Page 6 - EQTips_Eng

Learning

3 Earthquake Design
Earthquake Tip and

Construction

What are Magnitude and Intensity?

Terminology released goes into heat and fracturing the rocks, and
The point on the fault where slip starts is the Focus only a small fraction of it (fortunately) goes into the
or Hypocenter, and the point vertically above this on seismic waves that travel to large distances causing
the surface of the Earth is the Epicenter (Figure 1). The shaking of the ground en-route and hence damage to
depth of focus from the epicenter, called as Focal Depth, structures. (Did you know? The energy released by a
is an important parameter in determining the M6.3 earthquake is equivalent to that released by the
damaging potential of an earthquake. Most of the 1945 Atom Bomb dropped on Hiroshima!!)
damaging earthquakes have shallow focus with focal Earthquakes are often classified into different
depths less than about 70km. Distance from epicenter groups based on their size (Table 1). Annual average
to any point of interest is called epicentral distance. number of earthquakes across the Earth in each of
these groups is also shown in the table; it indicates that
Epicentral Distance on an average one Great Earthquake occurs each year.
Epicenter Place of Table 1: Global occurrence of earthquakes
Interest Group Magnitude Annual Average Number
Great 8 and higher 1
Fault Focal Major 7 – 7.9 18
Depth
Rupture Strong 6 – 6.9 120
800
Moderate 5 – 5.9 6,200 (estimated)
4 – 4.9
Light
Focus Minor 3 – 3.9 49,000 (estimated)
Very Minor < 3.0 M2-3: ~1,000/day; M1-2: ~8,000/day
Figure 1: Basic terminology Source: http::/neic.usgs.gov/neis/eqlists/eqstats.html
Intensity
A number of smaller size earthquakes take place Intensity is a qualitative measure of the actual
before and after a big earthquake (i.e., the Main Shock). shaking at a location during an earthquake, and is
Those occurring before the big one are called assigned as Roman Capital Numerals. There are many
Foreshocks, and the ones after are called Aftershocks. intensity scales. Two commonly used ones are the
Magnitude Modified Mercalli Intensity (MMI) Scale and the MSK
Scale. Both scales are quite similar and range from I
Magnitude is a quantitative measure of the actual (least perceptive) to XII (most severe). The intensity
size of the earthquake. Professor Charles Richter scales are based on three features of shaking –
noticed that (a) at the same distance, seismograms perception by people and animals, performance of
(records of earthquake ground vibration) of larger buildings, and changes to natural surroundings. Table
earthquakes have bigger wave amplitude than those of 2 gives the description of Intensity VIII on MSK Scale.
smaller earthquakes; and (b) for a given earthquake, The distribution of intensity at different places
seismograms at farther distances have smaller wave during an earthquake is shown graphically using
amplitude than those at close distances. These isoseismals, lines joining places with equal seismic
prompted him to propose the now commonly used intensity (Figure 2).
magnitude scale, the Richter Scale. It is obtained from
the seismograms and accounts for the dependence of
waveform amplitude on epicentral distance. This scale
is also called Local Magnitude scale. There are other
magnitude scales, like the Body Wave Magnitude,
Surface Wave Magnitude and Wave Energy Magnitude. X
These numerical magnitude scales have no upper and IX
lower limits; the magnitude of a very small earthquake
can be zero or even negative. VIII
An increase in magnitude (M) by 1.0 implies 10
times higher waveform amplitude and about 31 times VII
higher energy released. For instance, energy released Figure 2: Isoseismal Map of the 2001 Bhuj (India)

in a M7.7 earthquake is about 31 times that released in Earthquake (MSK Intensity)

a M6.7 earthquake, and is about 1000 (≈31×31) times Source:
that released in a M5.7 earthquake. Most of the energy http::/www.nicee.org/nicee/EQReports/Bhuj/isoseismal.html
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