Scientists say old Japanese papers confirm a huge magnitude nine earthquake struck north-western America 300 years ago.
The team modelled how the tsunami would have spread
The writings report damage from a five-metre-high tsunami that washed on to the Japan coast on 26 January, 1700.
Computer modelling allowed Kenji Satake of the Geological Survey of Japan and colleagues to model the size and source of the quake that created the wave.
A magnitude nine event would, in a few minutes, release about as much energy as the US now consumes in a month.
The Japanese writings fit neatly with what is already known from studying rocks in the region.
The simulations show the earthquake probably ruptured the full length of a fault, known as the Cascadia subduction zone, which extends more than 1,000 kilometres [600 miles] along the Pacific coast from southern British Columbia to northern California.
Until fairly recently, this fault was thought to be benign by most scientists. But then a number of discoveries in North America revealed the fault can produce earthquakes of magnitude eight or larger at irregular intervals, averaging about 500 years.
The most recent of the earthquakes, dated by radiocarbon methods, occurred between 1680 and 1720. It is the Japanese documentation that now ties the date specifically to 26 January, 1700.
Satake's team, which includes Kelin Wang of the Geological Survey of Canada, and Brian Atwater of the United States Geological Survey, reports its work in the Journal of Geophysical Research-Solid Earth, published by the American Geophysical Union.
Their findings are likely to affect the Pacific region's precautions against future earthquakes and tsunamis.
"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 nine earthquake in southern British Columbia, Washington, Oregon and northern California."
Wang also noted that the giant fault responsible for this earthquake is currently "locked," accumulating energy for a future destructive event.
"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 south-western Japan," he said.
"With a combination of a better understanding of the previous earthquake and modern observations, we hope to better define the potential rupture area of the future event."
The largest quake recorded anywhere this year was a magnitude 8.3 in Hokkaido, Japan. In 2002, the largest event was in Alaska, a 7.9.
A magnitude 9 is extremely rare. The last one hit Prince William Sound, Alaska, in March 1964.