Seismic Swarm PS20210521.2: Analysis of Events in Southern Qinghai
Southern Qinghai lies within the northeastern margin of the Tibetan Plateau, a region shaped by the ongoing collision between the Indian and Eurasian tectonic plates. This convergence drives crustal shortening, strike-slip faulting, and uplift, producing one of the most seismically active zones in Asia. The plateau's interior features extensive fault systems, including the Kunlun Fault and associated secondary structures, which accommodate lateral extrusion of material eastward.
The swarm designated PS20210521.2 began at 18:04 UTC on 21 May 2021 and concluded at 03:21 UTC on 22 May 2021. Over 9 hours and 17 minutes, seven earthquakes were recorded, all at a focal depth of 10 km. The sequence opened with a magnitude 7.3 event, followed within minutes by events of magnitude 5.2, 5.5, 5.5, and 4.9. Two additional events of magnitude 5.1 and 5.2 occurred later in the sequence. The mainshock was located approximately 23 km from the swarm centroid.
This temporal clustering without a single dominant aftershock sequence is characteristic of an earthquake swarm. Such patterns in the Tibetan Plateau often reflect distributed slip on interconnected fault segments or transient fluid pressure changes within the crust. The uniform 10 km depth across all events suggests rupture within the brittle upper crust, consistent with the regional seismogenic thickness.
Southern Qinghai has experienced recurrent moderate-to-large earthquakes throughout the instrumental era. The 21 May 2021 magnitude 7.3 mainshock fits within a broader pattern of thrust and strike-slip events driven by the plateau's eastward expansion. Historical records and paleoseismic studies indicate recurrence intervals on major faults ranging from centuries to millennia, underscoring the persistent tectonic loading in the area.
The swarm's rapid succession of events highlights the value of dense seismic monitoring for capturing short-term clustering. All recorded magnitudes remained below the initial 7.3 shock, and activity ceased within a single day, distinguishing the sequence from prolonged aftershock decay.
References
- China Earthquake Networks Center (CENC) preliminary reports
- United States Geological Survey Earthquake Catalog
- SeismoSight internal swarm classification PS20210521.2