Earthquakes

🧾 1. Definition and Causes

An earthquake is the shaking of the Earth caused by the release of energy in the Earth's crust.
The energy release generates seismic waves that travel in all directions.
Most earthquakes occur along faults due to tectonic movements.

🔄 Causes of Earthquakes

Cause	Details
Tectonic movement	Sudden slip along faults due to crustal stress.
Volcanic activity	During violent eruptions, e.g., in volcanic regions (volcanic earthquakes).
Collapse of underground mines	Called collapse earthquakes.
Explosion earthquakes	Result from nuclear or chemical explosions.
Reservoir-induced earthquakes	Large man-made reservoirs create pressure changes in Earth's crust.
Folding, faulting	Movement or displacement in rock strata.

📍 2. Focus and Epicentre

Term	Definition
Focus / Hypocentre	Point inside the Earth where energy is released (origin of quake). Usually ≤60 km deep.
Epicentre	Point on Earth's surface directly above the focus. Most severe shaking occurs here.

Earthquakes occur in the lithosphere (up to 200 km deep).
Seismic waves radiate outward from the focus in all directions.

🌊 3. Earthquake Waves

A. Types of Seismic Waves

Wave Type	Sub-Type	Description
Body Waves	P-waves	Primary waves, fastest, travel through solids, liquids, gases. Vibration is parallel to wave direction.
	S-waves	Secondary waves, slower, travel only through solids. Vibration is perpendicular to wave direction.
Surface Waves	-	Slowest but most destructive. Cause horizontal and vertical ground motion.

📊 4. Seismograph and Wave Recording

A seismograph is an instrument that records earthquake waves.
It displays three distinct wave sections: P, S, and Surface waves.
Seismographs help determine the epicentre and magnitude of an earthquake.

🚫 5. Shadow Zones

Zone	Wave Behavior
Within 105°	Both P and S waves detected.
Between 105°–145°	No P or S waves recorded (shadow zone).
Beyond 145°	Only P-waves recorded; S-waves blocked (they can’t pass through liquids).

Shadow zones helped confirm the liquid nature of the Earth’s outer core.

📏 6. Measuring Earthquakes

Scale	Measures	Range	Type
Richter Scale	Energy released (magnitude)	0 to 10	Logarithmic
Mercalli Scale	Damage and effect (intensity)	I to XII	Subjective, visual

⚠️ 7. Effects of Earthquakes

A. On Ground and Landscape

Ground shaking and displacement
Landslides, mudslides, avalanches
Soil liquefaction
River course changes and reservoir formation

B. On Man-Made Structures

Collapse of buildings, bridges, roads
Damage to dams, pipelines, railways
Fire outbreaks due to gas leaks
Disruption of electricity, communication, and transport

C. On Water Bodies

Tsunamis: Triggered if epicentre is under ocean floor (Richter 5+).
Cause sea-floor displacement and powerful waves.
Can travel thousands of km at high speed.

D. Socio-Environmental Impact

Homelessness, fear, trauma
Epidemics due to poor sanitation
Destruction of infrastructure and socio-economic losses

🌐 8. Global Distribution of Earthquakes

🔥 Earthquake Belts

Region	Characteristics
Circum-Pacific Belt	“Ring of Fire”; around the Pacific Ocean; many volcanoes and deep quakes.
Alpine-Himalayan Belt	Includes Himalayas and Alps; high seismicity due to plate collisions.
Mid-Oceanic Ridges	Shallow earthquakes; divergent boundaries in oceans like Atlantic and Indian.

🇮🇳 9. Earthquakes in India

📍 Seismic Zones of India (Zone V to Zone II)

Zone	Risk Level	Regions
Zone V	Very High Risk	North-east India, Northern Bihar (Darbhanga, Araria), Uttarakhand, Kashmir Valley, Kutch
Zone IV	High Risk	Remaining J&K, Himachal Pradesh, Delhi, Northern UP, Eastern Haryana, Punjab
Zone III	Moderate Risk	Parts of Gujarat, Maharashtra, Goa, Kerala, West Bengal
Zone II & I	Low/Very Low Risk	Parts of Central and Southern India (Peninsular region)

Himalayan arc and Northeast India are most active due to Indian Plate movement.

🛠️ 10. Earthquake Hazard Mitigation

✅ Preparedness Measures

Strategy	Details
Seismological Centres	For monitoring and early warning.
Vulnerability Mapping	Identify and educate at-risk zones.
Building Design Modifications	Encourage earthquake-resistant structures.
Construction Regulation	Discourage high-rise, heavy construction in high-risk areas.
Lightweight Materials	Use in buildings to reduce damage risk.
Public Awareness & Drills	Community education for evacuation and safety.

Phenomenon	Link to Earthquakes
Tsunami	Undersea earthquakes displace ocean floor → water displacement → massive sea waves.
Landslides	Triggered by ground shaking in hilly terrain (Himalayas, Western Ghats).
Volcanic Activity	Often linked, especially in tectonic zones like the Pacific Ring of Fire.