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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 up to the glacier calving viewpoints in the Johns Hopkins and Tarr Inlets. A 2018 article “Landslides and Giant Waves” by the National Park Service (NPS) states, “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 in the vicinity.
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 in 1958 from an M7.8 earthquake on the Fairweather fault that triggered a rockslide. The tsunami reached 524m (1,719 ft) 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 186 mi (300 km) northwest of GBNPP, travelled about 3 mi (5 km) into Taan Fiord and generated a tsunami that ran about 190m (623 ft) 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 100km (62 mi) 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, 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 (>1km or 0.6mi), and they can travel very fast (up to 100m/s or 224 mi/hr). And there is evidence that they are increasing in size and travel distance. The climate changes that are causing the more potent rock avalanches are mainly degradation and melting of high-elevation mountain permafrost, thinning and retreat of glaciers which removes support from mountain slopes, and more precipitation. Mountain permafrost is the ice in the cracks and crevices between the rocks that hold them together and help stabilize steep slopes. 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: 1) what areas of Glacier Bay can have landslides? 2) how likely and how large could they be? 3) what would the risk to park visitors be? 4) what can be done about it?
The USGS investigation summarized here and provided to the NPS addressed the first question, focusing on the West Arm of Glacier Bay where the cruise ship terminus points are located. The scientists determined where landslides could start and where they could travel. To determine where they could travel, they used a method that scientists in Norway use to model rock slides and avalanches in fjords with a similar setting to Glacier Bay. The Glacier Bay results indicate the 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 Glacier Bay National Park and Preserve.
The USGS scientists recommended that future field work be done to better determine geologic conditions that control landslide occurrence and size in order to estimate the potential landslide volumes and runout distances. They also suggested that a high-resolution lidar survey would provide better topographic data used for estimating the hazards. Tsunami modeling is needed to determine the size boat or ship that could be threatened by landslide-generated waves. Last, systematic monitoring of steep slopes could detect slow movements or deformations prior to a catastrophic event, perhaps providing a warning of impending rockslides before they happen.
- written by Lisa Wald, U.S. Geological Survey
Jeff Coe is a geologist who has been with the USGS for 30 years. Twenty-two of these years have been spent researching landslide processes and hazards. In his free time, Jeff enjoys building rock walls, hiking, and pretty much everything else outdoors.
Erin Bessette-Kirton Erin worked at the USGS for three years researching landslides in many parts of the U.S. and world. She recently began work on a PhD at the University of Utah She enjoys hiking, mountain biking, running, and skiing.
Robert Schmitt is a Geographer for the U.S. Geological Survey. He uses his skills in GIS to assist earthquake and landslide scientists. Robert is also involved in delivering GIS content to the internet via Web Applications and GIS services. In his free time he enjoys rock climbing, camping, hiking, and spending time with his family.