Alaska experiences widespread seismicity, and is prone to damaging events like the 1964 Alaska earthquake. As a consequence, seismologists have listened carefully for quaking over decades, serendipitously recording all sorts of interesting phenomena, including the shifting, cracking, and breaking of Alaska’s many glaciers.

A new Geophysical Research Letters paper, led by University of Alaska, Fairbanks doctoral student Sebin John, took a close look at seismically cataloged calving events at Alaska’s Columbia Glacier. The team connected records of glacial quakes to how Columbia’s leading edge launches icebergs into the sea.

Tidewater glaciers

Tidewater glaciers like Columbia travel from the mountains through valleys or fjords to the sea. Their ends are always touching salt water. Just like glaciers in other environments, tidewater glaciers have terminal moraines — ridges of unsorted rock piled at the glaciers’ ends. The terminal moraine for a tidewater glacier, however, is usually underwater, a shoal built by glacial processes.

Because the shoal moves along with the glacier as it advances, tidewater glaciers stay grounded (instead of floating) as they expand into their marine fjords, said Kurt Cuffey, a glaciologist at UC Berkeley who was not involved with John’s study.

This means that for an advancing or stationary tidewater glacier, the terminus is typically in shallow water, Cuffey explained. When retreat is triggered, the ice front moves behind the shoal and into deeper water. The ice begins to flow very rapidly as the entire glacier stretches and thins, he said. “That causes the front to retreat all the way through the deeper water of the fjord. It won’t stabilize until it’s back in shallower water, and that pattern repeats over time.”

This process, Cuffey said, is the tidewater glacier cycle.

For about 200 years, Columbia Glacier’s many branches wound their way through Alaska’s Chugach Mountains, joining each other near sea level and flowing, united, into Prince William Sound. Here, Columbia remained stably seated atop its moraine.

Then, the 1980s saw the beginning of retreat — the result of a glacier losing more ice than it gains from fresh snow. Tidewater glaciers lose much of their ice via calving, which can happen in different ways. In deep water, as the glacier thins and cracks, it is no longer buttressed by the terminal moraine. The underside of the glacier’s tongue is exposed to seawater, which is of course warmer than frozen ice. The water erodes the base. Massive icebergs form when huge stretches of the terminus detach. In shallow water, building-sized blocks of ice called seracs tend to topple from the front, hitting the water with a splash.

Since the 1980s, Columbia’s ice loss had moved in starts and stops. By 2010, the glacier retreated so far that it separated into two sections: the dominant east side of the glacier, which John and his colleagues refer to as Columbia Glacier, and the less voluminous western branch that the team calls Post Glacier. Since then, Columbia Glacier has retreated an additional 6.5 kilometers, and Post Glacier has retreated by more than 8 kilometers.

In spite of this, Columbia Glacier, a mere 30 kilometers west of Valdez (yes, that one), remains one of Alaska’s largest tidewater glaciers. Columbia’s dynamic terminus has been extensively studied via numerous techniques. In particular, satellite and photographic data have helped scientists see just how the glacier has sporadically, seemingly inexorably, backed away from the sea over the past few decades.

Glacial quakes

When a chunk of ice tumbles off the front of a glacier and into the ocean, the serac’s belly flop sends out seismic waves that cause the ground to shake tens to hundreds of kilometers away. This shaking is recorded as small, low-frequency earthquakes with emergent onset. Such calving-caused glacial quakes can reveal how the overall shape of a glacier’s tongue is changing. This process is distinct from other glacial quakes that rumble when crevasses form or when the glacier moves haltingly along its base in sudden jolts.

As it turns out, the Alaska Earthquake Center has recorded thousands of glacial quakes, with signals stemming from several tidewater glaciers in the southern part of the state, including Yahtse, Hubbard, La Perouse, Bering, and of course, Columbia. In Alaska’s earthquake catalog, glacial quakes are labeled as such so that they can be removed from earthquake studies.

Yet, only the most energetic calving events — the ones that make the biggest splash — produce signals detectable by Alaska’s permanent seismic network. In fact, most glacial quakes are too small to be detected. Nevertheless, Alaska’s earthquake catalog contains glacial quakes from as early as 1989, with detection rates increasing as both the network and detection algorithms improved.

Earthquakes over time

At Columbia Glacier, glacial quakes first appear in the 1990s but largely vanished by 2003 (the reasons for this aren’t clear). After 2006, the seismically quiescent ice awoke with a handful of glacial quakes spanning from that time until 2010. John’s study focused on the post-2006 era, taking a comprehensive look at what’s happened at the tongue of Columbia Glacier.

The location error of glacial quakes is much larger than for typical earthquakes, so John and colleagues examined glacial quakes within 15 kilometers of Columbia’s tongue. This whittled the number of events down to 1,520. Of those, the team successfully relocated 1,321 quakes, which clustered more tightly around the glacier’s tongue. Relocation involved the use of data from many seismic stations, including the NSF National Geophysical Facility data archive. Of these events, 1,167 occurred on Columbia Glacier, with the remainder tracing the tip of the western branch, Post Glacier.

The team compared glacial quake locations to the changing terminus position, determined from satellite data. They found that the quakes have been migrating to the northeast, tracking the glacier’s retreat, lending additional support to the link between glacial quakes and calving.

With bathymetric data, the team followed the fjords, finding that depth strongly correlates with glacial quakes. Between 2006 and 2010, when the glacier was barely awake, the terminus resided in deeper water, often more than 400 meters deep. But in the middle of August 2010, quaking and calving abruptly commenced, corresponding to the glacier’s retreat into shallow water less than 100 meters deep.

Seismicity reached a high in 2014 before dropping to a minimum in 2018. This drop coincided with the tongue’s return to deeper water. And again, when the terminus backed into shallow water in 2019, the number of glacial quakes picked up, gradually increasing through at least 2024.

The team also explored potential links between glacial quakes and environmental conditions, finding that quake rate correlates with factors like ocean temperature and precipitation.

Evolution of energy

Ice cantilevered over water is more likely to detach and drift away with little seismic fuss — what we’d expect when the tongue is in a deep part of the fjord. On the other hand, when the terminus sits in shallow water and rests on its shoal, calving tends to be dominated by seracs falling into the water with a seismic plop. “These events typically involve relatively low volumes of ice, but occur quite frequently,” the authors wrote.

The team also quantified the energy released from each glacial quake by analyzing records from station GLI, located near the glacier and continuously operated for the duration of the study period. Seismic waveform data from this station can be obtained through the NSF National Geophysical Facility.

Indeed, the team found that when the terminus was taller, the glacier produced higher energy quakes. Interestingly, the energy released by the glacial quakes changed over the years as well, with reduced energy after 2020. The authors posit that seracs are falling from lower elevations — the glacier was 35 meters higher prior to the 2020s — and so they release less energy as they plunk into the water.

Looking forward with legacy data

Today, Columbia glacier is carefully monitored compared with its initial retreat in 1980. Plus, existing catalogs of glacial earthquakes have already captured helpful information about long term changes of Alaska’s tidewater glaciers.

“This is just a case study proving what can be done,” John said. “This can be extended across multiple glaciers to learn about long term trends.” Now we know where to look, and how to read the information recorded within.