Derivation of 3-D coseismic surface displacement fields for the 2011 Mw 9.0 Tohoku-Oki earthquake from InSAR and GPS measurements

Interferometric synthetic aperture radar (InSAR) and global positioning system (GPS) have obvious deficiencies for monitoring surface deformations, for example, 1-D line-of-sight (LOS) measurements for InSAR and spatially very sparse observations for GPS. In this paper, InSAR and GPS measurements are integrated to derive spatially high-resolution 3-D coseismic surface displacement fields of the 2011 M-w 9.0 Tohoku-Oki, Japan earthquake. A unified simultaneous least squares (USLS) approach is developed to minimize the inconsistence between the InSAR results from adjacent paths. 3-D ground displacements are then derived by integrating the InSAR and the GPS measurements with the method of weighted least squares (WLS). Comparisons with independent GPS measurements show that the root mean square errors (RMSEs) of the derived 3-D displacements are 6.30, 4.57 and 1.29 cm for the vertical, east and north components, respectively. The 3-D coseismic displacement map shows that the Honshu Island moved eastwards towards the epicentre and subsided in the eastern part. The maximal displacements in the vertical, east and north directions are -1.5, 5.0 and -2.0 m, respectively. The effects of the density of GPS sites on the InSAR/GPS integration are also investigated. The experimental results reveal that lower to 70 km spatial resolution's GPS observations are adequate to guarantee the accuracies of the 3-D displacements for the Tohoku-Oki earthquake. This demonstrates the applicability of the developed WLS-based InSAR/GPS integration method, as in general the GPS observations are not as dense as those in this study area. Based on the spatially high-resolution 3-D surface displacement fields, we estimate the high-resolution and 3-D strain of the Tohoku-Oki earthquake. The preliminary results show that the Honshu Island suffers from an evident dilatation and shear during the seismic event.