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Lori Dengler | More tsunamis may be in our future – Times-Standard

The Barry Arm landslide can be seen on April 29, 2024. Red shows the approximate outline of the landslide area; the arrow is about 1.5 miles long. (Contributed)

Alaska’s landscape is impressive for everyone, but even more so for geoscientists. The mountains, insanely complex geology and more volcanoes, earthquakes and glaciers than any other part of the country make it a haven for geologists of all stripes. I was fortunate enough to spend the last week in Anchorage at the annual meeting of the Seismological Society of America and had the opportunity to experience a little piece of this extraordinary state.

The first five days were beautiful, sparkling clear and barely a breath of wind. The weather was at its best on the day of our Whittier-Prince William Sound excursion. Prince William Sound is the epicentral region of the 1964 M9.2 earthquake. The field trip notes include a discussion of what happened in ’64 and how the town of Whittier was affected, but our main focus was on future hazards, not Past.

Just reaching Whittier is exciting. Much of the hour and a half drive is along the Turnagain Arm, a long fjord that branches off at the head of Cook Inlet. The main purpose of James Cook’s third and final voyage was to find an inland passage from the Pacific to the Atlantic. As he entered the wide inlet that would later bear his name, Cook hoped they were on the right track. The head of the fjord came to a dead end and they were forced to “turn back”, which led to the name of the waterway.

The Turnagain Fjord is also famous for its tidal range. With a difference of 9 meters between the highest and lowest water levels, the currents can reach speeds of 19 kilometers per hour. The flowing inflow through the fjord creates a tidal bore. The leading edge of the incoming tide forms a wave with a distinct face as it pushes upstream against the current. We were lucky enough to pass the bore on the way. It seemed a few meters high, but can reach a height of 3 meters, high enough to attract a few intrepid surfers for a ride that can last up to an hour. Of course, if you miss the wave, you’ll have to wait 12 hours for the next one.

Turnagain’s arm ends at the now-abandoned town of Portage, so named because of the 10-mile walk on land to reach Prince William Sound. Portage was destroyed in 1964 when the land sank about six feet, flooding most of the buildings. The transfer would not be easy as Maynard Mountain, a 3,000-foot ridge, separates Portage from Whittier.

The Army had plans to develop the Whittier site before World War II as a port with water deep enough for submarines to enter stealthily. After the war started, construction accelerated to complete the harbor and tunnel through the mountain so trains could travel from Whittier to Anchorage. After the war and fifty years of controversy, the tunnel was modified so that trucks, buses and cars could finally travel the 4 km under the mountain.

It’s actually still a train tunnel. Traffic is allowed one-way every half hour during the day and is stopped periodically to allow scheduled trains to pass. A ventilation system with jet fans and continuous monitoring systems now keeps the air quality breathable. It is the longest motorable tunnel in North America.

Getting to Whittier isn’t the only unique thing about this city. Almost everyone lives in one building: the Begich Towers. For a community with only 200 permanent residents, seeing a 14-story building in the center of town is a surprise. Designed in 1953, it was built as the headquarters of the Army Corps of Engineers and as part of the planned expansion of military facilities. That expansion never materialized and was converted for non-military use after the earthquake and tsunami of ’64.

I digress. The purpose of our excursion was to highlight how climate change could impact the hazards of earthquakes and tsunamis. At first glance, you might think that earthquakes have little connection with a changing climate. Earthquakes occur kilometers below the surface, where changing surface temperatures never reach. The frequency and magnitude of earthquakes are mainly caused by tectonic forces deeper within the Earth.

But changes at the surface can have consequences at depth. Melting of glaciers causes land to rise (isostatic recovery), causing stresses that may be partly responsible for some earthquakes. Melting of the permafrost and changing groundwater distribution changes pore pressure and can push some faults closer to failure, and certainly makes areas more vulnerable to behaving like a fluid (liquefaction) when ground shaking occurs.

The link between climate change and tsunamis is clearer. The obvious connection is that higher sea levels mean more coastal areas are vulnerable to flooding. And it’s not just the water level that is important. Higher water in ports and harbors means a tsunami will cause stronger currents. Recent studies suggest that fifty years from now, a magnitude 8.8 earthquake in Alaska at the ports of Los Angeles would produce a current equivalent to what an M9 would produce today.

A second consequence of a warming climate is less obvious, but potentially more dangerous. The retreat of glaciers means that valleys that were once filled with ice are now exposed. The previously ice-covered slopes are now much more vulnerable to landslides and some of them can be large enough to cause extremely large tsunamis.

This is not a purely hypothetical argument. In the past 200 years, Lituya Bay in southeastern Alaska has had four tsunamis with heights exceeding 100 feet, all caused by landslides at the head of the bay. The 1958 earthquake-induced landslide produced waves of more than 500 meters above sea level, the highest ever recorded in modern times.

I knew the history of the Lituya Bay tsunami, but didn’t realize it was likely related to climate change until I spoke with colleague Bretwood Higman, who pointed out that the melting and retreat of glaciers in this area about 400 years ago caused the slopes above the bay have disappeared. much more vulnerable to landslides.

The highlight of the excursion was a boat ride to Barry Arm, where another Lituya Bay scenario could unfold. The three glaciers that feed Barry Arm are all retreating rapidly, exposing slopes previously supported by ice. In 2019, a local artist noticed flaws in the slope wall that attracted the attention of scientists and the media.

The USGS instrumented the landslide site with a suite of instruments that, combined with NASA images, show the landslide shifted nearly 400 feet between 2010 and 2017. The current movement has been relatively slow, but the entire mile-long-wide zone could fail catastrophically, creating a tsunami large enough to swamp maritime traffic in the region and damage the Whittier harbor area.

The Barry Arm landslide is just one of hundreds of potentially vulnerable slopes in Alaska. Globally, this number is much higher as glacier retreat accelerates. Today, about 95% of all tsunamis are caused by earthquakes. That ratio could change significantly as more frequent and larger landslides create significant waves.

Note: Two sites for further exploration: The Whittier https://www.npr.org/2015/01/18/378162264/welcome-to-whittier-alaska-a-community-under-one-roof and https:// agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2020GL089800?campaign=woletoc

Lori Dengler is a professor emeritus of geology at Cal Poly Humboldt, an expert on tsunami and earthquake hazards. Do you have any questions or comments about this column, or would you like a free copy of the preparedness magazine ‘Living on Shaky Ground’? Leave a message at 707-826-6019 or email [email protected].