Cascadia quake not chief among county’s natural disaster worries

Large coastal quake would likely have minimal direct impact on county

Hood River county would likely feel only a "light" shaking with some cracking of buildings and other light damage due to a large Cascadia Subduction Zone quake, according to the Oregon Resiliency Report.

Credit: Oregon Resiliancy Report
Hood River county would likely feel only a "light" shaking with some cracking of buildings and other light damage due to a large Cascadia Subduction Zone quake, according to the Oregon Resiliency Report.

The good news for Hood River residents is the effects in Hood River County of a large earthquake off the Oregon coast might not keep you up at night.

The other side of the coin is that there are plenty of other dangers in the area to keep county Emergency Management Coordinator Karl Tesch busy.

The statement sounds fairly ominous: “When, not if, the next great Cascadia subduction zone earthquake strikes the Pacific Northwest, Oregon will face the greatest challenge in its history. Oregon’s buildings, transportation network, utilities, and population are simply not prepared for such an event.”

That is the conclusion reached by the Oregon Resilience Plan, a report prepared for the state legislature outlining the potential impacts of 9.0 Earthquake in the Cascade Subduction Zone.

A large quake in the zone could cause havoc on the Oregon Coast and Portland area, potentially kill thousands and destroy critical infrastructure in the I-5 corridor.

While a 9.0 quake is formidable, its direct impact on Hood River County would likely be fairly minimal.

Hood River County prepares a natural hazards mitigation plan which was most recently updated in August.

The plan is required by the Federal Emergency Management Agency and lists the most probable natural disasters facing the county. While earthquakes make the list, it comes in behind disasters the county faces practically every year: wildfire and severe storms.

It’s also behind flood and drought — two disasters that, while they may not happen every year, are more likely to devastate the agriculture industry and county economy in general.

The county has two seperate faults running through it, one along the east edge of the county and another on the northwest flanks of Mount Hood.

According to Jason McClaughry, head of the Eastern Oregon office of the Oregon Department of Geology and Mineral Industries, both faults are classified as “young,” meaning they have seen activity at some point in the last 11,000 years.

“In recorded history there haven’t been a high number recorded in the area, McClaughry said. “But we’ve identified some areas some that may have potential for earthquake.”

While a large earthquake from either fault is unlikely, should one occur, it could have a large impact on the county.

The county’s emergency mitigation plan lists nearly every item of critical infrastructure as being vulnerable to a quake.

That includes a large amount of infrastructure on a liquefactions zone — soil particularly susceptible to the affects of an earthquake — such as port facilities and the city sewer treatment plant near the Columbia River, the Hood River distillery on the waterfront, the county administration building on State Street — which is built from unreinforced masonry — water supply facilities, and gas lines on the Hood River bridge.

Tesch has a plan for nearly every conceivable disaster in his office, from a tornado to a large oil spill in the Columbia River. It’s his job to make sure the county gets the resources and help it needs in the event of a disaster.

“I’m the link between the county and the state,” he said. “That includes being responsible for executing mutual aid agreements.”

Those plans and preparations include earthquake response.

However, Tesch spends more time preparing for and managing the impacts from events that have happened recently or are very likely to occur in the near future, such as wildfire and ice storms.

“There is really no past ‘recent’ history of earthquakes in Hood River County, but county residents have felt some earthquakes distant from Hood River County. Even with this lack of history, geology clearly shows that the county has been impacted by significant events in the last 500 years,” states the County’s hazard’s mitigation report.

A Cascadia Subduction zone quake is more likely to occur than one originating in or around Hood River County, and the county’s estimates for damage in the county from a Cascade quake estimate no deaths, no buildings extensively damaged and around $3.8 million in economic damage.

Most of the economic damage would come from infrastructure to the west of the county being badly damaged or destroyed.

“Within the limits of predictability, we must assume a moderate probability of occurrence for a damaging earthquake during the next 50 years,” the county report states. “A large earthquake centered in Western Oregon could have a minor impact on Hood River County suggesting moderate vulnerability.”

A sizable earthquake originating in one of the faults Hood River County would possibly spell bigger problems — such as an eruption on Mount Hood.

Even in the event of a significant eruption, the county could fare better than its neighbors.

Among the volcanic hazards identified by McClaughry and Oregon Geology include ash fall from an eruption at Hood or another Cascade Range volcano, lahar mud flows, debris and the fact that the historic lava flows, such as the Parkdale Lava flow are relatively young in the geologic sense, meaning they were active less than 1.7 million years ago.

“It’s the youngest lava flow in the valley,” McClaughry said of the Parkdale lava flow. “So there is the potential for lava flow-type eruptions.”

And should one occur, Tesch has a plan for that too.

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