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Background Note
BY
PROF. ANAND S. ARYA
CONSULTANT TO BMTPC, GoI.
1. INTRODUCTION
The Vulnerability Atlas of India (1997) was prepared by an Expert Group chaired by the author and
appointed by the Ministry of Urban Development, Govt. of India, with the Secretariat at BMTPC. In view
of new data and new knowledge having become available, the Atlas has been revised by a Peer Group
established by Ministry of Housing and Poverty Alleviation, Govt. of India, under the chairmanship of
the author and published by BMTPC in 2006.
The Earthquake Hazard rating of all the districts in India has been worked out by a small sub committee
under the chairmanship of the author based on the data published in Vulnerability Atlas of India (2006).
2. DESCRIPTION OF THE VULNERABILITY ATLAS
The Vulnerability Atlas contains the following information for each State and Union Territory of India.
1)
2)
3)
4)
Seismic hazard map
Cyclone and wind hazard map
Flood prone area map
Housing stock vulnerability table for each district, indicating for each house by wall and roof
type, the level of risk to which it could be subjected some time in the future.
5) The Landslide Hazard maps are given for three such major regions.
All hazard maps of the States and UTs, originally available in a smaller scale, after digitization have
been reproduced on larger scale of 1:2 million based on Survey of India map and show the district
boundaries and names of district towns for ease of identifying the hazard zone boundaries in the
districts. While printing, some maps of the larger States have been reduced to get accommodated
in A3 size. Maps of the Union Territories are drawn to different scales to suit A3 or A4 size. The
accuracy of the enlarged maps are limited to the accuracy of the small scale map.
The various parameters involved in the earthquake hazard, the sources of information used and the
limitations of the present maps and tables are described in the following paras:
3 Earthquake and Seismic Zones
The entire Indian landmass, susceptible to different levels of earthquake hazard, has broadly been
classified into four distinct Seismic Zones, referred to as Zones II to V as per the Seismic Zoning Map
of India (shown in figure I) contained in IS 1893:2002 (Part-I) Fifth Revision. According to the Foreword
to the Seismic Code IS 1893:2002, the general basis of the zones is as follows:
Zone V: Covers the areas liable to seismic Intensity IX and above on MSK (1964) Intensity Scale. This
is the most severe seismic zone and is referred here as Very High Damage Risk Zone.
Zone IV:, Gives the area liable to MSK VIII. This zone is second in severity to zone V. This is referred
here as High Damage Risk Zone.
Zone III: The associated Intensity is MSK VII. This is termed here as Moderate Damage Risk Zone.
Zone II: The probable Intensity is MSK VI or less. This zone is referred to as Low Damage Risk Zone.
Note: In the revision of the Seismic Zone Map given earlier in the Vulnerability Atlas of India 1997, the
seismic zone I has now been merged into Seismic Zone II and renamed as Zone II. Zone III has been
extended to cover more areas in Maharashtra, Andhra Pradesh and Tamil Nadu. Zones IV and V have
remained unchanged.
It may be mentioned here that the Intensity scale, called as MSK Intensity Scale 1964, is much more
detailed and quantitative in nature as compared to the Modified Mercalli (MM) though almost similar in
intensity. Hence MSK could be used in place of MM in the classification of the seismic zones given
above. The two intensity scales are reproduced in Annexures in IS: 1893-2002 (Part-I).
The following important comments, from the Foreword to the IS 1893:2002, are very relevant for clearer
understanding of the seismic zoning:
(a) “The Sectional Committee responsible for the formulation of this standard (IS 1893:2002 Part I) has
attempted to include a seismic zoning map. The object of this map is to classify the area of the
country into a number of zones in which one may reasonably expect earthquake shaking of more or
less same maximum intensity in future. The Intensity as per the comprehensive Intensity Scale
(MSK 64) broadly associated with the various zones is VI (or less), VII, VIII and IX (and above) for
Zones II, III, IV and V, respectively. The maximum seismic ground acceleration in each zone cannot
be presently predicted with accuracy either on a deterministic or on a probabilistic basis. The basic
zone factors included herein are reasonable estimates of effective peak ground accelerations for
the design of various structures covered in this standard.”
(b) “The Sectional Committee has appreciated that there cannot be an entirely scientific basis for
zoning in view of the scanty data available. Though the magnitudes of different earthquakes which
have occurred in the past are known to reasonable degree of accuracy, the intensities of the shocks
caused by these earthquakes have so far been mostly estimated by damage surveys and there is
little instrumental evidence to corroborate the conclusions arrived at. Maximum intensity at different
places can be fixed on a scale only on the basis of the observations made and recorded after the
earthquake and thus a zoning map which is based on the maximum intensities arrived at, is likely to
lead in some cases to an incorrect conclusion in view of (a) incorrectness in the assessment of
intensities, (b) human error in judgment during the damage survey, and (c) variation in quality and
design of structures causing variation in type and extent of damage to the structures for the same
intensity of shock. The Sectional Committee has therefore, considered that a rational approach to
the problem would be to arrive at a zoning map based on known magnitudes and the known
epicenters assuming all other conditions as being average and to modify such an idealized
isoseismic map in the light of tectonics, lithology and the maximum intensities as recorded from
damage surveys. The Committee has also reviewed such a map in the light of the past history and
future possibilities and also attempted to draw the lines demarcating the different zones so as to be
clear of important towns, cities and industrial areas, after making special examination of such
cases, as a little modification in the zonal demarcations may mean considerable difference to the
economics of a project in the area.”
(c) “In the seismic zoning map, Zones I and II of the contemporary map have been merged and
assigned the level of Zone II. The Killari area has been included in Zone III and necessary
modifications made, keeping in view the probabilistic hazard evaluation. The Bellary isolated zone
has been removed. The parts of eastern coast areas have shown similar hazard to that of the Killari
area, the level of Zone II has been enhanced to Zone III and connected with Zone III of Godawari
Graben area.”
4.
Epicentres of Earthquakes of M > 5.0
All earthquakes of M > 5.0 on Richter open ended logarithmic scale have been plotted along with the
seismic- intensity zones. The catalogue of earthquakes prepared by India Meteorological Department,
Govt. of India has been utilised for the purpose. The Magnitude of the earthquake as well as the year of
occurrence are shown along with the location on the maps. With regard to earthquakes of lower
magnitudes, it is known that their frequency of occurrence is much higher than the larger earthquakes.
Also lower the magnitude, the closer must be the installations of seismological instruments and better
should be their installation to permit higher gain, so as to be able to record the shocks and find their
location. The present seismological network in India is not so capable and is non-uniform in its
capability as well. No doubt, small magnitude earthquakes have been recorded in several parts of the
country through local, small aperture networks of high gain instruments by some organisations and
institutions, but the non-uniformity of the data and time gaps as well, may convey an unrealistic picture
of relative seismic activity in different areas, that is, areas having dense local instrument network
showing more activity than those where such networks do not exist. No attempt was therefore made to
present this available information on the general purpose hazard maps which are meant here
specifically for prevention, mitigation and preparedness concerning housing and related infrastructure.
As recommended in the Code itself, in the case of special structures, detailed investigations (site
related geologic, seismotectonic, geotechnical) should be undertaken. Such special structures will
include very tall buildings, say more than 90 m in height; very long span, special type and important
bridges, major dams, major power plants, hazardous/risky structures, etc. The seismic risk to such
structures can not be worked out from the data presented in this Atlas.
5.
Earthquake Magnitude and Intensity
The magnitude M of an earthquake is denoted by a number which is a measure of energy released
during the earthquake occurrence. It is now measured in different ways, the most commonly used is the
Richter Scale according to which “the magnitude of an earthquake is the logarithm to the base 10 of the
maximum trace amplitude, expressed in microns, with which the standard short period torsion
seismometer (with a period of 0.8 second, magnification of 2800 and damping nearly critical) would
register the earthquake at an epicentral distance of 100 km”. The scale being logarithmic, the energy of
earthquake magnitude ‘m+1’ is about 31 times the energy released in earthquake of magnitude ‘m’.
Magnitude scale is open ended, denoted numerically to one place of decimal (5.6, 8.3, etc.). “The
intensity of an earthquake at a place is a measure of the effects of the earthquake”. A number of
intensity scales have been in vogue in different times, namely Rossi-Forel (RF), Modified Mercalli (MM),
MSK 1964 and Japan Meteorological Agency (JMA) scales. All the scales are close-ended stepped
scales, RF having 10 points (I to X), MM and MSK with 12 points (I to XII) and JMA with 7 points (I to
VII). Presently MSK 12 point scale is the most used, JMA being used in Japan. In historical
earthquakes in India such as 1905 Kangra earthquake and 1934 Bihar- Nepal earthquake, RF intensity
scale was used for drawing the isoseismal. While for a given earthquake, the magnitude has one
unique value and epicentral location, the intensity varies from the maximum in the epicentral area to
smaller values at increasing distances from the epicenter. Isoseismals derived from the observed
damages in an earthquake as per the intensity scale show the intensity distribution caused in the
earthquake. The relationship between the earthquake magnitude and the maximum intensity caused is
not precise.
6.
MSK Intensity Scale
The MSK intensity scale describes the generally observed grades of damage to buildings and
structures in various intensity levels. For convenience of reference, the damage vulnerability of the
various building types in MSK seismic Intensities VII, VIII and IX is presented here in tabular form:
Table- Seismic Intensity vs. Damage to Buildings
Building Type
MSK Intensity VII
(Zone-III)
MSK Intensity VIII
(Zone-IV)
A. Mud and Adobe
houses, random- stone
constructions
B. Ordinary brick buildings,
building of large blocks
and prefab type, poor
half timbered houses
C. Reinforced buildings,
well built wooden
buildings
Most have large deep
cracks, Few suffer
partial collapse
Many have small cracks
in walls.
MSK Intensity IX
(Zone-V)
Most suffer partial
collapse
Most suffer complete
collapse.
Most have large and
deep cracks.
Many show partial
collapse. Few
complete collapse
Many have fine plaster
cracks
Most may have small
Many may have large
cracks in walls. Few
deep cracks,
Few may have partial
may have large deep
cracks.
collapses.
Note: In MSK Intensity VI area, Many buildings of Type A sustain slight damage and Few may even
have moderate damage like small cracks in walls and fall of fairly large pieces of plaster, Buildings
of Types B and C remain almost free of damage except occasional fine cracks.
* Most = about 75%, Many = about 50%, Few = about 5%
Source: Arya A.S., Bulletin ISET, September, 1990.
7.
Earthquake Hazard Rating
The sub-committee appointed by NDMA (Geo hazard)discussed the criteria which should be
adopted for rating the hazard intensity of a District to earthquakes and came to the conclusion
that a system of rating has to be devised taking into account (i) the hazard intensity in the district,
and (ii) different partial areas of the District covered by the hazard intensities.
7.1
Rating for Hazard Intensity
Reference to BIS Code IS: 1893 Part 1—2002 shows the following Maximum Considered
Earthquake Peak Ground Acceleration ‘Z’ for the four seismic zones:
Zone V 0.36g
Zone IV 0.24g
………………………………………………...(1)
Zone III 0.16g
Zone II
0.10g
Choosing a rating of 10 for Zone V, the rating of the other zone is to be taken in the ratio of their
‘Z’ values as 6.67 for Zone IV, 4.44 for Zone III and 2.78 for Zone II. The Sub-Committee
decided to adopt the following rating values for the Zones:
Zone V
Zone IV
Zone III
Zone II
10
7
5
3
……………………………………………..(2)
7.2
Rating for Hazard Prone Areas in Districts
The Seismic Zoning State’s maps in Vulnerability Atlas of India show that the districts have part
areas having different hazard intensities. The sub committee decided that the rating of the district
may be worked out by linearly interpolating the intensity areas. That is if A1 is in seismic zone V
and A2 is in seismic zone IV, then the rating may be worked out as follows:
R = 10A1+7A2
…………………………………….(3)
AD
where AD is the whole area of the district = A1+A2.
Based on the decisions of the sub committee under 7.1 and 7.2, the earthquake hazard ratings of
all the districts in various states and UTs of India have been worked out (See Tables 1 to 4).
7.3
Relative Hazard Rating in Various Districts
In order to fix the priorities of earthquake disaster mitigation actions, it was necessary to fix
some relative grades among the R values of the districts worked out as above. It is now
considered by the author that the relative hazard ratings between the districts may be worked out
on the consideration of the district areas subjected to the highest intensities in more than 50% of
the areas of the districts. On this basis, the following procedure has been considered
quantitavely.
i)
ii)
iii)
Districts having 100% area in seismic Zone V that is with rating of 10 should be in the
highest rating category.
Districts having more 50% area in seismic Zone V (rating 10) and the rest in seismic Zone
IV (rating 7) may be in the next lower rating category
Districts having less than 40% area in Zone V (rating 10) and more than 50% area in Zone
IV (rating 7) may be in the next lower rating category, and so on.
A similar approach may be adopted for districts partly in Zone IV and partly in Zone III OR partly in
Zone III and partly in Zone II. In this way the rating categories will become too many. So as to reduce
the categories to four only, that is R1, R2, R3 and R4, a slightly different scheme has been adopted as
explained below:
R1 for R>8.4, R2 for R from 6.0 to 8.4, R3 for R from 4.0 to 5.9 and R4 for R<4.0 where R the rating
calculated for a district using Eq. (3)
R1(>8.4) will mean
i)
at least 50% of Area of District lies in Seismic Zone V, the rest in Zone IV, e.i.
R1 = 10A5+7A4
AD
A5 = Part area of District in Zone V
A4 = Part area of District in Zone IV
AD = A5+A4= Total area of District
R2 (6.0- 8.4) means
i)
For District having part areas, A5 < 50% and A4>50%
ii)
R2 = 10A5 +7A4
AD
For District having part areas, A4 and A3 with A4>50% and A3<50%
R2 = 10A4 +7A3
AD
R3 (4.0-5.9) means
i)
For District having part areas A4 and A3 with A4 <50% and A3>50%
ii)
R3 = 7 A4+5A3
AD
For Districts having part areas A3 and A2 with A3>50% and A2<50%
R3 = 5A3+3A2
AD
R4 (<4.0) means
i)
For Districts having part areas A3 and A2 with A3<50% and the balance in A2
R4 = 5A3+3A2
AD
Based on this scheme of calculating relative earthquake hazard ratings, the Tables 1 to 4 have been
prepared which show under each relative rating all the districts taken state wise.
8.
Conclusions
The suggested earthquake hazard rating of each district based on the areas of the districts under
different earthquake intensities Zones as well as the relative earthquake hazard ratings of the districts in
four categories R1 to R4 are shown in Tables 1 to 4. These tables will be useful for State Disaster
Management Authorities as well as the National Disaster Management Authority for fixing priority of
disaster mitigation and preparedness activities among the various districts of the country.