Scientists Search for New Faults in Puget Sound
For more than two years, scientists have known that an earthquake about 1,100 years ago altered much of the landscape around Seattle.
Now they are finding evidence of huge land shifts and tsunamis from the same period from Olympia to Everett, a 60-mile stretch of the Puget Sound region occupied by about 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 Thomas L. Pratt, a U.S. Geological Survey geophysicist.
Pratt and other scientists from the USGS, University of Washington and state Department of Natural Resources are taking to air, sea and land to map the shallow faults that spawn such quakes.
During their work, there was a 4.9-magnitude tremor June 23 and aftershocks for days beneath the southern tip of Bainbridge Island.
Like the big one a millennium ago, they occurred within the Seattle Fault Zone, an east-west band about three miles wide with three main strands broken by two north-south fault lines.
A geologic structure some scientists have nicknamed the “Legislative Fault” runs southeast to northwest, passing just north of Olympia, the state capital. A second trends generally east-west beneath Tacoma. A third appears farther north, beneath Vashon Island.
Shawn Dadisman, a USGS scientist from Menlo Park, Calif., said the lines appear in charts of gravitational and magnetic force, some reflecting aerial surveys, but have yet to be confirmed as faults.
“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.”
USGS scientists chartered the 120-foot MV Robert Gray for an around-the-clock mapping project June 23-July 2 between Seattle and Olympia.
Basic tools included a 70-cubic-inch air gun, audible for miles under water, and 72 microphones mounted in a yellow streamer trailing 1,000 feet behind the vessel.
Echoes bouncing back from the seabed were recorded, yielding an image of the top 300 to 650 feet of underwater sediment and rock, said Samuel Y. Johnson, a USGS sedimentary geologist in Denver.
Rather than smooth, uninterrupted layers as would be expected without earth movement, he explained, the pictures show “lots of warped, truncated and deflected bands” that often indicate faults.
Similar mapping was done from Seattle northward to the San Juan Islands and in Hood Canal two years ago.
Next spring, scientists hope to map deeper layers using an air gun 100 times more powerful.
Another focus of research is the type and intensity of ground motion in different kinds of quakes, information critical to building safety standards, 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.
Puget Sound is much more seismically complex than California and its faults less known.
Instead of clearly visible faults and a dominant structure like the San Andreas Fault in California, the Puget Sound region is covered by glacial till, forests, lakes and marine inlets.
“We’re about three decades behind fault-mapping in California,” Johnson said.
There are subduction quakes off the coast, where the North American plate of the Earth’s crust is overriding the Juan de Fuca plate; temblors from about 30 miles beneath cities like Seattle and Portland, Ore., where the overridden plate is disintegrating; shallow tremors in the top 12 miles of the crust, and volcanic quakes around peaks like Mount St. Helens.
Craig Weaver, the top USGS seismologist in Seattle, said the research could lead to Puget Sound’s elevation from a level three quake-danger rating to a level four, the same as San Francisco Bay.
More than a millennium ago, a quake estimated at a magnitude 7 on the Seattle Fault thrust land upward as much as 23 feet and submerged 200-acre chunks of forest in central Puget Sound and Lake Washington.
Joanne “Jody” Bourgeois, a University of Washington scientist, recently found buried trees, sand deposits and other clues suggesting a nearly simultaneous downward shift and tsunami deposits in the Snohomish River delta near Everett.
Scientists also have found signs of sudden massive land subsidence around Olympia, Shelton and the Nisqually River delta between Olympia and Tacoma from the same period.
“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 north of Seattle called the Whidbey Island Fault may be part of a quake-generating system extending 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.”
Also poorly understood is the interplay between quakes along different faults.
Many scientists believe a 7.3 shock in the Landers area of San Bernardino County east of Los Angeles and a 6.5 jolt near Big Bear in the San Bernardino Mountains, both on June 28, 1992, led to quakes beneath Mammoth Lakes, Calif., near the Nevada border, and in other geological structures more than 100 miles away.