📖 PART 1: Meaning and Causes of Earthquakes
An earthquake is a sudden trembling or shaking of the Earth's surface caused by the sudden
release of energy stored in the rocks of the Earth's crust. The energy travels outward as seismic
waves.
Causes of Earthquakes
- Tectonic Activity (most common cause): Movement of tectonic plates — plates sliding
past each other, colliding, or pulling apart at fault lines causes stress to build up and then suddenly
release as an earthquake.
- Volcanic Activity: Movement of magma and volcanic eruptions cause earthquakes,
especially near active volcanoes.
- Collapse: Underground cave collapses, mine collapses can cause small local earthquakes.
- Man-made causes: Nuclear explosions, large dam reservoirs (water weight pressing on
rocks), deep mining activities — these cause minor earthquakes.
📍 PART 2: Important Terms
| Term |
Definition |
| Focus (Hypocentre) |
The point INSIDE the Earth where the earthquake originates — where the energy is first released.
Also called the hypocentre. Can be shallow (less than 70 km), intermediate (70–300 km), or deep
(300–700 km). |
| Epicentre |
The point on the Earth's SURFACE directly above the focus — receives the most intense shaking.
Damage is greatest at the epicentre. |
| Seismic Waves |
Waves of energy that travel through the Earth from the focus: P-waves (Primary/Push waves —
fastest; travel through solids and liquids), S-waves (Secondary/Shake waves — slower; travel
only through solids), Surface waves (travel along surface — cause most damage). |
| Seismograph |
An instrument that detects and records seismic waves. |
| Seismogram |
The record/printout produced by a seismograph. |
📏 PART 3: Measurement of Earthquakes
- Richter Scale (developed by Charles F. Richter, 1935): Measures the magnitude
(energy released) of an earthquake on a scale from 0 upwards. Each unit increase = 10x greater ground
motion and ~31.6x more energy released. A magnitude 7.0 = moderate to severe earthquake; magnitude 8+ =
major catastrophic earthquake.
- Modified Mercalli Intensity Scale: Measures the intensity (damage/experiences
at a particular location) on a scale of I–XII. Unlike Richter (fixed per earthquake), Mercalli values
change with distance from epicentre.
💥 PART 4: Effects of Earthquakes
Destructive Effects
- Ground shaking and structural damage: Buildings, bridges, roads collapse. The 2001 Bhuj
earthquake (Gujarat, India), 2015 Nepal earthquake, 2011 Japan earthquake and tsunami caused massive
destruction.
- Tsunamis: Undersea earthquakes displace water → generate massive ocean waves
(tsunamis). 2004 Indian Ocean Tsunami (triggered by 9.1 magnitude earthquake off Sumatra) killed
~2,27,000 people across 14 countries.
- Landslides: Earthquake shaking on steep hillsides triggers massive landslides.
- Liquefaction: Water-saturated loose soils lose their solid strength during shaking →
buildings sink or topple.
- Fires: Gas mains break and catch fire — impossible to fight during/after shaking. 1906
San Francisco earthquake caused devastating fires.
Constructive Effects
- Can create new hills, ridges, or change river courses.
- Can expose mineral deposits at the surface.
- Seismic waves help geologists study Earth's interior structure.
🗺️ PART 5: Earthquake Zones of the World
| Zone |
Location |
Notes |
| Circum-Pacific Belt (Ring of Fire) |
Around the Pacific Ocean |
Most active earthquake zone — accounts for ~80% of world's earthquakes. Chile, Japan,
Philippines, Alaska, western USA. |
| Alpine-Himalayan Belt (Mid-World Belt) |
From Mediterranean through Middle East to Himalayas |
Second most active zone. Italy, Greece, Turkey, Iran, Afghanistan, Pakistan, India, Nepal,
China. |
| Mid-Atlantic Ridge |
Atlantic Ocean floor mountain range |
Moderate earthquakes — plates pulling apart at divergent boundary. |
| Intra-Plate zones |
Interior of plates |
Less frequent but can be very destructive — 1819 Kutch earthquake (India), 1811–12 New Madrid
earthquakes (USA). |
🇮🇳 India's Earthquake Zones: India is divided into Seismic Zones II (low risk) to Zone V
(highest risk). Zone V includes J&K, Himachal Pradesh, Uttarakhand, parts of NE India, Kutch (Gujarat),
Andaman & Nicobar Islands. Zone IV includes Delhi, Jammu region, parts of Maharashtra. The 2001 Bhuj
earthquake (Gujarat) — 7.7 magnitude — was one of India's most devastating.
📌 Chapter Summary
- Earthquake = sudden release of energy from Earth's crust; travels as seismic waves. Main causes:
tectonic plate movement at faults.
- Focus = point underground where energy originates; Epicentre = point on surface directly above focus
(most damage).
- Seismic waves: P-waves (fastest; through solids + liquids), S-waves (slower; only through solids),
Surface waves (most destructive at surface).
- Measurement: Richter Scale (magnitude; energy released); Mercalli Scale (intensity; damage at
location, I–XII).
- Effects: Destructive (building collapse, tsunamis, landslides, fires, liquefaction); Constructive
(landform changes, mineral exposure, helps study Earth's interior).
- Zones: Ring of Fire (80% of world's EQs); Alpine-Himalayan Belt; Mid-Atlantic Ridge.