On December 4, 1992, the prestigious journal Science publishes five papers on what is now known as the Seattle Fault Zone, a previously undescribed, several-mile-wide area of crustal weakness running from about Issaquah, under Seattle's Seahawks Stadium and Safeco Field, through Alki Point and Restoration Point to Hood Canal. Using an array of seemingly disconnected evidence the reports’ authors show that roughly 1,100 years ago a massive earthquake rippled across the Seattle landscape. It is startling news because until that time few had suspected the existence of the fault.
An Abrupt Uplift: Restoration Point
Realization that a huge fault moved relatively recently in Seattle first began to gel in the late 1980s when geologist Robert Bucknam discovered that Restoration Point on Bainbridge Island had been thrust out of the water sometime less than 1,500 years ago. (Captain George Vancouver first noted this anomalous point of land during his round the world voyage of discovery. In 1792, he and his crew were some of the first Europeans to see Puget Sound. Vancouver wrote: "We arrived off a projecting point of land, not formed by a low sandy spit, but rising abruptly in a low cliff about ten or twelve feet from the water side" [Meany]).
In order to better understand what had happened at Restoration Point, Bucknam teamed with University of Washington paleobotanist Estella Leopold and Eileen Hemphill-Haley, of the United States Geological Survey. The trio began by studying marsh, peat, and beach deposits from Bainbridge Island, which revealed a rapid change in elevation as one site dropped below sea level and a corresponding site popped out of the water. The evidence came from humus, peat, charcoal, and diatoms (single-celled organisms that secrete complex skeletons). Using radiocarbon dating, Bucknam, Leopold, and Hemphill-Haley bracketed the change at Restoration Point at between 500 and 1,700 years ago.
Avalanches in the Olympics
The four other papers presented additional lines of evidence. The most distant data came from the Olympic Mountains. Five, large rock avalanches had dammed streams, which created lakes that had submerged trees. Snags collected from three of the lakes, Jefferson, Lower Dry Bed, and Spider, indicated that an earthquake occurred between 1,000 and 1,300 years ago.Geologists in Seattle also examined a layer of fine sediment in Lake Washington, deposited by multiple, same-age, subsurface landslides that could only be set in motion by some event as ground shaking as an earthquake. The mean age for organic matter in the sediments was 1,117 years before present, plus or minus 142 years.
Trees Submerged in Lake Washington
An analysis of Douglas fir trees from Seattle also gave a date for the last movement of the Seattle fault at about 1,100 years ago. The initial evidence came from three groves, which a fault-induced landslide had carried from their original habitat on the shores of Lake Washington to their present resting place 90 feet underwater. One stand slid off the southeast corner of Mercer Island. Another settled on the west side of the island, across from the south end of Seward Park, and the third slumped between Holmes Point and North Point near St. Edward Park, north of Kirkland.
In 1990, three researchers pulled up several trees from the Seward Park grove, which they used to obtain radiocarbon dates. Once again, the dates were between 1,000 and 1,300 years ago. But this was not the only intriguing piece of information from these Douglas firs. Researchers also analyzed the trees' annual growth rings.First, they determined that the firs all died in the same year and season. Next, they compared their tree ring data with a Douglas fir found at West Point, the western most point of Magnolia Bluff. This tree, discovered in February 1992 by USGS geologist Brian Atwater, was one part of an extensive archaeological and geological record removed from a trench dug for an effluent pipe connected with the Metro sewage treatment plant. Atwater’s work showed that the dead Douglas fir had been carried by a tsunami to the beach.
When Atwater found the tsunami-deposited log, it rested on a patch of the sand sheet and on toppled, flattened bulrush stems. Radiocarbon dating put the tree's death at between 850 and 1,350 years ago. In order to narrow this window of death and to pinpoint the date of the tsunami, Atwater compared his tree-ring widths with data from the submerged forest trees. The trees died within a half year or less or each other, most likely between 900 and 928 AD.
Combining all the various lines of evidence, researchers concluded that one day about 1,100 years ago the land south of the Seattle Fault line shot up 20 feet and the area north of it dropped at least 3 feet during a magnitude 7.5+ earthquake. Geologists also concluded that this earthquake occurred less than 10 miles beneath Seattle.
A shallow earthquake such as one that would be produced by the Seattle Fault Zone has the potential for significant damage. When a fault moves deep underground, most of its high frequency energy dissipates before reaching the surface, which reduces the level of ground shaking. In contrast, a shallow earthquake's high frequency energy remains strong at the surface. During the 1995 Kobe, Japan, and 1994 Northridge, California, earthquakes, both shallow, ground shaking ranged between 2.5 and 5 times greater than what has been recorded in subduction generated faults.
The other problem with the Seattle Fault relates to Restoration Point; a shallow fault can pierce the surface. Considering that the last movement on the fault thrust rock 20 feet out of the Sound, ground ruptures could sever natural gas, liquid fuel, sewer, and water supply pipelines, all of which cross through the Seattle Fault zone. And this does not even address the numerous bridges and roads that an earthquake could destroy.Since the publication of the five papers in 1992, geologists have continued to study the Seattle Fault Zone. They have found several additional strands and learned that it has moved many times in its history. They cannot predict when it will move again; they know only that it will.