Nepal-Himalaya Region Middle-Upper Crust Noise Imaging

The Himalayas orogenic belts are located in the southern edge of the Qinghai-Xizang Block in China,Pakistan and India.At the junction of the Indian and Eurasia plates,the Himalayas orogenic belt is the region where the Indian and Eurasia plates hit the most.The collision between the plates caused the uplift of the Himalayan orogens,it's average elevation is 6km.The high Himalaya is 5km higher than low Himalaya.and the topography is very complicated.Therefore,the Himalayas is an important area for studying the collision between the Indian plate and the Eurasian plate.The analysis of the velocity structure of the crust in the region is conducive to explaining the collision process between the Indian plate and the Eurasian plate.The use of seismology to obtain the velocity structure inside the Earth is a fundamental method to geophysicists.Previous researches on this area have many achievements,such as inversion of crust-mantle structures by remote seismic body wave tomography,but teleseismic body wave imaging is not precise enough for inversion of shallow crust,and some people use acceptance function to invert the platform.The speed under the station,but the acceptance function just inverts the structure below the station.The method used in this paper is ambient seismic noise tomography.Because the ambient seismic noise attenuation during the propagation of two stations is much smaller than that of natural earthquakes and artificial earthquakes,the empirical Green's function obtained by cross-correlation calculation can extract short-period surface wave velocity(group velocity)dispersion curves.It can invert the velocity structure of the shallow crust beneath the entire area.The inversion result of this method is only related to the coverage and concentration of stations,and does not depend on natural earthquakes and artificial earthquakes.In this paper,we used the continuous waveform data recorded by 28 stations of the HIMNT network in the Nepal-Himalayan.The instrument recording time was from December 2001 to December 2002.The empirical Green's function of the two stations was obtained by performing cross-correlation calculations on the two stations,and the phase velocity dispersion curve of the Rayleigh wave with periods of 6-26 s was extracted using image analysis techniques.Then,the phase velocity distribution of the Rayleigh wave at 6-26 s below the station is inverted through surface wave tomography.The results showed that the Indian plate subducted below the Eurasian plate,with its subduction direction roughly perpendicular to the Himalayas.A survey line perpendicular to the Himalayas was obtained,and the transverse wave velocity from 0 to 30 km depth was inversed using the 6-26 s dispersion curve,and the profile of the shear wave velocity below the line was plotted.It can be clearly seen from the figure that the Indian plate and the Eurasian plate interface,the Indian plate dive below the Eurasian plate,and with the plate inserted,the temperature of the MHT gradually increases.And found that the Indian crust has a partial deflection.