Relating 3D surface displacement from satellite- and ground-based InSAR to structures and geomorphology of the Jettan rockslide, northern Norway

Abstract

This study combines remote sensing data from ground- and satellite-based radar to calculate 3D displacement vectors for the Jettan rockslide, Troms, northern Norway. Using 3D displacement vectors, aspect data and strain rates in conjunction with structure (foliation, faults, fractures), geomorphological elements (ridges, scarps, terraces, depressions), topography and borehole data, we identify zones undergoing displacement, e.g., extension and compression, displacement into- or out-of-the-slope and/or various degrees of tilting. Our results show variable 3D displacement velocities, from north to south, that segment the rockslide into distinct domains. Displacement patterns are structurally controlled, as spatial variation in azimuth and plunge of 3D displacement vectors can be related to variation in attitudes of the host-rock foliation, faults and fractures. In the north, a complex graben system surrounded by orthogonal NW–SE and NE–SW-trending geomorphological elements, shows a repeated stepping 3D displacement pattern. This may indicate a complex fault geometry at depth, including stepped and discontinuous slide surfaces. We interpret 3D displacement into-the-slope in the upper part, and out-of-the-slope in the lower part, to be back-rotation of antithetic blocks with planar fractures becoming curved/listric gliding surfaces with depth. Downslope reduction in velocity indicates compression and stacking of blocks. In the southern area, N–S-trending geomorphological elements are arranged parallel to the hillslope. 3D displacement vectors show a more homogenous displacement pattern indicating movement along planar, hillslope-parallel, fracture sets at depth. We propose a structuralcontrolled slope displacement model including alternate planar- and wedge-failure, in addition to displacement along planar and listric fractures merging into foliation at depth. Using the Jettan rockslide as a case study, we show how remote sensing data may aid examination of structural and topographic controls on rockslide kinematics, thus giving new insights into subsurface geometry.