For the first time in 20 years, Portlanders are getting a detailed look at the damage that will be caused by a catastrophe we all know is coming — a major earthquake striking the metro area — and the prospects are grim.

The assessment comes in a new report published Thursday by the Oregon Department of Geology and Mineral Industries and includes some stark numbers. In a worst-case scenario, the metro area could see more than $80 billion in building damage, tens of thousands of people wounded or killed and more than 250,000 people facing long-term displacement.

"The damage estimates are significantly higher than those given in previously published studies for the area," the report states, as it uses better data about the buildings in the metro area, many of which were built using unreinforced masonry. The authors of the report stressed that the document should not be seen as an end-goal, but "as a platform for counties, jurisdictions, and communities to better understand their needs to prepare for, respond to, and recover from a major earthquake."

Dan Douthit, a spokesman for the Portland Bureau of Emergency Management, agreed, saying the report was not intended to scare people, but to reinforce the need to prepare.

"We hope that this report gives people a sense of urgency," he told The Oregonian/OregonLive.

The metro area is home to 44 percent of Oregon's total population and 50 percent of the state's jobs. The 3,076-square-mile study area, which includes Multnomah, Clackamas and Washington counties, is also home to all of Oregon's major liquid fuel port terminals and the state's largest airport. All of those critical pieces of infrastructure, as well as the 1.6 million people who live in the region, would be affected by a major earthquake.

Not every earthquake is the same, however, and the report laid out a few different scenarios for what could be waiting for Portland and the surrounding area.

Different quakes, different shakes

The earthquake that Pacific Northwesterners have long known about and feared is one caused by a rupture of the Cascadia Subduction Zone, the 600-mile-long fault that sits ominously off the west coast, running from Northern California up to Vancouver, B.C.

The subduction zone, and the danger it poses, was cast into stark relief by a Pulitzer prize-winning New Yorker story in 2015.

The fault is divided into sections, each of which could produce a moderate quake, but the report is modeled for the so-called "big one," in which the whole fault slips and a massive magnitude 9.0 temblor shakes the entire region.

But the fault buried under the ocean is not the only one that threatens the metro area. The Portland Hills Fault, which runs from Oregon City to Scappoose, sits directly under some of the city's most important infrastructure and densest population centers. While a quake on that fault is less likely than a subduction zone quake, a large temblor on a fault running right through Portland could be far more deadly.

The report produced damage estimates for both a magnitude 9.0 quake on the Cascadia Subduction Zone and a magnitude 6.8 quake on the Portland Hills Fault. It also accounted for a few other variables as well. An earthquake during the day, when most people are out and about, could be worse than one in the middle of the night. A shaker in winter, when topsoil is likely to be wet and prone to landslides, could be worse than one during dry months.

The Cascadia Subduction Zone

The last major rupture of the Cascadia Subduction Zone was in January of 1700.

The last time the whole Cascadia Subduction Zone ruptured was January 26, 1700, more than 300 years ago, and at least 40 large-magnitude earthquakes have occurred along the fault in the past 10,000 years. Studying core samples dug up from the ocean floor, seismologists peg the likelihood of a major quake occurring on the whole fault at 10 to 14 percent in the next 50 years.

Those numbers are slightly higher for the the central and northern Oregon coast, where experts say the chances of a large quake in the next 50 years are between 15 and 20 percent.

The impacts such an event would cause on the coast, from landslides to tsunamis, have been well documented, but this report focused solely on the impacts in the metro area.

If a magnitude 9.0 earthquake were to strike along the Cascadia Subduction Zone at 2 a.m., there would be between 4,400 and 10,400 casualties, ranging from minor injuries to fatalities. Between 9 and 14 percent of the buildings in the area, which number more than 600,000, would be lost. Monetary estimates for repairs and rebuilding range from $23 billion to $36 billion.

Up to 85,000 people could be displaced for long periods of time and the quake would produce between roughly 13 and 18 million tons of debris. For context, a Nimitz class aircraft carrier weighs approximately 100,000 tons.

If the quake were to strike during the day, at 2 p.m., things could be much worse.

The damage estimates for buildings stay the same, but a daytime temblor would cause between 18,000 and 27,000 casualties, the report said, nearly triple that of a quake in the dead of night. The biggest difference being that, at night, the vast majority of the population is at home sleeping in wood-framed houses, which are less vulnerable to earthquakes.

One of the advances in the most recent report, compared to previous studies, is the ability of researchers to detect which areas are prone to liquefaction, wherein saturated soil behaves more like a liquid than a solid. Vast swaths of Portland's west side, a large stretch of the south bank of the Columbia River and portions of the city's inner east side are all at high risk of liquefaction or landslides, especially if the quake strikes during the wet winter months, the report said.

A map of the metro area shows which areas are susceptible to liquefaction and landslides during a major earthquake.

"Although damage estimates vary widely throughout the study area, no community will be unharmed," the report said.

The Portland Hills Fault

While the Cascadia Subduction Zone has garnered more public attention in recent years, the lesser-known Portland Hills Fault has the potential to cause more damage, according to the report, though it is far less likely to occur.

While the subduction zone has seen some 40 major quakes over the last 10,000 years, the Portland Hills Fault is thought to have only produced two sizable seismic events over the last 15,000 years. Still, due to its location, the fault poses a grave danger to the residents of Portland.

It starts roughly on the northern edge of Forest Park and runs along the foot of Portland's West Hills before turning east on West Burnside Street for a few blocks and then turning southeast again through the heart of downtown. The 30-mile-long fault, which is actually a complex "fault zone" containing multiple fractures, then crosses the Willamette River between the Marquam and Ross Island bridges to Milwaukie and ends about a mile south of the Clackamas River near Oregon City and Gladstone.

"Other active crustal faults exist in the Portland Metropolitan Region, but a rupture on the Portland Hills fault would be the most impactful, given its position directly underneath downtown Portland and the population centers of Clackamas County," the report reads.

If a magnitude 6.8 quake were to strike this fault, 23 to 32 percent of the 615,000 buildings in the study area could be lost with damage estimates ranging from $60 billion to $83 billion. Some 26 to 34 million tons of debris would be created and between 96,000 and 257,000 people would face long-term displacement. If that quake were to strike at 2 a.m., the area would see between 16,000 and 29,000 casualties, again ranging from minor injuries to fatalities. If the earthquake came during the day, that number jumps to between 48,000 and 63,000.

Assuming the lower end of the casualty spectrum, that equates to the entire population of Tigard either injured or dead.

A path forward

The numbers put forward in the report are striking and they come just months after an audit found Oregon to be woefully unprepared for a major earthquake.

Still, Douthit stressed some of the damage could be mitigated on a local level.

Unreinforced masonry buildings

Source: City of Portland Bureau of Development Services

The city is home to an estimated 100,000 wood-framed homes that are in need of retrofitting. Another 1,600 structures are built of unreinforced masonry, Douthit said, and are particularly vulnerable to shaking.

"Seismic upgrades to buildings, or replacement of older buildings, can significantly reduce loss and casualties," the report said. One of the recommendations included in the report was to offer incentives to building owners to speed up the retrofitting process.

Beyond costly upgrades to structures, there are a number of ways individuals can prepare for a major earthquake. Having a well-stocked earthquake kit is a good place to start.

The Portland Bureau of Emergency Management has been working to train a cadre of volunteers under the moniker Neighborhood Emergency Teams, or NETs. The idea is to have volunteers on the ground in every corner of the city who can be self-sufficient for up to two weeks after a disaster. The teams are trained to provide assistance to neighbors, help untrained volunteers and, perhaps most importantly, assess and treat those with minor injuries in their immediate area.

That last point will take on special significance in a mass casualty event, Douthit said, as it will keep those with minor injuries from flooding to emergency rooms. Medical facilities are likely to overburdened with serious injuries after a major quake and otherwise hobbled by power loss and emergency transportation routes that could be impassable.

Douthit also pointed to the development of an early warning system for earthquakes, which is still in the testing phase. Though it is still in development, and waiting for the necessary funds for completion, the system has already proven effective.

"If we had that system in place," Douthit said, "we could get a lot of people to safety that might otherwise be in harm's way."

-- Kale Williams

kwilliams@oregonian.com

503-294-4048