Did North American Quake Cause 1700 Japanese Tsunami?
Stefan Lovgren in Los Angeles
for National Geographic News
|December 8, 2003|
An international team of scientists says that a great earthquake of magnitude 9 struck the Pacific Northwest in 1700, and created a tsunami that caused flooding and damage on the Pacific coast of Japan.
The study, published in the Journal of Geophysical ResearchSolid Earth, shows how a great earthquake along the Cascade subduction zone, which runs for 1,100 kilometers (680 miles) down the Pacific coast of North America, warped the ocean floor and briefly changed the shape of the overlying sea surface, sending large waves crashing onto Japanese shores only 10 to 20 hours later.
The study not only highlights the trans-Pacific connection between earthquakes and tsunamis, but also shows that earthquakes occurring along North America's west coast can be more powerful than previously thought.
"The findings leave little room to doubt that an earthquake in the Pacific Northwest occasionally attains magnitude 9," said Brian Atwater, a geologist with the United States Geological Survey based at the University of Washington in Seattle, and one of the study's authors.
A Massive Rupture
The relatively obscure Cascadia subduction zone, which extends from southern British Columbia in Canada to northern California, is a huge fault trace located under the Pacific Ocean, between two tectonic plates.
Great earthquakes occur on a shallow part of the fault boundary between the downward-moving oceanic plate, called the Juan de Fuca plate, and the overriding continental plate, known as the North America plate.
Until 20 years ago, the Cascadia fault was believed to be benign by most scientists. But then several discoveries in North America showed that the fault produces earthquakes of magnitude 8 or larger at irregular intervals, averaging about 500 years.
Radiocarbon evidence suggested that at least 900 kilometers (560 miles) along the fault ruptured between 1690 and 1720. However, no one knew the magnitude of the rupture.
Then, in 1996, Japanese researchers stunned their North American colleagues by linking a tsunami that struck Honshu Island in 1700 to geologic reports at the Cascadia subduction zone. Tree-ring dating in North America further suggested that an earthquake struck between August 1699 and May 1700.
Japan has a documented history of tsunamis, including trans-Pacific events, dating back to the 1500s. From the tsunami's time of arrival in Japan, the researchers concluded the Cascadia earthquake must have occurred in the evening of Tuesday, January 26, 1700.
The latest research shows the 1700 tsunami cresting as much as five meters (16 feet) in Japan. Scientists used computer simulations of trans-Pacific tsunamis to correlate the Cascadia earthquake size with the tsunami's heights in Japan.
"We have collected more evidence, made rigorous interpretation of it, and modeled the earthquake source and tsunami propagation by using the latest techniques," said Kenji Satake, a seismologist with the Geological Survey of Japan, and the lead author of the report. "We confirmed that the 1700 earthquake was magnitude 9."
They also found that the fault must have ruptured all at once, rather than in smaller segments.
In a few minutes, a magnitude 9 earthquake would release as much energy as the United States now consumes in a month. It would also produce low-frequency shaking, threatening tall buildings from British Columbia to northern California.
No other fault in the conterminous United States in the lower 48 states is believed to be capable of producing a magnitude 9 earthquake. A similar fault off Alaska produced a 9.2 earthquake in 1964.
A Train of Ocean Waves
Tsunamis are sometimes erroneously referred to as tidal waves. While tides result from the gravitational influences of the moon, sun, and planets, tsunamis are caused mainly by earthquakes, but could also result from landslides, volcanic eruptions, or even the impact of meteorites.
A giant rupture in the Cascadia fault would cause the underwater part of the continental plate to bounce upward. This warping of the sea floor happens so quickly that the sea water has nowhere to flow away, but will instead raise the sea level. The collapse of this raised sea level sets off a train of ocean wavesthe tsunamiwhich could reach the far side of the Pacific Ocean.
The rupture also stretches the overriding plate, whose upper surface subsides along the Pacific Coast and beneath near-shore waters. This also contributes to the tsunami.
Tsunamis are common in Japan. In the last century, more than 6,000 people have been killed by tsunamis.
It is possible that a Japanese earthquake would set off a tsunami that could hit the United States. Tide gauges in California and Oregon recorded, in 1854, a pair of tsunamis from magnitude 8.5 earthquakes in southwest Japan.
But those tsunamis were only a foot (0.3 meter) high, causing no damage. Scientists believe the faults in southwestern Japan face the wrong direction to cause severe tsunamis in North America, while the long fault off northeast Japan usually ruptures in segments too short to produce significant American tsunamis.
"Most importantly, observed earthquake size, or magnitude, has been at maximum 8.5 around Japan," said Satake. "We need a magnitude 9 event to cause trans-Pacific damage."
Storing Up Energy
Of course, a tsunami caused by a rupture along the Cascadia fault would first hit the coast nearest to itNorth America.
"At issue for North Americans," said Atwater, "is how to adjust building codes and tsunami-evacuation plans to reduce losses of life and property in the event of a future magnitude 9 earthquake in southern British Columbia, Washington, Oregon and northern California."
Under construction standards adopted since 1996, engineers have already sought to design buildings to withstand giant earthquakes in the northwestern United States and southwestern Canada. State and local officials have devised evacuation routes from areas that may be subject to a tsunami from a Cascadia earthquake of magnitude 9.
The giant Cascadia fault is currently "locked," accumulating energy for a future destructive event.
The chances of another earthquake occurring along the Cascadia fault is "100 percent," said Kelin Wang of the Geological Survey of Canada, who is also a co-author of the report. "The question is when, and whether the entire length of the subduction zone will rupture like in 1700."
Scientists in the United States, Canada, and Japan are carefully monitoring the fault's activities using seismological and geodetic methods, and making comparisons with a similar fault in southwestern Japan.
"We hope that understanding these modern activities of the plate boundary will eventually help us to identify warning signals before a major earthquake occurs," said Wang.
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