PALMER -- State geologists studying a 450-square-mile swath of the Talkeetna Mountains last year turned up previously undiscovered faults near the proposed Susitna River dam.
But none of the faults appear to have been recently active, the scientists say, indicating a relatively low risk that they would generate major earthquake activity.
The Alaska Division of Geological & Geophysical Surveys released the preliminary mapping report Thursday for an area 50 miles east of Talkeetna and about 6 miles south of the dam site.
"The mapping area that we worked in only contained very old faults that are not currently active or have not been active in the last several million years," said division director Steve Masterman, who is based in Fairbanks. "While there are faults out there, there are none that would appear to be of concern for being currently active."
The Susitna-Watana Hydroelectric Project was one of three mega-projects Gov. Bill Walker halted in December amid the state budget crisis. Last week, Walker said he would allow the dam to proceed using $6.6 million in unspent funding. Should the state get the requisite federal permit, the dam is expected to become operational anywhere from 2027 to 2030, according to Emily Ford, energy policy and outreach manager for the Alaska Energy Authority, the state entity behind the dam.
Located 90 miles upstream from Talkeetna, the project would include a dam at least 700 feet tall -- making it one of the tallest in the U.S. -- plus a 39-mile-long reservoir and power plant with a road and transmission line connecting to the Alaska Intertie. Supporters say the Susitna dam would provide reliable, low-cost power and could supply about half the Railbelt's electricity needs. Critics say better power sources exist and the project could jeopardize abundant fish and wildlife, including the Susitna's prized salmon.
The prospect of destructive earthquakes in the vicinity of the dam is one of many concerns raised by critics. They point to a phenomenon linked to hydro projects known as "induced seismicity:" Water pressure from reservoirs leaking into tiny cracks can lead to minor earthquakes.
Dam opponents at the Susitna River Coalition said in an email Friday that seismic risks "remain a serious concern."
Coalition board member Whitney Wolff said concerns include not only reservoir-induced seismicity, but previously unknown faults and the potential for seismic seiche activity, or the potential for even moderate seismic activity to generate oscillating reservoir waves or landslides.
"(A)ll pose a significant threat -- both to downstream communities and the overall public investment," Wolff wrote.
Masterman, addressing only the reservoir-induced seismicity question, said the phenomenon is complicated to predict and tends to result in lower magnitude, "not structure-threatening" quakes.
"Our surface mapping doesn't shed much light on that particular issue," he said, given that earthquakes occur below the surface.
The Alaska Energy Authority, as part of the federal hydro permitting process, is conducting geologic and seismic studies within the project area, Ford said. The state mapping and the project study area overlap by about 20 square miles.
Studies will help determine the overall risk but also the optimal design for the safest construction of the dam to "withstand the largest possible earthquake in the region," she said.
The Alaska Earthquake Information Center is also studying local seismic activity within the last three years. State seismologist Mike West wasn't available for comment Friday afternoon.
The map released this week is a part of the division's multiyear assessment of the strategic and critical minerals of the western Wrangellia geologic province, an approximately 4,000-square-mile area stretching from Talkeetna to Paxson, according to a news release. The report has yet to undergo peer review.
The division is on an quest to map unexplored regions around the state, a process that could take centuries. While detailed geologic maps are available for most of the Lower 48, many areas of Alaska, including the Talkeetna Mountains, have only reconnaissance-scale geologic mapping.
This particular area rose to the top, scientists say, because of the platinum, palladium and copper potential in the region. The proximity of the dam site was also a factor.
As far as mining interest goes, the researchers found the same kind and age of rock that holds platinum in the Yukon near Kluane Lake, but no evidence of platinum itself, according to report author Evan Twelker.
The report bears the functional title "Geologic map of the Talkeetna Mountains C-4 Quadrangle and adjoining areas, central Alaska." It's highly technical and intended for use by geologists and engineers. The cover photo features state geologist Larry Freeman clad in yellow rain gear and a gray pack, looking northeast "across upper Paleozoic to Late Triassic rocks of Wrangellia" as he stands atop a boulder-strewn knoll gazing down a wild valley bisected by a river below, the mountain tops hidden in low clouds.
The geologic mapping was funded by the Legislature through the Alaska Airborne Geophysical/?Geological Mineral Inventory Program and the Alaska Strategic and Critical Minerals Assessment Capital Improvement Project, according to a news release from the division. Additional funding came through the U.S. Geological Survey's National Cooperative Geologic Mapping Program STATEMAP component.
The map and accompanying booklet may be downloaded from the DGGS website.