Evidence of Transtensional Deformation Associated with Neotectonics: Insights into the Velocity Structure and Source Mechanism of Nanga-Parbat Syntaxis, Northwestern Himalayas

Abstract

Comprehensive information of the seismicity originating from Nanga Parbat syntaxis in northwestern Himalayas is crucial in understanding the fault systems associated with neotectonics. Continuous monitoring of the small magnitude earthquakes using local stations can provide precise and accurate velocity structure analysis of the subsurface variations. Seismogram data of 244 small magnitude earthquakes has been utilized to delineate the crustal thickness in Nanga Parbat syntaxis through coupled hypocenter velocity inversion analysis. This velocity inversion proposed that the Moho discontinuity lies at the depth of 60 km with an average value of 1.74 ±0.023 computed for the Vp/Vs ratio. Predicted velocity models having the least RMS error are further utilized in determining the source mechanism solution. Previous studies have reported the presence of several shear zones within Nanga Parbat syntaxis dominates the strike-slip sense of displacement which is facilitated by the reverse component of slip. Contrary to this, the current study presents evidence of the transtenssive deformation patterns with the application of time domain moment tensor solution. The findings of the source mechanism for the selected earthquakes unveiled that the oblique-slip deformation in the region is significantly controlled by the shear stresses coupled with normal component of dip-slip movement. This is further verified by the higher values of double-couple moment tensor (85%) representing shear deformation and positive value of compensated linear vector dipole (15%) validates the presence of normal component. The combination of transpressive and transtenssive stresses with shallow hypocenter depths and high amplitudes of tangential waveforms can cause devastating impact in the surroundings of Nanga Parbat syntaxis region.

Keywords: Nanga Parbat Syntaxis, Neotectonics, Velocity Structure Analysis, Moment Tensor Solution, Transtensional Deformation