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The Big Slide
The “hazard cascade” at Elliot Creek—a landslide followed by a 100-meter-high tsunami and a raging debris flow—was a rare geological event that has sparked interest from researchers around the world. It has also turned out to be a lingering cultural catastrophe for the local Homalco First Nation.
by Tyee Bridge
MARCH 29, 2022
Göran Ekström follows earthquakes the way some people read the daily news.
An earth sciences professor at the Lamont-Doherty Earth Observatory at Columbia University in New York, he learns about seismic activity in the planet’s most remote areas—earthquakes in the Baikal Rift Zone in northern Mongolia, a landslide in Alaska’s Taan Fiord—before just about anyone else.
Ekström checks two lists regularly: one a global roundup of earthquakes, the other a shorter tally of probable landslides and other “peculiar events.”
Both lists are generated at Columbia from data amassed by the Global Seismographic Network (GSN). The GSN monitors Earth’s shakes and shudders at more than 150 stations around the world, kicking out measurements known as seismograms (Ekström and his colleagues refer to them as “wiggles”). These are the digital versions of those scrolling ink-on-paper seismic readings familiar from earthquake disaster films, and Ekström scours them for anomalies.
“I am fascinated by finding things that nobody else has found before,” says Ekström. “When I find something on these lists that looks a bit strange, I look at it more carefully.”
What separates the signature of a fault-based earthquake from less common seismic events—a colossal landslide, the calving of a Greenland glacier, the collapse of a volcanic caldera—is a fairly technical distinction that can involve phrases like “azimuthal anisotropy” and “laterally heterogeneous phase velocity structure.”
But at its core, the difference is between the jolting waves of a typical fault-slip earthquake—known as body waves, which travel through the Earth—and the slower-moving surface waves created by the impact of a falling mass.
Unlike seismic surface waves, “body” waves associated with earthquakes—primary (P) waves and secondary (S) waves—can move through the Earth‘s mantle and even through the core. Video courtesy National Science Foundation/Incorporated Research Institutions for Seismology
In late November 2020, one of the wiggles on the list of surface waves caught Ekström’s attention. Located in the Pacific Range of British Columbia’s Coast Mountains, it was of significant size: measurements ultimately judged it comparable to a Magnitude 5.0 earthquake.
This doesn’t mean that the ground in the vicinity shook as it would during a Magnitude 5.0 earthquake but that the signal was of the same strength. As Ekström says, comparing the energy of an earthquake with that of a landslide is “complicated.”
However you measure it, it was a very big boom. The event’s signature showed up at sensor stations around the world, including in Germany, Japan and Australia.
A fly-through shows the location of the Elliot Creek slide in Homalco First Nation territory on British Columbia‘s Central Coast.