U.S. scientists studied Cascadia quakes going back 10,000 years and believe that a major quake is likely within 50 years

Two separate geological studies released this week suggest the earth-quake hazard in the transboundary region of the Pacific Coast of North America - including southern British Columbia - is significantly greater than previously believed.

Both teams of U.S. scientists are urging heightened readiness throughout the region for a future offshore "mega-thrust" event that could compare with the one that triggered Japan's earth-quake-tsunami-nuclear catastrophe last year.

In one study - a 13-year comprehensive analysis of the Cascadia earth-quake-prone zone between Vancouver Island and Northern California - a team of researchers concluded the "clock is ticking" ahead of a potentially devastating earthquake in the region within the next 50 years.

In a 184-page report published this week by the U.S. Geological Survey, the team, led by Oregon State University earth scientist Chris Goldfinger, compiled a detailed record of earth-quakes in the Cascadia region going back to 8000 B.C.

"Over the past 10,000 years, there have been 19 earthquakes that extended along most of the margin, stretching from southern Vancouver Island to the Oregon-California border," Goldfinger stated in a summary of the study. "These would typically be of a magnitude from about 8.7 to 9.2 - really huge earthquakes."

He added that the southern margin of Cascadia, encompassing southern Oregon and Northern California, "has a much higher recurrence level for major earthquakes than the northern end and, frankly, it is overdue for a rupture."

But, he cautioned, "That doesn't mean that an earthquake couldn't strike first along the northern half, from Newport, Ore. to Vancouver Island."

Co-author and OSU geologist Jay Patton also emphasized the likelihood of a major quake in the Pacific North-west in the coming decades.

"By the year 2060, if we have not had an earthquake, we will have exceeded 85 per cent of all the known intervals of earthquake recurrence in 10,000 years," he stated in the overview.

"The interval between earthquakes ranges from a few decades to thousands of years. But we already have exceeded about three-fourths of them."

The second study, which appears in the latest issue of the journal Geology, concludes that the probable impact area of the next megathrust quake in the Pacific Coast region could extend as much as 55 kilometres farther east than previous studies have suggested - raising the spectre that coastal cities such as Vancouver, Victoria, Seattle and Portland would experience significantly greater-than-expected seismic impacts when the next "Big One" strikes.

"If one is closer to the rupture area of an earthquake, then the ground shaking will be more intense, all other things being equal," said study co-author Glenn Spinelli, a research scientist with the New Mexico Institute of Mining and Technology.

Spinelli - who published the study with former institute scientist Brian Cozzens, and recently completed a year of research with the Geological Survey of Canada in B.C. - added further research will be required to assess "just how much of a difference for potential ground shaking in Victoria, Vancouver, Seattle, etc., may result from an approximately 50-km landward shift" of the expected impact zone.

Goldfinger said in an interview that previous earthquake-hazard studies based on specific stretches of the Pacific Coast - such as parts of Washington state, where there have been long intervals between major quakes - have led some researchers to "downplay the hazard unknowingly," pegging the likelihood of a disastrous event at only 10 to 15 per cent within the next 50 years.

He said his team's findings place that risk at up to 37 per cent in some of the most vulnerable areas, such as parts of southern Oregon.

"At 10-or 15-per-cent [risk], earth-quakes kind of fall into the category of potholes and repairing bridges and other things that need to be done over time," said Goldfinger. "But if the real probability is something more like 30 to 40 per cent, then this may happen long before we even start preparing for it."

A Canadian earthquake-hazard study published in 2010 in the Geological Society of America Bulletin also concluded the risk of a megathrust event in southern Cascadia was about twice as high as that in the northern part of the zone. But even if the epicentre of the next monster quake were off the coast of California the wall of sea water triggered by the event could well reach Canada, researcher Lucinda Leonard said at the time.

"It's fair to say that the probability of a major subduction earthquake is higher in the southern area, but we cannot be complacent in the north," she said.

"Even if the next one ruptures only the southern part of the margin, the resultant tsunami will likely be hazardous to the B.C. coast," she said.

Among the features examined by the Canadian team were cross-sections of soil from marshlands along the Pacific Coast that showed clearly where a major tsunami in the year 1700 had deposited sediments.

The researchers also identified "ghost forests" with the remains of ancient trees inundated by flood waters following the undersea mega-quake three centuries ago.

Scientists have previously identified the date of the last megathrust earth-quake in the region at Jan. 26, 1700.

The degree of precision is partly due to the 2003 discovery of shipping records in Japan detailing the sinking of a rice barge in the harbour of Hitachinaka, Japan - evidently the result of the tsunami unleashed when the seabed ruptured 7,000 kilometres away, off the North American coast.