中国南海地区重力剥离对莫霍面反演的影响分析

Influence of gravity stripping in the South China Sea area on Moho inversion

确定更为精细的中国南海地区莫霍面对认识该地区内部结构、形成机制及相关动力学问题具有重要意义.重力剥离的精度直接影响莫霍面反演结果的质量.本文将由XGM2019e解算的自由空气重力扰动进行地形水深、沉积层、固体地壳密度异常和岩石圈热膨胀重力剥离,提取出莫霍面重力扰动,结合线性回归法和地震资料估计反演参数,并采用直接迭代法反演南海莫霍面模型,最后利用地震资料对反演结果进行精度评定.研究结果表明:仅顾及地形水深重力剥离的莫霍面反演结果整体精度较高,为1.94 km;在沉积物较厚的陆坡盆地,忽略沉积层的影响可使莫霍面最大加深6.30 km,但在沉积物较薄的南海海盆,考虑沉积层重力剥离会降低反演精度;利用CRUST1.0进行固体地壳重力剥离会使反演结果的精度降低0.22 km,表明CRUST1.0在南海地区并不准确;热膨胀重力剥离对反演精度的影响不大,但若忽略该效应,可能会高估洋壳区的莫霍面深度.针对南海重力莫霍面研究,笔者建议:若无精细地壳资料,忽略固体地壳重力剥离;若主要关注南海海盆可忽略沉积层的影响,但应考虑热膨胀重力剥离;若研究区覆盖沉积盆地,则应考虑沉积层重力剥离,并仔细选取密度-深度函数.

Determining more refined Moho surface in the South China Sea is of great significance to understand the interior structure,formation mechanism and related dynamic problems of the region.The accuracy of gravity stripping directly affects the quality of gravimetric Moho.In this paper,the topography/bathymetry,sediments,solid crustal density anomaly and thermal expansion of the lithosphere are stripped from the free-air gravity disturbance derived by XGM2019e,and the Moho-source gravity disturbance is extracted.The inversion parameters are estimated by combining the linear regression method and seismic data,and the direct iterative method is used to invert the Moho model of the South China Sea.Finally,the seismic data is used to evaluate the accuracy of the inversion results.The results reveal that the overall accuracy of the Moho depth inversion result only considering the topographic/bathymetric gravity stripping is the highest,at 1.94 km.In the South China Sea basin with thin sediments,considering the gravity stripping of sedimentary layer will reduce the inversion accuracy.Using CRUST1.0 for solid crust gravity stripping will reduce the accuracy of the inversion by 0.22 km,indicating that CRUST1.0 is inaccurate in the South China Sea.Thermal expansion gravity stripping has little effect on the inversion accuracy,but if this effect is ignored,the Moho depth in the oceanic crust may be overestimated.For the research on the gravimetric Moho in the South China Sea,the author suggests:if there is no fine crustal data,the gravity stripping of the solid crust should be ignored;if the main focus is on the South China Sea basin,the influence of the sedimentary layer can be ignored,but the thermal expansion gravity stripping should be considered;if the study area covers the sedimentary basin,the Gravity stripping of sediments should be considered and the density-depth function chosen carefully.