Fishing for evidence of prehistoric earthquakes and sockeye salmon in Eklutna Lake

EKLUTNA -- The two granite hills that Eklutna Village was named after were originally washed out of Eklutna Lake by a very large and angry fish. That's according to an etiological myth told by the Dena'ina, the people who have inhabited upper Cook Inlet since before Vitus Bering "discovered" Alaska.

And that's why, on a mythical plane of existence, it didn't surprise me on a recent visit to Eklutna Lake to see what appeared to be the world's largest ice-fishing setup perched on the ice about a half mile from shore. If there's a big fish to be caught, someone is going to try to land it.

As it turned out, what looked like an ice-fishing rig was a platform for obtaining sediment cores, and the big fish being sought was scientific knowledge.

Sediment cores from a frozen lake

A team of researchers from Belgium's Ghent University is looking for prehistoric traces of Alaska earthquakes, like the 1964 "Good Friday" Earthquake, with unusually high magnitude and destructive power, so-called megathrust earthquakes. I found three scientists fiddling with a long plastic tube packed with Eklutna lake sediment on a sunny day in late February when the ambient air temperature was 10 degrees.

Nore Praet and Maarten Van Daele suspended operation long enough to explain how the coring rig takes samples from the lake's bottom. Meanwhile, every metal and plastic object they had just pulled out of the water froze solid and, wherever possible, adhered to one or more other pieces of equipment. The scientists shrugged and smiled.

That's what happens when wet equipment is exposed to frigid air, as any ice angler will tell you. But they seemed to be enjoying their work and the sunny day on the ice all the same.

Sediment cores are collected by pounding plastic tubes -- several inches in diameter and reinforced by a metal sleeve -- into the lake bottom. The team is collecting short cores, about 3 feet long, and longer cores with lengths of about 10 feet. A series of long cores will be used to sample overlapping depths extending down to about 50 feet.

The coring platform consists of an aluminum deck that supports a pyramidal structure. It appeared to be about 18 feet tall. The same platform is used in summer and winter -- though winter is the best time to obtain sediment cores from deep lakes because the platform is more stable on ice than floating on the surface.

In winter, after the parts are hauled onto the ice by snowmachines and assembled, a couple of large pontoons are strapped under the deck. This is a prudent measure given the bouts of unseasonably warm weather in Alaska this winter. The researchers have compiled a scrapbook of photos showing the process of setting up the coring platform on Eklutna Lake.

The team also planned to collect cores from Kenai and Skilak lakes this winter, but those lakes were not frozen in late February, an unexpected development.

Megathrust earthquakes

Megathrust earthquakes don't occur every day. In fact, they may happen only once every few hundred years at a specific subduction zone. These earthquakes tend to be the most powerful because they are triggered where the gargantuan tectonic plates that comprise the Earth's crust collide, with one plate forced beneath another. Due to tremendous friction, the plates resist sliding, essentially becoming stuck together, which builds stress that is released when a fault gives way and the earth moves quickly. Megathrust earthquakes are so massive that the Earth's axis can shift slightly due to the redistribution of mass. The 2011 earthquake in Japan knocked the planet's axis more than six inches off kilter.

The 1964 Alaska earthquake, which measured 9.2 on the Richter scale, was the second most powerful to be measured by a seismograph, following the Great Chilean Earthquake of 1960, which measured 9.5. Because the Richter scale is logarithmic, the Chilean quake was 2.8 times stronger than Alaska's. Both Alaska and Chile have had more than their share of megathrust earthquakes, a fact not lost on the Ghent University researchers. The team has also taken core samples in Chilean lakes to study prehistoric seismic and volcanic events.

With more accurate, long-term measurements of the recurrence of these earthquakes in a variety of locales, scientists can better predict, roughly, when the next megathrust earthquake might occur.

Accurate historical records don't go back much more than 200 years in Alaska, and recording seismometers have only been available for about half that time. However, paleoseismologists have found a way to measure timing and magnitude of megathrust earthquakes in the distant past by using lake sediments.

Seismic shaking causes subaquatic landslides that generate well-defined deposits of sediments on the bottom of the lake. These are easy to differentiate from annual sediment layers in glacial lakes, which consist of fine-grained sediments deposited in winter and coarser sediments in summer. In cross section annual sediment layers look like tree rings, and the chronology of large seismic events can be aged much like a forester can document past wildfires based on burns buried deep inside a tree trunk.

A study of underwater sediment layers conducted in southern British Columbia identified 22 large earthquakes over the past 11,000 years. In that study area, the recurrence interval for large and megathrust earthquakes appears to be about 500 years. City officials in nearby Vancouver and Seattle are planning for the next big one, less than two centuries away.

During a reconnaissance survey of Eklutna Lake in 2012, the team from Ghent University collected short sediment cores that represented approximately the last 150 years. Those core samples contained a clear imprint of the coarse sediment left by the 1964 quake. Seismic profiles were collected from over 60 miles of transects on the lake the same summer using a sub-bottom profiler, which operates acoustically like a fish finder to locate sediment layers below the lake floor. The seismic profiles also revealed an imprint of the 1964 quake, as well as even older quake-related deposits. But the resolution of seismic profiles isn't as sharp as sediment cores.

Praet says the sediment cores they are collecting now should help untangle the seismic history recorded during the past 5,000 to 6,000 years in the three Alaska lakes.

A previously undocumented megathrust earthquake may account for the angry fish legend. One might speculate that the legend was originally conceived to explain how so much water was emptied from Eklutna Lake that it flooded the river canyon and unleashed an otherwise inexplicable primordial deluge upon a Dena'ina settlement located nine miles downstream.

Prehistoric sockeye salmon

Eklutna Village elders also tell tales of a time when large runs of sockeye salmon returned annually to Eklutna Lake. Unlike the legendary angry fish, there is every reason to believe that the sockeye salmon run existed. But, until now, there has been no way to prove it.

In this case, prehistoric means before 1929, when the first dam built on the Eklutna River would have stopped any migrating salmon from reaching the lake. Despite a concerted effort by fisheries biologists, no written eyewitness accounts have been found of sockeye salmon in the lake prior to 1929.

This is not an academic question. An agreement signed in 1991, when Eklutna Lake's hydropower facilities were transferred from the federal Alaska Power Authority to several local electric utilities, requires that the new owners mitigate previous damages and rehabilitate fish and wildlife populations in the watershed. Proving that a sockeye salmon run once took place may be the first step towards reestablishing salmon in Eklutna Lake.

In a cooperative effort, the sediment cores collected by the Belgian researchers will be used by Alaska Pacific University student Rich Johnson to document the former existence of salmon runs. Johnson, who is also on the lake helping to collect the sediment cores, plans to travel to Ghent University this spring, after the layers have been assigned ages, to take samples from years prior to 1929 for analysis.

Johnson will be able to estimate not only the presence but also the relative abundance of salmon that once inhabited the lake. Wild salmon spend years at sea before returning to spawn in freshwater. During their growing years salmon consume marine organisms. Marine nutrients have a high level of 15N, an isotope of nitrogen, compared with terrestrial sources typically found in lakes without salmon. Because you are what you eat, salmon also accumulate high levels of 15N. When salmon die after spawning, the 15N is released into the freshwater environment by decomposition, with some locked in bottom sediments. Levels of 15N in lake sediment are highly correlated with the density of salmon that spawned and died in the lake that year.

Reconstructing prehistoric climate changes

Michael Loso, an associate professor of earth sciences at Alaska Pacific University, is the principal investigator on the fish research. He's also coordinating with the U.S. Geological Survey and another university project that will use the sediment cores. David Fortin and Darrell Kaufman, paleoclimatologists from Northern Arizona University, are looking for prehistoric geological evidence of climate change.

Sediment core samples from deep lakes can be used to reconstruct climate changes thousands of years ago. These sediments are among the most accurately dated sources of proxy climate data, according to the researchers. Fortin and Kaufman will analyze sediment layers from the same cores that the Ghent University team pulls from Skilak and Eklutna lakes.

Before the Great Alaska Earthquake rocked Southcentral Alaska on March 27 nearly 50 years ago, very few Alaskans were concerned about megathrust earthquakes, climate change, or restoring salmon runs in Eklutna Lake. Two of the studies are small cogs in the global search for better understanding of earthquakes and climate change. The biggest catch, the project with the most immediate impact in Alaska, might be the fairly simple analysis to document prehistoric sockeye salmon runs in Eklutna Lake.

In a sense, the scientists really are fishing -- but the fish they expect to catch have been dead for the better part of century. If the current research leads to restoration of salmon in Eklutna Lake, perhaps its legendary giant fish will have a good excuse to simmer down. Unfortunately, that won't get us off the hook where earthquakes are concerned.

Rick Sinnott is a former Alaska Department of Fish and Game wildlife biologist. The views expressed here are the writer's own and are not necessarily endorsed by Alaska Dispatch. Contact him at rickjsinnott(at)gmail.com.

Rick Sinnott

Rick Sinnott is a former Alaska Department of Fish and Game wildlife biologist. Email him: rickjsinnott@gmail.com