Ask the Expert: How Can the Chizhnik, Alaska Earthquake Help Predict Future Hazards? | today

Ask the Expert articles provide information and insights from MSU scholars, researchers, and scholars on national and global issues, complex research, and topics of general interest based on their areas of academic expertise and study. It may contain historical or background information, search results, or provide advice.

Earthquakes are caused by the movement of the tectonic plates that make up the Earth’s crust. Off the coast of Alaska, the Pacific Plate is pushing under the North American Plate creating enormous pressure at the Alaska-Aleutian Rift. Between 2020 and 2021, the two plates slid along this fault, triggering a series of earthquakes, including the Chineik, Alaska, earthquake on July 29, 2021, which measured 8.2 – the largest inland earthquake in 58 years.

Jeffrey Freimüller, a professor in Michigan State University’s College of Natural Sciences, is looking at this earthquake to learn more about where (and how much) this slip occurred to better understand how the faults work and to help assess the risk of future earthquakes and tsunamis. Freymueller is an internationally recognized expert in geodesy, or the study of the size and shape of the Earth, and serves as MSU Chair of Solid Earth Geology. This research recently appeared in the journal Science Advances. What are the challenges in studying the Alaska-Aleutian fault? The biggest challenge is that the rift comes to the surface on the ocean floor far from shore, and there are kilometers of water in the way! We need to measure how permanently the earth is moving due to an earthquake, and we really need measurements that are made directly over the part of the fault that has slipped.

To measure underwater earthquakes, researchers must combine data from satellites in the sky with signals from transceivers on the ocean floor. Credit: Mai Napura

On Earth, we can set up global positioning systems or global navigation satellite systems — GPS and GNSS, respectively — instruments and record plate positions fairly easily, but the part of the fault that slipped in the earthquake is very far from Earth. Radio signals from GPS satellites will not travel through water, so to obtain any data, we must combine GPS positioning of a floating platform with sound or sound waves, and positioning of the same platform relative to a set of transceivers, which pick up and emit signals at the bottom of the sea. The technique is called acoustic GPS positioning, and by repeating survey measurements before and after the earthquake, we can measure how much the sea floor has moved, and use that to better determine how the fault is moving.

Why is the study of the Chijnick, Alaska earthquake important for earthquake research? The most important thing we learned is that the total motion in the offshore acoustic GPS location was much greater than previous models of the earthquake had predicted. Large earthquakes are important to study because they produce the largest movements and cause the largest stress changes within the Earth. This means a great signal for us to measure, which is always useful. GNSS acoustic position measurements of seafloor motion are very noisy because the speed of sound through water is very sensitive to ocean temperature and varies greatly over time. This means that it is difficult to measure the vocal range as accurately as we can measure the GPS part. Therefore, it is useful to have a larger movement to measure when the noise level is high.

How does this research help in evaluating tsunami risks? This research helps assess risk and risk. An interesting feature of this earthquake is that it did not generate a large tsunami. Why didn’t you? The reason is that most of the slip on the interface between the plates occurred only on the part of the interface that was still very deep – the earthquake had not broken off on or even near the sea floor.

But we really need to know how close it came to the surface, and whether the shallow part of the fault that didn’t slide in this earthquake is able to do so in the future. If so, the risk of a major tsunami in the future is high. If the shallow part of the fault does not slip in earthquakes, but instead creeps steadily, the risk of a large tsunami from this part of the fault is much lower. We’re still not sure, but we now know more than we did before about what happened in the earthquake. We’re continuing to dig into other data, and hopefully we’ll get more GPS audio data as well to see how the shallow part of the fault behaves.

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