基于球面重力反演苏拉威西地区莫霍面结构

基于球坐标系的地球物理反演能有效避免地球曲率的影响, 适用于大尺度构造研究.本文基于重力异常数据在球坐标系下反演莫霍面深度, 结合数据误差及光滑正则化项建立反演目标函数并求解, 同时将该方法应用于苏拉威西地区.苏拉威西地区具有复杂的断裂系统、年轻的俯冲带, 是研究俯冲起始机制等前沿科学问题的理想场所.目前研究区的地球物理观测尚不充分, 缺乏对莫霍面形成有效约束的地震数据, 对研究区莫霍面的整体认识较少.本文基于卫星重力观测数据, 通过匹配滤波方法提取与研究区莫霍面结构相关的重力异常, 并结合频谱分析确定该地区的莫霍面深度参考值.在反演中, 通过两次随机子抽样交叉验证选择最优的超参数, 包括正则化因子、莫霍面密度差以及参考莫霍面深度, 迭代反演获得莫霍面深度.反演结果表明: 研究区莫霍面平均深度为20.0 km, 深度变化范围为9.2~33.3 km.总体上, 海区莫霍面浅, 约10.0~20.0 km, 陆区莫霍面深, 约25.0~33.0 km, 该结果与Crust1.0全球模型、前人重力反演结果以及地震数据基本相符, 总体上反映了苏拉威西地区的莫霍面变化特征.

Moho structure in Sulawesi area based on spherical gravity inversion

The geophysical inversion based on spherical coordinate system is widely used in regional scale research, which can take the Earth's curvature into account. This paper presents the method of calculating Moho depth variations based on spherical gravity inversion. The inversion objective function is established by considering data error and the smoothness regularization. In addition, the method is applied to study the Moho depth variations in Sulawesi area. Sulawesi area is located in the convergence zone of the Pacific Plate, Philippine Sea Plate, Eurasian Plate and Indo-Australia Plate with young subduction zones and complex fault systems which make it an ideal place to study the initiation mechanism of subduction and many other cutting-edge scientific issues. At present, there are not many geophysical observations in the study area, so we lack reliable seismological data to constrain the Moho depth. There are few overall knowledges of the Moho structure in the study area. In this study, we extract the gravity anomaly that reflects the Moho structure from the Bouguer gravity field by matched filtering. And the reference Moho depth in the study area is given by spectrum analysis. In the inversion, three hyperparameters including the regularization parameter, the Moho density-contrast, and the reference Moho depth are estimated using the method of hold-out cross-validation twice. At last, Moho depth is obtained by iterative inversion. According to our result, the average Moho depth in Sulawesi area is about 20.0 km. The Moho depth ranges from 9.2 km to 33.3 km, which is shallower in the sea area, about 10.0~20.0 km, while deeper in the land area, about 25.0~33.0 km. Our result is in general consistent with that in Crust1.0 model, previous gravity inversion research and seismological data, so it can be seen as a reasonable result in Sulawesi area.