At 8:19 p.m. Alaska Daylight Time on 17 October 2015, the side of a mountain collapsed on the western end of the head of Taan Fiord, a finger of Icy Bay. The resulting landslide generated a megatsunami in Taan Fjord. The area is uninhabited and no one was visiting it at the time, and the event went undetected for several hours until its signature was noted as a 4.9-magnitude event on the Richter scale on seismograms at Columbia University in New York City. Scientists visited the fjord in the spring and summer of 2016 to gather data on the event. Their studies revealed that the landslide consisted of about of rock with a volume of and lasted 60 to 100 seconds, reaching a maximum speed of. Some of the landslide came to rest on the foot of Tyndall Glacier, but about of rock with a volume of about entered the fjord, where it dislodged another of material from the bottom of the fjord. Some of the landslide′s debris traversed the deep water at the head of the fjord and then climbed to reach a final resting place at an elevation of about on the opposite shore. Altogether, the landslide debris covered. scientists survey run-up damage from the 17October 2015 megatsunami in Taan Fiord. Based on visible damage to trees that remained standing, they estimated run-up heights in this area of. The landslide generated a megatsunami with an initial height of about that struck the opposite shore of the fjord, with a run-up height there of, At a speed of up to, the megatsunami then continued down the entire length of the fjord – about – and into Icy Bay. In the uppermost of the fjord, run-up exceeded along the shore, and run-up heights in the upper part of the fjord otherwise varied between on the southeast shore and on the northwest side. In the middle part of the fjord, run-up heights varied greatly, dropping to as low as in some places but reaching in others. In the lower fjord, run-up heights on both sides were, increasing to at the entrance to the fjord. Along its path, the wave inundated an overall area of and left a trim line at its run-up height, stripping away all vegetation, including alder forests, and leaving behind barren beaches that reached elevations of. The wave may have been about tall when it entered Icy Bay itself about 12 minutes after the landslide, and it inundated the bay′s coastline with run-up levels of as much as in some places, although the run-up diminished to below the normal high-tide level in Icy Bay at distances greater than from the mouth of Taan Fiord. When it reached the nearest tide gauge, located to the southeast of the landslide near Yakutat, Alaska, the wave had diminished to a height of. The Taan Fiord event bore a strong similarity to the July 1958 landslide and megatsunami in Alaska′s Lituya Bay. The Taan Fiord landslide was the largest recorded in North America since the eruption of Mount St. Helens in May 1980, and the largest non-volcanic landslide in North America ever recorded. The megatsunami was the largest known marine tsunami worldwide since the Lituya Bay wave; although the Taan Fiord landslide was larger than the one at Lituya Bay, the Lituya Bay wave was larger than the one in Taan Fiord because the landslide in Taan Fiord did not fall from as great a height and landed in shallower water. The Taan Fiord wave also was the fourth-largest megatsunami of any type over the previous 100 years, with the fourth-highest run-up ever recorded anywhere in the world. Scientists assessed the landslide occurred because the retreat of Tyndall Glacier between 1961 and 1991 had left the mountainside unsupported by what had once been about of glacial ice. They also noted that heavy rains may have weakened the mountainside further, and that seismic waves from a 4.1-magnitude earthquake centered about away arrived two minutes before the landlside began and may also have contributed to the event.
