Natural Hazards: Causes and Effects - Disaster Management Center ...

Measuring Earthquakes
The severity of an earthquake can be expressed in several ways. The magnitude of an earthquake, as
expressed by the Richter scale, is a measure of the amplitude (total range of fluctuation) of the seismic
waves. Magnitude is related to the amount of energy released—an amount that can be estimated from
seismograph recordings. The intensity, as expressed by the modified Mercalli scale (see Fig. 2.4), is a
subjective measure that describes how severe a shock was felt at a particular location. Damage or loss
of life and property is another, and ultimately the most important, measure of an earthquake’s severity.
The Richter scale is the best known scale for measuring the magnitude of earthquakes. The scale is
logarithmic so that a recording of 7, for example, indicates a disturbance with ground motion 10 times as
large as a recording of 6. A quake of magnitude 2 is the smallest quake normally felt by humans.
Earthquakes with a Richter value of 6 or more are commonly considered major in magnitude.
The modified Mercalli scale expresses, in values ranging from I to XII, the intensity of an earthquake’s
effects in a given locality. The most commonly used adaptation covers the range of intensity from the
condition of “I.—Not felt except by a very few under especially favorable conditions,” to “XII.—Damage
total. Lines of sight and level are distorted. Objects thrown upward into the air.” Evaluation of
earthquake intensity can be made only after eyewitness reports and results of field investigations are
studied and interpreted. (See Fig. 2.5 for comparison between the scales.)
An earthquake’s destructiveness depends on many factors. In addition to magnitude, these include the
focal depth, the distance from the epicenter, local geologic conditions, and the design of buildings and
other human works. The extent of damage also depends on the density of population and construction in
the area shaken by the quake. 6
Intensity Scale
Modified
Mercalli
Scale
Perceived
by:
Damage
To:
Destruction
To:
The Measurement of an Earthquake
I II III IV V VI VII VIII IX X XI XII
----------------------------- Persons -----------------------------
None Few Some Many Most All
Magnitude Scale
Richter
Number: 1-2 3
Energy
Release in
ERGS:
In Multiples
of Base
Glass
Plaster
4
Furniture
Chimneys
---------------------------- Structures ----------------------------
Poor Ordinary Resistant Many Most All
Some Many Most
5 6 7 8
4.47x10 12 7.94x10 14 2.51x10 16 7.94x10 17 2.51x10 19 7.94x10 20 2.51x10 22
1.31.6 1,000 31,600 1,000,000 31,600,000 1,000,000,000 31,600,000,000
Intensity is a measure of the human experience and impact of earthquakes; magnitude is an estimate of energy
release. They are roughly comparable, as shown. With remote seismographs magnitude can be estimated for
almost all earthquakes but, in the absence of people or their property, there is no meaningful measure of intensity.
Source: Ian Burton, Environment as Hazard, Kates, page 25.
Figure 2.5