Humboldt due for the Big One

Paul Mann
Mad River Union

REDWOOD COAST – The sea level threat to the Redwood Coast is multiplied to an extreme degree by the region’s historical pattern of magnitude 8-9+ earthquakes.

NOAA’s National Ocean Service says fieldwork along the coasts of northern California, Oregon and Washington State indicates that these giant earthquakes are accompanied by shoreline subsidence on the order of three feet or more.

Major tsunami flooding compounds the threat to life and property.

If past is prologue, the coast’s next big one is due anytime. The last great earthquake occurred here a little more than 300 years ago – on the historical cycle of a catastrophe every 300-500 years.

That “strongly suggests that the present regime of relatively quiescent sea-level rise along the California coast north of Cape Mendocino will change virtually instantaneously when the next great earthquake occurs,” the National Ocean Service says. “While the timing of such an event is impossible to predict, the fact that this phenomenon has repeatedly occurred over thousands of years means that it must be taken as a serious threat.”

The region’s extreme vulnerability to quakes, tsunamis, rising sea levels and tectonic land subsidence stems from what the ocean service calls “a wide range of evidence” about the behavior of the Cascadian Subduction Zone.

The Redwood Coast’s hazardous geography connects with a 620-700 mile mega-thrust fault from Cape Mendocino to Northern Vancouver Island.

“Great Subduction Zone earthquakes are the largest in the world, and are the only source zones that can produce earthquakes greater than magnitude 8.5,” according to the Pacific Northwest Seismic Network, a research organization. “The Cascadian Subduction has produced magnitude 9.0 or greater earthquakes in the past, and undoubtedly will in the future.”

In-depth Humboldt State University research has helped to document the historical prevalence of tremors and aftershocks.

“In the past 150 years, nearly 40 earthquakes of magnitude 6 or larger have affected Northern California,” says the university’s informational portfolio, Living on Shaky Ground.

Most of the tremors centered on faults nearby, “but very large earthquakes located elsewhere in the Pacific basin, like the 1964 magnitude 9.2 Alaska earthquake, can generate tsunamis” and gravely damage the region.

Warming oceans magnify the ascending sea level threat. Besides melting ice, says NOAA, the other major cause of rising seas is the thermal expansion of seawater. (Water swells as it warms.)

“The oceans are absorbing more than 90 percent of the increased atmospheric heat associated with emissions from human activity,” says the agency. As continuing ocean and atmospheric warming reinforce each other, “sea levels will likely rise for many centuries at rates higher than that of the current century.”


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