On the northern flank of Mount Hood, a scar on the earth stretches miles beneath the fir trees that hide it from airborne observers.

It is the mark of a previously undetected seismic fault that appears to be active — and dangerous. The last time it ruptured, sections of the Earth's crust jolted 6 feet apart horizontally.

"It was probably a magnitude 6 or 7 earthquake," says Ian Madin, chief scientist with the Oregon Department of Geology and Mineral Industries.

The fault was discovered by Madin's team, which is using aircraft equipped with lidar, a system that bounces laser flashes off the ground, to search for hidden faults.

The team is trying to figure how big a hazard newly discovered faults pose.

Disaster preparedness in Oregon has largely focused on massive quakes on the Cascadia Subduction Zone, where an ocean-spanning slab called the Juan de Fuca Plate is plunging beneath the North American plate along the coast. A subduction quake could cause magnitude-9 shaking that persists several minutes across Oregon and Washington.

On shallow crustal faults, quakes last for seconds, not minutes, and the shaking is more limited in reach. But shallow faults can be deadly.

The magnitude 6.3 earthquake in February on a shallow fault beneath Christchurch, New Zealand, killed more than 180 people and collapsed modern, seismically reinforced buildings.

Geologists don't know how often the newly discovered Blue Ridge Fault causes quakes, or how big and damaging they might be. They do know that the fault caused a substantial quake sometime in the past 12,000 years because it shifted rock and debris.

Lidar imaging revealed another fault near Mount Hood that runs to within a mile or two of the Columbia River Gorge, which raises questions about the earthquake resistance of hydropower dams.

"None of the dams were designed with this kind of fault in the analysis," Madin says. "If we find evidence that these faults produce large earthquakes, it might be worth re-examining dam safety."