Tsunami May Help Pinpoint Last Great Northwest Quake

It is about 9 p.m. on Jan. 26, 1700.

Along the Pacific Northwest coast, the ground shakes violently. In some places, it sinks and is covered by water rushing in from the Pacific Ocean, swamping stands of red cedar and Sitka spruce.

Hours later, an earthquake-generated wave up to 10 feet high hits the central Japanese coast, damaging homes and inundating rice paddies.

In a new study, Japanese researchers, working backward from historical records of that wave, or tsunami, have pinpointed the alleged time and date of what many scientists believe was the last giant earthquake to occur in the Northwest.

The study, in the Jan. 18 issue of the journal Nature, postulates that the quake would have registered about 9 on the Richter scale. It provides another piece of evidence that the area from Northern California to southern British Columbia is subject to huge earthquakes that occur as plates in the Earth’s crust collide beneath the Pacific in what is called the Cascadia subduction zone.

Other research in recent years, using soil studies, tree-ring samplings and other methods, has indicated that a large earthquake or series of quakes hit the region within a couple of decades of 1700. But with no local historical records from that time other than Indian legends, the date or dates had not been pinpointed more precisely.

But using historical records in Japan, researchers found that tsunamis of unknown origin struck at least five locations on Japan’s Pacific coast on Jan. 27-28, 1700. At Miyako, 20 houses were damaged and the wave height was estimated at 6 1/2 to about 10 feet. At Otsuchi, a 10-foot wave damaged two houses and inundated rice paddies.

There were no records of local quakes occurring at that time, indicating that the tsunami came from a Pacific subduction zone other than Japan’s Nankai trough: the zones off South America, Alaska, Kamchatka or the Pacific Northwest.

Such zones produce the strongest quakes known. A 1960 quake off Chile, for example, had a magnitude of 9.5 and generated a tsunami that traveled across the Pacific in about 24 hours, causing extensive damage and killing 140 people in Japan.

But there is no historical or geological evidence at the South America, Alaska or Kamchatka locations for a large quake around 1700, leaving the Cascadia zone as the likely culprit, the study concludes.

A tsunami from there would take about 10 hours to reach Japan, so researchers calculated the quake’s time at 9 p.m. on Jan. 26--consistent with Indian legends that an earthquake occurred on a winter night.

The researchers also used the alleged size of the wave to calculate the strength of the quake, concluding that it would have been about magnitude 9.

But there has been a dispute over whether the entire 600-mile-long Cascadia zone could rupture at once, creating a monster quake of that size, or whether it would fracture in a series of quakes of about magnitude 8, at most--still large, but creating a smaller tsunami and a much smaller area of damaging effects.

Chris Goldfinger, a marine geologist at Oregon State University who subscribes to the latter theory, said the study did not take into account the geological structures of the Cascadia zone. For example, he said, the Cascadia zone off Oregon ruptures east to west, while the zone off Washington ruptures west to east--arguing against a single rupture.

“Those kind of structural things are what actually controls the size of tsunami that is generated,” he said.

The study also fails to consider the possibility of a “tsunami quake”--a smaller quake that generates a tsunami out of proportion to its size, possibly because of a coincidental underwater event such as a landslide.

A Sept. 1, 1992, quake in Nicaragua may have been such an event, he said. That quake measured about 7 in magnitude but generated a 40-foot tsunami that killed at least 105 people and left dozens more missing along a 150-mile stretch of coast in Nicaragua, according to news reports at the time.

A 125-mile stretch off the southern Oregon coast, Goldfinger noted, contains a series of large slope failures that might have contributed to a tsunami quake.

“It’s intriguing,” Goldfinger said of the new study. “I think it’s a good start. The idea of using Japanese tsunamis is a good one and it may be right.”