Multicomponent Ambient Noise Adjoint Tomography of the Main Himalayan Thrust

We construct a 3-D crustal shear wave velocity model of the Main Himalayan Thrust (MHT) beneath central Nepal using ambient noise adjoint tomography. The MHT is the major plate boundary fault accommodating India–Eurasia convergence and was the source of the 2015 Mw 7.8 Gorkha earthquake. This full-waveform inversion method uses the spectral-element method (SEM) and updates shear wave velocity directly from multicomponent empirical Green’s functions (EGFs). Previous studies suggest that the MHT exhibits substantial lateral variation and consists of north-dipping, imbricate thrust faults forming a duplex structure. These faults influence strain accumulation and rupture dynamics but their geometries remain poorly constrained by ray-based seismological imaging methods. 

We analyze 11 months of continuous data from 42 stations of the NAMASTE (Nepal Array Measuring Aftershock Seismicity Trailing Earthquake) network, deployed about 50 days after the Gorkha earthquake and spanning the rupture zone with an average spacing of 20 km. From multicomponent ambient noise cross-correlations, we extract EGFs in the 5–40 s period band. Frequency-dependent traveltime misfits between EGFs and synthetic Green’s functions from SEM simulations are iteratively minimized using finite-frequency sensitivity kernels. The resulting model will provide improved constraints on MHT geometry and contribute to a better understanding of Himalayan tectonics and seismic hazard.