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Abstract. Quick-clay landslides are common geohazards in Nordic countries and Canada. The presence of potential quick clays is confirmed using geotechnical investigations, but near-surface geophysical methods, such as seismic and resistivity surveys, can also help identify coarse-grained materials associated with the development of quick clays. We present the results of reflection seismic investigations on land and in part of the Göta River in Sweden, along which many quick-clay landslide scars exist. This is the first time that such a large-scale reflection seismic investigation has been carried out to study the subsurface structures associated with quick-clay landslides. The results also show a reasonable correlation with radio magnetotelluric and travel-time tomography models of the subsurface. Other ground geophysical data, such as high magnetic values, suggest a positive correlation with an increased thickness of the coarse-grained layer and shallower depths to the top of the bedrock and the top of the coarse-grained layer. The morphology of the river bottom and riverbanks, e.g. subaquatic landslide deposits, is shown by side-scan sonar and bathymetric data. Undulating bedrock, covered by subhorizontal sedimentary glacial and postglacial deposits, is clearly revealed. An extensive coarse-grained layer (P-wave velocity mostly between 1500 and 2500 m s−1 and resistivity from approximately 80 to 100 Ωm) exists within the sediments and is interpreted and modelled in a regional context. Several fracture zones are identified within the bedrock. Hydrological modelling of the coarse-grained layer confirms its potential for transporting fresh water infiltrated in fractures and nearby outcrops located in the central part of the study area. The modelled groundwater flow in this layer promotes the leaching of marine salts from the overlying clays by seasonal inflow–outflow cycles and/or diffusion, which contributes to the formation of potential quick clays.

Quick-clay landslides are important geohazards in Sweden, Norway, and Canada. While they have been studied using various geotechnical and geophysical methods, only a handful of seismic surveys have been reported for their studies. Here, we reprocess active-source seismic data from a quick-clay landslide site in southwest Sweden, to complement earlier studies of reflection imaging and first-break traveltime tomography with surface-wave dispersion analysis. Results suggest extremely low shear-wave velocities, even as low as 60-100 m/s. From a geotechnical perspective, this implies that the region classifies as a high-risk zone for landslides and construction purposes. High or anomalous values of Poisson's ratio (or similarly P-and S-wave velocity ratio) depicts a zone within the normally consolidated sediments that likely represents a coarse-grained layer, thus confirming earlier results from a number of boreholes drilled in the study area. Overall, the results present further support to the previous This article is protected by copyright. All rights reserved.2 hypothesis that the coarse-grained layer plays a major role in the formation and creation of quick-clay landslides in the study area. Additionally, an attempt to model the distribution of potential quick clays along one of the seismic profiles is performed by combination of the modelled geophysical properties and soil textures. This study illustrates the potential of seismic methods, and how integration of multiple geophysical properties and different data handling strategies can help to accurately characterise regions susceptible to quick-clay landslides.

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