Multiscale Surface Wave Tomography of the Bhutan Himalayas using Ambient Seismic Noise and Teleseismic Earthquake Data

The tectonic framework of Bhutan Himalaya documents significant along-strike variability in crustal structure and deformation. To visualize this spatial and depth variability, we compile an extensive dataset of surface-wave phase velocities derived from seismic ambient noise and teleseismic earthquakes recorded by the temporary GANSSER network (2013-2014) in Bhutan, aiming to produce Rayleigh phase-velocity maps over the period range of 4 to 50 seconds. We translate the phase-velocity maps into a 3-D shear-wave velocity model stretching from the surface to a depth of 42 kilometres. The employed methodologies enable imaging of the upper to mid-crustal and lower crustal velocity anomalies with a lateral resolution of approximately 25 km. The obtained tomographic model fills a void in the prior established shear-wave velocity structure of Bhutan, encompassing depths from upper-crustal to lowermost crust. Our findings indicate notable mid-crustal to lower-crustal high phase velocity anomalies in central Bhutan (around 90.5). The presence of this significant anomaly within the mid- to lower crustal layer may indicate localized stress accumulation along the Main Himalayan Thrust (MHT) resulting from the interaction of the dipping and sub-horizontal Moho. This area might act as a stress concentration zone, resulting in increased deformation and enhanced shear-wave velocity in the crust. Minor fluctuations in velocity across latitude may result from variations in the local geometry of MHT (dip or ramp-flat transition). Localised high shear velocity in western Bhutan may indicate a zone of crustal thickening. Northeastern Bhutan exhibits modest shear velocity, possibly because of a flat Moho and the partial creeping behaviour of the MHT.