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

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.