| The eruption of the volcano will cause many geological hazards which have a significant impact on human life.To predict the eruption of the volcano,the eruption mechanism and the structure of the volcano should be studied first.The eruption of the volcano is closely related to the magma system beneath the volcano,therefore,studying how the magma is transported is an important part of volcanic eruption prediction.Seismology is a common means for humans to study the internal structure of the earth.The seismic data observed in the volcano area help scientists to study the volcano and assess the volcanic risk,which deepened their understanding of volcanoes.Volcanoes in China are widely distributed,and the Changbai volcano is located in the northeast of China.The Changbai volcano which is the largest active volcano in China located in the northeast and is also a research hotspot in the world.The eruption of the Changbai volcano that is historically recorded had a huge impact on the surrounding environment and even the world,and volcanic ash from the"Millennium eruption"was found in many places in Japan 1000 km away.During 2002and 2005,scientists found a sudden increase in seismic activity in the Changbai volcano,accompanied by abnormal gas overflows in surface hydrothermal vents and overall topographic uplifts.After 2005,the seismic activity gradually returned to the state that was stable before the unrest.The unrest,which lasted for about 3 years,attracted the attention of scientists in different fields.And based on the results of different methods,scientists speculated that the 3-year unrest represented the activity of magma beneath the Changbai volcano.In order to study the distribution of magma chambers beneath the Changbai volcano,assess the risk of volcanic disasters,and provide a geophysical basis for subsequent research on the Changbai volcano,this paper used the short-period dense array observation data laid in the Changbai volcano and used ambient noise tomography method to construct a high-accuracy three-dimensional S-wave velocity model of the Changbai volcano.The data used in this study came from a short-period dense array deployed by the research group from July 2020 to August 2020.The dense array consists of two seismometers.A total of 361short-period seismometers are laid along the northern,western,and southern roads of the Changbai volcano,and finally gathered in the Tianchi crater.In this paper,the method of extracting the dispersion curve is the"frequency-time analysis method",and we used the direct inversion method to inverse the dispersion.This paper used about 30-day continuous waveform data to extract 4948 dispersion curves and used the direct inversion method to obtain a three-dimensional Vs structure in 0–15 km depths beneath the Changbai volcano.The results show that there are four low Vs anomalies in the shallow crust(～7 km)and one in the middle crust(9–15 km).Combined with the previous studies of the Changbai volcano and other volcanos in the world,three of the five anomalies are determined to be magma chambers.The others are fluid-filled fracture zones and melt residue.In addition,the chamber in the shallow crust is about 90 km~3.The belt fraction of them is calculated to be 15–26%.In ambient noise tomography,some areas have low ray density because of the topography and the noise source distribution,which make the results here inaccurate.To supplement these areas and expand the imaging area,a dispersion curve interpolation method based on kriging is developed in this paper.Through the tests,the root mean square error of the predicted value and the real value is less than 0.1;the relative error is less than 10%,which illustrates the method is feasible.After that,the method is applied to the Changbai volcano.The imaging area is expanded and the expansion multiplier can reach 2.4.The interpolation of the dispersion curve in space can supplement part of the spatially missing data and lay the foundation for subsequent temporal interpolation. |
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