Geologists want to learn more about Baker Valley's earthquake risk

Jul. 15—Wildfires and weather are more frequent threats to Baker Valley, but scientists say earthquakes pose a potential risk here as well.

They don't know a lot about the level of that risk, though, compared with other parts of Oregon.

Jason McClaughry would like to change that. McClaughry, who lives and works in Baker City, is the geological survey manager for the Oregon Department of Geology and Mineral Industries (DOGAMI).

He said the last full-scale survey of the Baker Valley was done by the U.S. Geological Survey (USGS) in the 1970s, when geologists had fewer weapons in their seismological quivers. It's not that those scientists were wrong — these studies accurately describe rock structure and fault placement, McClaughry said — but it's getting to be "vintage work."

"Major advances in the geological sciences in the last 50 years would allow us to put together a much better picture and understanding of faulting, earthquake history and potential risk in Baker County," McClaughry said.

Since those initial surveys, more earthquake-prone areas of the state have taken priority, leaving geologic knowledge about Baker City in the past.

Recent projects include surveys near the Columbia Gorge and on the south coast. McClaughry said that since Oregon is a large state and USGS has limited staffing, surveying the entire state is a "long-term process."

Most of the attention on temblors in Oregon has been on the Cascadia subduction zone off the coast, where one tectonic plate is dipping below another.

Scientists say the subduction zone has produced multiple quakes in the past of around 9.0 magnitude, and based on historical records, another quake could happen any time.

Baker County officials had a planning exercise in late June to prepare for the potential effects of that quake, which likely will cause catastrophic damage along the coast, due to tsunamis, and in the Willamette Valley from severe shaking.

The situation is less dire in Baker Valley.

Just a few earthquakes with magnitudes larger than 3.0, which only cause minor shaking, have occurred in Baker Valley in the past 150 years, according to a DOGAMI publication.

But that doesn't mean the seismic and geologic nature of the valley shouldn't be investigated on a deeper level, McClaughry said.

Baker Valley's geological history

The valley is what geologists call a graben — a German word meaning trench or ditch — a slab of the Earth's crust situated below two other adjacent landmasses.

In Baker Valley's case, those higher chunks of crust are the Elkhorn Mountains to the west, and the much lower hills rising east of the valley.

Those hills, made up of volcanic basalt 10 to 12 million years old, are a relatively young part of the Columbia River Flood Basalts, a massive outpouring of lava, much of it erupting from vents in the Wallowa Mountains, that flowed west along the current route of the Columbia River.

These basalts, which are a few thousand feet thick in places, were later cut through by the Columbia River, and the dark brown rocks make up the towering slopes of the river's gorge.

Some basalt flows extended to the Pacific Ocean, where they form some prominent headlands.

McClaughry uses the analogy of three building blocks to describe the relationship between the valley and the adjacent mountains or hills.

The valley is the middle block, and it's dropping, at the imperceptible rate of most geologic movements, relative to the mountains.

It's this slippage — friction between adjacent chunks of crust that occasionally is released — that causes earthquakes along what geologists call faults.

According to a 2017 USGS report partially authored by McClaughry, the fault on the western edge of the valley, at the foot of the Elkhorns, is the most recently active, in geologic time anyway.

The report says this fault has created roughly 3,000 feet of vertical displacement over the past 7 million years — the reason for the Elkhorn's prominence, with the highest peak, Rock Creek Butte, rising more than 5,000 feet above the valley floor.

Additionally, there's evidence of fault activity there within the last 2.5 million years, meaning the fault can be considered "potentially active," McClaughry said.

Light Detection and Ranging (LIDAR) evidence shows some other, smaller faults near the Elkhorns to be 120,000 years or younger, based on the fact that they cut through similarly aged glacial deposits.

Detailed geologic studies planned

With other statewide geologic studies completed, McClaughry said Baker County is near the top of the state's priority list for a survey using current technology.

"The target goal within the next five years is to begin the project," McClaughry said. "Then it would take us about three to four years to complete an adequate geologic assessment of the larger Baker Valley."

USGS geologists would use high-resolution mapping and geophysics to get a better understanding of rock formations in the valley, as well as what's underneath them. McClaughry and his colleagues from the federal agency will be able to see further into earth's crust and potentially recognize different types of fault systems — or potentially new faults they didn't know existed.

"There are certainly ones buried at depth below the valley, beneath the river fill, that we don't know their location, orientation or what they look like," McClaughry said.

They'll also get a better understanding of when the faults were last active, if they are still active, how big the potential earthquakes could be and what type of risk that might pose to the valley.

"You want to know two things: what is the largest earthquake that could develop, and how often do they occur," McClaughry said. "I don't have the answer to that at this point."

That is vital data, because the magnitude of a quake largely determines the severity of damage.

In Baker City, older, unreinforced multistory stone buildings, of which there are many in the historic downtown district, are particularly vulnerable to shaking.