Recent Delhi-NCR tremors do not signal a big event

New Delhi, June 19, 2020: In the wake of the recent series of
tremors in Delhi-NCR, Wadia Institute of
Himalayan Geology, an autonomous institute of
the Department of Science and Technology, has
said that such tremors are not unusual in the
Delhi-NCR region, but indicate that strain energy
is built up in the region.
They have said that since the seismic network is
quite good, present micro to minor earthquakes in
and around Delhi-NCR could be recorded.
Though our understanding, in terms of when,
where and with how much energy (or magnitude)
an earthquake can occur, is not clear, but the vulnerability
of a region can be understood from the
past seismicity, calculation of strain budget, mapping
of active faults etc.
The Delhi-NCR has been identified as the second
highest seismic hazard zone (Zone IV).
Sometimes, a vulnerable zone remains quiet,
experiences small magnitude earthquakes that do
not indicate any bigger earthquake, or receives a
sudden jolt by a big earthquake without any call.
Out of 14 small magnitude earthquakes in the
Delhi-NCR, the May 29 Rohtak earthquake had a
magnitude of 4.6. The recent events cannot be
defined as the ‘foreshocks’. If a big earthquake
takes place in a region, all smaller events that
occurred in the immediate past at that region are
categorized as the foreshocks. Therefore, scientifically
all tremors in Delhi-NCR can be demarcated
as the foreshocks only after a big earthquake
takes place immediately.
Though it cannot be predicted, a stronger earthquake
posing a threat to people and properties
cannot be ruled out. Since an earthquake cannot
be predicted by any mechanism, the tremors cannot
be described as the signal of a big event.
Why do earthquakes happen in Delhi-NCR?
All the earthquakes in Delhi-NCR are due to the
release of strain energy, which have been accumulated
as a result of northward movement of
Indian plate and its collision with the Eurasian
plate, through the fault or weak zones.
There are so many weak zones and faults in the
Delhi-NCR: Delhi-Haridwar ridge, Mahendragarh-
Dehradun subsurface fault, Moradabad fault,
Sohna fault, Great boundary fault, Delhi-Sargodha
ridge, Yamuna river lineament etc.
We must understand that the Himalayan seismic
belt, where the Indian plate collided with the
Eurasian plate and underthrusted beneath the
Himalayan wedge, accumulates strain energy at
the plate boundary due to relative movement of
plates against each other causing crustal shortening
and deformation of rocks.
These energies can be released through the
weak zones and faults in the form of earthquakes
The small magnitude earthquakes are frequent,
but large magnitude earthquakes are rare to very
rare. It is the large earthquakes that cause severe
damages both to structures and properties.
Impact of Earthquakes in the Himalaya to
Delhi-NCR:The Isoseismals of the 1905 Kangra
(7.8), 1934 Bihar-Nepal (8.0), 1950 Assam (8.6),
2005 Muzaffarabad (6.7) and 2015 Nepal (7.8)
earthquakes in the Himalayan arc are bounded by
the Main Central Thrust (MCT) to the north and
the Himalayan Frontal Thrust (HFT) to the south.
These earthquakes are the result of slip on a
décollement surface i.e. the contact between the
under thrusting Indian plate and overlying
Himalayan wedge, which extends southward
from 16-27 km depth beneath the MCT to its surficial
expression as the HFT at a distance of 50-
100 km from MCT.
The rupture areas due to large earthquakes show
gaps along the Himalayan arc, which have not
experienced great earthquakes for a long time,
and are identified as the future potential zones for
great earthquakes.
Three main seismic gaps have been identified in
the Himalaya: the Assam Gap between the 1950
Assam earthquake and the 1934 Bihar-Nepal
earthquake; the Kashmir Gap between the 1905
Kangra earthquake and the 1975 Kinnaur earthquake;
and the ~700 km long Central Gap
between the 1905 Kangra earthquake and the
1934 Bihar-Nepal earthquake. The entire NW-NE
Himalayan belt lies in the highest seismic potential
zone V and IV, where major to great earthquakes
can take place.
Neighbouring faults and ridges: There are so
many faults, ridges, and lineaments transverse to
the Himalayan arc, large sediment thickness in
the Ganga Alluvium Plains to the north of Delhi-
NCR.
Again, the Delhi-NCR is ~200 km away from
the Himalayan arc. Therefore, a major earthquake
in the Himalayan seismic belt may also be a
threat to Delhi-NCR. The Garhwal Himalaya,
lying in the Central Seismic Gap and north of
Delhi-NCR, has experienced the 1991 Uttarkashi
earthquake (6.8), 1999 Chamoli earthquake (6.6)
and 2017 Rudraprayag earthquake (5.7), and is
due for a major to great earthquake. Such a scenario
can make a pronounced impact to the north
India and Delhi-NCR.
Precautions: The subsurface structures, geometry,
and disposition of faults and ridges are to be
investigated thoroughly using Geo-scientific studies
in and around Delhi and NCR.
Since the soft soils do not support the structures’
foundations, structures anchored to bedrock
or stiff soils in earthquake-prone areas suffer less
damage. Thus, soil liquefaction studies are to be
carried out to know the thickness of soft soils.
Active faults are to be delineated, and lifeline
structures or other infrastructures are to be avoided
from nearby active faults, and to be constructed
as per the guiding principles of the Bureau of
Indian Standard (BIS). The outcome of recent
micro zonation studies for Delhi-NCR by IMD
should be considered for important construction.
Message to Common People:Earthquakes are
not predictable but there lies a probability of a
large to great earthquake with magnitude 6 and
more in the highest seismic potential zone V and
IV, which fall in the entire Himalaya and Delhi-
NCR. The only solution to minimise the loss of
lives and properties is the effective preparedness
against the earthquake.
Countries like Japan have proved this; where
earthquakes are common phenomenon, yet the
losses are negligible. Annual mock-drill is a regular
feature there. People’s participation, cooperation
and awareness are the key to success of this.
Some of the precautions and preparedness are:
Before an earthquake: Perform earthquake
mock-drill annually. Incorporate earthquakeresilient
construction to new buildings and retrofitting
existing structures
Preparation as an individual (in a family or
society): Sit together and chalk out mobile numbers
for neighbours, society/colony, nears &
dears, emergencies etc. Prepare a backup supply
kit that includes food (biscuit packets etc.), water,
medications and first aid supplies, flash light,
essential clothing and personal toiletries.Update
the first-aid kit regularly.
Choose at least two family meeting places: easy
to identify, open and accessible places that are
approachable. Identify a common place in society/
colony/street to assemble for shelter, kitchen
and first-aid.
During an earthquake:Remain calm, as the
ground shaking lasts for less than a minute.
Indoor: Stay inside-“DUCK, COVER and
HOLD”. Place yourself under sturdy furniture,
cover as much of your head and upper body as
you can. Hold onto the furniture. If you cannot
get under sturdy furniture, move to an inside wall
or archway and sit with your back to the wall,
bring you knees to your chest, cover your
head.Keep away from mirrors and windows. Do
not come out from the building during shaking.
Outdoor: Rush to an open area away from all
structures, especially buildings, bridges and overhead
power lines.
Driving: Stop immediately at the road-side
preferably in an open area away from any structure
especially a bridge, overpass, tunnel and
overhead power line. Stay as low as possible
inside the vehicle.
If trapped in debris: Do not light match
box/lighter. Do not shake body unnecessary and
do not remove dust, it can create problem for
breathing. Cover your face, if possible, with
handkerchief/cloth. Hit something on pipe/wall
etc. so that rescue teams can find you. Do not
shout unnecessarily because it will tire you and
dust/gases can go inside the body with breathing.
After the Earthquake: Remain calm. Move
cautiously, check for unstable objects and other
hazards around you. Check your body for
injuries. Help those around you. Inspect gas,
water and electric lines. If there are leaks or if
there is any doubt about leaks shut of mains.

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