---
title: "Short note: Seismicity"
subject: "Engineering Geology II"
chapter: 5
key: "semester_4/engineering_geology_ii:short-note-seismicity"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/short-note-seismicity
site: BCT Engineering
---

# Short note: Seismicity

Engineering Geology II · Chapter 5: Geological Hazards

- Asked in: 2079 Jestha, 2 marks
- Video: [Earthquakes and Seismology in Earth’s Interior](https://www.youtube.com/watch?v=Prey5z8WQiU) — Professor Dave Explains
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/short-note-seismicity
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-5

## Question

Write a short note on seismicity.

## Short answer

Seismicity denotes the frequency, distribution and magnitude of earthquakes in an area, used for hazard assesment. Most events occur along plate boundaries; Nepal's high seismicity stems from the Indian-Eurasian collision, with major faults and past great earthquakes indicating a potential seismic gap.

## Answer

**Seismicity** is the frequency, distribution and magnitude of earthquakes occurring in a region over a period of time. It shows how active an area is and is the basis of seismic hazard assessment.

- Earthquakes are measured by **magnitude** (energy released, Richter or moment magnitude) and **intensity** (effects at a place, Modified Mercalli scale).
- Seismicity follows the Gutenberg-Richter relation $\log N = a - bM$, where $N$ is the number of events with magnitude $\geq M$.
- Seismicity is shown on **epicentre maps** and **seismic zoning maps**. Most earthquakes occur along plate boundaries and active faults.

**Nepal:** the Himalaya is one of the most seismically active belts because of the collision of the Indian and Eurasian plates. The activity is concentrated along the Main Central Thrust, Main Boundary Thrust and Main Frontal Thrust. Great events in 1255, 1344, 1408, 1833, 1934 and 2015 show a repeat cycle, and the central Himalaya is regarded as having a seismic gap that may still hold stored strain.

Seismicity data are used to design earthquake-resistant structures (NBC 105) and to select safe sites for dams, tunnels and cities.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does seismicity describe?
   A. The depth of the Moho
   B. The frequency, distribution and magnitude of earthquakes in a region over time
   C. The speed of seismic waves only
   D. The strength of a building
   Answer: B. Seismicity shows how active an area is and is the basis of seismic hazard assessment. (Key point: Definition)

2. Which pair is correct?
   A. Magnitude measures effects at a place; intensity measures energy
   B. Both measure energy
   C. Magnitude measures energy released; intensity measures effects at a place (Modified Mercalli)
   D. Both measure effects at a place
   Answer: C. Magnitude (Richter/moment) is energy; intensity (MMI) is local effects. (Key point: Magnitude vs intensity)

3. In log N = a - bM, what is N?
   A. Number of events with magnitude greater than or equal to M
   B. Total depth of events
   C. Number of faults
   D. Intensity at the epicentre
   Answer: A. N counts events of magnitude at least M; larger events are fewer. (Key point: Gutenberg-Richter)

4. Where is Himalayan seismic activity concentrated?
   A. Along the Indus river
   B. Along the Main Central, Main Boundary and Main Frontal Thrusts
   C. Only in the Terai plains
   D. Along volcanic arcs
   Answer: B. The thrust system from plate collision hosts most activity, with a repeat cycle of great events. (Key point: Nepal seismicity)