The Jettan and Gámanjunni landslides are located in Troms County, Northern Norway (Fig. ). The topography surrounding the landslides exhibits a rounded relief with erosional (e.g., paleo) surfaces on the plateaus and long, U-shaped valleys formed during the Quaternary glaciations. The lithology in the area belongs to the Caledonian thrust sheets, specifically the Kåfjord and Vaddas nappes . Table  provides a comparative overview of the properties of the two landslides.

The Norwegian Water Resources and Energy Directorate (NVE) manages the early warning systems for the two landslides. The Jettan and Gámanjunni landslide monitoring systems have been improved over time with several instruments added over the years. The Jettan landslide dataset consists of a wide range of instruments that measure displacement, groundwater level, seismic activity, and meteorology (for additional information, see Table A1). The Gámanjunni landslide dataset is limited to surface displacement and meteorological data, with no borehole or seismic measurements available to constrain subsurface conditions (for additional information, see Table A2). Borehole installations at the Jettan landslide site proved technically challenging, while at the Gámanjunni site, drilling was not undertaken due to prohibitive costs.

At the Jettan landslide, we study the period from 1 January 2015 to 12 August 2025, where the mean rate of horizontal displacement across all GNSS receivers (except GPS-3) is 8 mmyr-1, varying from 6 mmyr-1 in 2015–2016 to 11 mmyr-1 in 2025. The displacement time series shows a distinct seasonal trend for all GNSS receivers, which is most pronounced at GPS-3 (Fig. ). GPS-3 is situated on a kinematically distinct block that is structurally separated from the main landslide body. As this block moves independently from the main landslide, we excluded it to calculate the mean displacement rate of the landslide. The location of this block is highlighted in Fig. .

At the Gámanjunni landslide, we study the period from 1 January 2021 to 12 August 2025, where the mean horizontal displacement rate for all GNSS receivers is 28 mmyr-1, fluctuating from 24 mmyr-1 in 2022–2023 to 39 mmyr-1 in 2025. There is a distinct seasonal component in the displacement data, most noticeable at GPS-10 where the frontal lobe of the landslide accelerates in late spring. The seasonality is less pronounced for the other GNSS receivers relative to the Jettan landslide (Fig. ). The mean displacement rate since 2021 is more than three times that of the Jettan landslide. Both Jettan and Gámanjunni display clear seasonal fluctuations in the GNSS data, with peak velocities in late spring and late autumn. For both landslides, we report mean horizontal displacement rates calculated across all GNSS receivers, excluding GPS-3 at Jettan (Fig. ).

At the Jettan landslide, we compare the cumulative displacement over all inclinometers in the boreholes and the nearest GNSS receivers over the period from 1 January 2020 to 1 January 2024. Borehole BH-1 displays a cumulative displacement of 36 mm, while GPS-4 and GPS-5 display 46 and 40 mm, respectively. Borehole BH-2 displays a cumulative displacement of 163 mm, while GPS-3 displays 157 mm, and borehole BH-3 displays a displacement of 59 mm, while GPS-8 displays 46 mm (Fig. ). We can detect the shear planes from the cumulative displacement plots in Fig. . Analysis of the shear planes shows that borehole BH-1 is predominantly concentrated within the localized shear zone, whereas in BH-2 and BH-3, deformation is more broadly distributed and only weakly localized within the sliding zone. When we compare the displacement in sliding zones, we observe that boreholes BH-1 and BH-3 move at a similar rate (5 mmyr-1) over the period from 2018 to 2020. Borehole BH-2 upper and lower shear zones have a slightly lower displacement rate (3 and 5 mmyr-1). In 2020, borehole BH-1 started accelerating (7 mmyr-1) and accelerated again in 2022 (21 mmyr-1). During the first acceleration, borehole BH-3 also accelerated (6 mmyr-1), but slowed down in 2022 (6 mmyr-1). At the same time as the last acceleration of borehole BH-1, the amplitudes of the seasonality of borehole BH-2's upper shear plane almost doubled (from 3 mmyr-1 in 2019 to 6 mmyr-1 in 2022), and nearly tripled in 2025 (9 mmyr-1). During this period, GPS-4 and GPS-8, located at boreholes BH-1 and BH-3, continued to move at a steady displacement rate (11 ± 1 mmyr-1 from 2018 to 2024). Similarly, the lower shear zone of BH-2 maintained a displacement rate consistent with that of the upper shear zone, but without the seasonal fluctuations (4 ± 1 mmyr-1 from 2018 to 2024).

The seasonal trends visible in the GNSS time series of Jettan are harder to spot in the borehole data. A clear seasonal pattern of increased velocity in late spring is pronounced for the upper shear zone in borehole BH-2, and infrequent bursts of slip acceleration occur in boreholes BH-1, BH-2 lower shear zone, and BH-3 (Fig. ).

Over the ten years from 1 January 2015 to 12 August 2025 at the Jettan landslide and the four years from 1 January 2021 to 12 August 2025 at the Gámanjunni landslide, the daily average median air temperature was 0.2 °C at Jettan, with a maximum of 23.9 °C in 2018 and a minimum of -18.9 °C in 2016, and -1.8 °C at Gámanjunni, with a maximum of 19.6 °C in 2022 and minimum of -19.0 °C in 2024. Modeled ground temperatures at borehole BH-1 (see Sect. , Appendix, Sect. A2) provided long-term trends beyond the borehole measurement period. The simulated mean annual ground temperature (MAGT) at 1 m depth over the last decade (2015–2025) was 2.4 °C, reflecting the thermal offset relative to air temperature. The model shows positive MAGT at both 45 and 115 m depths, indicating that no permafrost is present within the landslide body, which is consistent with recent borehole observations. Over the last 50 years, the simulated MAGT at 1 m depth increased by 0.2 °C per decade.

The freeze index for the two landslides shows shorter warm periods compared to cold periods for the Gámanjunni landslide on average, with a median of 0.51 ranging from 0.11 (2023) to 0.61 (2025), while the Jettan landslide has similar durations of warm and cold periods on average with a median of 1.24 ranging from 0.57 (2023) to 1.43 (2024) over the period from 1 January 2021 to 12 August 2025 (Fig. ).

The median yearly rain and snowmelt is 940 mm, ranging from 659 mm in 2021 to 1254 mm in 2022 for the Jettan landslide area, and 1324 mm, ranging from 905 mm in 2021 to 1700 mm in 2022 for the Gámanjunni area. At the Jettan landslide site, water pressure recorded in borehole BH-1 consistently indicates minimal or absent pore pressure. In contrast, borehole BH-2 exhibits pronounced seasonal variations of up to 17 m, with peak levels occurring in spring and remaining elevated throughout the year. Borehole BH-3 shows a similar timing of the peak, but with a smaller amplitude of up to 5 m, followed by a sharp decline in water level (Fig. ) The relative seismic velocity variation (dv/v) exhibits a recurring seasonal pattern. It drops every spring and reaches two distinct minima below the two standard deviation threshold, one in summer and another in autumn. The signal then gradually recovers to near-baseline levels in mid-winter before the cycle begins again in spring (Fig. ). For the period from 1 January 2016 until 31 July 2025, the dv/v trend is decreasing by -0.23 %yr-1 (Fig. A3).

To better understand landslide driving mechanisms in Arctic settings, we compare the kinematics of the Jettan and Gámanjunni landslides. For each site, we compute the mean horizontal displacement across all GNSS receivers to highlight overall kinematic trends and reduce noise, acknowledging that this approach does not capture local variations. Both landslides exhibit similar seasonal patterns: accelerations in spring and autumn, reduced velocities in summer, and a minimum in winter (Fig. ). Velocity signals are strongly correlated, with Jettan's peak spring acceleration occurring approximately 17 d earlier than Gámanjunni's. The following discussion examines the similarities and differences between the landslides.

At Jettan, spring slip velocities exceed those in autumn, whereas Gámanjunni shows a less consistent pattern, with some years exhibiting higher autumn accelerations. These accelerations correlate strongly with water availability, as indicated by responses to rain and snowmelt in both seasons, suggesting hydrological forcing as a key driver of short-term dynamics. This interpretation aligns with established links between infiltration, groundwater pressure, and landslide displacement . Subsurface thermal conditions differ markedly between the two sites. At Gámanjunni, discontinuous permafrost is inferred from temperature sensors and geophysical observations. proposed that spring accelerations may result from meltwater input from ice and snow in fractures. During colder climatic periods, such as the Little Ice Age, permafrost formation at the Jettan landslide was likely . At present, however, the landslide lies below the regional mountain permafrost limit, and evidence from temperature sensors, borehole measurements, and CryoGrid modeling indicates that permafrost is absent. Nevertheless, multi-year ice persists within the backscarp, which may influence hydrological processes by delaying and amplifying infiltration as it melts .

In autumn, infiltration is lower than in spring because snow has melted. Increased slip velocity in autumn correlates with rain events at Jettan, while the relationship is more complex at Gámanjunni. Air temperature and permafrost cooling may influence autumn velocities, as maximum temperature is critical for rock glaciers deformation and likely for landslides affected by permafrost . Notably, in some years, the annual peak velocity occurs in autumn, suggesting that permafrost may influence seasonal acceleration (Fig. ).

The Gámanjunni landslide moves three times faster than Jettan. Several factors may contribute to this difference, including geological structural characteristics, slope angle (29° at Jettan and 32° at Gámanjunni), and ground thermal conditions. The landslides occur in different lithological units: Jettan lies within the Nordmannvik Nappe, whereas Gámanjunni is in the Kåfjord Nappe. Jettan consists of garnet–quartz–mica schists with amphibole-rich layers in the lower parts and alternating layers of calc-silicate gneiss and calcite marble in the uppermost parts . Gámanjunni comprises mica schists dominated by biotite relative to muscovite and includes amphibole-rich layers and lenses . Lithological heterogeneity at Jettan introduces multiple potential weakness zones, whereas the more homogeneous mica schist at Gámanjunni favors the development of a single dominant shear zone. Accordingly, Gámanjunni deforms as a fractured yet largely coherent rock mass along a curved failure plane , in contrast to the Jettan landslide, which exhibits a compartmentalized structure with blocks moving along listric failure planes . In addition, landslide thickness at Jettan likely shows greater spatial variability than at Gámanjunni, further promoting differential movement (see Figs. , , and ). A coherent displacement pattern suggests a more mature landsliding system , evident from displacement vectors following the fall line of Gámanjunni's failure surface at the backscarp, where the block is displaced by 150 m . Comparatively, Jettan's failure surface appears less developed and dips more steeply than displacement vectors, with little vertical offset (Figs.  and ). At Jettan, foliation dips in the same direction as the slope, but at a gentler angle, intersecting the surface downslope. In contrast, Gámanjunni's inward-dipping foliation does not intersect the surface down-slope, implying that outcropping requires a lower-angle sliding surface, likely involving internal deformation or shearing within the rock mass, as indicated by the bulging toe.

Permafrost at the Gámanjunni landslide may contribute to temperature-dependent creep . A long-term relationship has been observed between increasing landslide velocity and rising mean annual rock surface temperature as permafrost thaws . Permafrost is likely absent at Jettan, eliminating this mechanism. In contrast, spatial variability in landslide thickness at Jettan, together with listric sliding surfaces, likely enhances kinematic segmentation and internal deformation, thereby reducing slip rates. A combination of a well-developed shear plane, coherent rock mass movement, and thawing permafrost inside the landslide could be the key factors driving Gámanjunni's faster displacement. However, the role of permafrost cannot be conclusively assessed without borehole data and should be the focus of future investigations.

We interpret that the earlier onset of annual warming at Jettan, due to its lower altitude, leads to earlier snowmelt and infiltration, initiating slip acceleration 17 d before Gámanjunni in average (Fig. ). This observation highlights that landslides influenced by permafrost and those without permafrost can accelerate at similar times yet exhibit different velocities. Importantly, higher velocities do not necessarily indicate greater hazard. At Gámanjunni, increased velocity may result from temperature-dependent creep, which is unlikely to cause an abrupt failure given the pressure and temperature conditions at the shear-plane depth. In northern Norway, rock slope failures are considerably less frequent than in southern Norway . This contrast between active landslides and the absence of historic large-scale slope failures may reflect the stabilizing influence of permafrost or the mechanical properties of mica-rich bedrock, both of which can inhibit rapid failure . These same factors may also promote the persistence of active landslides. Although both landslides exhibit comparable surface kinematics, only Jettan has detailed subsurface data, enabling deeper investigation of its seasonal dynamics in the following sections.

Subsurface borehole data from the Jettan landslide provide insight into its deformation characteristics. Borehole instruments record displacement along their entire depth, whereas GNSS measures cumulative surface displacement of the whole landslide body. The similar total displacement observed between the two datasets indicates that the boreholes capture the majority of landslide movement.

Borehole data reveal that most displacement occurs within the rock volume rather than along a discrete shear zone, as shown by cumulative displacement profiles in Fig. . The Jettan landslide comprises both mica-rich rocks, which are known to exhibit slow creeping behavior , and alternating layers of gneiss and calcite. This deformation may be facilitated by multiple planes of weakness within the rock mass , enabling distributed strain along structurally weakened pathways .

Comparable volumetric displacement has been documented at the Åknes landslide in borehole KH-02-06 , which uses the same instrumentation as Jettan. However, newer boreholes at Åknes do not exhibit similar patterns, suggesting that instrument-specific limitations or installation conditions may influence the observed deformation.

After characterizing the overall volumetric deformation, we now discuss the development and acceleration of the localized shear zone in borehole BH 1, which exhibits the most pronounced deformation (Fig. ). Borehole BH-2 upper shear zone and BH-3 are referenced for comparison, while the lower shear zone in BH-2 remains steady during this analysis and is not discussed further.

Prior to the onset of shear-zone accelerations in 2020, 2022, and 2025, displacement in borehole BH-1 was predominantly accommodated within the rock mass. These acceleration phases lead to higher deformation rates and generally coincide with years of deeper snowpacks and more intense snowmelt, with the exception of 2021 (Fig. ). Although relocation of the Jettan weather station may introduce uncertainty in long-term trends, comparison with the Lyngseidet station shows similar maximum snow depth patterns (Fig. A4), supporting the observed correlation between snow depth and displacement rates. Displacement in borehole BH-1 also correlates moderately with the 14 d running sum of rain and snowmelt, with a 23 d lag, indicating that water infiltration is a key driver. This extended lag may reflect prolonged saturation, possibly linked to shear-plane depth or multi-year ice in the back scarp. Comparable relationships between groundwater pressure and landslide velocity have been documented in other slow-moving landslides . However, open boreholes may not accurately reflect in situ groundwater flow paths or pore-pressure conditions because they can intercept multiple fractures and hydraulically connect otherwise distinct aquifers . As a result, hydraulic conditions observed within the borehole may not represent those in the surrounding rock mass. Furthermore, because the magnitude of water flow along the shear planes is unknown, the relative contributions of seepage forces and pore-pressure effects cannot be quantified. These processes are therefore not considered further in the present analysis.

Initial displacement behavior in borehole BH-1 (Fig. ) differs from the expected seasonal acceleration pattern. The absence of acceleration phases in the years before 2020 indicates that meteorological forcing alone cannot explain the three acceleration periods. A plausible explanation is structural modification within the shear zone, driven by progressive damage . ARDL modeling supports this interpretation by identifying the dominant role of endogenous factors in controlling displacement. These findings suggest that internal processes increasingly govern deformation, which is consistent with a maturing and structurally evolving shear zone. This evolution may also explain the increased sensitivity to water infiltration observed in later years. Further evidence emerged after the spring acceleration of 2025, when BH-1 displayed a seasonal acceleration pattern similar to the upper zone in BH-2, characterized by rapid acceleration followed by deceleration.

A notable pattern appears when comparing boreholes BH-1 and BH-3, which are aligned along the landslide slope. Both initially show similar displacement rates, followed by acceleration in 2020. BH-3 then decelerated while BH-1 maintained high velocity. This difference might reflect transient loading associated with strain accumulation and horizontal stress redistribution , culminating in a burst of acceleration in BH-3, before slowing. A similar imbalance recurs in 2025, as BH-3 accelerates following an earlier acceleration in BH-1 (Fig. ). The relationship between these two boreholes is also supported by the ARDL model. Comparable patterns of transient acceleration and strain redistribution have been documented at the Åknes landslide .

The Jettan landslide comprises several individual blocks that exhibit different deformation patterns and hydrological sensitivities. At times, boreholes BH-1 and BH-3 appear mechanically coupled, while at other times they behave independently due to diverging velocities. We infer that the boreholes sample local asperities within a potential plane that is yet to coalesce into one through-going shear zone, where progressive damage modifies mechanical parameters locally, whereas strain imbalances act at a larger scale to influence the overall evolution of the shear zone. The increasing occurrence of simultaneous slip across these asperities suggests a maturing failure surface with an increasing likelihood of sudden failure.

Ambient seismic noise analysis reveals a clear cyclical seasonal pattern in apparent seismic velocity changes (dv/v) (Fig. ). Seismic velocity reductions typically begin in mid-spring and persist until early winter, with two distinct minima in summer and autumn. Because ambient noise is sensitive to changes within the landslide volume, these variations likely reflect changes in water content, temperature, or mechanical rigidity .

Cross-correlation analysis shows strong negative relationships between dv/v and shallow ground temperature (1 m depth), the 14 d sum of rain and snowmelt, and borehole BH-1 velocity (Table ). The long-term cyclicity of dv/v suggests a similarly fluctuating controlling mechanism. From 2013 to 2020, borehole BH-1 displacement exhibited a predominantly linear, non-seasonal trend, indicating that dv/v does not directly reflect shear zone stability (Fig. ). While both shallow temperature and water availability display seasonal signals, only rain and snowmelt exhibit two distinct annual peaks (Fig. ). Previous studies have shown a dv/v decreases with wetting (causing stiffness reduction) and increases with drying of the medium . At the Åknes landslide (fractured gneiss), reported dv/v drops between 2.6 % and 3.9 % associated with water-level increases of several meters at approximately 80 m depth. Here, the first drop in dv/v (between day 8 and 9 in Fig. b) coincides with peak velocities in GNSS and borehole shear zones, whereas the second drop (day 10 in Fig. b) aligns with GNSS acceleration but not with borehole acceleration. This pattern suggests that spring acceleration reflects localized shear deformation, while autumn acceleration represents volumetric deformation of the landslide body. Collectively, these observations support an interpretation of an annual rheological transition from rigid to compliant behavior driven by progressive wetting of the landslide.

From a monitoring perspective, 4 % to 7 % decreases in dv/v have been observed in other landslides prior to collapse or acceleration , although such signals are not consistently observed across landslide sites . At Jettan, 4 % drops in dv/v precede acceleration events, but thresholds proposed in previous studies or values exceeding two standard deviations, do not appear to serve as reliable precursors to sudden failure in this case. Distinguishing between drying and wetting, as well as isolating landslide failure effects within the expected wetting and drying range (i.e. ±2) is not feasible. However, decreases exceeding 4 % may indicate reduced landslide stability.

A gradual negative trend in dv/v is observed since 2019. Linear regression indicates a consistent rate of approximately -0.23 %yr-1. This trend may reflect progressive internal changes within the landslide system, such as increased water content or reduced structural integrity, which could facilitate future failure. However, this trend applies only to the observed period and should not be extrapolated, as future environmental or mechanical conditions may alter system behavior.

The seasonal signal in dv/v provides a valuable proxy for tracking water infiltration within the landslide body. These observations offer a physical explanation for the annual kinematic behavior of the landslide, where water saturation drives a transition from rigid block-like movement to compliant volumetric deformation. This rheological shift highlights the role of hydrological forcing in modulating landslide dynamics on a seasonal basis.

The observations presented above indicate that hydrological forcing is the primary control on seasonal acceleration of these Arctic landslides, whereas geological structure and permafrost conditions modulate deformation style and long-term creep rates. Despite contrasting thermal regimes, both landslides exhibit similar seasonal responses to snowmelt and rainfall, while the higher slip velocity and more coherent displacement pattern at Gámanjunni likely reflect a more mature failure geometry and potentially the influence of permafrost-related creep. At Jettan, deformation is distributed throughout the rock mass, and progressive damage together with increasing interaction between localized shear zones suggests ongoing failure-plane development. Ambient seismic noise observations further indicate a seasonally recurring rheological transition associated with water infiltration, linking reductions in material stiffness to a shift from localized shear deformation toward more volumetric deformation. Collectively, these results highlight the coupled roles of hydrology, structural evolution, and thermal conditions in governing the dynamics of Arctic rock slope instabilities.