Along the southern Washington coast, "ghost forests" stand skeletal and half-submerged in water, their spindly roots and branches clawing the air and catching the attention of scientists like Brian Atwater. "They scream at you, 'Interpret me! How did I die?'" says Atwater, a U.S. Geological Survey researcher and affiliate professor at the University of Washington in Seattle.
Those ghostly trees, it turns out, died when coastal land collapsed in a massive earthquake 300 years ago, plunging entire forests under the sea. Atwater has used the buried ecological landscape of the year 1700 to rewrite earthquake history in the Pacific Northwest.
Until the mid-1980s, scientists had no records of quakes along the Cascadia subduction zone, a fault running from British Columbia to Northern California. Atwater and his colleagues discovered that seven massive temblors have shaken the coast in the last 3,500 years, and went on to date the most recent one with astonishing precision: a magnitude 9 quake that struck on January 26, 1700, at 9 p.m.
Subduction-zone earthquakes occur in places where two shifting blocks of the Earth's crust&emdash;an oceanic plate and a continental plate--converge and rupture. As the downward-plunging oceanic plate yields to the overriding continental plate, the shift raises the seafloor and lowers coastal land. The lurching of the ocean floor creates tsunamis. One swell, sometimes more than 50 feet high, rushes toward the nearby coast, while another chain of waves rolls toward distant shores at speeds approaching 500 mph.
When tsunamis wash over the land, plants are suffocated and--if trapped in an airless layer of mud--preserved. Even centuries later, grass stems may remain intact, still pressed in the direction of the waves.
That's what Atwater found when he dug into marshland beneath the ghost forest. Using radiocarbon dating, his team placed the last quake between 1680 and 1720. Japanese scientists found village records of a January 1700 tsunami there, and U.S.-based researchers traced the final growth ring of submerged western red cedars to 1699. Scientists declared the dates a match, and the "orphan tsunami" that hit Japan three centuries ago finally had parents: two tectonic plates off the Washington coast.
"Brian put together a coherent story by paying attention to marsh ecology," says fellow USGS researcher John Tinsley, MS '72, PhD '75.
Atwater first became captivated by geology as a Stanford sophomore on a field trip to the Grand Canyon. He's currently working on two new projects--studying Japanese written records of the tsunami generated by the 1700 Cascadia earthquake, and assessing the hazards threatening Seattle from sandy floods running off Mount Rainier, the largest volcano in the Cascade chain.
When he isn't digging in marshes or his garden, Atwater devours nonfiction. But his own saga is as groundbreaking as any--the story those dead trees were screaming, in a language he discovered how to read.
--Christina McCarroll, '00, MA '01
