Assessment of multitemporal INSAR data for establishing regional landslides inventories

Over 2700 gravitational movements are recorded as polygons in the inventory of the Aosta region (3261 km2, Northern Italy). The surface affected by gravitational processes is about 20% of the overall surface area of the Aosta region and corresponds mostly to deep seated slope deformations, landslides and rock slope collapses. In addition, a complete set of multitemporal INSAR data has been recently made available for the same area (SqueeSAR processing by TRE, for both ascending and descending orbits, from October 2014 to February 2020).

In a first step, the distribution of INSAR data was analyzed with respect to landcover and radar geometric deformations. Main outcomes are:

• About 732’000 points were found by INSAR corresponding to a total average density of ~220 pts/km2.
• 0% of points have velocities below 1 mm/y, 20.4% between 1 and 10 mm/y, and only 0.6% more than 1 cm/y.
• The landcover is forested over 30% of the surface, covered by low vegetation on steep slopes for 46% and unvegetated for 24%, Points density are respectively 146, 200 and 369 pts /km2.
• Less than 5% of the Aosta region is affected by radar layover or shadowing. But, considering the slope direction as possible vector of displacement, 60% of INSAR velocities are underestimated of 50% or more when projected on the line of sight of the satellite (of course most of the time these are not the same slopes for ascending and descending orbits).

In a second step, we assessed the information provided by INSAR for the landslides recorded in the IFFI inventory:

• 29% of the polygons of the IFFI inventory do not include INSAR pts. However, those are mostly small zones, corresponding to only 9% of the total surface mapped as affected by gravitational movements. Most of large instabilities have INSAR points. 52% of the polygons have INSAR points from both ascending and descending orbits, and 19% from only one orbit.
• 68% of IFFI polygons have all their INSAR velocities slower than 5mm/y (for both orbits). It doesn’t mean automatically that these instabilities are dormant or slow moving, because for about half of them INSAR velocities strongly underestimate expected real velocities because of unfavorable projection on the line of sight of the satellites.
• 55 instabilities show INSAR velocities between 50 and 10 mm/y, and 31 faster than 10 mm/y.

Finally, an independent inventory was made using only the INSAR data and then compared to the IFFI inventory. In order to handle the data, a minimum velocity 2.5 mm/y was selected.

• 1437 instabilities were mapped in this inventory, covering 308 km2, for 2702 instabilities over 604 km2 in the IFFI inventory.
• About 60% of the moving area detected looking only at the INSAR data are visible on only one orbit (ascending or descending).
• 62 clusters of INSAR points with velocities higher than 1 cm/y and not in the IFFI polygons were detected. Among them, 4 sites with significant extensions will require further geological investigations.