In 1958, Southeast Alaska was rocked by a 7.8 magnitude earthquake. The quake triggered a rockslide and a 1,700 foot tsunami wave which slammed against the coastline before ebbing back to sea. Earthquakes are common in Alaska but the strong ones can wreak havoc on local communities. In July of 2021, the largest earthquake since 1964 hammered the Alaskan peninsula triggering a tsunami that ultimately did not threaten the residents of the peninsula.
Floating Tectonic Plates Shaped Our World
Alaska lies at the intersection of two tectonic plates, the Pacific plate and the North American plate. All life on earth is supported by floating plates on the Earth’s crust –relatively thin mineral deposits atop the molten, liquified layers that compose most of the planet below our feet. And occasionally, these pieces bump into each other. Depending on how quickly they are moving and how they collide they may form mountains, trenches, or cause earthquakes.
In Alaska, the height of the North American plate relative to the height of the Pacific plate may determine how strong the earthquakes in the region are. If the North American plate was taller than the Pacific plate, when they slide past each other this may trigger a larger, and consequently more dangerous earthquake.
But there is a structure so large and so heavy resting on the North American plate that it causes the plate to sink, shortening the gap between the two plates. What structure could be so large and powerful?
When Geological Processes Collide
Numerous glaciers rest on the North American plate in Alaska. Glaciers are formed in areas where it is difficult for snow to melt and each year more and more snow piles on top of the previous year’s snow. Eventually, the pressure of decades (or centuries!) of unmelted snow crushes the bottom layers to form the beautiful clear blue glacial ice.
But, since the Little Ice Age ended in the 1800s, Alaska’s glaciers have been retreating. Their retreat has only accelerated in recent decades due to climate change — warmer temperatures mean less unmelted snow and less glaciers. In a new study from the University of Alaska Fairbanks, Chris Rollins and his collaborators set out to understand how retreating glaciers will change earthquakes in Alaska.
Previous researchers observed a change in tectonic plate activity after the loss of glacial ice. When the ice is solid, the extreme weight of it pushes the land down but as it melts, the ground springs back up. It’s like dropping a brick onto a pillow. The brick smashes the pillow down but once it is removed, the pillow slowly regains its original height. This is called uplifting. Glacier Bay, Alaska has the fastest uplift rates in the world, perhaps due to deglaciation. Rollins and his team sought to build on that research by first determining if melting glaciers was related to the devastating 1958 earthquake and then calculate the number of earthquakes in Alaska that were related to melting ice. They hypothesized that by using historical earthquake data, plate tectonic movement models, and glacial retreat models they could prove that the epicenter, or start, of the 1958 earthquake was in the area with the most glacial ice loss.
Their results showed that although this area was known for having earthquakes, the loss of glacier ice was a key contributor to the 1958 earthquake. The 1958 earthquake, the like recent one in July 2021, occurred in the summer when ice loss is the most rapid. 1958 was also the warmest year on record since 1944. And indeed, the researchers were correct with their hypothesis — based on historical climate and glacier ice data, the epicenter of the earthquake occurred near the area with the most ice loss. The rapid loss of ice caused the land to spring up and contributed to the earthquake caused by the two plates sliding past each other.
Since the late 1700s, this area of Alaska has seen several 7.0 or greater earthquakes, Using the same historical data and models they used to estimate the effect of glaciers on the 1958 earthquake, Rollins and his team extrapolated the data all the way back to the 1770s — near the end of the little ice age and the beginning of glacial retreat in Southeast Alaska. The models estimated that glacial ice loss contributed to 23 out of the 30 high magnitude earthquakes since 1770. As the climate continues to warm and glaciers continue to retreat causing the land to spring higher and higher above the Pacific plate, we may see more frequent and higher magnitude earthquakes in Southeast Alaska.
The loss of glaciers is a well-known side effect of climate change, but little is known about the indirect effect of glacier loss. If the loss of massive chunks of ice can change the dynamics of plate tectonics, what else might they be changing? These are questions scientists are now setting out to answer.
Citation: Rollins, C., Freymueller, J. T., & Sauber, J. M. (2021). Stress promotion of the 1958 Mw∼7.8 Fairweather Fault earthquake and others in southeast Alaska by glacial isostatic adjustment and inter-earthquake stress transfer. Journal of Geophysical Research: Solid Earth, 126, e2020JB020411.