A new federal study says the Teton Fault could produce ground shaking under the Jackson Lake dam in the US equivalent to a magnitude 9 earthquake lasting three minutes.

The fault is considered capable of producing only a 7.5 magnitude quake. But the forces from such a quake would be amplified and prolonged by the river and lake silt under the dam, according to the US Bureau of Reclamation study in June 2003 titled ‘Final Report, Jackson Lake Dam Ground Motion Evaluation’.

The dam, located 11.3km east of the fault in Grand Teton National Park, consists of a concrete section containing outlet works and spillway, plus an earth dike extending over a kilometre north. It rises 14.9m above the bed of the Snake river and holds 1Bm3 of water in Grand Teton National Park.

Its failure would likely inundate large portions of Jackson Hole, excluding the town of Jackson but covering most of the west bank of the Snake from Moose to Wilson and south to Munger Mountain.