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Research highlights Vashon’s geologic vulnerability
For two days last week, the fire station on Bank Road was transformed into an emergency response headquarters as Vashon Island Fire & Rescue took part in SoundShake, a region-wide exercise in which emergency response agencies simulated potential reactions to a large earthquake.
In the scenario, a magnitude 6.8 tremor on the Seattle Fault rattled the region. On Vashon, buildings were damaged, utilities went down, fires broke out, Islanders were injured or dead, and the ferry system came to a standstill.
Around 20 volunteers from VashonBePrepared, an Island non-profit focused on emergency preparedness, worked under the leadership of Fire Chief Hank Lipe to coordinate mock relief efforts and communicate with King County officials involved in the exercise to get needed supplies.
When King County couldn’t send a helicopter to Vashon, volunteers immediately searched for another solution. When it offered to send a barge, one innovative participant suggested injured Islanders be sent back on the empty boat. And when it was announced that one more Islander had died, the room reacted almost as though it had been a real death.
The bi-annual Sound-Shake, which prepares volunteers to better respond in the case of an actual earthquake, also serves as a chilling reminder of Vashon’s geologic vulnerability. While the simulated Seattle Fault tremor would no doubt wreak havoc on Vashon, recent research has shown that a similar fault poses an even greater threat to Vashon due to its close proximity to the Island.
A study completed by the U.S. Geological Survey (USGS) in April simulated an earthquake on the Tacoma Fault. While both the Seattle and Tacoma faults run close to the Earth’s surface and are capable of producing large quakes, the Tacoma Fault has strands that run directly through southern Vashon and Maury Island. Researchers found that in the event of a magnitude 7.1 quake on the fault, Vashon, along with parts of the Kitsap Peninsula, would experience the region’s most severe shaking.
Joan Gomberg, a USGS geologist who took part in the study, said that although the fault looks as though it has broken several times in the past, the last known quake it experienced was 1,100 years ago. And although earthquakes are difficult to predict, it isn’t likely to shake again anytime soon.
When it does, however, the subsequent damage on Vashon would likely be far more extensive than that of a Seattle Fault quake.
“If there was an earthquake on the Seattle Fault, you’d certainly get shaken up, but you wouldn’t be quite smack in the red zone,” Gomberg explained. Shaking from the Tacoma Fault, however, would be strong enough to topple buildings on Vashon.
William Steele, a spokesman for the Pacific Northwest Seismic Network, compared the Tacoma Fault scenario to Vashon’s most recent large earthquake, the Nisqually quake of 2001. He said shaking from an earthquake centered on the Tacoma Fault near Vashon would be stronger than Nisqually and last for about three minutes instead of 20 seconds.
“Vashon looks clobbered,” he said as he referenced a shake map that shows the Island as one of the hardest hit areas. “If you’re in central Vashon, you’ll be very close to the rupture when it does break. You’ll get very strong shaking. … There’s going be a lot of damage.”
Steele said the Tacoma Fault earthquake is certainly a worst-case scenario for the Island. However, like the Seattle Fault quake simulated in the SoundShake exercise, it’s not a likely scenario. “The probability of that occurring is a 5 percent chance in the next 50 years,” he said, something to prepare for but not to count on.
The chance that the Island will experience a less severe, Nisqually-like earthquake in the same time period, however, is 87 percent. “That’s almost a sure thing,” he said.
Unlike a tremor from the Seattle or Tacoma faults, a Nisqually-style quake occurs deep below the Earth’s surface. Thus, while the magnitude can be high, like the 6.8 Nisqually, its waves are suppressed by about 30 miles of rock, resulting in far less shaking and damage on the surface.
USGS geologist Brian Atwater said these earthquakes, which are comparatively small but still cause millions of dollars of damage, are the most common in the Puget Sound region. Before the Nisqually quake of 2001, similar ones occurred in 1965 and 1949.
“The most frequent threat is the most tolerable, … and it’s the one for which society has more or less taken precaution,” he said.
However, Atwater said, either type of earthquake could trigger another problem that would be of particular concern to Vashon. The shaking from a quake or subsequent landslides could trigger a tsunami in the Puget Sound.
“Not just any earthquake will succeed in making a tsunami, ... but it’s important for people to know that is in this area’s history,” Atwater said.
In the USGS’s Tacoma Fault earthquake simulation, researchers estimated that a magnitude 7.1 quake on the fault could generate a tsunami that would raise water as much as 12 feet in some parts of Vashon in a matter of minutes, with the swiftest waves rushing into Quartermaster Harbor.
Rick Wallace, vice president of VashonBePrepared, said the organization takes the threat of tsunamis seriously. However, the multitude of variables involved in a tsunami makes its effects more difficult to predict than that of an earthquake.
A tsunami could damage one or both of Vashon’s ferry docks and even take out facilities the Island depends on, Wallace said. However, he believes the biggest concern are the many waterfront homes on the Island that would be struck by a giant wave and floating debris. “Anybody who lives on the water would be concerned about that,” he said.
Wallace said VashonBePrepared volunteers learned of USGS’s recent findings related to the Tacoma Fault as it was preparing for this year’s SoundShake exercise.
The facts that SoundShake participants worked with, such as the percent of Island homes damaged and the number of residents injured, were based on a 2005 study in which USGS geologists helped the organization estimate the effects that four different Puget Sound-area earthquakes could have on Vashon.
Wallace said the damage from the newly studied Tacoma Fault earthquake scenario would be at least as severe, if not worse, than the earthquakes that VashonBePrepared examined. “If you had a 7.1 that basically was centered across the middle of the Island, the shaking we be so devastating, we would have horrific consequences on the Island,” he said.
Wallace said the organization would consider the USGS’s findings in its future planning. He said the organization could even use the scenario in a future preparedness exercise, although he believes Vashon’s response would be very similar to the one it just practiced.
Fire Chief Hank Lipe said some of the greatest challenges in any earthquake scenario are the huge task of assessing the damage to the Island’s infrastructure and the possible need for the Island to be self-sustaining until it received aid from King County.
“I think we made tremendous progress in our resource acquisition, supplies request and identifying supplies we’re going to be needing if we become isolated and how to acquire those supplies. … If we’re cut off from the mainland we’re going to have to be pretty self-sufficient on the Island for three to five days,” Lipe said.
The fact that Vashon is an island has driven its residents to consider the real possibility of being cut off from the mainland in the case of an earthquake and to take disaster preparedness seriously, Lipe said. He is comforted by what he sees, from the multitude of volunteers with VashonBePrepared and similar organizations, to the families that have equipped themselves to better withstand an emergency.
“Historically, disasters play havoc on any community,” Lipe said. “The communities that are most prepared tend to recover quicker.”
To learn more about emergency preparedness on Vashon, visit www.vashonbeprepared.org.