORK
• Study for Quiz
• Place the following words in your
glossary.
• Primary wave, Secondary wave, Surfaces
wave, Longitudinal wave, Transverse
wave, Richter Scale, Seismograph,
Seismogram, Magnitude, and Intensity.