Water-Serpent stories of Puget Sound Natives may refer to the A.D. 900 Seattle Earthquake (PDF)

Native Americans have resided by Puget Sound for thousands of years and must have witnessed many geologic & events. They described these events using their own cultural concepts, and incorporated the stories into their oral tradition.
Traditions about the A.D. 900 Seattle earthquake, handed down by storytellers for 1,100 years may survive in stories about water-serpents near the Seattle fault. A horned water serpent was said to have its home in Seattle by the shore of Lake Washington, near landslides dated to the A.D. 900 earthquake. Another story, about an earthquake- and landslide-causing horned water-serpent on the eastern shore of Puget Sound in the Fauntleroy neighborhood of West Seattle, is close to a large undated landslide visible in LIDAR images but not easily seen on the ground. Finally, on the west side of Puget Sound, a story about the deepening of Agate Pass (located on the downthrown side of the Seattle Fault) tells of an underwater battle between a water-serpent and a mythic bird, resulting in ground shaking, churning of the waters, and permanent ground level change.

A’yahos is a “Doctor” spirit power associated with shaking and rushes of turbid water. It
comes simultaneously from land and sea, and is malevolent dangerous, and possibly
fatal to encounter. The a’yahos is a shape-shifter, often appearing as an enormous
serpent, sometimes double headed with blazing eyes and horns, or a composite
monster having the fore-quarters and head of a deer and the tail of a snake.