Crustal Magma Chamber Beneath The Qiongbei Volcano Field In Hainan Island Unveiled By Seismic Ambient Noise Tomography

We collected the continuous seismic ambient noise data of 32 permanent stations located in the Hainan Island,Guangxi Province,and Guangdong Province.The data are recorded about two years from 2014.We extracted the Rayleigh wave dispersion curves with the group velocity from 5 to 22 s period and the phase velocity from 6 to11 s period based on the cross-correlation functions,then obtained the ambient noise tomography on a 0.25° × 0.25° grad.At the same time,the 3D S-wave velocity structure up to the depth of 25 km was derived based on the joint inversion of group and phase velocity dispersion data.Then we estimated the anomaly distribution of Rayleigh wave group velocity and phase velocity in different periods based on 3D S-wave velocity structure model and compared it with the results of the surface wave tomography.The results show that the imaging results of different periodic surface waves have little change.In most regions,the difference is less than 0.04 km/s(~1%),indicating the 3D S-wave velocity structure model could explain the observed data well.In addition,we found that there is a significant low velocity anomaly from the surface to a depth of 15 km in the region of Qiongbei.We speculate that the low velocity anomaly at a depth above 5 km may be caused by fluid filling in the fissure after volcanic eruption.As for the depth of 5~15 km,there is a more significant low velocity anomaly,and the low value anomaly is less than-10% or even lower.We propose that there may be a partially melted upper crust magma chamber.Based on the results of the quantitative relationship between the low velocity S-wave anomaly and the melting percentage of magma chamber,we calculated the maximum percentage is about 22~24% and the molten volume is about 270~340 km3 using the low velocity anomaly model whit a depth of 6 to 16 km.Due to the lack of longer period surface wave data,it is not yet possible to constrain the S-wave velocity structure of a larger depth.Therefore,whether there is a larger scale magma chamber at depths below 20 km in the region of Qiongbei is still needed for further study.At the same time,in order to obtain a more detailed shallow crustal structure and surface dry hot rock distribution in Qiongbei area,we installed 35 mobile dense arrays in the vicinity of Qiongbei Haikou,and obtained the surface of Haikou through seismic background noise imaging.The fine S-wave velocity structure distribution in the lower 5 km depth range is compared with the regional scale inversion results.It is found that there is an obvious low-speed anomaly in the middle and upper crust of Qiongbei area.