FIELD OBSERVATIONS FROM A MASSIVE LANDSLIDE TSUNAMI IN TAAN FJORD, WRANGELL ST. ELIAS NATIONAL PARK, AK

The October 17^th, 2015 Tyndall Glacier landslide at the head of Taan Fjord in Wrangell St. Elias National Park was one of the largest non-volcanic landslides in the history of North America and generated a tsunami with a maximum runup of ~180 m. Seismic records were used to identify the landslide and the event was confirmed by satellite imagery several days later. Fallen trees and scoured sediments visible in the satellite imagery indicated that the landslide induced a significant tsunami.

In the spring and summer of 2016 an interdisciplinary group of scientists from universities and government agencies travelled to remote Taan Fjord to conduct a post tsunami and landslide survey. Indicators of the limit of tsunami inundation in Taan Fjord are generally consistent with a sharp trimline of stripped forest (cottonwood, alder, willow, and spruce) between 40 m and 100 m above mean high tide inside the fjord. Fallen but still rooted trees are typically oriented in the final direction of flow but scoured bark and twisted trunks indicate that they experienced multiple directions of strong flow. Sediment grains embedded in the rooted wood suggests a sediment-laden flow and sediment transport is confirmed by tsunami deposits that blanket four deltas within the fjord that are as thick as 2 m nearest the slide and tens of cm 10 km down the fjord from the landslide. Two layers were identified in the tsunami deposit; the upper layer is composed of large boulders with interstitial sand and the lower layer includes sand to cobbles with few fine silts and entrained vegetation. These overlie a clearly identifiable substrate with in place plant roots.

Rapid retreat of the tidewater Tyndall Glacier, over 6.5 km in the last 20 years, has produced over-steepened slopes near deep water that contributed to the magnitude of the landslide and tsunami. Similar glacial retreat has resulted in landslide tsunami events in Glacier Bay National Park and other fjords around the globe. Increasing tourism in these fjords makes identifying the hazards associated with landslide tsunami events critical.