Nepal

Nepal sits directly on the Main Frontal Thrust of the Himalayas, the most active segment of the world's longest mountain-building collision zone. The Indian Plate pushes northward into Asia at 45–50 millimetres per year, and Nepal absorbs roughly 20 millimetres per year of this motion as crustal shortening across the thrust fault stack. The Main Frontal Thrust (MFT), Main Boundary Thrust (MBT), and Main Central Thrust (MCT) are the principal active [[fault]] systems, stacked from south to north as successive slices of thrust sheets that have carried Himalayan rocks hundreds of kilometres southward over the Indian Plate foreland. Paleoseismic trenching across the MFT reveals evidence of great prehistoric earthquakes exceeding magnitude 8.5, with the most recent in 1100 AD in the central Nepal segment and earlier events in approximately 1255 AD and 1344 AD. GPS measurements confirm that the Nepal Himalaya is currently locked over a large area, storing elastic strain energy that will eventually be released.

Nepal's modern seismic history is dominated by the 2015 Gorkha earthquake sequence. The April 25, 2015 mainshock (magnitude 7.8) killed nearly 9,000 people, injured 22,000, and destroyed or damaged approximately 750,000 houses. The [[epicenter]] was approximately 80 kilometres northwest of Kathmandu, and the rupture propagated eastward beneath the capital. A major [[aftershock]] of magnitude 7.3 struck on May 12, 2015, causing additional casualties and further damaging structures already weakened by the mainshock. The total [[aftershock]] sequence included over 300 magnitude 4.5+ events in the subsequent year. Earlier disasters include the 1934 Bihar-Nepal earthquake (magnitude 8.1), which killed over 10,000 people in Nepal and Bihar combined, and the 1988 Udayapur earthquake (magnitude 6.9), which killed 721 people. [[intensity]] maps from the 2015 Gorkha earthquake showed focused destruction in hilly terrain north of Kathmandu and in the Sindhupalchok district, where unreinforced stone masonry houses collapsed almost universally.

The deeper tectonic setting of Nepal's seismicity involves the architecture of the entire Himalayan collision system. The Indian Plate underthrusts Tibet at a gentle dip — approximately 5–10 degrees — beneath Nepal, creating an exceptionally long locked fault interface that can accumulate elastic strain over centuries. The geometry of this interseismic locking, inferred from GPS measurements and satellite radar, shows that the locking front passes approximately beneath the southern rim of the Kathmandu Valley, meaning that rupture of a future great earthquake would likely produce maximum ground motion close to the capital. The Kathmandu Valley itself is filled with ancient lake sediments that amplify [[seismic-wave]] energy substantially — a phenomenon well documented by strong-motion records from the 2015 event. The combination of the large locked fault area, the amplifying basin sediments, and the vulnerable building stock of Kathmandu creates a hazard scenario that earthquake engineers consider one of the most severe globally.