Combining InSAR and seismology to study the 2003 Siberian Altai earthquakes-dextral strike-slip and anticlockwise rotations in the northern India-Eurasia collision zone

Abstract

SUMMARY The 2003 September 27 M w 7.2 Siberian Altai earthquake was the largest to have struck the Altai mountains in more than seventy years, and was closely followed by two M w 6.2 and 6.6 aftershocks. We use radar interferometry, seismic bodywaves and field investigations to examine the source processes of these earthquakes. The main shock of the initial earthquake ruptured a subvertical, ∼NW‐SE striking dextral strike-slip fault. The fault was previously unrecognised; although it approximately follows the southwestern boundaries of two intermontane depressions within the interior northwestern Altai, it has very little topographic expression. A ∼NE-dipping M w ∼ 6.7 reverse subevent, possibly triggered by shear waves from the main shock, occurred ten seconds afterwards strike to the southeast. The later M w 6.2 and 6.6 aftershocks were dextral strike-slip events which contributed further to deformation in the northwest part of the fault zone. However, interferometric and bodywave models disagree significantly on the source parameters of the earthquakes, in particular the total moment released and the dip of the fault planes. Trade-offs of fault dip with moment and centroid depth in the bodywave modelling can account for some, but not all, of these discrepancies. The interferometric data is unevenly distributed, containing many more data points on one side of the fault zone than the other; however, on the basis of calculations with synthetic data we rule this out as a reason for the discrepancies in fault parameters. The lower moment predicted by interferometry could be explained by the lack of coherent data close to the faulting, if slip was concentrated at very shallow depths. The dip yielded by the interferometric modelling might be influenced by lateral changes in elastic properties, although these would also affect the bodywave solutions. The earthquake sequence occurred close to recent palaeomagnetic measurements of late Cenozoic anticlockwise rotations. These suggest that the right-lateral strike-slip faulting that ruptured in the 2003 earthquakes accommodates regional ∼NNE‐SSW shortening by rotating anticlockwise over time. The reverse subevent is a rare case of pure shortening perpendicular to the trend of the Altai range.