Location:

Period:

12 May 2015 07:22:15 - 14 May 2015 22:21:00 (2 days 14 hours 58 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

M 7.0+:

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

3 swarms found nearby.

2015

PS20150425.1(34.8km)

25 Apr

1 day 10 hours

16 earthquakes

PS20150425.2(95.7km)

25 Apr

12 hours

5 earthquakes

PS20150512.1(6.8km)

12 May

1 hours

10 earthquakes

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Seismic Swarm S20150512.1 in Eastern Nepal: Geological Context and Event Analysis

The seismic swarm designated S20150512.1 occurred in a tectonically active segment of the Himalayas, centered 39 km southeast of Kodari, Nepal. This sequence unfolded over 62 hours and 58 minutes from 07:22 on 12 May 2015 to 22:21 on 14 May 2015, registering 40 earthquakes. The events clustered immediately following the magnitude 7.3 earthquake of 12 May 2015, whose epicenter lay only 12 km from the swarm centroid.

Eastern Nepal lies within the central Himalaya, where ongoing convergence between the Indian and Eurasian plates drives crustal shortening at rates of approximately 40–50 mm per year. The primary structure accommodating this motion is the Main Himalayan Thrust, a gently north-dipping décollement that has produced repeated large thrust earthquakes. Kodari sits near the surface trace of the Main Central Thrust and associated splays, placing the swarm within a zone of distributed deformation between the Higher and Lesser Himalaya.

The swarm initiated within hours of the M 7.3 mainshock. Magnitudes ranged from 4.0 to 5.0, with the largest event (M 5.0) recorded at 21:38 on 13 May at a depth of 8 km. The majority of hypocenters concentrated between 7 and 15 km depth, consistent with rupture on or near the mid-crustal ramp of the Main Himalayan Thrust. Temporal distribution showed highest activity during the first 12 hours, followed by a gradual decline, typical of aftershock sequences modulated by static stress changes.

Since 1 January 2000, three swarms have been identified in the broader region; S20150512.1 represents the earliest. The 2015 sequence thus marks a notable departure from background seismicity patterns observed over the preceding fifteen years. Depths and focal mechanisms align with thrust faulting on planes striking roughly east–west, mirroring the regional structural grain.

Post-swarm monitoring indicates that such clusters can illuminate stress redistribution along locked or partially locked segments of the plate boundary. Continued geodetic and seismic observation remains essential for refining hazard estimates in this densely populated corridor of the Himalaya.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
Department of Mines and Geology, Nepal (dmgnp.gov.np)
SeismoSight internal swarm classification records