Complete three-dimensional near-field surface displacements from imaging geodesy techniques applied to the 2016 Kumamoto earthquake

Abstract

The recent development of imaging geodesy, an advanced technique with a high spatial resolution and large-scale coverage, has enabled researchers to obtain multiple high-quality surface displacement estimates at low labor-cost, thereby improving the capability to monitor and manage geological disasters. The different sources (e.g., radar, optical and LiDAR sensors) and analysis approaches (e.g., differential interferometric synthetic aperture radar, DInSAR; multiple-aperture InSAR; pixel offset tracking; and iterative closest point, ICP) in imaging geodesy used to derive displacement estimates have unique benefits and drawbacks. However, the inherent differences among these data sources and methods in the construction of three-dimensional (3D) deformation maps, particularly in the near field, remain poorly understood and require further discussion. In this study, we acquired three pairs of ALOS-2 stripmap mode images, two pairs of Sentinel-1 TOPS mode images and pre- and post-event LiDAR data for the 2016 Kumamoto earthquake to explore the 3D near-field displacements using various imaging geodesy techniques with different types of image information, i.e., SAR phase data, SAR amplitude data and LiDAR point cloud data. Our results show that each image type is independently capable of producing a high-quality 3D deformation map for the 2016 Kumamoto earthquake with an on-fault accuracy of <43 cm determined from field work measurement, and an off-fault accuracy of <14 cm determined from GNSS observations. The major contributors to the uncertainty in the 3D deformation estimates from the SAR phase, SAR amplitude, and LiDAR data methods are the effective Doppler bandwidth, pixel resolution and topographic roughness, respectively. In the near-field deformation region, more secondary fault ruptures were revealed by SAR amplitude and LiDAR information than by the SAR phase information, thereby overcoming the sensitivity of the SAR phase signal to incoherence. Finally, an integrated complete 3D map was generated to constrain the coseismic rupture behavior of the Kumamoto earthquake sequence associated with the Futagawa and Hinagu faults. Our slip model suggests the main slip rupture a length of 32 km and terminated near the rim of the Aso caldera; additionally, the slip pattern delineated by three asperities that was dominated by right-lateral strike-slip with a minor normal slip component at a depth of 7 km. Furthermore, the main rupture triggered two secondary faults with right-lateral strike-slip and normal slip. Our source model yields a geodetic moment of 5.77 × 1019 N·m, which corresponds to Mw 7.11.