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

Structurally controlled rock slope deformation in northern Norway

“…We interpret morphostructure and failure mechanisms based on a 0.5-m resolution aerial laser scanning (ALS) digital elevation model (DEM) (Kartverket 2019a), geological maps (Zwaan 1988;Zwaan et al 1998Zwaan et al , 2006Quenardel and Zwaan 2008) and extensive data sets (Nordvik et al 2010;Husby 2011;Rasmussen 2011;Hannus 2012;Bunkholt et al , 2013aBlikra and Christiansen 2014;Böhme et al 2016aBöhme et al , 2019Bredal 2016;Eriksen et al 2017b;Eriksen 2017;Eriksen et al 2017a;Andresen 2018) including field data collected sporadically by the authors from 1969 and intensely from 2008. We compared the structural setting to kinematic information using (1) in situ differential Global Navigation Satellite Systems (dGNSS) measurements when available and (2) synthetic aperture radar interferometry (InSAR) measurements based on 2015-2018 Sentinel-1 satellite images (free and open InSAR Norway mapping service insar.no; NGU 2019b) processed with a persistent scatterer interferometry algorithm (Ferretti et al 2000(Ferretti et al , 2001.…”

“…Areas without any InSAR information do not correspond to stable parts but to areas where no displacement information can be retrieved due to geometrical effects or decorrelation due to vegetation, snow, wet or quickly changing surfaces. For more information about the potential and limitations of InSAR for the characterisation of RSDs, see Barboux et al (2015), Carlà et al (2019), Eriksen et al (2017a), Frattini et al (2018), Lauknes et al (2010) and Schlögel et al (2016).…”

“…The site has a > 1/100-year probability of catastrophic failure and is considered a very high-risk object due to its characteristics and its location above dwellings, an access road and the Manndalselva River. The site has been the subject of ongoing investigations and continuous monitoring in recent years (Henderson et al 2011;Bunkholt et al , 2013bBöhme et al 2016aBöhme et al , b, 2019Eriksen 2017;Eriksen et al 2017b). The failure scenario that is considered most likely and which has been accounted for the hazard assessment is estimated to be 26 Mm 3 in volume (NGU 2019a).…”

“…The Nappe boundary daylights in the slope at ca 275 m asl, with garnet schist of the Kåfjord Nappe below (Zwaan et al 2006). Foliation dips gently towards the fjord (NW) ≤ 17° (Braathen et al 2004;Skrede 2013;Eriksen et al 2017a). The unstable mass is delineated by prominent steep orthogonal backscarps dipping NW and SW forming a 'V' shape (Fig.…”

“…It is understood that the behaviour of RSDs is largely controlled by pre-existing bedrock structure (Cruden and Varnes 1996;Agliardi et al 2001;Glastonbury and Fell 2010;Jaboyedoff et al 2009;Humair et al 2013;Stead and Wolter 2015;Pedrazzini et al 2016) and understanding the dominant controlling structures of an unstable slope increases the accuracy of volume and runout predictions of failure scenarios (Oppikofer et al 2016). Local structural and geomorphological studies combined with measurements of displacement provide a frame to discuss initiation, deformation and movement processes for various types of RSD in Troms (Lauknes et al 2010;Henderson et al 2011;Böhme et al 2016bBöhme et al , 2019Eriksen 2017;Eriksen et al 2017a, b). However, the characterisation of these slopes is complicated by complex deformation morphology, multiple failure mechanisms, ongoing rockfall and superficial and periglacial processes.…”