Seismic Swarm S20080512.2: Analysis of Activity Near Jiangyou, China

A seismic swarm designated S20080512.2 was recorded 50 km NNE of Jiangyou in Sichuan Province, China. The sequence began at 07:06 on 12 May 2008 and concluded at 06:35 on 13 May 2008, spanning 23 hours and 28 minutes. During this interval, 25 earthquakes were registered, with the majority occurring at shallow depths around 10 km and magnitudes ranging from 3.7 to 4.9.

The swarm exhibited a rapid onset of moderate events followed by a gradual decline in frequency and intensity. Initial shocks reached 4.9 within the first two minutes, succeeded by additional events of 4.8 and 4.7 within the first hour. Later activity included isolated deeper events, such as one at 30 km depth, and several smaller aftershocks below magnitude 4.5. One entry recorded a magnitude of 0.0, consistent with internal classification thresholds for the dataset.

The Jiangyou region lies within the Longmen Shan fault zone, a northeast-trending thrust system marking the eastern margin of the Tibetan Plateau. This tectonic boundary accommodates ongoing convergence between the Indian and Eurasian plates, producing crustal shortening and uplift at rates of several millimeters per year. The fault zone has hosted recurrent large earthquakes throughout the Quaternary, reflecting its role as a major seismogenic structure.

On 12 May 2008, the devastating Wenchuan earthquake (Mw 7.9) ruptured approximately 240 km of the Longmen Shan fault system southwest of Jiangyou. The timing of swarm S20080512.2 coincides with this mainshock, indicating that the recorded events form part of the broader seismic sequence associated with the regional stress release. Historical records show that only one swarm has occurred in the area since 1 January 2000, underscoring the rarity of such clustered activity outside major mainshock-aftershock sequences.

Geological mapping and geophysical studies confirm that the shallow focal depths observed in the swarm align with the brittle upper crust of the Longmen Shan, where most strain accumulates before release in moderate to large events. The predominance of 10 km depths suggests activation along secondary faults or distributed fractures rather than the primary basal detachment.

This swarm provides insight into the spatiotemporal evolution of seismicity along an active collisional margin. The clustered pattern, dominated by events between 4.0 and 4.9, illustrates how moderate shaking can occur in rapid succession without exceeding the energy threshold of a larger mainshock. Continued monitoring of the Longmen Shan remains essential for assessing residual stress and potential future activity in this densely populated region.

References

USGS Earthquake Hazards Program – Wenchuan Earthquake Summary
China Earthquake Administration – Regional Seismicity Reports
Sichuan Geological Survey – Longmen Shan Fault Zone Documentation