By Leander Kahney WiredNews.com
Geologists have discovered evidence of a massively powerful earthquake zone beneath the Pacific Northwest just offshore from the Seattle area. They made the discovery by piecing together ancient accounts of a giant Japanese tsunami and a computer simulation of a huge tremblor in the 17th century.
Thought to be inactive, the earthquake zone runs 600 miles up the Pacific Coast from Northern California to southern British Columbia. It appears to be subject to monster quakes every 500 years.
... the paper's authors present a solid case that a magnitude-9 quake on Tuesday, January 26, 1700, sent a 15-foot tsunami into Japan about 14 hours later...
"It flooded farmed fields, ruined salt kilns, damaged fishermen's shacks, ascended a castle moat, entered a government warehouse, drove people to high ground, and probably ran 2 kilometers up a river," the paper says. "It wrecked houses not only by flooding them but also by starting a fire. It contained multiple waves that range in reported time from midnight until the following noon. The tsunami initiated a nautical accident in which were lost two crew members and tons of rice."
... Atwood said the link was provided by stands of dead trees along the coast that drowned in seawater between August 1699 and January 1700. Their time of death was determined by the trees' last growth ring.
... During the quake, the sea floor stretched 60 feet and dropped 5 feet. "It was a sudden lowering of a sizeable chunk of the ocean floor," Atwood said, "The ocean rushed in, and that's what created the tsunami."
... Atwood said the geological record indicates the fault ruptures about once every 500 years and is capable of unleashing "truly giant earthquakes."
He said only three quakes this century compare in magnitude -- a 9.0 quake in Kamchatka in 1952, a 9.5 quake in Chile in 1960 and a 9.2 tremblor in Alaska in 1964.
"The 1906 earthquake (in San Francisco) was a very big earthquake, but this thing in 1700 was in a different class," said Atwood.
He said the fault, which is long and slopes gently under the North American tectonic plate, is characterized by tremors that last for several minutes. "It is very challenging for building design," he said. "Most earthquakes don't last that long. We have no experience for building to withstand it." ...