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Source: Asian Disaster Preparedness Center
DHA/South Pacific Regional Environmental
Programme/Emergency Management Australia
Legend: S = severe; M = moderate: L = low
Thus observing the Asian scenario, it may be concluded that India has
a moderately risk of facing natural events which may result in disasters. The risk of such events is quite high for floods or droughts and
is moderate for earthquakes or cyclones. However, considering the heavy damages earthquakes and cyclones inflict
on society, one has to be concerned about not only the
degree of risk but also the magnitude of impact of the event.
3.
Risk Assessment for the State of Maharashtra
“Risk is the probability that injury to life or damage to property
and the environment will occur. The extent to which risk is either
increased or diminished is the result of the interaction of a multitude
of causation chains of events.” (Terry Jeggle and Rob Stephenson,
Concepts of Hazard and Vulnerability Analysis).
The dominant physical trait of the state of Maharashtra is its plateau
character. The state is covered by the Satpura range on its northern
side while Ajanta and Satmala ranges run through the central part
of the state. The Arabian sea guards the western boundary
of Maharashtra with a coastal line of 720 kilometres.
Gujarat and Madhya Pradesh form the state’s boundaries on its northern
side with the latter also covering the eastern region while
Goa, Karnataka, and Andhra Pradesh are on the Southern side.
The state receives its rainfall chiefly from the southwesterly winds.
Normally, there are heavy rains in the coastal region (around 2000 mm), scanty rains in the rain-shadow
regions in the central part (around 500 mm) and medium rains in the
eastern part of the state (around 1000 mm).
3.1
Basic Statistics of Maharashtra
· Area
: 3,07,713 sq.Km
· Number
of administrative divisions : 6
· Number
of districts : 31
· Number
of talukas : 325
· Number
of city and town units : 336
· Number
of inhabitated villages : 40,412
· Total population (census 1991) : 78,937,000
Maharashtra is the country’s leading industrial state, accounting
for 23 per cent of the gross value output from the industrial sector.
Its main areas of strength are textiles, pharmaceuticals, petrochemical
industries, heavy chemical industries, electrical automobile industries,
engineering and food processing. The state is a leading manufacturer
of three wheelers, jeeps, synthetics, cold rolled steel products,
industrial alcohol and plastic. Around 30 per cent of the sugar output
of the country is from Maharashtra. The organisational structure for the
state is given in Appendix V. As a part of risk analysis, effort is
being made to look at the available data and assess the possible risk
for each of the hazards.
3.2
Earthquakes
3.2.1 Nature
and Occurrence
Of all natural hazards, earthquakes seem the most terrifying. They can inflict tremendous damage within seconds and without warning at
any time of day, on any day of the year. Ground shaking and surface faulting are often just the forerunners of secondary damage, such as fires, floods (caused by dam bursts), landslides, quick soil and tsunamis (seismic sea waves).
Earthquakes are caused by the movement of massive land area called plate on the earth's crust. Often covering areas larger than the continent, these plates are in a constant state of motion, acted upon by the periodic forces of the solar system and
movement caused by the rotation of the earth. As the plates move in to relation to one another, stresses form and accumulate
until a fracture or abrupt slippage occurs. This sudden release of stress is called an earthquake.
The place at which the stress is released is known as the focus of an earthquake. From this point, mechanical energy is initiated in the form of waves that radiate in all directions through the earth. When this energy arrives at the earth's surface, it forms secondary surface waves. The frequency and amplitude of the vibrations produced at the surface, indicating the severity of the earthquake, depends on the amount of mechanical energy released at the focus, the distance and depth
of the focus, and the structural properties of the rock or soil
on or near the surface.
Earthquakes can occur anywhere. They may occur in an area not known to have experienced previous
activity and may suggest a temporary increase in risk of hazard in the area. Or they may occur in areas which have a previous history of subterranean sounds and seismic activities. Another important indication as to where earthquakes may occur human activity like the construction of
a reservoir. However, it may be noted that some of the physical phenomena causing earthquake
are still poorly understood.
3.2.2 Nature
of Impact
Earthquakes have several distinct effects that can damage structures,
disrupt and even endanger our lives. An understanding of these natural hazards and
how they result in damage can lead to more effective safety planning
at the city level, and to better decisions about the uses of individual
lots.
· Ground Shaking is
the most geographically widespread effect of earthquakes, occurring
throughout the region.
· Surface rupture may
occur directly along the fault line.
· Ground failure occurs
when soil that is saturated with water, is on a slope, is otherwise weak, cannot support structures, or even itself, after
being subjected to ground shaking.
· Other hazards that
can result from earthquakes include tsunami and seiches.
3.2.2.1 Ground
shaking
Most earthquake damage is due to ground shaking which occurs in all earthquakes. The impacts of ground shaking will
be quite widespread, and are much less predictable than those of surface
rupture. The severity of ground shaking varies considerably
over the effected region, depending on:
· the size of the earthquake;
· the distance from the causative
fault;
· the nature of the soil at
the site;
· the nature of the geologic
material between the site and the fault topography.
In general, sites with stronger soil will experience shaking of
less intensity than those in low-lying areas. Some sites, particularly
those with poor soils, will experience strong ground shaking even
in distant earthquakes. Sites with stronger soil will
experience strong ground shaking only when a nearby fault is involved.
3.2.2.2 Surface
rupture
The most obvious and direct effect of earthquake is
the rupture of the ground surface along the fault. Ground rupture occurs in some, but not all large earthquakes.
Structures are often not able to withstand surface rupture.
Streets, utilities and other lifelines that cross an earthquake
fault are at great risk of damage. The impacts of fault
rupture, while locally severe, are not widespread and are relatively
predictable.
3.2.2.3 Ground
failure
Ground failure means that the soil is weakened so that can it no
longer support its own weight or the weight of structures.
The major types of ground failure associated with earthquakes
are liquefaction, landslides and settlement.
3.2.3 Possible
Damages
In addition to the characteristics of the earthquake and of the
site (such as the magnitude, duration, soils type), a structure's
characteristics, including structural type, materials, design, and
quality of construction and maintenance, will determine how well it
will perform. After San Francisco's 1865 earthquake, the front page of a local newspaper
observed, "Well-built structures on good ground survived the
shaking effects of the earthquake better than structures on made ground.
It may be noted that buildings having walls properly
secured and lain in cement, with sound foundations,
suffer the least damage during earthquakes.
3.2.3.1 Potentially
Hazardous Building Types
There are other building types, in addition to unreinforced masonry
buildings, that have not performed well in earthquakes. The
most serious hazard, and also the most difficult policy issues, may
be posed by non-ductile concrete frame structures. In many of these buildings, the frame was not designed or constructed
to allow it to move without fracturing. As a result,
these are susceptible to collapse in strong earthquakes. There were many falls of such buildings in the San Fernando 1971
earthquake. Many deaths in the Mexico City 1985 , Armenia 1988, Northridge 1994 and Kobe
1995 earthquakes resulted from the fall of non-ductile concrete frame
buildings.
“Soft-storey” buildings or those in which at least one storey often
the ground floor has much less rigidity and/or strength than the rest
of the structure, are significant hazards. The 1974
Blume report identified smaller wood-frame buildings
with soft storeys as having the potential to collapse during an earthquake.
During both the Loma Prieta and the Northridge 1994 earthquakes, soft-storey residential buildings
failed, resulting in deaths. Small wood-frame residential buildings
can be very resistant to earthquake ground shaking, especially if
they are bolted to their foundations and have strong first storeys.
Typically, in Latur, the housing material consisted of stone and
mud resulting in total or sizeable loss of houses along
with the loss of nearly 8,000 people in the affected villages. The
data on vulnerable houses is given in the vulnerability analysis.
The impact of earthquakes differ for urban and rural areas, primarily
because of the nature of infrastructure, quality of housing and occupational
differences. In rural areas, it is primarily the housing and physical
structures (including irrigation infrastructure) which may suffer
extensive damage, without necessarily destroying the crops.
In urban areas, in addition to housing and physical infrastructures,
it may also disturb the service infrastructure such as water supply,
sewage, telephones, electricity, piped gas supply etc., which are
essentially underground installations and hence exposed to a direct
impact. The disruption, therefore, in urban areas and consequent investments
for rehabilitation becomes a major challenge. So, more emphasis
may be given on mitigative and preparedness measures to minimise
the disastrous effects of an earthquake.
Earthquakes’ most profound impacts are deaths and serious injuries. Number of casualties largely depend on the number of people in the area
at the time and the types of structures that they occupy.
Most deaths and injuries are caused by the failure of buildings
and structures. The number of casualties also depend upon the time
of occurrence of the earthquake.
Important variables could substantially reduce the toll.
The failure (or lack of failure) of a few high-occupancy or critical facilities such as arenas, theaters, or dams could influence
the final casualty count significantly. The degree of water saturation
of the ground will influence the occurrence of landslides and the
area subject to liquefaction. Weather conditions, especially
wind speed and direction, will affect the spread of fire and the ability
of emergency responders to control fires.
3.2.4 Indian Peninsula and Maharashtra
During its evolution, the Indian peninsula was subjected to intense tectonic forces due to which numerous folds,
faults and fractures have been developed in the ancient rocks of this land
mass. peninsular India is classified
as a stable continental
region.
The earthquakes in a stable continental area, such as the one at Killari on 30th September 1993, were caused by adjustments of crustal blocks along such pre-existing weak
zones. Isolated seismic activity within the shield is indicative of
the movements going on along some of the old basement faults,
perhaps at a very slow rate.
The state of Maharashtra and its adjoining areas form part of the peninsular shield of India. The
state of Maharashtra occupies the central-western portion of peninsular India, technically an intraplate continental area. Most of Maharashtra is covered by the deccan traps, a sequence of basalt flows placed about 65 million years ago. In most of the area, these flows are nearly horizontal, demonstrating that tectonic deformation accumulated is very little or nil. Though
this area was treated as seismically stable with no potential
for disastrous earthquakes, this belief was shattered by the Koyna earthquake of December 11, 1967, with magnitude
of 6.5 on the Richter Scale. Recent Latur earthquake
of September 30, 1993, having magnitude 6.4 which occurred well within the central part of the shield led to a review of the seismic activity
in the stable shield zone.
Thus, tectonic stability in the deccan plateau may appear to be inconsistent with the observed level
of seismicity. This apparent
contradiction reflects the primitive stage of knowledge
about seismogenesis in stable continental region, and is the root of problem of rise assessment of an earthquake. Thus the potentially active tectonic features which could produce earthquakes with
engineering consequences need to be studied.
It may be noted that stable continental region worldwide are characterized by a very large number of unknown faults that can at some point produce an earthquake. Many of these faults may be located in areas that have low or no seismicity and are technically stable.
Maharashtra and adjoining regions are prone to earthquakes of moderate magnitude as can be seen from
the experience of several years. Koyna regions experiences the maximum number
of tremors in Maharashtra. Excluding the Koyna region, and other regions
of Killari, Khardi (Bhatsa) and Medhi (Surya), appendix
III lists 137 earthquakes of which 121 earthquakes
of magnitude about 3.0 or above are in Maharashtra
and 16 are in the adjoining regions. Table II gives
a list of the major earthquake occurrences in Maharashtra.
Table
II:
Major Earthquakes in Maharashtra
3.2.5 Pattern
of seismicity
The Deouskar committee report of 1995 presents the pattern of seismicity
in Maharashtra. The above map indicates the location
of areas which have experienced earthquakes till date. The study reports
that :
· Earthquakes in Maharashtra show major alignment along the west coast and western ghats region.
Seismic activity can be seen near Ratnagiri,
along the western coast, Koyna Nagar, Bhatsa
and Surya areas of Thane district.
· The north - south trend further continues deep inside Gujarat. The striking characteristic of this narrow region is its alignment with the hot spring belt. It
appears that the off - coast activity is associated
with submerged faults along the west coast of Maharashtra.
· In north Maharashtra,
the seismic activity near Dhule, Akola, Jalgaon and Amravati could be due to movements on the faults present in the area associated
with the complex system of Narmada, Tapi and Purna lineaments. However, the exact seismic status of these lineaments needs to be evaluated with extensive monitoring.
· In north - east corner of
Maharashtra, the earthquake activity in Nagpur and Bhandara districts may be associated with Deolapar thrust or
sheared and faulted zones of Ramtek and Sakoli Basins. This needs to be confirmed.
· Isolated activity is seen near Beed, Nanded, Ujjani and Solapur in eastern Maharashtra and Uran, Kolhapur and Sindhudurga in south-west Maharashtra. These activities may be due to movements on local faults in the basement.
· Seismicity is also seen near Bhatsanagar and Suryanagar. Recently, isolated activity also occurred in Latur-Osmanabad districts
in south-east Maharashtra.
3.2.6 Future Risk Analysis
Based on the earthquakes occurred so far in the
state and considering the seismic pattern, a rezoning, for new dam
designs only, has been proposed by the Deouskar Committee
for the state of Maharashtra. The existing and the proposed zoning
map is presented below. It may be noted that even after using sophisticated techniques
like carbon dating, it has not been possible to identify whether the fault is active. If some seismicity
is associated with a major lineament, it can be considered as an active tectonic feature
for the purpose of engineering seismic risk analysis and these regions can be considered as risk prone.
The west coast - Western Ghats seismogenic region is the
most active area in the Maharashtra state.
The Koyna-Warna and the Bhatsa areas are located in this region. Even before the occurrence of the Koyna earthquake of 11 December, 1967, with
magnitude 6.5, earthquakes with magnitude of about 6.0
are known to have occurred in this region. The report further observes
that :
· The activity in the Koyna-Warna region has been continuing for the past three decades with occasional
spurts, producing events with magnitude above 5.0.
This trend is expected to continue in future.
· Bhatsa region experienced
a swarm of seismic activity during 1983-84, with a maximum magnitude of 4.9. The activity has died down since then with only a temporary spurt in 1990.
· The Surya area about 50 km.
north-west of Bhatsa, has recently shown increase in
seismic activity. The energy in Bhatsa-Surya
region may not have been fully released and possibility of an earthquake with a magnitude of around
6.0 in future, cannot be ruled out.
· Further south of Warna, some
micro seismic activity has been reported in the past. Because this region lies in the active west coast-Western
Ghats, the possibility of a maximum magnitude earthquake
of around 6.0 cannot be ruled out in future.
· There was no evidence of any
significant seismic activity in the Latur-Osmanabad area in the known past.
Also, the tectonic features to which the Killari earthquake of 30 September,
1993, could be attributed are not known sufficiently.
However, the occurrence of Killari earthquake
of moderate magnitude of 6.4 gives an indication of neotectonic activity in the area. Though, the stress has been released at Killari, a similar magnitude earthquake at some other place in the area cannot
be ruled out.
· Before the Killari earthquake, infrequent tremors and subterranean sounds had been reported in the area. Similar instances were also
reported from many other locations in Beed, Parbhani
and Nanded districts. In the absence of knowledge
about any seismotectonic features in this area, low to moderate earthquakes can be experienced.
· In addition to the above seismogenic
regions in Maharashtra, the Narmada-Tapi region covering the border areas of Maharashtra and Madhya Pradesh states has been reported to be active since historical times. Tense
fracturing, alignment of hot springs and
locations of epicenters in these areas leads to the
conclusion that these two are the zones of crustal weakness in Maharashtra.
· The Tapi and the Purna lineaments in the northern part of Maharashtra have been recognized as active faults with potential
to generate low magnitude earthquakes. The Tapi lineament represents a line of
crustal weakness along which activity has
been recurrent during different periods. Earthquakes above magnitude
6.0 are known to have occurred in the northern parts of this region
across the border of Maharashtra.
· According to a scientist, there is a major anticipated zone of uplift in the Sangola area and another to the east
of Sholapur. Vertical movements of these crustal blocks may cause seismic disturbances in the areas, as has happened in Killari .
· Due to increasing trend of
seismic activity noticed in Maharashtra in recent times, earthquakes
with low magnitudes around 4 to 4.5 may occur in areas where there has been no
seismic activity in the past.
3.2.7 Reservoir
Induced Seismicity
While doing a risk assessment for earthquakes, mention
has to be made of reservoir induced seismicity (RIS).
The 1967 Koyna earthquake of 6.5 magnitude is contended
to be due to RIS. Between 1963 to 1998, the Koyna region has faced 102715
tremors, of which 79 were above magnitude of 4 (Richter scale) and seven were above magnitude
of 5 (Richter scale) as listed below.
Table III: Intensity of Earthquakes in Koyna Region (1963 to 1998) on
Richter scale
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