Using seismic data of about one year recorded by 18 broadband stations of ASCENT project,we obtained 2547 receiver functions in the northeastern Tibetan Plateau.The Moho depths under 14 stations were calculated by applying the H-κ domain search algorithm.The Moho depths under the stations with lower signal-noise ratio(SNR) were estimated by the time delay of the PS conversion.Results show that the Moho depth varies in a range of ~40–60 km.The Moho near the Haiyuan fault is vague,and its depth is larger than those on its two sides.In the Qinling-Qilian Block,the Moho becomes shallower gradually from west to east.To the east of 105°E,the average depth of the Moho is 45 km,whereas the west is 50 km or even deeper.Combining our results with surface wave research,we suggest a boundary between the Qinling and the Qilian Mountains at around 105°E.S wave velocities beneath 15 stations have been obtained through a linear inversion by using Crust2.0 as an initial model,and the crustal thickness that was derived by H-κ domain search algorithm was also taken into account.The results are very similar to the results of previous active source studies.The resulting figure indicates that low velocity layers developed in the middle and lower crust beneath the transition zone of the Tibet Block and western Qinling,which may be related to regional faults and deep earth dynamics.The velocity of the middle and lower crust increases from the Songpan Block to the northeastern margin of Tibetan Plateau.Based on the velocity of the crust,the distribution of the low velocity zone and the composition of the curst(Poisson's ratio),we infer that the crust thickening results from the crust shortening along the direction of compression.
We estimate the focal depths and fault plane solutions of 46 moderate earthquakes in the Himalayan- Tibetan region by modeling the broadband waveforms of teleseismic P waves. The depths of 38 of these earth-quakes range between 0-40 km, with a peak at ～5 km. One earthquake is located within the lower crust of the Indian shield. The remaining eight earthquakes occurred between depths of 80-120 km and are all located in the Pamir-Hindu Kush and the Indo-Myanmar deep seismic zones. None of the earthquakes outside these deep seismic zones are located in the mantle. Global centroid moment tensor (CMT) solutions indicate that most earthquakes in northern Tibet and northern India had thrust-faulting mechanisms and that normal and strike-slip faulting earthquakes occurred primarily in central Tibet. These mechanisms are consistent with the predominantly NNW-SSE compression in the direction of current Himalayan-Tibetan continental collision.
