Earthquakes such as the one we recently experienced in Anchorage can certainly generate a lot of conversation about a range of issues such as how we were prepared in our homes, in our offices and in our schools.
Here at Pebble, we fully expected questions about what all of this could mean for our designs, including our tailings storage facilities. A recent opinion piece attempted to convey to Alaskans that somehow the Pebble team has not taken seismicity or seismic design seriously. This assertion is simply not true.
We recognized the seismic issues in Alaska from the very beginning of our involvement at Pebble. We have studied this issue at some length to understand what types of events could occur near Pebble and what ground waves could be generated by such earthquakes. This data have then been incorporated into our design of the facilities at site. We have assessed the impact of four seismic events:
• A magnitude 9.2 megathrust event where the Pacific Plate subducts beneath the North American Plate, 200 to 300 miles east of the site (essentially what happened in the 1964 Good Friday earthquake)
• A magnitude 8.1 intra-slab event within the subducting Pacific plate (this is approximately where a magnitude 7.1 event occurred on the Iniskin Peninsula in 2016 and is apparently the same type of earthquake experienced in Anchorage last month)
• A magnitude 7.5 event on an unidentified extension of the Lake Clark fault, 6 or so miles east of the site (in contradiction of the assertions of the recent opinion piece, we have mapped the surface geology at the site and have found no evidence of movement along the Lake Clark fault)
• A magnitude 6.5 event on a “floating” fault immediately beneath our tailings storage facility (to emphasize, even though there is no evidence of such a fault we are designing for the possibility that such a feature exists)
So, what does this mean for the lay person? It means that we have taken an appropriate approach to design our facilities for earthquakes. The recent experience in Anchorage demonstrates that with proper design, buildings and other infrastructure can pass the earthquake test. Alaskans learned a lot from the 1964 Good Friday earthquake, and as a result have incorporated seismic design requirements into most construction in the area. Alaskans should take comfort that such a mindset has extended into the regulation of tailings facilities as well.
Our data and assumptions will be thoroughly vetted in the federal and state permitting process. This is the “don’t take our word for it” message and an illustration as to why we have an objective permitting process to review the information brought forward by project applicants. Alaskans must be aware the Alaska Dam Safety Program imposes a significant regulatory regime on the design, construction, and operation of all dams in Alaska and will play very critical role in permitting at Pebble. Alaska’s Dam Safety process, perhaps one of the most stringent in the nation, will commence several years prior to construction and will continue through the operating life of the mine and into closure. The seismicity issues will be a significant component of the analysis. No tailings facility will be built at Pebble until it passes this scrutiny.
At Pebble, we will have two tailings facilities – one for what we call bulk tailings and one for what we call pyritic tailings. Both will be designed under the criteria I outlined above. The pyritic tailings storage facility will be lined to prevent acid generation of these tailings. The pyritic tailings will be relocated into the pit for subaqueous storage upon mine closure and the facility will be decommissioned. The bulk tailings facility contains essentially inert material. The facility is designed to withstand seismic events during operations, but it will actually grow more stable over time after closure as the water drains out (and is captured for treatment and release in accordance with Alaska water quality standards) and the tailings consolidate.
There is one last point that is important in this discussion. There seems to be an underlying question as to whether mines can be built to withstand seismic events. In 2010, there was a magnitude 8.8 earthquake in Chile. It had a devastating impact on most civilian infrastructure – roads, buildings, the power grid. The tailings storage facilities at the Chilean mines, however, performed exactly as designed and there were no recorded failures.
In conclusion, it is important for Alaskans to ask about our plan and we welcome the opportunity to share our approach. When it comes to earthquakes, it is an issue we have spent a lot of time on and look forward to having our work reviewed via the permitting process.
Stephen Hodgson is Senior Vice President Engineering and Project Director for the Pebble Partnership, where he oversees all aspects of engineering for the Pebble Project. He has been leading engineering and design work on the Pebble Project since 2005 and has 40 years of mining engineering experience, including the Red Dog zinc mine in Northwest Alaska.
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