14.5 Earthquake Magnitude
Earthquake
magnitude
is the amount of energy released by the earthquake. The
intensity
of an
earthquake refers to the effects (damage) observed at the surface. There are two measures of the
energy released, or
magnitude
, of an earthquake:
1.
The
Richter magnitude
of an earthquake is determined from the maximum amplitude of ground
motion as recorded by seismic waves registering on a seismograph along with the distance to the
epicenter (also determined from the seismograph).
An earthquake with maximum amplitude of 1
mm on a seismograph 100 km from the epicenter is assigned a Richter magnitude of 3.0.
The
Richter magnitude is a logarithmic scale meaning that an increase of 1 on the Richter scale is a
tenfold increase in magnitude.
A graphical device called a nomogram is used to relate the maximum
amplitude of seismic waves and epicenter distance to the Richter magnitude.

The Richter scale is a logarithmic scale. Increasing the magnitude by one corresponds to a tenfold increase in amplitude.
The seismic waves
are 10 times
as large and 30 times
as much seismic energy
was released by the earthquake. In other
words a 7.0 earthquake is 10 times stronger than a 6.0 earthquake; a 8.0 earthquake is 100 times stronger than a
6.0 earthquake, etc.

The maximum amplitude of the seismic waves and distance to the earthquake epicenter (as indicated by the S-p
interval) is determined from an earthquake seismograph. A graphical device called a nomogram relates distance and
amplitude to Richter magnitude. A straight line drawn from the left (distance) axis to the right (amplitude) axis will
cross the middle axis at the earthquake’s Richter magnitude.

Note that a seismograph closer to a given earthquake records a greater amplitude of seismic waves than a seismograph
further away.
Because the amplitude of ground shaking depends on the material the seismic waves are passing
through, the Richter magnitude is only an estimate of the energy released by an earthquake. The
moment magnitude
is a true measure of the energy released by an earthquake and is based on
geophysical measurements of the amount of displacement along the fault, the strength of the rocks
that ruptured, and the area of the fault that ruptured.
Determining the moment magnitude of an
earthquake can take months of field measurements and laboratory work, while the Richter
magnitude can be determined from seismographs within minutes of an earthquake.
14.6. Earthquake Intensity
Earthquake
intensity
is a measure of the effects of an earthquake and is based on ground
observations, instead of instrument measurement, including people’s perceptions and reactions to
the earthquake and the damage caused by the earthquake. Damage depends on earthquake’s
magnitude, duration, distance from the epicenter, geological conditions, and the type and condition
of buildings, highways, and other infrastructure, so the intensity of the same earthquake will vary
from place to place. The effects of an earthquake in a given area are assigned an intensity from one
to twelve using the Modified Mercalli Intensity Scale.

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- Fall '08
- GODDARD
- Earthquakes, Seismic Waves