InSAR DETECTION OF RENEWED MOVEMENT OF A LARGE ANCIENT LANDSLIDE IN THE COLUMBIA RIVER GORGE, WASHINGTON

The Red Bluffs landslide, part of the 36 km² Cascades landslide complex, on the Washington side of the Columbia River Gorge near Stevenson, Washington, originally mobilized southward-dipping late Tertiary to early Quaternary lava flows (basalts, andesites) and associated clay-rich, Tertiary volcaniclastic sedimentary units. Initial movement within the complex is presumed to have begun during Pleistocene time but has not been dated. A catastrophic failure of the adjacent Bonneville landslide occurred between about AD 1550 and 1750, and minor localized ground motion continued intermittently on that slide through the 20^th century. The Red Bluffs landslide has had no known historical movement.

Reactivation of about 10 km² of the Red Bluffs landslide during the winter of 2007-08 was detected from InSAR interferograms assembled from L-band satellite radar data. It was the only detectable motion within the Cascades landslide complex during that period. No motion was detected for the previous winter of 2006-07 following an intense and prolonged regional rainstorm in November 2006—a storm that triggered extensive shallow landsliding, debris flows, and flooding throughout the southern Washington-northern Oregon Cascades. The question of motion during other years awaits further data analysis. Field reconnaissance has revealed recently formed small vertical scarps 5-15 cm high and tension cracks about 1 m wide near the head of the active zone, while the InSAR data show 15-25 cm of slope-parallel motion from 06 Nov 2007 to 23 Mar 2008. InSAR and GPS monitoring of the slide is ongoing. These results indicate that InSAR can be useful in tracking movements of at least several centimeters on large, low-angle, forested landslides, as long as the density of vegetation cover does not impede image coherence.