Location:

Period:

25 Apr 2015 06:11:25 - 26 Apr 2015 16:26:06 (1 day 10 hours 14 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

M 7.3; Nepal; (12 May 2015) (30km from the swarm center)

4 swarms found nearby.

2015

PS20150425.2(63.5km)

25 Apr

12 hours

5 earthquakes

PS20150512.1(31.7km)

12 May

1 hours

10 earthquakes

S20150512.1(34.8km)

12 May

2 days 14 hours

40 earthquakes

2025

PS20250107.1(178.4km)

7 Jan

31 minutes

7 earthquakes

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Seismic Swarm PS20150425.1 in Central Nepal: Geological Context and Event Analysis

The Himalayan region of central Nepal experiences persistent seismic activity driven by the ongoing collision between the Indian and Eurasian tectonic plates. Convergence rates of approximately 40–50 mm per year along the Main Himalayan Thrust generate frequent earthquakes, ranging from moderate swarms to great megathrust events. The area 67 km north-northeast of Bharatpur lies within this active fold-and-thrust belt, where crustal shortening and uplift continue to shape the landscape.

Seismic swarm PS20150425.1 began at 06:11 on 25 April 2015 and concluded at 16:26 on 26 April 2015, spanning 34 hours and 14 minutes. During this interval, 16 earthquakes were recorded. The sequence initiated with a magnitude 7.8 event at 8 km depth, followed rapidly by events of magnitudes 5.4, 5.1, 5.2, 5.0, 5.1, 6.6, 5.0, 5.5, 5.1, 5.0, 5.3, 5.1, 5.1, 6.7, and 5.0. Depths remained predominantly shallow, between 8 km and 22 km, consistent with rupture along the plate interface and overlying crustal faults.

This swarm provides insight into clustered seismicity in the Himalaya. Unlike typical mainshock-aftershock sequences, the activity displayed multiple large-magnitude events distributed over a short period without a single dominant trigger beyond the initial rupture. The presence of two events exceeding magnitude 6.5 within 25 hours highlights the capacity for compound ruptures in this tectonic setting. Shallow focal depths amplified ground shaking across central Nepal, exacerbating impacts in populated valleys.

The broader seismic history of the region includes recurrent great earthquakes. Since 2000, the most significant nearby event was a magnitude 7.3 earthquake on 12 May 2015, located 30 km from the swarm center. Earlier large events, such as the 1934 Bihar-Nepal earthquake, demonstrate that the plate boundary can release strain in both isolated great shocks and prolonged sequences.

Geological mapping indicates that the swarm epicentral area coincides with the transition zone between the Main Central Thrust and the Main Boundary Thrust. Strain accumulation along these structures, combined with possible fluid migration or stress transfer, can promote swarm-like behavior. Updated monitoring networks continue to record background seismicity in the same corridor, underscoring the persistent hazard.

In summary, swarm PS20150425.1 illustrates the dynamic nature of Himalayan seismicity, where rapid sequences of moderate-to-large events can occur within hours. Continued study of such clusters improves understanding of fault interactions and supports refined hazard assessments for Nepal.

References:
USGS Earthquake Catalog
SeismoSight internal swarm classification PS20150425.1
Nepal Department of Mines and Geology seismic bulletins