Why Earthquake resistant structures are needed? Discuss the geological considerations required for developing the Earthquake resistant structures.

Why Earthquake-Resistant Structures Are Needed

The necessity for earthquake-resistant structures stems from several critical factors:

• Preservation of Life: The primary objective is to prevent fatalities and injuries by ensuring buildings do not collapse during seismic events, allowing occupants time to evacuate.
• Minimizing Economic Loss: Earthquakes can cause massive economic damage through infrastructure failure, building collapse, and business disruptions. Resilient structures reduce these losses and aid faster recovery.
• Protecting Critical Infrastructure: Essential services like hospitals, fire stations, power plants, and communication networks must remain operational post-earthquake. Earthquake-resistant design ensures their continuity.
• Ensuring Structural Integrity: These structures are designed to deform and sway without collapsing, absorbing and dissipating seismic energy. This prevents complete structural failure and makes repair feasible after moderate quakes.
• Increasing Property Value and Sustainability: Earthquake-resistant buildings are generally more durable, sustainable, and retain higher property values, offering long-term benefits to owners and communities.
• Addressing India's Seismic Vulnerability: According to the updated Earthquake Design Code 2025 by the Bureau of Indian Standards (BIS), approximately 61% of India's landmass lies in moderate to high hazard zones (Zones III, IV, V, and the newly introduced Zone VI). About 75% of India's population now resides in seismically active regions, making robust construction practices imperative.

Geological Considerations for Developing Earthquake-Resistant Structures

Designing earthquake-resistant structures requires a deep understanding of the geological context and geotechnical conditions of the site. Key geological considerations include:

1. Seismic Zonation and Regional Seismicity

India is classified into seismic zones based on the expected intensity of earthquakes. The new Earthquake Design Code 2025 has updated India's seismic zonation map, introducing a new highest-risk Zone VI, which now includes the entire Himalayan arc. Understanding the seismic zone of a construction site is fundamental as it dictates the minimum seismic design forces buildings must withstand.

Seismic Zone (as per BIS 2025)	Description	Key Regions (Examples)
Zone II	Low intensity damage risk	Parts of Karnataka, Maharashtra, Rajasthan, Central India
Zone III	Moderate intensity damage risk	Kerala, Punjab, Haryana, parts of Gujarat, Uttar Pradesh
Zone IV	High intensity damage risk	Delhi, Chandigarh, Bihar, parts of Jammu & Kashmir, Himachal Pradesh, Uttarakhand
Zone V	Very High intensity damage risk	Northeast India, Rann of Kutch, Andaman & Nicobar Islands, parts of Himalayan belt (older classification)
Zone VI (New in 2025)	Ultra-High intensity damage risk	Entire Himalayan arc (Jammu & Kashmir, Ladakh, Himachal Pradesh, Uttarakhand, Sikkim, Arunachal Pradesh)

The revised IS 1893 (Part 1):2025 mandates significantly higher earthquake resistance (50-60% higher than old Zone V) for structures in Zone VI and near-fault provisions for structures within 20-30 km of active faults.

2. Local Soil Conditions and Site Effects

The type of soil beneath a structure plays a crucial role in how seismic waves are transmitted and amplified. Site-specific ground response studies and liquefaction checks are now mandatory, especially in areas like the Gangetic plains and Kashmir Valley, as per the 2025 code.

• Soil Amplification: Soft, loose soils (like alluvial deposits, soft clays) can amplify seismic waves, leading to greater ground shaking compared to bedrock. Buildings on such soils experience higher forces. Engineers must conduct soil testing to determine the dynamic properties of the soil.
• Soil Liquefaction: This phenomenon occurs when saturated granular soils (like loose sands and silts) temporarily lose their strength and stiffness due to increased pore water pressure during an earthquake, behaving like a liquid. This can cause buildings to tilt, sink, or collapse. Areas with high groundwater tables are particularly susceptible. Mitigation techniques include ground improvement (e.g., compaction, stone columns) and appropriate foundation types.
• Ground Settlement and Lateral Spreading: Earthquakes can cause differential settlement of the ground or lateral spreading, where liquefied soil flows horizontally on gentle slopes. This can severely damage foundations and lead to structural failure.
• Bearing Capacity of Soil: The ability of the soil to support the structural load must be assessed. Weak or inconsistent bearing capacity requires deeper foundations (e.g., pile foundations) or spread foundations (e.g., mat/raft foundations) to distribute the load over a larger area.

3. Presence of Active Faults

Proximity to active fault lines significantly increases seismic hazard. Structures located directly over or very close to an active fault are at risk of ground rupture, where the ground is physically torn apart. Geological surveys are essential to identify such faults and avoid building directly on them. Even near-fault areas experience specific seismic effects like high-velocity ground pulses that require special design considerations.

4. Topography and Geomorphology

• Slopes and Hillside Construction: Structures on slopes or hillsides are vulnerable to landslides and rockfalls triggered by earthquakes. Detailed geological and geotechnical investigations are needed to ensure slope stability. Terracing and retaining structures may be required.
• Valley Effects: Structures located in river valleys can experience amplified ground motion due to the basin's geometry and sediment infill, a phenomenon known as "basin effect."

5. Groundwater Conditions

High groundwater levels are a critical factor contributing to soil liquefaction and can affect the stability of foundations, especially in alluvial plains and coastal areas. Geotechnical investigations should include groundwater monitoring to assess its impact on seismic performance.

6. Rock Formations

Building on competent bedrock generally offers better seismic performance than on soft soils, as bedrock tends to transmit seismic waves with less amplification. However, certain rock types (e.g., fractured rock masses) may still pose challenges due to potential rockfalls or instability.

Indian Seismic Codes and Guidelines

The Bureau of Indian Standards (BIS) has a comprehensive set of codes for earthquake-resistant design and construction. Key codes include:

• IS 1893 (Part I): 2025 (Revised): Criteria for Earthquake Resistant Design of Structures. This is the main code providing the seismic zonation map and specifying seismic design forces.
• IS 4326: 1993: Code of Practice for Earthquake Resistant Design and Construction of Buildings.
• IS 13920: 2016: Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces. This code ensures that concrete structures have the necessary ductility to withstand large deformations without collapsing. It has been made mandatory for structures in Zones III, IV, V, and VI.
• IS 13827: 1993 & IS 13828: 1993: Guidelines for Improving Earthquake Resistance of Earthen and Low-Strength Masonry Buildings, respectively, applicable in Zones III, IV, V, and VI.
• IS 13935: 1993: Guidelines for Repair and Seismic Strengthening of Buildings.

These codes emphasize aspects like good structural configuration, adequate lateral strength and stiffness, and good ductility, tailored to local seismology, accepted risk levels, and building typologies. The updated IS 1893:2025 also places a significant focus on non-structural safety and mandatory retrofitting for existing buildings in high-risk zones, particularly schools, hospitals, and old masonry structures in the Himalayan region.