M A S T E R P A G E
Teacher Background Reading
Richter vs. Mercalli
3.3a
When an earthquake occurs, we often hear news
reporters describing it in terms of magnitude. Perhaps
the most common question at a news conference is
“What was the magnitude of the quake?” In addition
to calculating the magnitude of an earthquake,
however, we can describe the effect it had at a
particular location by measuring its intensity.
Magnitude and intensity are both measures of an
earthquake, but they describe different characteristics.
Each measurement has its uses.
Magnitude is a measurement of the amplitude of the
earthquake waves, which is related to the amount of
energy the earthquake releases. Magnitude is
calculated from the size of the earthquake waves
arriving at a seismic station. The most commonly used
scale for magnitude is the Richter scale, developed by
Charles Richter in 1935. The Richter scale is a
logarithmic measurement of the maximum wave
amplitude recorded at a seismograph station, corrected
for distance from the epicenter. It is theoretically
open-ended, although the largest quakes recorded in
this century had a magnitude of 8.9. Each whole
number increase in Richter magnitude indicates an
increase in the severity of the ground shaking by a
factor of 10. Thus a magnitude 6 earthquake will pro-
duce shaking 10 times more severe than that produced
by a magnitude 5 earthquake. Magnitude can also be
related to the amount of energy an earthquake releases.
Each whole number increase in Richter magnitude
indicates an increase in the amount of energy released
by a factor of roughly 30. Thus a magnitude 6
earthquake releases about 30 times more energy than a
magnitude 5 earthquake, and roughly 900 times as
much as a magnitude 4 earthquake. These factors are
seldom described correctly in news accounts.
Each earthquake has a single magnitude that is inde-
pendent of the location of the observer. If a magnitude
6.0 event strikes some location in the South Pacific,
then that event will be described as a magnitude 6.0
by observers all over the world. Because magnitude is
independent of observer location, it is a convenient
measure to use in reporting the occurrence of an
earthquake. No matter where it happened, or where
you are, you get a feeling for the relative size of the
earthquake by simply knowing its magnitude. This is
why the press is so quick to report this number.
Intensity is a measure of the effect that the vibration
had on natural and human-made structures. The most
common measurement of intensity is the Modified
Mercalli Intensity scale, originally developed in 1902
by Giuseppi Mercalli, an Italian geologist. Wood and
Neumann adapted it to “modern” conditions in 1931.
The intensity scale ranges from I, the lowest
perceptible intensity, to XII, the greatest intensity.
Intensity is a function of many variables, including
magnitude, depth of the earthquake, distance from the
earthquake, local geological conditions, and local
construction practices. Generally speaking, the
intensity felt at a given location will increase with
increasing magnitude, decreasing depth, decreasing
distance from the earthquake, and a decrease in the
quality of construction. If an earthquake is shallow, its
intensity will be greater. If it affects an area built on
soft sediments, such as landfills or sedimentary basins,
the intensity will also be greater. A single quake will
produce a range of intensities that typically decrease
with increasing distance from the earthquake. An
isoseismal map illustrates this range.
Intensity is more useful than magnitude as a measure
of the impact that an earthquake had at any given
location. Consequently, it is important to the
professionals who establish building codes and
insurance rates. If the maximum expected intensity in
a given area is VII, for example, the building codes
should specify construction practices that make
buildings able to withstand this intensity, and property
insurance rates will probably be high. If the maximum
expected intensity is only III, the area can relax its
building codes, and property insurance rates will be
moderate.
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