Sunday, May 31, 2026 21:15 [IST]
Last Update: Saturday, May 30, 2026 15:56 [IST]
Sikkim is highly
vulnerable to earthquakes due to its geographical location within the young
Himalayan mountain range. The state lies in a tectonically active region where
the Indian Plate is continuously converging with the Eurasian Plate. This
continuous plate interaction generates significant tectonic stresses within the
Earth’s crust, making the Himalayan belt one of the most seismically active
regions in the world.
The
seismic vulnerability of the region was prominently demonstrated during the
earthquake of 18th September 2011, when a moment magnitude (Mw) 6.9 earthquake
struck Sikkim. The epicentre of the
earthquake was located near the India–Nepal border, approximately 68 km
northwest of Gangtok, with a focal depth of 19.7 km, as reported by the United States Geological Survey. Within 30
minutes of the main seismic event, three significant aftershocks of magnitudes
5.7, 5.1, and 4.6 were also experienced across the region. The earthquake
caused widespread panic, infrastructural damage, landslides, and further
highlighted the seismic sensitivity of the Himalayan terrain.
As
per the seismic zoning provisions of Bureau of
Indian Standards under IS 1893:2016,
the majority of Sikkim falls under Seismic
Zone IV, while certain adjoining Himalayan regions fall under Seismic Zone V,
indicating very high seismic susceptibility. Due to these geological and
tectonic conditions, even moderate-intensity earthquakes may result in severe
structural damage if buildings are not properly designed and detailed in
accordance with earthquake-resistant design principles.
A
post-earthquake study conducted by the National
Information Centre of Earthquake Engineering at Indian Institute of Technology Kanpur revealed
that the collapse and damage of reinforced concrete (RC) buildings in the
affected areas were primarily attributed to structural deficiencies prevalent
in regional construction practices. These deficiencies included:
The
findings of the study clearly emphasize the critical importance of adopting
scientifically engineered seismic design methodologies and strict quality
control measures for construction in earthquake-prone regions such as Sikkim.
A
significant increase in seismic activity was observed in Sikkim during February 2026. A total of 41
earthquake events were recorded within a span of 18 days, from 9th February to
27th February 2026. The recorded earthquake magnitudes ranged from Mw 1.9 to Mw
4.6.
The
occurrence of multiple low to moderate magnitude earthquakes within a short
duration, without the presence of a single dominant mainshock event, is
technically identified as a seismic swarm. Such seismic
swarms generally indicate active stress adjustment within the tectonic regime
and reflect the highly active seismic nature of the Himalayan region.
Further,
the Bureau of Indian Standards published
the revised IS 1893:2025 (Part 1 to Part
5) on 03.11.2025, wherein Sikkim was
classified under Seismic Zone VI. The revised code also introduced separate
provisions specifically dedicated to building structures for improved
earthquake-resistant design practices.
However,
the revised IS 1893:2025 was subsequently
withdrawn by the Bureau of Indian Standards
on 03.05.2026 followingconcerns raised by the Ministry
of Housing and Urban Affairs. The concerns primarily related to
substantial escalation in construction costs, estimated at approximately 10–15%
for buildings located in Seismic Zones V and VI, and up to 50% for major
infrastructure projects. Additional concerns were also raised regarding
inadequate stakeholder consultation prior to finalization of the revised
provisions. Accordingly, with the withdrawal of IS 1893:2025, the provisions of
IS 1893:2016 shall continue to remain
applicable until further notification by the competent authority.
Despite
the withdrawal of the revised seismic code, the recent seismic activity
observed in Sikkim highlights the critical
importance of adopting earthquake-resistant planning, structural analysis,
ductile detailing, and proper construction practices to ensure structural
safety and resilience in seismically vulnerable regions.
During
an earthquake, buildings are subjected to sudden ground vibrations generated
due to the release of energy within the Earth’s crust. Unlike gravity loads,
which act vertically downward, earthquake forces predominantly act in the
horizontal direction, causing the structure to sway, vibrate, and deform
rapidly.
When
the ground shakes, the foundation moves along with the soil; however, due to
inertia, the upper portions of the structure tend to resist this movement. This
differential movement between the foundation and the superstructure generates
lateral forces, commonly referred to as seismic forces, within the structural
members.
The
seismic behaviour of a building depends upon several parameters, including:
Buildings
oscillate horizontally during seismic motion. Taller and relatively flexible
buildings generally experience larger lateral displacements and longer
vibration periods.
According
to Newton’s Second Law:
F=ma
the
seismic force developed within a structure is directly proportional to its mass
and acceleration. Consequently, heavier buildings attract larger seismic
forces.
Excessive
tensile and shear stresses generated during earthquake shaking may result in
cracks in:
Diagonal
cracking is commonly observed in masonry walls due to shear action.
If
a structure is not properly designed and detailed for seismic loads, the
following failures may occur:
Among
these, column failure is particularly critical, as columns are the primary
vertical load-carrying members of a structure.
If
the centre of mass and centre of rigidity of a building do not coincide,
torsional effects may develop during seismic excitation, resulting in twisting
of the structure, uneven stress distribution, and severe localized damage.
When
the natural time period of a structure approaches the predominant period of
ground motion, resonance may occur, leading to amplification of vibration and
severe structural damage.
Earthquake-resistant
design does not necessarily imply that a building will remain completely
undamaged during a severe seismic event. The primary objectives of seismic
design are:
Modern
seismic design codes such as IS 1893and IS 13920 emphasize:
In
general engineering practice, structural design is carried out in accordance
with the relevant Indian Standard Codes published by the Bureau of Indian Standards, including IS 456:2000, IS
800, IS 1893, and IS 13920.
However, from a structural engineering perspective, it has been observed
that in many ordinary residential buildings and small-scale construction
projects, critical structural detailing practices and codal provisions are
frequently overlooked during construction. Although such provisions may appear
insignificant at the execution stage, they are fundamentally important for
ensuring the overall structural stability, durability, serviceability, and
seismic resilience of the building.
Improper or inadequate implementation of structural detailing,
particularly in seismic-prone regions, can adversely affect the ductile
behaviour and load-resisting capacity of the structure during earthquake
events. Therefore, strict adherence to the relevant provisions of Indian
Standard codes and proper construction practices is essential to achieve safe
and earthquake-resistant structural performance.
For instance, as per IS 456:2000, the minimum grade of concrete
recommended for reinforced cement concrete (RCC) structural members under
normal exposure conditions is M20. However, IS 13920 mandates a minimum
concrete grade of M25 for ductile detailing of reinforced concrete structures
located in seismic-prone regions. The adoption of higher-grade concrete
significantly improves the compressive strength, stiffness, durability, energy
dissipation capacity, confinement behaviour, and overall seismic performance of
the structure under dynamic loading conditions.
Similarly, the reinforcement detailing requirements specified in IS
13920 differ substantially from those prescribed in IS 456:2000. IS 13920
primarily focuses on ductile detailing provisions intended to enhance the
deformation capacity and earthquake resistance of structural components such as
beams, columns, beam-column joints, shear walls, and foundations. These
provisions include requirements related to confinement reinforcement, spacing
of stirrups, anchorage lengths, lap splice locations, special confining
reinforcement, strong column–weak beam philosophy, and detailing at critical
zones.
Further, IS 1893 (Part 1):2016 explicitly specifies that structures
located in seismic regions shall be detailed in accordance with the provisions
of IS 13920 to ensure adequate ductility and seismic safety. However, in
practice, it is frequently observed that many residential and small-scale
construction projects continue to be executed primarily based on the general
provisions of IS 456:2000 without incorporating the mandatory ductile detailing
requirements of IS 13920. Such non-compliance can significantly compromise the
seismic performance and structural safety of buildings during earthquake
events.
Neglecting
such provisions, even though they may appear minor during construction, can
significantly affect the seismic behaviour and overall structural integrity of
buildings during strong ground motion. Here is the Structural Drawing of Column
& Beam as per IS 13920 2016:
In highly seismic regions such as Sikkim, strict adherence to seismic
design provisions, ductile detailing requirements, material specifications, and
quality construction practices is of paramount importance to minimize
earthquake-induced damage and to ensure the safety, stability, serviceability,
and long-term resilience of structures. Proper implementation of codal
provisions significantly enhances the ductile behaviour and energy dissipation
capacity of structures during seismic events, thereby reducing the probability
of catastrophic structural failure.
Further, the Bureau of Indian Standards has upgraded the seismic design
code from IS 1893 (Part 1):2016 to IS 1893 (Part 1):2025 to incorporate updated
seismic hazard assessments, revised design methodologies, and improved
performance-based considerations. However, it is evident that in many
construction practices, even the provisions and mandatory requirements
prescribed in IS 1893 (Part 1):2016 have not been adequately implemented.
Although IS 1893:2025 has subsequently been withdrawn for further review
and revision, the seismic vulnerability of Sikkim and the Himalayan region
remains unquestionable. The region continues to experience frequent seismic
activities due to its tectonic setting within the active Himalayan seismic
belt. Historical seismic events, including the 2011 Sikkim Earthquake and
recurring moderate tremors, clearly indicate the high earthquake susceptibility
of the region. Therefore, irrespective of codal revisions, the necessity for
rigorous seismic design, ductile detailing as per IS 13920, and strict quality
control during construction cannot be overemphasized in order to achieve
earthquake-resistant and disaster-resilient infrastructure.
In
seismic regions, IS 1893 recommends that
reinforced concrete structures shall be detailed in accordance with the
provisions of IS 13920 to ensure adequate
ductility and seismic resistance.
Accordingly,
reinforcement detailing of structural members such as columns, beams,
beam-column joints, and shear walls shall strictly conform to the ductile
detailing requirements specified in IS 13920.
The major detailing provisions include:
Although
these detailing provisions may appear minor during execution, they
significantly improve the ductility, energy dissipation capacity, and overall
seismic performance of the structure. Proper ductile detailing enables
structural members to undergo large inelastic deformations during earthquake
shaking without sudden or brittle failure.
Therefore,
such detailing practices play a vital role in minimizing structural damage,
preventing catastrophic collapse, and improving the overall safety and
resilience of buildings during seismic events.
