Sn attenuation tomography of southeastern Tibet: New constraints on lithospheric mantle deformation

We have formulated a 2-D Sn attenuation tomographic model to investigate the uppermost mantle shear wave Q and its tectonic implications beneath southeastern Tibet near Namche Barwa. To achieve our objective, we first compute interstation Q values using the two station method (TSM) analysis on 618 station pairs obtained from 26 regional earthquakes (Mw ≥5.5) with epicentral distances ranging from 5° to 15° recorded at 47 seismic stations belonging to the Namche Barwa network (XE network, 2003-2004). Furthermore, the QSn tomographic model is generated by utilizing these interstation Q values. QSn values are varying from 101 to 490 in the region. The tomography image reveals high attenuation (≤200 Q values) in the central region. Regions of low attenuation (>200 Q values) are observed in the southern part and in some small regions beneath the northern side of the study area. Consecutive high-low-high QSn values have been observed in the south part of the Lhasa block. The obtained QSn values, along with the prior isotropic Pn velocity model of the study area, indicate that the scattering effect is causing significant Sn wave energy dissipation due to structural heterogeneity present in the uppermost mantle beneath the region. This may be the result of the break-up of the subducting Indian Plate beneath the area.