Key Points at a Glance

Earthquake — Definition and Structure

• Sudden release of energy in Earth's crust or mantle due to movement along a fault
• Creates seismic waves: P-waves, S-waves, Surface waves
• Focus/Hypocenter — underground point of energy release
• Epicentre — point directly above the focus on the surface

Earthquake Measurement Scales

• Richter Scale (logarithmic, 1935) — each unit = 10× ground motion amplitude, ~32× energy release
• Moment Magnitude Scale (Mw) — now the scientific standard globally
• Modified Mercalli Intensity Scale (I–XII) — measures felt shaking intensity at specific locations

Circum-Pacific Belt — Ring of Fire (PYQ 2021)

• Encircles the Pacific Ocean for ~40,000 km; accounts for ~80% of all world earthquakes and ~75% of all active volcanoes
• Extends: New Zealand → East Asia (Japan, Philippines, Indonesia) → Alaska → American Pacific coasts (Cascades, Andes)
• Most likely sub-topic to reappear in 2026

Earthquake Classification by Depth of Focus

• Shallow-focus (0–70 km) — most destructive; energy reaches surface quickly
• Intermediate-focus (70–300 km) — moderate surface effect; along subducting slabs
• Deep-focus (300–700 km) — felt over wide area; less catastrophic at surface; occur at subduction zones

Volcanoes by Activity Status

• Active — recently erupted (Etna, Stromboli, Kilauea)
• Dormant/Sleeping — no recent eruption but not extinct (Mt. Rainier, Vesuvius)
• Extinct — no likelihood of eruption (Edinburgh Castle Rock, Saddleback)
• India's only active volcano: Barren Island (Andaman Sea, last major eruption 2017); Narcondam is dormant

Earthquake Types by Cause

• Tectonic earthquakes — most common (~90%); caused by plate movement along faults
• Volcanic earthquakes — associated with magma movement; precede eruptions
• Collapse earthquakes — cave or mine roof collapse; small, local
• Reservoir-induced seismicity — Koyna dam (1967, Richter 6.5) — India's largest reservoir-induced earthquake; 180+ deaths

Tsunamis — Key Facts

• Seismic sea waves triggered by submarine earthquakes, underwater landslides, or volcanic eruptions
• Speed: 700–900 km/h in open ocean; wave height amplifies dramatically near shore
• 2004 Indian Ocean Tsunami (26 Dec, Mw 9.1–9.3, epicentre off Sumatra): 2.27 lakh deaths in 14 countries
• Triggered by Sunda Megathrust fault; waves reached 30 m height in Banda Aceh

India's Seismic Zones (BIS IS-1893)

• India divided into Zones II, III, IV, and V (Zone V = highest risk; Zone I deprecated)
• Zone V (very high seismicity): entire northeast, J&K, Himachal Pradesh, Uttarakhand, northern Bihar, Andaman
• Rajasthan: mostly Zone II–III (moderate-low); Jaisalmer area is Zone II (lowest)

Major Recent Earthquakes for RPSC

• Nepal (April 2015, Mw 7.8, ~9,000 deaths)
• Türkiye-Syria (Feb 2023, Mw 7.8+7.7, ~58,000 deaths)
• Japan 2011 (Mw 9.0 Tōhoku, 15,000+ deaths, Fukushima nuclear disaster)
• Gujarat Bhuj (26 Jan 2001, Mw 7.7, ~20,000 deaths)

Volcanic Products

• Lava — molten rock at surface; temperature 700–1,200°C
• Magma — molten rock below surface
• Pyroclastic material — tephra, bombs, lapilli, ash
• Volcanic gases — SO₂, CO₂, H₂S, water vapour; Lahars — volcanic mudflows
• Basaltic lava flows at 10–30 km/h; rhyolitic lava barely moves (very viscous)

Mediterranean-Himalayan (Alpide) Belt

• Second major earthquake/volcanic zone; accounts for ~15% of world earthquakes
• Runs from Atlantic → Mediterranean → Middle East → Himalayas → Southeast Asia
• Caused by Africa-Eurasia and Indian-Eurasian plate collisions
• Major events along this belt: Bhuj 2001, Nepal 2015, Turkey 2023

Positive Impacts of Volcanoes

• Fertile soils — basaltic volcanic soils richest for agriculture; Java/Indonesia rice productivity exceptional
• Geothermal energy — Iceland generates 66% electricity and 90% home heating from geothermal
• Mineral deposits — copper, gold, sulphur associated with volcanic activity
• New land creation — Hawaiian Islands created entirely by volcanic activity