Measuring the Magnitude of an Earthquake
Magnitude: the amount of seismic energy released by an earthquake
There are 2 different scales which are used to measure the magnitude of earthquakes:
a. Richter Scale: traditional
b. Moment Magnitude Scale : modern
The Richter Scale
Developed by Charles Richter (American seismologist) in the 1930s.
The "Richter Scale" is the common name for the Richter Magnitude or Local Magnitude Scale.
It is a logarithmic scale - as each number increases, the change in magnitude is 10x the previous number.
Problems with the Richter Scale
1.
The magnitude of an earthquake is compared to the magnitude of another earthquake - the actual
magnitude is not measured.
2.
The scale was developed in Southern California. The conditions in the rest of the world may not be the
same as they are in California.
Moment Magnitude Scale
Was developed in 1979 in order overcome the problems of the Richter scale.
Seismologists can directly measure where the rupture occurs in order to calculate the amount of energy released.
The energy is called the seismic moment (the total energy of an earthquake).
The Moment Magnitude Scale was created so that earthquakes of more intensity can be measured.
This scale is now preferred over the Richter scale by seismologists.
Most news broadcasts now state only a number value when reporting
about a recent earthquake without reporting which scale was used to
measure it.
Differences Between the Richter Scale and the Moment Magnitude Scale
RICTHER SCALE
MOMENT MAGNITUDE SCALE
1
Records the responses of seismographs
and the distance from the epicenter
Records the physical property (the
seismic moment) of the earthquake
2
Fails in accuracy for earthquakes over 7
Capable of recording larger
magnitudes with accuracy
3
Can't record accurately for earthquakes
that are far away
Can reliably record earthquake data
over large distances
4
Can provide a detailed picture for low
magnitude earthquakes
Gives a blurred reading for low
magnitude earthquakes
5
Ideal for measuring low magnitude
earthquakes
Good for measuring medium and large
magnitude earthquakes
Questions
Name
1. How is the Richter scale measurement different from
the Moment Magnitude scale?
Period
The Richter scale is
Answer Key
1
2
Score
3
4
/14
March 5, 2014
Grade
%
used to compare one earthquake to another, the
Moment Magnitude scale actually measure the amount of energy (intensity) released during an earthquake
2. Why do seismologists now prefer the Moment Magnitude scale over the traditional Richter scale?
The actual intensity of the earthquake is measured and it can measure earthquakes of higher intensities
3. Look at the table, “Differences between the Richter scale and the Moment Magnitude scale”. Using this
information, complete the tables below for each scale.
Richter Scale:
Advantages
1.
It can record responses from the seismographs
Disadvantages
1.
and the distance from the epicenter
2. It is very accurate for low magnitude
It can’t accurately measure for earthquakes
above 7 or far away
2. It doesn’t measure the energy of an
earthquakes
earthquake, only makes a comparison to others
3. Can provide detailed picture of low magnitude
3. Intended to be used in Southern California, not
earthquakes
in other areas of the world
Moment Magnitude Scale:
Advantages
1.
Measures the actual energy of an earthquake
2. Accurately measures earthquakes of high
magnitude
3. Can record earthquake information from far
away
Disadvantages
1.
Can’t accurately read low magnitude
earthquakes
2. NA
3. NA