Earthquakes
Rupture Mechanics · Five Genetic Types · Magnitude vs. Intensity · Seismic Zoning (28 Micro-Realms)
The Sudden Release of Planetary Strain
An earthquake is simply the shaking of the Earth caused by a sudden release of energy along a fault plane. From the subterranean Focus (Hypocentre), seismic waves radiate in all directions to strike the surface Epicentre, producing tremors classified by origin (Tectonic, Volcanic, Collapse, Explosion, and Reservoir-Induced) and measured via logarithmic Magnitude (Richter) and observational Intensity (Mercalli).
I. Rupture Mechanics, Focus & Epicentre Geometry
What is an Earthquake?
- Shaking of the Earth (NCERT)
- Release of accumulated strain
- Waves travel in all directions
Focus (Hypocentre)
- Point where energy is released
- Also called the Hypocentre
- Subsurface seismic origin point
The Epicentre
- Point on the surface directly above
- First to experience the waves
- Directly perpendicular to Focus
Quake Cluster Sequences
- Foreshocks: Warning tremors
- Mainshock: Highest magnitude
- Aftershocks: Stress adjustment
Focal Depth Zones
- Shallow (0–70 km): 85% of total
- Intermediate (70–300 km)
- Deep-focus (300–700 km slab)
Rupture Directivity
- Rupture unzips along fault
- Doppler energy concentration
- Supershear exceeds S-speed
Seismogram Recording
- Inertial mass pendulum rig
- Records ground motion amplitude
- Yields P, S and Surface phases
II. Types of Earthquakes: The Five NCERT Categories
1. Tectonic Earthquakes
- Most common type (NCERT)
- Generated by sliding of rocks
- Along a geological fault plane
2. Volcanic Earthquakes
- Special class of tectonic quake
- Confined to active volcanoes
- Harmonic tremor & conduit gas
3. Collapse Earthquakes
- Areas of intense mining activity
- Roofs of underground mines collapse
- Produces minor tremors (NCERT)
4. Explosion Earthquakes
- Detonation of chemical explosives
- Or nuclear underground devices
- Pokhran-I & II seismic triggers
5. Reservoir-Induced (RIS)
- Occurs in areas of large reservoirs
- Pore-pressure reduces fault friction
- Koyna Dam, Maharashtra (1967 · M6.3)
Isostatic Rebound Quakes
- Melting of Pleistocene ice sheets
- Crust springs back upward
- Scandinavia & Canada intraplate
Intraplate Quakes (Peninsula)
- Within stable continental cratons
- Reactivated ancient rift zones
- Latur (1993) & Bhuj (2001) in India
III. Measuring Earthquakes: Magnitude (Richter) vs. Intensity (Mercalli)
Magnitude vs. Intensity
- Magnitude: Absolute energy source
- Intensity: Visible surface damage
- Single magnitude, varied intensity
Richter Scale (0–10)
- Charles Richter (1935)
- Relates to energy released (NCERT)
- Expressed in numbers 0–10
Logarithmic Energy Jumps
2 UNITS = 1,000x
- Wave amplitude jumps 10x per unit
- Energy jumps $\approx 31.6\times$ ($10^{1.5}$)
- M7 releases $1,000\times$ energy of M5
Moment Magnitude ($M_w$)
- Hiroo Kanamori & Thomas Hanks
- Replaces Richter for $M > 7.5$
- Measures fault area $\times$ slip length
Mercalli Scale (I–XII)
- Giuseppe Mercalli (1902)
- Takes into account visible damage
- Range from 1 to 12 (Roman I–XII)
Isoseismal Lines & Soil
- Contours of equal shaken intensity
- Site amplification in soft silt
- Bedrock shakes less than alluvium
Soil Liquefaction
- Saturated loose sand loses shear
- Behaves like a boiling liquid
- Buildings sink, sand boils erupt
IV. Earthquake Hazards, Tsunami Trigger & Indian Seismic Zones
NCERT 12 Hazard Impacts
- Ground shaking & settlement
- Landslides, mudslides, avalanches
- Fires, floods & structural collapse
Tsunami Generation
- Triggered when epicentre is in ocean
- Magnitude exceeds $M > 5.0$ (NCERT)
- Vertical seabed displacement wave
BIS Seismic Zonation (IS 1893)
- Bureau of Indian Standards map
- Zone I merged into Zone II
- Classified from Zone II to Zone V
Seismic Zone V (Very High)
- Entire North-East India & J&K/Ladakh
- Himachal, Uttarakhand & N. Bihar
- Rann of Kutch & Andaman-Nicobar
Zones IV, III & II
- Zone IV: Delhi-NCR, Gangetic basin
- Zone III: Western coast, Mumbai, Kolkata
- Zone II: Stable Peninsular plateau
Early Warning (EEW)
- P-waves travel faster than S-waves
- Sensors detect P-wave arrival
- Sends seconds of alert before S hits
Aseismic Engineering
- Base isolators & elastomeric pads
- Tuned mass dampers in skyscrapers
- Ductile shear-wall design
Core Distinctions
Focus (Hypocentre): The subterranean point where the actual fracture begins and energy is released. Epicentre: The point on the Earth's surface directly, perpendicularly above the Focus; it is the first surface point to experience seismic waves.
2. Magnitude (Richter) vs. Intensity (Mercalli)• Richter Scale: Measures the magnitude (absolute energy released at the source); expressed in whole numbers 0–10; logarithmic scale where each whole unit increase represents a $\sim 31.6\times$ jump in energy.
• Mercalli Scale: Measures the intensity (visible destruction and human impact); expressed in Roman numerals I–XII (1 to 12); intensity decreases with distance from the epicentre.
1) Tectonic (sliding along faults — most common), 2) Volcanic (confined to active eruptive areas), 3) Collapse (mining cavern cave-ins), 4) Explosion (chemical or nuclear blasts), and 5) Reservoir-Induced (stress from artificial dam impoundments, e.g., Koyna 1967).
Seismic Zones of India
Bureau of Indian Standards (BIS IS 1893:2016)