Spatial variation of crustal anisotropy in the Arunachal Himalaya inferred from splitting of local S waveforms

Shear wave anisotropy of the crust beneath the Arunachal Himalaya is investigated by analysing the splitting of steeply incident (i(c) <= 45 degrees) direct S-phases of 396 local earthquakes, recorded at 32 stations. The 76 well constrained splitting measurements reveal that the delay times between fast and slow waves range from 0.02 to 0.30 s, clustered around 0.07 s. A large variation in the orientation of fast polarization directions (FPDs) is found. The smallest delay times (mostly < 0.10 s) and large variation in the orientation of FPDs are found at stations located in the western part of the Arunachal Himalaya. This is likely associated with heterogeneities in the lithological properties and a combined effect of stress-induced and structure-induced anisotropy. For a small crustal block in the central part of Arunachal, the FPDs are parallel to the strike of the Himalayan arc and are associated with structure-induced anisotropy. In this region, the FPDs representing the upper mantle are also found to be parallel to the strike of the Himalayan arc, suggesting a coherent deformation. In the eastern part, a variation in the FPDs is observed from north to south, being parallel to the strike of the Siang river in the former and mostly parallel to the average maximum horizontal stress in the latter. The delay times show a correlation with depth when the observations in the central and eastern parts of the Arunachal Himalaya are combined.

Automated summary

The shear wave anisotropy of the crust beneath the Arunachal Himalaya was investigated using the splitting of direct S-phases from local earthquakes. The study found that there is a range of delay times between fast and slow waves, with the majority clustered around 0.07 seconds. The orientation of fast polarization directions was also found to vary significantly, with the smallest delay times and largest variation observed in the western part of the Arunachal Himalaya. This variation is likely due to heterogeneities in lithological properties and a combined effect of stress-induced and structure-induced anisotropy.