Glacier Bay National Park and Preserve protects 3,283,000 acres of spectacular Alaskan scenery (an area roughly the size of the state of Connecticut), so even some pretty spectacular geologic events in remote sections of the park can go unnoticed for a while. Thanks to remote sensing equipment, however, scientists have confirmed that "largest known landslide on Earth since 2010" occurred in the park earlier this month.
The slide occurred at about 5:24 a.m. local time on February 16 on the flanks of Mount La Perouse. According to Columbia University seismologist Colin Stark, the slide was triggered by the collapse of a near-vertical mountain face at an elevation of 9,200 feet. The sediment slid in a southeasterly direction, stretching across 4.6 miles and mixing with ice and snow in the process.
Preliminary estimates suggest the landslide involved 68 million metric tons of material, which Stark says would make it the largest known landslide on Earth since 2010.
If any humans were within sight or sound of the slide, they haven't reported it, so how do scientists learn about such events in remote locations?
The landslide was discovered by Stark when a rapid detection tool that sifts through data collected by a global earthquake monitoring network picked up a signal indicative of a fairly significant event. Earthquake sensors on that global network detect seismic waves—vibrations that radiate through Earth’s crust because of sudden movements of rock, ice, magma, or debris.
While both earthquakes and landslides produce both high-frequency and low-frequency waves, landslides produce more low-frequency waves on balance than earthquakes. Most earthquake detection tools are focused on high-frequency waves, but the detection tool Stark was using—the Global Centroid-Moment-Tensor (CMT) Project—also looks closely at low-frequency waves, meaning it is more likely to detect landslides than other tools.
Since the Global CMT tool had detected a significant event, while other tools managed by the U.S. Geological Survey and the Alaska Earthquake Information Center had not, Stark suspected he had found a landslide.
Inquiring minds went to work, and additional analysis of the seismic data by Columbia scientists Göran Ekström and Clément Hibert confirmed Stark’s hunch, narrowing the location down to a 10-square-mile area in southeastern Alaska.
The scientists then turned to imagery from the Landsat 8 satellite, and confirmed the location and approximate size of the slide.
Remote sensing tools such as satellites are a great aid to scientific studies, but it's hard to beat direct observation by human eyes. The first such visual confirmation of the slide occurred on February 22, 2014, when helicopter pilot Drake Olson flew over and photographed the landslide. You can view some of his dramatic images of the area at this link.
A day after Olson's flight, Landsat 8 passed over the area again, offering yet another view of the slide, and allowing further analysis of the data.
This area is no stranger to impressive landslides. In July 2012, the Traveler reported on a similar event which covered a roughly 5-mile stretch of the Johns Hopkins Glacier in the park. That slide, referred to on some scientific blogs as the Mount Lituya rock avalanche, is only about 10 kilometers from the February 16, 2014, event.