5.2 Measuring
Earthquakes
Introduction to Earthquakes
• In order to know where an earthquake was
centered, you need to know where it began!
• Earthquakes always begin in rock below the
surface (in the lithosphere)
• The focus is the point beneath Earth’s surface
where rock under stress breaks, triggering the
earthquake
• The epicenter is the point on the surface directly
above the focus
Where Do Earthquakes Start?
• What is the difference
between the Focus
and Epicenter?
Seismic Waves
•
•
•
Seismic waves are vibrations that travel
through Earth carrying energy released during
an earthquake
Seismic waves carry the energy of an
earthquake away from the focus, through
Earth’s interior, and across the surface
There are 3 types of seismic waves
1. Primary waves (P waves)
2. Secondary waves (S waves)
3. Surface waves
Primary Waves
• P waves compress
and expand the
ground like an
accordion.
• Travel the fastest
and first to arrive
• Can travel through
liquids (the mantle)
Secondary Waves
• S waves vibrate
from side to side
and up and down.
• Second fastest type
of seismic wave
• Can not travel
through liquids (the
mantle)
Surface Waves
• Slowest type of seismic
wave
• Can make the ground
roll like an ocean wave
• Produce the most
severe ground
movements (and cause
the most structural
damage)
Detecting Seismic Waves
• Seismologists
– Scientists who study earthquakes and seismic
waves
• Seismograph
– An instrument used to record waves
generated by earthquakes
– How does a seismograph work?
Detecting Seismic Waves
Detecting Seismic Waves
• The pattern of lines, called a seismogram, is
the record of an earthquake’s seismic waves
produced by a seismograph.
Measuring Earthquakes
•
There are 3 ways to measure
earthquakes:
1. The Mercalli Scale
2. The Richter Scale
3. The Moment Magnitude Scale
•
Magnitude the energy released
during an earthquake
The Mercalli Scale
• The Mercalli scale
was developed to
rate earthquakes
according to the
amount of damage
at a given place
• Uses Roman
numerals to rank
earthquakes by how
much damage they
cause
The Richter Scale
• Describes how much energy is released
by the earthquake
• About 32 times as much energy is
released for every increase of 1.0 on the
scale
– Example – a magnitude of 8.5 releases 32xs
more energy as a 7.5 magnitude
Moment Magnitude Scale
• Takes into account the size of the fault
rupture, the amount of movement along
the fault, and the rocks’ stiffness
• Values are estimated from the size of
several types of seismic waves produced
by an earthquake
Locating the Epicenter
• Each circle shows the distance from one
seismograph station to all the points where
the epicenter could be located.
• The single point where the three circles
intersect is the location of the earthquake’s
epicenter.
Locating the Epicenter