For more than two years, scientists have known that an earthquake thrust land upward as much as 23 feet and dropped 200-acre chunks of forest into the water near Seattle about 1,100 years ago.
Now evidence of similarly huge land shifts, slides and tsunamis, or sea waves, at about the same time is being found southward toward Olympia and northward around Everett, a 90-mile stretch of the Puget Sound region that is now home to more than 2.7 million people.
It may have been “one big quake or a whole series of earthquakes … four or five earthquakes, all triggering each other,” said Dr. Thomas L. Pratt, a U.S. Geological Survey geophysicist.
Pratt is one of the scientists from the USGS, University of Washington, state Department of Natural Resources and other agencies aboard a chartered research vessel, the MV Robert Gray, that is mapping the little-known, shallow faults that spawn such quakes.
More common examples include the tremors felt in Bremerton and much of the rest of the central Puget Sound area this week. Like the big one of a millenium ago, they occurred within the Seattle Fault Zone, an east-west band with three principal sets of fault lines broken by two north-south fault lines.
Another geologic structure nicknamed the “Legislative Fault” by some of the scientists may extend from the southeast to the northwest, passing just north of Olympia. Another runs more generally east-west beneath Tacoma. A third appears farther north, beneath Vashon Island.
Shawn Dadisman, a USGS scientist from Menlo Park, Calif., who is one of the coordinators of the project, said the lines appear in charts of minute differences in gravity and magnetism in the region.
“We’re going to investigate it to see what is causing these anomalies,” she said.
“We do not know how active any of these are,” Pratt added. “We are just starting to get some idea of how active the Seattle Fault is.”
Other research is needed to determine the type and intensity of shock waves generated by various kinds of quakes. That information will be used to develop safety standards for new construction and retrofits for existing structures, said Stephen Malone, a University of Washington professor who is developing a network of earthquake-monitoring devices.
“The problem is that, in the Pacific Northwest, we don’t know all that much about how earthquakes shake the ground,” Malone said.
Similar fault mapping was done from Seattle northward to the San Juan Islands and in Hood Canal two years ago. The current project, operating around the clock, began Wednesday and runs through next Wednesday from Seattle southward to Olympia.
The basic tools include a 70-cubic-inch air gun that can be heard a few miles away under water and 72 microphones mounted in a yellow streamer trailing 1,000 feet behind the vessel as it traverses a carefully marked grid over Commencement Bay.
Echoes bounce back from the bottom of the sound and are recorded by a bank of computers for progressively more detailed analysis over the next two years.
The result is a detailed model of the top 300 to 650 feet of rock and sediment layers beneath the water, said Samuel Y. Johnson, a USGS sedimentary geologist in Denver.
Rather than the relatively flat layers that would be expected without earth movement, he explained, the resulting pictures show “lots of warped, truncated and deflected bands” that indicate the location of faults.
Using the boat costs about $75,000 for the week, compared with $2 million to $3 million for the year or more that would be required to map faults over a comparable area on land, Dadisman said.
The Puget Sound region is much more seismically complex and its faults less well identified than is California, scientists aboard the vessel said.
Unlike drier regions to the south, where faults are readily visible on land, bedrock in the Puget Sound region is often hidden beneath glacial till and forests.
“We’re about three decades behind fault mapping in California,” Johnson said.
The region also has more kinds of shakes.
There are subduction quakes off the coast where one plate of the Earth’s crust is being overridden by another; deep temblors from an area about 30 miles down where the overridden plate is disintegrating; shallow tremors that may be the most dangerous of all, the kind the scientists aboard the vessel are measuring, and volcanic quakes around peaks like Mount St. Helens.
Some of the mapping may shed more light on discoveries since the big quake, estimated to be about magnitude 7, that altered much of the Puget Sound landscape.
Joanne “Jody” Bourgeois, a University of Washington scientist, has found buried trees, sand deposits and other evidence of a sudden downward shift and a tsunami deposit in the Snohomish River delta between Everett and Marysville about 1,100 years ago.
Scientists also have found massive subsidence of land at about the same time around Olympia, Shelton and the Nisqually River delta, between Olympia and Tacoma.
“People in Tacoma shouldn’t think that if there’s an earthquake on the Seattle Fault, they’re going to be OK,” Pratt said.
That fault, and another to the north known as the Whidbey Island Fault, may be part of a quake-generating system that could extend a dozen miles or so beneath Tacoma, he explained.
“This is a theory that we’re testing,” Pratt said. “The Seattle Fault isn’t just isolated. It goes somewhere.”