by David Williams
In November 1965, Professor of Physics Zdenko Frankenburger Danes published a short paper in the Journal of Geophysical Research that presented an unsettling new theory: An active fault cuts east-west across Elliot Bay and south Seattle.
“Most geophysicists and geologists said, ‘So what,’ but real estate agents were quite upset when I told people they were sitting on a fault,” says Danes, who retired from the faculty in 1984. Geologists now consider this feature, called the Seattle fault zone, one of the greatest potential hazards in the region.
Few suspected that Seattle was in danger of an earthquake when Danes began the work that appeared in his report, but he’d had an intimation. His clue came from an elegant little tool called a gravimeter. Basically a weight suspended on a spring, it can detect the subtle difference in gravitational pull between a dense rock, like a basalt, and a low-density sandstone, thus providing some insight into the subsurface geology.
In 1961 Danes had borrowed a gravimeter from the University of Minnesota, which he used to measure the gravity near his home on Mercer Island. “I ran a little survey on the island, and it was pretty obvious that something was going on, but I had to return the gravimeter before I could finish my work,” he says. Not one to give up on a problem, Danes mentioned his interest in acquiring a gravimeter from Hunt Oil Company to UPS Professor of Chemistry Bob Springer, and soon Danes had the tool he needed to figure out what was going on under Puget Sound.
There were nine co-authors on his paper, all of them high school students. They helped him as part of a grant focused on introducing kids to natural sciences by having them work on a real science project.
Working from Tacoma up to Seattle, Danes and his team found three areas where the gravitational field differed distinctly from an adjacent location. “We traveled together and the kids took turns taking the readings. We found a huge anomaly (or consistent change in gravity) at a line that ran roughly between Hood Point and south Seattle. It was obvious that something was going on and I knew that either a big fault or an intrusion had drastically altered the subsurface,” says Danes.
To account for the anomalies, Danes hypothesized that several active faults, which crisscrossed the Puget Sound lowlands, had ratcheted the subsurface rocks out of their original positions. The most active and most significant structure was a “double fault striking approximately at azimuth 105 degrees through Hood Point, Bremerton, southern Seattle, and Renton.” North of the fault, the evidence pointed to the six-mile-deep basin filled with relatively soft, sedimentary rocks, whereas the rocks in the uplift south of the fault were hard, dense igneous basalt and gabbro.
“Danes’ notion that the very steep gravity gradient in the Seattle area resulted from faulting was very insightful,” says USGS geologist Sam Johnson, whose research has helped refine Danes’ work. “It appears to have had minimal immediate influence. Once interest in the local earthquake hazard became more acute (mid ’80s to the present), Danes’ regional geophysical work was regarded as an important first step in the effort to find and document local faults.”
During the intervening 40 years, geologists have made huge strides in understanding when the Seattle fault zone last moved, where it runs underground, and what drives the movement. Modern tools include bouncing sound waves off subterranean rocks, reading the magnetism of these rocks, and using lasers to see through the vegetation, along with traditional field examination of traces of the fault on the surface.
Geologists have also started to probe further afield. Recent evidence shows that Tacoma also sits astride a massive fault complex, like the one in Seattle.
“All of this work began with Danes because the geophysical data gives us the tools to know where to look in the field,” says USGS geologist Brian Sherrod. “The more we look the more we find. Danes found the faults, and we’re just proving they’re active.”