Northwest In for a Monster Quake? : Seismologists Say the Evidence Makes It Seem Likely

Seismologists with the U.S. Geological Survey have concluded that the Pacific Northwest, an area not previously regarded as prone to earthquakes, actually has the potential for a quake more powerful than even the mighty San Andreas fault is capable of producing in California.

Such a quake, which could exceed 9 on the Richter scale, could devastate large areas of Oregon and Washington and set off tidal waves that would impact on much of the Pacific region.

“Every piece of evidence makes it hard to rule it out,” said Thomas H. Heaton, chief scientist of the survey’s field office in Pasadena and co-author, with Stephen H. Hartzell, of a report on the findings in Friday’s issue of the journal Science.

However, the scientists stop short of actually predicting an earthquake, saying the historical record is too inconclusive to know whether such giant quakes have hit the area in the past, and if so, when. The lack of historical evidence also means it is impossible at this point to know whether such a quake is due soon, or more likely to hit in the distant future.

Striking Similarities

The two scientists concluded that a great quake should be expected at some time in the Pacific Northwest because the area is strikingly similar geologically to other regions of the Earth that have produced giant earthquakes, including southern Chile, southwestern Japan and Colombia. Those areas all have been struck by earthquakes that have caused extensive damage and claimed thousands of lives.

“The suggestion of similar events in the Pacific Northwest is disturbing,” the scientists concluded.

“Although (the comparison study) does not prove that great earthquakes will occur (in the area) it does suggest that it is inappropriate to assume that great earthquakes will not occur,” the scientists said in their report.

“Most seismologists are taking this very seriously,” Heaton added in an interview.

The area under study is known as the Cascadia subduction zone, an 800-mile-long ridge just offshore from Oregon and Washington where the Pacific and Juan de Fuca tectonic plates are being sucked under the North American plate. Tectonic plates are giant slabs of the Earth’s crust that move slowly over the planet’s molten mantle.

Chain of Volcanoes

The violent clash of the plates is the source of the dynamics and the energy that created the chain of volcanoes throughout the Cascade range, including Mt. St. Helens, which erupted with devastating fury in 1980.

However, the region has been devoid of earthquakes in historical times, a fact that Heaton finds extremely disturbing. That lack of activity has led to a popular assumption that California has more to worry about from earthquakes than the Pacific Northwest. But Heaton warns that the seismic inactivity in the Cascadia zone may be the most ominous warning of all.

That lack of activity may simply mean that the zone is storing up energy in the form of seismic strain and, when it finally breaks loose, it will do so with a vengeance.

He noted that in California, some sections of the San Andreas fault are constantly producing small to moderate earthquakes, but those are not the areas seismologists worry about the most. It is in areas where the fault appears “locked,” and no slippage or seismic activity occurs, where the strain is building to the point of a giant quake, Heaton said.

“The quiet areas are the most dangerous,” he said.

Another Explanation

Heaton and Hartzell concede that the Cascadia subduction zone could be quite different from the other areas that have produced massive quakes, and that perhaps there is no strain building up in the system. That would mean slippage between the tectonic plates is so smooth that they simply glide past each other like skaters on a frozen lake. But that would be most extraordinary, Heaton said.

“It could be that this is somehow a very different zone, with a slow, creeping movement,” he said.

“But when we look at other subduction zones, there are many similarities between this zone and others that have produced very large earthquakes,” he said. The similarities include the age of the sediments being subducted, the rate of convergence, heat flow, volcanic activity and the size of the tectonic plates.

And although the relatively new science of attempting to date past earthquakes has not produced a record of any major quakes in the Pacific Northwest, current studies under way in that region suggest that some of the land formations there were created by major temblors, Heaton said.

Scientists are studying coastal Northwest bogs that were suddenly submerged at some time in the last few hundred years, indicating a rapid drop in the ground level like that produced by major earthquakes, he said.