When a wall of rock collapsed into Southeast Alaska’s Tracy Arm last August, it triggered one of the highest-reaching tsunamis ever recorded in the world. The event happened to be in a place heavily visited by cruise ships.

A study newly published in the journal Science describes its massive magnitude.

The landslide dumped 64 million cubic meters of debris into the water, triggering a megatsunami with an initial wave of 328 feet, or 100 meters, the study said. The tsunami pushed water as high as 1,580 feet, or 481 meters, up the slope on the opposite side of the fjord and, as it traveled to Tracy Arm’s mouth, stripped the surface of the mountain face to bare rock.

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The tsunami had the second-highest slope run-up on record, with a height just below the record 1,730-foot run-up in Alaska’s Lituya Bay that was created in 1958 by an earthquake-generated slide.

Unlike the Lituya Bay event, the Tracy Arm slide had no connection to any earthquakes, even though it created its own seismic signal equivalent to a magnitude 5.4 quake.

Instead, the collapsed rock wall had been destabilized by the melt of the glacier that used to buttress it, coupled with rain. It is an example of a hazard that is becoming more prominent in coastal regions of Alaska that attract crowds of tourists.

“With fjord regions increasingly visited by cruise ships, and climate change making similar events more likely, this unanticipated, near-miss event highlights the growing risk from landslides and tsunamis in coastal environments,” said the study, which had coauthors from the University of Alaska Fairbanks.

No one was hurt in the Tracy Arm landslide and tsunami, a contrast with the 1958 earthquake and tsunami, which resulted in five deaths.

The only casualties from last year’s event were some material goods. A kayak and some other equipment that campers about 25 miles away from the landslide site were lost when the huge waves washed on the island where they had set up their tent – fortuitously, in the woods rather than directly on the beach.

The campers saw the results of the tsunami, and some boaters even farther away felt it raise the water level even as the tide was falling.

Alaskans were lucky to be left unscathed by the event, considering that Tracy Arm has long been a popular tourist and recreational destination, said study coauthor Ezgi Karasözen, a seismologist at UAF’s Alaska Earthquake Center and a study coauthor.

“It was a close call. I think we dodged a bullet here, because there were multiple cruise ships in the area the day before,” she said.

It helped that the landslide resulting in the tsunami happened at 5:26 a.m. on a rainy morning, when visitors were not in the area, Karasözen said.

“It’s the timing of the event that saved us all,” she said.

In the landslide’s aftermath, tour companies that used to ferry people to Tracy Arm to watch the calving glaciers there are now skipping the area.

Large cruise lines that previously listed Tracy Arm as a stop now show Endicott Arm, a fjord farther south, on their itineraries. Allen Marine Tours, historically the largest tourism operator in Tracy Arm, isn’t sailing there this year. Instead, its boats are traveling from Juneau to Endicott Arm, farther away from the capital city.

“To my knowledge, there is no cruise-related tourism activity in Tracy Arm this year, and I’m not aware of any commercial local operators in Tracy Arm this year,” said Liz Perry, director of Travel Juneau, the city’s tourism bureau.

Similar adjustments have been made by tour companies operating in Prince William Sound, about 100 miles east of Anchorage. They have swapped out different destinations for Barry Arm, a fjord where a mountain slope is already moving and is in danger of a massive, tsunami-generating collapse.

Karasözen said it makes sense for visitors to avoid Tracy Arm for now, as there are still portions of rock face that may be in perilous shape and could collapse in new tsunami-producing landslides.

But Tracy Arm and Barry Arm are not the only coastal Alaska areas with landslide and tsunami dangers, she said.

“There are many fjords in Alaska that have steep slopes, retreating glaciers and similar conditions, so it can produce these kinds of landslides, probably, and a tsunami. So in that sense, the hazard is not confined to a single location,” she said.

How to keep safe is a “difficult question,” she said. Avoiding coastal areas entirely may be unrealistic, given that they are the areas where people want to live and visit, she said.

The Alaska Earthquake Center is working on establishing a landslide early warning system to keep people safe even as they move around coastal areas.

Already, the center has created a landslide-detection system developed by Karasözen and Mike West, Alaska’s state seismologist. The system enables scientists to identify slides and possible tsunamis within a few minutes. It expanded the monitoring program established at heavily instrumented Barry Arm, a site considered to pose tsunami risks to the community of Whittier and to the many users of western Prince William Sound. That area of the sound is heavily trafficked by vessels of different kinds, including cargo ships, tour boats, fishing boats and recreational watercraft.

The Alaska Earthquake Center scientists’ intention is to create a system to warn people about landslides before they happen, something that does not yet exist in the United States.

The Tracy Arm event provides some information that would help in that effort, according to the Science study. For several days before the big landslide, there was “microseismicity,” or movements of magnitudes too small to be felt by people, the study said. And the shrinking of Tracy Arm’s Sawyer Glacier, along with the heavy late-summer rainfall, could be seen as precursors to a slide, the study said.

James Brooks contributed to this story from Juneau.