PORTLAND, Ore. - Geologists said Tuesday they have confirmed suspicions that a fault capable of generating large destructive earthquakes runs through downtown Portland.

A state geologist who happened to be conducting tests at a suburban Milwaukie middle school about two weeks ago noticed some distortions in the sediment at a trench dug for a retaining wall.

Studies of the sediment at Rowe Middle School led to the first scientific confirmation that a fault zone running through the heart of the city has been geologically active during the past 12,000 years.

"They're hard to study because someone inconveniently built a city on top of these faults," said Ian Madin, the geologist who discovered the distortions.

The fault lines that pass through downtown are several thousand feet wide and stretch about 30 to 50 miles, from the Clackamas River to the south to the tip of Forest Park to the north, Madin said.

The sediment breaks were about six feet, which indicates one or more past earthquakes of magnitude 6 or above, Madin said at a news conference with other geologists from the Oregon Department of Geology and Mineral Industries.

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

It's not as big a problem as in the San Francisco Bay area, where faults are active every century, but it is a major concern for the Portland metro area, the most seismically active area of the state, geologists said.

Mosts faults in other parts of Oregon have not been active for millions of years, Madin said, "so we don't worry about them."

But evidence of activity even 12,000 years ago is relatively recent in terms of geological time, he said.

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