Finding Fault with Seattle

Most northerners enjoy an occasional visit to Seattle. The place was long the real capital of Alaska, economically and practically if not politically, so we have good reason to feel the Emerald City is really an overgrown and civilized version of home.

However, it may be more like home than we---or Seattleites---would like. According to the 4 December 1992 issue of the journal Science, Seattle also lies at the heart of earthquake country. We're talking big earthquakes, too.

This may not seem like a startling discovery. After all, every section of the coast of North America shakes at some time as the great plates composing our planet's crust grind past or under one another. And the city of Seattle itself is no stranger to earthquakes. A moderate quake, with magnitude 7.1 and centered near Olympia, hit there in 1949, and one of magnitude 6.5 with epicenter nearly halfway between Seattle and Tacoma hit there in 1965; that one cost about $12 million in damage.

Most of the shaking in the Puget Sound area was known to come from fairly distant activity, usually in the Cascadia subduction zone well to the west of Seattle. Geological studies during the 1980s indicated that great earthquakes along the Cascadia zone struck different portions of the Pacific Northwest from time to time, most recently about 300 years ago.

There were some exceptions, though. The slippage causing the 1965 Seattle earthquake occurred almost directly under the city, but at nearly 60 kilometers depth.

The discoveries encouraged scientists to take a hard look at the history of earthquakes in the northwestern corner of the contiguous United States. It became a field laboratory for the relatively new field of paleoseismology, the study of earthquakes taking place before means existed to record or observe them.

Recognizing a prehistoric earthquake takes a lot of different skills and some blind luck. The Science articles were contributed by people whose specialties included not only geology and seismology but also sedimentology and dendrochronology---put simply, expertise in mud layers and tree rings. Working together, the specialists built a strong case that a major earthquake, perhaps a great earthquake like the 1964 event that scarred Alaska, had taken place about a thousand years ago close to the surface of where Seattle now stands.

Their evidence was pretty strong. With the help of radiocarbon dating techniques, researchers identified marshes buried under the kind of sand layer left by a big tsunami occurring between 1000 and 1100 years ago. Within five kilometers of Seattle' s high-rise buildings, they found shorelines uplifted more than 20 feet, marked by the remains of sea-dwelling animals that had died close to 900 years ago. They found trees drowned between 1000 and 1300 years ago in avalanche-dammed lakes on the Olympic Peninsula, and at least three of the avalanches seemed to have happened at the same time.

The sediment studies were close to home: Lake Washington's bottom showed that fine particles had been resuspended and moved downslope in turbidity currents that had multiple but simultaneous origins. That is, something had shaken the whole lake, and the shaking could be dated to between 940 and 1280 years ago.

Luck joined expertise in the tree-ring studies. A forested slope slid into Lake Washington during the same season of the same year that a tsunami washed trees upshore off Puget Sound. That's what the dendrochronologists found, and they pinpointed the time of life and the dates of death for all the trees to around 1000 to 1300 years ago. The trees were available for study because old-growth timber has become increasingly valuable---even if it's been brine-cured for ten centuries. Log salvagers were dredging up the long-drowned trees, and permitted the scientists to take samples.

So, if Seattle is better prepared for an earthquake, it may be partly because the need to protect the spotted owl made old dead trees worth saving.