Last reviewed 16 Sept 2026 · 8 min read
Earthquakes — geological causes
(For waves, magnitude and intensity see Seismology in Earthquake Engineering.)
- Most earthquakes occur along plate boundaries — convergent, divergent and transform.
- Active faults — faults showing movement in recent geological time (commonly the Holocene/late Quaternary) and capable of future movement; evidenced by offset young sediments, fault scarps, displaced streams, and seismicity.
- Intraplate earthquakes — within plates along old zones of weakness (e.g. rifts).
- Reservoir-induced seismicity — increased pore pressure and loading from reservoir filling (e.g. Koyna, 1967).
- Volcanic and collapse earthquakes (local).
Seismotectonic setting of India
| Region | Geological setting |
|---|---|
| Himalayan belt | Collision of the Indian plate with the Eurasian plate; major thrusts such as the Main Central Thrust (MCT), Main Boundary Thrust (MBT) and Himalayan Frontal Thrust (HFT); great earthquakes |
| North-east India | Complex convergence (Himalaya and Indo-Burman ranges); Shillong Plateau; very high seismicity (1897, 1950) |
| Andaman–Nicobar | Subduction zone — source of the 2004 earthquake and tsunami |
| Kachchh (Gujarat) | Rift basin reactivated under compression — 1819 and 2001 earthquakes |
| Peninsular (stable continental) India | Lower but not negligible seismicity along old rifts and faults — Koyna, Latur, Jabalpur |
| Indo-Gangetic plains | Thick alluvium — amplification of shaking and liquefaction potential |
Geological effects of earthquakes
| Effect | Description / engineering concern |
|---|---|
| Surface fault rupture | Ground offset along active faults — structures across faults are torn apart; avoid siting dams, bridges and pipelines across active faults, or design for displacement |
| Ground shaking and site amplification | Soft soils and thick alluvium amplify shaking; basin effects; resonance of buildings with site period |
| Liquefaction | Loose, saturated, fine sands and silty sands lose shear strength when pore pressure rises under cyclic loading — sand boils, settlement, tilting of buildings, lateral spreading, floating of buried tanks |
| Landslides and rockfalls | Triggered on steep slopes, especially in the Himalaya |
| Tsunami | Seafloor displacement from undersea earthquakes |
| Ground subsidence/uplift | Regional elevation changes |
| Changes in groundwater | Springs, wells, river courses |
Geological considerations in seismic areas
- Detailed seismotectonic study and identification of active faults for major projects (dams, nuclear plants).
- Site-specific seismic hazard analysis for important structures.
- Avoid construction on active faults, loose saturated sands, steep unstable slopes, reclaimed fills; or apply ground improvement (compaction, stone columns, drainage), deep foundations.
- Microzonation of cities based on soil and geology.
Landslides and mass movements
Mass movement (mass wasting) is the downslope movement of soil and rock under gravity; a landslide is a mass movement along a definite failure surface (commonly used for all types).
Varnes classification (simplified)
| Type | Movement | Examples / features |
|---|---|---|
| Falls | Free fall, bouncing, rolling of detached blocks | Rockfall from cliffs and road cuts |
| Topples | Forward rotation of blocks about a pivot | Steeply dipping joints facing the slope |
| Slides — rotational | Movement along a curved (concave) surface | Slumps in homogeneous soils/weak rocks; back-tilted blocks |
| Slides — translational (planar) | Movement along a planar surface (bedding, joints, faults) | Beds dipping towards the valley; wedge failures on two intersecting joints |
| Lateral spreads | Extension on gentle slopes over liquefied or soft layers | Earthquake-induced spreading |
| Flows | Movement like a viscous fluid | Debris flows, mudflows, earthflows; rapid and destructive in monsoon/cloudbursts |
| Creep | Very slow, imperceptible continuous movement | Evidence: tilted trees, poles, fences, bent walls, curved tree trunks |
| Complex | Combination of types |
Causes of landslides
Preparatory (internal/predisposing) factors:
- Steep slopes; weak or weathered rocks (shale, clay, schist); unfavourable structures — bedding or joints dipping out of the slope, faults, shear zones.
- Loss of vegetation (deforestation).
- Weathering reducing strength.
Triggering (external) factors:
- Heavy or prolonged rainfall — rise in pore water pressure (the most common trigger in India).
- Earthquakes — shaking.
- Undercutting of the toe by rivers, waves or excavation (road cutting).
- Loading at the top — buildings, fills, waste dumps.
- Rapid drawdown of reservoirs.
- Blasting and vibrations; leaking water pipes and poor drainage.
Recognition of landslide-prone areas
- Crescent-shaped scarps, tension cracks at the crown, hummocky ground, bulging toe.
- Seepage and springs, marshy patches on slopes.
- Tilted trees and poles; displaced drains and roads; cracks in structures.
- Past landslide scars on aerial photographs and satellite imagery.
Landslide hazard zonation (LHZ)
- Mapping of areas into zones of very low to very high hazard using factors such as lithology, structure (relation of discontinuities to slope), slope angle, relative relief, land use/land cover, groundwater conditions, and past landslides.
- Guidelines exist in Indian Standards (IS 14496, Part 2) for landslide hazard zonation in mountainous terrain; agencies such as the Geological Survey of India (GSI) prepare landslide susceptibility maps.
- Used for land-use planning, route selection of roads/railways, and early warning.
Prevention and control of landslides
| Measure | Examples |
|---|---|
| Drainage (most effective and economical in many cases) | Surface drains and catch water drains to divert runoff; sealing of cracks; sub-surface drainage — horizontal drains (weep holes, perforated pipes), trench drains, drainage galleries — to lower pore pressure |
| Slope modification | Flattening the slope; benching/terracing; removing weight from the head (unloading); adding weight at the toe (toe berm/buttress) |
| Retaining structures | Retaining walls, gabion walls, crib walls, piles and anchored walls |
| Reinforcement of rock/soil | Rock bolts, rock anchors, soil nails, shotcrete with mesh, grouting |
| Rockfall protection | Wire mesh drapes, rockfall barriers/catch fences, catch ditches, rock sheds, scaling of loose blocks |
| Bioengineering | Vegetation (grasses, shrubs, trees), coir/jute geotextiles, live fascines — reduce erosion and add root reinforcement |
| Avoidance and management | Re-route roads; restrict construction; monitoring (inclinometers, piezometers, extensometers) and early warning |
Infinite slope stability
FormulaFactor of safety — infinite slope in cohesionless soil
Dry (or submerged, no seepage):
Seepage parallel to slope with water table at the surface:
= angle of internal friction; = slope angle; . Seepage parallel to the slope roughly halves the factor of safety — explaining why slopes fail during heavy rain.