## 3 Methods
### Ground Motion
Average motion derived is colour-coded, with red indicating motion away from the sensor and blue towards the sensor. Points A, B and P identify features of interest referred to in the text. A notable observation is that there is very little motion towards the sensor (i.e., blue) in either ground motion map. This is in part due to the presence of layover, which may obscure foreslopes, but may also be due to the fact that the sensor is less sensitive to motion on foreslopes than backslopes. This is because it takes a larger amount of motion on a foreslope to elicit the same LOS change seen by a small shift on the backslope. This is clear from Figure 5, where the same motion down a west-facing slope elicits a larger signal in the descending pass than the ascending pass. Considering the known landslide areas (adapted from [12] and shown in Figure 7), motion is observed within most of those locations, but not in both ascending and descending geometries simultaneously. On the whole, this is due to occlusion by layover and shadow in one result or the other. Other significant landslide areas are also observed outside of the known locations. One of these is a large area on the north shore that appears in both surveys, between  
Figure 7: ISBAS average velocities as calculated from the (**a**) ascending (track N\({}^{\circ}\) 100) and (**b**) descending (track N\({}^{\circ}\) 5) geometries. Known landslide areas are indicated (after [8]). Points A, B and P identify features of interest referred to in the text. the Bazayatash and Batasayf landslide areas (indicated as location P in Figures 6 and 7). This motion occurs on a steep south-facing slope that is not subject to significant layover or shadow. The observation that this area is subject to motion away from the sensor in both surveys implies that there is no strong lateral motion and the downward motion of the landslide is far greater than its motion towards the sensor in either of the viewing geometries. In fact, the presumption that this is a landslide can only be made based on the fact that it occurs on a steep slope, not from any notion of lateral motion. This illustrates a major drawback with the use of satellite InSAR from a single platform in a polar orbit; ascending and descending geometries may not detect the lateral movement of landslides on the north- or south-facing slopes as there will be little motion in the LOS direction. Possible solutions to this may be the integration of observations from different satellite platforms [8; 51] or using the multi-aperture interferometry (MAI) technique [52]. Time-series of ISBAS-derived measurements corresponding to points A and B (locations indicated in Figures 6 and 7) are shown in Figures 8 and 9. Point A is located on the Right Bank landslide (also known as the Pravoberezhniy landslide), which has an area of around 3 km\({}^{2}\) and lies approximately 5 km east of the Usoy Dam. This is an unstable slope with numerous cracks that has attracted the attention of researchers since the beginning of Lake Sarez investigations [12]. Cracks on the slope have been monitored for many years and the main landslide area was recorded as experiencing movement of up to 100 mm/year between 1985 and 1990 [53]. Significant downslope movement (~47 mm/year on average) is clearly detectable using InSAR (Figures 7 and 8), with motion on parts of the mass reaching a maximum velocity of 105 mm/year. This is in accordance with localised field measurements of 10-20 mm/year [22] and ENVISAT-derived InSAR measurements up to 120 mm/year [35], both made during the period 1998-2006. Moreover, this supports the notion that the rate of movement on the Right Bank has been constant over the past 15-20 years. It is also noted that the motion, although clear from both geometries, has a smaller range and is noisier in the ascending geometry, which again illustrates the above discussion regarding the relative lack of sensitivity over foreslopes.