Conventional wisdom among the very small numbers of scientists who study the history of underwater earthquakes off the West Coast has held that 19 massive earthquakes have shaken the area from San Francisco to Vancouver over the past 10,000 years.
That works out to, on average, about one every 500 years.
A new paper suggests those assumptions could be wrong. If they are, scientists could be left with no pattern to predict when the next so-called Big One might hit.
“We ought to be using the best information we can have and what we’ve used so far hasn’t been as good as it ought to be,” said Bobb Carson, of Lehigh University in Pennsylvania, a co-author of the paper published last month in the journal Geology.
The estimate on how often these large earthquakes occur is hotly debated within the tiny field of underwater paleoseismology.
In the science of finding 10,000-year-old earthquakes, researchers pull up long tubes of mud that have built up slowly over time at the bottom of the ocean. Periodically, thick bands of a sand-like substance occur. Researchers say underwater landslides triggered by large earthquakes could have caused those bands.
Studies of core samples that were taken thousands of kilometres apart off the coast of the Pacific Northwest have shown such bands occurred at around the same time, indicating a large-scale earthquake.
Many of the samples show evidence of 19 earthquakes over the past 10,000 years.
Caused by the meeting of the plate under the Pacific Ocean and a part of the North American plate, the great earthquakes that are believed to have struck the area would have had a magnitude of 9.0 and above. Known as megathrust earthquakes, the last was in 1700. Records kept by monks in Japan confirm a tsunami arrived on the other side of the Pacific after the quake.
Timing the next earthquake is important for emergency services preparing for an event that could devastate the Pacific Northwest.
The paper is the latest contradicting the conventional view from the lead author of the new paper, Brian Atwater of the University of Washington.
According to Dr. Carson and his fellow researchers, the existing record “just isn’t dependable.”
The current controversy concerns samples taken in the late 1960s and early 1970s. According to the new paper, those older samples do not show the telltale signs of 19 earthquakes.
One of the proponents of the currently accepted theory dismisses the paper as “seriously flawed.”
Chris Goldfinger of the University of Oregon has published several papers on earthquakes in the Pacific Northwest. He is writing a rebuttal letter for the next edition of Geology.
The disagreement boils down to where the samples were taken. To capture the presence of the underwater landslides, known as turbidity currents, the samples must be from near underwater canyons. According to Dr. Goldfinger, the old samples used in Dr. Atwater’s study were taken without the aid of modern navigational systems, making it impossible to tell exactly where they were from.
“Geology is like real estate: location, location, location. You really need to be spot on,” Dr. Goldfinger said. “I work on these all the time. It’s frustrating because these older cores have great data, but you just don’t know where they came from.”
Audrey Dallimore of Royal Roads University is one of Canada’s leading researchers in underwater geology. While she welcomed the questioning of the current theory, she called Dr. Goldfinger’s existing research “stellar.”
While the field barely existed before 1990, correctly interpreting the signs in the mud at the bottom of the ocean could help save millions of lives. Dr. Goldfinger said newer research is quickly getting closer to guessing how often the big earthquakes occur.
“For Vancouver, Seattle and other cities, it’s quite important that we get this right,” he said.