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Earthquakes

ICSE · Class 9 · Geography

Quick revision notes for Earthquakes — ICSE Class 9 Geography. Key concepts, formulas, and definitions for last-minute revision.

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A cross-section diagram showing the Earth's crust and mantle, clearly indicating the seismic focus (hypocenter) where an earthquake originates, and the epicenter on the Earth's surface directly above
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Key Topics to Revise

1

Key Terminology – Focus, Epicentre, and Seismograph

  • The FOCUS (also called the hypocentre) is the point inside the Earth's crust or mantle where the earthquake originates — where rocks first begin to fracture.
  • The EPICENTRE is the point on the Earth's surface directly above the focus. The intensity of shaking is maximum at the epicentre.
  • Seismic waves (shock waves) travel outward from the focus in all directions and are recorded by a Seismograph (also called Seismometer).
2

Causes of Earthquakes

  • There are four main causes of earthquakes: (i) Volcanic Eruptions, (ii) Folding and Faulting, (iii) Plate Tectonics, and (iv) Anthropogenic (human-made) Factors.
  • VOLCANIC ERUPTIONS: Gas explosions during volcanic activity trigger earthquakes. The eruption of Krakatoa (between Sumatra and Java) caused 30–40 metre high tsunamis, killing over 36,000 people. Its i
  • FOLDING AND FAULTING: A fault is a fracture in rock along which observable displacement has occurred. Earthquakes along faults are called tectonic earthquakes. The San Andreas Fault in California caus
3

Measurement of Earthquakes – Richter Scale and Intensity

  • The magnitude of an earthquake is most commonly measured by the RICHTER SCALE, devised by Charles F. Richter in 1935 and modified in 1965 by Richter and Beno Gutenberg.
  • The Richter Scale is an open-ended, logarithmic scale — it has no fixed maximum or minimum.
  • The scale relates to the energy released at the earthquake centre and estimates the severity.
4

Distribution of Earthquakes – Major Earthquake Belts

  • Earthquakes follow a definite pattern of distribution worldwide. There are THREE major earthquake belts and one additional zone.
  • 1. THE CIRCUM-PACIFIC BELT (Pacific Ring of Fire): The most active belt; accounts for about 66% of all earthquakes. It extends along the west coasts of North and South America, through Alaska, Japan,
  • 2. THE MID-ATLANTIC BELT: Extends along mid-oceanic ridges in the Atlantic Ocean. Caused by seafloor spreading (divergent plate boundaries). Earthquakes are shallow focus (less than 70 km) and of mode

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Full Notes

Key Concepts

An earthquake is a sudden vibrationThere are four main causesThe magnitude of an earthquakeEarthquakes follow a definite geographic patternEarthquakes cause wide

Frequently Asked Questions

What are the important topics in Earthquakes for ICSE Class 9 Geography?
Key topics in Earthquakes include Earthquakes – Complete Chapter Overview, Earthquakes — Chapter Overview Mind Map, Earthquakes – Full Chapter Concept Hierarchy. These are the concepts ICSE Class 9 examiners draw on most — study them first, then practise related questions.
How to score full marks in Earthquakes — ICSE Class 9 Geography?
Start by understanding all key concepts. Practise previous year questions from this chapter. Revise formulas and definitions regularly. Use flashcards for quick revision before the exam.

Sources & Official References

Content is aligned to the official syllabus. Refer to the board website for the latest curriculum.

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