USGS landslide scientists recently completed an investigation that provides an initial assessment of areas where landslides could enter the water of Glacier Bay and generate tsunamis.
Potential Landslides and Tsunami Hazards in Glacier Bay, Alaska
This article is part of the August-September 2020 issue of the Sound Waves newsletter.
Glacier Bay and its inlets are a popular destination for cruise ships and passenger boats; about 540,000 people visited Glacier Bay National Park and Preserve (GBNPP) in 2017. A typical tour of the Bay traverses the entire length of the Bay to the glacier calving viewpoints in the Johns Hopkins and Tarr Inlets. In 2018, the National Park Service (NPS) article “Landslides and Giant Waves” said, “The combination of recent deglaciation, relatively frequent earthquakes, steep rocky slopes, and narrow inlets suggests that many locations in Glacier Bay have the potential for generating large tsunami waves” that could pose a threat to ships or boats nearby.
In recent years, there has been a spate of large landslides in GBNPP that corresponded to record-breaking warm temperatures in Alaska. With the recognition that these conditions pose a possible risk to GBNPP visitors, USGS landslide scientists recently completed an investigation and published a 2019 report in the NPS “Alaska Park Science” series that provides an initial assessment of areas where landslides could enter the water of Glacier Bay and generate tsunamis.
Although landslide-generated tsunamis are historically uncommon in GBNPP, there have been plenty of landslides in and near the park, and some of them have produced tsunamis. Lituya Bay, on the west side of the park (part of Glacier Bay National Monument in 1958), has experienced at least 3 tsunamis, the largest occurring in 1958 from an M7.8 earthquake on the Fairweather fault that triggered a rockslide. The tsunami reached 524 m above sea level on the opposite shoreline and killed two people in a small boat. More recently, in 2015, a rock avalanche in Wrangell-St. Elias National Park and Preserve about 300 kilometers northwest of GBNPP, travelled about 5 kilometers into Taan Fiord, and generated a tsunami that ran about 190 meters up the shoreline. The largest recent landslide in GBNPP was a 2016 rock avalanche on the Lamplugh glacier with the equivalent volume of roughly 28,000 Olympic-sized swimming pools. This rock avalanche occurred during the warmest year on record in Alaska.
Although there have been at least 90 M>4 earthquakes within 100 kilometers of Glacier Bay since 1958, none have been large or close enough to trigger landslides in the Park. Instead, all the recent rockslides and rock avalanches were apparently caused by climatic conditions, totaling at least 24 between 1984 and 2016. Rock avalanches are particularly dangerous because they involve large volumes of earth material, they can move long distances (>1 kilometer), and they can travel very fast (up to 100 meters per second). There is evidence that they are increasing in size and travel distance. With increased effects of climate change predicted for the future, it is important to investigate the potential impacts to landslide hazards and any associated risks to park visitors. USGS scientists want to answer these questions:
- What areas of Glacier Bay can have landslides?
- How likely and how large could they be?
- What would the risk to park visitors be?
- What can be done about it?
The USGS investigation determined where landslides could start and where they could travel within GBNPP. Areas with the most potential for landslides entering the water (where they could possibly generate waves that could affect boats) are the Johns Hopkins, Tarr, Rendu, and Tidal Bay Inlets.
An interesting find emerged during the study—Bathymetry data used during the research revealed a previously hidden landslide at the junction of Johns Hopkins and Tarr inlets that appears to have occurred sometime after 1892 when glaciers withdrew from the area. It originated above land, but most of the deposit is underwater in Glacier Bay. It’s unknown whether or not it created a tsunami, but its size makes it the largest known landslide within GBNPP.
Based on this initial investigation, USGS scientists recommended more fieldwork to determine geologic conditions that control landslide occurrence and size, systematic monitoring of steep slopes to detect slow movements or deformations that could possibly provide warning of landslide events, and tsunami modeling to determine the boat size that could be threatened by landslide-generated waves. There is clearly more work to be done to understand the risk posed by these natural hazards.
Online Tool for Landslide Risks
In October 2019, the USGS unveiled a new web-based interactive map that marks an important step toward mapping areas that could be at higher risk for future landslides. In collaboration with State geological surveys and other Federal agencies, the USGS has compiled much of the existing landslide data into a searchable, web-based interactive map called the U.S. Landslide Inventory Map.
“Although landslides occur in every State, our understanding of landslide hazards at the national scale is limited because landslide information across the United States is incomplete, varies in quality, accessibility, and extent, and what is known is not collected in a central location,” said Jonathan Godt, USGS program coordinator for Landslide Hazards.
Until now, no Federal agency has taken on the monumental task of systematically cataloging landslide occurrence across the United States. Existing digital data on landslide occurrence are held by a range of Federal, State, and local government agencies, and no central point of access has previously been available.
Potential Hazard in Prince William Sound
In May 2020, a large but slow-moving landslide, known as the Barry Arm landslide, was discovered 31 miles from Whittier, AK, on Prince William Sound. If this landslide were to fail and move rapidly into the fjord, it could trigger a tsunami that would threaten local communities. However, there is no evidence a significant failure is imminent, or that one will happen anytime soon. Still, Alaskans should understand the risk and follow advice from emergency managers to prepare for a tsunami in the unlikely event that one occurs.
Since public safety is the priority, the USGS and the Alaska Division of Geological and Geophysical Surveys have been remotely monitoring movement of the large landslide (Landslide A), a smaller landslide (Landslide B), and the northwest-facing slope on the opposite side of Barry Glacier every 24 days using satellite radar images. In addition, the National Weather Service National Tsunami Warning Center is working to put a tsunami warning system in place.
The most recent satellite data were obtained between July 13 and August 13. The results indicate that Landslide A has shown little to no movement, with less than an inch of change. Portions of the head and toe of the smaller Landslide B have shown a relatively small 3 inches of movement during the same period.
Additional radar images will be compared throughout the coming months until snowpack obscures the slope faces. See the data in the report, "Interferometric synthetic aperture radar data from 2020 for landslides at Barry Arm Fjord, Alaska."
Researchers have also completed other surveys of the landslide areas. These baseline data will serve to assess future landslide movement and released when analyzed.
Local, state, and federal partners are working to understand the likelihood of rapid movement of the landslide, to put in place a tsunami warning system, and to educate people about steps to take should such a tsunami occur.
Get more information on the interagency web site, "Barry Arm Landslide and Tsunami Hazard."