Almost 37,000 residents of Multnomah County live in areas highly susceptible to landslides — including 6,700 who live on sites of earlier slides.

The new number comes from the Oregon Department of Geology and Mineral Industries, which indexed 7,000 past landslides using laser-surveying technology. It's an unprecedented look into where landslides have occurred in the past, and as a result are likely to occur in the future.

"We spent a lot of time investigating the past — the recent past, the historic past, and then deep into the geologic record," said Bill Burns, a state engineering geologist and the study's lead author. "That's really the key to understanding what might happen in the future.

And in addition to the thousands of residents living on historic landslide sites, the study found that $1.65 billion of building and land value sits on top of previous slides.

A 2014 landslide in Oso, Washington, killed 43 people and destroyed 49 buildings. That slide, the deadliest in U.S. history, occurred on a site with a recorded history of landslide activity.

That's the kind of risk researchers are trying to identify, Burns said.

Much of the highest-risk areas are on Portland's west side. More than half of Northwest Portland is at high risk of a landslide, as well as nearly a quarter of Southwest Portland (excluding the downtown area, which is lower-risk).

The department has posted a map of the historic landslides and risk zones online.

The resulting data, Burns said, could help municipalities regulate future development in landslide-prone areas and mitigate risks in areas that are already developed.

There are also steps homeowners can take to mitigate their risk, he said, including planting trees and directing stormwater away from slopes. (The agency, in partnership with Washington state officials, has published a guide for homeowners.)

Nearly all of the historical slides recorded in the Oregon survey were the result of heavy precipitation. Nearly 10 percent of them occurred in the not-too-distant past, after the heavy storms of 1996 and 1997 that flooded the Willamette Valley.

That storm season helped raise the profile of landslides as a hazard in the Northwest, Burns said.

A major earthquake, however, would also likely trigger landslides on a massive scale. Burns said that in some areas, nearly a quarter of the damage caused by a quake would be the result of slides rather than the shaking itself.

Burns said the agency planned similar studies in landslide-prone areas across the state. It's already completed a similar study in the urbanized part of Clackamas County and is now working on the Eugene metro area. Washington County is also a priority.

-- Elliot Njus

enjus@oregonian.com

503-294-5034

@enjus