Seismic Swarm Near Patan, Nepal, April 2015

On 25 April 2015, a seismic swarm was recorded 5 km southeast of Patan, Nepal. The sequence began at 06:15 and concluded at 18:58 local time, lasting 12 hours and 43 minutes. Five earthquakes were registered during this period, with the largest event reaching magnitude 6.1 at a depth of 10 km. Subsequent events included magnitudes 5.6, 5.0, 5.2, and 4.2, the final shock occurring at 14 km depth.

This swarm occurred within the Himalayan orogenic belt, where the Indian Plate continues to converge northward with the Eurasian Plate at approximately 4–5 cm per year. The resulting crustal shortening is accommodated primarily along the Main Himalayan Thrust and associated splay faults. The April 2015 activity took place in a region of high seismic hazard characterized by frequent moderate-to-large thrust earthquakes.

The 2015 Gorkha earthquake sequence provides the immediate tectonic context. A magnitude 7.8 mainshock struck on 25 April 2015, followed by a magnitude 7.3 aftershock on 12 May 2015. Both events ruptured segments of the Main Himalayan Thrust east of the swarm location. The swarm’s timing and proximity indicate it formed part of the distributed aftershock field generated by stress redistribution following the mainshock.

Historical records show that Nepal has experienced repeated large earthquakes. Notable events prior to 2015 include the 1934 Bihar–Nepal earthquake (magnitude ~8.0) and the 1833 Kathmandu earthquake (magnitude ~7.7). These events demonstrate the recurring nature of great Himalayan ruptures and the potential for clustered seismic activity along the plate boundary.

Since 2000, only one swarm has been identified in the immediate area according to internal classification records. The 2015 sequence therefore represents an uncommon but documented mode of seismic energy release in this segment of the Himalaya.

References
USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
Department of Mines and Geology, Nepal (dmgnp.gov.np)