Where to build the Gearhart firehouse?
Since the bond for a new firehouse was defeated in 2006, the topic has simmered in Gearhart as the city seeks a replacement for its current firehouse, constructed in 1958 and deemed structurally inadequate in a Cascadia Subduction Zone earthquake and tsunami.
Consultant Tom Horning of Horning Geosciences in Seaside presented an analysis of potential firehouse locations at a January town hall. Stewart Schultz, professor of biology at the University of Zadar, Croatia, submitted a contradictory analysis to the city in April. “At the town hall, the information presented to the city was completely different from that in his report, and incorrect,” Schultz said.
Both Horning and Schultz state their findings are based on data from the state’s Department of Geology and Mineral Industries.
With each report citing DOGAMI, we reached out to the agency and spoke with Jonathan Allan, Ph.D, a coastal geomorphologist. Allan is the Oregon Science Lead, National Tsunami Hazard Mitigation Program, Oregon Department of Geology and Mineral Industries. He reviewed both documents.
Q: As I understand it, Stewart Schultz’s report draws conclusions based on a medium-sized Cascadia Subduction Zone event, while Horning’s conclusions are based on risk for a large-scale scenario? Can it be boiled down to that?
Allan: My understanding is that there are at least four sites that were being considered: the existing site; one up on the dune west of the existing fire station (Gearhart Park); one at the intersection of 10th, and Marion; and one on 13th (High Point).
What it mainly comes to is: what is the acceptable level of risk to the community in order for that fire station to also meet its immediate strategic needs, which is servicing the local area.
From our standpoint that’s a reasonable approach to take.
Q: How vulnerable is the current fire station site?
Allan: The existing fire station site is inundated by the medium, the large, extra-large, and extra-extra-large events. Four out of five events actually inundate the existing site.
The actual water level at the fire station for the large scenario reaches around 31 feet.
But then it climbs dramatically to closer to about 56 feet for the XL. And it’s slightly larger than that for the extra-extra large.
The (Gearhart) park site is high and dry for medium.
At the two higher elevations sites, 10th Street and 13th Street, large doesn’t inundate either of those sites. They’re basically high and dry. The water levels reach 62 feet for an extra-large scenario at the 10th Street site, given the ground elevation at the site. The 13th Street site has really similar numbers.
Those two sites are only exceeded by XL and XXL scenarios.
Q: Is it the frequency of tsunamis over time that tells us when the next tsunami will hit?
Allan: You know, there’s a lot of debate about that one in terms of the possibility of sort of a cyclical nature here.
We have essentially 46 events defined over a 10,200-year period. You can certainly see some evidence of grouping, but really how real that is, is debatable.
Q: In other words, the timing is random.
Allan: Exactly. It’s a very challenging area.
What we can say is we know the number of very big events that impacted Cascadia over the last 10,000 years, 19 of them which are the full margin rupture. The last one took place in 1700 when that was a magnitude 9 event. It’s been 319 years since the last event.
Strain is already accumulating, and has been since the last event. So we’re building to the next event.
We know that the interval between ruptures can be anywhere from 100 years to as long as 1,000 years. But on average they’ll go off an average of 500 years spread across the entire Coast.
(Oregon State University professor) Chris Goldfinger’s most recent work, which was published in 2017, indicates that Cascadia has something like an 18 percent chance of rupturing in the next 50 years.
Q: Is that a number that DOGAMI tends to go with?
Allan: That’s the number that I’ve been using since it reflects the most up-to-date results from Chris’s work.
He’s saying it’s about 16-22% chance in the next 50 years. The odds of it taking place on the South Coast are a little bit higher.
Q: Looping back to the Horning and Schultz firehouse studies, is either one definitive?
Allan: Tom’s comments are right on in that he’s basing a lot of his comments on existing information.
For example, he cites the work of Kurt Peterson, who did some paleo-tsunami coring work in histories along the coast.
He knows that run-up elevations defined in Cannon Beach reached about 50 feet, so based on that he argues that’s strong grounds to go with the large approach. That’s a perfectly reasonable approach to take using geological data.
Fundamentally what it comes down to what is the level of risk the community is willing to take.
Q: Can you analyze Schultz’s conclusions?
Allan: He’s kind of cherry-picking some of his comments here in terms of statistics.
He’s right — in a large event, there’s a 5 percent it being exceeded.
A medium event, there’s around 21 percent chance of it being exceeded. Those are based on our work.
But keep in mind the approach we developed is not truly a probabilistic approach. Hundreds of thousands of simulations are run of earthquakes, where you can truly extrapolate the recurrence and exceed its values associated with those.
Q: What does that mean? You’re more interested in complex modeling than numbers?
Allan: Yes. In the best of all worlds, you would actually run many, many, many simulations to try to better constrain the uncertainty portion.
This is something that the American Society of Civil Engineers has actually been working on the last several years now, in developing design standards for tsunamis for critical and essential buildings.
They’ve come up with 2,500-year design wave in a tsunami inundation zone. While that type of approach has its own inherent problems with the modeling that was done, it needs to be truly implemented to begin to really define what these recurrences are.
Another comment — Schultz made a statement about the existing site and how a medium would only reach a certain height above the ground and then people could basically escape out the top of the fire station.
I thought that was somewhat naive, because to be honest, existing buildings are not designed to withstand the types of forces that are thrown at them by a tsunami.
I thought it was a highly generalized statement to be making.
In reality, when you look at what happened in Japan, unless a building was specifically designed to withstand the forces, effectively the buildings in the inundation zone, certainly multistory buildings up to two stories, were decimated.
It was only the truly multistory four-, five- and greater story buildings that tended to survive the event. We’re talking about structures that are much more heavily designed and robust for those conditions.
It’s another consideration and again it goes to what is the purpose of the fire station: Is there a dual purpose, one being to serve the strategic needs of the community for fire and risk and so on, and/or designing it as a vertical evacuation structure?
For the latter, you’d have to truly engineer it using much more robust modeling and engineering guidelines to build it to sufficient height and to withstand the forces. That takes a lot more time and effort to come up with those designs standards.
Q: Are you speaking as a representative of DOGAMI?
Allan: I am a member of the agency of Oregon Department of Geology and Mineral Industries.
Q: Will you be weighing in on the Gearhart Firehouse as it goes forward?
Q: So wherever they put it, you would remain silent unless they asked for guidance? Would you intervene?
Allan: We’ve already had discussions with the city of Gearhart over the last couple of years in terms of providing detailed information about the various sites.
It’s my understanding the two higher elevation sites, 10th Street and 13th Street, are outside our regulatory responsibility. In that respect, there’s no requirement. We don’t have a regulatory responsibility at that point.
Q: But at the current site at 670 Pacific Way, if they built there, you would have a regulatory responsibility?
Allan: That’s a little more complicated. The initial decision is made by the local building codes official.
If they determine it’s within the zone or not the city is required to have a consultation with us. The city can choose to request an exception based on a strategic need and they can request that exception from our Oregon Department of Geology governing board. But it’s my understanding that cities can choose to bypass that explicit path based on a strategic need for that particular site.
Q: Will you be in contact with Gearhart over this?
Allan: We’re obviously available to them any time, or are looking for additional information.
Q: You’ve gone through this with other cities?
We’ve worked with other cities in terms of providing information. Each community has taken a slightly different approach. We’ve worked with, for example, the communities of Nehalem and Manzanita when they were establishing their fire station. The city of Yachats has gone through this process, when they made a decision to move outside of the large inundation zone for their purposes.
It again comes down to fundamentally the specific level of risk that is acceptable to a community.