Last weekend’s undersea volcanic explosion near Tonga devastated the island nation and sent small tsunami waves to Washington’s ocean coast. Those waves took about 12 hours to reach the state and gave residents plenty of time to prepare if they had been bigger.
Those same residents would have only 10 minutes to evacuate for waves up to 100-feet-high that would hit them following a massive earthquake in the Cascadia subduction zone. Some might not get that much time. Ground sinking below their feet might flood during a magnitude 9 quake.
That’s what computer modeling shows, according to a report released Jan. 10 by the Washington Geological Survey. The report illustrates what would happen to cities, river mouths, beaches and other low-lying areas on the Olympic Peninsula. Previously, the Geological Survey released maps for the southwest Washington coast, San Juan islands and Puget Sound.
The report includes detailed maps from just north of Grays Harbor to Port Townsend. The goal is to prepare both officials charged with community protection as well as to warn the public of potential hazards.
THE BIG, LONG SHAKE
The modeling used for the report assumes the Big One hits in the subduction zone, 80-100 miles off Washington’s coast, where the Juan de Fuca tectonic plate is sliding under the North American plate. The “full rip” event would run along the fault’s entire length from northern California to north of Vancouver Island.
It’s been 322 years since that last happened and only a matter of time before it happens again.
“There’s lots of geologic evidence that these quakes and tsunamis have happened many times,” said Corina Allen, chief hazards geologist for the Geologic Survey.
There would be no sleeping through this quake. The strong ground shaking would likely last between three and six minutes and serve as an immediate call to seek higher ground.
“The earthquake is your warning,” Allen said. “Get to high ground.”
By comparison, the region’s last major temblor, 2001’s Nisqually Quake, lasted about 45 seconds.
It’s during the earthquake itself, Allen said, when coastal Washington will drop in relationship to areas west of the fault. The change in sea level would flood vulnerable areas up to five feet, modeling shows.
It’s happened before, as evidence from the Copalis River ghost forest shows. Trees killed by a saltwater inundation from the 1700 earthquake still stand along the river and helped geologist Brian Atwater prove that the quake was responsible.
In the 1700 quake, a tsunami struck Japan and killed thousands of people. Its source remained a mystery until Atwater made the connection.
Following a mega quake on the Cascadia fault, simulations show that the tiny town of La Push would get hit first by a tsunami, 10 minutes after shaking started.
Those Hollywood depictions of a giant wave rising from the sea are inaccurate, Allen said. Think wall of water instead. And it comes very fast.
“In deep water it travels about the speed of a jet plane,” she said. “When it gets close to land it slows down.”
Within 30 minutes, many parts of the coast would be hit by waves. Wave heights can vary but they’re predicted to be 30 feet or higher. Most Pacific coast beaches and campgrounds would be under 60 feet of water.
The report said waves of 60 feet or higher could hit the Hoh Indian Reservation, Queets, Taholah on the Quinault Indian Reservation, Moclips, Pacific Beach, Iron Springs, Copalis Beach and Ocean City.
The mouth of the Hoh River could be flooded to a depth of 100 feet.
Within an hour, a 20-foot-high wave would hit Port Angeles. The U.S. Coast Guard Air Station there would be under 15 feet of water a little more than an hour after the quake.
Waves would continue to hit for eight hours and be a hazard for a full day after the quake.
Messaging to coastal residents on Saturday following the volcanic eruption near Tonga that the first wave might not be the biggest holds true for all tsunamis, Allen said. Flood levels can also vary depending on tidal levels.
That holds true in Puget Sound where modeling shows the fourth wave to hit Olympia would likely be the biggest.
“We have such a complicated waterway in the Puget Sound,” she said. “As this wave travels through, there’s lots of sloshing going on. (The) wave is bouncing off of our islands and our peninsulas and our inlets.”
The speed and depth of tsunami waves make them dangerous, along with potential debris the waves might be pushing.
“At inundation depths greater than 6 feet, survival is unlikely for persons out in the open or within or on most conventional structures,” the report said. “Fortunately, survival remains highly likely within or on a reinforced and specially designed building, such as a vertical evacuation structure.”
One such structure is currently under construction on the Shoalwater Bay Indian Reservation at Willapa Bay.
For some people caught in the open, climbing to the upper story or roof of a sturdy building could be a last resort. Even climbing a tree is better than being out in the open, the report said.
It’s not just earthquakes that can cause tsunamis, as last weekend showed. Landslides and even a meteor strike could cause one. Tsunami forecasting was made more difficult Saturday due to the lack of modeling using an undersea volcano as a tsunami source.
“A volcanic eruption in Tonga was not on my radar as a tsunami event to be thinking about,” Allen said. [ChronLine]
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