High-resolution velocity structure in the Bomi area, Eastern Himalayan Syntaxis, constrained from multiscale ambient noise tomography

Bomi area in the north of Eastern Himalayan Syntaxis is traversed by the large strike-slip Jiali fault. This study aims to investigate fault zone structure and local tectonic stress in the Bomi area. The 3D high-resolution S-wave velocity (Vs) and anisotropy models of the shallow crust (0–4 km) are obtained using the ambient noise recordings from Bomi dense array (207 short-period seismometers) based on a multiscale inversion scheme. The azimuthal anisotropy of the group velocities shows that the fast directions change from N–S to E–W or NW–SE from west to east. This variation is probably affected by the northeastward tectonic stress from the India-Asia collision, eastward extrusion of crustal materials, and the dextral strike-slip activity of the Jiali faults. Vs model suggests that southern low-velocity belts along the Jiali main faults are primarily affected by rock mass fragmentation, and the northern low-velocity feature surrounding the Jiali secondary faults is probably related to the loose sedimentary rock and numerous geothermal waters. The high velocity in the central part may reflect the presence of intrusive Gangdese granite. Additionally, the sediment thickness distribution in the river valleys (Parlung Tsangpo and Bodui Tsangpo) is highly uneven. Sediment thickness in the Bodui Tsangpo is generally greater than that in Parlung Tsangpo. On the western side of Bodui Tsangpo, the sedimentary deposits are thicker than those on the eastern side.