Crustal structure and earthquake focal depths beneath northeastern India and southern Tibet

Abstract

We use broad-band teleseismic data recorded at eight sites along a north-south profile from Karimganj (24.84°N, 92.34°E), south of the eastern Shillong Plateau, to Bomdilla (27.27°N, 92.41°E) in the eastern Lesser Himalaya, to determine the seismic characteristics of the crust in northeastern India. We also analyse data from the Chinese Digital Seismic Network station at Lhasa and INDEPTHII stations located on the southern Tibetan Plateau north of our profile, to extend the seismic images of the crust further northwards. Although the northeastern Indian stations and the Tibetan stations do not lie along a linear profile across the Himalaya, the well-recognized uniformity of the Himalaya along strike make this comparison of the two profiles meaningful. Receiver functions calculated from these data show that the crust is thinnest (~35-38 km) beneath the Shillong Plateau. Receiver functions at Cherrapunji, on the southern edge of the Shillong Plateau, have a strong azimuthal dependence. Those from northern backazimuth events show that the Moho beneath the southernmost Shillong Plateau is at a depth of ~38 km while receiver functions from southern backazimuth events indicate that the Moho beneath the northernmost Bengal Basin is at a depth of ~44 km. Receiver functions from sites on the Brahmaputra Valley demonstrate that the Moho is deeper by ~5-7 km than below the Shillong Plateau, a result which agrees with the hypothesis that the Shillong Plateau is supported by shearing stress on two steep faults that cut through the crust. Further north of the eastern Himalayan foredeep, the Moho dips gently northwards, reaching a depth of ~48 km beneath Bomdilla in the Lesser Himalaya, and 88 km below Lhasa in Tibet. Using the crustal velocity models obtained from receiver function inversions, we redetermined focal depths of well-recorded earthquakes across this part of the Indo-Tibetan collision zone and find all of these to occur within the crust. Hence we find no evidence for bimodal depth distribution of earthquakes beneath this region of northeastern India.