## 5 Results and Discussion  
Figure 8 shows the total displacement distance from the beginning of the monitoring. There is a visible decrease in movement speed on both devices. Although, in the case of HIE3 this is less pronounced. Both 3D dilatometers HIE2 and HIE3 showed permanent very slow creep movement during the analyzed period, generally in the downslope sense. It can be noted from Figure 8c, that on HIE3 device the horizontal component of the movement is parallel to the general slope. On HIE2 device  
Figure 8: Cumulated spatial displacement since the beginning of the monitoring: (**a**) HIE2 dilatometer; (**b**) HIE3 dilatometer; (**c**) spatial interpretation: red line—horizontal plane, blue line—vertical plane. Figure 7: Cumulated PGA five-day sums and its moving 30-day average (red line). the horizontal component of the movement is almost perpendicular to the general slope (sinistral strike slip). This is probably a long-term behavior, which was already documented by [47] and it is recognizable on a step-like shape of the gully below the device. On both dilatometric devices the movement direction corresponds to the El Hierro earthquake and volcanic activity epicenters, which are concentrated to the west and southwest from the SAL landslide. The speed of the movement and its direction changed over time. Movement trend on the HIE2 device is well described by a second order polynomial (Figure 9a-c). The X and Y axes show a close-to-linear decrease in the speed of the movement. Analysis of GNSS and InSAR data from the beginning of the 3D dilatometer monitoring period [29] shows large absolute movements of the island in the period from July 2012 to March 2014. Although there are no permanent GNSS receivers on the SAL body, the nearest permanent GNSS stations show northeast shift and uplift of several centimeters. Similar results are shown from InSAR data, which are  
Figure 9: Decomposition of movements on the different axes showing the general decrease of the speed of the movement, black line—raw data; blue line—detrended data; red line—trend; green line—velocity. : (**a**) HIE2 \(X\)-axis; (**b**) HIE2 \(Y\)-axis; (**c**) HIE2 \(Z\)-axis; (**d**) HIE3 \(X\)-axis; (**e**) HIE3 \(Y\)-axis; (**f**) HIE3 \(Z\)-axis. available for the island. The results, however, show largest surface movements in the western and southwestern parts of the island, while the SAL body on the northeast was much more stable at that time. It has to be noted, however, that HIE2 and HIE3 automatic devices are measuring relative movements on the detachment plane, while GNSS and InSAR are showing absolute movements of the island. PS InSAR analysis performed for the period from October 2013 to October 2015 [65] shows rather different behavior on different parts of the SAL body, suggesting that the landslide moves as separate creeping blocks (as also shown in [32]). The speed of the movement during the two-year analyzed period reaches few mm.a\({}^{-1}\) downslope. It has to be noted that a principal road connecting the La Estaca port with the island's capital Valverde is crossing the SAL detachment plane just below the HIE3 dilatometer. This road has been damaged by cracks during 2012-2014 with general direction along the detachment plane, and the tarmac had to be repaired. Movement trends on the HIE3 device are much more fluctuating (Figure 9d-f). This is not surprising considering the fact, that this device is installed in the open air and thus more prone to climatic influence (i.e., temperature changes, humidity). In the case of the \(Z\)-axis, the movement speed seems to slightly increase. This is most probably due to the fact, that this device is measuring a separated block within the entire landslide body, which exhibits slightly different behavior than the rest of the landslide [32]. It can be seen, that most significant changes in the direction of the movement happened in December 2013, at the very beginning of the monitoring.