Fan deltas could preserve evidence of landslide-triggered tsunamis

The October 17, 2015, Taan Fiord landslide and tsunami generated a runup of 193 meters (633 feet), nearly an order of magnitude greater than most previously surveyed tsunamis. The tsunami changed the shape of several low-gradient fan deltas (fan-shaped piles of sediment shed from adjacent highlands) within Taan Fiord, making it an excellent laboratory for characterizing signatures of a landslide tsunami. A new publication documents those changes, some of which include complete vegetation loss over more than 0.6 square kilometer (about 0.2 square mile) of fan surfaces, formation of steep fan-front scarps up to 10 meters (33 feet) high, and formation of new tsunami return-flow channels. Two relatively stable fan deltas in Taan Fiord were heavily vegetated before the Taan event and may preserve features of tsunami modification for decades to centuries. Fans in poorly monitored regions, such as Greenland, could thus hold evidence of previously unidentified recent landslide tsunami events.

Overview of Taan Fiord. Inset showing location of Taan Fiord, Alaska in Wrangell St. Elias National Park and Preserve. Taan Fiord is about 100 km from the nearest town, Yakutat, AK. Landsat 8 image of Taan Fiord acquired in 2016. Vegetation loss is clear near the water line. The evacuated subaerial landslide area and runout are visible to the left of the fjord head. Approximately 90% of the landslide entered the fjord, generating a tsunami. The other ~10% of slide material ran out on the Tyndall Glacier at the top of the image. Basemap is a spring 2016 Landsat 8 Image. Inset image is a spring 2015 Worldview Image.

(Public domain.)