Earthquakes — Types, Measurement, and Global Distribution

2.1 Causes and Types of Earthquakes

Tectonic Earthquakes (most common — ~90%)

Caused by movement along geological faults where stress accumulates until it exceeds the friction holding rocks together — then sudden slip occurs, releasing energy as seismic waves.

• At divergent boundaries — Normal faults (tensional); relatively minor earthquakes (e.g., Mid-Atlantic Ridge, Iceland)
• At convergent boundaries — Thrust faults (compressional); most powerful earthquakes (Mw 9+); subduction zone megathrusts (Japan 2011, Indonesia 2004, Chile 1960)
• At transform boundaries — Strike-slip faults; powerful, shallow earthquakes (San Andreas Fault, California; North Anatolian Fault, Turkey)

Volcanic Earthquakes

Triggered by magma movement through rock — swarms of small earthquakes precede volcanic eruptions (key early warning signal).

• Mt. St. Helens (1980, USA) — earthquakes gave warning before eruption
• Pinatubo (1991, Philippines) — warning allowed millions to evacuate

Collapse Earthquakes

Occur when underground caves or mine tunnels collapse. Small magnitude, very local effect.

Reservoir-Induced Seismicity

Large reservoirs increase pore water pressure in rocks, lubricating pre-existing faults.

• Koyna Earthquake (1967, Maharashtra): Mw 6.5 — triggered by Koyna Dam reservoir; India's largest reservoir-induced earthquake; 180+ deaths
• Wenchuan, China (2008) possibly reservoir-influenced

2.2 Earthquake Measurement Scales

Richter Local Magnitude Scale (ML)

• Developed by Charles F. Richter in 1935 at Caltech using seismographs
• Logarithmic scale: Each 1.0 increase = 10× ground motion amplitude and approximately 31.6× energy released
• Practically used for earthquakes up to Mw ~7.0; less accurate for very large events

Moment Magnitude Scale (Mw) — Current Standard

• Developed 1970s; based on seismic moment = rigidity of rock × area of fault × average slip
• More accurate for large earthquakes; now the scientific standard globally
• Key values: 2004 Indian Ocean = Mw 9.1–9.3; 2011 Tōhoku Japan = Mw 9.0; 1960 Chile (largest ever) = Mw 9.5

Modified Mercalli Intensity Scale (MMI)

• Scale I to XII measuring subjective felt shaking intensity at a specific location
• I = Not felt; IV = Felt indoors; VII = Standing difficult; X = Most masonry destroyed; XII = Total destruction
• Different from magnitude — the same earthquake may have intensity X near epicentre and IV at 200 km away

Magnitude (Mw)	Description	Energy Equivalent	Average Annual Frequency
< 3.0	Micro	—	~8,000/day
3.0–3.9	Minor	—	~1,000/day
4.0–4.9	Light	6 tonnes TNT	~135/day
5.0–5.9	Moderate	199 tonnes TNT	~1,319/year
6.0–6.9	Strong	4 kiloton TNT	~134/year
7.0–7.9	Major	32 kiloton TNT	~17/year
8.0–8.9	Great	1 megaton TNT	~1/year
9.0+	Exceptional	32 megatons TNT	Rare — few per century

2.3 Earthquake Focus Depth Classification

Type	Depth	Location	Characteristics
Shallow-focus	0–70 km	All plate boundaries; intraplate zones	Most common; most destructive (energy reaches surface quickly)
Intermediate-focus	70–300 km	Subduction zones	Moderate surface effect; occur along subducting slabs
Deep-focus	300–700 km	Deep subduction zones (Pacific rim)	Felt over vast area; less destructive at surface due to distance

2.4 Seismic Wave Types

Body Waves (travel through Earth's interior)

• P-waves (Primary/Compressional) — Push-pull motion; fastest (6–8 km/s in crust, 13 km/s in mantle); travel through solids, liquids, gases; arrive first at seismographs; less destructive
• S-waves (Secondary/Shear) — Side-to-side motion; slower (3.5–5 km/s); travel only through solids; arrive second; more destructive than P-waves; do not pass through outer core

Surface Waves (travel along Earth's surface)

• Love Waves — Horizontal shear movement; fast; highly destructive to buildings
• Rayleigh Waves — Elliptical rolling motion (like ocean waves); slower; cause most earthquake damage

2.5 Global Distribution — Seismic Belts

1. Circum-Pacific Belt — "Ring of Fire"

• ~80% of all world earthquakes; ~75% of all active volcanoes
• Follows the Pacific Plate margins where it converges with surrounding plates
• Extends 40,000 km in a rough horseshoe shape: New Zealand → Pacific Islands → East Asia (Tonga, Samoa, Fiji) → Indonesia → Philippines → Japan → Kuril Islands → Kamchatka → Aleutian Islands → Alaska → Pacific Coast of North and South America (Cascades, Andes)

Key facts about this belt:

• Countries with highest earthquake frequency: Japan (~1,500 earthquakes/year), Indonesia, Philippines, USA (Alaska/West Coast), Chile, Peru
• Home to world's deepest ocean trench: Mariana Trench (11,034 m, Pacific) near Japan
• World's most active individual volcano: Kilauea (Hawaii — though Hawaii is a hotspot, not subduction)

2. Mediterranean-Himalayan (Alpide) Belt

• ~15% of world earthquakes
• Extends from Atlantic coast of Portugal → Mediterranean → Middle East → Iran → Afghanistan → Himalayan arc → Southeast Asia (connecting to Circum-Pacific)
• Caused by Africa-Eurasia and Indian-Eurasian plate collisions

Major earthquakes along this belt:

• Turkey (2023, Mw 7.8), Iran (2003 Bam, Mw 6.6; 26,271 deaths), Nepal (2015, Mw 7.8), India Bhuj (2001, Mw 7.7), Kashmir (2005, Mw 7.6)
• Includes active volcanoes: Etna (Italy), Vesuvius (Italy), Santorini (Greece), Elbrus (Russia)

3. Mid-Oceanic Ridge Belt

• ~5% of world earthquakes
• Along divergent plate boundaries — Mid-Atlantic Ridge, Indian Ocean Ridge, East Pacific Rise
• Mostly shallow, moderate earthquakes; less destructive (remote ocean locations)
• Iceland sits on Mid-Atlantic Ridge — volcanically very active; associated volcanoes are effusive (basaltic lava)

2.6 India's Seismic Zones

The Bureau of Indian Standards (BIS) IS-1893 classifies India into seismic zones based on historical earthquake data and tectonic setting:

Zone	Risk Level	States/Regions
Zone V	Very High (Highest)	Entire NE India, J&K, Himachal Pradesh, Uttarakhand, northern Bihar, Andaman & Nicobar Islands
Zone IV	High	Delhi, Sikkim, northern UP, Kutch (Gujarat), Himalayas foothills
Zone III	Moderate	Rest of Gujarat, western Rajasthan, parts of MP, Maharashtra (Pune, Nashik)
Zone II	Low (Lowest)	Most of Rajasthan (Jaisalmer, Barmer, Bikaner region), southern India, central India

Rajasthan's Seismicity

Mostly Zone II–III. The 2001 Bhuj earthquake (Zone V, Gujarat) did cause tremors in Rajasthan. The Aravalli-Delhi fold belt is a seismically sensitive zone with ancient faults.

Major Indian Earthquakes

• Bhuj, Gujarat (26 Jan 2001): Mw 7.7; ~20,000 deaths; 400,000 homes destroyed; caused by the East-Patli fault in Kachchh Rift Basin
• Kashmir (8 Oct 2005): Mw 7.6; ~80,000+ deaths (combined Pakistan-India); Himalayan collision zone
• Latur, Maharashtra (1993): Mw 6.2; ~10,000 deaths — unusual intraplate event (Deccan Plateau); triggered concern about intraplate seismicity
• Koyna, Maharashtra (1967): Mw 6.5; ~180 deaths; reservoir-induced; still ongoing moderate seismicity

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