Integration of GPS and InSAR Measurements for Kinematic Analysis of Crustal Deformation in Taiwan

Abstract:

GPS (Global Positioning System) and InSAR (Interferometric Synthetic Aperture Radar) observations have their own characteristic: GPS can measure three-dimensional (East, North, Up) ground motion in high accuracy but has relatively sparse station coverage, while InSAR can provide densely spatial measurements of surface displacement but only in one dimension (LOS direction). In this study we adapt a velocity field method to combine InSAR measurements of LOS velocities with available GPS data. This allows us to form a velocity field in the GPS reference frame with higher spatial resolution than is available from the sparse GPS data alone. We first divide study areas into triangular meshes, and assume that the velocity varies linearly with latitude and longitude (i.e., homogeneous strain) within each triangle. Therefore, the geodetic observations within each triangle are related to the velocities of its vertices by an interpolation shape function. Given observed GPS ENU and InSAR LOS velocities, we can solve a system of linear equations for unknown velocities of the triangular vertices. With this velocity field model, we can get a continuous velocity field by interpolation of the vertex velocities and further calculate the strain rate of each triangular mesh. An additional advantage of this method is that it can provide ground motion (and their uncertainties) even in areas of InSAR incoherence. We apply this method to the Taipei area and the northern Longitudinal Valley (LV) of Taiwan by combining continuous GPS data and PSInSAR (Permanent Scatter InSAR) measurements observed by Envisat and ALOS satellites. Results show that the vertical motion in the Taipei basin is episodic, with the largest strain-rate magnitude shown along the hanging-wall of the Sanchiao normal fault and the south of the Tatun volcano area. In the northern LV, ground subsidence of ~5 mm/yr is revealed, with the largest strain-rate magnitude near 23.4°~23.6°N that is consistent with an area of tectonic transition from arc-continent collision to the south to the plate subduction to the east.