Seasonal and precipitation-triggered movements of the Almenningar and Tungnakvíslarjökull landslides, Iceland, monitored by low-cost GNSS observations

Landslides show various characteristics of spatio-temporal distribution of movement. For example, nearby parts of the same landslide may respond differently to heavy rainfall. We report here on measurements of various episodic and transient movements, using low-cost continuously recording GNSS instruments, in two landslides areas in Iceland.

 

In the Tungnakvíslarjökull landslide, which lies on the west flank of the Katla Volcano in south Iceland, two GNSS instruments were installed in 2019 and 2020, at 830 and 650 m a.s.l. height, respectively. This landslide mass has subsided gradually by approximately 200 m in the past 70 years and has a scarp approximately 1.5 km long. The GNSS stations show movements of several decimeters per year, with most movement confined to late summer and fall each year. The lower station of the two shows distinct "jerky" motion, with instantaneous movements of 5-15 cm each time. These offsets are sometimes accompanied by regionally located seismic events occurring within seconds of the offsets. The upper station, however, moves more continuously. The landslide region experiences heavy rain in the fall season, however, also in the spring when little movement is observed. One possibility explaining the lack of motion in the spring time that frozen surface layers in spring to mid-summer may hinder precipitation from entering the landslide mass.

 

The Almenningar landslide region in north Iceland is composed of three main landslides spanning ~5 km distance. The fastest moving part is ~0.3 km wide and moves by ~1 m per year. A main road traverses the landslide area and needs frequent repairs because of differential motion. In the summer of 2022, nine continuously running GNSS stations were installed along the main road in the landslide region at 50 – 60 m a.s.l. height, with eight stations located in active parts, and one acting as a local reference station for monitoring purposes. Since the installation, three distinct movement episodes have been recorded, all following heavy rain, recorded by local and regional meteorological stations. However, different segments of the landslide area respond differently to the rain forcing, starting and stopping at different times, with some stations showing abrupt start with near-exponential decay, while some show gradual acceleration, followed by deceleration. We suggest that hydrological pressure inside the landslide governs much of its behavior.

 

In summary, the continuous low-cost GNSS observations complement spatially dense deformation techniques, such as using InSAR, differential DEM, or feature tracking. The continuous GNSS monitoring allows for great potential in understanding the time-dependent mechanics of landslides, and contributing to early warning of excessive motion.