Seismic Swarm PS20080512.2: Aftershock Sequence Following the 2008 Wenchuan Earthquake

The seismic swarm designated PS20080512.2 occurred in the Longmen Shan region of Sichuan Province, China, approximately 37 km northwest of Tianpeng. This sequence began at 06:28 UTC on 12 May 2008 and concluded at 02:54 UTC on 14 May 2008, encompassing 23 earthquakes over 44 hours and 26 minutes. The events align closely with the mainshock of the Wenchuan earthquake, a magnitude 7.9 event located 58 km west of Tianpeng and roughly 26 km from the swarm centroid.

Geologically, the Longmen Shan fault zone marks the eastern margin of the Tibetan Plateau, where the Indian Plate continues to converge with the Eurasian Plate at rates of 4–6 cm per year. This compression has produced a series of thrust faults that accommodate crustal shortening. The 2008 Wenchuan mainshock ruptured approximately 240 km along the Yingxiu–Beichuan and Pengguan faults, generating intense aftershock activity as stresses readjusted across the fault network.

The swarm initiated with the magnitude 7.9 mainshock at a focal depth of 19 km. Subsequent events clustered predominantly at depths of 9–34 km, with the majority near 10 km. Early aftershocks included a magnitude 5.7 at 06:41 and a magnitude 5.8 three minutes later, both at 10 km depth. Activity continued with multiple magnitude 5.0–5.5 shocks throughout 12 May, including a magnitude 6.1 event at 11:11 UTC. Depths varied modestly, with several events recorded between 17 km and 34 km on the evening of 12 May.

On 13 May, the sequence produced a magnitude 5.8 shock at 07:07 UTC at 9 km depth, followed by additional magnitude 5.0 events. The final recorded event was a magnitude 5.4 earthquake at 02:54 UTC on 14 May at 18 km depth. Magnitudes generally declined after the initial 24 hours, consistent with typical aftershock decay patterns governed by Omori’s law.

The broader tectonic setting features Paleozoic to Mesozoic sedimentary rocks overlying Precambrian basement, folded and thrust eastward during the Cenozoic. Historical seismicity in the region includes moderate events, yet the 2008 sequence stands as the most energetic since instrumental recording began. Post-2000 monitoring has documented only this magnitude 7.9 mainshock of comparable size within 100 km of Tianpeng.

This swarm illustrates the prolonged seismic response of the Longmen Shan faults following a major continental thrust earthquake. Continued monitoring by regional networks helps refine models of stress transfer and aftershock hazard in this densely populated portion of western China.

References
USGS Earthquake Catalog (earthquake.usgs.gov)
China Earthquake Administration annual reports
Global Centroid Moment Tensor Project (www.globalcmt.org)