Does the recent earthquake in Anchorage tell us anything about earthquake risk at Pebble Mine?
The Nov. 30 earthquake was a big one – magnitude 7, or the equivalent of 1,000 magnitude 5 earthquakes. Though such earthquakes aren’t all that uncommon in Alaska, its location made it a monster: The epicenter was a bit north of Anchorage, and the fault it happened on extended to the south, perhaps even directly under the city.
Now that it’s happened, we know such an earthquake is possible. Infrastructure can be constructed to withstand it. Fortunately, in Anchorage, it already was. While similar-size quakes have caused huge casualties elsewhere in the world, Anchorage came out with minimal damage, thanks to post-1964 design standards.
The fault that produced the quake, however, was unknown to science. That should not be surprising. These active, buried, unknown faults are common and often produce major earthquakes. For example, recent earthquakes in Haiti; Christchurch, New Zealand; Northridge, California; Kobe, Japan; and Bam, Iran, were all on mystery faults, despite the fact that many of these places are better studied by geologists than Alaska is.
Herein lies a critical issue for Pebble Mine – how do we prepare for these earthquakes? The chances are good that a given fault would not have produced an earthquake recorded by seismologists, even if it is active. At the Pebble Mine site, tailings dams must hold waste in perpetuity. Therefore, any active fault in the area should be expected to rupture sometime in the life of the facility. Exactly where those faults are determines how much shaking the facility must be designed for.
How do scientists and engineers estimate this risk? One approach is to guess at seismicity based on nearby areas, but this is very imprecise and will never tell you whether there is a fault directly under a facility. The other is to look at geologic evidence of past earthquakes. As a scientist who studies geologic hazards, looking for such evidence is one of the things I do.
Often this search involves hunting for traces of faults – things like cracks and steps in the ground, or places where different types of bedrock come into . Once faults have been identified, geologists can look for evidence of times when the fault shifted, providing a sense of how active the fault is.
Unfortunately, earthquakes like what happened Nov. 30 don’t leave this type of evidence. If such an earthquake had instead happened 500 years ago, and we needed to know about it to be able to build a safe mine, what would we find? If we relied only on the standard approach of looking for surface traces of faults, the answer is that we’d probably find nothing.
So far, the scientists working for Pebble have studied the local fault lines and have found no evidence of recent earthquakes. They hope to use this lack of evidence to justify making facilities that are only built to withstand relatively small or distant earthquakes. However, there is a real risk they’ve missed past earthquakes that simply left less obvious evidence.
So – what could Pebble do better? Even sneaky earthquakes do leave other evidence, especially if they’re dangerously large. Shaking causes liquefaction and landslides that can be identified and dated to estimate the danger of hidden faults. The ground above earthquakes can bulge or tilt, shifting lake shores and stream channels in ways that might be identified later.
The area near Pebble Mine provides excellent opportunities for such investigation, but as yet, Pebble hasn’t done it. I am certain that if they did additional fieldwork, they’d find something worth analyzing, because I have found such evidence. Currently, I am writing a paper describing large-scale liquefaction features in numerous locations not far from Pebble. Who knows what other clues are hidden in the landscape, waiting to help untwist the story of past earthquakes and help tell Pebble what they need to do to make their proposed mine safer.
If the U.S. Army Corps of Engineers and the state of Alaska let Pebble get away with their current incomplete studies, their dams might be designed to fail in future earthquakes.
Bretwood “Hig” Higman, Ph.D., is a geologist who studies geologic hazards including earthquakes, tsunamis and landslides. He lives in Seldovia.
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