Evidence of transtensional deformation associated with neotectonics: insights into the velocity structure and source mechanism of nanga-parbat syntaxis, northwestern himalayas

A detailed understanding of seismicity originating from the Nanga Parbat syntaxis in the northwestern Himalaya is crucial for characterizing the active fault systems and associated neotectonic processes in the region. Continuous earthquake monitoring through local seismic stations enables high-precision results by constraining the velocity structure. In this study, seismogram data from 244 small-magnitude earthquakes are analyzed to delineate the crustal thickness and investigate the source mechanisms beneath the Nanga Parbat syntaxis. The results are achieved with the application of Coupled Hypocenter Velocity Inversion (CHVI) analysis and Time Domain Moment Tensor (TDMT) analysis. The velocity inversion proposed that the Moho discontinuity lies at 60 km depth with an average v_P/v_S ratio of 1.735 ± 0.017. The minimum 1D velocity model obtained through velocity inversion with least RMS error is further utilized in determining the source mechanism solution. In contrast to earlier studies, which highlighted strike-slip displacement accompanied by reverse dip-slip components, the present research provides a revised interpretation. The moment tensor analysis conducted in this study provides evidence of transtensional deformation associated with neotectonics, attributed to the presence of multiple shear zones. The results of the source mechanism for the selected earthquakes unveiled that the oblique-slip deformation is significantly controlled by the shear stresses coupled with the normal component of dip-slip movement. This is further supported by the higher values of the double-couple moment tensor (85%), which indicate shear deformation, while the positive value of the compensated linear vector dipole (15%) confirms the presence of a normal component. The coexistence of transpressive and transtensive stresses, together with shallow hypocentral depths and high-amplitude tangential waveforms, can potentially cause devastating impacts in the surroundings of the Nanga Parbat syntaxis.