Beginning in April 2024, the Alaska Earthquake Center started hearing reports from residents around Fairbanks who were feeling shaking that wasn’t quite what they were used to from earthquakes. Instead, they were experiencing shock waves from explosions related to mining in the Goldstream Valley (Figure 1).

“It shakes the windows and the house,” says resident Christy Happ, “We worry about the windows being broken. I feel it through the house.” Some of our social media subscribers asked why they felt a quick jolt and rattle of windows that was unlike the rocking, rolling, and swaying they associate with earthquakes.

A distinct seismic couplet

Anything that shakes the ground generates seismic waves that are detected by seismometers, and the Alaska Earthquake Center monitors such activity 24/7. “We routinely record mining blasts,” says Natalia Ruppert, senior scientist with the Earthquake Center. In fact, if you pull up the legend for the interactive map on the Earthquake Center’s website, you will see that analysts categorize seismic activity in Alaska into earthquakes, landslides, explosions, and glacial and volcanic events (Figure 1). What the latter four have in common is that they are much more likely to happen right at Earth’s surface than an earthquake, which results from slip along a fault that penetrates deep into the crust.

In addition to being right at the ground surface, “mining blasts generate seismic waves that look different from earthquake waves,” says Ruppert. All types of ground shaking form the same type of compressional seismic (or P) waves that travel through the earth. Ground-surface explosions also generate a hallmark shock wave from the sudden gust of compressed air. This wave, which rattles neighborhood windows, also travels along the air–ground interface, slower than the P-waves. The shock wave has a sharp onset because of the sudden change in atmospheric pressure that the explosion generates, and it also dissipates quickly in the ground. Together the two wave types create a distinctive couplet in the seismic record (Figure 2).

Location accuracy

While seismic analysts can readily pick out the signature of mining blasts, they can be more challenging to locate accurately because they are typically small. The Goldstream events have registered as earthquake-equivalent magnitudes of 2 or less. The Alaska Earthquake Center auto-detection system (Figure 3) will populate an event directly onto our website as soon as it happens, but, Ruppert says, “because these are so small, they are not as precise, and could be off by a couple of miles.” Seismic event locations come from comparing how long it takes the seismic waves to reach different stations. The smaller the event, the weaker the seismic waves it generates and the fewer seismic stations the waves reach. Fewer stations means larger location uncertainty. The auto-detection system also has greater difficulty in accurately marking the arriving signals from explosive events because they look different from those of earthquakes. Our analysts sift through all incoming seismic waveform data in a process called relocating—marking wave arrival types to help the computer program better triangulate the origin of the shaking.

Why track blasts?

The Fairbanks area is no stranger to mining blasts, which are common occurrences at Fort Knox Gold Mine. The Earthquake Center also commonly locates explosions from Usibelli Coal Mine north of the Alaska Range, Red Dog Mine in northwest Alaska, and the Man Choh Project near Tok. This activity is not typically reported as felt. The April and May events, however, came from an unexpected locale, and were felt across a wide area.

Why does the Earthquake Center put so much effort into cataloging events as small as these? For one, says Ruppert, because they are reported by residents, and we want them to better understand what is generating the shaking that they feel. This is important in citizen science and educating the public. Happ, for example, was concerned about how such shaking might impact birds and mammals during nesting season, and can use the public information provided by the Earthquake Center to hold companies to their permitting commitments.

Additionally, by examining explosions locally and globally, we are better equipped to contribute to worldwide nuclear monitoring, which dates back to the 1960s.

Another important aspect is making sure we understand the tectonic framework that generates the truly damaging seismic activity across Alaska. “We don’t want to mistakenly label an event as an earthquake and try to interpret it as a tectonic feature,” says Ruppert. “Someone might look at the location and say, ‘Wow, there’s a new volcano here.’”

Whatever their origins, the Alaska Earthquake Center helps keep an ear on the rumblings of Alaska.