For nearly two decades scientists have known about the Seattle fault and have been refining their understanding of the danger it presents to the Puget Sound region if it ruptures in a major earthquake.
They have seen evidence that a major quake, perhaps with a magnitude of 7 to 7.5, struck the fault more than a millennium ago and raised the south end of Bainbridge Island and Alki Point in West Seattle right out of the water. They know the quake triggered a tsunami on Puget Sound, and they have been able to track the fault zone east through Seattle and across Lake Washington.
But there is little understanding of what the fault zone looks like west of Bainbridge Island as it crosses the Kitsap Peninsula and heads toward the Olympic Mountains. A big reason is topography — to the east the evidence is contained in terraces pushed well above water level by the shaking 11 centuries ago, while to the west many of the signs are not as dramatic.
New research by Beth Martin, a UW doctoral student in Earth and space sciences, examined evidence in saltwater marshes on the Kitsap Peninsula, including marsh bottom that, while currently within reach of high tides, shows signs of being raised at least 6 feet.
Working in trench excavations and examining core samples, she found fossil remains of clams and oysters in a layer raised substantially higher than the tide flat where the creatures would have lived. Directly above that layer she found a peat bog containing remnants of cedar and hemlock trees and seeds from plants typically found in freshwater swamps.
The evidence, she said, implies that the tide flat is at least 6 feet higher now than it was when the last major Seattle fault earthquake struck between A.D. 900 and 930.
“I’m trying to paint a better picture of what actually happened in these earthquakes,” Martin said. “I’m trying to say, ‘If this were to happen again, what would that day be like?’”
As research for her doctoral dissertation, Martin set out to investigate about a dozen locations from the Skokomish River delta on the Kitsap Peninsula west of Puget Sound to Lake Sammamish State Park some 50 miles to the east, sites where reports indicated there might be telltale deposits from historic tsunamis.
She presented her findings Tuesday, Oct. 20., during the Geological Society of America annual meeting in Portland.
Tsunamis, powerful earthquake-generated waves that can cause massive destruction, leave broad sand deposits of relatively uniform thickness over an entire landscape. Another sign of a potent earthquake, liquefaction, or liquefying soil, also leaves sand deposits. But in the wall of an excavation, liquefaction can look something like a volcano that has erupted sand up through the soil, with the deposit shaped like a dome that is much thicker in the center and thinner toward the edges.
Martin could confirm only one tsunami deposit that had not been documented by scientists previously. It is at the Kitsap Peninsula hamlet of Gorst, at the head of a Puget Sound arm called Sinclair Inlet.
“In some cases it was too hard to tell whether it was a tsunami or something else,” such as the remnants from a landslide or a deposit from a major flood, she said.
Evidence from Gorst and from the Skokomish River delta on Hood Canal indicated tectonic forces had pushed the surface at least 6 feet higher, she said. A site in between the two, Lynch Cove near Belfair at the end of Hood Canal, appears to be at least 10 feet higher.
Martin noted that, in some cases, the tectonic activity responsible for the changes she observed might not have originated on the Seattle fault. Lynch Cove, for example, more likely was affected by activity on the Tacoma fault, which is less understood and its path less defined than the Seattle fault.
The evidence in the Skokomish River delta, she said, could be associated with the Tacoma fault, the Olympia fault or a smaller local tectonic feature called the Saddle Mountain fault.
“In the Puget Sound region the record for tsunamis isn’t as extensive as you would think it should be, given the number of faults,” she said.
The research, Martin said, is a step toward understanding the relationship between the crustal faults crossing various parts of the Puget Sound region.