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IITK-BMTPC Earthquake Tip 3

What are Magnitude and Intensity? page 2
Table 2: Description of shaking intensity VIII as per enclosed by the isoseismal VIII (Figure 2) may have
MSK scale experienced a PGA of about 0.25-0.30g. However, now
Intensity VIII - Destruction of Buildings strong ground motion records from seismic
(a) Fright and panic. Also, persons driving motorcars are instruments are relied upon to quantify destructive
disturbed. Here and there branches of trees break off. Even ground shaking. These are critical for cost-effective
heavy furniture moves and partly overturns. Hanging earthquake-resistant design.
lamps are damaged in part.

(b) Most buildings of Type C suffer damage of Grade 2, and Table 3: PGAs during shaking of different intensities
few of Grade 3. Most buildings of Type B suffer damage of MMI
Grade 3, and most buildings of Type A suffer damage of V VI VII VIII IX X
Grade 4. Occasional breaking of pipe seams occurs. PGA 0.03-0.04 0.06-0.07 0.10-0.15 0.25-0.30 0.50-0.55 >0.60
(g)
Memorials and monuments move and twist. Tombstones
overturn. Stonewalls collapse. Source: B.A.Bolt, Earthquakes, W.H.Freeman and Co., New York, 1993

(c) Small landslips occur in hollows and on banked roads on Based on data from past earthquakes, scientists
steep slopes; cracks develop in ground up to widths of Gutenberg and Richter in 1956 provided an
several centimeters. Water in lakes becomes turbid. New
reservoirs come into existence. Dry wells refill and existing approximate correlation between the Local Magnitude
wells become dry. In many cases, changes in flow and level M L of an earthquake with the intensity I 0 sustained in
of water are observed. the epicentral area as: M L ≈ 3 I 0 + 1. (For using this
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Note:
• Type A structures - rural constructions; Type B - ordinary equation, the Roman numbers of intensity are replaced
masonry constructions; Type C - Well-built structures with the corresponding Arabic numerals, e.g., intensity
• Single, Few – about 5%; Many – about 50%; Most – about 75% IX with 9.0). There are several different relations
• Grade 1 Damage – Slight damage; Grade 2 – Moderate proposed by other scientists.
damage; Grade 3 – Heavy damage; Grade 4 – Destruction;
Grade 5 – Total damage

Basic Difference: Magnitude versus Intensity 100 Watt Bulb
Magnitude of an earthquake is a measure of its size.
For instance, one can measure the size of an
earthquake by the amount of strain energy released by
the fault rupture. This means that the magnitude of the
earthquake is a single value for a given earthquake. On Near
the other hand, intensity is an indicator of the severity
of shaking generated at a given location. Clearly, the Bright
severity of shaking is much higher near the epicenter (100 lumens)

than farther away. Thus, during the same earthquake
of a certain magnitude, different locations experience
different levels of intensity. Normal
To elaborate this distinction, consider the analogy (50 lumens)
of an electric bulb (Figure 3). The illumination at a Far
location near a 100-Watt bulb is higher than that
farther away from it. While the bulb releases 100 Watts Dull
of energy, the intensity of light (or illumination, (20 lumens)
measured in lumens) at a location depends on the Figure 3: Reducing illumination with distance

wattage of the bulb and its distance from the bulb. from an electric bulb

Here, the size of the bulb (100-Watt) is like the
magnitude of an earthquake, and the illumination at a Reading Material
location like the intensity of shaking at that location. Richter,C.F., (1958), Elementary Seismology, W. H. Freeman and
Magnitude and Intensity in Seismic Design Company Inc, USA (Indian Reprint in 1969 by Eurasia Publishing
House Private Limited, New Delhi)
One often asks: Can my building withstand a http://neic.usgs.gov/neis/general/handouts/magnitude_intensity.
magnitude 7.0 earthquake? But, the M7.0 earthquake html
causes different shaking intensities at different
locations, and the damage induced in buildings at Authored by:
these locations is different. Thus, indeed it is particular C.V.R.Murty
levels of intensity of shaking that buildings and Indian Institute of Technology Kanpur
structures are designed to resist, and not so much the Kanpur, India
magnitude. The peak ground acceleration (PGA), i.e., Sponsored by:
maximum acceleration experienced by the ground Building Materials and Technology Promotion
during shaking, is one way of quantifying the severity Council, New Delhi, India
of the ground shaking. Approximate empirical This release is a property of IIT Kanpur and BMTPC New
correlations are available between the MM intensities Delhi. It may be reproduced without changing its contents
and the PGA that may be experienced (e.g., Table 3). and with due acknowledgement. Suggestions/comments
may be sent to: nicee@iitk.ac.in. Visit www.nicee.org or
For instance, during the 2001 Bhuj earthquake, the area www.bmtpc.org, to see previous IITK-BMTPC Earthquake Tips.

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