The 2015 Nepal Earthquake: Geological Setting and Event Details

The magnitude 7.8 earthquake that struck 67 km NNE of Bharatpur, Nepal, on 25 April 2015 at 06:11 UTC represents one of the most significant seismic events in the region in recent decades. The earthquake occurred at a shallow depth of 8.2 km, consistent with typical rupture characteristics along the Himalayan thrust system. Nepal lies at the convergent boundary where the Indian tectonic plate collides with the Eurasian plate, driving the ongoing uplift of the Himalayas at rates of approximately 4–10 mm per year. This collision produces the Main Himalayan Thrust, a major décollement that accommodates much of the convergence and periodically releases strain through large earthquakes. The 2015 event nucleated within this fold-and-thrust belt, where the Indian plate underthrusts the Himalaya. Historical records document recurrent great earthquakes in Nepal, including the 1934 Bihar-Nepal event (estimated Mw 8.0–8.4) and earlier events inferred from paleoseismic trenches. These quakes demonstrate a pattern of strain accumulation and release along the locked portion of the Main Himalayan Thrust. Since 2000, the 2015 mainshock stands as the sole magnitude 7.8 event recorded in the region, highlighting its exceptional size relative to background seismicity. The shallow focal depth amplified ground shaking across the Kathmandu Valley and surrounding districts, where thick sedimentary basins further intensified local intensities. Aftershocks, including a Mw 7.3 event on 12 May 2015, delineated the rupture extent along the thrust interface. Ongoing monitoring by regional seismic networks continues to refine understanding of post-seismic deformation and stress transfer along the Himalayan arc, informing updated hazard assessments for central Nepal.

References

USGS Earthquake Catalog (event parameters).
Geological Survey of India and Department of Mines and Geology, Nepal (regional tectonic framework).