Short note: Seismicity

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.