Earthquakes: Causes, Measures, Impacts and Mitigation

Definition of Earthquake

An earthquake, also known as Tremor, is one of the most powerful and devastating natural phenomena experienced on Earth. It occurs when sudden energy is released in the Earth’s lithosphere (Earth’s crust or upper mantle) due to stress accumulation along geological faults, which overcomes friction that holding the rocks together and a sudden slip (rupture) occurs along that fault plane due to movement of rocks beneath the surface of the earth called Craton. This release generates seismic waves that cause the ground to shake. The Epicentre refers to the point on the Earth’s surface directly above the earthquake’s origin, while the Hypocentre (or focus) is the point within the Earth where the seismic rupture begins.

Before the main shock, small tremors known as foreshocks often occur, warning of an impending earthquake. Similarly, aftershocks follow the major event, sometimes lasting for weeks or even months, adding to destruction. The areas of equal seismic intensity during an earthquake are mapped as isoseismal lines or lines of equal intensity, whereas homoseismal lines or lines of equal magnitude indicate points on the surface where seismic waves from the same focus arrive simultaneously.

Together, these terms form the scientific basis of understanding earthquakes, allowing seismologists to track, measure, and predict their patterns.

Earthquake as an Endogenic Force (Catastrophic)

The Earth is constantly shaped by two types of geomorphic forces:

1. Endogenic Forces – internal forces originating from within the Earth.
2. Exogenic Forces – external forces such as weathering, erosion, and deposition that wear down the land.

Earthquakes fall under endogenic catastrophic forces or fast acting forces, alongside volcanic eruptions. Endogenic processes also include diastrophic forces or slow acting forces (responsible for crustal uplift or subsidence) and orogenic/epeirogenic movements (mountain building or continent-scale uplift), as well as catastrophic phenomena like volcanic activity and earthquakes. Unlike exogenic forces, which are gradual and constructive, earthquakes are sudden, violent, and often catastrophic.

---

Layers of the Earth

The occurrence and behavior of earthquakes are linked to the Earth’s internal structure.

• Crust :
  Crust is the top thin outer layer of rocks on earth surface. It contains cratons, shields etc. It consists of continental crust (Sial: Silica + Aluminium) or Felsic (Granitic) and oceanic crust (Sima: Silica + Magnesium) or Mafic (Basaltic). The thickness of crust is about 60-100 km with density 2 to 3. The continental crust is thicker with 30-50km, and made of Granitic lava or Felsic (high silica content), than Oceanic crust which is about 5-10km but denser. Continental crusts are made granitic lava and FelSiC mineral in nature, whereas Oceanic crusts are made of Basaltic lava and MaFiC mineral in nature. The is upper crust which contains the continent and new formed oceanic rocks and lower crust which contains cratons and old oceanic rocks. Both are separated by Conrade discontinuity.
  The crust also constitutes parts of Lithosphere which is the rigid outermost shell. Lithosphere contains crust and upper part of mantle which is separated by a layer from Lower crust, called Moho discontinuity. Here seismic waves increases from crust to mantle.
  The lithosphere thickness ranges from 5 km (young oceanic) to 250 km (old continental cratons). Tectonic plates are part of lithosphere that float on molten Asthenosphere.
  Beneath the lithosphere lies the asthenosphere, a semi-molten zone critical for plate movement which is mafic in nature that extends unto 300km-400km until middle mantle. It is rich in Sima and contains more amount of Ferro-magnesium silicate (MaFiC) compared to Lithosphere which is more of non-ferro magnesium silicate or FelSiC (Feldspars and silica). This the layer where convection current or plum formation, according th Aurther Holmes, takes place.
• Mantle:
  Composed of silicate minerals rich in iron and magnesium (SiMa). Convection currents within the mantle drive plate tectonics, which is the root cause of most earthquakes.
  Mantle consists of upper or outer mantle with thickness unto 300-400km. this is composed of Olivine mineral in peridotite rock formation, which is plastic in nature and tends to deform under heat and pressure. Hence, semi-liquid in nature of Asthenosphere,
  Middle Mantle unto 900 km. Which is separated from upper mantle by Repitte discontinuity.
  Inner Mantle unto 2900km. Solid and heavily dense due to peridotite rock type- Dynite and Granet.
• Core:
  The core is separated from Mantle by Gutenburg discontinuity. It forms the 32% of earth mass and 16% by volume due Nickel and Iron. The outer core is semi-liquid, composed mainly of nickel and iron (NiFe) with depth of 5000km, while the inner core is solid with depth unto 6,400km; both are separated by Lehmann discontinuity. Outer and inner cores play a major role in generating Earth’s magnetic field and affects seismic wave transmission.

---

Types of Seismic Waves

Earthquakes release seismic waves, categorized into two types:

1. Body Waves (travel through Earth’s interior):
   These waves generate at the focus or hypocentre and travel over the layers of earth.
   • P-waves (Primary waves): Longitudinal, compressional waves that travel fastest and speed is fastest in solid compared to liquid medium.
   • S-waves (Secondary waves): Transverse waves that move slower and cannot pass through liquids.
     S-waves cannot pass through the liquid asthenosphere or outer core because transverse waves require rigid media, whereas P-waves (longitudinal) can compress and expand even in liquids.
2. Surface Waves (travel along Earth’s surface):
   These arrive after body waves but die out at surface, nevertheless these are quite destructive in nature.
   • Love Waves (L-waves): Horizontal shearing motion, destructive.
   • Rayleigh Waves (R-waves): Rolling motion, cause maximum ground shaking.

Table: Comparison of Seismic Waves

Wave Type	Nature	Medium	Speed	Impact	Example Path
P-Waves	Longitudinal	Solid + Liquid	Fastest	Less destructive	Crust → Mantle → Core
S-Waves	Transverse	Solid only	Slower	Strong shaking	Crust + Mantle
Love Waves	Transverse (horizontal)	Surface	Slow	Highly destructive	Surface layers
Rayleigh Waves	Rolling (elliptical) type of transverse wave	Surface	Slowest	Most destructive	Surface layers

How waves propagate to create earthquakes-

As P and S-waves travel deeper inside the Earth, their velocities generally increase, causing them to bend outward.

In the continental crust, these waves move relatively slowly (P – 6 km/s). This velocity matches granitic rocks, and such waves are named Pg and Sg. The rocks here have a density of about 2.7 g/cm³.

At depths beyond 35 km in the continent, the velocity of P and S-waves rises sharply to about 7 km/s. This marks the Conrad discontinuity, which separates felsic rocks from mafic ones.

Crossing the Moho discontinuity wave velocities further increase to 8 km/s, indicating the beginning of the upper mantle.

Around 100 km depth, there is a sudden velocity drop (P – 7 km/s), which continues until 300- 400 km depth. This zone corresponds to the asthenosphere, characterized by its partially molten or plastic state. Because of the reduced velocities, it is also called the low-velocity zone (LVZ).

Below the asthenosphere, velocities steadily increase again, reaching 9-11 km/s in the lower mantle and up to 13 km/s in the deep mantle, known as the high-velocity zone (HVZ).

Shadow Zones of P and S Waves:

At about 2,900 km depth corresponding to seismic shadow angles of 103°-105°, S-waves vanish since the outer core is liquid. P-waves, however, continue to propagate, though they bend significantly, with maximum deflection around 145°, creating a shadow zone between angle 105° to 145°. A shadow zone is where seismic waves disappear.

The average density of the Earth is about 5.51 g/cm³. The deepest recorded earthquake focus has been measured at about 700 km depth. The source of Earth’s internal heat energy is mainly the disintegration and decay of radioactive elements, along with the conversion of gravitational energy into thermal energy.

Why S-waves cannot travel through liquids?

• S-waves (shear waves) move particles perpendicular to their direction of travel.
• They can pass through solids because rocks have shear strength (rigidity) that holds particles together.
• Liquids lack shear strength. Thus causing loss of energy transmission of S-waves.
• Since rigidity is required for S-waves to propagate, they cannot travel through liquids.

---

Types of Earthquakes

1. Natural Causes:
   • Tectonic: Caused by plate boundary interactions (most common).
   • Volcanic: Triggered by magma movement and volcanic eruptions which are explosive and fissure type forming extrusive rocks.
   • Isostatic: Due to crustal adjustments. Mostly takes place around mountain folds and faults, glacier movement, etc.
   • Plutonic: Deep underground stresses about 300-700km deep.
2. Anthropogenic Causes:
   Human-induced earthquakes arise from:
   • Dam construction and water reservoirs.
   • Deep mining and excavation.
   • Underground nuclear tests.
3. Other reasons can be: Meteoroid activity or asteroid impact. Reason for Cretaceous- Paleogene (K-T) mass extinction (66 million years ago) causing extinction of 80% species including dinosaurs.

---

Causes of Earthquakes

Tectonic Plate Movement

The Plate Tectonic Theory by Arthur Holmes (mantle convection currents) and Harry Hess (seafloor spreading) explains most earthquakes.

Arthur Holmes argued that mantle convection (slow circulation driven by internal heat) at astehenosphere drives large-scale crustal movements or tectonic plate movements. According to him, in asthenosphere mantle is plastic and molten in nature which circulates as convection cells, called plumes; these cells melt the lithosphere which ascends and deposit as lava from fissures, forming ridges, and vents. This simultaneous melting and ascending lead to the circular movement convention current, and hence movement of plates.

Harry Hess proposed sea-floor spreading based on Arthur Holmes postulates, molten material upwells at mid-ocean ridges, creating new ocean crust and ridges that moves laterally, helping to drive plates apart and interact at boundaries.

Movements occur at:

• Convergent boundaries (responsible for most major quakes)- where plates collide or one plate dives beneath another. These produce the deepest and most powerful earthquakes (and most tsunamis when undersea). These can be Continent-Oceanic plate or Continent-continent or Ocean-oceanic plate collision. Examples: the Japan trench, the Peru–Chile trench and Indian Ocean tsunami was due oceanic-continent interaction, Indian plate-Eurasian plate interaction.
• Transform boundaries – plates slide past one another; produce shallow, frequently repeating quakes. Example, San Andreas Fault, California;
• Divergent boundaries– plates pull apart ; quakes are usually smaller but numerous. examples, mid-ocean ridges.

Divergent plates cause moderate tremor with shallow focus as spreading of plates and creating faults and ridges through upwelling of magma from asthenosphere are not fast and abrupt in nature compared to events at convergents plates which are prone to explosive volcanic activities as well.

Such interactions can trigger tsunamis when underwater earthquakes displace ocean water violently.

Isostatic Movement

The Elastic Rebound Theory (by H.F. Reid) explain that the earthquake can also occur due to faulting and folding of crustal rocks. It states that rocks bend under stress until they rupture suddenly, releasing stored energy as an earthquake.

The Himalayan region is a prime example:

• MBT (Main Boundary Thrust)
• MCT (Main Central Thrust)
• HFT (Himalayan Frontal Thrust)
  These faults explain frequent seismic activity in northern India.

Volcanism

Explosive volcanic eruptions or magma intrusions into fissures cause localized earthquakes. Volcanic induced earthquakes are more prevalent at convergent boundaries, as explosive composite type and cinder type are prevalent around convergent boundaries, for instance, eruptions in the Pacific plate “Ring of Fire”, which a chain of volcanic cones along faults due to the subduction of pacific plate under Philippines and North American plates, are often accompanied by strong quakes. Eruption of Etna in 1968 is one of the devastating examples of volcanic induced earthquakes.

---

Measurement of Earthquakes

1. Mercalli Intensity Scale:
   Measures the observed effects and damage (qualitative). Ranges from Roman numeralsI (not felt) to XII (total destruction) level of intensity by observing the visible damage of the event. It is not based on any mathematical or statistical approach rather a subjective order based. Normally, damage is associated with intensity level V, it is not used as standard measure now.
   
2. Richter Scale:
   Developed by Charles Richter, it measures the magnitude based on seismic wave amplitude (quantitative) in logarithmic scale, i.e. the magnitude increases 10 times of wave amplitude at each consecutive scale. It is used as standard scale to measure earthquake. It is expressed in nominal numbers from 0 to maximum 10, however no higher limit is set but scale 10 is considered as the maximum devastation a civilisation can sustain.
   At each Richter scale the energy released 31.5 kJ times the previous level.
   example, at Richter (RR) Magnitude 1 : Energy released is 31.5kJ with wave amplitude of 10.
   Richter (RR) magnitude 2: Energy released 31.5² kJ with wave amplitude of 10² (magnitude increases 10 times).
   Richter (RR) magnitude 3: Energy released 31.5³ kJ with wave amplitude of 10³.
   

---

Classification of Earthquakes

• Based on Hypocentre Depth:
  • Shallow focus: 0–70 km (most destructive).
  • Intermediate: 70–300 km.
  • Deep focus: 300–700 km.
• Based on Magnitude:
  • Minor: <4.0
  • Moderate: 4.0–6.0
  • Major: 6.0–7.9
  • Great: ≥8.0

---

World Distribution of Earthquakes

Earthquakes are concentrated along plate boundaries:

• Circum-Pacific Belt (Ring of Fire): the world’s most active seismic and volcanic arc: Japan, Indonesia, Pacific Northwest, west coasts of North & South America. It hosts most volcanic activity and 70% of earthquakes.
• Alpide-Himalayan Belt/ Mid-Continental: stretches westward from the Mediterranean through the Alpine system and the Himalaya- Alps, Apennines, Pyrenees and Balkan Mountains in Europe; Taurus, Zagros, Caucasus, Albroz and Hindu Kush in middle and central Aisa; and Suleiman, Karakoram, Pamir and Himalayas in South Asia ; major fold-belt seismicity.
• Mid-Atlantic Ridge/ Mid-Oceanic Ridge belt: divergent boundary seismicity distributed along ridge crests.

Zones also include:

• Fault lines along fold mountains.
• Volcanic arcs.

Examples of Major Earthquakes:

• 1960 Chile (Valdivia) — Mw 9.5, largest instrumentally recorded earthquake.
• 2004 Indian Ocean — Mw 9.1 (Sumatra–Andaman); caused a catastrophic tsunami.
• 2011 Tohoku, Japan — Mw 9.0; major tsunami and nuclear plant impacts.
• 2015 Gorkha (Nepal) — Mw 7.8; devastating in the Himalayan foothills.

---

Seismic Zones of India

India’s seismicity is strongly influenced by the ongoing collision between the Indian and Eurasian plates. Maps produced by geological agencies and disaster authorities divide India into seismic hazard zones (commonly shown as Zones II–V, where Zone V is the highest hazard). The highest hazard is concentrated in:

• The Himalayan belt and parts of northeastern India (Zone IV and V).
• The Indo-Gangetic plain (vulnerable because of high population and soft sediments that amplify shaking).
• Parts of the west coast, Andaman & Nicobar and certain portions of peninsular India (e.g., Koyna) have shown significant seismicity due to local faults and reservoir.

Why the Himalayan belt is earthquake-prone: India’s northward motion and collision with Eurasia has folded and thrusted crust for tens of millions of years. Forming 3 parallel folds along the subduction boundary which are- Himalayan Frontal Thrust (HFT), Main Boundary Thrust (MBT) and Main Central Thrust (MCT). The frontal thrusts and deep crustal ramps lock, accumulate strain, and release it in sudden, often shallow earthquakes — precisely the recipe for high seismic hazard.

Recent earthquakes:

• 2015 Nepal (M 7.8).
• 2001 Bhuj, Gujarat (M 7.7).
• 2023 Turkey-Syria earthquake (impact felt globally in seismology).

(Insert Map: India Seismic Zone Map showing intensity ranges.)

---

Earthquake Hazards

• Loss of life and property.
• Tsunamis, landslides, avalanches.
• Soil liquefaction.
• Damage to dams, power plants, and urban infrastructure.

---

Mitigation Programs

Worldwide

• Global seismic networks and rapid data sharing (USGS, IRIS, EMSC) allow quick magnitude/impact estimates.
• Early warning systems (Japan, Mexico, parts of the U.S.) detect P-waves and send automatic warnings before the slower, damaging S- and surface waves arrive — providing seconds to minutes for automated shutdowns and protective action.
• Seismic design codes (modern building codes and retrofit programmes) reduce casualties when enforced.
• Public education and preparedness drills (drop, cover, hold on).

India

• NDMA (National Disaster Management Authority) issues policies, guidelines and preparedness plans.
• Bureau of Indian Standards (BIS) and IS codes specify seismic design criteria for buildings and critical infrastructure (e.g., IS 1893).
• Seismic zonation maps and hazard assessments guide land-use planning and building regulations.
• Early warning research (sensor networks, sea-level monitors near subduction zones) is expanding, but India’s diverse exposure (Himalaya, Andaman subduction, peninsular faults) makes integrated preparedness essential.

---

Conclusion

An earthquake is not just a scientific event but a powerful, emotional, and catastrophic force that reshapes civilizations, landscapes, and history. Understanding its causes, measurement, and mitigation strategies allows humanity to face this force with resilience. While earthquakes cannot be prevented, their impact can be reduced through scientific planning, preparedness, and strong disaster management policies.

---