长乐—南澳断裂带两侧地壳结构差异的地震-重力联合反演

Imaging crustal structure variation across the Changle-Nan'ao fault zone by the joint inversion of seismic and gravity data

长乐一南澳断裂带出露于福建沿海地区,由于海陆过渡带的特殊性,地球物理探测受到许多限制,难以获得由陆到海的清晰而准确的深部构造形态.2014年福建省地震局采集了横跨长乐一南澳断裂带的广角反射/折射剖面(HX-6),由于观测系统的缺陷和原始资料信噪比等问题,单纯使用地震数据反演长乐一南澳断裂带的深部地壳结构有很强的不确定性,无法解答断裂带两侧地壳结构存在何种差异,影响了对断裂带构造属性和区域构造演化的正确认识.基于岩石波速和密度有良好的对应关系,地震-重力联合反演可以有效降低多解性.本文采用地震走时拟合和重力异常拟合同步进行的方法,利用最新采集的高质量P波地震走时数据与高精度实测重力数据,反演得到了连城一厦门一金门外海剖面的二维地壳波速-密度结构模型.联合反演结果显示:长乐一南澳断裂带两侧地壳厚度差约3 km,壳内分层结构和上地幔顶部波速-密度无显著变化,推断长乐一南澳断裂带是华南地块正常陆壳和台湾海峡减薄陆壳的分界,本研究结果为进一步研究该区深部构造环境和长乐一南澳断裂带的地球动力学意义提供了新的地球物理学证据.

The Changle-Nan＇ao fault zone （CNFZ） is an important tectonic unit in the coastal area of Fujian, where the accurate deep tectonic structure is difficult to obtain due to the limitations and restrictions on geophysical survey in the coastal zone. A wide angle reflection/refraction profile （HX-6） was carried out in 2014 across the CNFZ by the Earthquake Administration of Fujian Province, CEA. Velocity estimation by inversion of seismic data only has its intrinsic defect such as acquisition geometry and signal to noise ratio, so it was unable to determine the difference between the two sides of CNFZ due to the uncertainty of inversion, which would have an adverse impact on the research of the tectonic attribute of CNFZ and the structural evolution in this area. On account of the good correspondence between the wave speed and density of rocks, joint inversion of seismic and gravity data can decrease non unicity of the solution. And we illustrated that the fitting of seismic travel times and gravity data should be carried out synchronously, which is the key premise of reducing the multiplicity of joint inversion. A 2-D crustal velocity-density model is acquired along the Liancheng-Xiamen-Chinmen profile based on joint interpretation of P-wave travel times with good quality and gravity field data with high accuracy. The results show that the crustal thickness decreases about 3 km beneath the CNFZ from west to east; while the lateral velocity and density have no significant changes in the crust and upper mantle, respectively. Thus we can deduce that the CNFZ is a boundary between the normal continental crust of South China and the thinned continental crust of Taiwan Strait. The final crustal velocity-density model beneath the CNFZ provides new geophysical evidences for the further research on the geodynamic significance of CNFZ and the tectonic background in this region.