Earthquakes
What is an Earthquake?
An earthquake is defined as a tremor below the surface of the Earth that causes shaking of the crust. These devastating tremors are caused by stresses in the crust that build up and are released suddenly.
About 100,000 earthquakes occur every year worldwide (as stated in the transcript).
Causes of Earthquakes
(i) Volcanoes
Volcanic eruptions are one of the main causes of earthquakes. Hot gases push the crust upward and cause seismic shocks.
The occurrence is interrelated: an earthquake may follow a volcanic eruption and a volcanic eruption may follow an earthquake.
Example: The violent eruption of Krakatau in the Sunda Strait caused severe earthquakes around the region.
Tsunami can be generated by volcanic activity as well as by earthquakes.
(ii) Plate Tectonics
The Earth’s crust consists of moving plates. The edges slide past one another or collide, creating faults and earthquakes (tectonic earthquakes).
Most earthquakes occur at plate boundaries; they are especially common along the Pacific Ring of Fire, including regions like the western edge of the Pacific Ocean.
Notable India example: Bhuj, Gujarat (2001) occurred due to the Indian plate dipping below the Asiatic plate.
(iii) Folding and Faulting
Internal horizontal and vertical movements due to compressional and tensional forces cause folding and faulting in the crust, displacing rocks.
This imbalance triggers earthquakes. Example: San Andreas Fault, California (USA) 1906, segment about 430 km long, magnitude ~7.8 on the Richter scale; followed by a devastating fire and a destruction zone about 400 miles by 25 miles.
(iv) Man-made Causes (Anthropogenic)
Human activities near fault zones can trigger earthquakes: large dams, nuclear explosions, blasting of rocks, mining, etc.
Examples referenced include the Koyna dam in Maharashtra and underground chemical wastes, which have been linked to seismic events.
Anatomy of an Earthquake
The tension from plate movement builds up and is released as powerful vibrations that travel as waves.
The three main types of waves:
(i) P-waves (Primary waves):
Recorded first on seismograms.
Compressional waves, particles move in the direction of wave travel.
Can travel through solids, liquids, and gases.
(ii) S-waves (Secondary or Shear waves):
Recorded after P-waves.
Distortional waves; particles vibrate at right angles to the direction of travel.
Travel through the Earth's interior but cannot pass through liquids.
(iii) L-waves (Surface or Long waves):
Travel along the Earth's surface and are recorded after P and S waves.
Two kinds: Love waves and Rayleigh waves.
Focus and Epicentre:
Seismic focus: the point within the Earth where the earthquake waves originate.
Epicentre: the point on the Earth’s surface directly above the seismic focus.
Depth: Most earthquakes have a seismic focus at depths between 8 and 35 km.
Movement through the Earth:
P-waves and S-waves travel through the interior of the Earth.
L-waves travel along the surface and are often responsible for the most intense ground motion felt near the surface.
Seismographs and Measurement Scales
Seismograph: An instrument with a pen attached that records ground motion on a moving strip of paper. It detects and records the arrival times of seismic waves and calculates the P–S arrival time interval to estimate distance to the epicentre.
Richter Scale (Magnitude):
Measures the energy/power of an earthquake on a scale roughly from 1 to 9.
Each whole-number increase represents about a ten-fold increase in the amplitude of the shock waves.
Relationship between amplitudes: if magnitudes are M1 and M2, the amplitude ratio is
Mercalli Scale (Intensity):
A 12-point scale based on observed effects and damage.
The Richter scale is more commonly used for magnitude, whereas Mercalli focuses on observed intensity.
Depth and location information are inferred from P–S timing and wave behavior.
Notable magnitudes and examples:
Assam earthquake (1897): magnitude ~8.7 on the Richter scale.
Gujarat earthquake (2001): magnitude ~7.7 on the Richter scale but caused extensive damage.
Nepal earthquake (2015): magnitude ~7.8 on the Richter scale; major aftershock on May 12, 2015 with magnitude ~7.3.
Earthquake Scales: Magnitude and Energy
Moment Magnitude (Mw) vs Richter Magnitude:
Mw is considered the most accurate and reliable for large earthquakes; based on the same underlying principle as the Richter scale.
Mw measures the total energy released during an earthquake.
The text notes that Mw is preferred for comparing large events.
The text references Fig. 8.4 (Earthquake Magnitude Scale) and how it relates to observed damage.
Distribution of Earthquakes and Major Belts
Most earthquakes originate at plate boundaries.
Submarine ridges form where oceanic plates converge with continental plates, creating unstable rocks that trigger earthquakes.
Major earthquake belts:
(i) Circum-Pacific Mountain Belt (the Pacific Ring of Fire): ~70% of all earthquakes; includes California (San Andreas Fault), Japan, the Philippines, Indonesia, and related regions.
(ii) Mid-World Mountain Belt: extends from Eastern Europe through the Alpine–Himalayan ranges; about 20% of earthquakes.
(iii) Mid-Atlantic Ridge: along mid-ocean ridges and adjacent islands; about 10% of earthquakes; the East African Rift Valley is considered an extension of this belt.
The text describes the distribution with reference to global tectonics and plate boundaries.
Tsunamis
Definition: A tsunami is a series of waves generated by a disturbance that rapidly displaces a large mass of water.
Etymology: The term comes from Japanese words "tsu" (harbour) and "nam(i)" (wave) meaning harbour waves.
Primary causes: Undersea earthquakes are the most common trigger; tsunamis can also be caused by undersea landslides, volcanic eruptions, or asteroid impacts.
Geographic occurrence: Most frequent in the Pacific Ocean but can occur globally; can occur in inland waters like large lakes if landslides occur.
Notable events referenced:
2004 Indian Ocean earthquake: triggered a devastating tsunami that killed approximately 275,000 people.
2018 Java–Sumatra event: a tsunami in Java and Sumatra in Indonesia on December 22, 2018, resulted in more than 300 deaths (linked to Anak Krakatau eruption in the Sunda Strait).
Other Effects of Earthquakes
Constructive effects (geological perspective):
Energy release helps the Earth release stored energy; most earthquakes occur around plate margins.
These processes contribute to long-term geological reshaping and faulting.
Sometimes vertical/displacement changes can uplift land (e.g., Sagami Bay uplift of ~200 m in 1923).
Destructive effects (human-focused):
Collapse of structures due to ground shaking; leading to loss of life and property.
Submergence: uplift/subsidence can cause coastal areas to submerge (e.g., Dwarka, Gujarat, now submerged).
Changes in river courses can render areas unsuitable for irrigation/agriculture.
Direct danger to human life from falling structures, landslides, and associated hazards.
Landslides destabilize rock and soil, damaging buildings and transport networks (Himalayas, Alps, Andes).
Fires can start from broken gas lines, ruptured oil refineries, or other flammable materials.
Tsunamis and landslides can cause flash floods; strong ground shaking can damage dams and embankments and divert river flows.
Predicting Earthquakes
Historical context: Earthquake prediction moved from astrology to scientific pursuit.
Key scientific approaches:
P- and S-waves discovery (1960s): These seismic waves form the basis of earthquake prediction efforts.
Plate tectonics provides a framework for anticipating where earthquakes are more likely (e.g., at plate boundaries).
Japan’s approach includes measuring sea-level changes and variations in Earth's magnetic field to predict earthquakes.
Limitations: No method is foolproof; earthquakes continue to occur unpredictably in many regions, as seen in Japan, India, and the USA.
Top 10 Earthquakes Ever Recorded (Mw scale reference)
The text introduces a list of the top 10 earthquakes by moment magnitude (Mw), noting that Mw is used because it provides a more accurate measure for large events than the traditional Richter scale.
Examples listed in the transcript include:
Valdivia, Chile — 22-05-1960 — Mw in the vicinity of 9.1–9.5 (as given in the text: "9.1 to 9").
Alaska, USA — 27-03-1964 (date shown; Mw around 9+).
Sumatra, Indonesia — 26-12-2004 — Mw around 9.1–9.3.
Tohoku, Japan — 11-03-2011 — Mw around 9.0.
Kamchatka, Russia — 05-11-1952 — Mw around 9.0.
Chile — 27-02-2010 — Mw around 8.8.
Rat Island (Alaska) — 03-02-1965 — Mw around 8.6–8.7.
Assam–Tibet region — 15-08-1950 — Mw around 8.6.
Aleutian Islands — 01-04-1946 — Mw around 8.6.
Indian Ocean event — 11-04-2012 — Mw around 8.6.
Note: The transcript lists magnitudes and dates with some formatting irregularities; the items above reflect the locations and dates as presented, with Mw values inferred from the context of well-known large events.
Predictive and Practical Implications
Predicting earthquakes remains imperfect; scientific methods improve understanding of risks but cannot reliably forecast precise events.
Preparedness and engineering practices (building codes, early warning systems, land-use planning) are essential to mitigate impacts in high-risk regions.
Quick Reference: Key Definitions and Numbers
Seismic focus: the origin point within the Earth where earthquakes originate.
Epicentre: the point on the Earth's surface directly above the seismic focus.
Depth range for most earthquakes:
Types of seismic waves:
P-waves: fast, travel through solids, liquids, gases; compressional.
S-waves: slower, travel only through solids; shear.
L-waves: surface waves; Love and Rayleigh; cause significant ground motion.
Magnitude vs Intensity:
Magnitude (Richter/Mw): measures energy released; typically on a 1–9 (Richter) or Mw scale for large quakes.
Intensity (Mercalli): observed effects, 12-point scale.
Relationship of magnitude increments to amplitude:
Major earthquake belts: Circum-Pacific Belt (~70%), Mid-World Mountain Belt (~20%), Mid-Atlantic Ridge (~10%).
Notable consequences: 2004 Indian Ocean tsunami (≈275,000 deaths), 2018 Java–Sumatra tsunami (hundreds of deaths).