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Major fault found from Mount Hood to Cascade Locks

September 3, 2011

If you feel uncomfortable about passing through the full-body scanners at the airport, what you read next may cause you to squirm.

Hood River is the first county in the state of Oregon to be scanned in its entirety with cutting edge technology known as LIDAR - right down to its geologic "skivvies."

And, what scientists uncovered is not necessarily pretty.

Nestled amidst a network of small earthquake faults surrounding Mount Hood, the geologic mapping program team discovered a significant fault oriented between Mount Hood and Cascade Locks, approximately 10 miles west of Parkdale.

The Blue Ridge fault, as it has been named, "is significant enough that it could produce an earthquake as large as a magnitude 7. That is a number that catches geologists' eyes," said Ian Madin, chief scientist with the Oregon Department of Geology and Mineral Industries and leader of the multi-agency team conducting the work.

"A quake at that level causes significant damage," Madin said. "That is the reason we are investigating this so thoroughly."

To answer further questions about the fault and better determine its risk level and activity, Madin's team will begin digging an investigative trench along one of the fault's sections on Sept. 12, Dollar Fire permitting.

"We were to have begun next week, but the dig site is within the fire closure area," said Madin. The trench will be placed in an existing clear cut area and refilled following data collection.

"We will examine soil strata to see if fault lines cut across geologic layers, indicating one single large earthquake or multiple small ones."

The data will allow the team to adjust the magnitude 7 risk estimate either up or down, depending on what they learn.

According to Madin, the fault is at least 8 miles long, oriented northwest to southeast, and is 10-15 miles deep. At most points, you can stand with feet on either side of the fault line.

Where the fault's potential danger is perhaps more evident, can be found in the scarps (ground-level cliffs) that are left behind - results of previous fault slippage. The Blue Ridge fault has multiple evident scarps that can span 20-30 feet in width.

The fault's north end appears to terminate about 20 miles from Cascade Locks and Bonneville Dam.

"What matters most in terms of potential damage is where the epicenter of the fault lies," said Madin. The closer to the epicenter, the greater potential hazard.

For now, with additional Blue Ridge fault information due to be updated and detailed, Madin shared some advice to residents.

"Anyone living in western Oregon should be aware that we live in a seismically active region. Everyone should be taking sensible precautions. Do what you can do to prepare at home and in your workplace - and then get on with your life."

Some of those sensible precautions include securing heavy furniture and objects to structures, anchoring chimneys and foundations, maintaining home and work evacuation plans and keeping a family disaster kit. More details are available through the Red Cross.

The fault discovery is part of a larger project however whose results will become public within the next month.

"We have already completed a thorough assessment of volcanic, landslide, flood and earthquake hazard zones for the entire Hood River Valley," said Madin.

Using LIDAR (light detection and ranging) - a high-tech laser-based imaging technology cousin of RADAR - Madin's team began aerial fly-overs in 2007 to collect the finely detailed images, allowing scientists to accurately evaluate the geologic composition and evolution of the Valley.

In addition to providing accessible land hazard information, data from the project will be used to help locate water-bearing rock and improve future construction siting, among other applications.

To view a current interactive map of LIDAR images for Oregon, detailed down to the street level, visit www.oregongeology.org/dogamilidarviewer/

Comprehensive Hood River Valley maps will be available soon online in an interactive format that will allow government agencies and residents alike to view detailed geologic hazard assessments for every Valley property.

Lot by lot and home by home, viewers may see recorded volcanic eruption, flood, earthquake and landslide risk assessments. The final interactive hazard maps, once completed and reviewed, will be posted at: www.oregongeology.org.

Hood River County is working in cooperation with Madin to incorporate collected data points when possible, into its existing Webmap Online Parcel Viewer program available through the County's website: www.co.hood-river.or.us, tabbed under maps.

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