HAMILTON, Mont. (AP) — A mapping method used to look at floodplains in the Bitterroot Valley turned up something unexpected: a new fault line that just might cross under the Lake Como Dam.

Jeff Lonn, a geologist with the Montana Bureau of Mines and Geology in Butte, said Ravalli County was using Light Detection and Ranging — or LIDAR — equipment as part of a floodplain study. LIDAR uses laser to map the ground, showing what the land is like devoid of homes, grasses and trees.

Mike Stickney, a senior research geologist and director of earthquake studies at the Bureau of Mines was watching a pre...