Ancient Fault Puts Portland at Risk

Geologists said Tuesday they’ve discovered evidence that a major earthquake hit what is now downtown Portland about 12,000 years ago along a fault now considered an active threat.

Geologists have long known about the so-called Portland Hills fault beneath the city, but did not know how big a risk it posed.

John Beaulieu, the chief state geologist, said at a Tuesday news conference, “There’s nothing here that should cause undue alarm.” Still, he said, “it’s a risk we simply can’t afford to ignore.”

Surface evidence of the ancient quake was discovered by Ian Madin, a state geologist, who recently noticed distortions in sediment exposed by workers digging a trench at a suburban Portland school.

Further study of the sediment indicated that a quake occurred about 12,000 years ago--roughly the end of the last Ice Age.

It may seem like a long time, Madin said, but in geologic time, it’s “almost yesterday.”

The fault zone that triggered the ancient quake is several thousand feet wide and passes directly through the heart of downtown, stretching about 30 to 50 miles.

Beaulieu said the Portland fault has the potential for an earthquake similar to the 1994 Northridge quake that did more than $15 billion in damage and killed 72 people, or the Feb. 28 quake centered near Olympia, Wash., that caused more than $2 billion in damage and injured more than 400 people in the Seattle area.

The Northridge quake reached magnitude 6.7 while the Seattle-area quake was 6.8--likely the maximum force the Portland Hills fault could generate, geologists said.

Beaulieu said more studies are needed to determine the risk the Portland Hills fault poses, but he said it will help engineers decide what steps to take to retrofit older buildings and schools.

“The discovery is important in a number of ways but needs to be kept in perspective,” Beaulieu said. “We’ve long been saying that people in the Portland area--and in fact all of Oregon--need to be prepared for destructive earthquakes.”