GNSS imaging analysis of vertical deformation in Australian continental crust

In the study of vertical crustal deformation, the GNSS technique and InSAR technique have insufficient spatial and temporal resolutions, respectively. To better explore the continuous spatial characteristics of crustal vertical deformation, images of crustal vertical motion can be generated based on discrete GNSS station velocities; thus, the continuous spatial characteristics of crustal vertical motion can be directly revealed.

In this work, the vertical deformation of the Australian continental crust is studied via GNSS imaging. GNSS imaging was first proposed by Professor Hammond of the Nevada Geodesy Laboratory, who used this method to obtain high-resolution images of crustal vertical deformation (GNSS images) in California and Nevada, USA. As a method to obtain images of continuous crustal vertical deformation with the help of image processing technology, it can automatically eliminate the influence of abnormal observations in the study area and reveal the spatiotemporal variation characteristics of crustal vertical deformation. First, the coordinate time series of the GNSS station in Australia are used to obtain the station velocities and uncertainties via a robust nonparametric estimation method, namely, the median interannual difference adjusted for skewness (MIDAS); second, the relationships between stations with the spatial structure function (SSF) are constructed; third, a median spatial filter (MSF) is constructed and applied to eliminate velocity outliers and enhance regional common characteristics; finally, the velocity field is densified using image processing technology, and spatially continuous GNSS images in the study areas are generated. In addition, checkerboard tests and cross-checks are carried out to verify the reasonability of GNSS imaging and the reliability of the GNSS images generated with the stations in these areas. Moreover, when the velocities before and after MSF were compared, the necessity of MSF in GNSS imaging was verified, and the causes of oversmoothing and the formation of arcuate abrupt boundaries were analyzed.

The findings of this study indicated that 98% of the Australian continent and its surrounding regions experienced subsidence. In contrast, only certain areas in northern Australia and a small portion of eastern Australia exhibited an increase in crustal deformation. Notably, the subsidence rate in the eastern part of the region was higher than that observed in the central and western areas. This pattern aligns with certain impacts from climate-related load sources but contradicts the effects associated with glacial isostatic adjustment.The mean and median values of vertical deformation in and around Australia are −0.76 mm/a and −0.72 mm/a, respectively, and the vertical deformation ranges from −3 mm/a to 1 mm/a. Moreover, through checkerboard tests and cross-checks, we can conclude that MSF for sites can effectively eliminate some effects of gross errors and effectively reduce the problems of fragmentary pattern spots and regular circular abrupt edges in GNSS images. However, gross errors cannot be well identified with peak and mutation values in the filtering process when sites are sparse. In the filtering process, some peaks and mutation values may be eliminated, which makes the generated image too smooth.

Based on the research in this paper, it is concluded that GNSS images from the Australian continent accurately capture the overarching trends across extensive areas, demonstrating their reliability and correctness. Furthermore, these images effectively represent the temporal and spatial distribution characteristics of crustal vertical deformation. This method is helpful for studying the temporal and spatial distributions of crustal vertical deformation.