Northwest on 500-year cycle for big quake

An earthquake of magnitude 8.0 or even 9.0 will likely occur sometime in the future along the Cascadia Subduction Zone, which runs from British Columbia to Northern California, says a group of scientists.

The international seismic experts, writing in the Journal of Geophysical Research — Solid Earth, noted that it has been authenticated that a quake in the 9 magnitude range struck off Washington and Oregon on Jan. 26, 1700, sending a destructive tsunami across the Pacific to Japan.

The interval between such temblors in the past has been about 500 years, so the next one may not arrive in this century, according to the scientists.

Still, said Brian Atwater, a U.S. Geological Survey scientist based in Seattle, it is not too early to explore "how to adjust building codes and tsunami evacuation plans to reduce losses of life and property" from such a quake.

A subduction zone is where one tectonic plate is diving, over many thousands of years, under another. In this case, the Juan de Fuca plate is subducting the North American. Such processes cause great quakes and, inland, volcanic eruptions.

Kenji Satake, a seismologist with the Geological Survey of Japan who is the lead author of the article, said that such a huge quake releases as much energy as the United States now consumes through electricity, gas and other means in a month.

Satake said it would be marked by low-frequency shaking that would threaten tall buildings all along the Northwest coast.

There have been quakes stronger than magnitude 9.0 off Chile in 1960 and Alaska in 1964. Both sent destructive tsunamis across the Pacific. But no other fault in or near the lower 48 U.S. states is believed capable of producing as powerful a quake.

The scientists said that radiocarbon dating techniques indicated the quake rupture in 1700 extended at least 560 miles, a considerably longer rupture than that which marked the San Francisco quake of 1906 on California’s San Andreas fault.