‘The clock is ticking’: ‘Megathrust’ West coast earthquake could resemble Japan’s, studies say

by Randy Boswell

Two separate geological studies suggest the earthquake hazard in the transboundary region of the Pacific Coast of North America — including southern British Columbia — is significantly greater than previously believed, with both teams of U.S. scientists urging heightened readiness for a future offshore “megathrust” event that could compare with the one that triggered Japan’s catastrophe last year.

In one study, a 13-year, comprehensive analysis of the Cascadia earthquake-prone zone between Vancouver Island and Northern California, a team of researchers led by Oregon State University earth scientist Chris Goldfinger concluded that 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 compiled a detailed record of earthquakes in the Cascadia region going back to 8,000 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,” Prof. 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, Oregon, to Vancouver Island.”

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

“By the year 2060, if we have not had an earthquake, we will have exceeded 85% of all the known intervals of earthquake recurrence in 10,000 years,” he said 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, interprets new fault-zone temperature data along the Pacific Coast to conclude that the probable impact area of the next megathrust quake in the 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,” study co-author Glenn Spinelli, a research scientist with the New Mexico Institute of Mining and Technology, told Postmedia News. “Therefore, if the rupture area of a Cascadia subduction zone earthquake is closer to land, then the shaking on land will be greater.”

Mr. 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. — noted that the goal of preparing for the next major Pacific quake is why U.S. and Canadian officials “have put considerable effort into estimating the potential … limit of seismicity for a potential large earthquake on the plate interface in the Cascadia subduction zone.”

He added that 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.

Prof. 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% within the next 50 years.

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

“At 10% or 15% [risk], earthquakes kind of fall into the category of potholes and repairing bridges and other things that need to be done over time,” said Mr. Goldfinger. “But if the real probability is something more like 30 to 40%, 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 that 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, GSC researcher Lucinda Leonard told Postmedia News at the time that the wall of sea water triggered by the event could well reach Canada.

“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 following the publication of that study. “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.”