Walla Walla-area earthquake disaster just waiting to happen

Starting around the Wallula Gap and running east along the southern mountains all the way to Milton-Freewater is the active Wallula Fault Zone.

Advertisement

photo

The photo shows the Wallula Fault Zone and its relationship to U.S. Highway 12 and the Walla Walla River.

photo

The Daily Bulletin on July 16, 1936, gave a report on an earthquake.

WALLA WALLA - The likelihood and affects of a natural or man-made disaster to the area were discussed at Wednesday's City Council meeting, as County Emergency Management staff presented and received unanimous approval for its Hazard Mitigation Plan.

At the top of the list of probable disasters detailed by County Emergency Management Director Gayla Erst was the possibility of a severe storm, such as the windstorm of 2008.

But right along with the threat of high winds, hail, excessive snow, flooding and even F2 tornadoes - that last of which occurred here in 1958 - the most likely disaster to cause extensive damage, numerous injuries was earthquakes.

"We say it is one of our highest hazards," Ernst said, adding that a survey at the county fair last summer showed most residents had no idea of the danger that lurks a mere 15 miles underground.

"I looked at some of the responses and most of them couldn't believe that we had earthquakes," she said.

Ernst described how a 6.2 magnitude earthquake would likely cause 13 deaths, 140 injuries and extensive damage to buildings, infrastructure and roads, crippling the city for days, maybe even weeks.

But nobody can predict when such earthquakes will hit, other than saying "we're due" or "sometime in the next 25 years."

What isn't a matter of speculation is that starting around the Wallula Gap and running east along the southern mountains all the way to Milton-Freewater is the active Wallula Fault Zone.

And as far as faults go, 15 miles underground and several miles to the south is not much buffer to diminish the shock waves of a major earthquake, noted Whitman College professor and geologist Kevin Pogue, who pointed to the Nisqually earthquake of 2001 for proof.

"That was a magnitude 6.8 or something like that. And the epicenter was about 40 kilometers down and 80 kilometers south of Seattle. But they still call it the Seattle earthquake," Pogue said.

That earthquake caused one indirectly related death, 407 injuries and $2 billion in damage, as well as damaging the Alaskan Way Viaduct and cracking the dome of the capital, according to Washington state reports.

The greatest damage was to unreinforced cement and masonry buildings, especially those closer the epicenter.

So what would happen if a 6.8 magnitude earthquake happened here?

David Collette of the city and county building department is fairly sure most buildings would be affected and some would come down.

"We are in a fairly historic area, especially dealing with our downtown area, pretty much everything downtown is unreinforced masonry," he said.

If you take a walk along Main Street, most every coffee shop, restaurant, retail store and office is made of unreinforced masonry. Collette said very few have been seismically retrofitted over the years.

One that has been is the Marcus Whitman Hotel & Conference Center. But even in that building, the bricks of the facade would still come falling down from several stories up, Collette said.

"If and when we have a major magnitude earthquake there is not going to be much left downtown," he added.

So when will such an earthquake hit?

The only inkling to the answer is in the past.

According to Pogue and other experts, the region has a significant earthquake every several years.

That last one was in 2000, when a 3.0 seven miles west, northwest of Walla Walla. In 1991 and 1992 a few earthquakes, including a 4.1 and a 4.3, hit the area. History shows that about every eight years these smaller earthquakes happen.

The last major earthquake was in 1936, when 15 miles under Umapine the ground shifted.

According to a local newspaper article, chimneys toppled, brick buildings were damaged, items were knocked over, and the ground rose in parts. What surprises Pogue is that one headline refers to the 1936 earthquake as being the "most severe here in last 30 years."

Of course, that same headline also said, "Not much damage."

Pogue and Collette believe there are several factors as to why damage was minimal, and some of those factors are no longer an option should another major earthquake hit.

The Umapine earthquake of 1936 happened in the summer, which is a time of year when the ground is relatively dry.

Pogue has no doubts that if a similar earthquake hit in the winter or spring, when the ground is saturated and allows liquefaction to occur, the damage would be much greater.

"With liquefaction, the surface material turns to goo and buildings collapse. And all of downtown Walla Walla has a very high liquefaction ability," Pogue said.

Collette pointed out that there are more people living in the area, and many of those unreinforced masonry buildings have severely deteriorated mortar.

"Any time you have a brick and mortar building you got a concern, especially when the mortar is getting near 100 years old," Collette said.

He said many of the downtown buildings he inspected have mortar that was almost like sand and severely in need of replacement.

Pogue said geologist aren't certain of the magnitude of the 1936 earthquake, but they are fairly certain it was in the sixes. And Pogue is certain that if a similar earthquake hit today in the wetter months, the results might even be worse than 13 deaths and 140 injuries.

"There is one building downtown on Main Street where the bricks have obviously been repaired. And it makes me wonder every time I see that, was that from the '36 earthquake? ... I always tell people earthquakes don't kill people. Buildings falling down kill people," he said.

Comments

Use the comment form below to begin a discussion about this content.

Sign in to comment