Seismic Swarm S20080513.1: Analysis of Activity Near Tianpeng, China

Seismic swarm S20080513.1 was recorded from 07:56 on 12 May 2008 until 03:34 on 16 May 2008, spanning 91 hours and 37 minutes. The events were located 38 km north of Tianpeng in Sichuan Province, China, with a total of 53 earthquakes registered during this interval. This sequence represents the first swarm documented in the region since 1 January 2000, when systematic records began, underscoring its significance within the local seismic catalog.

The swarm exhibited a range of magnitudes, predominantly between 3.0 and 5.1, with the majority of events clustered at depths of 10 km. Notable higher-magnitude occurrences included a 5.1 event on 14 May at 09:26 and multiple 4.9 and 4.8 shocks distributed across 12–16 May. Several events registered at 0.0 magnitude, likely reflecting detection thresholds or microseismicity. Depths remained consistent near 10 km for most activity, though a few reached 30 km and one at 7 km, indicating shallow crustal involvement overall. The temporal distribution showed peak intensity on 12 May with repeated events exceeding magnitude 4.5, tapering gradually through 15 May before concluding early on 16 May.

This activity unfolded within the Longmen Shan fault system, a major thrust belt marking the eastern margin of the Tibetan Plateau. The region experiences ongoing compression from the collision between the Indian and Eurasian plates, driving uplift and frequent seismicity along northeast-trending faults. Historical records document recurrent large earthquakes in this zone, including the devastating 2008 Wenchuan event, which occurred on the same day the swarm initiated. Such swarms often accompany stress redistribution following major ruptures, highlighting the interconnected nature of fault segments in the area.

Geological studies emphasize the Longmen Shan’s role as a transitional zone between the rigid Sichuan Basin and the deformable plateau interior. Sedimentary layers overlying Precambrian basement rocks amplify ground shaking during events, while active thrusting sustains the potential for both mainshocks and associated swarms. Since 2000, only two swarms have been identified regionally, positioning S20080513.1 as a key indicator of post-2008 stress adjustment.

Insights from the sequence reveal a classic swarm pattern: rapid onset, clustered moderate-magnitude events without a dominant mainshock, and progressive decay. Most activity concentrated within the first 48 hours, consistent with fluid migration or aseismic slip triggering mechanisms common in compressional regimes. Depths averaging 10 km align with the seismogenic layer of the Longmen Shan, where brittle failure predominates.

Further monitoring of this fault system remains essential given its proximity to densely populated areas. Continued analysis of swarm statistics aids in refining probabilistic models for future seismic hazards along the plateau margin.

References

SeismoSight internal swarm classification database.
USGS Earthquake Catalog (regional tectonics summaries).
China Earthquake Administration historical reports on Longmen Shan activity.