This is the counterpart to the better known, and much better documented, Cascadia subduction zone quake of 1700. We have only geology and fairly hazy local Indian stories to confirm that the Seattle Fault released about a 7.3 magnitude quake in a year close to 900 A.D., with the results including a tsunami, land to the south of the fault being pushed up about 20 feet, and huge landslides that buried grown trees under the surface of Lake Washington.
It’s hard to visualize what such a quake could do today, although local agencies have produced a scenario for the havoc a somewhat smaller event on the Seattle Fault would produce. The Haiti earthquake was of similar size, and happened on a strike-slip fault, whereas the Seattle Fault is a reverse fault: land to the north of it drops in an earthquake, land to the south of it rises in an earthquake. The 6.9 magnitude Kobe earthquake in 1995 was also a strike-slip action, and the damage from it is probably the closest recent match for the sort of damage that would come from a circa 7 magnitude Seattle Fault quake.
Anyway, here is some information about those local Indian stories connected with the circa 900 quake. Back in 1893, the Seattle Post Intelligencer printed this explanation from Samuel Coombs, an early settler, of a long-ago earthquake and its associated ritual:
“During the past thirty-three years I have on many occasions endeavored to gather from the oldest and most intelligent Indians something for their earlier recollections; for instance, as to when the heaviest earthquake occurred. They said that one was said to have occurred a great many years before any white man had ever been seen here, when mam-ook tamah-na-wis was carried on by hundreds. This is the same performance they go through when they are making medicine men, and consists of shouting, singing, beating on drums and sticks and apparently trying to make as much noise as they can.”
Also, the Suquamish Tribe, located on the Kitsap Peninsula, had an oral tradition about the creation of Agate Passage, which separates the peninsula from Bainbridge Island:
“Long ago, when this land was new, the area we know as Agate Pass was much smaller than today. . . . There lived in this . . . body of water a . . . Giant Serpent. The Double Headed Eagle flew over the pass and the Giant Serpent came up very angry. The two began to fight, and the earth shook and the water boiled . . . the people began to scream and cry until it was as loud as thunder.
“Then, as if the earth was going to be swallowed by the waters, they began to boil and churn. Then, the Double Headed Eagle exploded out of the water and up into the sky with the body of the Giant Serpent in its claws. The Double Headed Eagle flew back into the mountain and behind him was left the wide pass . . . .”
The two above stories come from a 2005 research article, Serpent Spirit-power Stories along the Seattle Fault, which notes that “the description of the widened channel could reflect permanent ground-level change, and the sense of ground motion suggested by the story is accurate: Agate Passage is on the down-dropped northern side of the Seattle Fault.”
The article estimates that the circa 900 earthquake “caused 7 m of vertical uplift on the southern side [of Seattle Fault], sent massive block landslides tumbling into Lake Washington, and created a tsunami in Puget Sound that left sand deposits on Southern Whidbey Island.”
The article also links earthquakes and tsunamis to Indian stories about the a’yahos, a supernatural being thought of as either a huge serpent with fearsome eyes and horns [such as the one told of above] or as a composite monster with the head and toros of a deer and the tail of a snake. The a’yahos was “associated with shaking and rushes of turbid water and comes simultaneously from land and sea. At the spot where a’yahos came to a person the very earth was torn, land slides occurred and the trees became twisted and warped. Such spots were recognizable for years afterward.”
The article goes on to talk about a handful of a’yahos spirit boulders located near the Seattle Fault and thought to be markers for landslides and other damage wreaked by the huge circa 900 quake. It is one of the few documents that attest to not just the threat but the actual damage caused by ancient Northwest earthquakes that did not make it into the historical record. You can read a bit more about this in a University of Washington news release on Indian tribes and earthquake mythology in the Northwest.