October 18, 2015 Tsunami: Taan Fjord, AK, United States

A large landslide, estimated to be the largest detected in North America since the collapse at Mount St. Helens, occurred at approximately 20:19 AKST, October 17, 2015 (05:19 UTC 10/18/2015). The estimated 200 million metric tons of rock spilled onto the toe of Tyndall Glacier and into Taan Fjord, in Icy Bay, Alaska. This landslide produced a tsunami that was recorded at two NOS tide gages monitored by the National Tsunami Warning Center (NTWC). (ref #11514)

An approximately 100-m high landslide generated tsunami flooded the coastline of Taan Fiord in SW Alaska on October 17, 2015. Inundation reached 190 m at its highest point, and exceeded 50 m along over 10 km of coastline. Within the area of inundation, several alluvial fans provided both a source and depositional setting for sediment transported by the tsunami, and each fan is blanketed by tens of cm to meters of sediment. During a month of fieldwork in spring and summer of 2016, we sampled and documented these deposits. Where the wave was largest near the tsunami source, the deposit has two distinct layers: A) a basal layer with very poorly sorted sediment ranging from sand to boulders and B) an overlying layer composed of relatively well sorted cobbles and boulders where interstitial spaces are open except at the base of this layer, where they are packed with sand. (ref #11717)

On 17 October 2015, an estimated 60 × 106m3 rockmass (Haeussler et al., 2017; Higman et al., submitted for publication) detached from as high as ~850 m above sea level at the terminus of Tyndall Glacier (Figs. 1, 3), a site of previous landsliding (Meigs et al., 2006). Approximately 90% of the mass entered directly into the fjord, creating a tsunami with runup of up to 192 m (Higman et al., submitted for publication). Some debris re-emerged from the fjord on the opposite shore of Taan Fiord and on the eastern moraine, where it formed hummocky deposits typical of large landslides. Such re-emergence of debris onto land after travel throughwater has, to our knowledge, not been described before. A small portion of the landslide debris (~3%) slid onto Tyndall Glacier and a larger slide block remained within the source area (~7%). (ref #12007)

Glacial retreat in recent decades has exposed unstable slopes and allowed deep water to extend beneath some of those slopes. Slope failure at the terminus of Tyndall Glacier on 17 October 2015 sent 180 million tons of rock into Taan Fiord, Alaska. The resulting tsunami reached elevations as high as 193m, one of the highest tsunami runups ever documented worldwide. (ref #12261)

An estimated 180 million metric tons of rock broke free and plunged into the fjord waters. This ranks the event among the largest non-volcanic landslides in North American history by volume. The impact of the landslide into the fjord displaced an enormous volume of water, generating a tsunami. Field and remote-sensing analyses later revealed that the tsunami reached runup elevations up to ~193 m above sea level on the opposite fjord wall […] The 2015 Taan Fjord tsunami inundated an area of over 20 km² around the fjord’s periphery, tearing out coastal forests, stripping vegetation and soils, and depositing debris high on hillslopes. The tsunami’s impact was most violent within the inner fjord near the landslide source, but significant effects were observed throughout the fjord and even at the outer bay. (ref #14255)