Earthquakes

1. Origin, faults and earthquake terminology

Plate motions slowly deform rocks and accumulate elastic strain. When stress exceeds the resistance to slip on a fault, the fault ruptures and releases energy as seismic waves. Elastic rebound describes the sudden recovery of strained rocks towards a less deformed state. The theory developed from studies of the 1906 San Francisco earthquake. A fault may remain locked for a long period even while the surrounding crust continues to deform.

Normal faults develop mainly under extension, reverse and thrust faults under compression, and strike-slip faults under lateral shearing. Megathrust earthquakes occur on the gently dipping interface between a subducting plate and the overriding plate. These interfaces can rupture over enormous areas and generate the largest earthquakes. Earthquakes also occur within plates when stresses reactivate older weaknesses; distance from a plate boundary does not guarantee safety.

The hypocentre marks where rupture starts, not the whole ruptured fault surface. Foreshocks precede a larger earthquake but can only be identified as foreshocks retrospectively. Aftershocks follow the mainshock as stresses readjust and may cause additional damage to weakened buildings. Their frequency generally declines with time.

Most destructive earthquakes are tectonic. Others accompany volcanic activity, underground cavity collapse or human activities such as reservoir impoundment, mining and fluid injection. Reservoir-associated seismicity can involve changes in loading and pore pressure along existing faults; it is not simply the direct consequence of water weight.

• Shallow-focus earthquakes: approximately 0–70 km depth.
• Intermediate-focus earthquakes: approximately 70–300 km depth.
• Deep-focus earthquakes: approximately 300–700 km depth.

2. Seismic waves and the Earth’s interior

Body waves travel through the Earth. Primary or P waves are compressional waves: particles oscillate parallel to the direction of propagation. They are the fastest seismic waves and usually arrive first at a recording station. Secondary or S waves are shear waves: particle movement is perpendicular to propagation. Since fluids lack the shear rigidity needed to transmit them, S waves cannot travel through the liquid outer core.

Surface waves travel along the Earth’s surface and generally arrive after body waves. Love waves produce horizontal transverse motion, while Rayleigh waves produce rolling, elliptical particle motion. Their often large amplitudes and relatively long durations can make them especially damaging. However, damage also depends on frequency, local geology, building characteristics and distance; surface waves are not invariably the sole cause of destruction.

Seismic-wave refraction, reflection and travel times reveal internal layering. In the standard textbook model, direct P waves have a shadow zone roughly between 103° and 142° from the epicentre because of strong refraction at the core boundary. Direct S waves are absent beyond approximately 103°, supporting the inference of a liquid outer core. These angular distances are measured at the Earth’s centre.

A seismograph records ground motion as a seismogram. The difference between P-wave and S-wave arrival times helps estimate the distance to an earthquake. In the simplified triangulation method, distances from at least three suitably placed stations locate the epicentre; operational networks use many observations and velocity models to estimate location and depth.

3. Magnitude, intensity and associated hazards

Magnitude measures earthquake size from instrumental observations. The original Richter or local magnitude scale was designed for particular recording conditions and nearby earthquakes. Moment magnitude, Mw, is preferred for large earthquakes because it is derived from seismic moment, which depends on rock rigidity, rupture area and average fault slip. Magnitude scales are logarithmic, not linear.

Intensity describes the effects of shaking at a particular place. The Modified Mercalli scale ranges from I to XII and uses observed effects on people, objects, buildings and the landscape. One earthquake has different intensities at different locations. Depth, distance from the rupture, sediment thickness, topography and construction quality influence these variations. Soft sediments may amplify shaking, while resonance can increase building damage.

Earthquakes cause ground shaking, surface rupture, landslides, rockfalls, fires and damage to dams or essential services. Liquefaction occurs when shaking raises pore-water pressure in loose, saturated granular material, reducing its effective strength. Buildings may tilt or settle and the ground may spread laterally. Loose sandy deposits near rivers, coasts and reclaimed land are particularly susceptible.

A tsunami requires substantial displacement of water, commonly from sudden vertical movement of the seabed during a large, shallow submarine earthquake. Submarine landslides can also generate tsunamis. Not every undersea earthquake produces one: fault movement, depth, magnitude and seabed deformation matter. Tsunamis have long wavelengths and periods and can inundate extensive coastal areas.

4. Global earthquake belts and India

Earthquakes cluster along plate boundaries. The circum-Pacific belt includes subduction margins around Japan, Indonesia and the western Americas. The Alpine–Himalayan belt extends through the Mediterranean region and western Asia to the Himalayas. Mid-ocean ridges and associated transform faults also generate earthquakes, predominantly shallow ones. Convergent subduction margins display inclined zones of seismicity called Wadati–Benioff zones, extending from shallow to great depths.

India’s Himalayan seismicity reflects continuing convergence between the Indian and Eurasian plates. The Main Himalayan Thrust is a major earthquake-generating structure beneath the Himalayan belt. Northeast India has complex collision and subduction-related tectonics, while the Andaman–Nicobar region lies along an active subduction system. Kachchh and parts of peninsular India demonstrate the importance of damaging intraplate earthquakes.

For the widely used seismic zoning framework in IS 1893 (Part 1): 2016, India is divided into Zones II, III, IV and V, with Zone V representing the highest seismic design hazard among these zones. Zone I does not appear in this framework. Zoning represents broad regional design requirements, not a forecast of the next earthquake; code editions and official map revisions must be checked when using current classifications.

5. Monitoring, preparedness and disaster-risk reduction

India’s National Center for Seismology, under the Ministry of Earth Sciences, monitors earthquakes. INCOIS operates the Indian Tsunami Early Warning Centre in Hyderabad. Earthquake early warning exploits the interval between detection of initial waves and arrival of stronger shaking elsewhere. Warning time may be only seconds, and locations close to the rupture may receive little or no useful warning.

Risk reduction requires enforcement of earthquake-resistant construction standards, retrofitting, ductile detailing, safe land-use planning and protection of lifeline infrastructure. Seismic microzonation maps local variations in ground conditions and expected shaking. Public drills, secured furniture and emergency communication improve preparedness. During shaking indoors, Drop, Cover and Hold On is generally recommended; rushing towards stairs or lifts can increase injury risk.

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