Seismic Swarm PS20030927.1: Analysis of the 2003 Altai Sequence

The seismic swarm designated PS20030927.1 occurred on 27 September 2003 in the Altai Mountains of southern Russia. The sequence began at 11:33 local time and concluded at 20:30, encompassing eight earthquakes over 8 hours and 56 minutes. Its epicentral area lies approximately 28 km west of Kosh-Agach in the Altai Republic, near the borders with Kazakhstan, Mongolia, and China.

The events registered the following parameters: a magnitude 7.3 earthquake at 11:33:25 (depth 16 km), followed by a magnitude 5.3 at 11:58:06 (10 km), magnitude 4.9 at 13:03:07 (10 km), magnitude 5.4 at 13:16:43 (10 km), magnitude 5.1 at 13:38:37 (10 km), magnitude 5.0 at 15:31:21 (10 km), magnitude 6.4 at 18:52:46 (10 km), and magnitude 5.3 at 20:30:23 (10 km). Depths clustered predominantly at 10 km, with the initial mainshock occurring slightly deeper.

This swarm coincides closely with the strong regional event of magnitude 7.3 recorded 29 km south of Aktash, only 1 km from the swarm centroid. The Altai region forms part of the Central Asian orogenic belt, where active intraplate deformation arises from far-field stresses transmitted by the ongoing collision between the Indian and Eurasian plates. The resulting transpressional regime produces strike-slip and reverse faulting along inherited Paleozoic structures within the Altai-Sayan mobile belt.

Seismicity in the area reflects Quaternary reactivation of these faults, with historical records indicating recurrent moderate-to-large events. The 2003 sequence represents one of the most energetic episodes documented in the instrumental era for this sector of the Altai. Most events nucleated within the upper crust, consistent with the rheological properties of the continental lithosphere in this compressional setting.

The temporal clustering of foreshock-mainshock-aftershock activity within less than nine hours highlights the rapid stress transfer characteristic of the region’s fault networks. Depths remaining near 10 km suggest rupture along mid-crustal faults that accommodate both strike-slip and reverse motion under the prevailing northeast-directed compression.

Geological mapping confirms the presence of active segments of the Kurai and Teletskoye fault systems in the immediate vicinity, which have hosted similar sequences in the past. Continued monitoring is warranted given the region’s proximity to populated valleys and infrastructure corridors traversing the Altai Republic.

References

USGS Earthquake Catalog (2003 Altai event parameters)
Global CMT Catalog (focal mechanisms and depths)
Geological Survey of the Russian Academy of Sciences (regional tectonics summaries)