Seismic Swarm PS20150512.1: Insights into Nepal's Tectonic Activity

Nepal lies within the tectonically active Himalayan orogenic belt, formed by the ongoing collision between the Indian and Eurasian plates. This convergence occurs at rates of approximately 4–5 cm per year, generating substantial crustal stress that manifests in frequent seismic events. The region’s geology features thrust faults, including segments of the Main Himalayan Thrust, which accommodate much of the strain release.

On 12 May 2015, seismic monitoring recorded swarm PS20150512.1 near the epicentral area of a prior major event. The sequence began at 07:05 and concluded at 08:24, encompassing ten earthquakes within one hour and eighteen minutes. Magnitudes ranged from 5.0 to 7.3, with focal depths predominantly between 10 and 15 km, except for one deeper event at 100 km.

The initial shock registered magnitude 7.3 at 07:05:19, followed by events of 5.5, 5.0, 5.4, 5.3, 6.3, 5.0, 5.1, 5.2, and a final 5.0. This rapid succession of moderate to strong tremors illustrates typical swarm behavior, where clustered activity occurs without a single dominant mainshock-aftershock progression. Depths clustered in the upper crust suggest rupture along shallow thrust structures characteristic of the Himalayas.

Historical records since 2000 indicate only two such swarms in Nepal, with this event marking the first. It occurred 5 km from the epicenter of the magnitude 7.3 earthquake on the same date, linking the swarm directly to the broader 2015 seismic sequence triggered by the April Gorkha mainshock. The interplay of these events underscores how stress redistribution following large ruptures can initiate secondary clusters.

Such swarms provide valuable data on fault interactions and crustal heterogeneity in convergent margins. Continued monitoring enhances understanding of recurrence patterns in this high-hazard zone, supporting refined seismic hazard assessments for Nepal’s densely populated valleys and mountain communities.

References

SeismoSight internal classification records for swarm PS20150512.1.
USGS Earthquake Catalog (2000–present).
Nepal Department of Mines and Geology historical seismic database.