Identifying geological structures in the Pamir region using non-subsampled shearlet transform and gravity gradient tensor

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Summary The Pamir tectonic zone originates from the intense collision of the Indo-Eurasian plate. Identifying the faults in the Pamir region region is essential for elucidating the collision mechanism and seismic characteristics. This paper compares the effect of the two-dimensional discrete wavelet transform (DWT2D) and the non-subsampled shearlet transform (NSST) on gravity field separation through synthetic model gravity field experiments. The results show that NSST can avoid the Gibbs phenomenon of DWT2D and better maintain the gravity field distribution. The surface gravity disturbances data of the Global Gravity Model Plus (GGMplus) with a high-spatial resolution (7.2 arcsec or approximately 200 m) is employed to separate the region-residual gravity fields in the neighbouring domain of the Pamir region based on the NSST. Furthermore, the gravity gradient tensor (GGT) is computed, and the correspondence between the GGT and the location and strike of the surrounding faults is analyzed. The results show that the GGT component and its various combinations can effectively identify shallow and deep faults, the residual field GGT and its combinations can effectively identify the distribution and direction of shallow faults, and the regional field GGT and its combinations can effectively identify the distribution and direction of deep faults. The existence of north-south trending faults in the Pamir-Hindu Kush region is widely accepted. However, our study has revealed an east-west trending concealed fault in the deep areas of the Hindu Kush (Depth > 200 km). This finding provides significant insights for studying the bidirectional subduction of the Indian and Eurasian plates. This research not only helps us to analyze the tectonic characteristics of the shallow and deep parts of the region separately but also provides complementary information for investigating the distribution of deep underground faults, especially when fault inversion of intermediate to deep source earthquakes is limited by factors such as uncertainty in source depth and complexity of seismic wave velocities.