Among the many threats bedeviling Seattle residents these days, tsunamis are not high on the list. A tidal wave swamping Seattle? Isn’t that supposed to happen out at Ocean Shores or Moclips? Don’t they come from mega-quakes out in the subduction zone?
They do, but damage from quakes and tsunamis that could originate in Puget Sound represent a much greater threat.
The culprit in this case is not the north-south Cascadia Subduction Zone, where the Juan de Fuca plate of the Pacific Ocean floor, reaching from Vancouver Island to northern California, slides inexorably beneath the North American plate 80 miles west of the Washington coast.
The greater threat comes from the shallow Seattle Fault, a west-east zone of faults that reach from Hood Canal to Fall City. Most pass south of Restoration Point on Bainbridge Island across the Sound just north of Alki Point, across Elliott Bay and right through the downtown business district of Seattle. They parallel I-90 from the Rainier Valley to Lake Sammamish. Here is a description of it in action in 1872, recorded by Seattle Episcopal Priest, R. W. Summers:
…across the bay I heard a sound like the rumble of many freight trains rolled into one. It grew louder as it circled the shore towards me and finally struck the business part of the city and started back toward the interior. As it travelled up the hill, everything shook. The sidewalks, fences, buildings, trees all cracked and crackled like a great discharge of Minnie balls. As it touched church and church ground, the earth rolled under my feet in such waves as a carpet makes, shaken with regular motion on a greensward. Then the quaking and the earth-waves rolled on above the town and over the brow and off toward Lake Duwamish [Washington], until the terrible convulsion passed out of hearing. The giant firs and cedars lashed together back and forth and their trunks were reported as swaying a full fifteen feet. It was about nine o’clock [PM].
At the time, Seattle was not built up enough to sustain much damage beyond broken crockery, glass and residents’ nerves. The quake occurred probably no more than four kilometers below the surface, but the release of energy was enough to produce the dramatic phenomena Summers recorded. Frightening as that was, we know now that the fault is capable of far worse.
In the spring of 935 AD, it moved. A wedge of earth’s crust from Bainbridge Island to Seattle lunged upward. Intertidal beaches were lifted between 13 and more than 20 feet, becoming Alki Point in Seattle and Restoration Point on Bainbridge Island. At the same time, an adjacent northern block of earth fell roughly the same distance. Northern parts of the Kitsap Peninsula and Bainbridge Island sank beneath the waves as did West Point on the eastern shore. On Lake Washington and Lake Sammamish, crustal movement and terrible shaking sent forested hillsides plunging to the bottom.
On Puget Sound tsunamis generated by this terrific 10th-century event spread all directions: into Elliott Bay and Salmon Bay, depositing a long layer of sand as it engulfed Cultus Bay on Whidbey Island. Headlands collapsed and shorelines for miles were swept clean. In the Olympic and Cascade mountain ranges, huge avalanches buried valleys. And like the Reverend Summer, survivors left accounts that have been preserved in myths.
The Suquamish people tell a story about a battle between a whale and a great eagle at the site of a shallow tidal stream that drained a marsh near their main village of Dxw Su qwub (Suauamish). The battle was so terrible that marsh and stream sank into a wide saltwater channel—Agate Passage–separating the Kitsap Peninsula from Bainbridge Island. Along the faults, myth sites identify people who were turned to prominent beach stones during a cosmic transformation of the world. At various sites, horn-headed, serpentine earthquake monsters recall landslides and destruction. In the northern Olympics a story about Lake Sutherland tells how a great avalanche separated it from Lake Crescent, and further south other great avalanches forever isolated a Valley of Peace.
Four Mile Rock below Magnolia Bluff recalls the story of STAH kub, a hero who tossed a net onto the rock where it turned to stone during a great transformation of the world. Tossing the net may recall the huge rock having rolled from a collapsing bluff as a tsunami swept through. When you visit, note how the rock’s upper half over hangs the lower half—Stah kub’s overhanging net. Golfers at the Restoration Point Country Club still hack clams from their ancient burrows beneath the turf. At Me-kw Mooks park in West Seattle, a bedrock washboard in the north adjoins a southern sandy shore, revealing the fault itself.
A major quake on the Seattle Fault will produce far more local damage than a mega-quake on the Cascadia Subduction Zone. The Zone is 80 miles away from the Washington coastline, while the Fault bisects the downtown. Mega quakes occur on the Zone about 50 miles down, but quakes on the Seattle Fault happen only a few miles down. It is simple physics: the intensity of a source of energy changes in inverse proportion to the square of the distance from the source.
Three 6+ magnitude earthquakes that shook western Washington in 1945, 1965, and 2001 occurred about 40 miles from Seattle and 35 miles down. They may have been associated with the poorly understood Tacoma fault. Damage was in the billions; many died. The 2001 Nisqually quake voided the warranty for Seattle’s Alaskan Way Viaduct. The damage from such a quake three miles beneath Pioneer Square is difficult to imagine. A crustal displacement, will sever water, sewer, and power lines, and a tsunami will churn through the city’s historic district, cover Harbor Island, rage up the Duwamish River, surge up Commencement Bay, crash through downtown Olympia, rolling over the Bremerton Naval Shipyard, the nuclear submarine base at Banger, and Everett’s Navy facilities.
Geological evidence indicates that the Seattle Fault has slipped catastrophically 3,200 years ago, 1,700 years ago and 1,100 years ago. This suggests a frequency of about once every 750 years so the 1,100 years separating us from the last big one would indicate we are overdue. When I was a kid looking across Puget Sound at night from Edmonds, hardly a light marked the opposite shore. Today it blazes north to south. A large tsunami will carve away those homes like a steak knife trimming fat, and the shore will be dark once again. Even darker. Allowing homes to be built along the beach represents a zoning failure as catastrophic as the fate awaiting them.
A raspberry to the Dennys, Borens, and Bells for planting the city precisely on a catastrophically destructive fault. The danger is real and inevitable. How can one prepare for such a terrific event? We can prepare, and maybe we will.