Earthquakes
Measuring Earthquakes
Guide For Reading
• What are the different kinds of
seismic waves?
• How does the energy of an earthquake
travel through Earth?
• What are the scales used to measure
the strength of an Earthquake?
There are about ______
earthquakes per day,
worldwide.
• 8,000
Most earthquakes begin below
the earth’s surface in the
_______ within ______
kilometers from Earth’s
surface.
• Lithosphere
• 100
Focus
• The point beneath Earth’s surface
where rock breaks under stress and
causes an earthquake
Epicenter
• The point on Earth’s surface directly
above an earthquake’s focus
Seismic Waves
Seismic Waves
• A vibration that travels through
Earth carrying the energy released
during an earthquake
Figure 11: At what point do
seismic waves first reach the
surface?
• The epicenter
What determines how much the
ground shakes during an
earthquake?
• How close a location is to the
epicenter
• The types of rock and soil
surrounding the epicenter determines
how much the ground shakes
Guide For Reading: What are
the different kinds of seismic
waves?
• There are three types of seismic
waves:
• Primary Waves (P waves)
• Secondary Waves (S waves)
• Surface Waves
• An earthquake sends out two types of
waves P waves and S waves
• When the waves reach Earth’s
surface at the epicenter, surface
waves develop.
Primary Waves
Primary Waves (P Waves)
• A type of seismic wave that
compresses and expands the ground
• The first wave to arrive at an
earthquake
Describe how P waves move?
• P waves compress and expand like an
accordion
• When P waves arrive they vibrate the
particles of the crust forward and
back along the path of the wave
Secondary Waves
Secondary Waves (S Waves)
• A type of seismic wave that moves
the ground up and down or side to
side
How are P waves different
from S Waves?
• P waves compress and expand the ground
• These waves cause buildings to contract and expand
• P waves travel through solids and liquids
• S waves vibrate from side to side
• These waves vibrate from side to side and thrust the
ground up and down, or back and forth
• S waves can not move through liquids
Surface Waves
Surface Waves
• A type of seismic wave that forms
when P waves and S waves reach
Earth’s surface
Why do you think surface
waves produce more severe
ground movements than P waves
and S waves?
• Surface waves consist of loose soil,
sand, gravel, mud, small rocks, not
solid rock
• These more loose substances are
more likely shift and slide
Guide For Reading: How does
the energy of an earthquake
travel through Earth?
• Seismic waves carry the energy of an
earthquake from the focus, through
Earth’s interior, to the epicenter, and
across the surface
Detecting Seismic Waves
Seismograph
• A device that records ground
movements caused by seismic waves
as they move through Earth
Describe how a mechanical
seismograph records ground
movement.
• A heavy weight attaches to a frame by
spring or wire
• A pen connected to the weight rests its
point on a rotating drum
• During an earthquake the seismic waves
cause the drum to shake while the pen
stays in place
• The pen records lines on the paper around
the drum
Measuring Earthquakes
There are at least ______
different measures for rating
earthquakes.
• 20
Magnitude
• The measurement of an earthquake’s
strength based on seismic waves and
movement along faults
The Mercalli Scale
Mercalli Scale
• A scale that rates earthquakes
according to their intensity and how
much damage they cause
• Developed in the early twentieth
century
An earthquake’s ______ is the
strength of ground movement
in a given place.
• Intensity
The Mecalli scale has ______
steps and describes how an
earthquake affects ______,
______, and the ______.
• 12
• People
• Buildings
• Land surface
Figure 14: How would you rate
the damage to the Foligno city
hall on the Mercalli scale?
• The damage would probably rate VII
- VIII
The Richter Scale
Richter Scale
• A scale that rates seismic waves as
measured by a particular type of
mechanical seismograph
How does the Richter scale
measure an earthquake?
• The Richter scale measures seismic
waves using a seismograph
• Over time, mechanical seismographs
were replaced by electronic
seismographs
What is a strength of the
Richter scale when measuring
earthquakes? What is a
weakness?
• Strength: Provides accurate
measurements for small, nearby
earthquakes
• Weakness: Does not measure larger
and more distant earthquakes well
How are the Mercalli scale and
the Richter scale similar? How
are they different?
• Both measure the strength of an
earthquake
• The Mercalli scale measures the strength
in terms of extent people notice the
earthquake and the amount of damage
caused
• The Richter scale measures the size of
seismic waves
The Moment Magnitude
Scale
Moment Magnitude Scale
• A scale that rates earthquakes by
estimating the total energy released
by an earthquake
Why is the moment magnitude
scale used today by geologists
to measure earthquakes?
• The moment magnitude scale determines
the total energy released by an earthquake
• This scale uses a electronic seismograph
that can measure earthquakes that are big
or small, and near or far
• Geologist examine movement along the
fault and the strength of broken rock
• These two measurements give a more
accurate measurement of an earthquake
On which scale would an
earthquake’s strength vary
from one place to another?
Explain.
• The Mercalli scale because the
amount of shaking that people would
feel and the damage to objects would
be greater in a place closer to the
earthquake’s epicenter
Checkpoint:What are three scales for
measuring earthquakes? Explain what
each one measures.
• Mercalli Scale
• Measures earthquakes on how much damage they
cause
• Richter Scale
• Measures seismic waves using a seismograph
• Measures small and nearby earthquakes
• Moment Magnitude Scale
• Looks at the total energy released
• Measures large and distant earthquakes
• Helps scientists predict how much fault movement
their was
Locating the Epicenter
How do scientists calculate how
far a location is from the
epicenter of an earthquake?
• Scientists calculate the difference
between arrival times of the P waves
and S waves
• The further away an earthquake is,
the greater the time between the
arrival of the P waves and the S
waves
Figure 17: Use the map scale
to determine the distances
from Savannah and Houston to
the epicenter. Which one is
closer?
• Houston
• 800 Km
• Savannah
• 900 km