Focal mechanisms and the contemporary tectonic stress field of the Tashkent geodynamic polygon, Western Tien Shan

Twenty-one earthquake focal mechanism solutions (M 4.1–7.5, 1946–2024) from the Tashkent geodynamic polygon and its immediate surroundings, Western Tien Shan, were compiled and analysed. Focal depths range from 3 to 30 km with a median of 15 km, and 76 % of the events lie between 5 and 20 km. Classification by the plunges of the P and T axes shows that reverse faulting dominates (43 %), followed by obliquereverse, strike-slip and oblique-normal types (14 % each); compressional mechanisms account for 57 % of the sample. Linear inversion of slip directions with iterative fault-plane selection, constrained by bootstrap and jackknife tests, yields for the compressional population a sub-horizontal axis of maximum compression with an azimuth of 77 ± 16° and a plunge of 10°, a sub-vertical axis of minimum compression, a stress ratio of 0.58 and a mean slip-direction misfit of 13°, corresponding to a stable thrust to transpressional regime. The inversion also demonstrates that the field is not homogeneous: the median misfit increases from 11° in the western sector to 35° in the eastern sector, where a strike-slip regime prevails.

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